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Maximum Permissible Current

Capacitor units shall be suitable for continuous operation at an RMS current of 1.30 times the

current that occurs at rated sinusoidal voltage and rated frequency, excluding transients. a!ing

into account the capacitance tolerances of 1.1 "#, the maximum permissible current can be up to

1$3 %#.

hese overcurrent factors are intended to ta!e care of the combined effects of harmonics and

overvoltage&s up to and including1.10 '#, according to %S 133$0.

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Discharge Device

(ach capacitor unit or ban! shall be provided )ith a directly connected discharge device. he

discharge device shall reduce the residual voltage from the crest value of the rated value '# to

*0 + or less )ithin 1 min, after the capacitor is disconnected from the source of supply. here

must be no s)itch, fuse or any other isolating device bet)een the capacitor unit and the

discharge device.

discharge device is not a substitute for short-circuiting the capacitor terminals together and to

earth before handling.

Where:

t  time for discharge from '# /r to 'Rs,

R   equals discharge resistance

C  rated capacitance p2 per phase,

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UN  rated voltage of unit +,

UR   permissible residual voltage

k   coefficient depending on both resistance and capacitor unit connections, +alue of k  to be

ta!en as per %S133$0

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Configuration of Capacitor bank

delta-connected ban! of capacitors is usually applied to voltage classes of $00 volts or less.

%n a three-phase system, to supply the same reactive po)er, the star connection requires acapacitor )ith a capacitance three times higher than the delta connected capacitor. %n addition,

the capacitor )ith the star connection results to be sub4ected to a voltage 53 lo)er and flo)s

through by a current 53 higher than a capacitor inserted and delta connected.

For Three Phase STAR Connection

"apacity of the capacitor ban!

" 6c 7 82r 'r 

Rated current of the components %R"  82r "'r  7 53

9ine current % %R"

Three Phase Delta Connection

"apacity of the capacitor ban! " 6c 7 82r 'r 

.3Rated current of the components %R"  82r "'r 

9ine current % %R" 7 53

Where,

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Ur  rated voltage, )hich the capacitor must )ithstand indefinitely:

Fr  rated frequeny

Qc  generally expressed in !+R reactive power of the capacitor ban!

;hile deciding the si<e of capacitor ban! on any bus it is necessary to chec! the voltage rise due

to installation of capacitors under full load and light load conditions. %t is recommended to limit

the voltage rise to maximum of 3= of the bus voltage under light load conditions. he voltage

rise due to capacitor installation may be )or!ed out by the follo)ing expression.

op

Voltage Drop/Rise Due to SwitchingS)itching on or off a large bloc! of load causes voltage change. he approximate value can be

estimated by>

Voltage change ≅ load in MVA/fault level in MVA

S)itching a capacitor ban! causes voltage change, )hich can be estimated by>

Voltage change ≅ capacitor bank rating in MVA /system fault level in MVA

Where,

% VC  = voltage change or rise due to capacitor 

% X  = Reactance of equipment e.g. ransformer 

%f the capacitor ban! is SR connected than the required value of " )ill be higher in

comparison to the value of " in ?(9 connection for the same value of required !+R. @igher 

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value of " )ill cause higher voltage rise of the system causing nuisance tripping of the

equipment provided )ith over voltage protection.

%t is common practice to leave the star-connected capacitor ban!s ungrounded there are separate

reason for leaving it ungrounded )hen used in the system or use delta-connected ban!s to

 prevent the flo) of third harmonic currents into the po)er system through the grounded neutral.

9arge capacitor ban!s can be connected in SR ungrounded, SR grounded or delta.

@o)ever, the )ye ungrounded connection is preferable from a protection standpoint. 2or the

SR ungrounded system of connecting single capacitor units in parallel across phase-to-neutral

voltage the fault current through any incomer fuse or brea!er of capacitor ban! is limited by the

capacitors in the t)o healthy phases. %n addition the ground path for harmonic currents is not

 present for the ungrounded ban!.

2or SR grounded or delta-connected ban!s, ho)ever, the fault current can reach the full short

circuit value from the system because the sound phases cannot limit the current.

op

Detuning of Capacitor Banks%n an industrial plant containing po)er factor correction capacitors, harmonics distortions can be

magnified due to the interaction bet)een the capacitors and the service transformer. his is

referred to as harmonic resonance or parallel resonance. %t is important to note that capacitors

themselves are not main cause of harmonics, but only aggravate potential harmonic problems.

