HAL Project Report (1)

7/31/2019 HAL Project Report (1)

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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.

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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.


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.


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.


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.

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Transformer ES5TR4 of 1000KVA is installed to provide the power supply to CNC A/C plant and

other loads.


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.

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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.

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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

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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

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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

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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:

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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.

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Fig.5 Circuit diagram of single phase P.F. correction system

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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

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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.

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HAL Project Report (1)