rpm manual mnm

7/28/2019 Rpm Manual Mnm

1/69

CHAPTER - 1REACTIVE POWER FUNDAMENTALS

1.0 Introduction

Voltage is proportional to the magnetic flux in the power system element. Most of the PowerSystem elements are reactive in nature. They absorb / generate reactive power depending on systemloading conditions. The balance in reactive power availability and requirement at a node indicates steadyvoltage. Drawal of reactive power leads to reduction in voltage and supply of reactive power leads toincrease 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 reactivepower losses due to the series inductance of transformers, overhead lines and underground cables. Linesand cables also generate reactive power due to their shunt capacitance; this generation of reactive poweris, however, only of significance at high system voltages.

During the steady-state operation of an AC power system the active power production must match theconsumption plus the losses, since otherwise the frequency will change. There is an equally strongrelationship between the reactive power balance of a power system and the voltages. In itself, a reactivepower balance will always inherently be present, but with unacceptable voltages if the balance is not aproper 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, theheating 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 properreactive power balance can and often has to be effected both by the generators and by dispersed specialreactive devices, producing or absorbing reactive power. The use of shunt reactive devices. i.e. shuntcompensation, 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 thegenerators 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 thedevelopment 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 importantcompensation devices. The latest development is the Thyristor-controlled static var compensator, whichis 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 betweendifferent voltage ranges in terms of HV, EHV, etc. It should therefore be appropriate to explain brieflythese terms.

1


2/69

Classification of System Voltages

Voltage Level in kV Category of Voltage


3/69

of users are not well defined and there is not enough realization in this regard. Utilities have nowintroduced power factor clause in the tariff structure. However. It would be worthwhile to note that evena 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 ofthe voltage and the reactive (watt-less) component of the current, this reactive current component beingin quadrature with the voltage.

A single-phase circuit according to Figure 1.1 the reactive power Q is given by

Q=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 toan inductive element is positive. Consequently, the net reactive power supplied to capacitive element isnegative. 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:

G M

Vs Vr0I r

Sr

j X

Fi . 1.2 Sim le Transmission S ste

3


4/69

LetVs =Sending end voltageVr =Receiving end voltageSr =Receiving end complex power

Pr =Receiving end active powerQr =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

*cos

Xj

VSinjVV rss

=

X

VCosVVjSin

X

VV rrsrs2

Pr = srs PSinP

X

VV maxsin (2)

For a loss less line.

P and are closely related.

Qr = X

VCosVV rrs2

(3)

Qs = X

CosVVV rss 2

(4)

For small angles of

Qr =

X

VVV rsr (5)

Qs =

X

VVV rss

(6)

4


5/69

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

2. Magnitudes of V1 and V2 do not determine the MW flow direction

3. Though P1=P2, Q1 Q24. The reactive loss in line reactance is

x

rVsVQrQsQave

22

22

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

*V

JQP;

I* =

V

JQP

I2 =I.I* =

2

22

*. V

QP

VV

jQPjQP

Ploss =I2R = R

V

QP.

2

22

(7)

Qloss =I2X = X

V

QP.

2

22 (8)

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

5


6/69

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.

Vr X.QrV

EThX.Pr

V

Fig 1.3 Voltage regulation in a loss less system

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

*V

jQP rr

=V +V

XPj

V

XQ rr (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 powerand so reduces system voltage. Capacitor injects reactive power and so increases system voltage.

1.7 Short circuit capacity:

fsc IVS 3 MVA (10)Where

V = Phase to phase voltage in kVI f = The three phase fault current in k.A.

Expressed in p.u parameters

Ssc =(V0-)(If) p.u. =I f p.u. =

ThX

1 (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

.puS

QV

SC

6


7/69

Where

Q = Change in Q injectionSsc =Short circuit capacity

V =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.

S

P

Q

Fig 1.4. Physical analogy for Active and Reactive powers

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

1.9.1 Transformers

Fig 1.5 Equivalent Circuit of Transformer

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

7


8/69

1.9.2 Overhead lines

Overhead lines and underground cables are distributed-constant circuits, which have their seriesresistance, series inductance and shunt capacitance distributed uniformly along its length. Figure 1.6shows a lumped-constant equivalent circuit. If we assume constant operating voltages at the ends, thereactive 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 SurgeImpedance Load (SIL) P0 or natural load of an uncompensated line is a convenient value for referencepurposes. It is given approximately by:

MWx

bVPo

2 ------------------------------------------------(12)

whereV =voltage, line-line kVb =susceptance mho/kmx =reactance ohm/km

A loss less line (a reasonable approximation of an EHV line) transferring an active power P0 and withequal voltages at the line ends has reactive power balance. The reactive power loss due to the lineinductance is equal to the reactive power generated by the line capacitance.

Operating voltagekV

SILMW

Line chargingMvar/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 thesame while the line charging, X and X/R values are 20 per cent higher. The SIL is usually much lowerthan the thermal rating. Below 69 kV the line charging is usually negligible while it is a significantsource of reactive power for long lines of higher system voltages.

Paradoxically, the series reactance is fairly independent of the system voltage, assuming a singleconductor. The lower values at 400 kV, 500 kV and 750 kV illustrate the effect of the necessary use ofbundle conductors for these system voltages. In reality there is a great spread in the X/R values, for asystem 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.

8


9/69

1.9.3. Underground cables:Table 2 gives sample values of underground cable characteristics. The spread in parameter values for asystem voltage under consideration is very much larger than for overhead lines, depending on the cabletype, 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 introducedat ever higher system voltaes, is much lower, e.g. 50 per cent of that of paper-insulated cables.

Operating voltagekV

SILMW

Line chargingMvar/km

XOhm/km

X/R

0.410130220400

-3

50010003200

-0.012413

0.070.100.150.180.2

0.30.4269

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:

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 / 0.7

Plastic moulding plants 0.6 / 0.75Printing 0.55/0.7Quarries 0.5 / 0.7

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.

