CHAPTER - 1REACTIVE POWER FUNDAMENTALS

--M.N.Murthy, Director, PSTI, Bangalore

1.0 Introduction

Voltage is proportional to the magnetic flux in the power system element. Most of the Power System elements are reactive in nature. They absorb / generate reactive power depending on system loading conditions. The balance in reactive power availability and requirement at a node indicates steady voltage. Drawal of reactive power leads to reduction in voltage and supply of reactive power leads to increase in voltage at the node. Ideally, the reactive power balance should be effected within each region, within each distribution system.

Excess of MVAr high voltageDeficit of MVAr Low VoltageMVAR balance Good voltage low system losses

A great many loads consume not only active but also reactive power. The electric network itself both consumes and produces reactive power. Transmission and distribution of electric power involve reactive power losses due to the series inductance of transformers, overhead lines and underground cables. Lines and cables also generate reactive power due to their shunt capacitance; this generation of reactive power is, however, only of significance at high system voltages.

During the steady-state operation of an AC power system the active power production must match the consumption plus the losses, since otherwise the frequency will change. There is an equally strong relationship between the reactive power balance of a power system and the voltages. In itself, a reactive power balance will always inherently be present, but with unacceptable voltages if the balance is not a proper one. An excess of reactive power in an area means high voltages: a deficit means low voltages. The reactive power balance of a power system also influences the active losses of the network, the heating of components and, in some cases, the power system stability.

Contrary to the active power balance, which has to be effected by means of the generators alone, a proper reactive power balance can and often has to be effected both by the generators and by dispersed special reactive devices, producing or absorbing reactive power. The use of shunt reactive devices. i.e. shunt compensation, is a straightforward reactive-power compensation method. The use of series capacitors, i.e. series compensation is a line reactance compensation method.

No special reactive compensation devices were used in the early AC power systems, because the generators were situated close to the loads. As networks became more widespread, synchronous motors, small synchronous compensators and static shunt capacitors were adopted for power-factor correction. Ever larger synchronous compensators were installed in transmission systems. Along with the development of more efficient and economic capacitors, there has been a phenomenal growth in the use of shunt capacitors as a means of furnishing reactive power, particularly within distribution systems. With the introduction of extra-high-voltage lines, shunt reactors and series capacitors became important compensation devices. The latest development is the Thyristor-controlled static var compensator, which is now well established not only in high- power industrial networks but also in transmission systems.

In the following a distinction is made between transmission and distribution systems and also between different voltage ranges in terms of HV, EHV, etc. It should therefore be appropriate to explain briefly these terms.

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Classification of System Voltages

Voltage Level in kV Category of Voltage <33 kV Distribution System

33 kV to132 kV Sub. Transmission System230 kV to 400 kV HV Transmission System750 kV and above UHV System

Transmission systems form those parts of power systems conveying comparatively large amounts of electrical power. They link the generating sources with the distribution systems and interconnect parts of the power system or adjacent power systems. Distribution systems form the continued links to the consumers. The boundary between transmission and distribution systems is not very well defined. Systems for voltages higher than 132 KV are usually called transmission systems. Systems for voltages lower than 33 KV are usually called distribution systems. Systems in the range 33 to 132 kV are called distribution, sub transmission systems.All the figures given in this introduction refer to the highest voltage for equipment.

1.1 Need for management of reactive powerIn an integrated power system, efficient management of active and reactive power flows is very important. Quality of power supply is judged from the frequency and voltage of the power supply made available to the consumers. While frequency is the measure of balance between power generated (or power available) and MW demand impinged on the system, the voltage is indicative of reactive power flows.

In a power system, the ac generators and EHV and UHV transmission lines generate reactive power. Industrial installations whether small or large as also the irrigation pump motors, water supply systems draw substantial reactive power from the power grid.The generators have limited defined capability to generate reactive power- this is more so in respect of large size generating units of 210 MW/500 MW capacity. Generation of higher reactive power correspondingly reduces availability of useful power from the generators. During light load conditions, there is excess reactive power available in the system since the transmission lines continue to generate the reactive power thereby raising the system voltage and this causes reactive power flows to the generators.

Particularly in India, the load curves show wide fluctuations at various hours of the day and in various seasons of the year. When load demand is heavy, there is low voltage, which is harmful to the consumers as well as utility’s installations. Burning of motors occur. When load demand is very low, high voltage occurs in the system and this has harmful effect on insulation of power transformers. Failure of power transformers occur.

For better efficiency, it is necessary to reduce and minimize reactive power flows in the system.Besides harmful effects, the reactive power flows also affect the economy adversely both for the utility and the consumer. If reactive power flows are reduced i² R power losses as well as i² X losses are reduced. The generators can produce additional active power. If the consumer reduces reactive power requirement his demand KVA is reduced. For energy conservation also there is need to reduce reactive power demand in the system.

It is therefore very clear that for efficient management of power system and for improving the quality of electric supply, it is very essential to install reactive compensation equipment. Such installations are necessary and essential for utility as well as the consumer. Infact the utility should be made responsible

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for making available only the active power to the consumer. Unfortunately, in India, the responsibilities of users are not well defined and there is not enough realization in this regard. Utilities have now introduced power factor clause in the tariff structure. However. It would be worthwhile to note that even a 90% power factor load requires 43% reactive power from the grid.

1.2 Basic Principles:A phasor description of voltage and current, the reactive power supplied to an AC circuit is the product of the voltage and the reactive (watt-less) component of the current, this reactive current component being in quadrature with the voltage.

A single-phase circuit according to Figure 1.1 the reactive power Q is given byQ= VIsin ------------------------------------------------------(1)

Unit is volt-ampere reactive (VAR) The sign of Q is a matter of convention, it depends on the definition of the direction of . According to the IEC the sign shall be such that the net reactive power supplied to an inductive element is positive. Consequently, the net reactive power supplied to capacitive element is negative. In the past the opposite sign convention has also been used. With the sign convention as base, reactive power is said to be produced/generated by overexcited synchronous machines and capacitors, and consumed or absorbed by under excited synchronous machines, inductors, etc.