Aften, harmonic-related problems do not sho) up until capacitors are applied for po)er factor

correction.

%n de-tuned systems, reactors are installed in series )ith the capacitors and prevent resonance

conditions by shifting the capacitor7net)or! resonance frequency belo) the first dominant

harmonic usually the *th.

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%mpedance of the capacitor decreases )ith increase in frequency. "apacitor capacity to cancel

out harmonic decreases )ith increase in frequency. his offer the lo) impedance path to

harmonic currents. hese harmonic currents added to the fundamental current of capacitors can

 produce dangerous current overloads on capacitor. (ach of the harmonic currents causes the

voltage drop across the capacitor. his voltage drop is added to the fundamental voltage. hus in

 presence of harmonics higher voltage rating of capacitor is recommended. his overvoltage can

 be much above permissible 10= value )hen resonance is present.

nother important aspect is resonance )hich can occur )hen p.f. capacitors forms the series or

 parallel resonant circuit )ith impedance of supply transformer. %f the resonance frequency of this

9" circuit coincides )ith one of the harmonic present, the amplitude of the harmonic current

flo)ing through 9" circuit is multiplied several times damaging the capacitors, supplytransformer and other net)or! components.

op

Precautions to be taken while switching

! a capacitor bank

Ma!e sure that there is adequate load on the system. he normal current of the capacitor to be

s)itched A# at $$0 volts is say 100 amps. herefore the minimum load current at )hich the

capacitor should be s)itched A# is 130-1*0 amps.

%f one capacitor unit is already on and a second one is to be added then minimum load current on

this bus system must be equal to or more than the combined capacitor current of the t)o ban!s

 by at least a factor of 1.! to 1.!.

fter s)itching off the capacitor B )ait for at least one minute before s)itching it on. (arth all

the live terminals only after )aiting for one minute before touching these )ith spanner etc. %f

above precautions are not observed, this could lead to dangerous situations both for plant and

 personnel.

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S)itch off the capacitors )hen there is not enough load. his is a M'S. %f the capacitors are

!ept A# )hen there is no load or less load then Co)er factor goes to leading side and system

voltage increases )hich may cause damage to the capacitors as )ell as other electrical

equipments and severe disturbance can be caused.

%f the line voltages are more than the capacitor rated voltage, then do not s)itch on the

capacitors. s the load builds up, the line voltage )ill fall. S)itch on the capacitors then only.

op

peration of capacitor bank an" co relatation with

harmonics in the s#stem

@armonics can be reduced by limiting the non-linear load to 30= of the maximum transformer&s

capacity. Dy doing this )e ensure that po)er system does not exceeds the *= voltage distortion

level of %((( Standard *1E. @o)ever, )ith po)er factor correction capacitors installed,

resonating conditions can occur that could potentially limit the percentage of non-linear loads to

1*= of the transformer&s capacity.

'se the follo)ing equation to determine if a resonant condition on the distribution could occur>

2R   5!+S" 7 !+R"

Where,

FR   resonant frequency as a multiple of the fundamental frequency

kVAS" short circuit current at the point of study

kVARC  capacitor rating at the system voltage

%f FR  equals or is closed to a characteristic harmonic, such as the *th or Fth, there is a possibility

that a resonant condition could occur. lmost all harmonic distortion problems occur )hen the

 parallel resonance frequency is close to the fifth or seventh harmonic, since these are the most

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 po)erful harmonic current components. he eleventh and thirteenth harmonics may also be

)orth evaluating.

op

$rue an" "isplacement power factor speciall# with

regar"s to variable spee" "rives%

Co)er factor of variable speed drives B ;ith the six-step and current source inverters, the po)er

factor )ill be determined by the type of front end used. ;hen S"R&s are used, the po)er factor

)ill be relatively poor at reduced speeds. ;hen diodes )ith a dc chopper are used, the po)er

factor )ill be the same as a C;M inverter, )hich is relatively high near to unity at all, speeds.

rue po)er factor is the ratio of real po)er used in !ilo )atts !; divided by the total !ilo volt-

amperes. ?isplacement po)er factor is a measure of the phase displacement bet)een the voltage

and current at the fundamental frequency. rue po)er factor includes the effects of harmonics in

the voltage and current. ?isplacement po)er factor can be corrected )ith capacitor ban!s.

