reactive power document and voltage control of north ...nerldc.org/docs/webupload/ner reactive...
TRANSCRIPT
Reactive Power Document and
Voltage Control of North Eastern Region
December-2017
Edition-9
North Eastern Regional Load Despatch Centre
Shillong Power System operation Corporation Limited
(A Government of India Enterprise)
A Typical SVC Station
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 1 of 59
CONTENTS
CONTENTS .......................................................................................................................................................................1
List Details. ........................................................................................................................................................................2
List of Figures: ...................................................................................................................................................................2
List of Tables: ....................................................................................................................................................................3
1 REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL .....................................................................6
1.1 Introduction ................................................................................................................................ 6
1.2 Analogy of Reactive Power ................................................................................................... 8
1.3 Understanding Vectorally .................................................................................................... 10
1.4 Voltage Stability .................................................................................................................. 11
1.5 Voltage Collapse .................................................................................................................. 12
1.6 Proximity to Instability ....................................................................................................... 14
1.7 Reactive Reserve Margin .................................................................................................... 15
1.8 NER GRID – Overview ............................................................................................................. 18
1.9 Reliability Improvement Due to Local Voltage Regulation .............................................. 21
2 TRANSMISSION LINES AND REACTIVE POWER COMPENSATION ................................................................. 22
2.1 Introduction ........................................................................................................................... 22
2.2 Surge Impedance Loading (SIL) ........................................................................................... 23
2.3 Shunt Compensation in Line ............................................................................................. 23
2.4 Line loading as function of Line Length and Compensation ........................................... 24
3 SERIES AND SHUNT CAPACITOR VOLTAGE CONTROL ...................................................................................... 30
3.1 Introduction ...................................................................................................................... 30
3.2 MeSeb Capacity Building And Training Document Suggest (Sub Title As Given In The PFC
Document For Corporatization Of MeSeb): ................................................................................... 31
3.3 As Per The Assam Gazette, Extraordinary, February 10, 2005 ......................................... 31
4 TRANSFORMER LOAD TAP CHANGER AND VOLTAGE CONTROL ...................................................................... 33
4.1 Introduction ..............................................................................................................................33
4.2 As Per The Assam Gazette, Extraordinary, February 10, 2005 .............................................. 35
5 HVDC AND VOLTAGE CONTROL ........................................................................................................................... 37
5.1 Introduction .......................................................................................................................... 37
5.2 HVDC Configuration ............................................................................................................ 37
5.3 Reactive Power Source ...................................................................................................... 40
5.4 ±800 kV HVDC Bi-Pole ....................................................................................................... 40
5.5 Technical details of Biswanath Chariali –Alipurduar-Agra HVDC: ........................................ 41
5.6 Impact of Largest Filter Switching Under Different HVDC Power Order. ........................... 43
6 FACTS AND VOLTAGE CONTROL ..................................................................................................................... 44
6.1 Introduction ...................................................................................................................... 44
6.2 Static Var Compensator (SVC) ............................................................................................... 44
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 2 of 59
6.3 Converter-based Compensator ............................................................................................. 45
6.4 Series-connected controllers ............................................................................................... 46
7 GENERATOR REACTIVE POWER AND VOLTAGE CONTROL ......................................................................... 47
7.1 Introduction .......................................................................................................................... 47
7.2 Synchronous Condensers .................................................................................................... 49
8 CONCLUSION ........................................................................................................................................................ 50
9 SUMMARY .............................................................................................................................................................. 51
10 STATUTORY PROVISIONS FOR REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL .......... 54
10.1 Provision in the Central Electricity Authority (Technical Standard for connectivity to the
grid) Regulations 2007 [8]: ............................................................................................................ 54
10.2 Provision in The Indian Electricity Grid Code (IEGC), 2010: .......................................... 54
11. BIBLIOGRAPHY: ................................................................................................................................................. 59
List Details.
List 1 International connectivity of NER at 400kV (Charged at 132kV) ....................................... 25
List 2 International Connectivity of NER at 132kV ......................................................................... 25
List 3 +/- 800 kV HVDC Lines Agra-BNC ..................................................................................... 25
List 4: Fixed, Switchable and Convertible Line reactors in North Eastern Region ......................... 26
List 5: Bus Reactors in North Eastern Region .................................................................................. 28
List 6: List of Upcoming Bus Reactors in North Eastern Region……………………………………………. 29
List 7: Tertiary Reactors on 33kV side of 400/220/33 kV ICTs in North Eastern Region .............. 29
List 8: Shunt Capacitors details in North Eastern Region ............................................................... 32
List 9: Transmission/Transformation/VAR Compensation Capacity of North Eastern Region ... 36
List of Figures:
Figure 1 Voltage and Current Waveform ............................................................................................ 6
Figure 2 Power Triangle ....................................................................................................................... 7
Figure 3 Boat Pulled by Horse ............................................................................................................ 8
Figure 4 Direction of Pull .................................................................................................................... 8
Figure 5 Vector Representation of Analogy ........................................................................................ 8
Figure 6 Labyrint Spel ......................................................................................................................... 9
Figure 7 Vector Representation ......................................................................................................... 10
Figure 8 Time frames for voltage stability phenomena ..................................................................... 13
Figure 9 PV curve and Voltage stability margin under different conditions .................................... 14
Figure 10 Average cost of Reactive power technologies .................................................................... 17
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 3 of 59
Figure 11 NER Grid map..................................................................................................................... 18
Figure 12 Switching principle of LTC .................................................................................................33
Figure 13: An example of voltage scatter plot…………………………………………………………………………35
Figure 14 HVDC Fundamental components ..................................................................................... 39
Figure 15: Schematic Diagram of HVDC-BNC ................................................................................... 41
Figure 16 Static VAR Compensators (SVC): TCR/TSR, TSC, FC and Mechanically Switched
Resistor ............................................................................................................................................... 45
Figure 17 STATCOM topologies:(a) STATCOM based on VSI and CSI (b) STATCOM with storage
............................................................................................................................................................ 45
Figure 18 Series-connected FACTS controllers: (a) TCSR and TSSR; (b) TSSC; (c) SSSC .............. 46
Figure 19 D-Curve of a typical Generator .......................................................................................... 47
List of Tables:
Table 1: Reactive power compensation sources ................................................................................. 16
Table 2 : Line Parameters & Surge Impedance Loading of Different Conductor Type ................... 24
Table 3: Equipment preference ......................................................................................................... 30
Table 4: AC Filter Bank at HVDC Agra ............................................................................................. 42
Table 5: AC Filter Bank at HVDC BNC. ............................................................................................ 42
Table 6: Impact of Largest Filter Switching under different HVDC Power order. .......................... 43
Table 7: List of units in NER required to be normally operated with free governer action and AVR
in service ............................................................................................................................................ 49
Table 8: IEGC operating voltage range .............................................................................................. 56
ANNEXURES
Annexure I: Fault levels of major substation in NER
Annexure II: List of Lines in North Eastern Region
Annexure III: List of ICTs in North Eastern Region
Annexure IV: Substations in North Eastern Region
Annexure V: Capability curves of various generators in NER GRID
Annexure VI: The AEGCL Gazette, Extraordinary, February 10, 2005
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 4 of 59
EXECUTIVE SUMMARY Quality of power to the stakeholders is the question of the hour worldwide. Enactment of
several regulations viz. IE act – 2003, ABT, Open access regulations, IEGC, DSM and
several other amendments are in the direction towards improvement of system reliability
and power quality.
It is also significant to mention that due to the massive load growth in the country, the
existing power networks are operated under greater stress with transmission lines
carrying power near their limits. Increase in the complexity of network and being loaded
non-uniformly has increased its vulnerability to grid disturbances due to abnormal
voltages (High and Low). In the past, reason for many a black outs across the world have
been attributed to this cause.
Three objectives dominate reactive power management. Firstly, maintaining adequate
voltage throughout the transmission system under normal and contingency conditions.
Secondly, minimizing congestion of real – power flows. Thirdly, minimizing real – power
losses. Also with dynamic ATCs, var compensation, congestion charges, if not seriously
thought, it may have serious commercial implications in times to come due to the amount
of bulk power transfer across the country.
Highlights of rolling year of NER grid include commercial operation of 400/220 kV, 315
MVA ICT 2 at BgTPP, 132 kV Pasighat- Roing S/C line and 132 kV Roing – Tezu S/C line.
BgTPP ICT 2 commissioning has fulfilled the N-I contingency requirement of existing
400/220 kV 315 MVA ICT at Bongaigaon. Commissioning of 132 kV Pasighat – Roing –
Teju has enabled Roing and Teju areas of Arunachal Pradesh to get connected with the all
India Grid. NTPC second unit of capacity 250 MW declared commercially operational in
the month of November’ 2017.
Other major elements commissioned during current year were 20 MVAR Line Reactor in
220 kV Mariani(PG)- AGBPP at AGBPP , 20 MVAR Bus reactor at Roing(PG), 20 MVAR
Bus reactor at Teju(PG), 132/33 kV, 3x5 MVA ICT I & II at Roing (PG), 132/33 kV, 3x5
MVA ICT I & II at Teju (PG) , 132 kV Doyang - Wokha which was further LILO at Sanis,
LILO of 132 kV Aizwal- Zuangtui at Melriat(PG) and 420 kV 63 MVAR Line Reactors (to
be used as Bus Reactor) connected to 400 kV Lower Subansiri – Biswanath Chariali – I
Line Bay & 400 kV Lower Subansiri – Biswanath Chariali - III Line Bay at Biswanath
Chariali (POWERGRID).
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 5 of 59
Commissioning of Melriat substation has strengthened the connectivity of Mizoram with
NER grid. Wokha Substation commissioning has improved connectivity in Nagaland and
it has also provided another evacuation path for DHEP (Doyang Hydro-Electric Power
Project) of NEEPCO. With the commissioning of these elements state network
connectivity with NER grid has further been strengthened. With the increase in
controllability compared to earlier years, grid operation has been smooth and grid
parameters were maintained within the prescribed IEGC limits.
Even though there has been improvement in connectivity of NER grid, this year NER grid
experienced a unique event of sudden reduction in voltage to a very low level without loss
of connectivity with the grid, commonly known as “BROWNOUT”. The event occurred in
Tripura system at 16:11 Hrs on 23-Sep-2017. Before the incident AGTCCPP Power Station,
Tripura Power System and South Comilla (Bangladesh) Power System were connected
with rest of NER Grid through 400/132 kV, 125 MVA ICT- 2 at Palatana, 132 kV
AGTCCPP - Kumarghat I line and 132 kV P K Bari - Kumarghat line. 125 MVA ICT-I at
Palatana was under planned shutdown. After shutdown was withdrawn, while closing HV
side CB of ICT – I, B-phase CB of ICT-I failed to close due to unhealthy operating
mechanism resulted in unbalanced neutral current in ICT-II at Palatana. Before operation
of Pole Discrepancy relay of ICT-I HVCB, ICT-II tripped on Back up Earth fault
Protection. After tripping of both ICTs at Palatana, thus removing the reactive power
support being provided by Palatana generation, Voltage Collapse was observed in Tripura
Power System. Sharp decline in voltage observed at Agartala Bus, voltage went down to
around 5 kV. All state generation were de-synchronized due to low voltage from the Grid.
Although the voltage dropped down to 5 kV, Tripura Power System was still connected to
the rest of NER Grid, thus causing Brownout. Loads were manually disconnected to
improve voltage. This event of Brownout caused huge load and generation loss in Tripura
and South Comilla (Bangladesh). The event has raised the need of sufficient local reactive
power supports like capacitor banks in Tripura system.