9


10/69

Both controlled rectifiers and arc furnaces of steel mills have a reactive power consumption characterizedby a high average value and fast variations. Purely resistive loads, like filament lamps and electricheaters, 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 powerwhen under excited and produces reactive power whenoverexcited. Synchronous motors are usually operatedoverexcited and thus usually produce reactive power.Individual loads may, of course, vary within short or long timeranges. The composite loads of a power system. Each onebeing the total load of a certain area, usually vary with thetime of the day, the day of the week and the season of the yearand may also grow from year to year. The consumer demandfor reactive power varies in a somewhat similar way to thedemand for active power. Figure 1.7 illustrates how the activeand the reactive power supplied from a transmissionsubstation into a load area, with mixed industrial and domesticloads, may vary during a Sunday and a Monday.

The resultant active power demand of a power system variesroughly as the variation of total toad. The resultant reactivepower demand may vary considerably more due to thechanging series reactive power losses in the networks.

Fig.1.7 Examples of load

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

V=V1 V2--------------(13)

The Phasor diagram of Figure 1.8, for a case with laggingpower factor, shows that it can be approximately expressedby 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. Theequations are usually sufficiently accurate for calculations concerning a single link with lagging powerfactor. 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 computerpower flow programs.

10


11/69

The equation (15) is, however, generally useful for qualitative discussions of voltage versus reactivepower. For transformers, R can always be disregarded. For transmission (not distribution) lines andcables. 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 onV per kvar of reactive power than per kW of active powertransmitted.

When power is supplied through a single link, Figure 1.8, assuming V1 constant, V2 varies with changesin 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 turns 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.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 showhow shunt compensation influences the voltage. The voltage-change directions mentioned arise becausethe network equivalent impedance has an inductive character at the fundamental frequency. The shuntcompensation 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 approximatelyexpressed by;

V =scS

Q------------------(16)

Where

Q- change in nominal three phase reactive power injection MvarSsc- Short-circuit capacity in MVAAdjacent 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 seriescompensation is meant compensation of line inductive reactance by means of a capacitor in series withthe 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 formingan 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.

11


12/69

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

In PSSTPLAN, PV curves are provided as a distinct Analytical Engine. As such it is provided withpowerful features:

Easy setup Comprehensive results Adaptive step size. You define a range for the transfer increment, and PSSTPLAN 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.

12


13/69

Maximize the existing reactive power resources to minimize investment in additionalfacilities.

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 precedingsubsections, should not be unnecessarily transferred. Ideally, a reactive power balance should be effectedwithin each region of a power system, within each transmission system and within each distributionsystem. In practice, however, this principle is not always followed for one reason or another. The subjectof reactive power compensation is easy to understand if we consider a single link of a power system, butquite complex when we consider an entire power system with its different conditions and behaviors.

13


14/69

CHAPTER - 2REACTIVE 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 thepower system and to bring about, or at least contribute to, the desired reactive power balance in the areasadjacent to the generating stations. A generator absorbs reactive power when under excited and itproduces reactive power when overexcited. The reactive power output is continuously controllablethrough varying the excitation current. The allowable reactive power absorption or production isdependent 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 ratedpower factor of generators usually lies within the range 0.80 to 0.95. Generators installed remotely fromload centers usually have a high rated power factor; this is often the case with large hydro-turbinegenerators. Generators installed close to load centers usually have a lower rated power factor. In somecases of large steam-turbine generators the rated power factor may have been selected at the lower end ofthe above range in order to ensure reactive power reserve for severe forced outage conditions of thepower 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

14


15/69

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. Theterminal voltage of a large generator is usually allowed to be controlled within a 5% range around thenominal voltage, at rated load. In most countries the generator step-up transformers are usually notequipped with on-load tap changers.

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

When the unit generates MVAR and supplies to the system, the system voltage profile around thegenerating 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 unitexcitation. Hence the influence of a unit on voltage profile in the system is local in nature. The load endvoltages 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 inthe unit, it can be used to control the system voltages in its vicinity.

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

WhereV =change in bus voltage in puQ =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 reactorA shunt reactor is a reactor connected in shunt to a power system for the purpose of absorbing reactivepower. In some cases where a fixed or mechanically switched shunt reactor can be used with regard tothe voltage control requirements. It is usually the most economic special means available for reactivepower absorption. The majority of shunt reactors are applied in conjunction with long EHV overheadlines. 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.

15


16/69

Shunt reactors are necessarily installed to suppress high voltage during light load conditions. For400kV and UHV lines, shunt reactors are directly connected on line. This is for the purpose ofcompensating leading charging MVAR released by the line. Shunt reactors are also connected on tertiarydelta 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 theother end. It draws the magnetizing current, which is purely inductive, from the system and hence formsan inductive load at the point of connection. Hence the reactor absorbs reactive power from the systemas 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 byV =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 thisvoltage is quite significant, it increases the receiving end voltage to unacceptable limits under light loadconditions. A 400 kV line generates about 55 MVAR per 100 km and hence this Ferranty effect is highfor 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 fromthe system depending upon their capacity.

The bus reactors are switchable and hence are cut-in whenever the system voltage is higher and can becut-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 to

a) Reduce the effect of line chargingb) Provide a least impedance path for the switching over voltages generated in the system due toinductive load currents switching. The switching over voltages are of power frequency andequal 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 isimplemented, the abnormal voltages developed across the circuit breaker can be contained onlywith a line reactor on the line side.

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

2.3 Shunt capacitors

A shunt capacitor is a single capacitor unit or, more frequently, a bank of capacitor units connected inshunt to a power system for the purpose of absorbing reactive power. When a fixed or mechanicallyswitched shunt capacitor can be used with regard to the voltage control requirements, it is the mosteconomic means available for reactive power supply. The majority of shunt capacitors are applied withindistribution systems of different types: Industrial, urban, residential and rural. They have a widespreaduse 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 ofunits, rated hundreds of Mvar.

16


17/69

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

The output of a capacitor bank is Qc =V2c

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 capacitorbank is connected reduces to 0.9 p.u. the MVAR generated by the capacitor reduces to 0.81 p.u. Hencethe performance of a capacitor bank will be poor under low voltage conditions, at which time it isrequired most.