+ariable speed drives have different displacement po)er factor characteristics, depending on the

type of rectifier.

C;M type variable speed drives use a diode bridge rectifier and, have displacement po)er

factors very close to unity. @o)ever, the input current harmonic distortion can be very high for

these variable speed drives, resulting in a lo) true po)er factor. rue po)er factor is

approximately G0= despite the fact that the displacement po)er factor is very close to unity. he

true po)er factor can be improved substantially in this case through the application of input

cho!es or transformers )hich reduce current distortion.

"apacitor ban!s provide no po)er factor improvement for this type of variable speed drives and

can ma!e the po)er factor )orse by magnifying the harmonic levels.

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Capacitor Banks In Power System (part two)Posted Mar 1 2012 by Asif Eqbal in Energy and Power , Electrical Lectures with 11

Comments

Automatic caacitor ban!s consist of stages controlled by a ower factor controller

which ensures that the required caacitor ower is always connected to the system,

it means that always would be otimal correction "hoto credit# energolu!ss$l%&

"ontinued from part one B "apacitor Dan!s %n Co)er System part one

Si&ing of switching "evice for Capacitor

banks

%t should be noted that in an inductance the current lags the voltage by E0 degrees and in a

capacitor the current leads the voltage by E0 degrees. hese relationships are very important for

dra)ing phasor diagrams.

 "t is very convenient to remember these relationships by the #ord $"V"% as follo#s:

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 &ence $urrent dra#n from $apacitor bank '

Since sin() ' 1 hence the equation for current dra)n can be re)ritten as>

he relevant Standards on this device recommend a continuous overload capacity of 30=.

capacitor can have a tolerance of up to +1% in its capacitance va!ue. ll current-carrying

components such as brea!ers, contactors, s)itches, fuses, cables and busbar systems associated

)ith a capacitor unit or its ban!s, must therefore be rated for at least 1.* times the rated current.

The rating of a capacitor unit will thus vary in a square proportion of the eective

harmonic voltage and in a direct proportion to the harmonic frequency. This rise in

kVAR, however, will not contribute to improvement of the system power factor.  

but only of the overloading of the capacitors themselves.

herefore it may, ho)ever, sometimes be desirable to further enhance the overloading capacity

of the capacitor and so also the rating of the current-carrying components if the circuit conditions

and type of loads connected on the system are prone to generate excessive harmonics.

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(xamples are )hen they are connected on a system on )hich )e operating static drive and arc

"urnaces. %t is desirable to contain the harmonic effects as far as practicable to protect the

capacitors as )ell as inductive loads connected on the system and the communication net)or!, if 

running in the vicinity.

1$ 'ence as er abo%e discussion when determining the actual load current of a

caacitor unit in oeration, a factor of 1$1( is additionally considered to

account for the allowable tolerance in the caacitance %alue of the caacitor

unit$

2$ Efective kVAR = 1. ! 1. I" = 1." times t#e rate$ kVAR  and for which

all switching and rotecti%e de%ices must be selected$

$aking care of harmonics

%t is common practice to leave the star-connected capacitor ban!s ungrounded )hen used in the

system or use delta-connected ban!s to prevent the flo) of third harmonic currents into the

 po)er system through the grounded neutral.

'se of filter circuits in the po)er lines at suitable locations, to drain the excessive harmonicquantities of the system into the filter circuits.

 A lter circuit is a combination of capacitor and series reactance, tuned to a particular

harmonic frequency (series resonance), to oer it the least impedance at that

 frequency and hence, lter it out.

Say, for the fth harmonic, Xc5 X!S.

he use of a reactor in series )ith the capacitors )ill reduce the harmonic effects in a po)er

net)or!, as )ell as their effect on other circuits in the vicinity, such as a telecommunication

net)or!. he choice of reactance should be such that it )ill provide the required detuning by

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resonating belo) the required harmonic, to provide a least impedance path for that harmonic and

filter it out from the circuit.

he basic idea of a filter circuit is to ma!e it respond to the current of one frequency and re4ect

all other frequency components. t po)er frequency, the circuit should act as a capacitive load

and improve the p.f. of the system.