This manual is in continuation to the previous edition for understanding the basics of
reactive power and its management towards voltage control, its significance and
consequences of inadequate reactive power support. It also includes details of reactive
power support available at present and efforts by planners from future perspective in
respect of NER grid.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 6 of 59
1 REACTIVE POWER MANAGEMENT AND VOLTAGE
CONTROL
1.1 Introduction
1.1.1 hat is Reactive Power ? Reactive power is a concept used by engineers to
describe the background energy movement in an Alternating Current
(AC) system arising from the production of electric and magnetic fields.
These fields store energy which changes through each AC cycle. Devices which
store energy by virtue of a magnetic field produced by a flow of current are said
to absorb reactive power (viz. Transformers, Reactors) and those which store
energy by virtue of electric fields are said to generate reactive power (viz.
Capacitors).
1.1.2 Power flows, both actual and potential, must be carefully controlled for a
power system to operate within acceptable voltage limits. Reactive power flows
can give rise to substantial voltage changes across the system, which means that
it is necessary to maintain reactive power balances between sources of
generation and points of demand on a 'zonal basis'. Unlike system frequency,
which is consistent throughout an interconnected system, voltages experienced
at points across the system form a "voltage profile" which is uniquely related to
local generation and demand at that instant, and is also affected by the
prevailing system network
arrangements.
1.1.3 In an interconnected AC grid, the
voltages and currents alternate up
and down 50 times per second (not
necessarily at the same time). In
that sense, these are pulsating
quantities. Because of this, the power being transmitted down a single line also
“pulsates” - although it goes up and down 100 times per second rather than 50.
1.1.4 To distinguish reactive power from real power, we use the reactive power unit
called “VAR” - which stands for Volt-Ampere-Reactive (Q). Normally electric
power is generated, transported and consumed in alternating current (AC)
W
Figure 1 Voltage and Current Waveform
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 7 of 59
networks. Elements of AC systems supply (or produce) and consume (or absorb
or lose) two kinds of power: real power and reactive power.
1.1.5 Real power accomplishes useful work (e.g., runs motors and lights lamps).
Reactive power supports the voltages that must be controlled for system
reliability. In AC power networks, while active power corresponds to useful
work, reactive power supports voltage magnitudes that are controlled for
system reliability, voltage stability, and operational acceptability.
1.1.6 VAR Management? It is defined as the control of generator voltages,
variable transformer tap settings, compensation, switchable shunt capacitor
and reactor banks plus allocation of new shunt capacitor and reactor banks in a
manner that best achieves a reduction in system losses and/or voltage control.
1.1.7 Although active power can be transported over long distances, reactive power is
difficult to transmit, since the reactance of transmission lines is often 4 to 10
times higher than the resistance of the lines. When the transmission system is
heavily loaded, the active power losses in the
transmission system are also high. Reactive power
(vars) is required to maintain the voltage to deliver
active power (watts) through transmission lines.
When there is not enough reactive power, the voltage
sags down and it is not possible to push the power demanded by loads through
the lines. Reactive power supply is necessary in the reliable operation of AC
power systems. Several recent power outages worldwide may have been a result
of an inadequate reactive power supply which subsequently led to voltage
collapse.
1.1.8 Voltage and current may not pulsate up and down at the same time. When the
voltage and current do go up and down at the same time, only real power is
transmitted. When the voltage and current go up and down at different times,
reactive power is also gets transmitted. How much reactive power and
which direction it is flowing on a transmission line depend on how different
these two items are.
Although AC voltage and current pulsate at the same frequency, they peak at a
different time. Power is the algebraic product of voltage and current. Over a
cycle, power has an average value, called real power (P), measured in volt-
amperes, or watts. There is also a portion of power with zero average value that
is called reactive power (Q), measured in volt-amperes reactive, or vars. The
total power is called apparent power or Complex power, measured in volt-
amperes, or VA.
Figure 2 Power Triangle
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 8 of 59
1.2 Analogy of Reactive Power
1.2.1 Why an analogy? Reactive Power is an essential aspect of the electricity system,
but one that is difficult to comprehend by a lay man. The horse and the boat
analogy best describe the Reactive Power aspect.
Visualize a boat on a canal, pulled by a horse on the bank of the canal.
The horse is not in front of the boat to do a meaningful work of pulling it in a
straight path. Due to the balancing compensation by the rudder of the boat, the
boat is made to move in a straight manner rather deviating towards the bank.
This is in line with the understanding of the reactive power.
1.2.1 In the horse and boat analogy, the horse’s objective (real power) is to move the
boat straight. The fact that the rope is being pulled from the flank of the horse
and not straight behind it, limits the horse’s capacity to deliver real work of
Figure 4 Direction of Pull
Figure 5 Vector Representation of Analogy
Figure 3 Boat Pulled by Horse
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 9 of 59
moving straight. Therefore, the power required to keep the boat steady in
navigating straight is delivered by the rudder movement (reactive power).
Without reactive power there can be no transfer of real power, likewise without
the support of rudder, the boat cannot move in a straight line.
1.2.2 Reactive power is like the bouncing up and down that happens when we walk
on a trampoline. Because of the nature of the trampoline, that up-down
bouncing is an essential part of our forward movement across the trampoline,
even though it appears to be movement in the opposite direction.
1.2.3 Reactive power and real power work together in the way that’s illustrated very
well by the labyrinth puzzle, LABYRINTSPEL:
The description of the puzzle begins to show
why this game represents the relationship
between real and reactive power:
The intent is to manipulate a steel ball
(1.2cm in diameter) through the maze by
rotating the knobs – without letting the ball
fall into one of the holes before it reaches the
end of the maze. If a ball does fall prematurely
into a hole, a slanted floor inside the box
returns the ball to the user in the trough on the
lower right corner of the box.
1.2.4 The Objective is to twist the two knobs to adjust the angle of the platform in two
directions, in order to keep the ball rolling through the maze without falling
into any holes. Those twists are REACTIVE POWER, which helps propel the
real power through to its ultimate goal, which is delivery to the user. Without
reactive power, ball falls into holes along the way, which are NETWORK
failures.
1.2.5 Both of these examples illustrate how important it is to understand the system
and how it works in order to meet our objectives effectively. In the
LABYRINTSPEL game, if the structure of the system is not taken into account,
winning would be really easy because one knob would be turned all the way in
one direction, and the other knob all the way in the other direction, and the ball
would merely roll across the platform. If that’s the model how electricity works,
then that would deliver the electrons to the end user in the form of real power.
Figure 6 Labyrint Spel
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 10 of 59
But in the game, on the trampoline, and in the electric power network, the
system has more going on that means it’s essential to do things that seem
counterintuitive, like bouncing up and down on the trampoline or turning the
platform in the game towards west to avoid the hole to the east, even though we
have to go east to win.
1.2.6 In electric power, the counterintuitive thing about reactive power is to use some
power along the path to balance the flow of electrons and the circuits.
Otherwise, the electricity just flows from the generator to the largest consumer
(that’s Kirchhoff’s law, basically). In this sense, reactive power is like water
pressure in a water network.
1.2.7 LABYRINTSPEL game and the trampoline are good examples that they capture
the fact that mathematically, real power and reactive power are pure
conjugates.
1.3 Understanding Vectorally
1.3.1 In practice circuits are invariably combinations of resistance, inductance and
capacitance. The combined effect of these impedances to the flow of current is
most easily assessed by expressing the power flows as vectors that show the
angular relationship between the powers waveforms associated with each type
of impedance. Figure 7 shows how the vectors can be resolved to determine the
net capacity of the circuit needed to transfer the power requirements of the
connected equipment.
1.3.2 The useful power that can be drawn
from the electricity distribution system
is represented by the vertical vector in
the diagram and is measured in
kilowatts (kW).The reactive or wattless
power that is a consequence of the
inductive load in the circuit is
represented by the horizontal vector to
the right and the reactive power
attributable to the circuit capacitance by
the horizontal vector to the left. These
are measured in kilovars (kVAr).
Figure 7 Vector Representation
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 11 of 59
1.3.3 The resolution of these vectors, which is the diagonal vector in the diagram is
the capacity required to transmit the active power, and is measured in kilovolts-
ampere (kVA). The ratio of the kW to kVA is the cosine of the angle in the
diagram shown as theta, and is referred to as the “power factor”.
1.3.4 When the net impedance of the circuit is solely resistance, so that the
inductance and capacitance exactly cancel each other out, then the angle theta
becomes zero and the circuit has a power factor of unity. The circuit is now
operating at its highest efficiency for transferring useful power. However, as a
net reactive power emerges the angle theta starts to increase and its cosine falls.
1.3.5 At low power factors the magnitude of the kVA vector is significantly greater
than the real power or kW vector. Since distribution assets such as cables, lines
and transformers must be sized to meet the kVA requirement, but the useful
power drawn by the customer is the kW component, a significant cost emerges
from having to over-size the distribution system to accommodate the
substantial amount of reactive power that is associated with the active power
flow.
1.4 Voltage Stability
1.4.1 Power flows, both actual and potential, must be carefully controlled for a
power system to operate within acceptable voltage limits and vice versa. Not
only is reactive power necessary to operate the transmission system reliably,
but it can also substantially improve the efficiency with which real power is
delivered to customers. Increasing reactive power production at certain
locations (usually near a load center) can sometimes alleviate transmission
constraints and allow cheaper real power to be delivered into a load pocket.
1.4.2 Voltage control (keeping voltage within defined limits) in an electric power
system is Important for proper operation of electric power equipment and
saving it from imminent damage, to reduce transmission losses and to maintain
the ability of the system to withstand disturbances and prevent voltage collapse.
In general terms, decreasing reactive power causes voltages to fall, while
increasing reactive power causes voltages to rise. A voltage collapse occurs
when the system is trying to serve much more load than the voltage can
support.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 12 of 59
1.4.3 As voltage drops, current must increase to maintain the power
supplied, causing the lines to consume more reactive power and the voltage to
drop further. If current increases too much, transmission lines trip, or go off-
line, overloading other lines and potentially causing cascading
failures. If voltage drops too low, some generators will automatically
disconnect to protect themselves.
1.4.4 Usually the causes of under – voltages are:
Overloading of supply transformers
Inadequate short circuit level in the point of supply
Excessive voltage drop across a long feeder
Poor power factor of the connected load
Remote system faults , while they are being cleared
Interval in re-closing of an auto-reclosure
Starting of large HP induction motors
1.4.5 If the declines continue, these voltage reductions cause additional elements to
trip, leading to further reduction in voltage and loss of load. The result is a
progressive and uncontrollable decline in voltage, all because the power system
is unable to provide the reactive power required to supply the reactive power
demand.
1.5 Voltage Collapse
1.5.1 When voltages in an area are significantly low or blackout occurs due to the
cascading events accompanying voltage instability, the problem is considered to
be a voltage collapse phenomenon. Voltage collapse normally takes place when
a power system is heavily loaded and/or has limited reactive power to support
the load. The limiting factor could be the lack of reactive power (SVC and
generators hit limits) production or the inability to transmit reactive power
through the transmission lines.
1.5.2 The main limitation in the transmission lines is the loss of large amounts of
reactive power and also line outages, which limit the transfer capacity of
reactive power through the system.
1.5.3 In the early stages of analysis, voltage collapse was viewed as a static problem
but it is now considered to be a non linear dynamic phenomenon. The dynamics
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 13 of 59
in power systems involve the loads, and voltage stability is directly related to
the loads. Hence, voltage stability is also referred to as load stability.
