Capacitor Banks In Power System (part one)

Metal enclosed capacitor banks (MECB) - ABB

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 capacitance

of a capacitor which stores a charge of one coulomb when a voltage of one volt is applied across

its 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.

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.

Dielectric Strength

The 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.

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 – 10

3 Paraffin Wax 8

4 Porcelain 9 – 20

5 Transformer Oil 13.5

6 Bakelite 20 – 25

7 Glass 50 – 120

8 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 one

minute through a current limiting resistance of 100 Ω per volt.

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 distribution system can be placed in one of three categories:

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

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 point of view, than

raising 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 of

heating elements – absorb from the network not only the active energy they convert into

mechanical 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 the

system 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]

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.

Capacitor Banks In Power System (part two)

Automatic capacitor banks consist of stages controlled by a power factor controller which

ensures that the required capacitor power is always connected to the system, it means that

always would be optimal correction (photo credit: energolukss.lv)

Continued from part one – Capacitor Banks In Power System (part one)

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.

It is very convenient to remember these relationships by the word CIVIL as follows:

Hence Current drawn from 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.

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 harmonic

quantities 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 variation in 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 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 in

the 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 each

harmonic, 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 than measured 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.

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

and 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 correction relays are normally available in three 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).

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. The timer

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 the applied voltage. This

feature ensures safety against an overvoltage.

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.

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.

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.

Power Factor Correction of Induction Motor

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.

At unity power factor, the residual voltage of a capacitor is equal to the system voltage. It rises

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 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.

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.

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 reactive component, 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.

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.

To be continued in 3rd part – Capacitor Banks In Power System (part three)

Capacitor Banks In Power System (part three)

Low Voltage Power Capacitor

Continued from part two – Capacitor Banks In Power System (part two)

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. The

discharge 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 flows

through by a current √3 higher than a capacitor inserted and delta connected.

For Three Phase STAR Connection

Capacity of the capacitor bank C = Qc / (2πFrUr2)

Rated current of the components IRC = 2πFrCUr / √3

Line current I = IRC

Three Phase Delta Connection

Capacity of the capacitor bank C = Qc / (2πFrUr2.3)

Rated current of the components IRC = 2πFrCUr

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/fault 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

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 the capacitors 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.

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 all

the 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 distortion level 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

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

worth evaluating.

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 power

factor will be determined by the type of front end used. When SCR’s are used, the power factor

will 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

Capacitor Banks In Power System (part four)

Alternator capability curve - Green area is normal operating range of a typical synchronous

machine, yellow is abnormal but not damaging and operating in red regional will cause

damage or misoperation.

Continued from technical article: Capacitor Banks In Power System (part three)

PF correction for loads connected on captive Diesel Generator (DG)

Let us consider that there is a captive diesel generator the rating of which is specified as

1000kVA and PF 0.85. Rating in kVA specifies the maximum current the alternator can deliver

at the system voltage.

In the previous parts of this article we have seen that the role of power capacitors in improving

the power factor and reducing total cost of electricity in an industrial installation is well

established with regard to supply of power from the Utilitys/utilities.

Hence it seems logical to extend the above application of power capacitors when power is

drawn from captive diesel generator to optimize their performance.

It is however a common practice that DG set users generally switch off capacitors or do not

install capacitors at all when the DG set is in use because of the following reasons:

1. Apprehension that the DG set may get over loaded due to the fact that the kVA rating

or current delivered by the DG set is generally considered as the indicator of output of

DG set. It is well known that use of capacitors will reduce the kVAR requirement from

DG and hence kVA requirement will go down which in turn will reduce the current

drawn from the DG set and could thus tempt the to add more loads on a given DG set.

.

2. The other reason for such an opinion is related to the risks arising due to sustained

leading power factor conditions that would occur with the use of fixed capacitors in

variable load situations.

However with meticulous application of PF correction capacitor we can improve the overall

efficiency of DG set operation and result in considerable economic benefits to the DG set user.

This article tries to analyze the same in the following paragraphs.

Diesel Generator Set Rating in kVA

As we have considered 1000kVA DG. This way of specifying the DG rating is very logical

because specifies the maximum current the alternator can deliver at the system voltage.

DG set Rating in kVA at a particular PF

The diesel generator which we had assumed was of 1000kVA at 0.85PF. The relevance of PF in

case of DG rating is as follows:

1. To find mechanical power rating of a diesel engine for a particular diesel generator,

first convert kVA to kW and thereafter kW to BHP. This can only be done if we assume a

certain average Power Factor (PF) under which the DG set would operate.

