capaciters in ps

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CAPACITOR BANKS IN POWER SYSTEM

Capacitance

When a charge is delivered to a conductor its potential is raised in proportion to the quantity of

charge given to it. At a particular potential a conductor can hold a given amount of charge.

Capacitance is the term to indicate the limited ability to hold charge by a conductor.

Let charge given to a conductor be = q

Let V be the potential to which it is raised.

Then q α V, or

q = CV

C is constant for a conductor depending upon its shape size and surrounding medium. This

constant is called capacitance of a conductor.

If V = 1 Volt than C = Q, thus capacitance is defined as the amount of electric charge in

coulomb required to raise its potential by one volt.

If V = 1 Volt than C = Q, and Q = 1 Coulomb than C = 1 Farad thus one Farad is capacitanceof a capacitor which stores a charge of one coulomb when a voltage of one volt is applied acrossits terminal.

Capacitor

A capacitor or condenser is a device for storing large quantity of electric charge. Though the

capacity of a conductor to hold charge at a particular potential is limited, it can be increased

artificially. Thus any arrangement for increasing the capacity of a conductor artificially is called

a capacitor.

Capacitors are of many types depending upon its shape, like parallel plate, spherical and

cylindrical capacitors etc….

In capacitor there are two conductors with equal and opposite charge say +q and – q.Thus q is called charge of capacitor and the potential difference is called potential of

capacitor.

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Principle of Capacitor

Let A be the insulated conductor with a charge of +q units. In the absence of any other conductor

near A charge on A is +q and its potential is V. The capacity of conductor A is therefore given

by: :C = qV

If a second conductor B is kept closed to A than electrostatic induction takes place. – q units of

charge are induced on nearer face of B and +q units of charge is induced on farther face of B.

Since B is earthed the charge +q will be neutralized by the flow of electrons from the earth.

Potential of A due to self charge = V

Potential of A due to – q charge on B = -V’

Thus net potential of A = V + (-V’) = V -V’ which is less than V

Hence potential of A has been decreased keeping the charge on it fixed, hence capacitance has

been increased.

With the presence of B the amount of work done in bringing a unit positive charge from infinity

to conductor A decreases as there will be force of repulsion due to A and attraction due to B.

Thus resultant force of repulsion is reduced on unit positive charge and consequently the amount

of work doe is less and finally due to this potential of A decreases.

Therefore capacity of A to hold charge (Capacitance ) is increased.

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Dielectric StrengthThe material between the two conductors A and B as shown in figure above is always some

dielectric material. Under normal operating conditions the dielectric materials have a very few

free electrons. If the electric field strength between a pair of charged plates is gradually

increases, some of the electrons may be detached from the dielectric resulting in a small current.

Formatted: Font: (Default) Times NewRoman, 12 pt

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When the electric filed strength applied to a dielectric exceeds a critical value, the insulating

properties of the dielectric material gets destroys and starts conducting between the two

conductors A and B.

This is called breakdown of dielectric which is fault condition for a capacitor bank. The

minimum potential gradient required to cause such a break down is called the dielectric strength

of the material. It measures the ability of a dielectric to withstand breakdown. It is expressed as

kV/mm.

It is reduced by moisture, high temperature; aging etc. Below table gives dielectric strength of

some dielectrics.

Si.No. Dielectric Material Dielectric strength [kV/mm]1 Air 3

2 Impregnated Paper 4 – 103 Paraffin Wax 8

4 Porcelain 9 – 205 Transformer Oil 13.5

6 Bakelite 20 – 257 Glass 50 – 1208 Micanite 30

9 Mica 40 – 150

Dielectric Strength for capacitor is the maximum peak voltage that the capacitor is rated to

withstand at room temperature. Test by applying the specified multiple of rated voltage for oneminute through a current limiting resistance of 100 Ω per volt.

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Sizing of Capacitor banks for power factor improvement

The Power Factor Correction of electrical loads is a problem common to all industrial

companies. Every user which utilizes electrical power to obtain work in various forms

continuously asks the mains to supply a certain quantity of active power together with reactive

power.