1.5.4 There are other factors which also contribute to voltage collapse, and
are as below:
Increase in load
Action of tap changing transformers
Load recovery dynamics
All these factors play a significant part in voltage collapse as they effect the
transmission, consumption, and generation of reactive power.
Usually voltage stability is categorized into two parts
Large disturbance voltage stability
Small disturbance voltage stability
1.5.5 When a large disturbance occurs, the ability of the system to maintain
acceptable voltages falls due to the impact of the disturbance. Ability to
maintain voltages is dependent on the system and load characteristics, and the
Figure 8 Time frames for voltage stability phenomena
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 14 of 59
interactions of both the continuous and the discrete controls and protections.
Similarly, the ability of the system to maintain voltages after a small
perturbation i.e. incremental change in load is referred to as small disturbance
voltage stability. It is influenced by the load characteristics, continuous control
and discrete controls at a given instant of time.
1.6 Proximity to Instability
1.6.1 Static voltage instability is mainly associated with reactive power
imbalance. Thus, the loadability of a bus in a system depends on the reactive
power support that the bus can receive from the system. As the system
approaches the maximum loading point or voltage collapse point, both real and
reactive power losses increase rapidly.
1.6.2 Therefore, the reactive power supports have to be locally adequate. With static
voltage stability, slowly developing changes in the power system occur that
eventually lead to a shortage of reactive power and declining voltage.
1.6.3 This phenomenon can be seen from a
plot of power transferred versus
voltage at the receiving end. These
plots are popularly referred to as P–V
curves or ‘Nose’ curves. As power
transfer increases, the voltage at the
receiving end decreases. In the fig(9)
eventually, a critical (nose) point, the
point at which the system reactive
power is out of usage, is reached
where any further increase in active
power transfer will lead to very rapid
decrease in voltage magnitude.
1.6.4 Before reaching the critical point, a large voltage drop due to heavy reactive
power losses is observed. The only way to save the system from voltage collapse
is to reduce the reactive power load or add additional reactive power prior to
reaching the point of voltage collapse.
Knee
point
∆v
Figure 9 PV curve and Voltage stability
margin under different conditions
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 15 of 59
These are curves drawn between V and P of a critical bus at a constant load
power factor.
These are produced by using a series of power flow solutions for different load
levels.
At the knee point or the nose point of the V-P curve, the voltage drops rapidly
with an increase in the load demand.
Power flow solution fails to converge beyond this limit which indicates the
instability.
1.7 Reactive Reserve Margin
1.7.1 The amount of unused available capability of reactive power static as well as
dynamic in the system (at peak load for a utility system) as a percentage of total
capability is known as Reactive reserve margin.
1.7.2 Voltage collapse normally occurs when sources producing reactive power reach
their limits i.e. generators, SVCs or shunt reactors, and there is not much
reactive power to support the load. As reactive power is directly related to
voltage collapse, it can be used as a measure of voltage stability margin.
1.7.3 The voltage stability margin can be defined as a measure of how close the
system is to voltage instability, and by monitoring the reactive reserves in the
power system, proximity to voltage collapse can be monitored.
1.7.4 In case of reactive reserve criteria, the reactive power reserve of an individual or
group of VAr sources must be greater than some specified percentage (x %) of
their reactive power output under all contingencies. The precincts where
reactive power reserves were exhausted would be identified as critical areas.
1.7.5 Reactive power requirements over and above those which occur naturally are
provided by an appropriate combination of reactive source/devices which are
normally classified as static and dynamic devices.
STATIC SOURCES: Static sources are typically transmission and distribution
equipments such as Capacitors and Reactors that are relatively static and can
respond to the changes in voltage – support requirements only slowly and in
discrete steps. Devices are inexpensive, but the associated switches, control,
and communications, and their maintenance, can amount to as much as one
third of the total operations and maintenance budget of a distribution system.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 16 of 59
DYNAMIC SOURCES: It includes pure reactive power compensators like
synchronous condensers, Synchronous generators and solid-state devices
such as FACTS, SVC, STATCOM, D-VAR, and SuperVAR which are normally
dynamic and can respond within cycles to changing reactive power
requirement. These are typically considered as transmission service devices.
1.7.6 Static devices typically have lower capital costs than dynamic devices, and from
a system point of view, they are used to provide normal or intact-system voltage
support and to adapt to slowly changing conditions, such as daily load cycles and
scheduled transactions. By contrast, dynamic reactive power sources must be
deployed to allow the transmission system to respond to rapidly changing
conditions on the transmission system, such as sudden loss of generators or
transmission facilities. An appropriate combination of both static and dynamic
resources is needed to ensure reliable operation of the transmission system at an
appropriate level of costs.
1.7.7 Reactive power absorption occurs when current flows through an inductance.
Inductance is found in transmission lines, transformers, and induction motors
etc. The reactive power absorbed by a transmission line or transformer is
proportional to the square of the current.
Sources of Reactive Power Sinks of Reactive Power
Static:
Shunt Capacitors
Filter banks
Underground cables
Transmission lines (lightly
loaded)
Dynamic:
Synchronous
Generators/Synchronous
Condensers
FACTS (e.g.,SVC,STATCOM)
Transmission lines (Heavily
loaded)
Transformers
Shunt Reactors
Synchronous Generators
FACTS (e.g.,SVC,STATCOM)
Induction generators (wind plants)
Loads
Induction motors (Pumps, Fans
etc)
Inductive loads (Arc furnace etc)
Table 1: Reactive power compensation sources
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 17 of 59
1.7.8 A transmission line also has capacitance. When a small amount of current is
flowing, the capacitance dominates, and the lines have a net capacitive effect
which raises voltage. This happens at night when current flows/Load is low.
During the day, when current flow/load is high, inductive effect is greater than
the capacitance, and the voltage sags.
Figure 10 Average cost of Reactive power technologies
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 18 of 59
1.8 NER GRID – Overview
1.8.1 NER grid with a maximum peak requirement of around 2700 MW and installed
capacity of 3769 MW caters to the seven north eastern states (namely Arunachal
Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland and Tripura). It is
synchronously connected with ER Grid through 400 kV BONGAIGAON – NEW
SILIGURI D/C, 400 kV BONGAIGAON – ALIPURDUAR D/C, 220 kV BIRPARA –
ALIPURDUAR D/C and internationally through 132 kV SALAKATI –
GELYPHU(Bhutan) , 132 kV RANGIA – DEOTHANG (Bhutan), 132 kV D/C
SURAJMANINAGAR – COMILLA (Bangladesh) and 11 kV MOREH –
TAMU(Myanmar). Also, it is connected to NR grid through ±800kV HVDC Bipole
Biswanath Charali-Agra link. The bottle neck of operating the NER grid arises because
of the brittle back bone network of about 8172 Ckt Kms of 132 KV lines, 3410 Ckt Kms
of 220 KV lines and 4295 Ckt Kms of 400 KV lines compared to other regional grids.
1.8.2 With Commissioning of first 800kV multi-terminal HVDC between Biswanath Charali,
Alipurduar and Agra , NER grid is directly connected with NR grid by this HVDC link.
The capacity of each terminal at Biswanath Charali(NER) and Alipurduar (ER) is 3000
MW and at Agra it is 6000 MW, at ± 800 kV voltage level.
Figure 11 NER Grid map
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 19 of 59
1.8.3 Highlights of NER grid for current year include commercial operation of 400/220 kV
315 MVA ICT 2 at BgTPP, 132 kV Pasighat- Roing S/C line and 132 kV Roing – Tezu
S/C line. BgTPP ICT 2 commissioning shall fulfill the N-I contingency requirement of
existing 400/220 kV 315 MVA ICT at Bongaigaon. Commissioning of 132 kV Pasighat
– Roing –Teju has enabled Roing and Teju areas of Arunachal Pradesh to get
connected with the all India Grid. NTPC second unit of capacity 250 MW declared
commercially operational in the month of April. Other major elements commissioned
during current year were 20 Mvar Line Reactor in 220 kV Mariani(PG)- AGBPP at
AGBPP , 20 MVAR Bus reactor at Roing(PG), 20 MVAR Bus reactor at Teju(PG),
132/33 kV, 3x5 MVA ICT I & II at Roing (PG), 132/33 kV, 3x5 MVA ICT I & II at Teju
(PG) , 132 kV Doyang - Wokha which was LILO at Sanis, LILO of 132 kV Aizwal-
Zemabawk at Melriat(PG) and 420 kV 63 MVAR Line Reactor (to be used as Bus
Reactor) connected to 400 kV Lower Subansiri – Biswanath Chariali - I Line Bay &
400 kV Lower Subansiri – Biswanath Chariali - III Line Bay at Biswanath Chariali
(POWERGRID). Commissioning of Melriat substation has strengthened the
connectivity of Mizoram with NER grid. Wokha Substation commissioning has
improved connectivity in Nagaland and it has also provided another evacuation path
for DHEP (Doyang Hydro-Electric Power Project) of NEEPCO.
1.8.4 Almost 50% of the total NER load is spread out in 132 kV pocket of southern part of
NER which were without the direct support of major EHV trunk lines. This part of the
network was highly sensitive and was susceptible to grid disturbance in the past and
demanded more operational acumen. Increase in the loading of major 132 kV trunk
lines, in particular 132 kV DIMAPUR – IMPHAL S/C,132 kV JIRIBAM – LOKTAK S/C
and 132 kV BADARPUR – KHLIEHRIAT S/C in peak hours has led to many a grid
incidents in the past in the form of cascade tripping accompanied by voltage sag.
However, with system augmentation grid incidence in this part of the grid has become
a matter of past.
1.8.5 Relationship between frequency and voltage is a well-known fact. Studies have
revealed that though voltage is a localized factor, it is directly affected by the frequency
which is a notional factor. Any lopsidedness in the demand/generation side leading to
fluctuations in NEW grid frequency affects NER grid immensely, in particular the
voltage profile of the grid, leading to sagging and swelling of voltage heavily during
such occasions. Ironically, NER was synchronously connected with NEW grid for
stretching the transmission capability to reduce the load – generation mismatch of the
country.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 20 of 59
1.8.6 FSC’s have been integrated with the NER system in the 400 kV Balipara – Bongaigaon
III & IV at Balipara end.
1.8.7 Presently NER Grid is supported by 3055 MVAr from shunt reactors and 273 MVAr
from shunt capacitors spread across the region.
1.8.8 Skewness in the location of hydro stations and load centers in NER is another obstacle
which aggravates the voltage problem further. Lines are long and pass through
difficult terrains to the load centers. Northern part of NER grid which is well
supported by some strong 400 KV and 220 KV network faces high voltage regime
during lean hydro period as the corridor is not fully utilized and is usually lightly
loaded. Supports from hydro stations in condenser mode are not available for
containing low voltage conditions. D curve optimization is yet to be realized fully due
to technical glitches.
1.8.9 Reactive power management and voltage control are two aspects of a single activity
that both supports reliability and facilitates commercial transaction across
transmission network. Controlling reactive power flow can reduce losses and
congestion on the transmission system.
1.8.10 Operationally in NER, Voltage is normally controlled by managing production and
absorption of reactive power in real time :
a. By Switching in and out of Line reactance compensators such as capacitors
and shunt reactors (Line/Bus Reactors) as and when system demands in co-
operation with the constituents and the CTU.
b. By Circuit switching: Mostly one circuit of the lightly loaded d/c line is kept
open keeping in mind the n-1 criterion during high voltage and high
frequency period. Voltage differences as well as fault level of stations are
taken into account before any switching operation of circuits. Fault Level at
important substations of NER is mentioned in Annexure I.
c. By using automatic voltage regulators (AVR), the generating units control
field excitation to maintain the scheduled voltage levels at the terminals of
the generators. In real time operation, the generation/consumption of
reactive power must be within the capability curve of generator.
d. By generation re-dispatch/rescheduling.
e. By regulating voltage with the help of OLTC’s.
f. By load staggering/shedding.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 21 of 59
1.9 Reliability Improvement Due to Local Voltage Regulation
1.9.1 Local voltage regulation to a voltage schedule supplied by the utility can have a
very beneficial effect on overall system reliability, reducing the problems caused
by voltage dips on distribution circuits such as dimming lights, slowing or
stalling motors, dropout of contactors and solenoids, etc.