.

2. The power factor so assumed should be in line with the average power factor prevalent

in the industry. A typical industrial load comprises of induction motors (typical PF of 0.8

to 0.85), non-linear loads (typical PF of 0.5 to 0.6) and combination of unity PF loads

(Resistive heating and incandescent lighting). Hence assuming an average power factor

of 0.85 for typical industrial loads is considered acceptable by convention.

.

3. Consequently a power factor of 0.85 is used for calculating the kW, which is then

converted to the BHP rating of the prime mover. BHP rating so obtained is the output

of the prime mover. Considering suitable engine losses it becomes possible to calculate

the power rating of the engine.

Now after understanding the DG set name plate rating parameter, let us come back to the

question should we connect the Capacitor Banks in parallel to the loads conned to DG?

Answer is YES, It is however, important to ensure that under actual operating conditions the kW

loading and current loading should not be exceeded.

Power Factor of loads supplied by DG sets can therefore be improved closer to unity by use of

suitable Reactive Power Compensation Systems keeping in view the rated current loading is not

exceeded.

Let us consider an example for the same:

** Any industry has a 1000 kVA DG set which is loaded at an average of 600 kW at 0.7 PF. In

addition, there are 125 kW of other loads within the same installation, which are not loaded on

the DG set due to capacity restrictions that arise during occurrence of short-term peak loads,

such as motor starting, and intermittent welding load. Due to this, productivity in the Industry is

lowered when the DG Set is in operation.

During the period when Utility supply is available all loads can be operated. Is it possible to

improve productivity when DG Set is in operation?

** A well designed power factor correction capacitor bank panel can improve the cost of

electricity consumed from utility as well as improve productivity when DG Set is in operation.

• DG rated capacity = 1000 kVA

• kW of load connected to DG = 600 kW

• Average load power factor in industry where DG is installed = 0.7

• kVA drawn at normal condition = 600 / 0.7 = 857 kVA

Hence percentage load on DG without Capacitor bank = 857 /1000 = 85.7%

Now if we connect the suitably sized and designed (already discussed in part1 to 3) capacitor

bank in parallel to the loads connected to DG and improve the average overall load power factor

from 0.7 to 0.85 than for the same percentage loading of 85.7% that is 857kVA the active power

that can be drawn is = 857 x 0.85 = 728.45 kW

Hence one can see the moment capacitor bank is connected in parallel to the loads connected to

the DG the additional requirement of 125kW is comfortably met without exceeding the

percentage loading on DG.

During the period when the Industry is using supply from the Utility the Capacitor banks system

can ensure consistently high PF, thereby achieving demand savings and reduction in losses and

elimination of any PF penalty. Consequently, cost of electricity consumed from the EB will be

minimized.

The same Capacitor banks system can be also used when the Industry is using supply from the

DG set. The fast acting property of the Capacitor banks system will reduce the peak load

requirements that are to be met from the DG set. This is achieved by providing instantaneous

compensation from the Capacitor banks system during conditions when motors are started and /

or welding machines are being operated. This will enable the Industry to transfer the 125 kW of

additional load on to the DG set and ensure that productivity is improved when the DG set is in

operation.

Due to better loading, the DG set efficiency will improve as for same 857 kVA; Active power

now delivered is now 728.45 kW instead of 600 kW.

REACTIVE POWER COMPENSATION SYSTEMS by Capacitor Banks can enable D.G set

users to reconfigure their loads / D.G sets to achieve better percentage loading and efficiency on

the machines. As a result reduction in cost / kWh can be attained.

Impact of leading kVAR on generators

Now since we have very well established that a suitably designed Capacitor Banks can be

connected in parallel to the loads connected to DG. However what is the impact if one keeps on

improving the power factor and the power factor goes on leading side.

Some inherent characteristics of an alternator limit the amount of leading kVAR that can be

absorbed by a DG. We cannot go on switching ON the Capacitor Banks as and when required,

this can create over voltage condition in DG and subsequently over fluxing.

There is a reverse kVAR limit of every generator.

The ability of any generator to absorb the kVAR is termed as reverse kVAR limit. This ability is

defined as reactive capability curve. Below figure shows typical generator reactive capability

curve. X axis is the kVAR produced or absorbed (positive to the right). Y axis indicates the kW

(positive going up). kVAR and kW are shown as per unit quantities based on the rating of the

alternator (not necessarily the generator set, which may have a lower rating.