Reactive power can be considered as a convenient evaluation quantity, giving information about the watt-less current, which greatly influences voltages, active losses.

1.3 Sources and sinks of Reactive power :

S.No. Sources (Q- Generation) Sinks (Q – Absorption)1 Gen. Over excited Gen. Under excited2 Transmission Lines - charging Transmission Lines - series reactance drop3 Shunt Capacitors Shunt Reactors4 Static Var Compensators (Q –gen mode) Static Var Compensators (Q – absorb

mode)5 Series Capacitors (Cse) -

6 Synchronous Condenser over excited Synchronous Condenser under excited

7 Loads -Capacitive Loads - Inductive

1.4 Power transmission in a Transmission line:

3

G M

VsM

Vr0Ir

Sr

j X

Fig. 1.2 Simple Transmission System

Let Vs =Sending end voltageVr =Receiving end voltageSr = Receiving end complex powerPr = Receiving end active power Qr = Receiving end reactive power = The angle difference between Vs and VrIr = Receiving end currentX = Line reactancePs = Sending end active powerQs = Sending end reactive power

Sr = Pr +j Qr = Vr . Ir* (1)

= Vr

=

Pr = (2)

For a loss less line.P and are closely related.

Qr = (3)

Qs = (4)

For small angles of

Qr = (5)

Qs = (6)

Q and V are closely coupled.

Inferences:If V1and V2 are the sending end and receiving end voltages

The transmission capacity increases as the square of the voltage level1. the direction of MW flow is determined by

V1 leading V2 P is 1 2V1 lagging V2 P is 2 1

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2. Magnitudes of V1 and V2 do not determine the MW flow direction3. Though P1=P2, Q1 Q24. The reactive loss in line reactance is

5. If Vs Vr the MVAR flows 1 2If Vr Vs the MVAR flows 2 1

1.5 Power Losses in a Transmission line:

Losses across the series impedance of a transmission line are I2 R and I2 X.

Where I = ;

I* =

I2 = I.I* =

Ploss = I2R = (7)

Qloss =I2X = (8)

Hence in order to minimise losses we have to minimise the transfer of Q.

1.6 Voltage Regulation:

Voltage regulation is defined as the change of voltage at the receiving end when rated load is thrown off, the sending end voltage being held constant.

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Vr X.Qr V

EThX.Pr V

Fig 1.3 Voltage regulation in a loss less system

ETh =V0 +j X I =V + j X

= V + (9)

The voltage rise term in phase with V depends on Q. The angle, depends mainly on the quadrature term involving P.

Three methods of system voltage control are available : (a) Varying excitation of generators, (b) Varying the turns ratio of transformers by OLTC and (c) Varying shunt compensation.

Shunt compensation is drawing or injection of reactive power at a node. Reactor absorbs reactive power and so reduces system voltage. Capacitor injects reactive power and so increases system voltage.

1.7 Short circuit capacity: MVA (10)

Where V = Phase to phase voltage in kVIf = The three phase fault current in k.A.

Expressed in p.u parameters

Ssc = (V0-)(If) p.u. = If p.u. = (11)

V0- =The prefault voltage in p.u. = 1.0 p.u.

XTh = Thevinin impedance = Driving point impedance of the network.The change in voltage when certain quantity of reactive power is supplied to the system is given by

WhereQ = Change in Q injectionSsc =Short circuit capacityV = Change in voltage in per unit

1.8 Reactive power - physical analogy

The reactive power is the extra effort needed to pull a load along the rail when the effort, s is at an

angle, to the rails.

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P

Q

Fig 1.4. Physical analogy for Active and Reactive powers

S

1.9 Power transfer componentsTransformers, overhead lines and underground cables make up the major AC power transfer components and are discussed in this subsection.

1.9.1 Transformers

Figure 1.5 shows a simple equivalent circuit of a two-winding transformer. The series reactance X is of main interest, usually lying within the range 0.05 to 0.15 p.u. based on the transformer power rating, with low values for small and high values for large transformers. The resistance is usually negligible. The total reactive power losses due to the magnetizing shunt reactance Xm of many small transformers within a distribution system can, however, be of some importance. The magnetizing reactive power may also increase rapidly with the voltage level, due to core Saturation.1.9.2 Overhead lines

Overhead lines and underground cables are distributed-constant circuits, which have their series resistance, series inductance and shunt capacitance distributed uniformly along its length. Figure 1.6 shows a lumped-constant equivalent circuit. If we assume constant operating voltages at the ends, the reactive power generated due to the capacitance, the charging reactive power, is practically independent of the power transferred. Particularly when we are dealing with long EHV lines, the so-called Surge Impedance Load (SIL) P0 or natural load of an uncompensated line is a convenient value for reference purposes. It is given approximately by:

------------------------------------------------(12)

where V = voltage, line-line kV

b = susceptance mho/kmx = reactance ohm/km

A loss less line (a reasonable approximation of an EHV line) transferring an active power P0 and with

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Fig 1.5 Equivalent Circuit of Transformer

equal voltages at the line ends has reactive power balance. The reactive power loss due to the line inductance is equal to the reactive power generated by the line capacitance.

Operating voltage kV

SILMW

Line charging Mvar/km

XOhm/km

X/R

0.410130220400500750

--

501305509102200

--

0.050.140.61.02.3

0.400.400.400.400.330.300.28

0.50.536151630

Table 1. Typical values of overhead line characteristics at 50Hz.

Table 1 gives typical values of overhead line characteristics at 50Hz. At 60 Hz the SIL values are the same while the line charging, X and X/R values are 20 per cent higher. The SIL is usually much lower than the thermal rating. Below 69 kV the line charging is usually negligible while it is a significant source of reactive power for long lines of higher system voltages.