1.9.2 In past years a voltage drop would inherently reduce load, helping the situation.
Light bulbs would dim and motors would slow down with decreasing voltage.
Dimmer lights and slower motors typically draw less power, so the situation
was in a certain sense self-correcting. With modern loads, this situation is
changing.
1.9.3 Today many incandescent bulbs are being replaced with compact fluorescent
lights, LED lamps that draw constant power as voltage decreases, and motors
are being powered with adjustable-speed drives that maintain a constant speed
as voltage decreases. In addition, voltage control standards are rather
unspecific, and there is a tremendous opportunity for an improvement in
efficiency and reliability from better voltage regulation. Capacitors supply
reactive power to boost voltage, but their effect is dramatically diminished as
voltage dips.
1.9.4 Capacitor effectiveness is proportional to the square of the voltage, so at 80%
voltage, capacitors are only 64% as effective as they are at normal conditions.
As voltage continues to drop, the capacitor effect falls off until voltage collapses.
The reactive power supplied by an inverter is dynamic, it can be controlled very
rapidly, and it does not drop off with a decrease in voltage. Distribution systems
that allow customers to supply dynamic reactive power to regulate voltage
could be a tremendous asset to system reliability and efficiency by expanding
the margin to voltage collapse.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 22 of 59
2 TRANSMISSION LINES AND REACTIVE POWER
COMPENSATION
2.1 Introduction
2.1.1 In moving power from generators to loads, the transmission network introduces
both real and reactive losses. Housekeeping loads at substations (such as security
lighting and space conditioning) and transformer excitation losses are roughly
constant (i.e., independent of the power flows on the transmission system).
Transmission-line losses, on the other hand, depend strongly on the amount of
power being transmitted.
2.1.2 Real-power losses arise because aluminum and copper (the materials most often
used for transmission lines) are not perfect conductors; they have resistance. The
consumption of reactive power by transmission lines increases with the square of
current i.e., the transmission of reactive power requires an additional demand for
reactive power in the system components.
2.1.3 The reactive-power nature of transmission lines is associated with the geometry of
the conductors themselves (primarily the radius of the conductor) and the
geometry of the conductor configuration (the distances between each conductor
and ground and the distances among conductors).
2.1.4 The reactive-power behavior of transmission lines is complicated by their inductive
and capacitive characteristics. At low line loadings, the capacitive effect dominates,
and generators and transmission-related reactive equipment must absorb reactive
power to maintain line voltages within their appropriate limits. On the other hand,
at high line loadings, the inductive effect dominates, and generators, capacitors,
and other reactive devices must produce reactive power
2.1.5 The thermal limit is the loading point (in MVA) above which real power losses in
the equipment will overheat and damage the equipment. Most transmission
elements (e.g., conductors and transformers) have normal thermal limits below
which the equipment can operate indefinitely without any damage. These types of
equipment also have one or more emergency limits to which the equipment can be
loaded for several hours with minimal reduction in the life of the equipment.
2.1.6 If uncompensated, these line losses reduce the amount of real power that can be
transmitted from generators to loads. Transmission-line capacity decreases as the
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 23 of 59
line length increases if there is no voltage support (injection or absorption of
reactive power) on the line.
2.2 Surge Impedance Loading (SIL)
2.2.1 Transmission lines and cables generate and consume reactive power at the
same time. The reactive power generation is almost constant, because the
voltage of the line is usually constant, and the line’s reactive power
consumption depends on the current or load connected to the line that is
variable. So at the heavy load conditions transmission lines consume reactive
power, decreasing the line voltage, and in the low load conditions – generate,
increasing line voltage.
2.2.2 The case when line’s reactive power produced by the line capacitance is equal to
the reactive power consumed by the line inductance is called natural loading or
surge impedance loading (SIL) , meaning that the line provides exactly the
amount of MVAr needed to support its voltage. The balance point at which the
inductive and capacitive effects cancel each other is typically about 40% of the
line’s thermal capacity. Lines loaded above SIL consume reactive power, while
lines loaded below SIL supply reactive power.
2.2.3 A 400 kV, line generates approximately 55 MVAR per 100 km/Ckt, when it is
idle charged due to line charging susceptance. This implies a 300 km line
generates about 165 MVAR when it is idle charged.
2.3 Shunt Compensation in Line
2.3.1 Normally there are two types of shunt reactors; Line reactor and bus reactor.
Line reactors are normally used to control over voltage due to switching and
load rejection whereas Bus reactors are normally used to control the steady
state over voltages during light load conditions.
2.3.2 The degree of compensation is decided by an economic point of view between
the capitalized cost of compensator and the capitalized cost of reactive power
from supply system over a period of time. In practice a compensator such as a
bank of capacitors (or inductors) can be divided into parallel sections, each
Switched separately, so that discrete changes
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 24 of 59
in the compensating reactive power may be made, according to the
requirements of the load.
2.3.3 Reasons for the application of shunt capacitor units are :
Increase voltage level at the load
Improves voltage regulation if the capacitor units are properly switched.
Reduces I2R power loss in the system because of reduction in current.
Increases power factor of the source generator.
Decrease kVA loading on the source generators and circuits to relieve an
overloaded condition or release capacity for additional load growth.
By reducing kVA loading on the source generators additional kilowatt
loading may be placed on the generation if turbine capacity is available.
2.4 Line loading as function of Line Length and Compensation
2.4.1 The operating limits for transmission lines may be taken as minimum of
thermal rating of conductors and the maximum permissible line loadings
derived. SIL given in table below is for uncompensated line.
Table 2 : Line Parameters & Surge Impedance Loading of Different Conductor Type
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 25 of 59
List 1: International connectivity of NER at 400kV (Charged at 132kV)
SR.
NO
.
FROM TO UTILITY KM CKT CONDUCTOR
1 Comilla Surajmani
Nagar POWERGRID 47 1 ACSR Twin Moose
2 Comilla Surajmani
Nagar POWERGRID 47 2 ACSR Twin Moose
List 2 : International Connectivity of NER at 132kV
SR.
NO
.
FROM TO UTILITY KM CKT CONDUCTOR
1 Gelyphu(BH
U)
Salakati(IND
) POWERGRID 49.2 1 ACSR Panther
2 Motonga(BH
U) Rangia(IND) AEGCL 49 2 ACSR Panther
List 3 : +/- 800 kV HVDC Lines Agra-BNC
SR.
NO
.
FROM TO UTILITY KM CKT CONDUCTOR
1 Agra Biswanath
Charali POWERGRID 1728 1
Hexa Lapwing
2 Agra Biswanath
Charali POWERGRID 1728 2
Hexa Lapwing
List of Transmission Lines in NER GRID along with their line length and conductor type is
given in ANNEXURE II
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 26 of 59
List 4 : Fixed, Switchable and Convertible Line reactors in North Eastern Region
SR.
NO. UTILITY FROM TO
INSTALLED
AT (STATION) KV MVAR KM CONVERTIBLE SWITCHABLE FIXED
1 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 50 289.9 .... …. TRUE 2 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 50 289.9 .... …. TRUE 3 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 289.9 TRUE …. …. 4 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 289.9 TRUE …. …. 5 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 63 305 TRUE …. …. 6 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 63 305 TRUE …. …. 7 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 305 TRUE …. …. 8 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 305 TRUE …. …. 9 POWERGRID BONGAIGAON BINAGURI(ER) BONGAIGAON 400 63 218 .... …. TRUE 10 POWERGRID BONGAIGAON BINAGURI(ER) BONGAIGAON 400 63 218 .... …. TRUE 11 POWERGRID MISA NEW MARIANI MISA 220 50 222.7 .... …. TRUE 12 POWERGRID MISA MARIANI MISA 220 50 220 …. …. TRUE 13 POWERGRID PALATANA SILCHAR SILCHAR 400 50 247 …. TRUE …. 14 POWERGRID PALATANA SILCHAR SILCHAR 400 50 247 …. TRUE …. 15 OTPC PALATANA SILCHAR PALLATANA 400 63 247 …. TRUE …. 16 OTPC PALATANA SILCHAR PALLATANA 400 63 247 …. TRUE ….
17 NEEPCO RANGANADI BISWANATH
CHARALI RANGANADI 400 50 204 TRUE …. ….
18 NEEPCO RANGANADI BISWANATH
CHARALI RANGANADI 400 50 204 TRUE …. ….
19 POWERGRID BISWANATH
CHARALI BALIPARA BALIPARA 400 50 65 TRUE ….
....
20 POWERGRID BISWANATH
CHARALI BALIPARA BALIPARA 400 50 65 TRUE ….
.... 21 POWERGRID SILCHAR BYRNIHAT SILCHAR 400 63 217.14 …. TRUE ….
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 27 of 59
22 MeECL SILCHAR BYRNIHAT BYRNIHAT 400 63 217.4 TRUE …. …. 23 POWERGRID SILCHAR AZARA SILCHAR 400 63 264 …. TRUE …. 24 AEGCL SILCHAR AZARA AZARA 400 63 264 …. …. TRUE 25 POWERGRID BANGAIGAON BYRNIHAT BANGAIGAON 400 63 167 TRUE …. …. 26 POWERGRID BANGAIGAON AZARA BANGAIGAON 400 63 118 TRUE …. ….
27 NEEPCO NEW
MARIANI AGBPP AGBPP 220 20 160.54 TRUE …. ….
NOTE: CONVERTIBLE: LINE REACTORS WHICH CAN BE OPERATED UPON ONLY WHEN LINE IS IN OUT CONDITION.
SWITCHABLE: LINE REACTORS WHICH CAN BE OPERATED EVEN WHEN LINE IS IN SERVICE.
FIXED: LINE REACTORS WHICH ARE FIXED AND CANNOT BE OPERATED UPON AS A BUS REACTOR.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 28 of 59
List 54: Bus Reactors in North Eastern Region
SR. NO. UTILITY INSTALLED AT
(STATION) KV
RATING STATUS
MVAR MAKE
1 POWERGRID BALIPARA 400 50 BHEL IN SERVICE
2 POWERGRID BALIPARA 400 80 BHEL IN SERVICE
3 POWERGRID BONGAIGAON 400 2 X 50 BHEL IN SERVICE
4 POWERGRID BONGAIGAON 400 2 X 80 BHEL IN SERVICE
5 POWERGRID MISA 400 50 BHEL IN SERVICE
6 POWERGRID SILCHAR 400 2 X 63 CGL IN SERVICE
7 POWERGRID BISWANATH
CHARALI 400 2 X 80 …...