The normal operating range of a generator set is between zero and 100 percent of the kW rating

of the alternator (positive) and between 0.8 and 1.0 power factor (green area on curve). The

black lines on the curves show the operating range of a specific alternator when operating

outside of normal range. Notice that as power factor drops, the machine must be de-rated to

prevent overheating. On the left quadrant, you can see that near-normal output (yellow area) can

be achieved with some leading power factor load, in this case, down to about 0.97 power factor,

leading. At that point, the ability to absorb additional kVAR quickly drops to near zero (red

area), indicating that the AVR is “turning off” and any level of reverse kVAR greater than the

level shown will cause the machine to lose control of voltage.

A good rule of thumb for generators is that it can absorb about 20% of its rated kVAR output in

reverse kVAR without losing control of voltage. However, since this characteristic is not

universal, it is advisable for a system designer to specify the reverse kVAR limit used in his

design, or the magnitude of the reverse kVAR load that is expected.

Note that this is not specified as a leading power factor limit, but rather as a maximum

magnitude of reverse kVAR.

Defining Size and Location of Capacitor in Electrical System (1)

Defining Size and Location of Capacitor in Electrical System (1)

Content

Type of Capacitor Bank as per Its Application:

1. Fixed type capacitor banks

2. Automatic type capacitor banks

3. Types of APFC – Automatic Power Factor Correction

Type of Capacitor as per Construction

Selecting Size of Capacitor Bank

Selection of Capacitor as per Non Liner Load

Configuration of Capacitor:

1. Star-Solidly Grounded

2. Star-Ungrounded

3. Delta-connected Banks

Effect of series and Parallel Connection of capacitor:

1. Parallel Connection

2. Series Connection

Type of Capacitor Bank as per Its Application

1. Fixed type capacitor banks

The reactive power supplied by the fixed capacitor bank is constant irrespective of any

variations in the power factor and the load of the receivers. These capacitor banks are switched

on either manually (circuit breaker / switch) or semi automatically by a remote-controlled

contactor.

This arrangement uses one or more capacitor to provide a constant level of compensation.

These capacitors are applied at the terminals of inductive loads (mainly motors), at bus bars.

Disadvantages:

Manual ON/OFF operation.

Not meet the require kvar under varying loads.

Penalty by electricity authority.

Power factor also varies as a function of the load requirements so it is difficult to

maintain a consistent power factor by use of Fixed Compensation i.e. fixed capacitors.

Fixed Capacitor may provide leading power factor under light load conditions, Due to

this result in overvoltages, saturation of transformers, mal-operation of diesel

generating sets, penalties by electric supply authorities.

Application:

Where the load factor is reasonably constant.

Electrical installations with constant load operating 24 hours a day

Reactive compensation of transformers.

Individual compensation of motors.

Where the kvar rating of the capacitors is less than, or equal to 15% of the supply

transformer rating, a fixed value of compensation is appropriate.

Size of Fixed Capacitor bank Qc ≤ 15% kVA transformer

2. Automatic type capacitor banks

The reactive power supplied by the capacitor bank can be adjusted according to variations in

the power factor and the load of the receivers.

These capacitor banks are made up of a combination of capacitor steps (step = capacitor +

contactor) connected in parallel. Switching on and off of all or part of the capacitor bank is

controlled by an integrated power factor controller.

The equipment is applied at points in an installation where the active-power or reactive power

variations are relatively large, for example:

At the bus bars of a main distribution switch-board,

At the terminals of a heavily-loaded feeder cable.

Where the kvar rating of the capacitors is less than, or equal to 15% of the supply transformer

rating, a fixed value of compensation is appropriate.

Above the 15% level, it is advisable to install an automatically-controlled bank of capacitors.

Control is usually provided by contactors. For compensation of highly fluctuating loads, fast

and highly repetitive connection of capacitors is necessary, and static switches must be used.

Types of APFC – Automatic Power Factor Correction

Automatic Power Factor correction equipment is divided into three major categories:

1. Standard = Capacitor + Fuse + Contactor + Controller

2. De tuned = Capacitor + De tuning Reactor + Fuse + Contactor + Controller

3. Filtered = Capacitor + Filter Reactor + Fuse + Contactor + Controller.

Advantages:

Consistently high power factor under fluctuating loads.

Prevention of leading power factor.

Eliminate power factor penalty.

Lower energy consumption by reducing losses.

Continuously sense and monitor load.

Automatically switch on/off relevant capacitors steps for consistent power factor.

