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POWER ENGINEERING JOURNAL JUNE 2000 129
Power flow management
Innovative power flowmanagement and voltagecontrol technologies
Power flow control concepts
Power flows can be influenced bycontrolling the basic electrical
parameters, namely impedance of the
transmission line and system voltages,
as shown in eqn. 1:
PS = Re US
QS = Im US(1)
wherePS = active power across the transmission line
QS = reactive power at the sending end
US = sending end voltage
UR = receiving end voltage
Xl = impedance of the transmission line
To be able to control the flows of active power
P and/or reactive power Q, one or several of
these parameters can be controlled by power
equipment already available or under
development. The control of the basic electrical
parameters can be achieved using a shunt
control device, series control device, shunt
current injection device, series voltage
injection device or a combination of these.
In this article the following model
assumptions are used:
4 lossless transmission lines
4 sending and receiving ends are stiff nodes and
their voltages are equal in magnitude
4 performance characteristics are drawn for
midpoint location of control devices.
Shunt control device
The impact on power flow due to a capacitive
shunt device with a reactance ofXC can be
investigated using the transmission model,
vector diagram and mathematical relationsshown in Fig. 1. Shunt devices basically impact
the voltage at the point of connection. When
connected to weak nodes in the power system,
for example in the midpoint or in the receiving
end of a long transmission line, the power flow
can be influenced substantially by the change of
voltage due to the shunt device.
Series control device
The voltage in series with the line can be
created by the natural voltage drop caused by
the line current across an impedance element
with a capacitive reactance ofXC. The insertion
of a series compensation device in a
transmission line directly impacts the power
Driven by ever increasing energy demands, environmental constraints, deregulation
and privatisation of the power supply industry, existing transmission systems are
often operated and stressed to the limit and occasionally beyond the performance
capability of their original design in order to maximise asset utilisation. To ensurethat under these conditions the economical, reliable and secure operation of the
grid is maintained, the need for various aspects of power flow management within
the power systems is becoming increasingly evident.
by E. Wirth and A. Kara
US URjXl
*
US URjXl
*
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130 POWER ENGINEERING JOURNAL JUNE 2000
1 Transmission-line
model, power flow
equations and vector
diagram of the system
with a shunt controldevice
2 Transmission-line
model, power flow
equations and vector
diagram of the system
with a series control
device
flow on the line. The influence of a capacitive
element providing the series voltage can be
investigated using the equations shown in
Fig. 2.
Shunt current injection device
Power flow control devices can utilise the
physical principles described above, or
depending on their construction and operating
mode, can be based on the concepts of
controllable shunt current injection and
controllable series voltage injection. The
concept of a device based on shunt current
injection can be demonstrated using the system
shown in Fig. 3. Ii is the controllable shunt
current injected to the midpoint of the
transmission system.
Series voltage injection device
As mentioned already, the series voltage can be
provided by a controlled voltage source. The
series voltage device can be constructed such
that the injected voltages magnitude UTand/or
phase angle can be varied. The impact onpower flow can be investigated by using the
transmission model, vector diagram and
equations shown in Fig. 4.
Impact of power flow control and reactive
power compensation devices on system
performance
By employing devices that can control the basic
electrical parameters, power system
performance can be significantly improved.
One of the major aims of improving a
transmission systems performance is to
increase its power transfer capability. By usingthe concepts discussed above, it is possible to
quantify the impact that shunt and series
control devices have on power transfer
Power flow management
Um
IcXc
*
US
PS = Re US
US URUm
2Xc
Xl1 UR
UR
Xl/2 Xl/2 IR
IRIS
Ic
IS
( ( jXl 4XcXl(1 )*
QS = Im US
US
2Xc
Xl1 UR( (US
jXl 4Xc
Xl(1 )
Um1
*
Um2US
PS = Re US
US URUm2 Um1
UR
XcXl/2 Xl/2 I
I
I
US URj(Xl Xc)*
QS = Im US US URj(Xl Xc)
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POWER ENGINEERING JOURNAL JUNE 2000 131
capability and reactive power requirements in
transmission systems. The reactive power
balance is one of the many requirements that
enforces a practical limit on how much activepower P can be transferred over a system. Series
devices providing a specific amount of
compensation in general enable more active
power to be transferred with less sending end
reactive power supply requirements as
compared with a shunt device. Capacitive
series devices increase the transfer capability
(their reactive power output increases also with
line loading) and, in addition to increasing the
stability limit, the voltage regulation
capabilities of the system are significantly
improved.
