Download - HAL Project Report (1)
-
7/31/2019 HAL Project Report (1)
1/19
1. ELECTRIC POWER SYSTEM
An electric power system is a network of electrical components used to supply, transmit
and use electric power.
An example of an electric power system is the network that supplies a region's homes and
industry with power - for sizable regions, this power system is known as the grid and can be
broadly divided into the generators that supply the power, the transmission system that carries the
power from the generating centres to the load centres and the distribution system that feeds the
power to nearby homes and industries.
Smaller power systems are also found in industry, hospitals, commercial buildings and
homes. The majority of these systems rely upon three-phase AC power - the standard for large-
scale power transmission and distribution across the modern world.
1.1. POWER SYSTEM COMPONENTS
1.1.1. SUPPLIES
All power systems have one or more sources of power. For some power systems, the source of
power is external to the system but for others it is part of the system itself.
Direct current power can be supplied by batteries, fuel cells or photovoltaic cells. Alternating
current power is typically supplied by a rotor that spins in a magnetic field in a device known as
a turbo generator. There have been a wide range of techniques used to spin a turbine's rotor, from
steam heated using fossil fuel (including coal, gas and oil) or nuclear energy, falling water
(hydroelectric power) and wind (wind power).
The speed at which the rotor spins in combination with the number of generator poles determines
the frequency of the alternating current produced by the generator. If the load on the system
increases, the generators will require more torque to spin at that speed and, in a typical power
station, more steam must be supplied to the turbines driving them. Thus the steam used and the
fuel expended are directly dependent on the quantity of electrical energy supplied.
1
-
7/31/2019 HAL Project Report (1)
2/19
1.1.2. LOADS
Power systems deliver energy to loads that perform a function. These loads range from
household appliances to industrial machinery. Most loads expect a certain voltage and, for
alternating current devices, a certain frequency and number of phases. The appliances found in
home, for example, will typically be single-phase operating at 50 or 60 Hz with a voltage between
110 and 260 volts (depending on national standards).
Making sure that the voltage, frequency and amount of power supplied to the loads is in
line with expectations is one of the great challenges of power system engineering. However it is
not the only challenge, in addition to the power used by a load to do useful work (termed real
power) many alternating current devices also use an additional amount of power because they
cause the alternating voltage and alternating current to become slightly out-of-sync
(termed reactive power). The reactive power like the real power must balance (that is the reactive
power produced on a system must equal the reactive power consumed) and can be supplied from
the generators, however it is often more economical to supply such power from capacitors.
A final consideration with loads is to do with power quality. Power quality issues occur
when the power supply to a load deviates from the ideal: For an AC supply, the ideal is the current
and voltage in-sync fluctuating as a perfect sine wave at a prescribed frequency with the voltage ata prescribed amplitude. For DC supply, the ideal is the voltage not varying from a prescribed level.
Power quality issues can be especially important when it comes to specialist industrial machinary
or hospital equipment.
1.1.3. CONDUCTORS
Conductors carry power from the generators to the load. In a grid, conductors may be
classified as belonging to the transmission system, which carries large amounts of power at high
voltages (typically more than 50 kV) from the generating centres to the load centres, or
the distribution system, which feeds smaller amounts of power at lower voltages (typically less
than 50 kV) from the load centres to nearby homes and industry.
2
-
7/31/2019 HAL Project Report (1)
3/19
Choice of conductors is based upon considerations such as cost, transmission losses and
other desirable characteristics of the metal like tensile strength. Copper, with lower resistivity than
aluminium, was the conductor of choice for most power systems. However, aluminum has lower
cost for the same current carrying capacity and is the primary metal used for transmission line
conductors. Overhead line conductors may be reinforced with steel or aluminum alloys.
1.1.4. CAPACITORS AND REACTORS
The majority of the load in a typical AC power system, is inductive; the current lags behind
the voltage. Since the voltage and current are out-of-sync, this leads to the emergence of a
"useless" form of power known as reactive power. Reactive power does no measurable work but is
transmitted back and forth between the reactive power source and load every cycle. This reactive
power can be provided by the generators themselves but it is often cheaper to provide it through
capacitors, hence capacitors are often placed near inductive loads to reduce current demand on the
power system.