Most loads on an electrical distribu tion system can be placed in one of three categori es:

Resistive

Inductive

Capacitive

The most common of these on modern systems is the inductive load. Typical examples includes

transformer, fluorescent lighting, AC induction motors, Arc/induction, furnaces etc. which draw

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not, only active power from the supply, but also inductive reactive power (KVAr). Common

characteristics of these inductive loads is that they utilize a winding to produce an

electromagnetic field which allows the motor or transformer to function and requires certain

amount of electrical power in order to maintaining the field.

Therefore Active Power (KW) actually performs the work whereas Reactive Power (KVAr)

sustains the electro-magnetic field. This reactive power though is necessary for the equipment to

operate correctly but could be interpreted as an undesirable burden on the supply.

If we quantify power factor improvement aspect from the utility company’s po int of view, thanraising the average operating power factor of the network from 0.7 to 0.9 means:

Cutting costs due to ohmic losses in the network by 40% Increasing the potential of production and distribution plants by 30%.

These figures speak for themselves: it means saving hundreds of thousands of tons of fuel and

making several power plants and hundreds of transformer rooms available.

Thus in the case of low power factors utility companies charge higher rates in order to cover the

additional costs they must incur due to the inefficiency of the system that taps energy. It is a

well-known fact that electricity users relying on alternating current – with the exception ofheating elements – absorb from the network not only the active energy they convert intomechanical work, light, heat, etc. but also an inductive reactive energy whose main function is to

activate the magnetic fields necessary for the functioning of electric machines.

Power Factor is also defined as cos Ø = kW / KVA

One can see after compensation requirement of kVAR (equal to kVAR1 – kVAR2) from thesystem has gone down.

Since kVA = kW + kVAR decreased kVAR requirement from the system has will result in

decreased kVA requirement, which will consequently result in lower current consumption from

the source.

Point to be noted in this case that any load which was operating at a power factor of 0.85 before

compensation continues to operate on same power factor of 0.85 even after compensation. It is

the source power factor which has been improved by compensating the kVAR requirement of

that particular load (or group of loads) from parallel connected capacitor banks. The source is

now relieved of providing some amount of kVAR (=kVAR1 – kVAR2).

compensated kvar = kvar1 – kvar2 = kw tanø1 – tan ø2 = kw [tanø1 - tan ø2]


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Power Factor Triangle

Hence Required Rating of Capacitor banks to be connected = kW [tanØ1 - tan Ø2]

Where,

cos Ø1 = Operating Power Factor

cos Ø2 = Target Power Factor or Power Factor after improvement.


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Sizing of switching device for Capacitor banks

It should be noted that in an inductance the current lags the voltage by 90 degrees and in a

capacitor the current leads the voltage by 90 degrees. These relationships are very important for

drawing phasor diagrams.

I t is very convenient to remember these relati onships by the word CIVI L as fol lows:

Hence Cur rent drawn fr om Capacitor bank =

Since sin90 = 1 hence the equation for current drawn can be rewritten as:

The relevant Standards on this device recommend a continuous overload capacity of 30%. A

capacitor can have a tolerance of up to +15% in its capacitance value. All current-carrying

components such as breakers, contactors, switches, fuses, cables and busbar systems associated

with a capacitor unit or its banks, must therefore be rated for at least 1.5 times the rated current.

The rating of a capacitor unit will thus vary in a square proportion of the effective 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. Therefore it may, however, 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.

Examples are when they are connected on a system on which we operating static drive and arc

furnaces. It is desirable to contain the harmonic effects as far as practicable to protect the

capacitors as well as inductive loads connected on the system and the communication network, if

running in the vicinity.

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1. Hence as per above discussion when determining the actual load current of a capacitor

unit in operation, a factor of 1.15 is additionally considered to account for the allowable

tolerance in the capacitance value of the capacitor unit.

2. Effective kVAR = 1.3 x 1. I5 = 1.5 times the rated kVAR and for which all switching and

protective devices must be selected.