Ensures easy user interface.

Automatically variation, without manual intervention, the compensation to suit the load

requirements.

Application:

Variable load electrical installations.

Compensation of main LV distribution boards or major outgoing lines.

Above the 15% level, it is advisable to install an automatically-controlled bank of

capacitors.

Size of Automatic Capacitor bank Qc > 15% kVA transformer.

Method Advantages Disadvantages

Individual capacitors

Most technically efficient, most flexible Higher installation & maintenance cost

Fixed bank Most economical, fewer installations Less flexible, requires switches and/or circuit breakers

Automatic bank

Best for variable loads, prevents over voltages, low installation cost

Higher equipment cost

Combination Most practical for larger numbers of motors

Least flexible

Type of Capacitor as per Construction

1. Standard duty Capacitor

Construction: Rectangular and Cylindrical (Resin filled / Resin coated-Dry)

Application :

1. Steady inductive load.

2. Non linear up to 10%.

3. For Agriculture duty.

2. Heavy-duty

Construction: Rectangular and Cylindrical (Resin filled / Resin coated-Dry/oil/gas)

Application:

1. Suitable for fluctuating load.

2. Non linear up to 20%.

3. Suitable for APFC Panel.

4. Harmonic filtering

3. LT Capacitor

Application:

Suitable for fluctuating load.

Non linear up to 20%.

Suitable for APFC Panel & Harmonic filter application.

Selecting Size of Capacitor Bank

The size of the inductive load is large enough to select the minimum size of capacitors that is

practical.

For HT capacitors the minimum ratings that are practical are as follows:

System Voltage Minimum rating of capacitor bank

3.3 KV , 6.6KV 75 Kvar

11 KV 200 Kvar

22 KV 400 Kvar

33 KV 600 KvarUnit sizes lower than above is not practical and economical to manufacture.

When capacitors are connected directly across motors it must be ensured that the rated current

of the capacitor bank should not exceed 90% of the no-load current of the motor to avoid self-

excitation of the motor and also over compensation.

Precaution must be taken to ensure the live parts of the equipment to be compensated should

not be handled for 10 minutes (in case of HT equipment) after disconnection of supply.

Crane motors or like, where the motors can be rotated by mechanical load and motors with

electrical braking systems, should never be compensated by capacitors directly across motor

terminals.

For direct compensation across transformers the capacitor rating should not exceed 90 % of

the no-load KVA of the motor.

Selection of Capacitor as per Non Liner Load

For power Factor correction it is need to first decide which type of capacitor is used.

Selection of Capacitor is depending upon many factor i.e. operating life, Number of Operation,

Peak Inrush current withstand capacity.

For selection of Capacitor we have to calculate Total Non-Liner Load like: UPS, Rectifier,

Arc/Induction Furnace, AC/DC Drives, Computer, CFL Blubs, and CNC Machines.

Calculation of Non liner Load, Example: Transformer Rating 1MVA,Non Liner Load

100KVA

% of non Liner Load = (Non Liner Load/Transformer Capacity) x100 = (100/1000)

x100=10%.

According to Non Linear Load Select Capacitor as per Following Table.

% Non Liner Load Type of Capacitor

<=10% Standard Duty

Up to 15% Heavy Duty

Up to 20% Super Heavy Duty

Up to 25% Capacitor +Reactor (Detuned)

Above 30%

Configuration of Capacitor

Power factor correction capacitor banks can be configured in the following ways:

1. Delta connected Bank.

2. Star-Solidly Grounded Bank.

3. Star-Ungrounded Bank.

1. Star-Solidly Grounded

Initial cost of the bank may be lower since the neutral does not have to be insulated

from ground.

Capacitor switch recovery voltages are reduced

High inrush currents may occur in the station ground system.

The grounded-Star arrangement provides a low-impedance fault path which may

require revision to the existing system ground protection scheme.

Typically not applied to ungrounded systems. When applied to resistance-grounded

systems, difficulty in coordination between capacitor fuses and upstream ground

protection relays (consider coordination of 40 A fuses with a 400 A grounded system).

Application: Typical for smaller installations (since auxiliary equipment is not required)

2. Star-Ungrounded

Industrial and commercial capacitor banks are normally connected ungrounded Star, with

paralleled units to make up the total kvar.

It is recommended that a minimum of 4 paralleled units to be applied to limit the over voltage

on the remaining units when one is removed from the circuit.

If only one unit is needed to make the total kvar, the units in the other phases will not be

overloaded if it fails.