Shunt compensation and control devices
improving voltage maintenance and power
transfer capability through reactive current
supply
Shunt devices help maintain the system voltage
when transferred power is varied. Shuntreactors are used to compensate for the reactive
power surplus in case of reduced power transfer
or open transmission lines. In case of long
transmission lines, some of the shunt reactors
are permanently connected to the system to
give maximum security against overvoltages in
the event of sudden load rejection or opening of
lines. The conventional shunt capacitor
compensation provides the most economical
reactive power source for voltage control in
cases when additional voltage support is
required.Conventional shunt control devices and
modern shunt current injection devices, e.g.
the STATCOM, can also control the power flow
Power flow management
3 Transmission-line
model, power flowequations and vector
diagram of the system
with a shunt current
injection device
4 Transmission-line
model, power flow
equations and vector
diagram of the system
with a series voltage
injection device. The
equations are based on
the concept with active
powerPT drawn from
the network andreactive powerQTgenerated locally,
shown in Fig. 6.
Um1 Um2US
Um1= UR +jX2IR UT
Um1*
UR
UTjX1IS
US
jX2IR
Um2
Um1
UR
X1 X2 IRIS UT
US = Um1 +jX1 IR + Re(UTIR*)
Um1*IS = IR +
PS = Re (US IS*)
QS = Im (US IS*)
Re(UTIR*)
Um
*
US
PS = Re US
UR
IR
Ii
IS
US UR IijXl 2*
QS = Im US US UR IijXl 2
Xl/2 Xl/2
US URUm
IS IR
Ii
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through a transmission system in a limited
range by supplying or absorbing reactive
current at the point of connection to the
system.
Series voltage injection devices improving
flexibility and enhancing system performance
In the case of the series voltage injection
devices, further system performance
improvement can be achieved by providing
greater operational flexibility in addition to
increasing power transfer capability. There are
basically two ways of generating this series
voltage. One way is to draw all the active power
PT and reactive power QT requirements needed
to generate this voltage from the network, as
shown in Fig. 5. The other way is to draw only
the active power from the network and provide
the reactive power required locally as in Fig. 6.
The power flow control capabilities of
devices capable of coupling a series voltage
with a variable phase angle are shown in
Fig. 7. The impact on power flow control of theformer concept is shown by the green curve and
of the latter by the purple one. Both the curves
are for series voltages of 20% of the nominal
sending end system voltage and a transmission
angle of 60 between the receiving andsending end voltages. As the phase angle ofthe series injected voltage is varied between 0
and 360, the active power flowing through the
transmission system can theoretically be
controlled for a range from a maximum
through to minimum values. From Fig. 7, it can
also be seen that the locally provided reactive
power concept has a bigger impact on power
flow control compared with obtaining reactive
power from the network.
Fig. 8 shows the improvement and the limits
(0 and 180) in power transfercapability with the different series voltage
injection concepts, for an injected voltage
magnitude of 20% of nominal system voltage,
over a range of transmission angles . Thepurple band shows the operating capability of
the series voltage injection device with locally
supplied reactive power QT whilst the green
meshed band is due to a device drawing activeand reactive power from the power system. The
bands indicate the control ranges of devices for
varying between 0 and 180.