Taking care of harmonics

It is common practice to leave the star-connected capacitor banks ungrounded when used in the

system or use delta-connected banks to prevent the flow of third harmonic currents into the

power system through the grounded neutral.

Use of filter circuits in the power lines at suitable locations, to drain the excessive harmonicquantities of the system into the filter circuits.

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

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

and hence, filter it out. Say, for the fifth harmonic, Xc5 = XLS.

The use of a reactor in series with the capacitors will reduce the harmonic effects in a power

network, as well as their effect on other circuits in the vicinity, such as a telecommunication

network. The choice of reactance should be such that it will provide the required detuning by

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

filter it out from the circuit.

The basic idea of a filter circuit is to make it respond to the current of one frequency and reject

all other frequency components. At power frequency, the circuit should act as a capacitive load

and improve the p.f. of the system. For the fifth harmonic, for instance, it should resonate below

5 x 50 Hz for a 50 Hz system, say at around 200-220 Hz, to avoid excessive charging voltages

which may lead to:

Overvoltage during light loads Overvoltage may saturate transformer cores and

Failure of capacitor units and inductive loads connected generate harmonics in the

system.

It should be ensured that under no condition of system disturbance would the filter circuit

become capacitive when it approaches near resonance. To achieve this, the filter circuits may be

tuned to a little less than the defined harmonic frequency. Doing so will make the Land hence

XL, always higher than Xc, since This provision will also account for any diminishing variationin C, as may be caused by ambient temperature, production tolerances or failure of a few

capacitor elements or even of a few units during operation.

The power factor correction system would thus become inductive for most of the current

harmonics produced by power electronic circuits and would not magnify the harmonic effects or

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

tuned to the lowest (say the fifth) harmonic produced by an electronic circuit. This is because LT

capacitors are normally connected in delta and hence do not allow the third harmonic to enter the

circuit while the HT capacitors are connected in star, but their neutral is left floating and hence it

does not allow the third harmonic to enter the circuit.

In non-linear or unbalanced loads, however, the third harmonic may still exist. For a closer

compensation, uni-frequency filters can be used to compensate individual harmonic contents by

tuning the circuit to different harmonics.

For more exact compensation, the contents and amplitudes of the harmonic quantities present inthe system can be measured with the help of an oscilloscope or a harmonic analyzer before

deciding on the most appropriate filter circuit/circuits. Theoretically, a filter is required for eachharmonic, but in practice, filters adjusted for one or two lower frequencies are adequate to

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

If we can provide a series reactor of 6% of the total kVAR of the capacitor banks connected on

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

the system would be tuned to below the fifth harmonic (at 204 Hz) for a 50Hz system.

Working of APFC Relay

The basic principle of this relay is the sensing of the phase displacement between the

fundamental waveforms of the voltage and current waves of power circuit. Harmonic quantities

are filtered out when present in the system. This is a universal practice to measure the p.f. of a

system to economize on the cost of relay. The actual p.f. of the circuit may therefore be less thanmeasured by the relay. But one can set the relay slightly higher (less than unity), to account for

the harmonics, when harmonics are present in the system. From this phase displacement, a D.C.

voltage output is produced by a transducer circuit.

The value of the D.C. voltage depends upon the phase displacement, i.e. the p.f. of the circuit.

This D.C. voltage is compared with a built-in reference D.C. voltage, adjustable by the p.f.

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

Corrective signals are produced by the relay to switch ON or OFF the stage capacitors through a

built-in sequencing circuit to reach the desired level of p.f. A little lower p.f. then set would

attempt to switch another unit or bank of capacitors, which may overcorrect the set p.f. Now the

relay would switch off a few capacitor units or banks to readjust the p.f. and so will commence a

process of hunting, which is undesirable. To avoid such a situation the sensitivity of the

comparator is made adjustable through the knob on the front panel of the relay.