Paradoxically, the series reactance is fairly independent of the system voltage, assuming a single conductor. The lower values at 400 kV, 500 kV and 750 kV illustrate the effect of the necessary use of bundle conductors for these system voltages. In reality there is a great spread in the X/R values, for a system voltage under consideration, in particular at low system voltages. The figures are however, included in order to illustrate that the X/R ratio increases rapidly with the system voltage.1.9.3. Underground cables:Table 2 gives sample values of underground cable characteristics. The spread in parameter values for a system voltage under consideration is very much larger than for overhead lines, depending on the cable type, size and conductor geometry and spacings. Except for low voltage cables, the SIL is usually much larger than the thermal rating. The line charging of polyethylene insulated cables, now being introduced at ever higher system voltaes, is much lower, e.g. 50 per cent of that of paper-insulated cables.

Operating voltage kV

SILMW

Line charging Mvar/km

XOhm/km

X/R

0.410130220400

-3

50010003200

-0.01

24

13

0.070.100.150.180.2

0.30.4269

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Table.2 Sample values of underground cable characteristics at 50 Hz. 0.4. 10 kV:PVC, 132,400kV paper-insulated cables.

1.10 LoadsA great many loads consume not only active but also reactive power. The Industry wise power factor is generally observed to be as follows:

Some typical values of reactive power consumption of individual loads are given below: Induction motors 0.5 to 1.1 kvar/kW, at rated output. Uncontrolled rectifiers 0.3 kvar/kW. Controlled rectifiers usually consume much more kvar/kW than uncontrolled ones and with

dependence on the rectifier delay angle. Arc furnaces around 1 kvar/kW.

Both controlled rectifiers and arc furnaces of steel mills have a reactive power consumption characterized by a high average value and fast variations. Purely resistive loads, like filament lamps and electric heaters, do not, of course, consume reactive power.The synchronous motor is the only type of individual load, which can produce reactive power. it consumes reactive power when under excited and produces reactive power when overexcited. Synchronous motors are usually operated overexcited and thus usually produce reactive power.Individual loads may, of course, vary within short or long time ranges. The composite loads of a power system. Each one being the total load of a certain area, usually vary with the time of the day, the day of the week and the season of the year

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Fig.1.7 Examples of load

curves

INDUSTRY POWER FACTOR

Textiles 0.65/0.75Chemical 0.75/0.85Machine shop 0.4 / 0.65Arc Welding 0.35/ 0.4Arc Furnaces 0.7 / 0.9Coreless induction furnaces and heaters 0.15/0.4Cement plants 0.78/0.8Garment factories 0.35/0.6Breweries 0.75/0.8Steel Plants 0.6 / 0.85Collieries 0.65/0.85Brick Works 0.6 / 0.75Cold Storage 0.7 / 0.8Foundries 0.5 / 0.7Plastic moulding plants 0.6 / 0.75Printing 0.55/0.7Quarries 0.5 / 0.7Rolling Mills (i.e. ,Paper, Steel , etc.) 0.3 / 0.75

and may also grow from year to year. The consumer demand for reactive power varies in a somewhat similar way to the demand for active power. Figure 1.7 illustrates how the active and the reactive power supplied from a transmission substation into a load area, with mixed industrial and domestic loads, may vary during a Sunday and a Monday.The resultant active power demand of a power system varies roughly as the variation of total toad. The resultant reactive power demand may vary considerably more due to the changing series reactive power losses in the networks.

1.11. Relationship of voltage to reactive powerAs regards the study of terminal voltages of a transmission or a distribution link, the link can be represented by the series impedance only if the shunt admittances of the equivalent circuit are included in

the treatment of the connecting parts of the power system, Fig. 1.8. The link may be an overhead line, an underground cable, or a transformer. The voltage drop, i.e. the scalar voltage difference, is defined by:

V= V1 – V2--------------(13)

The Phasor diagram of Figure 1.8, for a case with lagging power factor, shows that it can be approximately expressed by the following equations:

V=RI cos+XI sin --------------- ----- ------------------(14) V= (RP+XQ) / V2 --------------------------------------- (15)

The accuracy of the equations (14) and (15) is better, the less the voltage-angle difference is. The equations are usually sufficiently accurate for calculations concerning a single link with lagging power factor. The equations are less accurate and should not be used in calculations for -leading power factor. Precise calculations concerning a complete network are, nowadays, performed by means of computer power flow programs.

The equation (15) is, however, generally useful for qualitative discussions of voltage versus reactive power. For transformers, R can always be disregarded. For transmission (not distribution) lines and cables. X is usually much larger than R. For all these many links, where X is -much larger than R, there will evidently be a much greater influence on V per kvar of reactive power than per kW of active power transmitted.

When power is supplied through a single link, Figure 1.8, assuming V1 constant, V2 varies with changes in P and Q. Load variations create voltage variations if not counteracted. This is a general, and sometimes -troublesome, operation feature of AC power systems.There are three major methods of power system voltage control:

Varying the excitation of the generators by means of their excitation systems. Varying the turn’s ratio of transformers by means of their on-load tap changers. Varying the shunt compensation, where applied.

By shunt compensation is meant drawing or injection of reactive power, at a point of a power system by means of a shunt-connected device, which is installed for this sole purpose. Drawing reactive power. e.g.

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absorption by means of a shunt reactor, effects voltage reduction. Injection of reactive power, e.g. production by means of a shunt capacitor, effects voltage rise. The equation (15) and Figure 1.8 show how shunt compensation influences the voltage. The voltage-change directions mentioned arise because the network equivalent impedance has an inductive character at the fundamental frequency. The shunt compensation may be fixed, switchable in steps or continuously controllable. Around the nominal voltage, the voltage change V, when the shunt compensation is changed in step, is approximately expressed by;

V = ------------------(16)

WhereQ- change in nominal three phase reactive power injection MvarSsc- Short-circuit capacity in MVA

Adjacent generators with voltage regulators and adjacent transformers with voltage-relay controlled on-load tap changers will, of course, more or less reduce the voltage change after a certain time. By series compensation is meant compensation of line inductive reactance by means of a capacitor in series with the line, thus reducing the effective inductive reactance of the line and the effects thereof.

1.12 PV Curves

PV Curves are the product of parametric analysis. Take into consideration the system shown at right. Power is transferred from the Sending Area to the Receiving Area via a set of transmission lines forming an Interface. As the transfer increases, the conditions on the lines and buses along the transfer path, including those within the Sending and Receiving area, change. The voltages may drop, flows on branches may increase or decrease.