For the "i"th har#onic$ "or instance$ it shou!d resonate e!ow & ' () "or a ' () s*ste#$

sa* at around ''-' ()$ to avoid e&cessive charging vo!tages which #a* !ead to,

• )%er%oltage during light loads

•

)%er%oltage may saturate transformer cores and

• *ailure of caacitor units and inducti%e loads connected generate harmonics

in the system$

+t should be ensured that under no condition of system disturbance would the lter

circuit become caaciti%e when it aroaches near resonance$ -o achie%e this, the

lter circuits may be tuned to a little less than the dened harmonic frequency$

?oing so )ill ma!e the 9and hence H9, al)ays higher than Hc, since his provision )ill also

account for any diminishing variation in ", as may be caused by ambient temperature,

 production tolerances or failure of a fe) capacitor elements or even of a fe) units during

operation.

he po)er factor correction system )ould thus eco#e inductive "or #ost o" the current

har#onics produced * power e!ectronic circuits and )ould not magnify the harmonic effects

or cause disturbance to a communication system if existing in the vicinity filter circuit can be

tuned to the lo)est say the fifth harmonic produced by an electronic circuit. his is because 9capacitors are normally connected in delta and hence do not allo) the third harmonic to enter the

circuit )hile the @ capacitors are connected in star, but their neutral is left floating and hence it

does not allo) the third harmonic to enter the circuit.

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+n non.linear or unbalanced loads, howe%er, the third harmonic may still e/ist$ *or a

closer comensation, uni.frequency lters can be used to comensate indi%idual

harmonic contents by tuning the circuit to dierent harmonics$

2or more exact compensation, the contents and amplitudes of the harmonic quantities present inthe system can be measured )ith the help of an oscilloscope or a harmonic analy<er before

deciding on the most appropriate filter circuit7circuits. heoretically, a filter is required for each

harmonic, but in practice, filters ad4usted for one or t)o lo)er frequencies are adequate to

suppress all higher harmonics to a large extent and save on cost.

%f )e can provide a series reactor of G= of the total !+R of the capacitor ban!s connected on

the system, most of the harmonics present in the system can be suppressed. ;ith this reactance,

the system )ould be tuned to belo) the fifth harmonic at 0$ @< for a *0@< system.

'orking of (P)C Rela#

he basic principle of this relay is the sensing o" the phase disp!ace#ent etween the

"unda#enta! wave"or#s of the voltage and current )aves of po)er circuit. @armonic quantities

are filtered out )hen present in the system. his is a universal practice to measure the p.f. of a

system to economi<e on the cost of relay. he actual p.f. of the circuit may therefore be less than

measured by the relay.

ut one can set the relay slightly higher (less than unity), to account for the harmonics,

when harmonics are present in the system. !rom this phase displacement, a ".#. voltage

output is produced by a transducer circuit.

he value of the ?.". voltage depends upon the phase displacement, i.e. the p.f. of the circuit.

his ?.". voltage is compared )ith a built-in reference ?.". voltage, ad4ustable by the p.f.

setting !nob or by selecting the operating band provided on the front panel of the relay.

"orrective signals are produced by the relay to s)itch A# or A22 the stage capacitors through a

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 built-in sequencing circuit to reach the desired level of p.f. little lo)er p.f. then set )ould

attempt to s)itch another unit or ban! of capacitors, )hich may overcorrect the set p.f.

 #o) the relay )ould s)itch off a fe) capacitor units or ban!s to read4ust the p.f. and so )ill

commence a process of hunting, )hich is undesirable. o avoid such a situation the sensitivity of 

the comparator is made ad4ustable through the !nob on the front panel of the relay.

he sensitivity control can be built in terms of phase angle normally ad4ustable from $ to 1$

degrees electrical or percentage !+R. he sensitivity, in terms of an operating band, helps the

relay to avoid a marginal overcorrection or under correction and hence the hunting.

s soon as the system&s actual p.f. deviates from the pre-set limits, the relay becomes activated

and s)itches in or s)itches out capacitor units one by one, until the corrected p.f. falls )ithin the

sensitivity limit of the relay.