The influence of a capacitor bank on the system voltage is again local like in case of a generator. It ismost 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 relationV =Q/SWhereV =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)whereQ 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 thatthey provide the least support at the very time when it may be most needed, because the reactive poweroutput 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 shuntcapacitors 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 staticcapacitor owing to its low losses, simplicity and high efficiency, is finding very wide and universal usefor pf correction.

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

2.4 Transformer Tap Changing: A transformer in the grid is like a node. Its voltage is maintained bythe requirement and availability of reactive power at its terminals. If the HV voltage is low, due tobucking tap at, say -5, for e.g. at 0.96 pu the HV bus will get a net reactive power in-flow of say 200MVAR 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.

17


18/69

If the transformer tap is raised to say 5, it is now boosting the HV voltage to say, 1.02 pu. Now thereactive 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 thanthe 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 HVbus.

2.5 Synchronous condensersSynchronous condenser is another reactive power device, traditionally in use since 1920s. Synchronouscondenser is simply a synchronous machine without any load attached to it. Like generators, they can beover-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 ofload, 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 voltagecollapse and to improve system stability.

Synonymous terms are synchronous compensator and synchronous phase modifier. The synchronouscompensator is the traditional means for Continuous control of reactive power. Synchronouscompensators are used in transmission systems: at the receiving end of long transmissions, in importantsubstations and in conjunction with HVDC inverter stations. Small synchronous compensators have alsobeen 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 usuallyair-cooled, while those above are usually hydrogen-cooled. Modern synchronous compensators are

18


19/69

usually equipped with a fast excitation system with a potential-source rectifier exciter. Various startingmethods 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 overloadcapability. The absorption capability is normally of the order of 60 per cent of the MVA rating, whichmeans that the control range is usually 160 per cent of the MVA rating. The reactive power output iscontinuously controllable. The step-response time with closed-loop voltage control is from a few tenths ofa second, and up. The losses of hydrogen-cooled synchronous compensators are of the order of 10 W/kvarat 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 ofnew installations, due to benefits in cost performance and reliability of the latter. One exception is HVDCinverter stations, in cases where the short-circuit capacity has to be increased. The synchronouscompensators 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 kVARequal to its rating and can absorb upto 100% ofits KVA rating

Shunt capacitor should be associated with areactor to give that performance

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

3. The output is not limited by the system voltagecondition. This gives out its full capacity even

when system voltage decreases

The capacitor output is proportional to V2of the system. Hence its performance

decreases under low voltage conditions

4. For short periods the synchronous condensercan supply KVAR in excess of its rating atnominal voltage

The capacitor can not supply more than itscapacity at nominal voltage. Its output isproportional to V2.

5. The full load losses are above 3% of itscapacity

The capacitor losses are about 0.2%

6. These can not be economically deployed atseveral locations in distribution

The capacitor banks can be deployed atseveral locations economically indistribution

7. The synchronous condenser ratings can not bemodular

The capacitors are modular. They can bedeployed as and when system requirementschange

8. A failure in the synchronous condenser canremove the entire unit ability to produceKVAR. However failures are rare insynchronous condensers compared tocapacitors

A failure of a single fused unit in a bank ofcapacitors affects only that unit and doesnot affect the entire bank

19


20/69

9. They add to the short circuit current of a systemand therefore increase the size of (11kV etc.)breakers in the neighbourhood.

The capacitors do not increase the shortcircuit capacity of the system, as theiroutput is proportional to V2

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

These are static and simple devices. HenceO&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 powergeneration or absorption of which can be varied by means of Thyristor switches. The adjective staticmeans that, unlike the synchronous compensator, it has no moving primary part. Because it is the latestdeveloped means of reactive compensation, it will be described and discussed in greater detail than theother devices. In a strict sense, the term static var compensator covers not only Thyristor-controlledcompensator but also other, types and in particular, the self-saturated iron-core reactor type. Even thoughthe self-saturated reactor compensators introduced before the Thyristor-controlled one, the latercompletely dominates the applications of compensators in transmission systems, covering more than 95per 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 industrialnetworks, particularly for the reduction of voltage fluctuations caused by arc furnaces. In transmissionsystems the breakthrough came at the end of the 1970s. Since then, there has been an almost explosiveincrease 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 controllableshunt compensation.

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

2.5.1. Function of SVCs in Power systems:SVCs are used to improve voltage regulations, improve power factor, reduction of voltage and currentunbalances, damping of power swings, reduction of voltage flicker, improved transient stability of thesystem etc. This can result in saving in operational costs, increased power transfer capability, reducedline 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 withadjustable voltage reference and the error signal is used to generate firing pulses. All three phases arefired at the same angle making a balanced control system. A voltage droop proportional to thecompensator current is added to the measured system voltage and filtered to get low ripple feed backvoltage signal.

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

20


21/69

2.5.1.2. Reactive Power Control for I ndustrial loads:SVC can be used to compensate the reactive power to the loads, like furnaces, roller mills. The loadpower factor is measured from voltage and current signals, compared with a reference signal. Error signalcontrols 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 ofseparate reactive power measurement control and firing pulse generation circuits for each phase to enableindividual phase control. The firing angle for each phase will be different depending on its loadconditions 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. Industrialsystem SVCs in conjunction with arc furnaces usually differ in some respects: No SVC transformer, fixedcapacitor (filter)/Thyristor-controlled reactor main circuit arrangement only, open-loop reactive-powercompensation 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 orinductive, 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 capacitorC and the bi-directional Thyristor switch TY. In addition, there is a minor component, the inductor L., thepurpose 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 Thyristorswitch is at a minimum, ideally zero. In Fig 2.5 the switching-on instant is selected at the time (t1) whenthe branch voltage has its maximum value and the same polarity as the capacitor voltage. This ensuresthat 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 remaincharged 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

21


22/69

Switching transients are negligible. No generation of harmonics

Fig. 2.5 operating principle of Thyristor-switched Capacitor.

2.5.3 Thyristor controlled reactor:

Fig. 2.6 Operating principle of Thyristo -controlled

22


23/69

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 thecurrent are controlled by delaying the closing of the thyristor switch with respect to the natural zeropassages.

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 beused (TSR mode of operation). The advantage of this mode of operation is that no harmonic current isgenerated. 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 takesplace within a maximum of one cycle of the network voltage.

