2/1/2014 Defining Size and Location of Capacitor in Electrical System (1) | EEP

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

2/1/2014 Defining Size and Location of Capacitor in Electrical System (1) | EEP

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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 variat ions

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

manually (ci rcui t 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), a t bus bars.

Disadvantages:

Manual ON/OFF operation.

Not meet the require kvar under varying loads.

Penalty by electricity authority.

Power factor a lso 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 l ight load conditions, Due to this

result in overvoltages, saturation of transformers, mal-operation of diesel generating sets,

penalt ies by electric supply authorit ies.

Application:

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Where the load factor is reasonably constant.

Electrical insta llations 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

Go to Content ↑

2. Automatic type capacitor banks

The reactive power supplied by the capacitor bank can be adjusted according to variat ions 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.

Go to Content ↑

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.

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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.

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

Ensures easy user interface.

Automatica lly variation, without manual intervention, the compensation to suit the load

requirements.

Application:

Variable load electrical instal lations.

Compensation of main LV distribution boards or major outgoing lines.

Above the 15% level, i t is advisable to insta ll an automatically-controlled bank of capacitors.

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

Method Advantages Disadvantages

Individual capacitorsMost technically efficient, most

flexible

Higher insta llat ion & maintenance

cost

Fixed bankMost economical, fewer installations Less flexible, requires switches

and/or circuit breakers

Automatic bankBest for variable loads, prevents

over voltages, low installation cost

Higher equipment cost

CombinationMost practical for larger numbers of

motors

Least flexible

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Type of Capacitor as per Construction

1. Standard duty Capacitor

Construction: Rectangular and Cylindrical (Resin f illed / Resin coa ted-Dry)

Application:

1. Steady inductive load.

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2. Non linear up to 10%.

3. For Agriculture duty.

2. Heavy-duty

Construction: Rectangular and Cylindrical (Resin f illed / Resin coa ted-Dry/oil/gas)

Application:

1. Suitable for fluctuating load.

2. Non linear up to 20%.

3. Suitable for APFC Panel.

4. Harmonic fil tering

3. LT Capacitor

Application:

Suitable for fluctuating load.

Non linear up to 20%.

Suitable for APFC Panel & Harmonic fi lter application.

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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 VoltageMinimum rating of capacitor

bank

3.3 KV , 6.6KV 75 Kvar

11 KV 200 Kvar

22 KV 400 Kvar

33 KV 600 Kvar

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Unit sizes lower than above is not practica l and economical to manufacture.

When capacitors are connected directly across motors i t 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 mechanica l 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.

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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 l iner 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

LoadType of Capacitor

<=10% Standard Duty

Up to 15% Heavy Duty

Up to 20% Super Heavy Duty

Up to 25% Capacitor +Reactor (Detuned)

Above 30%

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Go to Content ↑

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.

Go to Content ↑

1. Star-Solidly Grounded

Initia l 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 sys tem) .

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

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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 industria l 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

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

operation of ground fault relays.

Also, the protective relay scheme would be sensitive to system l ine-to-ground voltage Unbalance,

which could also result in false relay tripping.

Application: In Industria l and Commercia l.

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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.

Go to Content ↑

Effect of series and Parallel Connection of capacitor

Parallel Connection

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

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

voltage.

Go to Content ↑

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.

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One is that because of the series connection, in a short circuit condit ion 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 wil l lead to very low impedance and may cause very high currents to flow through the

lines.

Go to Content ↑

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