IN SERVICE
8 OTPC PALATANA 400 80 BHEL NOT IN
SERVICE
9 ASSAM MARIANI 220 2 X 12.5 .... IN SERVICE
10 ASSAM SAMAGURI 220 2 X 12.5 .... IN SERVICE
11 POWERGRID AIZWAL 132 20 .... IN SERVICE
12 POWERGRID KUMARGHAT 132 20 .... IN SERVICE
13 TRIPURA DHARMANAGAR 132 2 X 2 .... IN SERVICE
14 POWERGRID ZIRO 132 20 …. IN SERVICE
15 POWERGRID IMPHAL 132 20 …. IN SERVICE
16 POWERGRID NEW MARIANI 132 20 …. IN SERVICE
17 ASSAM SAMAGURI 132 2X12.5 …. IN SERVICE
18 ASSAM AZARA 400 63 BHEL IN SERVICE
19 MEGHALAYA BYRNIHAT 400 63 CGL NOT IN
SERVICE
20 POWERGRID ROING 132 20 …. IN SERVICE
21 POWERGRID TEJU 132 20 …. IN SERVICE
22 POWERGRID BISWANATH
CHARALI 400 2 X 63
…. IN SERVICE
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 29 of 59
List 6: List of Upcoming Bus Reactors in North Eastern Region
SR. NO. UTILITY
TO BE
INSTALLED AT
(STATION)
KV
RATING
(MVAR)
1 POWERGRID Namsai 132 20
2 POWERGRID Mokokchung 220 31.5
3 POWERGRID Bongaigaon 400 125
4 POWERGRID Balipara 400 125
5 AEGCL Rangia 400 80
6 POWERGRID Ranganadi 400 80
7 AEGCL Sonapur 400 80
8 POWERGRID Misa 400 80
9 POWERGRID New Mariani 400 125
10 POWERGRID Imphal 400 125
11 POWERGRID Imphal 400 80
12 POWERGRID Silchar 400 125
13 STERLITE PK Bari 400 125
14 STERLITE Surjamaninagar 400 125
List 7: Tertiary Reactors on 33kV side of 400/220/33 kV ICTs in North Eastern
Region
SR. NO. UTILITY
INSTALLED
AT
(STATION)
INSTALLED
ON
RATING
STATUS MVAR MAKE
1 POWERGRID BALIPARA 33 KV SIDE
OF ICT I 4 X 25 BHEL
IN
SERVICE
2 POWERGRID BONGAIGAON 33 KV SIDE
OF ICT I 2 X 25 BHEL
IN
SERVICE
3 POWERGRID MISA 33 KV SIDE
OF ICT I 4 X 25 BHEL
IN
SERVICE
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 30 of 59
3 SERIES AND SHUNT CAPACITOR VOLTAGE CONTROL
3.1 Introduction
3.1.1 Capacitors aid in minimizing operating expenses and allow the utilities to serve
new loads and consumers with a minimum system investment. Series and
shunt capacitors in a power system generate reactive power to improve power
factor and voltage, thereby enhancing the system capacity and reducing the
losses.
3.1.2 In series capacitors the reactive power is proportional to the square of the load
current, thus generating reactive power when it is most needed whereas in
shunt capacitors it is proportional to the square of the voltage. Series capacitors
compensation is usually applied for long transmission lines and transient
stability improvement. Series compensation reduces net transmission line
inductive reactance. The reactive generation I2XC compensates for the reactive
consumption I2X of the transmission line. This is a self-regulating nature of
series capacitors. At light loads series capacitors have little effect.
3.1.3 This is because the
protective equipment for a
series capacitor is often
more complicated. The
factors which influence the
choice between the shunt
and series capacitors are
summarized in Table 3.
3.1.4 Due to various limitations in
the use of series capacitors,
shunt capacitors are widely
used in distribution
systems. For the same
voltage improvement, the rating
of a shunt capacitor will be higher than that of a series capacitor. Thus a series
capacitor stiffens the system, which is especially beneficial for starting large
motors from an otherwise weak power system, for reducing light flicker caused
by large fluctuating load, etc.
Table 3: Equipment preference
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 31 of 59
3.2 MeSEB Capacity Building And Training Document Suggest (Sub
Title As Given In The PFC Document For Corporatization Of MeSEB):
3.2.1 Installation of Shunt-capacitors:
Installation of capacitors is a low cost process for reduction of technical losses.
The agricultural load mainly consists of irrigation pump motors. The PF of
pump motors are generally below 0.6, which means the total reactive power
demand of the system is high. The reactive power demand can be reduced by
installation of suitable capacitors. However, proper maintenance has to be
adopted to keep the system in order. In view of the maintenance problem,
reactive compensation technique could be installed at the distribution
transformer centers. Care has to be taken that it does not lead to over voltage
problems during the off peak hours. To avoid this there should be switch off
arrangement in the capacitor bank. The optimum allocation of LT capacitors at
distribution substation by minimizing a cost function, which includes loss cost
in the beneficiary system and the annual cost of the capacitor bank. The reactive
compensation can also be carried out at the primary distribution feeders (11
KV) lines. The optimum number, size and location of online capacitors will
depend on the following factors:
Type of load.
Quantum of load.
Load factor.
Annual load cycle.
Power factor.
3.3 As Per the Assam Gazette, Extraordinary, February 10,
2005
IN CHAPTER 9: FREQUENCY AND VOLTAGE MANAGEMENT
Sec 9.1 (d) System voltages levels can be affected by Regional operation. The SLDC shall
optimize voltage management by adjusting transformer taps to the extent
available and switching of circuits/ capacitors/ reactors and other operational
steps. SLDC will instruct generating stations to regulate MVAr generation
within their declared parameters. SLDC shall also instruct Distribution
Licensees to regulate demand, if necessary.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 32 of 59
List 8: Shunt Capacitors details in North Eastern Region
SR. NO. UTILITY SUBSTATION INSTALLED
ON
CAPACITY
(MVAR)
1 MeECL MAWLAI 132 KV BUS BAR 12.5
2 MeECL EPIP I 132 KV BUS BAR 20
3 MeECL EPIP II 132 KV BUS BAR 20
4 MeECL EPIP II 33 KV BUS BAR 15
5 MeECL EPIP II 33 KV BUS BAR 15
6 AEGCL BAGHJAB 33 KV BUS BAR 2X5
7 AEGCL KAHELIPARA 33 KV BUS BAR 3X5
8 AEGCL BARNAGAR 33 KV BUS BAR 2X5
9 AEGCL GOSAIGAON 33 KV BUS BAR 1X5
10 AEGCL GAURIPUR 33 KV BUS BAR 1X10
11 AEGCL RANGIA 33 KV BUS BAR 2X10
12 AEGCL MARGHERITA 33 KV BUS BAR 2X5
13 AEGCL N LAKHIMPUR 33 KV BUS BAR 1X5
14 AEGCL DULLAVCHERRA 33 KV BUS BAR 1X5
15 AEGCL DEPOTA 33 KV BUS BAR 2X5
16 AEGCL SARUSAJAI 33 KV BUS BAR 2X10
17 AEGCL ROWTA 33 KV BUS BAR 2X5
18 AEGCL DIPHU 33 KV BUS BAR 2X5
19 AEGCL DIBRUGARH 33 KV BUS BAR 2X10
20 AEGCL SHANKARDEV
NAGAR 33 KV BUS BAR 2X5
21 AEGCL RUPAI 33 KV BUS BAR 2X5
22 AEGCL SRIKONA 33 KV BUS BAR 2X5
Total Capacity of NER
273
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 33 of 59
4 TRANSFORMER LOAD TAP CHANGER AND VOLTAGE
CONTROL
4.1 Introduction
4.1.1 Transformers provide the capability to raise alternating-current generation
voltages to levels that make long-distance power transfers practical and then
lowering voltages back to levels that can be distributed and used. The ratio of the
number of turns in the primary to the number of turns in the secondary coil
determines the ratio of the primary voltage to the secondary voltage. By tapping
the primary or secondary coil at various points, the ratio between the primary
and secondary voltage can be adjusted. Transformer taps can be either fixed or
adjustable under load through the use of a load-tap changer (LTC). Tap capability
is selected for each application during transformer design.
4.1.2 The OLTC alters the power
transformer turns ratio in a
number of pre-defined steps and
in that way changes the
secondary side voltage.
4.1.3 Each step usually represents a
change in LV side no-load voltage
of approximately 0.5-1.7%.
Standard tap changers offer
between ± 9 to ± 17 steps (i.e. 19
to 35 positions). The automatic
voltage regulator (AVR) is
designed to control a power
transformer with a motor driven
on-load tap-changer.
4.1.4 Typically the AVR regulates voltage at the secondary side of the power
transformer. The control method is based on a step-by-step principle which
means that a control pulse, one at a time, will be issued to the on-load tap-
changer mechanism to move it up or down by one position.
Figure 12 Switching principle of LTC
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 34 of 59
4.1.5 The pulse is generated by the AVR whenever the measured voltage, for a given
time, deviates from the set reference value by more than the preset dead band
(i.e. degree of insensitivity). Time delay is used to avoid Unnecessary operation
during short voltage deviations from the pre-set value.
4.1.6 Transformer-tap changers can be used for voltage control, but the control differs
from that provided by reactive sources. Transformer taps can force voltage up (or
down) on one side of a transformer, but it is at the expense of reducing (or
raising) the voltage on the other side. The reactive power required to raise (or
lower) voltage on a bus is forced to flow through the transformer from the bus on
the other side.
4.1.7 The reactive power consumption of a transformer at rated current is within the
range 0.05 to 0.2 p.u. based on the transformer ratings. Fixed taps are useful
when compensating for load growth and other long-term shifts in system use.
LTCs are used for more-rapid adjustments, such as compensating for the voltage
fluctuations associated with the daily load cycle. While LTCs could potentially
provide rapid voltage control, their performance is normally intentionally
degraded. With an LTC, tap changing is accomplished by opening and closing
contacts within the transformer’s tap changing mechanism.
4.1.8 Tap optimization study is done twice in a year for obtaining the optimal tap
position of a transformer. Voltages at a particular bus, where the transformer is
connected, is plotted over a period of time in a Scatter Plot. An example of scatter
plot in given in figure 14. If the density of plots is higher in the first quadrant, i.e,
HV side voltage is higher and LV side voltage is lower, then the tap setting in
increased from the present tap setting. If the density of plots is higher in the
Third quadrant, i.e, HV side voltage is lower and LV side voltage is higher, then
the tap setting in decreased from the present tap setting. In second and fourth
quadrant, tap changing will not help in improvement of voltage.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 35 of 59
List of ICTs in North Eastern Region is given in ANNEXURE III.
4.2 As Per The Assam Gazette, Extraordinary, February 10, 2005
IN CHAPTER 9: FREQUENCY AND VOLTAGE MANAGEMENT
Sec 9.1(d) System voltages levels can be affected by Regional operation. The
SLDC shall optimise voltage management by adjusting transformer taps to the
extent available and switching of circuits/ capacitors/ reactors and other
operational steps. SLDC will instruct generating stations to regulate MVAr
generation within their declared parameters. SLDC shall also instruct
Distribution Licensees to regulate demand, if necessary.
Figure 13: An Example of Voltage Scatter Plot
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 36 of 59
List 9: Transmission/Transformation/VAR Compensation Capacity of
North Eastern Region
TRANSMISSION LINE (CKT KM)
AGENCY HVDC 400 KV 220 KV 132 KV 66 KV
POWERGRID 3456 2755 1737 2568 NIL
NEEPCO NIL NIL NIL 68 NIL
NETC NIL 1328 NIL NIL NIL
ENCIL NIL 212 212 NIL NIL
STATES NIL 0 1461 5536 1492
TOTAL 3456 4295 3410 8172 1492
TRANSFORMATION CAPACITY (MVA)
POWERGRID/NEEPCO/OTPC/NHPC/
NTPC 3170/770/250/5/315 MVA
STATES 8923 MVA
TOTAL 13433 MVA
REACTIVE COMPENSATION (MVAR)
POWERGRID/NEEPCO/OTPC 2398/120/206MVAR
STATES 331 MVAR
CAPACITIVE COMPENSATION – 273 MVAR
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 37 of 59
5 HVDC AND VOLTAGE CONTROL
5.1 Introduction
5.1.1 Basically for transferring power over a long distance or submarine power
transmission, High voltage DC transmission lines (HVDC) are preferred which
transmits power via DC (direct current). They normally consist of two converter
terminals connected by a DC transmission line and in some applications, multi-
terminal HVDC with interconnected DC transmission lines. Back-to-Back DC and
HVDC Light are specific types of HVDC systems. HVDC Light uses new cable and
converter technologies and is economical at lower power levels than traditional
HVDC.