Monitoring voltage at a particular bus and plotting this against the power transfer produces a familiar diagram known as the PV Curve. A sample curve is shown below. When the voltage at the selected bus goes below some pre-defined criteria, then the transfer at which this occurs is the Low Voltage transfer limit for that bus. Ignoring the low voltage and continuing to increase transfer would eventually bring the curve to a point where the system collapses. The point of collapse can likewise be designated as the Voltage Collapse transfer limit.

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In PSS™TPLAN, PV curves are provided as a distinct Analytical Engine. As such it is provided with powerful features:

Easy setup Comprehensive results Adaptive step size. You define a range for the transfer increment, and PSS™TPLAN will select a

step size which will maintain the accuracy of the simulation at minimum loss of resolution. Non-divergent power flow. The last point on the curve is always accurately determined by a

special algorithm which can identify divergence.

1.13 Need to optimize reactive power resources:The need to optimize reactive power sources is essential to Capacity utilization of existing transmission facilities for power transfer. Maximize the existing reactive power resources to minimize investment in additional

facilities. Minimize transmission losses Improve system security Maintain power supply quality by maintaining bus voltages close to nominal value.

1.14. RemarksActive power must, of course, be transmitted from the generators to the loads. Reactive power need not, and with regard to voltage differences, losses and thermal loading as discussed in the preceding subsections, should not be unnecessarily transferred. Ideally, a reactive power balance should be effected within each region of a power system, within each transmission system and within each distribution system. In practice, however, this principle is not always followed for one reason or another. The subject of reactive power compensation is easy to understand if we consider a single link of a power system, but quite complex when we consider an entire power system with its different conditions and behaviors.

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CHAPTER - 2 REACTIVE POWER SOURCES AND SINKS

2.0 Introduction:Sources of reactive power are Generating units Synchronous condenser On-load tap changers and phase-shifting transformers. Capacitors and reactors Static compensators.

Power system component characteristicsA brief look at characteristics for power system components will help to explain reactive power matters.

The role of power system components in reactive power control are briefed below.

2.1 Generators The purposes of generators are to supply the active power, to provide the primary voltage control of the

power system and to bring about, or at least contribute to, the desired reactive power balance in the areas adjacent to the generating stations. A generator absorbs reactive power when under excited and it produces reactive power when overexcited. The reactive power output is continuously controllable through varying the excitation current. The allowable reactive power absorption or production is dependent on the active power output as illustrated by the power charts of Figures 2.1 and 2.2. For short-term operation the thermal limits are usually allowed to be overridden.

The step-response time in voltage control is from several tenths of a second and upwards. The rated power factor of generators usually lies within the range 0.80 to 0.95. Generators installed remotely from load centers usually have a high rated power factor; this is often the case with large hydro-turbine generators. Generators installed close to load centers usually have a lower rated power factor. In some cases of large steam-turbine generators the rated power factor may have been selected at the lower end of the above range in order to ensure reactive power reserve for severe forced outage conditions of the power system.

Fig 2.1 Typical Power chart for large steam turbine and gas turbine generatorswherea — Turbine power limitb — Stator winding thermal limitc — Field winding thermal limitd — Steady- slate stability limit with proper AVRe — Assumed intervention curve of under excitation limiter

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Fig.2.2. Typical power chart for large hydro-turbine generators (salient-pole machines)

Large generators are usually connected direct to transmission networks via step-up transformers. The terminal voltage of a large generator is usually allowed to be controlled within a ± 5% range around the nominal voltage, at rated load. In most countries the generator step-up transformers are usually not equipped with on-load tap changers.

Excitation Control: The MVAR output of a generator is dependent on its excitation. The MVAR is generated during over excitation and is absorbed during under excitation. The rotor current depends on the excitation. The rotor winding temperature, the air gap temperature and the machine temperature increase during over excitation. The winding temperature is limited to about 90oC during normal loading. It increases to 100 – 105oC during over loading. The machine which is already over heated due to MVAR generation can not take MW load to its full capacity. Hence MW load is to be compromised when the unit is excited beyond its normal limits.

When the unit generates MVAR and supplies to the system, the system voltage profile around the generating station increases. This increase in voltage is more in first neighbourhood. The load end voltages which are beyond, say second neighbourhood will not get effected because of this unit excitation. Hence the influence of a unit on voltage profile in the system is local in nature. The load end voltages can not be controlled by the generating units.

However depending on the capability curve of the generating unit and as long as margin is available in the unit, it can be used to control the system voltages in its vicinity.

The change in the voltage V in the first neighbourhood of the generating station depends on the relationV = Q/S in p.u.

Where V = change in bus voltage in pu Q = Amount of Q supplied through over excitation in p.u. S = Fault level of the system at first neighbourhood in p.u.

2.2 Shunt reactor A shunt reactor is a reactor connected in shunt to a power system for the purpose of absorbing reactive power. In some cases where a fixed or mechanically switched shunt reactor can be used with regard to the voltage control requirements. It is usually the most economic special means available for reactive power absorption. The majority of shunt reactors are applied in conjunction with long EHV overhead lines. They are also applied in conjunction with HV and EHV underground cables in large urban areas. Shunt reactors in use range in size from a few Mvar at low medium voltages and up to hundreds of Mvar.

Shunt reactors are necessarily installed to suppress high voltage during light load conditions. For

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400kV and UHV lines, shunt reactors are directly connected on line. This is for the purpose of compensating leading charging MVAR released by the line. Shunt reactors are also connected on tertiary delta windings of autotransformers so that these can be switched on during light load periods.

Reactor Operation: The shunt reactor is a coil connected to the system voltage and grounded at the other end. It draws the magnetizing current, which is purely inductive, from the system and hence forms an inductive load at the point of connection. Hence the reactor absorbs reactive power from the system as long as it is connected to the system. Hence it is complimentary to a capacitor bank in its function. The reduction in voltage at the point of connection is given by V = Q/S, all expressed in p.u. terms.The reactors are required to be used at EHV voltages of 400 kV and above, as the line charging at this voltage is quite significant, it increases the receiving end voltage to unacceptable limits under light load conditions. A 400 kV line generates about 55 MVAR per 100 km and hence this Ferranty effect is high for lines of 300 km and above.