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

23


24/69

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

Fig 2.9 SVC current verses voltage Characteristic.

24


25/69

Harmonics in SVC:A TSC does not produce harmonic currents, but a TCR does. All SVCs with continuous reactive powercontrol include one TCR or more thus they produce harmonic currents. The harmonics of zero sequencecharacter (eg. 3rd, 9th etc.) are eliminated by some delta connection. The 5th and 7th harmonics are in somecases eliminated by 12 pulse arrangement. As a last resort a filter is included. The allowable amount ofharmonic currents into the Power System expressed in terms of voltage distortion at the point of SVCconnection 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 itsstep-response time. It is defined here as the time required to achieve 90% of the called-for change involtage, for a step change in the reference voltage. The step change must be small enough for the SVCnot to reach a limit. The step-response time depends on the power-system equivalent impedance at theSVC point of connection. It is typically less than a few cycles of the power-frequency voltage at theminimum 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 voltagecontrol instability, this can be cured by a gain supervisor automatically reducing the gain in case ofinstability.

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

The above discussion is primarily referred to continuously acting SVCs, but does in principle also applyto 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 andmain circuits only. For a large voltage deviation, the SVC response time is typically of the order of onepower-frequency cycle, considering the power-frequency voltage component only.

25


26/69

Fig. 2.11 Illustrates the dynamic performance of an SVC for a large step change in the reference voltageIT, 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 inductivereactance and so reducing the transfer impedance.

The reactive power generated in a series capacitor is proportional to IL2 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 (Cse) can belocated 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 parallelacross full line voltage. The currentthrough the shunt capacitor is nearlyconstant as the supply terminal voltage andits reactance are constant.

The series unit is connected in series inthe circuit and therefore conducts fullcurrent

26


27/69

2. The voltage across the shunt capacitor issubstantially constant as it is equal to thesystem voltage and generally within

certain limits of say 0.9 to 1.1 pu.

The voltage across the series capacitorchanges instantaneously as it depends onthe load current through it, which varies

from 0 to I Lmax

3. The power developed across the shuntcapacitor is

Csh KVAR =CshcSH x

vv

x

v 2.

The power developed across the seriescapacitor is

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

4. The shunt capacitor supplies laggingreactive power to the system. Hencedirectly compensating the lagging KVARload. It improves the load power factor

substantially. Hence its main purpose is tocompensate the load Power factor

The series capacitor reduces the linereactance as it introduces leadingreactance in series of the line. Thus seriescapacitor at rated frequency Compensates

for the drop, through inductive reactanceof the feeder. Hence it is used to increasethe line transmission capacity.

5. The size and capacity of shunt capacitor isgenerally higher for the same voltageregulation

The size and capacity of a series capacitoris relatively lesser for the same voltageregulation

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

The voltage regulation due to seriescapacitor is proportional to the I L

2hence itmeets the requirements of transientvoltage changes

7. Performance is dependent on terminalvoltage. Hence not effective in fluctuatingvoltage conditions.

The performance does not depend on thesystem voltage variations. But dependson system load current. Hence gives fulloutput under low voltage and heavilyloaded conditions

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

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

9. The rating is based on

KVARCsh =KW(Tan1 - Tan2) where1 is the power factor angle beforecorrection, 2 is the pf angle aftercorrection

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 =

FFx

x 200

16314

101 6

10. The Ferranti effect is aggravated by shuntcompensation

The Ferranti effect is reduced by theseries capacitor

27


28/69

11. Power transferred through a line

P= SinX

VV rs

with shunt capacitor, Vr increases Pincreases

With Cse, Vr increases and X decreaseshence P increases much more.

12. The shunt compensation does not requirespecial protection arrangements as theterminal voltage of the capacitor bank fallsunder fault conditions

The voltage across series capacitorabnormally rises due to flow of faultcurrent through it. Hence it requiresspecial protection schemes.

The fig. 2.12 Shows the bypass arrangement series capacitor (Cse) in case of faults as large voltagedevelops across the series capacitor. But the transient stability warrants reinsertion of Cse into the systemat 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.

Csc

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

Zno arrestor is highly non linear. It is connected across the series capacitor in addition to the triggeredgap and by pass switch. The varistor clamps the capacitor voltage below its short time over voltagerating during the fault. The re-insertion is almost instantaneous. Thus both capacitor protection andsystem 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 or33 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.

28


29/69

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

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 bilateralcoupling 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 adisturbance, 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 tobe a promising counter measure.

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

29


30/69

CHAPTER - 3CAPACITORS

3.0 The Capacitance:Absolute permitivity () =(Electric Flux density/ Electric Field Intensity)

=or ; =Absolute permitivityPermitivity of free space =oRelative permitivity =rElectric field Intensity =(V/d) =( Voltage across the dielectric / Thickness of the dielectric)Electric Flux density =(Q/A) =( Charge in coulombs/ Area of the dielectric)Q =Charge accumulated in coulombs =(Current in Amperes X Time in seconds)C =Capacitance =(Q / V) farads

A Farad is the capacity of a capacitor between the plates of which there appears a difference ofpotential of one Volt when it is charged by a quantity of electricity equal to one coulomb.C =(or A / d) = A/d farads

A=area of the dielectric field in sq. mts.d =distance between plates in mts.

=Absolute permitivity

Capacitors in series = .....1111

321

CCCC

Capacitors in parallel = C=C1 +C2+C3 +

3.1 Capacitor in AC circuits:

Ic

V

Fig 3.1. Voltage and current relationships in a.c. capacitive circuits

Leading Ic

V

900Xc

Ic

3.2 Circuit containing Resistance & capacitance

VR =IRVC =IXC

22

22

222

)1

(1

1)()(

CR

VZ

CRIIXIRV C

30


31/69

Fig 3.2. circuit containing resistance and capacitance a) Circuit b) Phasor diagram

KVar =(IC) (V) =C V.V =2fC V2

KVA = =310))(( VI 22 KVarKW

KW =VI Cos x10-3

KVar =VI sin x10-3

Pf = CosKVA

KW

KVar =

TanKW

KWKWKVA

cos

sin22

3.3 Dielectric Loss: The dielectric loss is present due to dielectric in a capacitor instead of perfect

vacuum. The phase angle of current falls short of 90o by . Hence Power factor of capacitor =Cos (90-) =Sin tan.