5.2 HVDC Configuration
5.2.1 Bipolar
In bipolar transmission a pair of conductors is used, each at a high potential with
respect to ground, in opposite polarity. However, there are a number of
advantages to bipolar HVDC which can make it the attractive option.
Under normal load, negligible earth-current flows, as in the case of
monopolar transmission with a metallic earth-return. This reduces
earth return loss and environmental effects.
When a fault develops in a line, with earth return electrodes
installed at each end of the line, approximately half the rated power
can continue to flow using the earth as a return path, operating in
monopolar mode.
In very adverse terrain, the second conductor may be carried on an
independent set of transmission towers, so that some power may
continue to be transmitted even if one line is damaged.
A bipolar system may also be installed with a metallic earth return conductor. Bipolar systems may carry as much as 3000 MW at voltages of +/-800 kV (viz., 3000 MW +/- 800 KV Biswanath Charali-Alipurduar-Agra HVDC link in INDIA connecting NER GRID & ER GRID to NR GRID). Submarine cable installations initially commissioned as a monopole may be upgraded with additional cables and operated as a bipole.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 38 of 59
5.2.2 Back to back
A back-to-back station (or B2B for short) is a plant in which both static inverters and rectifiers are in the same area, usually in the same building. The length of the direct current line is kept as short as possible. HVDC back-to-back stations are used for
Coupling of electricity mains of different frequency (as in INDIA;
the interconnection between NEW GRID and SR GRID through
1000 MW HVDC BHADRAVATI and 1000 MW HVDC
GAZUWAKA)
Coupling two networks of the same nominal frequency but no fixed
phase relationship (viz., HVDC SASARAM, HVDC
VINDHYACHAL).
Different frequency and phase number (for example, as a
replacement for traction current converter plants)
5.2.3 A high voltage direct current (HVDC) link consists of a rectifier and an inverter.
The rectifier side of the HVDC link is equivalent to a load consuming positive real
and reactive power and the inverter side of the HVDC link as a generator
providing positive real power and negative reactive power (i.e. absorbing positive
reactive power).
5.2.4 Thyristor based HVDC converters always consume reactive power when in
operation. A DC line itself does not require reactive power and voltage drop on
the line is only the IR drop where I is the DC current. The converters at the both
ends of the line, however, draw reactive power from the AC system. The reactive
power consumption of the HVDC converter/inverter is 50-60 % of the active
power converted. It is independent of the length of the line.
5.2.5 The reactive power requirements of the converter and system have to be met by
providing appropriate reactive power in the station. For those reason reactive
power compensations devices are used together with reactive power control from
the ac side in the form of filter and capacitor banks.
5.2.6 Both AC and DC harmonics are generated in HVDC converters. AC harmonics are
injected into the AC system and DC harmonics are injected into the DC line.
These harmonics have the following harmful effects:
Interference in communication system.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 39 of 59
Extra power losses in machines and capacitors connected in the
system.
Some harmonics may produce resonance in AC circuits resulting in
over voltages.
Instability of converter controls.
5.2.7 Harmonics are normally minimized by using filters. The following types of filters
are used:
AC filters.
DC filters.
High frequency filters.
AC Filters
AC filters are RLC circuits connected between phase and earth. They offer low
impedance to harmonic frequencies. Thus, AC harmonic currents are passed to
earth. Both tuned and damped filter arrangements are used. The AC harmonic
AC Filter
DC Filter
DC FilterAC Filter
DC Filter
DC Filter
Converter Xmers
Valve Halls
-Thyristors
-Firing ckts
-Cooling ckt
Smoothing Reactor
Electrode station
Basic Components of HVDC TerminalBasic Components of HVDC Terminal
400 kV
DC Line
Control Room
-Control & Protection
-Telecommunication
AC PLC
Figure 12 HVDC Fundamental components
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 40 of 59
filters also provide reactive power required for satisfactory operation of
converters and also partly injects reactive power into the system.
DC Filters
DC filters are similar to AC filters. A DC filter is connected between pole bus and
neutral bus. It diverts DC harmonics to earth and prevents them from entering
DC lines. Such a filter does not supply reactive power as DC line does not require
reactive power.
HIGH FREQUENCY FILTERS
HVDC converters may produce electrical noise in the carrier frequency band from
20 Khz to 490 Khz. They also generate radio interference noise in the mega hertz
range of frequencies. High frequency (PLC-RI) filters are used to minimize noise
and interference with PLCC. Such filters are connected between the converter
transformer and the station AC bus.
5.3 Reactive Power Source
Reactive power is required for satisfactory operation of converters and also to
boost the AC side voltages. AC harmonic filters which help in minimizing
harmonics also provide reactive power partly. Additional supply may be obtained
from shunt (switched) capacitor banks usually installed in AC side.
5.4 ±800 kV HVDC Bi-Pole
HVDC Biswanath Charali-Alipurduar-Agra is first +/- 800 kV Multi-terminal
HVDC in India with terminals located at Biswanath Charali (NER), Agra(NR) and
Alipurduar(ER) and operating at +/- 800kV. This HVDC was planned in around
2006 to evacuate generation from NER, Sikkim and Bhutan to load centres in NR
and WR. This is the first HVDC designed to evacuate power from large hydro
projects of NER and ER region. Considering the seasonality of hydro generation
this is the first HVDC bipole having flows in either direction depending upon the
season and providing flexibility and function as a pseudo phase-shifter.
The capacity of each terminal at Biswanath Charali(NER) & Alipurduar (ER) is of
3000 MW.There are two terminals at Agra with capacity of 3000MW each. After
commissioning of this HVDC it has provided first interconnection between NER
and NR and additional interconnection between ER & NR.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 41 of 59
5.5 Technical details of Biswanath Chariali –Alipurduar-Agra HVDC:
The Schematic Diagram of HVDC BNC-Agra is as follow.
The technical details of the line are as follows:
a. Transmission Line
a. Voltage : +/-800 kV DC
b. Length : 1726 km
c. Conductor Type : Lapwing-Hexa bundled
d. Resistance in Ohms (Approx.) : ~12.310
b. Converter Transformers
a. Agra: 4 converter transformers each with capacity of
6*295.1 MVA
b. BNC: 2 converter transformers each with capacity of
6*295.1 MVA
c. Filter Banks
a. At Agra end (Total 3705 MVAR)
Figure 13: Schematic Diagram of HVDC-BNC
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 42 of 59
b. At BNC end (Total1983 MVAR)
S.NO Filter Bank(Z1
Capacity(MVAR)
Filter Bank(Z2
Capacity(MVAR)
Filter Bank(Z3
Capacity(MVAR)
Filter Bank(Z4
Capacity(MVAR)
Filter Bank(Z5
Capacity(MVAR)
1 Hp12 125 Hp12 125 Hp12 125 Hp12 125 Hp3 125
2 Hp12b 201 Hp12
b 201
Hp12b
201 Hp12
b 201
Hp12b
201
3 Shunt capacit
or 200
Hp24/36 b
200 Shunt capacitor
200 Hp24/36 b
200 Hp24/36
200
4 Shunt Capaci
tor 200
Shunt Capac
itor 200
Shunt Capac
itor 200
Shunt Capac
itor 200
Shunt Capac
itor 200
Table 4: AC Filter Bank at HVDC Agra
S.NO Filter
Bank(Z1 Capacity(MVAR)
Filter Bank(Z2
Capacity(MVAR)
Filter Bank(Z3
Capacity(MVAR)
1 HP12 125 HP12 125 HP12 125
2 HP12B 160 HP12B 160 HP12B 160
3 HP24/36 125 Hp24/36 125 HP24/36 125
4 HP3 159 HP24/36 125 HP3 159
5 Shunt
Capacitor 155
Shunt Capacitor
155
Table 5: AC Filter Bank at HVDC BNC.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 43 of 59
5.6 Impact of Largest Filter Switching Under Different HVDC Power
Order.
HVDC Power Order
Voltage at BNC before Switching of Filter
Bank
Voltage at BNC after Switching of Filter
Bank Rise in Voltage
0 417.6 440.6 23
500 426.2 453.8 27.6
1000 403.4 426.4 23
Table 6: Impact of Largest Filter Switching under different HVDC Power order.
Reactive power requirement of Converter transformer varies continuously depending
upon the power order. Reactive power generated by Switching of filters are in blocks so
at each set point there may be excess/deficit of reactive power resulting exchange of
reactive power with the grid causing bus voltage to change. Depending upon voltage to
be increased or decreased set point may be decided accordingly.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 44 of 59
6 FACTS AND VOLTAGE CONTROL
6.1 Introduction
6.1.1 The demands of lower power losses, faster response to system parameter change,
and higher stability of system have stimulated the development of the Flexible AC
Transmission systems (FACTS). Based on the success of research in power
electronics switching devices and advanced control technology, FACTS has
become the technology of choice in voltage control, reactive/active power flow
control, transient and steady-state stabilization that improves the operation and
functionality of existing power transmission and distribution system.
6.1.2 The achievement of these studies enlarge the efficiency of the existing generator
units, reduce the overall generation capacity and fuel consumption, and minimize
the operation cost. The power electronics-based switches in the functional blocks
of FACTS can usually be operated repeatedly and the switching time is a portion
of a periodic cycle, which is much shorter than the conventional mechanical
switches.
6.1.3 With the advanced semiconductor technology, the switching frequency and
voltage-ampere ratings of the solid switches has increased. For example, the
switching frequencies of Insulated Gate Bipolar Transistors (IGBTs) are from 3
kHz to 10 kHz which is several hundred times the utility frequency of power
system (50~60Hz).
6.2 Static Var Compensator (SVC)
6.2.1 Static Var Compensator is “a shunt-connected static Var generator or absorber
whose output is adjusted to exchange capacitive or inductive current so as to
maintain or control specific parameters of the electrical power system (typically
bus voltage)” .SVC is based on thyristors without gate turn-off capability.
6.2.2 The operating principal and characteristics of thyristors realize SVC variable
reactive impedance. SVC includes two main components and their combination:
(1) Thyristor-controlled and Thyristor-switched Reactor (TCR and TSR); and (2)
Thyristor-switched capacitor (TSC). Figure 15 shows the diagram of SVC.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 45 of 59
6.2.3 TCR and TSR are both composed of a shunt-connected reactor controlled by two
parallel, reverse-connected thyristors. TCR is controlled with proper firing angle
input to operate in a continuous manner, while TSR is controlled without firing angle control which results in a step change in
reactance.
6.2.4 TSC shares similar composition
and same operational mode as
TSR, but the reactor is replaced
by a capacitor. The reactance can
only be either fully connected or
fully disconnected zero due to the
characteristic of capacitor. With
different combinations of
TCR/TSR, TSC and fixed
capacitors, a SVC can meet
various requirements to
absorb/supply reactive power
from/to the transmission line.