Two types of reactor connection are adopted in EHV systems. A) The bus reactor, which is connected to the bus through a circuit breaker and hence can be

switched as and when required.B) The line reactor; which is connected to the line through only an isolator and hence can be

removed from the system only when the line is switched off.

The functions of both bus reactor as well as line reactor are same. They absorb the reactive power from the system depending upon their capacity.The bus reactors are switchable and hence are cut-in whenever the system voltage is higher and can be cut-off from the system whenever the system voltage reduces.

The line reactors are permanently connected to the lines and hence the system. Their role is toa) Reduce the effect of line chargingb) Provide a least impedance path for the switching over voltages generated in the system due to

inductive load currents’ switching. The switching over voltages are of power frequency and equal to 1.5 to 2.5 p.u. in magnitude.

c) When the EHV lines have single phase switching facility and auto reclose protection scheme is implemented, the abnormal voltages developed across the circuit breaker can be contained only with a line reactor on the line side.

d) The line reactors provide a least impedance path for low frequency (power frequency) switching over voltages. Hence they act as surge diverters for power frequency over voltages. The lightning over voltages cannot pass through the line reactor because of their high frequency.

2.3 Shunt capacitorsA shunt capacitor is a single capacitor unit or, more frequently, a bank of capacitor units connected in shunt to a power system for the purpose of absorbing reactive power. When a fixed or mechanically switched shunt capacitor can be used with regard to the voltage control requirements, it is the most economic means available for reactive power supply. The majority of shunt capacitors are applied within distribution systems of different types: Industrial, urban, residential and rural. They have a widespread use there, for power-factor correction. Some shunt capacitors are installed in transmission substations. Very large shunt capacitor banks (usually filters) are to be found in HVDC terminal stations.Shunt capacitors in use range in size from a single unit rated a few kvar at low voltage up to a bank of units, rated hundreds of Mvar.

Capacitor Operation: The capacitor banks are reactive power sources. They produce reactive power equal to their rating when connected to the bus. In order to keep the insulation costs less, they are connected to the system at distribution voltage levels, e.g. 0.4 kV, 11 kV, 33 kV etc.

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The output of a capacitor bank is Qc = V2 c

Where Qc = output in MVARV = the system voltage in k.V.C = in farads

Hence the output is proportional to the square of the voltage. If the system voltage to which the capacitor bank is connected reduces to 0.9 p.u. the MVAR generated by the capacitor reduces to 0.81 p.u. Hence the performance of a capacitor bank will be poor under low voltage conditions, at which time it is required most.

The influence of a capacitor bank on the system voltage is again local like in case of a generator. It is most pre dominent at the bus to which it is connected. Its effect gets reduced as we go to next neighbourhood. The change in voltage at the point of connection is governed by the relation V = Q/SWhere V = change in bus voltage in pu Q = Amount of Q supplied through the capacitor bank in p.u. S = Fault MVA of the bus in p.u.Hence it is possible to compute the capacitor requirement of the system at a location using

Q = (V)(S)where Q is the amount of Q to be supplemented

V is the voltage raise required to reach the nominal value in p.u.S is the fault level of the system in p.u.

Outstanding features of shunt capacitors are their low overall costs and their high application flexibility. An unfavorable characteristic, most important in conjunction with major outages and disturbances, is that they provide the least support at the very time when it may be most needed, because the reactive power output is proportional to the voltage squared. If used in a proper mix with other reactive power sources, this is, however, no obstacle to an extensive use of shunt capacitors. The losses of modern shunt capacitors are of the order of 0.2w/Kvar, including the losses of fuses and discharge resistorsShunt capacitors are useful in Power factor correction Voltage control and reactive power balance Reducing transmission losses Meeting requirements of reactive loads

Pf correction by shunt capacitors is by far the most satisfactory and economical method. The static capacitor owing to its low losses, simplicity and high efficiency, is finding very wide and universal use for pf correction.

A detailed description on construction, operation, protection and trouble shooting of capacitor banks is provided in Chapter 3.

2.4 Transformer Tap Changing: A transformer in the grid is like a node. Its voltage is maintained by the requirement and availability of reactive power at its terminals. If the HV voltage is low, due to bucking tap at, say -5, for e.g. at 0.96 pu the HV bus will get a net reactive power in-flow of say 200 MVAR through its EHV transmission network. The same reactive power flows towards the LV bus. The LV bus voltage now increases. This is illustrated in Fig 2.3.

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If the transformer tap is raised to say 5, it is now boosting the HV voltage to say, 1.02 pu. Now the reactive power in-flow reduces to HV bus, to say 20 MVAR. This reduced MVAR is flowing to LV bus. Hence the LV bus voltage reduces. This is illustrated in Fig 2.4. Hence the transformer tap only alters the number of turns in the HV winding there by altering the HV voltage. If this HV voltage is less than the neighbourhood voltage it receives MVAR, if it is more, then it pumps MVAR to its neighbourhood. The LV bus voltage is maintained only as a consequence of MVAR inflow or outflow to it from the HV bus.

2.5 Synchronous condensersSynchronous condenser is another reactive power device, traditionally in use since 1920s. Synchronous condenser is simply a synchronous machine without any load attached to it. Like generators, they can be over-exited or under-exited by varying their field current in order to generate or absorb reactive power, synchronous condensers can continuously regulate reactive power to ensure steady transmission voltage, under varying load conditions. They are especially suited for emergency voltage control under loss of load, generation or transmission, because of their fast short-time response. Synchronous condensers provide necessary reactive power even exceeding their rating for short duration, to arrest voltage collapse and to improve system stability.

Synonymous terms are synchronous compensator and synchronous phase modifier. The synchronous compensator is the traditional means for Continuous control of reactive power. Synchronous compensators are used in transmission systems: at the receiving end of long transmissions, in important substations and in conjunction with HVDC inverter stations. Small synchronous compensators have also been installed in high-power industrial networks of steel mills; few of these are in use today. Synchronous compensators in use range in size from a few MVA up to hundreds of MVA.