Tan =r.c =(90-)Power absorbed by capacitor =VI cos =VI tan for low values of

Tan =0.0006 Loss of 0.6 w/KVar

Tan =rC=90o -

Fig 3.3 Power absorbed by the capacitor equals VI cos VI tan

31


32/69

3.4 Charge and discharge of a capacitorCF

Fig 3.4. Charge / Discharge of a capacitor through a pure resistor

310xKVar

I =Single phase capacitor currentV

C

V

KVarIC.3

103 =Three phase capacitor Current

V =line to line voltage

tV CRech e

RI 1arg

CR =Time constant of the unit

CR

tV

edisch eR

I arg

Energy stored in a capacitor =J = CV2 joules

.5 Capacitor for Power factor correction

(tan -tan

Where C inF and V in kV

3

KVar1 =KW tan1;KVar2 =KW tan2 ;(KVar1-KVar2) =KW 1 2)

Fig 3.5.a Capacitor connected in parallel with load where V=supply ltage, IL=current taken by load,voIC=Current taken by capacitor, I=current drawn from supply.

32


33/69

Fig 3.5.b. Determination of shunt capacitor requirement

Fig 3.6. Phasor diagram showing the effect of adding capacitance where IL=current flowing when nocapacitor is connected. IC =current due to capacitor only; I=current taken from supply with capacitorconnected

3.6 Shunt Capacitors applied to Power Supply System:

33


34/69

Fig 3.7. Simplified distribution system a) System b) simplified circuit c) Phasor diagram lagging powerfactor d) phase diagram unity power factor with shunt capacitor bank.

The reactive power is generated at the receiving end. Hence the HV transmission and distribution systemis relieved of this reactive power flow in the system.

34


35/69

3.7 Series capacitors in Power Systems:

Fig 3.8. Simplified distribution system with series connected capacitors a) System b) Simplified circuitc) Phasor diagram without capacitor d) Phasor diagram with capacitor

3.8 Protection Of A Capacitor Bank

3.8.1 Design considerations for protection of HT Capacitors:The protection of HT Capacitors should consider abnormal voltage variations, distorted current and

voltage wave forms due to non linear loads, resonance, parallel switching, restrike of switching device,identification and elimination of failed capacitors etc.

a) BIS, IEC specify capacitors to be suitable to take care 10% over voltage for 12 hours a dayb) The harmonics can be taken care of by a series reactor in series with capacitor. The series

reactor shall be say 6%. This combination offers lower impedance at 5th harmonic. Netimpedance at 5th harmonic is inductive and hence no resonance takes place. Explained in section3.9.

c) The capacitor offers very low impedance at the time of switching. These in rush currents will belarger when subsequent banks are switched on. In order to limit this in rush current a 0.2 to 1%reactor is used in series.

35


36/69

d) Restrike: When capacitor is switched off, one side of the switch is system voltage and the otherside the charge. Hence double system voltage may appear across the contacts. If a restrikeoccurs the capacitor gets damaged. Hence the switch should be restrike free.

e) The failed units of the capacitor bank shall have to be eliminated in order to avoid over voltage

on the remaining units.

3.8.2. A one line diagram of a protection scheme of a capacitor bank is as shown in fig (3.9)As per IEC 70, Capacitor banks must

i) Withstand 10% over voltageii) Withstand 30% over load due to over voltage and/or harmonicsiii) Peak value of in rush current must not exceed 100 times the rated current of the capacitor.iv) Capacitor must not be re-energized until residual voltage falls below 10% rated voltage.v) CB must be restrike free

3.8.3 Over load protection (50):The capacitor bank tuning factor =8.9

Rated load current =I = A28910333

105.163

6

(1.1) Ir =1.1 x 289 =318 AmpsHence the CT ratio at 33kV is 320/5 Amps

The relay should trip at 30% over load 289 x 1.3/320 x 5 =5.87 Amps=117%

The above overload current is not sufficient for an 1DMT relay. Hence an O.C. relay withadjustable definite time delay is provided.Recommended settings

a) Current =1.3 x Irb) Timer =0.3 sec.

3.8.4 Over voltage protection (59):Recommended settings

a) Voltage =1.1 V ratedb) timer =7 to 10 sec.

36


37/69

3.8.5 Over current/Short circuit Protection:An IDMT relay is provided to take care of short circuits in the capacitor bank

3.8.6 Under voltage relay:

This is provided with bus PT and set at 60% of rated voltage. A time delay of 2 sec. is provided.This takes care of auto re-close CB on the in coming feeder.

3.8.7. Capacitor Unbalance protection:This is to disconnect the bank when a fault has occurred inside the bank in order to prevent a healthyunit from being exposed to more than 1.1 rated voltage.

a) Voltage unbalance scheme (60):The unbalance can be detected by sensing the residual voltage coming across the open delta ofPT secondary as shown in fig.3.10Disadvantages: i) The scheme can not distinguish between unbalance due to capacitor internalfault and unbalance due to external fault or unbalanced load.ii) Sensitivity of voltage unbalance scheme is always less than sensitivity of current unbalancescheme.

Fig. 3.10 Unbalance protection of 3 phase capacitor bank with RVT

b) Current unbalance scheme (61):3 phase capacitor banks are connected in double star. The CT between two neutrals detects theunbalance current and trips.

37


38/69

Delay in re-closing operation:Once tripped the capacitor should be allowed to discharge to an appreciable limit of 10% ofVrated within 5 mts. To facilitate this, a time delay interlock is provided to prevent the reclosure ofthe breaker within 5 minutes.

The CB should be re-strike free: It should be capable to break the capacitor current at maximumpermissible bus voltage

Inrush current: Since generally the capacitor banks are used in series with reactors as filter banks.Peak value of the in rush current is limited by the reactor within specified limit. If it is used alone a smallreactor is considered in series with the capacitor to limit the peak in rush current.