Reactors consume reactive power and are used to regulate voltage on long transmission
lines. Reactors installed in series in a power system also limit rushes of current flow, small reactors
are therefore almost always installed in series with capacitors to limit the current rush associated
with switching in a capacitor. Series reactors can also be used to limit fault currents.
1.1.5. PROTECTIVE DEVICES
Power systems contain protective devices to prevent injury or damage during failures. The
quintessential protective device is the fuse. When the current through a fuse exceeds a certain
threshold, the fuse element melts, producing an arc across the resulting gap that is then
extinguished, interrupting the circuit. Fuses two problems: First, after they have functioned, fuses
must be replaced as they cannot be reset. This can prove inconvenient if the fuse is at a remote site
or a spare fuse is not on hand. And second, fuses are typically inadequate as the sole safety device
in most power systems as they allow current flows well in excess of that that would prove lethal to
a human or animal.
3
-
7/31/2019 HAL Project Report (1)
4/19
The first problem is resolved by the use of circuit breakers - devices that can be reset after
they have broken current flow. In modern systems that use less than about 10 kW, miniature circuit
breakers are typically used. These devices combine the mechanism that initiates the trip (by
sensing excess current) as well as the mechanism that breaks the current flow in a single unit.
In higher powered applications, the protective relays that detect a fault and initiate a trip are
separate from the circuit breaker. Different relays will initiate trips depending upon
different protection schemes. For example, an overcurrent relay might initiate a trip if the current
on any phase exceeds a certain threshold whereas a set of differential relays might initiate a trip if
the sum of currents between them indicates there may be current leaking to earth. The circuit
breakers in higher powered applications are different too. Air is typically no longer sufficient to
quell the arc that forms when the contacts are forced open so a variety of techniques are used. Themost popular technique at the moment is to keep the chamber enclosing the contacts flooded
with sulfur hexafluoride (SF6) - a non-toxic gas that has superb arc-quelling properties.
2. ELECTRICAL SUBSTATION
A substation is a part of an electrical generation, transmission, and distribution system. Substations
transform voltage from high to low, or the reverse, or perform any of several other important
functions. Electric power may flow through several substations between generating plant and
consumer, and its voltage may change in several steps.
Substations generally have switching, protection and control equipment, and transformers.
In a large substation, circuit breakers are used to interrupt any short circuits or overload currents
that may occur on the network. Smaller distribution stations may use fuses for protection of
distribution circuits. Substations themselves do not usually have generators, although a power
plant may have a substation nearby. Other devices such as capacitors and voltage regulators may
also be located at a substation.
4
-
7/31/2019 HAL Project Report (1)
5/19
3. AC VOLTAGE STABILIZERS
3.1. ELECTROMECHANICAL
These operate by using a servomechanism to select the appropriate tap onan autotransformer with multiple taps, or by moving the wiper on a continuously variable
autotransfomer. If the output voltage is not in the acceptable range, the servomechanism switches
connections or moves the wiper to adjust the voltage into the acceptable region.
3.2. CONSTANT-VOLTAGE TRANSFORMER
The ferroresonant transformer or constant-voltage transformer is a type of saturating transformer
used as a voltage stabilizer. These transformers use a tank circuit composed of a high-voltageresonant winding and a capacitor to produce a nearly constant average output voltage with a
varying input current or varying load. The circuit has a primary on one side of a magnet shunt and
the tuned circuit coil and secondary on the other side. The regulation is due to magnetic saturation
in the section around the secondary. Saturating transformers provide a simple rugged method to
stabilize an AC power supply.
4. POWER FACTOR MANAGEMENT
4.1. INTRODUCTION
The power factor of an AC electric power system is defined as the ratio of the real
power flowing to the load to the apparent power in the circuit, and is a dimensionless number
between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time.
Apparent power is the product of the current and voltage of the circuit.