In industrial or commercial power systems the capacitors are not grounded for a variety of

reasons. Industrial systems are often resistance grounded. A grounded Star connection on the

capacitor bank would provide a path for zero sequence currents and the possibility of a false

operation of ground fault relays.

Also, the protective relay scheme would be sensitive to system line-to-ground voltage

Unbalance, which could also result in false relay tripping.

Application: In Industrial and Commercial.

3. Delta-connected Banks

Delta-connected banks are generally used only at distributions voltages and are configured with

a Single series group of capacitors rated at line-to-line voltage. With only one series group of

units no overvoltage occurs across the remaining capacitor units from the isolation of a faulted

capacitor unit.

Therefore, unbalance detection is not required for protection and they are not treated further in

this paper.

Application:   In Distribution System.

Effect of series and Parallel Connection of capacitor

Parallel Connection

This is the most popular method of connection. The capacitor is connected in parallel to the

unit. The voltage rating of the capacitor is usually the same as or a little higher than the system

voltage.

Series Connection

This method of connection is not much common. Even though the voltage regulation is much

high in this method,

It has many disadvantages.

One is that because of the series connection, in a short circuit condition the capacitor should be

able to withstand the high current. The other is that due to the series connection due to the

inductivity of the line there can be a resonance occurring at a certain capacitive value.

This will lead to very low impedance and may cause very high currents to flow through the

lines.

Defining Size and Location of Capacitor in Electrical System (2)

Defining Size and Location of Capacitor in Electrical System (2)

Continued from part 1: Defining Size and Location of Capacitor in Electrical System (2)

Content

Size of circuit breaker (CB), fuse and conductor of capacitor bank:

A. Thermal and magnetic setting of a circuit breaker

B. Fuse selection

C. Size of conductor for capacitor connections

Size of capacitor for transformer no-load compensation:

Fixed compensation

Sizing of capacitor for motor compensation:

1. If no-load current is known

2. If the no load current is not known

Placement of power capacitor bank for motor:

Location 1 (the line side of the starter)

Location 2 (between the overload relay and the starter)

Location 3 (the motor side of the overload relay)

Placement of capacitors in distribution system:

A. Global compensation

B. Compensation by sector

C. Individual compensation

Common capacitor reactive power ratings

Size of CB, Fuse and Conductor of Capacitor Bank

A. Thermal and Magnetic setting of a Circuit breaker

1. Size of Circuit Breaker1.3 to 1.5 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors

1.31×In for Heavy Duty/Energy Capacitors with 5.6% Detuned Reactor (Tuning Factor

4.3)

1.19×In for Heavy Duty/Energy Capacitors with 7% Detuned Reactor (Tuning Factor 3.8)

1.12×In for Heavy Duty/Energy Capacitors with 14% Detuned Reactor (Tuning Factor

2.7)

Note: Restrictions in Thermal settings of system with Detuned reactors are due to limitation of

IMP (Maximum Permissible current) of the Detuned reactor.

2. Thermal Setting of Circuit Breaker1.5x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors

3. Magnetic Setting of Circuit Breaker5 to 10 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors

Example: 150kvar,400v, 50Hz Capacitor

Us = 400V, Qs = 150kvar, Un = 400V, Qn = 150kvar

In = 150000/400√3 = 216A

Circuit Breaker Rating = 216 x 1.5 = 324A

Select a 400A Circuit Breaker.

Circuit Breaker thermal setting = 216 x 1.5 = 324 Amp

Conclusion: Select a Circuit Breaker of 400A with Thermal Setting at 324A and Magnetic

Setting (Short Circuit) at 324A

B. Fuse Selection

The rating must be chosen to allow the thermal protection to be set to:

1.5 to 2.0 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors.

1.35×In for Heavy Duty/Energy Capacitors with 5.7% Detuned Reactor (Tuning Factor

4.3)

1.2×In for Heavy Duty/Energy Capacitors with 7% Detuned Reactor (Tuning Factor 3.8)

1.15×In for Heavy Duty/Energy Capacitors with 14% Detuned Reactor(Tuning Factor 2.7)

For Star-solidly grounded systems:

Fuse > = 135% of rated capacitor current (includes overvoltage, capacitor tolerances, and

harmonics).

For Star -ungrounded systems:

Fuse > = 125% of rated capacitor current (includes overvoltage, capacitor tolerances, and

harmonics).