The sensitivity control can be built in terms of phase angle (normally adjustable from 4 to 14

degrees electrical) or percentage kVAR. The sensitivity, in terms of an operating band, helps the

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


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As soon as the system’s actual p.f. deviates from the pre -set limits, the relay becomes activatedand switches in or switches out capacitor units one by one, until the corrected p.f. falls within the

sensitivity limit of the relay.

The power factor corr ection r elays are normal ly availabl e in thr ee versions:

1. Electromagnetic (being quickly outdated). They are very slow, and may take up to 2

minutes or more to initiate a correction.

2. Solid state-based on discrete ICs.

3. Solid state-based on micro-controllers (microprocessors).

4. A time delay is built in to allow discharge of a charged capacitor up to 90% before it is

reswitched. This is achieved by introducing a timer into the relay’s switching circuit. Thetimer comes on whenever an OFF signal occurs, and blocks the next operation of a

charged capacitor, even on an ON command, until it is discharged to at least 90% of theapplied voltage. This feature ensures safety against an overvoltage.

5. Normally this time is 1-3 minutes for LT and 5-10 minutes for HT shunt capacitors

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

time. Fast-discharge devices are sometimes introduced to discharge them faster than these

stipulations to match with quickly varying loads. The ON action begins only when the

timer is released. The time of switching between each relay step is, however, quite short,of the order of 3-5 seconds. It includes the timings of the control circuit auxiliary relays

(contactors). It may be noted that of this, the operating time of the static relay is scarcely

of the order of three to five cycles.

6. In rapidly changing loads it must be ensured that enough discharged capacitors are

available in the circuit on every close command. To achieve this, sometimes it may be

necessary to provide special discharge devices across the capacitor terminals or a few

extra capacitor units to keep them ready for the next switching. It may require a system

study on the pattern of load variations and the corresponding p.f. Fast switching,however, is found more often in LT systems than in HT. HT systems are more stable, as

the variable loads are mostly LT.

7. The above discussion is generally related to IC-based solid-state relays and in most parts

to microprocessor based relays of the more rudimentary types.8.

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9. Power Factor Correction of Induction Motor10. The 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 difficult

and should be done with care because the reactive loading of the motor also fluctuates

accordingly.

A capacitor with a higher value of kVAR than the motor kVAR, under certain load

conditions, may develop dangerous voltages due to self-excitation.

11.

At unity power factor, the residual voltage of a capacitor is equal to the system voltage. Itrises at leading power factors. These voltages will appear across the capacitor banks

when they are switched off and become a potential source of danger to the motor and the

operator.

Such a situation may arise when the capacitor unit is connected across the motor

terminals and is switched with it. This may happen during an open transient condition

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while changing over from star to delta, or from one step to another, as in an A/T

switching, or during a tripping of the motor or even while switching off a running motor.

12. In all such cases the capacitor will be fully charged and its excitation voltage, the

magnitude of which depends upon the p.f. of the system, will appear across the motor

terminals or any other appliances connected on the same circuit. The motor, after

disconnection from supply, will receive the self-excitation voltage from the capacitor and

while 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 magnetizing current,

Im, of the motor, say, 90% of it.

13. If these facts are not borne in mind when selecting the capacitor rating, particularly when

the p.f. of the motor is assumed to be lower than the rated p.f. at full load, then at certain

loads and voltages it is possible that the capacitor kVAR may exceed the motor reactivecomponent, and cause a leading power factor. A leading p.f. can produce dangerous

overvoltages. This phenomenon is also true in an alternator. If such a situation arises with

a motor or an alternator, it is possible that it may cause excessive torques.

14. Keeping these parameters in mind, motor manufacturers have recommended

compensation of only 90% of the no-load kVAR of the motor. irrespective of the motor

loading. This for all practical purposes and at all loads will improve the p.f. of the motor

to around 0.9-0.95. which is satisfactory. Motor manufacturers suggest the likely

capacitor ratings for different motor ratings and speeds.