In an electric power system, a load with a low power factor draws more current than a load
with a high power factor for the same amount of useful power transferred. The higher currents
increase the energy lost in the distribution system, and require larger wires and other equipment.
5
-
7/31/2019 HAL Project Report (1)
6/19
Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge
a higher cost to industrial or commercial customers where there is a low power factor.
4.2. CAUSES OF LOW POWER FACTOR
The main causes are inductive loads which include induction motors, transformers,
induction generators etc. These inductive loads constitute major portion of the power consumed in
industrial complexes. Reactive power (KVAR) required by inductive loads increases the amount of
apparent power (KVA) in the distribution system. This increase in reactive and apparent power
results in a larger angle (measured between KW and KVA) and hence results in low power
factor.
4.3. ADVANTAGES OF MAINTAINING HIGH POWER FACTOR
4.3.1. ELIMINATION OF PENALTY AND REDUCED PEAK KW BILLING DEMAND
Utilities usually charge customers an additional fee when their power factor is less than
0.95. Thus, this additional fee can be avoided by increasing the power factor. By raising the power
factor, we use less KVAR. This results in less KW, which equates to a dollar savings from the
utility.
4.3.2. INCREASED SYSTEM CAPACITY
By adding capacitors (KVAR generators) to the system, the power factor is improved and
the KW capacity of the system is increased. For example, a 1,000 KVA transformer with an 80%
power factor provides 800 KW (600 KVAR) of power to the main bus. By increasing the power
factor to 90%, for the same amount of KVA, the KW capacity of the system increases to 900 KW
and the utility supplies only 436 KVAR.
4.3.3. INCREASED VOLTAGE LEVEL
Uncorrected power factor causes power system losses in the distribution system. As power
losses increase, you may experience voltage drops. Excessive voltage drops can cause overheating
6
-
7/31/2019 HAL Project Report (1)
7/19
and premature failure of motors and other inductive equipment. So, by raising your power factor,
you will minimize these voltage drops along feeder cables and avoid related problems.
4.4. METHODS OF IMPROVING POWER FACTOR
4.4.1. USING CAPACITORS SWITCHED BY CONTACTORS
An automatic power factor correction unit consists of a number of capacitors that are
switched by means of contactors. These contactors are controlled by a regulator that measures
power factor in an electrical network. Depending on the load and power factor of the network, the
power factor controller will switch the necessary blocks of capacitors in steps to make sure the
power factor stays above a selected value.
4.4.2. USING SYNCHRONOUS CONDENSERS
An unloaded synchronous motor can supply reactive power. The reactive power drawn by
the synchronous motor is a function of its field excitation. This is referred to as a synchronous
condenser. It is started and connected to the electrical network. It operates at a leading power
factor and puts VARs onto the network as required to support a systems voltage or to maintain the
system power factor at a specified level.
4.4.3. USING STATIC VAR COMPENSATORS
For power factor correction of high-voltage power systems or large, fluctuating industrial
loads, power electronic devices such as the Static VAR compensator are increasingly used. These
systems are able to compensate sudden changes of power factor much more rapidly than contactor-
switched capacitor banks, and being solid-state require less maintenance than synchronous
condensers.
7
-
7/31/2019 HAL Project Report (1)
8/19
5. ENERGY SOURCES & POWER SUPPLIES AT HAL
The electrical power to HAL Engine Division is provided from Main Receiving
Station(MRS) of HAL through two 11KV feeders. The total connected load of Engine Division is
12.5MVA.
Fig.I gives the schematic of HT Ring Main Distribution system at HAL Engine Division.
Fig.II gives the schematic of electrical distribution system at Main Receiving Station.
5.1. ES SUBSTATION
Fig.III gives the schematic of electrical distribution system at main power house(ES). At
ES, two 11KV feeders are coming from MRS, two feeders are going for ring main and fouroutgoing feeders for transformers at ES substation. All these feeders are provided with VCBs with
multifunction meters. From ES, one ring main feeder is going to ES-6 substation and the return
ring main feeder is from ES-1 substation.