3.9 Series reactor for harmonic suppression:When there are harmonic generators like rectifiers or arc furnaces present in the system, there is a

possibility of capacitors drawing much more current than permissible limit.A series reactor connected to a capacitor forms a circuit with tuning frequency, fo:

At tuning frequency X =X L XC 0fo XL =2foL; XC =1/(2foC)X = XL XC =0 2foL =1/2foC

fo2 =

LCfo

LC 2

1

2

12

Fig 3.12 the series reactor for harmonic suppression

The tuning frequency (fo) is the frequency at which the LC circuit offers least impedance. In order tosuppress harmonics, the series reactor is so chosen that tuning frequency falls below the harmonics of thelowest order that may be present in the system.

For example if 5th harmonic is the lower order present in the system then the tuning frequency should besay 240 Hz so that the LC circuit will offer higher impedance to the 7th, 11th . harmonics.

38


39/69

The tuning number =n =L

CO

X

X

LfCfLCff

f

1111 2.2

1

2

1

Where fo =tuning frequency

f1 =fundamental frequency i.e. 50 HzXC =capacitive reactanceXL =series reactor reactance

The frequency response characteristic of the series LC circuit with 6% series reactor:

n = 47.1606.01

C

C

L

Co

X

X

X

X

f

f.09

f XL XC X50 0.06 XC XC (1-0.06)XC =0.94XC100 0.12 XC XC/2 (1/2 0.12)XC =0.76XC150 0.18 XC XC/3 (1/3 0.18)XC =0.46XC200 0.24 XC XC/4 (1/4 0.24)XC =0.04XC250 0.3XC XC/5 (1/5 0.3)XC =-0.5XC

Fig 3.13 the frequency response characteristic of a series LC circuit

n =fo/f1 =205/50 4.09

The LC circuit does not offer high impedance to the harmonic currents close to the tuning number.Hence the series reactor and the capacitor should be designed to withstand these currents also.

S.No. f0 / f1 XL =XC / (f0/f1)2 XL as %ge of XC

1 1 XL =XC (1.0) 1002 3 XL =XC (0.11) 113 5 XL =XC (0.04) 44 7 XL =XC (0.02) 25 9 XL =XC (0.012) 1.26 11 XL =XC (0.0083) 0.83

39


40/69

3.10 Causes Of Capacitor Bank Failures And Remedial MeasuresIt has been found invariably, whenever capacitor bank failure takes place leading to failure of capacitorunits, the tendency of the user is to get the failed units replaced as soon as possible or to use the bankwith remaining lesser number of units without going into details of failure and causes thereof, Instead of

rushing to re-install the capacitor bank one must analyse the failure and arrive at the root cause of failureso that necessary remedial measures can be taken to avoid recurring of such failures in future.

Capacitor FailuresCan be segregated into following categories

Failures due to internal unit faults. Failures due to installation problems. Failures due to system problems.

3.11 Failures Due To Internal Unit FaultsFaults are generally due to defective material used or due to manufacturing defects.

Defective Materials include mainly POLYPROPY LENE FILM having voids or spaces where thicknessof the film is lower than average thickness stipulated by film manufacturer. This gives rise to highervoltage stresses thereby leading to puncture in the capacitor element and subsequent failure of the unit. Incase of units with internal element fuse protection, may be such defective elements are Isolated andbalance units continue to be in service. However, In case of units having external fuse protection, suchfaulty elements may lead to arcing, rise in Internal pressure, bulging (and may be unit bursting) beforethe external fuse can identify the fault to trip the unit.

Impregnate Oil is also important from the point of view of providing interlayer insulation and cooling.Any impurities in the oil are likely to give flashovers at lead wires/interconnections and containers andthereby failure of the unit.

CRCA Sheetsare used for containers because of their higher tensile strength, which leads to distortion inthe shape of the container in the event of abnormal internal pressure. If the sheet material is not CRCA,rusting of container, bursting due to internal pressure etc. are seen as reasons for unit failure.

Defective Workmanship include defects during the manufacturing process such as Element Winding,Impregnation. Container Welding. Sealing of bushings.

Element Winding is necessarily required to be done in dustfree atmosphere. Generally pressurisedrooms are used for this purpose to avoid dust entering into the winding room. Any dust particles in theelement give arcing in the elements and thereby failure of the unit

Impregnation process is most vital. Longer the impregnation cycle, better will he the quality ofcapacitor. During impregnation cycle full vacuum should be maintained in order to ensure completedrying of elements and then proper oil impregnation shall take place. If during the process vacuum is lostor oil impregnation is not done properly, premature failure of elements is likely.

If Container Welding & Sealing Of Bushingsis not done properly, oil leakages start and when oil leaksout, air contamination leads to subsequent failureof tank. Generally raw material and process problemsare identified during inspection stage and testing. Such portion of faulty unit or entire unit can be rejectedduring the process of manufacture. However, sometimes these units pass the in-house testing but do notsustain field conditions and lead to premature failure in operation. However, one must note, such failuresare only isolated cases and are restricted to one unit failure at a time and generally within one month of

commissioning of the bank.

40


41/69

3.12 Failures Due To Installation Problems:Capacitor Bank installations should he done properly as per SUPPLIERS DRAWINGS andINSTRUCTION MANUALS. Unit configuration and number of series groups should be strictly

followed as per drawings. Mass failures are likely to occur if the series groups and number of units perseries group are not installed properly.

If Handling of capacitors at the time of installation is done by dragging the units on the floor with thehelp of bushings, oil leakages from bottom welding portion or bushings solder may start leading tofailure of the units. Sometimes units damaged in transit with OIL LEAKED out completely are used inthe installation, which will cause subsequent failure. Interconnections between unit bushings and busbarsshould be done with L clamps using 2 spanner method to avoid breakage of solder joint of bushing.Sufficient space should be available between units for better COOLING of the units particularly forindoor banks

For open type banks live parts should be minimum 8 feet above ground level. Either elevated structuresor wire mesh enclosures should be used. This is important with more than one series group is involvedwhen the containers become live. Electrical Safety clearances should be maintained as per IE rules.

Earthingof installation is necessary but remember not to earth live structure or floating neutral point ofcapacitor bank.

At places where BIRD FAULTS are likely, insulate live parts with insulating tape, sleeving. Wheneverlive structures are involved with capacitor banks of more than one series groups, bird faults may lead tomass failure. Wire mesh may be used to avoid bird fault under such conditions.