6.3 Converter-based Compensator
6.3.1 Static Synchronous Compensator (STATCOM) is one of the key Converter-based
Compensators which are usually based on the voltage source inverter (VSI) or
current source inverter (CSI), as
shown in Figure 16 (a). Unlike SVC,
STATCOM controls the output
current independently of the AC
system voltage, while the DC side
voltage is automatically maintained
to serve as a voltage source. Mostly,
STATCOM is designed based on the
VSI (VOLTAGE SOURCE
INVERTER).
Figure 15 STATCOM topologies: (a) STATCOM based
on VSI and CSI (b) STATCOM with storage
Figure 14 Static VAR Compensators
(SVC): TCR/TSR, TSC, FC and
Mechanically Switched Resistor
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 46 of 59
6.3.2 Compared with SVC, the topology of a STATCOM is more complicated. The
switching device of a VSI is usually a gate turn-off device paralleled by a reverse
diode; this function endows the VSI advanced controllability.
6.3.3 Various combinations of the switching devices and appropriate topology make it
possible for a STATCOM to vary the AC output voltage in both magnitude and
phase. Also, the combination of STATCOM with a different storage device or
power source (as shown in Figure 16b) endows the STATCOM the ability to
control the real power output.
6.3.4 STATCOM has much better dynamic performance than conventional reactive
power compensators like SVC. The gate turn-off ability shortens the dynamic
response time from several utility period cycles to a portion of a period cycle.
STATCOM is also much faster in improving the transient response than a SVC.
This advantage also brings higher reliability and larger operating range.
6.4 Series-connected controllers
6.4.1 As shunt-connected controllers, series- connected FACTS controllers can
also be divided into either impedance type or converter type.
6.4.2 The former includes Thyristor-
Switched Series Capacitor (TSSC),
Thyristor-Controlled Series
Capacitor (TCSC), Thyristor-
Switched Series Reactor, and
Thyristor-Controlled Series
Reactor.
6.4.3 The latter, based on VSI, is usually
in the Compensator (SSSC). The
composition and operation of
different types are similar to the
operation of the shunt connected
peers. Figure shows the diagrams
of various series-connected controllers.
Figure 16 Series-connected FACTS
controllers: (a) TCSR and TSSR; (b)
TSSC; (c) SSSC
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 47 of 59
7 GENERATOR REACTIVE POWER AND VOLTAGE
CONTROL
7.1 Introduction
7.1.2 An electric-power generator’s primary function is to convert fuel (or other energy
resource) into electric power. Almost all generators also have considerable
control over their terminal voltage and reactive-power output.
7.1.3 The ability of a generator to
provide reactive support
depends on its real-power
production which is
represented in the form of
generator capability curve or
D - curve. Figure 18 shows the
combined limits on real and
reactive production for a
typical generator. Like most
electric equipment,
generators are limited by their
current-carrying capability.
Near rated voltage, this
capability becomes an MVA
limit for the armature of the
generator rather than a MW
limitation, shown as the
armature heating limit in the
Figure.
7.1.4 Production of reactive power involves increasing the magnetic field to raise the
generator’s terminal voltage. Increasing the magnetic field requires increasing
the current in the rotating field winding. This too is current limited, resulting in
the field-heating limit shown in the figure. Absorption of reactive power is limited
by the magnetic-flux pattern in the stator, which results in excessive heating of
the stator-end iron, the core-end heating limit. The synchronizing torque is also
reduced when absorbing large amounts of reactive power, which can also limit
Figure 17 D-Curve of a typical Generator
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 48 of 59
generator capability to reduce the chance of losing synchronism with the system.
7.1.5 The generator prime mover (e.g., the steam turbine) is usually designed with less
capacity than the electric generator, resulting in the prime-mover limit in Fig. 18.
The designers recognize that the generator will be producing reactive power and
supporting system voltage most of the time. Providing a prime mover capable of
delivering all the mechanical power the generator can convert to electricity when
it is neither producing nor absorbing reactive power would result in
underutilization of the prime mover.
7.1.6 To produce or absorb additional VARs beyond these limits would require a
reduction in the real-power output of the unit. Capacitors supply reactive power
and have leading power factors, while inductors consume reactive power and
have lagging power factors. The convention for generators is the reverse. When
the generator is supplying reactive power, it has a lagging power factor and its
mode of operation is referred to as overexcited. When a generator consumes
reactive power, it has a leading power factor region and is under excited.
7.1.7 Control over the reactive output and the terminal voltage of the generator is
provided by adjusting the DC current in the generator’s rotating field. Control can
be automatic, continuous, and fast. The inherent characteristics of the generator
help maintain system voltage.
7.1.8 At any given field setting, the generator has a specific terminal voltage it is
attempting to hold. If the system voltage declines, the generator will inject
reactive power into the power system, tending to raise system voltage. If the
system voltage rises, the reactive output of the generator will drop, and ultimately
reactive power will flow into the generator, tending to lower system voltage.
7.1.9 The voltage regulator will accentuate this behavior by driving the field current in
the appropriate direction to obtain the desired system voltage. Because most of
the reactive limits are thermal limits associated with large pieces of equipment,
significant short-term extra reactive-power capability usually exists. Power-
system stabilizers also control generator field current and reactive-power output
in response to oscillations on the power system. This function is a part of the
network-stability ancillary service.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 49 of 59
7.2 Synchronous Condensers
7.2.1 Every synchronous machine (motor or generator) has the reactive power
capability. Synchronous motors are occasionally used to provide voltage support
to the power system as they provide mechanical power to their load. Some
combustion turbines and hydro units are designed to allow the generator to
operate without its mechanical power source simply to provide the reactive-
power capability to the power system when the real power generation is
unavailable or not needed.
7.2.2 Synchronous machines that are designed exclusively to provide reactive support
are called synchronous condensers. Synchronous condensers have all of the
response speed and controllability advantages of generators without the need to
construct the rest of the power plant (e.g., fuel-handling equipment and boilers).
Because they are rotating machines with moving parts and auxiliary systems,
they may require significantly more maintenance than static alternatives. They
also consume real power equal to about 3% of the machine’s reactive-power
rating. That is, a 50-MVAR synchronous condenser requires about 1.5 MW of real
power.
7.2.3 As per planning philosophy and general guidelines in the Manual on
Transmission planning criteria issued by CEA (MOP, India), Thermal / Nuclear
Generating Units shall normally not run at leading power factor. However for the
purpose of charging unit may be allowed to operate at leading power factor as per
the respective capability curve. Capability curve of various generators of NER
region are given in Annexure V.
7.2.4 Generator capability may depend significantly on the type and amount of cooling.
This is particularly true of hydrogen cooled generators where cooling gas
pressure affects both the real and reactive power capability
SL. NO. STATION UTILITY UNIT NO.
UNIT
CAPACITY
(MW)
TYPE
1 KOPILI HEP NEEPCO 1,2,3 & 4* 50 HYDEL
2 RANGANADI
HEP NEEPCO 1,2 & 3 135 HYDEL
Table 7: List of units in NER required to be normally operated with free
Governor action and AVR in service.
*Units running in 132 KV pocket is exempt from FGMO.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 50 of 59
8 CONCLUSION
8.1 Generators, synchronous condensers, SVCs, and STATCOMs all provide fast,
continuously controllable reactive support and voltage control. OLTC
transformers provide nearly continuous voltage control but they are slow because
the transformer moves reactive power from one bus to another, the control
gained at one bus is at the expense of the other. Capacitors and inductors are not
variable and offer control only in large steps.
8.2 An unfortunate characteristic of capacitors and capacitor-based SVCs is that
output drops dramatically when voltage is low and support is needed most. The
output of a capacitor, and the capacity of an SVC, is proportional to the square of
the terminal voltage. STATCOMs provide more support under low-voltage
conditions than capacitors or SVCs do because they are current-limited devices
and their output drops linearly with voltage.
8.3 The output of rotating machinery (i.e., generators and synchronous condensers)
rises with dropping voltage unless the field current is actively reduced.
Generators and synchronous condensers generally have additional emergency
capacity that can be used for a limited time. Voltage-control characteristics
favour the use of generators and synchronous condensers. Costs, on the other
hand, favor capacitors.
8.4 Generators have extremely high capital costs because they are designed to
produce real power, not reactive power. Even the incremental cost of obtaining
reactive support from generators is high, although it is difficult to unambiguously
separate reactive-power costs from real-power costs. Operating costs for
generators are high as well because they involve real-power losses. Finally,
because generators have other uses, they experience opportunity costs when
called upon to simultaneously provide high levels of both reactive and real power.
8.5 Synchronous condensers have the same costs as generators but, because they are
built solely to provide reactive support, their capital costs do not include the
prime mover or the balance of plant and they incur no opportunity costs. SVCs
and STATCOMs are high-cost devices, as well, although their operating costs are
lower than those for synchronous condensers and generators.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 51 of 59
9 SUMMARY
9.1 The process of controlling voltages and managing reactive power on
interconnected transmission systems is well understood from a technical
perspective. Three objectives dominate reactive-power management. First,
maintain adequate voltages throughout the transmission system under current
and contingency conditions. Second, minimize congestion of real-power flows.
Third, minimize real-power losses.
9.2 This process must be performed centrally because it requires a comprehensive
view of the power system to assure that control is coordinated. System operators
and planners use sophisticated computer models to design and operate the power
system reliably and economically. Central control by rule works well but may not
be the most technically and economically effective means.
9.3 The economic impact of control actions can be quite different in a
restructured/regulated industry than for vertically integrated utilities. While it
may be sufficient to measure only the response of the system in aggregate for a
vertically integrated utility, determining individual generator performance will be
critical in a competitive environment.
9.4 While it reduces or eliminates opportunity costs by providing sufficient capacity,
it can waste capital. When an investor is considering construction of new
generation, the amount of reactive capability that the generator can provide
without curtailing real-power production should depend on system requirements
and the economics of alternatives, not on a fixed rule.
9.5 The introduction of advanced devices, such as STATCOMs and SVCs, further
complicates the split between transmission- and generation based voltage
control. The fast response of these devices often allows them to substitute for
generation-based voltage control. But their high capital costs limit their use. If
these devices could participate in a competitive voltage-control market, efficient
investment would be encouraged.
9.6 In areas with high concentrations of generation, sufficient interaction among
generators is likely to allow operation of a competitive market. In other locations,
introduction of a small amount of controllable reactive support on the
transmission system might enable market provision of the bulk of the reactive
support. In other locations, existing generation would be able to exercise market
power and would continue to require economic regulation for this service.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 52 of 59
9.7 A determination of the extent of each type within each region would be a useful
contribution to restructuring. System planners and operators need to work
closely together during the design of new facilities and modification of existing
facilities. Planners must design adequate reactive support into the system to
provide satisfactory voltage profiles during normal and contingency operating
conditions. Of particular importance is sufficient dynamic support, such as the
reactive output of generators, which can supply additional reactive power during
contingencies.
9.8 System operators must have sufficient metering and analytical tools to be able to
tell when and if the operational reactive resources are sufficient. Operators must
remain cognizant of any equipment outages or problems that could reduce the
system’s static or dynamic reactive support below desirable levels. Ensuring that
sufficient reactive resources are available in the grid to control voltages may be
increasingly difficult because of the disintegration of the electricity industry.
9.9 Traditional vertically integrated utilities contained, within the same entity,
generator reactive resources, transmission reactive resources, and the control
center that determined what resources were needed when. Presently, these
resources and functions are placed within three different entities. In addition,
these entities have different, perhaps conflicting, goals. In particular, the owners
of generating resources will be driven, in competitive generation markets, to
maximize the earnings from their resources. They will not be willing to sacrifice
revenues from the sale of real power to produce reactive power unless
appropriately compensated.