Transformer ESTR1 of 500KVA is installed for air compressors. Transformer ESTR2 of
750KVA is installed for broaching machines. Transformer ESTR3 of 150KVA is installed for
lighting purpose and Transformer ESTR4 of 250KVA is installed for boiler house, pump house,
security gate etc,.
5.2. ES-1 SUBSTATION
Fig.IV gives the schematic of electrical distribution system at ES-1 substation. At ES-1, the
11KV ring main is incoming from ES-2 substation and ring main outgoing is to ES main
substation. There are five transformers installed at this substation.
Transformer ES1TR1 of 750KVA and ES1TR2 of 750KVA are installed to cater the powersupply to CNC machines and other loads through 500KVA servo voltage stabilizer. Transformer
ES1TR3 of 1000KVA is installed for heat treatment furnaces. Transformer ES1TR4 of 150KVA is
installed for lighting purpose. Transformer ES1TR5 of 100KVA is switched off and the lighting
load is shifted on ES1TR4. Transformer ES1TR6 of 500KVA is installed for vacuum bracing
along with 250KVA servo voltage stabilizer.
8
-
7/31/2019 HAL Project Report (1)
9/19
5.3. ES-2 SUBSTATION
Fig.V gives the schematic of electrical distribution system at ES-2 substation. At ES-2, the
11KV ring main is incoming from ES-1 substation and ring main outgoing is to ES-3 substation.
Transformer ES2TR1 of 250KVA is installed for lighting purpose. Transformer ES2TR2 of
750KVA is installed along with 500KVA servo voltage stabilizer to provide power to the machines
at ES-2 substation.
5.4. ES-3 SUBSTATION
Fig.VI gives the schematic of electrical distribution system at ES-3 substation. At ES-3,
11KV ring main is incoming from ES-2 substation and ring main outgoing is to ES-4 substation.
Transformer ES3TR1 of 500KVA is installed to provide power to the machines at ES-3 substation
along with 500KVA servo voltage stabilizer. Transformer ES3TR2 of 150KVA is installed for
lighting purpose.
5.4. ES-4 SUBSTATION
Fig.VII gives the schematic of electrical distribution system at ES-4 substation. At ES-4,
the 11KV ring main is incoming from ES-3 substation and ring main outgoing is to ES-5
substation. Transformer ES4TR1 of 150KVA and ES4TR4 of 150KVA are installed for lighting
purpose. Transformer ES4TR2 of 500KVA is installed to cater the power supply to CNC machines
and other loads through 500KVA servo voltage stabilizer. Transformer ES4TR3 of 500KVA is
installed for machines and other loads.
5.5. ES-5 SUBSTATION
Fig.VIII gives the schematic of electrical distribution system at ES-5 substation. At ES-5,
the 11KV ring main is incoming from ES-4 substation and ring main outgoing is to ETBR&DC.
Transformer ES5TR1 of 500KVA is installed to cater the power supply to CNC machines and
other loads through 500KVA servo voltage stabilizer. Transformer ES5TR2 of 500KVA is
installed for new rig room. Transformer ES5TR3 of 150KVA is installed for lighting purpose.
9
-
7/31/2019 HAL Project Report (1)
10/19
Transformer ES5TR4 of 1000KVA is installed to provide the power supply to CNC A/C plant and
other loads.
5.6. ES-6 SUBSTATION
Fig.IX gives the schematic of electrical distribution system at ES-6 substation. At ES-6, the
11KV ring main is incoming from ES-5 substation and ring main outgoing is to ES main
substation. Transformer ES6TR1 of 1000KVA is installed to cater the power supply to CNC
machines and other loads through 500KVA servo voltage stabilizer. Transformers ES6TR2 and
ES6TR3 of 500KVA are installed to provide the power supply to CNC A/C plant and other loads
through 500KVA servo voltage stabilizer.