Both indoor and outdoor installations exist. Synchronous compensators below, say, 50 MVA are usually air-cooled, while those above are usually hydrogen-cooled. Modern synchronous compensators are usually equipped with a fast excitation system with a potential-source rectifier exciter. Various starting

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methods are used; the modern one is inverter starting.

The size of a synchronous compensator is referred to the Continuous MVA rating far the generation of reactive power. In the generating mode of operation it usually has a rather high short-time overload capability. The absorption capability is normally of the order of 60 per cent of the MVA rating, which means that the control range is usually 160 per cent of the MVA rating. The reactive power output is continuously controllable. The step-response time with closed-loop voltage control is from a few tenths of a second, and up. The losses of hydrogen-cooled synchronous compensators are of the order of 10 W/kvar at rated output. The losses of small air-cooled machines are of the order of 20 W/kvar at rated output.

In recent years the synchronous compensator has been practically ruled out by the SVC, in the case of new installations, due to benefits in cost performance and reliability of the latter. One exception is HVDC inverter stations, in cases where the short-circuit capacity has to be increased. The synchronous compensators can do this, but not the SVC.

Comparison between Synchronous Condenser and shunt capacitor:

Sl.No Synchronous condenser Shunt capacitor

1. Synchronous condenser can supply kVAR equal to its rating and can absorb up to 100% of its KVA rating

Shunt capacitor should be associated with a reactor to give that performance

2. This has fine control with AVR This operates in steps

3. The output is not limited by the system voltage condition. This gives out its full capacity even when system voltage decreases

The capacitor output is proportional to V2

of the system. Hence its performance decreases under low voltage conditions

4. For short periods the synchronous condenser can supply KVAR in excess of its rating at nominal voltage

The capacitor can not supply more than its capacity at nominal voltage. Its output is proportional to V2.

5. The full load losses are above 3% of its capacity

The capacitor losses are about 0.2%

6. These can not be economically deployed at several locations in distribution

The capacitor banks can be deployed at several locations economically in distribution

7. The synchronous condenser ratings can not be modular

The capacitors are modular. They can be deployed as and when system requirements change

8. A failure in the synchronous condenser can remove the entire unit ability to produce KVAR. However failures are rare in synchronous condensers compared to capacitors

A failure of a single fused unit in a bank of capacitors affects only that unit and does not affect the entire bank

9. They add to the short circuit current of a system and therefore increase the size of (11kV etc.)

The capacitors do not increase the short circuit capacity of the system, as their

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breakers in the neighbour-hood. output is proportional to V2

10. This is a rotating device. Hence the O&M problems are more

These are static and simple devices. Hence O&M problems are negligible

2.5 Thyristor-controlled static var compensators (SVCs) A Thyristor-controlled static var compensator is a static shunt reactive device, the reactive power generation or absorption of which can be varied by means of Thyristor switches. The adjective’ static’ means that, unlike the synchronous compensator, it has no moving primary part. Because it is the latest developed means of reactive compensation, it will be described and discussed in greater detail than the other devices. In a strict sense, the term static var compensator covers not only Thyristor-controlled compensator but also other, types and in particular, the self-saturated iron-core reactor type. Even though the self-saturated reactor compensators introduced before the Thyristor-controlled one, the later completely dominates the applications of compensators in transmission systems, covering more than 95 per cent of all compensators. Today, it also leads industrial applications in conjunction with arc furnaces. The following description is restricted to Thyristor-controlled compensators utilizing traditional Thyristor (not GTO Thyristor).

As early as the first half of the 1970s the SVC became a well-established device in high-power industrial networks, particularly for the reduction of voltage fluctuations caused by arc furnaces. In transmission systems the breakthrough came at the end of the 1970s. Since then, there has been an almost explosive increase in the number of applications, in the first place as an alternative to synchronous compensators, but also for a more extensive use of dynamic shunt compensation, i.e. of easily and rapidly controllable shunt compensation.

Compensators in use range in size from a few Mvar up to 650 Mvar control range, and with nominal voltages up to 765 kV.

2.5.1. Function of SVC’s in Power systems:SVCs are used to improve voltage regulations, improve power factor, reduction of voltage and current unbalances, damping of power swings, reduction of voltage flicker, improved transient stability of the system etc. This can result in saving in operational costs, increased power transfer capability, reduced line losses, higher availability of power etc.

2.5.1.1. Voltage control in Power systems : The voltage variations in power systems are caused due to load switching, power system elements’ switching. These variations are compensated by SVC. Three phase system voltages are compared with adjustable voltage reference and the error signal is used to generate firing pulses. All three phases are fired at the same angle making a balanced control system. A voltage droop proportional to the compensator current is added to the measured system voltage and filtered to get low ripple feed back voltage signal.

This way the SVC not only improves the voltage characteristic but also helps in damping oscillations during post fault period. This property is also used for damping of power swings. Damping of angular swings are improved by feeding a properly conditioned signal derived from power flow on the line to the voltage regulator.

2.5.1.2. Reactive Power Control for Industrial loads: SVC can be used to compensate the reactive power to the loads, like furnaces, roller mills. The load

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power factor is measured from voltage and current signals, compared with a reference signal. Error signal controls the firing angle of TCR or switching of TSC to generate the required reactive power.

2.5.1.3. Load Balancing for unbalanced systems: Unbalanced loads are created in traction loads, electric arc furnaces. The SVC regulator consists of separate reactive power measurement control and firing pulse generation circuits for each phase to enable individual phase control. The firing angle for each phase will be different depending on its load conditions thus effecting unbalanced control

2.5.1.4. Flicker control for electric arc furnaces:Arc furnaces used to melt scrap in steel mills represent highly unbalanced and rapidly fluctuating loads. They produce the following types of disturbances.- Rapid open/short circuit conditions during arc initiation in the furnace- Wide and rapid current fluctuations with unbalance between phases- Fluctuations in the reactive current resulting in voltage variation which causes flicker.These loads cause flicker in lamps, interference in TV reception and other electronic loadsTo control flicker, furnace voltage and current are measured and reactive power requirement calculated. Control of firing angle is done by open loop to get very fast response.