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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. Taking

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

143 IN.

These overcurrent factors are intended to take care of the combined effects of harmonics and

overvoltage’s up to and including1.10 UN, according to IS 13340

Discharge Device

Each capacitor unit or bank shall be provided with a directly connected discharge device. Thedischarge device shall reduce the residual voltage from the crest value of the rated value UN to

50 V or less within 1 min, after the capacitor is disconnected from the source of supply. There

must be no switch, fuse or any other isolating device between the capacitor unit and the

discharge device.

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

earth before handling.

Where:

t = time for discharge from UN Jr to UR(s),

R = equals discharge resistance

C = rated capacitance (pF) per phase,

UN = rated voltage of unit (V),

UR = permissible residual voltage

k = coefficient depending on both resistance and capacitor unit connections, Value of k to be

taken as per IS13340

Configuration of Capacitor bank

A delta-connected bank of capacitors is usually applied to voltage classes of 2400 volts or less.

In a three-phase system, to supply the same reactive power, the star connection requires a

capacitor with a capacitance three times higher than the delta connected capacitor. In addition,the capacitor with the star connection results to be subjected to a voltage √3 lower and flowsthrough by a current √3 higher than a capacitor inserted and delta connected.

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For Three Phase STAR Connection

Capacity of the capacitor bank C = Qc / (2πFr Ur 2)

Rated current of the components IRC = 2πFr CUr / √3 Line current I = IRC

Three Phase Delta Connection

Capacity of the capacitor bank C = Qc / (2πFr Ur 2.3)

Rated current of the components IRC = 2πFr CUr Line current I = IRC / √3

Where,

Ur = rated voltage, which the capacitor must withstand indefinitely;

Fr = rated frequeny

Qc = generally expressed in kVAR (reactive power of the capacitor bank)

While deciding the size of capacitor bank on any bus it is necessary to check the voltage rise due

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

the voltage rise to maximum of 3% of the bus voltage under light load conditions. The voltage

rise due to capacitor installation may be worked out by the following expression.

Voltage Drop/Rise Due to Switching

Switching on or off a large block of load causes voltage change. The approximate value can be

estimated by:

Voltage change load in MVA/f ault level in MVA

Switching a capacitor bank causes voltage change, which 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. Transformer

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If the capacitor bank is STAR connected than the required value of C will be higher in

comparison to the value of C in DELTA connection for the same value of required kVAR.

Higher value of C will cause higher voltage rise of the system causing nuisance tripping of the

equipment provided with over voltage protection.

It is common practice to leave the star-connected capacitor banks ungrounded (there are separate

reason for leaving it ungrounded) when used in the system or use delta-connected banks to

prevent the flow of third harmonic currents into the power system through the grounded neutral.

Large capacitor banks can be connected in STAR ungrounded, STAR grounded or delta.

However, the wye ungrounded connection is preferable from a protection standpoint. For the

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

voltage the fault current through any incomer fuse or breaker of capacitor bank is limited by thecapacitors in the two healthy phases. In addition the ground path for harmonic currents is not

present for the ungrounded bank.

For STAR grounded or delta-connected banks, however, the fault current can reach the full short

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

Detuning of Capacitor Banks

In an industrial plant containing power factor correction capacitors, harmonics distortions can be

magnified due to the interaction between the capacitors and the service transformer. This is

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

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

Often, harmonic-related problems do not show up until capacitors are applied for power factor

correction.

In de-tuned systems, reactors are installed in series with the capacitors and prevent resonance

conditions by shifting the capacitor/network resonance frequency below the first dominant

harmonic (usually the 5th).

Impedance of the capacitor decreases with increase in frequency. Capacitor capacity to cancel

out harmonic decreases with increase in frequency. This offer the low impedance path to

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

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

voltage drop across the capacitor. This voltage drop is added to the fundamental voltage. Thus in

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

be much above permissible 10% value when resonance is present.

Another important aspect is resonance which can occur when p.f. capacitors forms the series or parallel resonant circuit with impedance of supply transformer. If the resonance frequency of this

LC circuit coincides with one of the harmonic present, the amplitude of the harmonic current

flowing through LC circuit is multiplied several times damaging the capacitors, supply

transformer and other network components.