9.10 Similarly, transmission owners will want to be sure that any costs they incur to
expand the reactive capabilities on their system (e.g., additional capacitors) will
be reflected fully in the transmission rates that they are allowed to charge.
9.11 Failure to appropriately compensate those entities that provide voltage-control
services could lead to serious reliability problems and severe constraints on inter
regional links and other congested areas as TTC (Total Transfer Capability) has a
voltage limit function as a baggage with it which is directly linked to var
compensation. With dynamic ATC’s (Available Transfer capability), Var
compensation if not seriously thought of may have serious commercial
implications in time to come due to the amount of bulk power trading happening
across the country in today’s context.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 53 of 59
9.12 THINGS TO DO DURING HIGH VOLTAGE CONDITION:
9.12.1 Insert shunt Reactors
9.12.2 Remove shunt Capacitors
9.12.3 Close open-ended lines or remove from service all together
9.12.4 Remove lightly loaded transmission lines from service without
compromising grid security
9.12.5 Ask the generators to maximize Var absorption within their capability
curve i.e, lower the AVR set point.
9.13 THINGS TO DO DURING LOW VOLTAGE CONDITION:
9.13.1 Remove shunt Reactors
9.13.2 Insert shunt Capacitors
9.13.3 Energize open transmission lines
9.13.4 Ask the generators to maximize Var generation within their capability
curve i.e, raise the AVR set point.
9.13.5 Shed interruptible inductive loads.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 54 of 59
10 STATUTORY PROVISIONS FOR REACTIVE POWER
MANAGEMENT AND VOLTAGE CONTROL
10.1 Provision in the Central Electricity Authority (Technical
Standard for connectivity to the grid) Regulations 2007 [8]:
Extracts from this standard is as reproduced below for ready reference.
Part II: Grid Connectivity Standards applicable to the Generating Units The units at
a generating station proposed to be connected to the grid shall comply with the
following requirements besides the general connectivity conditions given in the
regulations and general requirements given in part-I of the Schedule:-
1. New Generating Units
Hydro generating units having rated capacity of 50 MW and above shall be capable
of operation in synchronous condenser mode, where ever feasible
2. Existing Units
For thermal generating unit having rated capacity of 200 MW and above and
hydro Units having rated capacity of 100 MW and above, the following facilities would be
provided at the time of renovation and modernization.
(1) Every generating unit shall have Automatic Voltage Regulator. Generators having rated capacity of 100 MW and above shall have Automatic Voltage Regulator
with two separate with two separate channels having independent inputs and
automatic changeover.
10.2 Provision in The Indian Electricity Grid Code (IEGC), 2010:
10. 2.1 As per sec 3.5 of IEGC planning criterion general policy
(a) The planning criterion are based on the security philosophy on which the ISTS
has been planned. The security philosophy may be as per the Transmission Planning
Criteria and other guidelines as given by CEA. The general policy shall be as detailed
below:
a) As a general rule, the ISTS shall be capable of withstanding and be secured
against the following contingency outages
a. without necessitating load shedding or rescheduling of generation during Steady
State Operation:
-Outage of a 132 kV D/C line or,
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 55 of 59
-Outage of a 220 kV D/C line or,
-Outage of a 400 kV S/C line or,
-Outage of single Interconnecting Transformer, Or
-Outage of one pole of HVDC Bipole line, or one pole of HVDC back to back
Station or
-Outage of 765 kV S/C line.
b. without necessitating load shedding but could be with rescheduling of
generation during steady state operation-
- Outage of a 400 kV S/C line with TCSC, or
- Outage of a 400kV D/C line, or
- Outage of both pole of HVDC Bipole line or both poles of HVDC back
to back Station or
- Outage of a 765kV S/C line with series compensation.
ii) The above contingencies shall be considered assuming a pre-contingency system
depletion (Planned outage) of another 220 kV D/C line or 400 kV S/C line in another
corridor and not emanating from the same substation. The planning study would
assume that all the Generating Units may operate within their reactive capability curves
and the network voltage profile shall also be maintained within voltage limits specified
(e) CTU shall carry out planning studies for Reactive Power compensation of ISTS
including reactive power compensation requirement at the generator’s /bulk
consumer’s switchyard and for connectivity of new generator/ bulk consumer to the
ISTS in accordance with Central Electricity Regulatory Commission ( Grant of
Connectivity, Long-term Access and Medium-term Open Access in inter-state
Transmission and related matters) Regulations, 2009.
10.2.2 As per Sec 4.6.1 of IEGC, Important Technical Requirements for
Connectivity to the Grid:
Reactive Power Compensation
a) Reactive Power compensation and/or other facilities, shall be provided by STUs, and
Users connected to ISTS as far as possible in the low voltage systems close to the load
points thereby avoiding the need for exchange of Reactive Power to/from ISTS and to
maintain ISTS voltage within the specified range. b) The person already connected to the grid shall also provide additional reactive
compensation as per the quantum and time frame decided by respective RPC in
consultation with RLDC. The Users and STUs shall provide information to RPC
and RLDC regarding the installation and healthiness of the reactive
compensation equipment on regular basis.
RPC shall regularly monitor the status in this regard.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 56 of 59
10.2.3 In chapter 5 of IEGC operating code for regional grids:
5.2(k) All generating units shall normally have their automatic voltage regulators
(AVRs) in operation. In particular, if a generating unit of over fifty (50) MW size
is required to be operated without its AVR in service, the RLDC shall be
immediately intimated about the reason and duration, and its permission
obtained. Power System Stabilizers (PSS) in AVRs of generating units (wherever
provided), shall be got properly tuned by the respective generating unit owner as
per a plan prepared for the purpose by the CTU/RPC from time to time. CTU
/RPC will be allowed to carry out checking of PSS and further tuning it, wherever
considered necessary.
5.2(o) All Users, STU/SLDC , CTU/RLDC and NLDC, shall also facilitate identification,
installation and commissioning of System Protection Schemes (SPS) (including
inter-tripping and run-back) in the power system to operate the transmission
system closer to their limits and to protect against situations such as voltage
collapse and cascade tripping, tripping of important corridors/flow-gates etc..
Such schemes would be finalized by the concerned RPC forum, and shall always
be kept in service. If any SPS is to be taken out of service, permission of RLDC
shall be obtained indicating reason and duration of anticipated outage from
service.
5.2(s All Users, RLDC, SLDC STUs , CTU and NLDC shall take all possible measures
to ensure that the grid voltage always remains within the following operating range.
Table 8: IEGC operating voltage range
Voltage – (KV rms)
Nominal Maximum Minimum
765 800 728
400 420 380
220 245 198
132 145 122
110 121 99
66 72 60
33 36 30
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 57 of 59
5.2(u) (ii) During the wind generator start-up, the wind generator shall ensure that the
reactive power drawl (inrush currents in case of induction generators) shall
not affect the grid performance.
10.2.4 In chapter 6 of IEGC Section-6.6 Reactive Power & Voltage Control:
1. Reactive power compensation should ideally be provided locally, by generating
reactive power as close to the reactive power consumption as possible. The Regional
Entities except Generating Stations are therefore expected to provide local VAr
compensation/generation such that they do not draw VArs from the EHV grid,
particularly under low-voltage condition. To discourage VAr drawals by Regional
Entities except Generating Stations, VAr exchanges with ISTS shall be priced as
follows:
- The Regional Entity except Generating Stations pays for VAr drawal when voltage
at the metering point is below 97% - The Regional Entity except Generating Stations gets paid for VAr return when
voltage is below 97% - The Regional Entity except Generating Stations gets paid for VAr drawal when
voltage is above103%
- The Regional Entity except Generating Stations pays for VAr return when voltage is
above 103% Provided that there shall be no charge/payment for VAr drawal/return
by a regional Entity except Generating Stations on its own line emanating directly
from an ISGS.
2. The charge for VArh shall be at the rate of 10 paise/kVArh w.e.f. 1.4.2010, and this will
be applicable between the Regional Entity, except Generating Stations, and the
regional pool account for VAr interchanges. This rate shall be escalated at
0.5paise/kVArh per year thereafter, unless otherwise revised by the Commission. 3 Notwithstanding the above, RLDC may direct a Regional Entity except Generating
Stations to curtail its VAr drawal/injection in case the security of grid or safety of any
equipment is endangered. 4. In general, the Regional Entities except Generating Stations shall endeavor to
minimize the VAr drawal at an interchange point when the voltage at that point is
below 95% of rated, and shall not return VAr when the voltage is above 105%. ICT taps
at the respective drawal points may be changed to control the VAr interchange as per a
Regional Entity except Generating Stations’s request to the RLDC, but only at
reasonable intervals.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 58 of 59
5. Switching in/out of all 400 kV bus and line Reactors throughout the grid shall be
carried out as per instructions of RLDC. Tap changing on all 400/220 kV ICTs shall
also be done as per RLDCs instructions only.
6. The ISGS and other generating stations connected to regional grid shall
generate/absorb reactive power as per instructions of RLDC, within capability limits
of the respective generating units, that is without sacrificing on the active generation
required at that time. No payments shall be made to the generating companies for
such VAr generation/absorption. 7. VAr exchange directly between two Regional Entities except Generating Stations on
the interconnecting lines owned by them (singly or jointly) generally address or cause
a local voltage problem, and generally do not have an impact on the voltage profile of
the regional grid. Accordingly, the management/control and commercial handling of
the VAr exchanges on such lines shall be as per following provisions, on case-by-case
basis:
i) The two concerned Regional Entities except Generating Stations may mutually
agree not to have any charge/payment for VAr exchanges between them on an
interconnecting line.
ii) The two concerned Regional Entities except Generating Stations may mutually
agree to adopt a payment rate/scheme for VAr exchanges between them identical
to or at variance from that specified by CERC for VAr exchanges with ISTS. If the
agreed scheme requires any additional metering, the same shall be arranged by the
concerned Beneficiaries.
iii) In case of a disagreement between the concerned Regional Entities except
Generating Stations (e.g. one party wanting to have the charge/payment for VAr
exchanges, and the other party refusing to have the scheme), the scheme as
specified in Annexure-2 shall be applied. The per kVArh rate shall be as specified
by CERC for VAr exchanges with ISTS.
iv) The computation and payments for such VAr exchanges shall be effected as
mutually agreed between the two Beneficiaries.
REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL IN NORTH EASTERN REGION
Page 59 of 59
11. BIBLIOGRAPHY:
1. Best practice manual of transformer for BEE and IREDA by Devki energy
consultancy pvt. ltd.
2. NERPC progress report August, 2010.
3. Document on MeSEB capacity building and training document
4. Manual on Transmission Planning Criteria, CEA, Govt. of India, June
1994
5. Indian Electricity Grid Code, CERC, India, 2010 with Amendment.
6. The Central Electricity Authority (Technical Standard for connectivity to
the grid) Regulations 2007.
7. Operation procedure for NER July 2017.
8. Document on Metering code for AEGCL grid.
9. Principles of efficient and reliable reactive power supply and
consumption, staff report, FERC, Docket No. AD05-1-000, February 4,
2005
10. Proceedings of workshop on grid security & management 28th and 29th
April, 2008 Bangalore.
11. Extra High Voltage AC transmission Engineering – R D Begamudre.
12. Electrical Engineering Handbook – SIEMENS.
13. C. W. Taylor, “Power System Voltage Stability”, McGraw-Hill, 1994.
14. THE AEGCL GAZETTE, EXTRAORDINARY, FEBRUARY 10, 2005
North Eastern Regional Load Despatch Centre
Shillong
Power System operation Corporation Limited (A Government of India Enterprise)
Dongtieh-Lower Nongrah –Lapalang
Shillong-793006