Care should be taken when using NEMA Type T and K tin links which are rated 150%. In this

case, the divide the fuse rating by 1.50.

Example 1: 150kvar,400v, 50Hz Capacitor

Us = 400V; Qs = 150kvar, Un = 400V; Qn = 150kvar.

Capacitor Current =150×1000/400 =375 Amp

To determine line current, we must divide the 375 amps by √ 3

In (Line Current) = 375/√3 = 216A

HRC Fuse Rating = 216 x1.65 = 356A to

HRC Fuse Rating = 216 x 2.0 = 432A so Select Fuse Size 400 Amp

Problems with Fusing of Small Ungrounded BanksExample: 12.47 kV, 1500 Kvar Capacitor bank made of three 3 No’s of 500 Kvar single-

phase units.

Nominal Capacitor Current = 1500/1.732×12.47 = 69.44 amp

Size of Fuse = 1.5×69.44 = 104 Amp = 100 Amp Fuse

If a capacitor fails, we say that It may approximately take 3x line current. (3 x 69.44 A =

208.32 A).

It will take a 100 A fuse approximately 500 seconds to clear this fault (3 x 69.44 A = 208.32 A).

The capacitor case will rupture long before the fuse clears the fault.

The solution is using smaller units with individual fusing. Consider 5 No’s of 100 kVAR

capacitors per phase, each with a 25 A fuse. The clear time for a 25 A fuse @ 208.32 A is below

the published capacitor rupture curve.

C. Size of Conductor for Capacitor Connections

Size of capacitor circuit conductors should be at least 135% of the rated capacitor current in

accordance with NEC Article 460.8 (2005 Edition).

Size of capacitor for Transformer No-Load compensation

Fixed compensation

The transformer works on the principle of Mutual Induction. The transformer will consume

reactive power for magnetizing purpose. Following size of capacitor bank is required to reduce

reactive component (No Load Losses) of Transformer.

Selection of capacitor for transformer no-load compensation

KVA Rating of the Transformer Kvar Required for compensation

Up to and including 315 KVA 5% of KVA Transformer Rating

315 to 1000 KVA 6% of KVA Transformer Rating

Above 1000 KVA 8% of KVA Transformer Rating

Sizing of capacitor for motor compensation

The capacitor provides a local source of reactive current. With respect to inductive motor load,

this reactive power is the magnetizing or “no load current“ which the motor requires to operate.

A capacitor is properly sized when its full load current rating is 90% of the no-load current of

the motor. This 90% rating avoids over correction and the accompanying problems such as

overvoltages.

1. If no-load current is known

The most accurate method of selecting a capacitor is to take the no load current of the motor,

and multiply by 0.90 (90%).

Example:

Size a capacitor for a 100HP, 460V 3-phase motor which has a full load current of 124 amps

and a no-load current of 37 amps.

Size of Capacitor = No load amps (37 Amp) X 90% = 33 Kvar

2. If the no load current is not known

If the no-load current is unknown, a reasonable estimate for 3-phase motors is to take the full

load amps and multiply by 30%. Then multiply it by 90% rating figure being used to avoid

overcorrection and overvoltages.

Example:

Size a capacitor for a 75HP, 460V 3-phase motor which has a full load current of 92 amps and

an unknown no-load current.

No-load current of Motor = Full load Current (92 Amp) x 30% = 28 Amp estimated no-load

Current.

Size of Capacitor = No load amps (28 Amp) X 90% = 25 Kvar.

Thumb Rule:It is widely accepted to use a thumb rule that Motor compensation required in kvar is equal to

33% of the Motor Rating in HP.

Placement of Power Capacitor Bank for Motor

Capacitors installed for motor applications based on the number of motors to have power factor

correction. If only a single motor or a small number of motors require power factor correction,

the capacitor can be installed at each motor such that it is switched on and off with the motor.

Required Precaution for selecting Capacitor for Motor:

The care should be taken in deciding the Kvar rating of the capacitor in relation to the

magnetizing kVA of the machine.

If the rating is too high, It may damage to both motor and capacitor.

As the motor, while still in rotation after disconnection from the supply, it may act as a

generator by self excitation and produce a voltage higher than the supply voltage. If the motor

is switched on again before the speed has fallen to about 80% of the normal running speed,

the high voltage will be superimposed on the supply circuits and there may be a risk of

damaging other types of equipment.

As a general rule the correct size of capacitor for individual correction of a motor should have a

kvar rating not exceeding 85% of the normal No Load magnetizing KVA of the machine. If

several motors connected to a single bus and require power factor correction, install the

capacitor(s) at the bus.