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Precautions to be taken while switching ON a capacitor bank

Make sure that there is adequate load on the system. The normal current of the capacitor to be

switched ON at 440 volts is say 100 amps. Therefore the minimum load current at which the

capacitor should be switched ON is 130-150 amps.

If 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 two banks

by at least a factor of 1.35 to 1.5 .

After switching off the capacitor – wait for at least one minute before switching it on. Earth allthe live terminals only after waiting for one minute before touching these with spanner etc. If

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

Switch off the capacitors when there is not enough load. This is a MUST. If the capacitors are

kept ON when there is no load or less load then Power factor goes to leading side and system

voltage increases which may cause damage to the capacitors as well as other electrical

equipments and severe disturbance can be caused.)

If the line voltages are more than the capacitor rated voltage, then do not switch on the

capacitors. As the load builds up, the line voltage will fall. Switch on the capacitors then only.

Operation of capacitor bank and co relatation with harmonics in the system

Harmonics can be reduced by limiting the non-linear load to 30% of the maximum transformer’s

capacity. By doing this we ensure that power system does not exceeds the 5% voltage distortionlevel of IEEE Standard 519. However, with power factor correction capacitors installed,

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

15% of the transformer’s capacity.

Use the following equation to determine if a resonant condition on the distribution could occur:

FR = √kVASC / kVARC

Where,

FR = resonant frequency as a multiple of the fundamental frequency

kVASC= short circuit current at the point of study

kVARC

= capacitor rating at the system voltage

If FR equals or is closed to a characteristic harmonic, such as the 5th or 7th, there is a possibility

that a resonant condition could occur. Almost all harmonic distortion problems occur when the

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


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powerful harmonic current components. The eleventh and thirteenth harmonics may also be

worth evaluating.

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True and displacement power factor specially with regards to variable speed

drives?

Power factor of variable speed drives – With the six-step and current source inverters, the powerfactor will be determined by the type of front end used. When SCR’s are used, the power factorwill be relatively poor at reduced speeds. When diodes with a dc chopper are used, the power

factor will be the same as a PWM inverter, which is relatively high (near to unity) at all, speeds.

True power factor is the ratio of real power used in kilo watts (kW) divided by the total kilo volt-

amperes. Displacement power factor is a measure of the phase displacement between the voltage

and current at the fundamental frequency. True power factor includes the effects of harmonics in

the voltage and current. Displacement power factor can be corrected with capacitor banks.

Variable speed drives have different displacement power factor characteristics, depending on the

type of rectifier.

PWM type variable speed drives use a diode bridge rectifier and, have displacement power

factors very close to unity. However, the input current harmonic distortion can be very high for

these variable speed drives, resulting in a low true power factor. True power factor is

approximately 60% despite the fact that the displacement power factor is very close to unity. The

true power factor can be improved substantially in this case through the application of input

chokes or transformers which reduce current distortion.

Capacitor banks provide no power factor improvement for this type of variable speed drives and

can make the power factor worse by magnifying the harmonic levels.

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How to Protect Capacitor Banks?

Introduction

Capacitor banks are used to compensate for reactive energy absorbed by electrical system loads,

and sometimes to make up filters to reduce harmonic voltage.

Their role is to improve the quali ty of the electrical system. They may be connected in star, delta

and double star arrangements, depending on the level of voltage and the system load.

A capacitor comes in the form of a case with insulating terminals on top. It comprises individual

capacitances which have limited maximum permissible voltages (e.g. 2250 V ) and are series-

mounted in groups to obtain the required voltage withstand and parallel-mounted to obtained the

desired power rating.

Capacitor bank

There are two types of capacitors:

1.

Those with no internal protection,2. Those with internal protection: a fuse is combined with each individual capacitance.