132 POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
7 Achievable
transmitted active
power for the different
series voltage injection
concepts and a
transmission angle of 60
8 Impact on power
transfer capability using
different series voltage
injection concepts for
transmission angles between 0 and 90
UT
PS
QS
PT,QT
PT,QT
1600
1200
800
PS,MW
400
0 30
, deg
60 90
PTfrom network, QTlocal generated
PT, QTfrom network
no control
180
0
1600
1200
800
PS,MW
400
0 90
PTfrom network, QTlocal generated
180
, deg
270 360
PT, QTfrom network
UT
PS
QS
PT
QT
5 Series voltage injection with Pand Q
from network
6 Series voltage injection with Ptaken
from network, Qgenerated locally
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POWER ENGINEERING JOURNAL JUNE 2000 133
Solutions to transmission system concerns
using power flow control technologies
Finding the most cost-effective solution to the
various issues limiting transmission
performance is attracting ever growing interest
as utilities deregulate and a competitive
electrical supply environment is becoming a
norm rather than an exception. Power flow
control technologies can provide the key to
these solutions. An overview of the
transmission issues and the possible effective
solutions are summarised in Table 1. These
solutions include both conventional as well as
innovative technologies, though they are by no
means exhaustive. In must be noted that, due to
the wide range of network configurations and
Power flow management
9 (a) TCSC system and
(b) its performance
characteristics
PSXC
a
US UR
X1 X2
thermal line overload issues tripping of parallel circuit
voltage and low voltage at heavy load
reactive power high voltage at light load control issues voltage deviation following outage
power flow parallel line load sharing issues post-fault sharing
power flow control
dynamic and lack synchronising torque stability issues dynamic flow control
and transient stabilitypower oscillations voltage stability
BSC
BSR
TCSC
ASC
SVC
STATCOM
TCPAR
QBT
IPC
UPFC
BDV = breaker switched capacitor
BSC = breaker switched reactor
IPC = interphase power controller
QBT = quadrature boosting transformer
STATCOM = static synchronous compensator
SVC = static VAr compensator
TCPAR = thyristor-controlled phase-angle regulator
TCSC = thyristor-controlled series capacitor
UPFC = unified power flow controller
12
10
08
06
04
02
0
200 40 60 80, deg
b
PS,pu
100 120 140
Xcmax compensation
Xcmincompensation
no compensation
160 180
ASC = advanced series compensator
Table 1 Overview of transmission system limitations and possible solutions using control devices
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system operation procedures, proper corrective
actions to deal with various issues are of
necessity application dependent.2
In steady-state conditions the total power
flow on all lines that connect two power
systems is determined by unbalance betweenpower production and load demand including
losses in the individual systems. On the other
hand, during transients the power flow control
equipment can also have an impact on the total
power exchange between the systems. Power
flow control technologies and equipment can
thus be generally categorised according to their
ability to solve steady-state or dynamic problem
domains.
Thermal issues are generally related to
thermal limits caused by a change in the
network configuration during outages and can
be overcome by rearranging the network or byadding a power flow control equipment.
Voltage and reactive power control issues are
related to voltage constraints in the power
system. Low voltage at heavy load can be a
limiting factor under steady-state conditions.
The corrective actions include correcting the
power factor and compensating the reactive
losses in lines by supplying reactive power.
High voltage at light load is an undesirableoccurrence in the transmission and distribution
systems and may be diminished using
mechanically switched shunt capacitors or
reactors to supplement the action of
tapchangers. Low voltage as well as high
voltage following outages can exceed the
voltage limits so that corrective actions have to
be taken to avoid further equipment damage.
Power flow issues are generally related to
controlling the active power in the power
system for better utilisation of the transmission
assets, minimisation of losses, limit flows to
contract paths, post contingency strategies etc.Dynamic and stability issues are related to
dynamic performance of the power system.