Where do not install Capacitor on Motor:Do not install capacitors directly onto a motor circuit under the following conditions:

1. If solid-state starters are used.

2. If open-transition starting is used.

3. If the motor is subject to repetitive switching, jogging, inching, or plugging.

4. If a multi-speed motor is used.

5. If a reversing motor is used.

6. If a high-inertia load is connected to the motor.

Fixed power capacitor banks can be installed in a non-harmonic producing electrical system at

the feeder, load or service entrance. Since power capacitor banks are reactive power generators,

the most logical place to install them is directly at the load where the reactive power is

consumed.

Three options exist for installing a power capacitor bank at the motor.

Installing a power capacitor bank at the motor

Location 1 (The line side of the starter)

Install between the upstream circuit breaker and the contactor.

This location should be used for the motor loads with high inertia, where disconnecting the

motor with the power capacitor bank can turn the motor into a self excited generator, motors

that are jogged, plugged or reversed, motors that start frequently, multi-speed motors, starters

that disconnect and reconnect capacitor units during cycling and starters with open transition.

AdvantageLarger, more cost effective capacitor banks can be installed as they supply kvar to several

motors. This is recommended for jogging motors, multispeed motors and reversing applications.

Disadvantages

Since capacitors are not switched with the motors, overcorrection can occur if all

motors are not running.

Since reactive current must be carried a greater distance, there are higher line losses

and larger voltage drops.

Applications

Large banks of fixed kVAR with fusing on each phase.

Automatically switched banks

Location 2 (Between the overload relay and the starter)

Install between the contactor and the overload relay.

This location can be used in existing installations when the overload ratings surpass the

National Electrical Code requirements.

With this option the overload relay can be set for nameplate full load current of motor.

Otherwise the same as Option 1.

No extra switch or fuses required.

Contactor serves as capacitor disconnect.

Change overload relays to compensate for reduced motor current.

Too much Kvar can damage motors.

Calculate new (reduced) motor current. Set overload relays for this new motor FLA.

FLA (New) = P.F (Old) / P.F (New) x FLA (Name Plate)

Application:Usually the best location for individual capacitors.

Location 3 (The motor side of the overload relay)

Install directly at the single speed induction motor terminals (on the secondary of the

overload relay).

This location can be used in existing installations when no overload change is required

and in new installations in which the overloads can be sized in accordance with reduced

current draw.

When correcting the power factor for an entire facility, fixed power capacitor banks are

usually installed on feeder circuits or at the service entrance.

Fixed power capacitor banks should only be used when the facility’s load is fairly

constant. When a power capacitor bank is connected to a feeder or service entrance a

circuit breaker or a fused disconnect switch must be provided.

New motor installations in which overloads can be sized in accordance with reduced

current draw

Existing motors when no overload change is required.

Advantage

Can be switched on or off with the motors, eliminating the need for separate switching

devices or over current protection. Also, only energized when the motor is running.

Since Kvar is located where it is required, line losses and voltage drops are minimized;

while system capacity is maximized.

Disadvantages

Installation costs are higher when a large number of individual motors need correction.

Overload relay settings must be changed to account for lower motor current draw.

ApplicationUsually the best location for individual capacitors.

Placement of capacitors in Distribution system

The location of low voltage capacitors in Distribution System effect on the mode of

compensation, which may be global (one location for the entire installation), by sectors

(section-by-section), at load level, or some combination of the last two.

In principle, the ideal compensation is applied at a point of consumption and at the level

required at any instant.

Placement of capacitors in distribution system

A. Global compensation

PrincipleThe capacitor bank is connected to the bus bars of the main LV distribution board to

compensation of reactive energy of whole installation and it remains in service during the

period of normal load.

Advantages

Reduces the tariff penalties for excessive consumption of kvars.

Reduces the apparent power kVA demand, on which standing charges are usually based

Relieves Reactive energy of Transformer , which is then able to accept more load if

necessary

Limitation

Reactive current still flows in all conductors of cables leaving (i.e. downstream of) the

main LV distribution board. For this reason, the sizing of these cables and power losses

in them are not improved by the global mode of compensation.

The losses in the cables (I2R) are not reduced.

Application

Where a load is continuous and stable, global compensation can be applied

No billing of reactive energy.

This is the most economical solution, as all the power is concentrated at one point and

the expansion coefficient makes it possible to optimize the capacitor banks

Makes less demands on the transformer.