Types of faults

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The main faul ts which are li able to aff ect capacitor banks are:

1. Overload,

2. Short-circuit,

3. Frame fault,

4. Capacitor component short-circuit

1. Overload

An overload is due to temporary or continuous overcurrent:

Continuous overcur rent li nked to:

Raising of the power supply voltage,

The flow of harmonic current due to the presence of non-linear loads such as

static converters (rectifiers, variable speed drives), arc furnaces, etc.,

Temporary overcurrent linked to the energizing of a capacitor bank step. Overloads result in

overheating which has an adverse effect on dielectric withstand and leads to premature capacitor

aging.

2. Short Circuit

A short-circuitis an internal or external fault between live conductors, phase-to-phase or phase-

to-neutral depending on whether the capacitors are delta or star-connected .

The appearance of gas in the gas-tight chamber of the capacitor creates overpressure which may

lead to the opening of the case and leakage of the dielectric.

3. Frame fault

A fr ame fault is an internal fault between a live capacitor component and the frame created by

the metal chamber.

Similar to internal short-circuits, the appearance of gas in the gas-tight chamber of the capacitorcreates overpressure which may lead to the opening of the case and leakage of the dielectric.

4. Capacitor component short-circuit

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A capacitor component short-circuit is due to the flashover of an individual capacitance.

With no internal protection: The parallel-wired individual capacitances are shunted by the faulty

unit:

The capacitor impedance is modified

The applied voltage is distributed to one less group in the series

Each group is submitted to greater stress, which may result in further,

cascading flashovers, up to a full short-circuit.

With internal protection: the melting of the related internal fuse eliminates the faulty individual

capacitance: the capacitor remains fault-free, its impedance is modified accordingly.

Top

Protection devices

Capaci tors should not be energized unless they have been discharged. Re-energizing must be

time-delayed in order to avoid transient overvoltage. A 10-minute time delay allows sufficient

natural discharging.

Fast discharging reactors may be used to reduce discharging time.

Overloads

Overcurrent of long duration due to the rai sing of the power supply voltage may be avoided by

overvoltage protection that monitors the electrical system voltage. This type of protection may be

assigned to the capacitor itself, but it is generally a type of overall electrical system protection.

Given that the capacitor can generally accommodate a voltage of 110% of its rated voltage for

12 hours a day, this type of protection is not always necessary.

Overcur rent of long duration due to the flow of harmonic curr ent i s detected by an overl oad

protection of one the foll owing types:

Thermal overload

Time-delayed overcurrent

provided it takes harmonic frequencies into account.

The amplitude of overcurrent of short duration due to the energizing of capacitor bank steps is

limited by series-mounting impulse reactors with each step.

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Short circuits

Short-circuits are detected by a time-delayed overcur rent protection device . Current and time

delay settings make it possible to operate with the maximum permissible load current and to

close and switch steps.

Frame faults

Protection depends on the grounding system. If the neutral is grounded, a time-delayed earth

fault protection device is used.

Capacitor component short -circui ts: Detection is based on the change in impedance created by

the short-circuiting of the component for capacitors with no internal protection by the

elimination of the faulty individual capacitance for capacitors with internal fuses.

When the capacitor bank is double star-connected , the unbalance created by the change in

impedance in one of the stars causes current to flow in the connection between the netural points.

This unbalance is detected by a sensiti ve overcu rr ent protection device .

Top

Examples of capacitor bank protection

Double star connected capacitor bank for reactive power compensation

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Double star connected capacitor bank for reactive power compensation

Filter

Filter

Setting information

Type of fault Setting

Overload

Overvol tage setti ng: ≤110% Vn Thermal overl oad: setting ≤1.3 In or overcurrent setting ≤1.3 In direct time or IDMT time delay 10 sec

Short-circuitOvercur rent di rect time setting: approximately 10 In time delay approximately 0.1 sec

Frame fault

Earth faul t direct time setting: ≤20% maximum earth fault current and ≥10% CT rating if suppied by 3 CTs time delayapproximately 0.1 sec

Capacitor component short

circuitOvercur rent direct time setting: < 1 ampere time delay approximately 1 sec

capaciters in ps

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