Transient stability describes the ability of the
134 POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
PS
US UR
X1
UT X2
a
10 (a) ASC system,
(b) its steady-state
operating and
(c) performance
characteristics
inductive capacitive UT, pu
I, pu
b
16
14
12
10
08
06
04
02
0
0
PS,pu
02
20 40 60 80
, deg
c
UT= 05 pucapacitive compensation
UT= 0no compensation
UT= 05 puinductive compensation
100 120 140 160 18004
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POWER ENGINEERING JOURNAL JUNE 2000 135
Power flow management
11 (a) SVC system,(b) its steady-state
operating and
(c) performance
characteristics
power system to survive the first few seconds
after a major disturbance and can be improved
by extracting energy from the sending end of
the network, supplying energy to the receivingsystem respectively by increasing the
synchronising power between sending and
receiving ends. Power system oscillation
describes sustained or growing power swing
oscillations (generally in range below 1.5 Hz)
between generators or group of generators,
initiated by a disturbance (fault, major load
changes etc.). Solutions to this problem lie in
the use of equipment that permits dynamic
damping of these oscillations. Voltage stability
problem is a slow process caused by progressive
increase in load and can be improved by voltage
support, e.g. by using reserve devices, co-
ordinating system load tapchangers, automatic
undervoltage load shedding or generator
control action.
Power flow control devices and their
performance characteristics
As Table 1 shows, solutions to the transmission
issues can be addressed by various power flowcontrol devices. Their application and
suitability to solve a particular problem depend
on many factors covering technical as well as
economical considerations. This section
provides brief descriptions of the technical
capability, technology and performance of each
of the devices listed in Table 1, allowing a first
estimate of device suitability for an intended
application.
Breaker switched capacitor and reactor (BSC,
BSR)
Shunt-connected equipment of these types
allow the reactive power to be supplied via
capacitor banks or absorbed via reactor banks
and thus have significant influence on the
PS
a
US UR
X1 X2
inductivecapacitive
Ush, pu
Ish, pu
b
12
10
08
06
04
02
0200 40 60 80
, deg
c
PS,pu
100 120 140
SVC on capacitive limit
no control
SVC on inductive limit
160 180
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136 POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
voltage at the point of connection. Series-
connected equipment allow the impedance
characteristics of the transmission system
where they are installed to be varied and thushave direct impact on the power transfer
capability. These devices can be permanently
connected to a system or are connected through
circuit breakers. Breaker switched devices offer
greater operational flexibility in terms of
allowing the operators to adapt to changing
reactive power requirements of their power
systems. Their performance is limited by their
step-wise control characteristics.
Thyristor-controlled series capacitor (TCSC)
The thyristor-controlled series capacitor system
is shown in Fig. 9 together with its performance
characteristics. The variation of capacitance
can be achieved by varying the thyristor-
controlled reactance that is connected in
parallel to the capacitor. The reactance is
determined by the thyristor valve firing angle.
The controllable parameter influencing the
power flow is the capacitance of the TCSC.
Advanced series compensator (ASC)
In contrast to the TCSC where the reactive
power is produced or consumed by capacitors
and reactors, advanced series compensators usepower electronics elements with turn-off
capability such as integrated gate commutated
thyristors (IGCT). By proper repetitive
switching of the IGCTs, the phases of the
system are connected and/or disconnected
causing reactive power to flow among them.
The main difference from the TCSC is that the
injected series voltage UT of the ASC does not
depend on line current. The controllable
parameter here is the series injected voltage and
is coupled in general to the power system via a
booster transformer. Fig. 10 shows an ASCsystem with its corresponding performance
characteristics.
Static VAr compensator (SVC)
An SVC consists of a combination of fixed
capacitors, thyristor-switched capacitors and
thyristor-controlled reactors connected in
parallel with the power system in most cases via
a step-up transformer. The maximum SVC
reactive currents are dependent on SVC
terminal voltage. The reactive power produced
or consumed by an SVC is generated or
absorbed by passive reactive components. The
controllable parameter in this equipment is the
parallel capacitive or inductive susceptance.