B. Compensation by sector

PrincipleCapacitor banks are connected to bus bars of each local distribution Panel.

Most part of the installation System can benefits from this arrangement, mostly the feeder

cables from the main distribution Panel to each of the local distribution panel.

Advantages

Reduces the tariff penalties for excessive consumption of kvar.

Reduces the apparent power Kva demand, on which standing charges are usually based.

The size of the cables supplying the local distribution boards may be reduced, or will

have additional capacity for possible load increases.

Losses in the same cables will be reduced.

No billing of reactive energy.

Makes less demands on the supply Feeders and reduces the heat losses in these

Feeders.

Incorporates the expansion of each sector.

Makes less demands on the transformer.

Remains economical

Limitations

Reactive current still flows in all cables downstream of the local distribution Boards.

For the above reason, the sizing of these cables, and the power losses in them, are not

improved by compensation by sector

Where large changes in loads occur, there is always a risk of overcompensation and

consequent overvoltage problems.

ApplicationCompensation by sector is recommended when the installation is extensive, and where the

load/time patterns differ from one part of the installation to another.

This configuration is convenient for a very widespread factory Area, with workshops having

different load factors

C. Individual compensation

Principle

Capacitors are connected directly to the terminals of inductive circuit (Near to motors).

Individual compensation should be considered when the power of the motor is

significant with respect to the declared power requirement (kVA) of the installation.

The kvar rating of the capacitor bank is in the order of 25% of the kW rating of the

motor.

Complementary compensation at the origin of the installation (transformer) may also be

beneficial.

Directly at the Load terminals Ex. Motors, a Steady load gives maximum benefit to

Users.

The capacitor bank is connected right at the inductive load terminals (especially large

motors). This configuration is well adapted when the load power is significant compared

to the subscribed power. This is the technical ideal configuration, as the reactive energy

is produced exactly where it is needed, and adjusted to the demand.

Advantages

Reduces the tariff penalties for excessive consumption of kvars

Reduces the apparent power kVA demand

Reduces the size of all cables as well as the cable losses.

No billing of reactive energy

From a technical point of view this is the ideal solution, as the reactive energy is

produced at the point where it is consumed. Heat losses (RI2) are therefore reduced in

all the lines.

Makes less demands on the transformer.

Limitations

Significant reactive currents no longer exist in the installation.

Not recommended for Electronics Drives.

Most costly solution due to the high number of installations.

The fact that the expansion coefficient is not incorporated.

ApplicationIndividual compensation should be considered when the power of motor is significant with

respect to power of the installation.

Common Capacitor Reactive Power Ratings

Voltage Kvar Rating Number of Phases

216 5, 7.5, 131/3, 20, 25 1 or 3

240 2.5, 5, 7.5,10, 25, 20, 25, 50 1 or 3

480 5, 10, 15, 20 25, 35, 50, 60, 100 1 or 3

600 5, 10, 15, 20 25, 35, 50, 60, 100 1 or 3

2,400 50, 100, 150, 200 1

2,770 50, 100, 150, 200 1

7,200 50, 100, 150, 200,300,400 1

12,470 50, 100, 150, 200,300,400 1

13,800 50, 100, 150, 200,300,400

How to Protect Capacitor Banks?

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 quality 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

The main faults which are liable to affect   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 overcurrent linked 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 frame 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 capacitor creates overpressure which may lead to the opening of the case and leakage of the dielectric.

4. Capacitor component short-circuit

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

Capacitors 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 raising 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.

Overcurrent of long duration due to the flow of harmonic current is detected by an overload protection of one the following 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.

Short circuits

Short-circuits are detected by a time-delayed overcurrent 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-circuits: 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 sensitive overcurrent protection device.

Examples of capacitor bank protection

Double star connected capacitor bank for reactive power compensation

Double star connected capacitor bank for reactive power compensation

Filter

Filter

Setting information

Type of fault Setting

Overload Overvoltage setting: ≤110% VnThermal overload:setting ≤1.3 In or overcurrentsetting ≤1.3 In direct timeor IDMT time delay 10 sec

Short-circuit Overcurrent direct time setting: approximately 10 In time delay approximately 0.1 sec

Frame fault Earth fault direct time setting: ≤20% maximum earth fault currentand ≥10% CT rating if suppied by 3 CTs time delay

approximately 0.1 sec

Capacitor component short circuit

Overcurrent direct time setting: < 1 ampere time delay approximately 1 sec

Resource: Protection Guide – Schneider Electric