Balancing Of Capacitor Units Per Group should be done before commissioning with the help of

capacitance meter or by applying low voltage single phase AC supply. Failure of any unit in the groupwill also give unbalance leading to overvoltage on balance units of the same group which may bedangerous enough to cause failure of the units.

3.13 Failures Due To System Problems:As mentioned earlier, generally the capacitor bank should stabilize within one month of operation. Ifhowever, it is found that the units fail one by one or mass failure occurs, system study like harmonics,load variations, power factor measurements at various loading conditions, voltages and voltage/currentsurges due to loads/capacitor banks switching, will have to be carried out, to ascertain cause of failure.

In case HARMONICS are present in the system, capacitor system should be designed to take care ofharmonics present since capacitor system offers lowest impedance path to harmonics. By adding

appropriate size of reactor in the capacitor system, we can increase the impedance and curtail harmonicsentering into capacitor thus reducing loading on capacitors. However, if harmonic contents are largeenough to give loading on capacitors more than designed value, we have to use capacitors in the form oftuned filter circuit designed to carry the required harmonics. If capacitors are not designed to take care ofthese aspects they are likely to fail due to harmonics.

3.14 Selection Of Capacitors:Rating of capacitors, basic technology and operating conditions are vital in selecting appropriate

capacitors.

Ratingshould be selected to ensure that the Power Factor does not go leading under all conditions ofloading. Leading powerfactor particularly under light load conditions is likely to rise system voltages as

41


42/69

also resonance phenomena between incoming transformer and capacitor bank may occur to build upvoltages and thereby failure of capacitors. Rated voltage of the capacitors should be selected based on thehighest system voltage keeping some safety margin and considering effect of harmonics.

As per the latest trend and due to lowest loss figures 100% PP film capacitors are used in HTapplications. However, in case of LT applications, selection of TECHNOLOGY will be vital. MetallisedPolypropylene (MPP) capacitors are available at moderate rates and loss figures. These are best suited forLT system where harmonics are not present and application does not involve frequent switching. Forsystem with harmonics or application involving frequent switching these capacitors output goes onreducing due to self healing property. Under such conditions either Mixed Dielectric (MD) capacitors ofPaper plus polypropylene film dielectric or latest version with very low losses, 100% polypropylene film(All PP) capacitors should be used. Generally MPP capacitors are provided with inductor coil to reduceeffect of switching surge currents thereby extending life against self healing under normal operatingconditions.

When capacitors are connected directly across MOTOR in individual feeder compensation, due careshould be taken to check that under all loading conditions of the motor, capacitors dont overcompensate.

Number of SWITCHING OPERATIONS should not exceed 3-4 per day. If the number of switchingoperations are likely to be more, the life of capacitor bank, reduces as each time the capacitors have tocarry high inrush currents. While SWITCHING ON the capacitor bank, it shall be ensured that systemvoltage is less than the rated voltage of the capacitor. Again, life expectancy goes down with switchingat higher voltages. In case capacitors are used near to transformer with on-load cap changer it would besafer to have OLTC on the primary side of the transformer and capacitor on secondary side as eachOLTC operation generates surges, dangerous to capacitors.

3.15 SELECTION OF ASSOCIATED EQUIPMENT:

Associated equipment selection is as import as capacitor selection for better performance of capacitorbank system.

In order to avoid overvoltages generated at the time of opening of BREAKER, the same should berestrike free. Necessary test certificates for breaker suitability for capacitor duty should be obtained.Sometimes it may be necessary to use surge absorbers With breakers used for capacitor duty. It has alsobeen noticed that switching of fully loaded inductive feeder by vacuum circuit breaker (VCB)gives riseto voltage surges. If capacitors are connected to the same bus, these surges are likely to damagecapacitors. Here also it is advisable to use surge absorbers.

Series Reactors are used with capacitors to (i) Limit switching surge currents particularly during parallelswitching (ii) Limit harmonic currents (iii) For tuned filter circuit. Depending upon the application

involved, the parameters of the series reactors are decided. Capacitor rated voltage should be increased tothe extent of drop in the series reactors. The series reactor current rating should be chosen to cover 130%continuous current rating of associated capacitor bank. The heat run test should be carried out at 130%current rating for series reactors, to ensure this compliance and to avoid failures due to higher currents.Series Reactors are available in magnetically & non-magnetically shielded versions. Generally in systemswith harmonics, non-magnetically shielded reactors are used to avoid failures due to harmonic fluxesflowing through shielding.

It is a common practice to reduce one unit from each phase if one of the unit fails and use reducedcapacity bank with same reactor. This should be avoided as reduced capacity bank has higher Xc valuethereby percentage of series reactor compared to Xc reduces. Reduced value of series reactor may not beeffective to curtail harmonics. A small value reactor at neutral end of the capacitor bank is always useful

42


43/69

to improve capacitor bank performance as it reduces considerably switching surges particularly duringparallel switching thereby reducing the duty on all the associated equipment including breakers.

Lightning Arrestors to some extent restrict switching surge voltages. Whenever high surges are expected,

Lightning arrestors of higher discharge handling capacity should be used. Also, Lightning Arrestorleakage currents should be periodically checked to confirm the same are not more than 150% of the valuerecorded at the time of installation.

If RVT windings are not mechanically strong enough to sustain voltage surges due to capacitor bankswitching, RVT is likely to fail.

3.16 Nature Of FailuresWhenever capacitor unit failure occurs, this necessarily gives rise to NDR (Neutral Displacement Relay)operation. In case of capacitor units protected with external expulsion fuse (or HRC FUSE), the fuse mayoperate to protect the unit, which will also give rise to NDR operation. NDR will also operate if RVT hasinternal fault. Therefore whenever capacitor bank trips on NDR operation, one has to find out if fuse hasblown but unit is intact. This can be due to (i) Transient overcurrent or (ii) Overheating of fuse due toloose end caps.

Capacitor bank may trip due to other protections offered like overcurrent, over voltage, under voltagewhich are common protections. Here again bank may trip without any failure of units, one has to identifyby checking the capacitance value of the unit and certify unit failure. From the protective relay operationone can identify the type of fault which might have caused failure of unit.