10
-
7/31/2019 HAL Project Report (1)
11/19
6. CONVENTIONAL CONTACTOR-SWITCHED P.F. CORRECTION SYSTEMS
In this, number of capacitors that are switched by means of contactors. These contactors are
controlled by a regulator that measures power factor in an electrical network. Depending on the
load and power factor of the network, the power factor controller will switch the necessary blocks
of capacitors in steps to make sure the power factor stays above a selected value.
In this method, power factor correction is slow. Total correction takes few minutes. It gives
slow response; P.F. correction is not as effective in reducing maximum demand, especially when
the load variations are fast.
7. PROPOSED METHODOLOGY FOR IMPROVING POWER FACTOR
7.1. INTRODUCTION
The proposed methodology employs one thyristor switched-capacitor bank to generate a
controllable static VAR for single phase AC system. The capacitor bank is constructed of five
binary weighted thyristor- switched capacitors. This arrangement leads to a capacitor bank capable
of generating stepping reactive power having thirty one equidistant non-zero levels. Thecontrolling circuit of the capacitor bank is designed such that maximum absolute deviation from
linear response is 1/62 of its rating. Each capacitor is controlled by a single thyristor shunted by a
reverse diode. The system is capable of correcting lagging power factor up to unity or adjusting it
according to user desire. Each capacitor is connected to a series reactor for protecting the solid
state combination from inrush current occurring at the first instant of compensator plug in to power
system network. The proposed system is characterized by negligible no load operating losses, no
generation of harmonics, energy saving, and reduction of transmission losses.
11
-
7/31/2019 HAL Project Report (1)
12/19
7.2. ADVANTAGES OF PROPOSED METHODOLOGY OVER CONVENTIONAL
CONTACTOR SWITCHED-CAPACITOR P.F. CORRECTION METHOD
Load P.F. correction is quick and consistently near to the set value. Total P.F. correction is
achieved within few hundred milliseconds.
Fast P.F. correction reduces maximum demand more effectively, hence more savings on
account of reduction in MD charges.
Capacitors are switched through thyristors at "zero current crossover threshold". Hence the
capacitor connection to the mains is always smooth, transient free and absolutely without
generation of harmonics and voltage spikes.
7.3. THE CAPACITOR BANK CONFIGURATION
The proposed capacitor bank is composed of five binary weighted capacitors as shown in
Fig.1. This configuration offers 31 non-zero levels of possible capacitive reactive current as shown
in Fig.2. Each capacitor is controlled by a single thyristor shunted by a reverse diode. The thyristor
handles the positive half cycle of the capacitor current and the diode deals with the negative half
cycle. Reactors LS1 to LS5 are current limiters.
Fig.1 Capacitor bank configuration
12
-
7/31/2019 HAL Project Report (1)
13/19
Fig.2 Expected reactive current response
7.4. THE PROPOSED SINGLE-PHASE SYSTEM
The single-phase power factor correction system block diagram is shown in Fig.3. The
capacitor bank triggering circuit is excited by two signals. The first signal is KiL, where K is the
attenuation factor of the current transformer (C.T) circuitry and iL is the instantaneous load
current. The second signal is K*v, where K* is the attenuation factor of the voltage transformer
(V.T) circuitry and v is the instantaneous phase voltage. The load voltage and current can be given
by: v=Vmsin(t) iL=Imsin(t-),
Where Vm is the load voltage amplitude in volts, Im is the load current amplitude in
amperes, is load current power factor angle in radians, and t is time in seconds. The first zero-
crossing detector in Fig.4 converts K*v to a rectangular waveform V1 which is then differentiated
and half-wave rectified by the first RC differentiator/rectifier, forming V2. The latter is a train of
13
-
7/31/2019 HAL Project Report (1)
14/19
pulses used to trigger the sample and hold circuit at t = n, where n is a positive odd integer. The
analogue differentiator converts K*v to the analogue signal V3 which is then zero-crossing
detected, forming the waveform V4. The latter is processed similar to V1, forming V5. These
waveforms are shown in Fig.4.