The following subsections 2.5.2 to 2.5.5 apply in the first place to transmission system SVCs. Industrial system SVCs in conjunction with arc furnaces usually differ in some respects: No SVC transformer, fixed capacitor (filter)/Thyristor-controlled reactor main circuit arrangement only, open-loop reactive-power compensation control instead of closed-loop voltage control.

Principles of operation:Two types of Thyristor-controlled elements are used in SVCs:

1. TSC — Thyristor-switched capacitor2. TCR — Thyristor- controlled reactor

From a power-frequency point of view they can both be considered as a variable reactance, capacitive or inductive, respectively.

2.5.2 Thyristor-switched capacitor:Fig. 2.5 shows the basic diagram of a TSC. The branch shown consists of two major parts, the capacitor C and the bi-directional Thyristor switch TY. In addition, there is a minor component, the inductor L., the purpose of which is to limit the rate of rise of the current through the Thyristor and to prevent resonance. Problems with the network.Fig. 2.5 illustrates the operating principle. The problem of achieving essentially transient-free switching on of the capacitor is overcome by choosing the switching instant when the voltage across the Thyristor switch is at a minimum, ideally zero. In Fig 2.5 the switching-on instant is selected at the time (t1) when the branch voltage has its maximum value and the same polarity as the capacitor voltage. This ensures that the switching on takes place with practically no transient.Switching off a capacitor is accomplished by suppression of the firing pulses to the Thyristor so that the Thyristor will block as soon as the current becomes zero (t2). In principle, the capacitor will then remain charged to the positive or negative peak voltage and be prepared for a new switching on.

The TSC is characterized by: Stepwise control Average one half-Cycle (maximum one cycle) delay for executing a command from the regulator,

as seen for a single phase Switching transients are negligible. No generation of harmonics

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2.5.3 Thyristor controlled reactor:

Fig. 2.6 shows the basic diagram of a TCR. The branch shown includes an inductor L and a bi-directional Thyristor switch TY. The current and there by also the power frequency component of the

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Fig. 2.6 Operating principle of Thyristor-controlled

reactor.

Fig. 2.5 operating principle of Thyristor-switched Capacitor.

current are controlled by delaying the closing of the thyristor switch with respect to the natural zero passages.

The TCR is characterized by: Continuous control. Maximum one half-cycle delay for executing a command from the regulator, as seen for a single

phase. Practically no transients. Generation of harmonics

If stepwise control is acceptable, a switched mode of operation with constant delay angle. = 90o, can be used (TSR mode of operation). The advantage of this mode of operation is that no harmonic current is generated. A sufficiently small SVC step size can usually be achieved by a few TSRs, sized and operated in a so-called binary system.

2.5.4 Static Var Compensator:

It is configured as FC + TCR or TSC + TCR.The TCR and TSC are connected in delta for trapping harmonic currents of zero sequence (3rd, 9th etc.)Fig 2.8 illustrates the operating performance of the compensator according to fig 2.7 (b)Most transmission applications require closed-loop bus voltage control by an AVR. For a rapid change of the control order the change from full lagging current to full leading current takes place within a maximum of one cycle of the network voltage.

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Fig 2.7 (a) SVC of the FC/ TCR type (b) SVC of the TSC / TCR type

Fig 2.8 Operating principle of a SVC of type TSC + TCR for a slow change of control order

2.5.5 SVC Characteristics:According to CIGRE an SVC shall be considered as a reactive load on the power system. That means the

reactive power, Q, of an SVC is positive when the SVC absorbs reactive power, and negative when the SVC generates reactive power.

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Fig 2.9 SVC current verses voltage Characteristic.

Harmonics in SVC:A TSC does not produce harmonic currents, but a TCR does. All SVCs with continuous reactive power control include one TCR or more thus they produce harmonic currents. The harmonics of zero sequence character (eg. 3rd, 9th etc.) are eliminated by some delta connection. The 5 th and 7th harmonics are in some cases eliminated by 12 pulse arrangement. As a last resort a filter is included. The allowable amount of harmonic currents into the Power System expressed in terms of voltage distortion at the point of SVC connection are :

The allowed voltage distortion caused by a single harmonic current =1.0% The allowed total voltage distortion caused by all harmonic currents=1.5%

Dynamic Performance:The small-signal performance of an SVC with closed-loop voltage control may be characterized by its step-response time. It is defined here as the time required to achieve 90% of the called-for change in voltage, for a step change in the reference voltage. The step change must be small enough for the SVC not to reach a limit. The step-response time depends on the power-system equivalent impedance at the SVC point of connection. It is typically less than a few cycles of the power-frequency voltage at the minimum short-circuit MVA level considered when choosing the voltage regulator gain.If there is a risk that the short-circuit MVA level can be even lower and thereby cause SVC voltage control instability, this can be cured by a gain supervisor automatically reducing the gain in case of instability.

If there are frequent wide variations in the short-circuit MVA level and if it is judged important to get as fast small-signal voltage control as possible for all operating conditions, this can be achieved by a gain optimizer, automatically and repeatedly adjusting the gain up or down versus the short-circuit MVA level.

The above discussion is primarily referred to continuously acting SVCs, but does in principle also apply to discrete acting SVCs (SVCs of TSC, TSR or TSC/TSR type in a binary arrangement).The large-signal performance is essentially characterized by the actuating time of the SVC triggering and main circuits only. For a large voltage deviation, the SVC response time is typically of the order of one power-frequency cycle, considering the power-frequency voltage component only.

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Fig. 2.11 Illustrates the dynamic performance of an SVC for a large step change in the reference voltage IT, IC and IB mean total, capacitor and reactor current respectively.