PS
US UR
X1 X2
a
12 (a) STATCOM system, (b) its steady-state operating and
(c) performance characteristics
inductivecapacitive
Ush, pu
Ish, pu
b
16
14
12
10
08
06
04
02
0
0
PS,pu
02
20 40 60
, deg
c
STATCOM on capacitive limit
no control
STATCOM on inductive limit
80 100 120 140 160 18004
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POWER ENGINEERING JOURNAL JUNE 2000 137
Power flow management
13 (a) TCPAR system,
(b) its steady-state
operating and
(c) performance
characteristics
Within the SVC rating, its susceptance can be
continuously controlled. When the SVC
reaches its capacitive or inductive limit, it then
acts as a parallel capacitor or reactor,
respectively. Fig. 11 shows a SVC system, its
steady-state operating and performance
characteristics.
Static synchronous compensator (STATCOM)
By employing power electronics elements with
turn-off capability as in the case of the ASC, the
SVC system can be similarly improved to
become a static synchronous compensator
(STATCOM). The STATCOM basically consists
of an IGCT converter and a DC circuit. The
reactive power generation or absorption is
performed by the system itself and in balanced
conditions reactive elements are necessary for
energy storage during short periods betweenpower electronic switching. From the
STATCOM operating characteristics in Fig. 12,
it is evident that it can supply constant reactive
current almost over the entire range,
independent of the terminal voltage. The
STATCOM controllable parameter is its reactive
current.
Thyristor-controlled phase angle regulator
(TCPAR)
Phase-shifting transformers (PST) are
transformers with complex turn ratios. The
phase difference between the PST terminal
voltages is achieved by connecting a boosting
transformer in series with the transmission
line, as shown in Fig. 13. The active and
reactive powers that are injected into the
transmission line must be taken from the
network by the shunt transformer and
redirected to the boosting transformer. If losses
are neglected, the PST does not produce or
consume reactive power.The thyristor-controlled phase angle
regulator is one type of PST with equal input
and output voltage magnitudes but with a
PS
UT
a
US URUM
X1
UT
US
UMUR
+
b
10
09
08
07
06
05
04
03
01
02
050 0 50 100
PS,pu
, deg
c
150 200 250
=40
=10
=0
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phase shift between these voltages. The TCPARis controlled extremely quickly by a static
thyristor based on-load tapchanger. The
controllable parameter of the TCPAR is the
voltage phase shift angle . Fig. 13 shows alsothe steady-state operating and performance
characteristics of the TCPAR.
Quadrature booster transformer (QBT)
The quadrature booster transformer is another
type of PST where the phasor of the injected
voltage is shifted by a constant angle withrespect to the input voltage vector. Various
types of QBT enable various angles. Thecontrollable parameter of the QBT is the
magnitude of the injected voltage UT. Fig. 14
shows a QBT system with =90, its steady-
state operating and performancecharacteristics.
Interphase power controller (IPC)
The interphase power controller is a series-
connected device, where the major
components in each of the phases are a reactor
and a capacitor subjected to individually phase-
shifted voltages provided by two phase shifting
transformers PAR1 and PAR2. There are many
IPC configurations, depending on specific
application requirements and on the method
used to implement the internal phase shifts. In
the case where the reactor (XA) and thecapacitor (XA) form a conjugate pair, each
terminal of the IPC will behave as a voltage-
dependent current source and provide the IPC
138 POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
14 (a
) QBT system,(b) its steady-state
operating and
(c) performance
characteristics
PSUT
a
US URUM
X1
b
UTUS
UM UR
12
10
08
06
04
02
00
UT=05 UT=05
UT=0
PS,pu
50
, deg
c
100 150 20050
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POWER ENGINEERING JOURNAL JUNE 2000 139
with the unique decoupling effect property, a
feature that is desirable. The controllable
parameters are the phase shift angles 1 and 2
of PAR1 and PAR2, respectively. Fig. 15 showsan IPC system and its performance
characteristics.
Unified power flow controller (UPFC)
The basic structure of the unified power flow
controller and its performance characteristics
are shown in Fig. 16. It consists of shunt
(exciting) and series (boosting) transformers.