Fig.3 Block diagram of proposed single phase system for p.f. correction
14
-
7/31/2019 HAL Project Report (1)
15/19
Fig.4 The basic voltage waveforms
The current signal KiL is sampled by the sample and hold circuit at t= n, where n is a
positive odd integer, yielding an analogue signal proportional directly to the reactive current
component of the load current as follows: KiL=Kimsin(t-)t=n
= Kimsin()
The latter signal is amplified and clamped upward by 0.15625 volts producing the analogue
voltage Vi which is proportional to the reactive current demand. Vi is the analogue input of the 8-
bit analogue-to-digital converter (8-bit ADC). For unity power factor correction and at full
compensator rating, Vi will have a magnitude of 10 volts. The 8-bit ADC starts conversion at t =
(2n-1)/2, where n is a positive odd integer. The five most significant digits (DB7, DB6, DB5,
DB4, and DB3) of the 8-bit ADC are employed for controlling the capacitor bank switching
devices (T5, T4, T3, T2, and T1) respectively. The instantaneous capacitor bank current iC is
determined by the logic status of the 8-bit ADC as follows:
15
-
7/31/2019 HAL Project Report (1)
16/19
Where, C is the basic capacitance of the capacitor bank. For DB7, DB6, DB5, DB4, and
DB3, logic zero refers to zero volts, while logic one refers to +5 volts. Note that when Vi is zero,
all the digital outputs of the 8-bit ADC are logic zero. When Vi is 10 volts, all the digital outputs
are logic one.
Table 1 shows the capacitor bank switching status as Vi varies from zero to 10 volts. The
driving circuit includes five sub-circuits; each one of them deals with one of the thyristor (T1 to
T5). Choosing appropriate switching instants will protect the switching devices from inrush
currents. The appropriate switching of thyristor occurs at t = (2n+1)/2, where n is a positive odd
integer. At these instants dv/dt is zero and each of the capacitors (C, 2C, 4C, 8C, and 16C) is
charged to -Vm through its corresponding diode and limiting reactor. Note that when this
compensator is started, each of the above capacitors charges through its individual diode and
limiting reactor to -Vm. No inrush current will associate the charging and switching mechanisms.
Table 1. The capacitor bank status as Vi varies from 0 to 10V.
16
-
7/31/2019 HAL Project Report (1)
17/19
Fig.5 Circuit diagram of single phase P.F. correction system
17
-
7/31/2019 HAL Project Report (1)
18/19
7.5. RESULTS
The basic capacitance C was chosen to be 50 F. The power system network has a frequency of 50
Hz and a phase voltage of 240 volts (r.m.s value). Consequently, the single-phase reactive current
rating is 165 A (peak value). Figure 8 shows PSpice tests for the single-phase system.
The first test corresponds to the case at which the load impedance (ZL) was 1.25 with angle 37o.
The power factor for this load is 0.8 lagging. The reactive component of the load current was 163A
(peak value) which was within the compensator rating. Consequently the compensator generated a
capacitive reactive current completely cancelled the load current reactive component yielding a
real total current (iT) as shown in Figure 7a. The second test corresponds to an inductive load of
1.25 with angle 53o . The reactive component for that load was 217.6A (peak value) which
exceeded the system rating by 52.6A (peak value). Therefore the power factor for that load was
only improved as shown in Figure 7b.
Fig.7 (a) UPF correction (b) Power Factor improvement
18
-
7/31/2019 HAL Project Report (1)
19/19
8. CONCLUSION
The proposed single-phase automatic power factor correction system has certain reactivecurrent or
reactive power ratings. When the detected reactive power absorbed by the load is greater than the
compensator rating, the power factor will not be corrected to unity, but certainly will be improved
and the apparent power supplied by the ac supply will be reduced. These systems respond almost
linearly throughout their pre-assigned areas of operation. They achieve better power quality by
reducing the apparent power drawn from the ac supply and minimizing the power transmission
losses. In addition, no harmonics disturbing the power system network are released, and hence no
filtering is required. The responses of both systems are settled down within the power system
network fundamental cycle. There is a feasibility of utilizing this technique for designing systems
with high voltage and current ratings.
19