2.6 Series Capacitor:It is a bank of capacitor units inserted in a line for the purpose of canceling a part of the line inductive reactance and so reducing the transfer impedance.The reactive power generated in a series capacitor is proportional to IL

2 and so increases with increasing transmitted power and thus influences the reactive power balance of the system.The typical uses are:

To increase the transmission loading capability as determined by Transient stability limits To obtain a desired steady state active power division among parallel circuits in order to reduce

overall losses To control transmission voltages and reactive power balance To prevent voltage collapse in heavily loaded systems To damp the power oscillations in association with Thyristor control

The degree of compensation is 20 to 70% of line inductive reactance. The series capacitor (C se) can be located at the ends of a long Transmission line or in a switching station in the middle of it. Considerations are voltage profiles, efficiency of compensation, losses, fault currents, over voltages, proximity to attended stations etc.

2.6.1. Comparison between shunt and series compensation

S.N Shunt compensation Series compensation

1. The shunt unit is connected in parallel across full line voltage. The current through the shunt capacitor is nearly constant as the supply terminal voltage and its reactance are constant.

The series unit is connected in series in the circuit and therefore conducts full current

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2. The voltage across the shunt capacitor is substantially constant as it is equal to the system voltage and generally within certain limits of say 0.9 to 1.1 pu.

The voltage across the series capacitor changes instantaneously as it depends on the load current through it, which varies from 0 to ILmax

3. The power developed across the shunt capacitor is

Csh KVAR =

The power developed across the series capacitor is

Cse KVAR = (IL XCse) (IL)= IL2 XCse

4. The shunt capacitor supplies lagging reactive power to the system. Hence directly compensating the lagging KVAR load. It improves the load power factor substantially. Hence its main purpose is to compensate the load Power factor

The series capacitor reduces the line reactance as it introduces leading reactance in series of the line. Thus series capacitor at rated frequency Compensates for the drop, through inductive reactance of the feeder. Hence it is used to increase the line transmission capacity.

5. The size and capacity of shunt capacitor is generally higher for the same voltage regulation

The size and capacity of a series capacitor is relatively lesser for the same voltage regulation

6. Not suitable for transient voltage drops caused by say, frequent motor starting, electric welding etc.

The voltage regulation due to series capacitor is proportional to the IL

2 hence it meets the requirements of transient voltage changes

7. Performance is dependent on terminal voltage. Hence not effective in fluctuating voltage conditions.

The performance does not depend on the system voltage variations. But depends on system load current. Hence gives full output under low voltage and heavily loaded conditions

8. The shunt capacitor need not be on the source side. But closer to the load point

The series capacitor should always be on the source side of the load.

9. The rating is based on KVARCsh = KW(Tan1 - Tan2) where 1 is the power factor angle before correction, 2 is the pf angle after correction

The rating is based on percentage compensation of the line reactance. Generally XCse = 0.3 to 0.4 of Xline Ex: A 220KV, 0.4/km, 100km line, 40%, XL = 0.4 X 100 = 40, Xcse = 0.4 x 40 = 16 = 1/2fCse Cse =

10. The Ferranti effect is aggravated by shunt compensation

The Ferranti effect is reduced by the series capacitor

11. Power transferred through a line With Cse, Vr increases and X decreases

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P=

with shunt capacitor, Vr increases P increases

hence P increases much more.

12. The shunt compensation does not require special protection arrangements as the terminal voltage of the capacitor bank falls under fault conditions

The voltage across series capacitor abnormally rises due to flow of fault current through it. Hence it requires special protection schemes.

The fig. 2.12 Shows the bypass arrangement series capacitor (C se) in case of faults as large voltage develops across the series capacitor. But the transient stability warrants reinsertion of C se into the system at the earliest. This is achieved by the Zinc Oxide (Zno) varistor. It provides instantaneous capacitor reinsertion after fault clearing. A triggered spark gap is provided to take care of excess energy absorbed

by Zno. Damping circuit (D) limits the discharge current.Zno arrestor is highly non linear. It is connected across the series capacitor in addition to the triggered gap and by pass switch. The varistor clamps the capacitor voltage below its short time over voltage rating during the fault. The re-insertion is almost instantaneous. Thus both capacitor protection and system stability aspects are taken care of.

Series Capacitor in radial distribution systems:A Series Capacitor is becoming popular in radial distribution systems because

Cse is a cost effective device of reducing voltage drops caused by steady loads on a 11 or 33 KV radial line with load Power factor of say 0.7 to 0.9

To take care of starting of a large motor and consequential voltage fluctuations To decrease line losses due to the lower current To increase load ability of the feeder Simple and reliable bypass systems are available Advanced resonance detectors are available.

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Fig.2.12 Series Capacitor with Zinc-oxide varistor by-pass system.

Csc

2.6.1. Sub Synchronous Resonance (SSR):The SSR is generated in radially connected turbo generators with a series Capacitor (Cse ) in the line.

Two basic phenomenon: The generator appears as an induction generator for sub synchronous armature currents If the difference between the synchronous frequency and the sub synchronous natural frequency of

the electrical system lies close to a natural frequency of the shaft mechanical system, the bilateral coupling between the two systems becomes strong. If the net damping of the two systems is negative, electrical and torsional oscillations will build up, either spontaneously or after a disturbance, e.g. a line fault.

In case of hydro-turbine generator units, the risk of torsional oscillation problem is practically negligible.Preventive Measures:

SSR detection and relaying leading to tripping of unit Compensating sub synchronous currents with Dynamic stability Pole-face amortizer winding against induction generator effect Thyrister Controlled Series Capacitor.

The use of a Thyristor-controlled module, appropriately controlled, of the series capacitor bank seems to be a promising counter measure.

Another subject often discussed is how to ensure correct operation of line relay protections in conjunction with series capacitors. According to service experience the risk of maloperation of line distance protections seems small. Ultra-high-speed line protections based on traveling wave detection can eliminate the possible problems of line protection in conjunction with series capacitors.

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Fig 2.13 System of the type most exposed to the sub-synchronous resonance

Ref: 1) Power capacitor hand book-T Longland, T W Hunt, W A Brecknell : Butterworths – 1984

2) Reactive Power Compensation- Tore Peterson, ABB Power systems, SWEDEN – 1993

3) Proceedings of Seminar on “CAPACITORS” during 18 – 19 January 2001.- A CBIP and MPEB publication – 2001.

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