Both of these are connected by two IGCT
converters and a DC circuit represented by the
capacitor. One difference between the UPFC
and a PST is that the UPFC reactive powerinjected into the line by the series branch does
not need to be transmitted from the parallel
branch. It is generated by the converter
connected to the series branch. The active
power injected into the system by the series
branch must be taken from the system by the
parallel branch and transmitted to the series
branch over the DC circuit. Additionally, the
reactive power of the parallel branch can be
controlled in the same manner as for the
STATCOM. The voltage UTcan be of any phase
with respect to the input voltage US and can
have any magnitude ranging from 0 to UTmaxcorresponding to the dimension of the UPFC.
The controllable UPFC parameters are phase
and magnitude of the injected voltage UT and
the magnitude of the parallel branch reactive
current.
Innovative system solutions the key to
cost-effective power flow control
Driven by ever increasing energy demands,
environmental constraints, deregulation and
privatisation of the power supply industry,
existing transmission systems are oftenoperated and stressed to the limit of, and
occasionally beyond, the performance
capability of their original design. To ensure
that under these conditions the economical,
reliable and secure operation of the grid is
maintained, power flow management concepts
employing innovative technologies have been
proposed.
Load sharing and loss minimisation,
regulating power flow through transmission
corridor, transient stability enhancement and
rapid power flow management to prevent
overloads as well as controlling power flow
patterns are transmission issues that are of
concern and interest to system operators
worldwide. Technical solutions for these
concerns have been proposed and discussed.
In Fig. 17 a 200MVA phase-shifting
regulating transformer for 240kV/132kV based
on a new compact concept is shown. The two
booster transformers for in-phase control and
quadrature control, normally connected in
series with the main transformer, are replaced
by only few extra windings inside the main
transformer tank. This considerably reduces
not only the investment costs but also the
operating costs. The main saving is in the
transformer cores and the copper windings.
Another key benefit is the significantly smaller
space that is required.
Utilities share many of the common energy
transmission problems yet have differenttechnical, economical and environmental
requirements. In order that their needs are
individually met, and cost-effective solutions
Power flow management
15 (a) IPC system and
(b) its performance
characteristics
PS
URUS
X1
PAR 1
a
XA
XA
PAR 2
1
2
12
10
08
06
04
02
090 0
, deg
PS,pu
XIPCX1
90
XIPC =XA
1 + 22 sin2
IPC =1 + 2
2
b
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are provided, the key lies in the application of
innovative power flow control technologies.
Co-operation between the power industry
partners can develop optimised solutions
capable of meeting the performance
requirements demanded in the new and
evolving electrical utility environment.
References
1 DUNLOP, R. D., GUTMAN, R., AND MARCHENKO,
R. P.: Analytical development of loadability
characteristics for EHV and UHV transmission lines,
IEEE Trans., March/April 1979, PAS-98, pp.606617
2 CIGRE TF 38-01-06: Load flow control in high
voltage power systems using FACTS controllers,
CIGRE, January 1996
3 WIRTH, E., and RAVOT, J. -F.: Regulating
transformers in power systems new concepts and
applicationsABB Review, 4/1997
4 JAUCH, T., KARA, A., KIEBOOM, G., and WIRTH,
E.: Operational aspects and benefits of interphase
power controllers with conventional or electronically
switched phase shifting devices a robust FACTS
application, CIGRE-Session, Paris, August 1998
5 LINDER, S. et al.: A new range of reverse conducting
gate-commutated thyristors for high-voltage
medium-power applications, Proceedings of the 7th
European Conference on Power Electronics and
Applications, Trondheim, Norway, September 1997
IEE: 2000
The authors are with ABB High Voltage Technologies
Ltd., Dept. AET, PO Box 8546, CH8050, Zurich,
Switzerland.
140 POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
16 (a) UPFC system
and (b) its performance
characteristicsPS UT
US UR
X1
a
17 200 MVA phase-
shifting regulated
transformer for 240
kV/132 kV based on a
new compact concept
12
10
08
06
04
02
00
UT=05 puP maximum
UT=05 puP minimum
UT=0
PS,pu
50
, deg
b
100 150 20050