3.17 Summary:Any unit failure should be analysed based on information given above and such failures can be attributedto (a) manufacturing defects or (b) wrong applications. Failures due to harmonics, switching over

voltages or inadequate protection shall not be attributed to manufacturers.

In order to find out cause of failure, full data of capacitor bank at the time of commissioning and at thetime of failure may be noted.

IEEE have standardised ratings of capacitor units to take care of normal site conditions and safetymargins in the form of adjusted rated voltages. After the capacitor bank system is fully stabilized, spareunits should be kept to replace any of the failed units under circumstances beyond control, but only afterascertaining the cause of failure.

Problems:1) A load of 85 KVA is working at pf of 0.6. The demand charges per KW of Maximum Demand per

month =Rs.180/-. If the power factor in a month is less than 0.9 the MD charges for that month areincreased by 1% for each 0.01 by which the pf is below 0.9. Find payback period if cost/KVar =Rs.100/- and the power factor is to be raised up to 0.95.

Soln: a) Total demand charges prior to correction:85 x 0.6 =51.0 KW x 180 = 9180pf penalty =30% of above = 2754

11934 per monthb) The cost of capacitor:Correction KVAR =Kw[Tan(cos-1pf1) Tan(cos-1pf2)]KVAR=51 [Tan(cos-1 0.6) Tan(cos-1 0.95)] =51 x 1.005Hence 51 KVar is required to correct the power factor to 0.95

43


44/69

Cost per KVar =Rs.100/-

51 KVar 100 x 51 =Rs. 5100/-c) Improved Load conditions:New total charges per month = 51 x 180 =9180

Difference/month =11934 9180 =2754 Rs.Hence the investment can be recovered in about 2.0 months

2) A HT consumer has 50 KW MD at 0.8 pf and average consumption of 5000 units per month. Thedistribution company penalizes @ 3 ps/kwh for each 1% decrease in power factor below 0.9. If the costof capacitor bank along with associated switchgear is Rs.200/kVar. In how many months the investmenton capacitor can be recovered if the power factor is raised to 0.98.

Solution: Percentage of pf inviting penalty =(0.9 0.8)/1 x 100 =10%

Energy charges penalty =10 x 3 x100

5000=1500/- Rs.

penalty is Rs.1500/ month on average consumption of 5000 unitsCorrection KVar =50 kW [(tan(cos-10.8) tan (cos-1 0.98)]=50 (0.541) =27.050 KVarCost of 27 KVar = 27 x 200 = 5400/Rs.

It can be recovered in1500

5400=3.6 months

3) Find the average reactive power flow through a 220 kV, 120 km line operating at Sending end voltage

(Vs) of 1.0 pu and Receiving end voltage (Vr) of 0.9 pu and the =30o.Soln : cos =cos 30 =0.866

Qs =

pu

xxxx

VsVrV s 22.07794.1)866.0)(9.0)(0.1(0.1cos22

Qr = xxxx

VrVsVr 03.081.078.0)9.0()866.0)(9.0)(0.1(cos 22

220 kV, 120 km x =48

Base impedance = 484100

22022

MVA

KV

Line impedance in pu = PU1.0484

48

Qs = pu

o

2.21.0

22.

=220 MVAR

Qr = pu3.01.0

03.0

Qaverage puQrQs

95.02

9.1

2

)3.0(2.2

2

=95 MVAR

4) A 100 MVAR capacitor is connected at a bus with 5000 MVA short circuit capacity what is theexpected voltage change.

Let 1 pu =100MVA;

44


45/69

V = ;scS

Q Q =100 MVAR,

Q in pu =100/100 =1 pu

Ssc =5000 MVASsc in pu =5000/100 =50 pu

V (pu) = puSscpu

QPu02.0

50

1%2

5) A bus experiences 3% voltage fluctuation. The Ssc is 5000 MVA. We wish to size the Static VarCompensator to smoothen the voltage fluctuation, what shall be the size of SVC?

Soln: V =0.03 pu; Ssc =5000/100 =50 puQ =(V )(Ssc) =(0.03)(50) =1.50 pu = 150 MVARHence the size of SVC required is 150 MVAR

***

45


46/69

CHAPTER 4

REACTIVE POWER COMPENSATION IN TRANSMI SSION SYSTEMS

4.0 IntroductionWell-planned and coordinated reactive power compensation is an indispensable element in the design andoperation of a reliable power system. The effectiveness of reactive power control on power system maybe of utmost importance not only under normal conditions, but also during major system disturbances.

It is often advantageous to operate the transmission parts of a power system. with a fairly flat voltage profile, in order to avoid unnecessary reactive power flows. With a relatively small supply of reactive power into the distribution systems. With reactive power capacity reserves available for use in connection with major

disturbances and under generator, transformer or line outage conditions.

4.1 Transmissions with long overhead lines

This section discusses transmission and sub transmission systems, where shunt compensation, in oneform or another, is necessary or useful for reactive power and voltage control and possibly also forsynchronous stability improvement. Problems of voltage control and synchronous stability are mostpronounced in systems with high transfer impedances. With low transfer impedances the question ismore that of only balancing the reactive loads by reactive power production. The heading transmissionswith long overhead lines has been chosen because long lines means high transfer impedances.

The discussion is grouped into the subjects of steady-state var and voltage control, prevention of voltagecollapse, reduction of temporary over voltages, other voltage quality improvements and synchronousstability improvement.

4.2 Steady-state var and voltage control

The aim of the steady-state voltage control is to keep the transmission bus voltages within fairly narrowlimits, while the load transferred varies. The desirable voltage range under normal operating conditions isusually defined by the nominal voltage +/- 5 to 10 per cent, usually with higher voltage during heavyload conditions than during light load conditions. Usually a larger voltage deviation is allowed undercircuit outage operating conditions than under normal operating conditions. The set voltages on thedifferent buses, for which the voltage can be controlled directly, should be such that the reactive powerflows are minimized.

Since the reactive power transmitted may greatly vary hour by hour, the variation of the reactive powerbalance of a lin

rpm manual mnm

Documents