he power "actor correction re!a*s are nor#a!!* avai!a!e in three versions,

1$ Electromagnetic "being quic!ly outdated&$ -hey are %ery slow, and may ta!e

u to 2 minutes or more to initiate a correction$

2$ olid state.based on discrete +Cs$

$ olid state.based on micro.controllers "microrocessors&$

time delay is built in to allo) discharge of a charged capacitor up to E0= before it is

res)itched. his is achieved by introducing a timer into the relay&s s)itching circuit. he timer

comes on )henever an A22 signal occurs, and bloc!s the next operation of a charged capacitor,

even on an A# command, until it is discharged to at least E0= of the applied voltage. his

feature ensures safety against an overvoltage.

$ormally this time is "#$ minutes for !T an% 5#"& minutes for 'T shunt capacitors 

unless fast%discharge devices are provided across the capacitor terminals to reduce this

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time. !ast%discharge devices are sometimes introduced to discharge them faster than

these stipulations to match with quic&ly varying loads.

he A# action begins only )hen the timer is released. he time of s)itching bet)een each relay

step is, ho)ever, quite short, of the order of 3-* seconds. %t includes the timings of the control

circuit auxiliary relays contactors. %t may be noted that of this, the operating time of the static

relay is scarcely of the order of three to five cycles.

%n rapidly changing loads it must be ensured that enough discharged capacitors are available in

the circuit on every close command. o achieve this, sometimes it may be necessary to provide

special discharge devices across the capacitor terminals or a fe) extra capacitor units to !eep

them ready for the next s)itching. %t may require a system study on the pattern of load variationsand the corresponding p.f. 2ast s)itching, ho)ever, is found more often in 9 systems than in

@. @ systems are more stable, as the variable loads are mostly 9.

he above discussion is generally related to %"-based solid-state relays and in most parts to

microprocessor based relays of the more rudimentary types.

Power )actor Correction of *n"uction

Motor 

he selection of capacitor rating, for an induction motor, running at different loads at different

times, due either to change in load or to fluctuation in supply voltage, is di""icu!t and shou!d e

done with care because the reactive loading of the motor also fluctuates accordingly.

 A capacitor with a hi(her value of kVAR than the motor kVAR , under certain load

conditions, may develop dangerous voltages due to self%e'citation.

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t unity po)er factor, the residual voltage of a capacitor is equal to the system voltage. %t rises at

leading po)er factors. hese voltages )ill appear across the capacitor ban!s )hen they are

s)itched off and become a potential source of danger to the motor and the operator.

uch a situation may arise w#en t#e capacitor %nit is connecte$ across t#e

motor termina&s and is switched with it$ -his may haen during an oen

transient condition while changing o%er from star to delta, or from one ste to

another, as in an A3- switching, or during a triing of the motor or e%en while

switching o a running motor$

.n a!! such cases the capacitor wi!! e "u!!* charged and its excitation voltage, the magnitude

of )hich depends upon the p.f. of the system, )ill appear across the motor terminals or any other 

appliances connected on the same circuit. he motor, after disconnection from supply, )illreceive the self-excitation voltage from the capacitor and )hile running may act as a generator,

giving rise to voltages at the motor terminals considerably higher than the system voltage itself.

The solution to this problem is to select a capacitor with its capacitive current slightly

less than the magnetiing current, m, of the motor, say, )&* of it .

%f these facts are not borne in mind )hen selecting the capacitor rating, particularly )hen the p.f.

of the motor is assumed to be lo)er than the rated p.f. at full load, then at certain loads and

voltages it is possible that the capacitor !+R may exceed the motor reactive component, and

cause a leading po)er factor. leading p.f. can produce dangerous overvoltages. his

 phenomenon is also true in an alternator. %f such a situation arises )ith a motor or an alternator, it

is possible that it may cause excessive torques.

Ieeping these parameters in mind, motor manufacturers have recommended compensation of

only E0= of the no-load !+R of the motor. irrespective of the motor loading. his for all

 practical purposes and at all loads )ill improve the p.f. of the motor to around 0.E-0.E*. )hich is

satisfactory. Motor manufacturers suggest the li!ely capacitor ratings for different motor ratings

and speeds.

To be continued in 3rd  part – $apacitor *anks "n +o#er ystem -part three

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