elect rical-engineering-po rt al.co mhttp://electrical-engineering-portal.com/defining-size-and-location-o f-capacitor- in-electrical-system-1

jiguparmar

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

De fining Size and Lo catio n o f Cap acito r in Ele ctrical Syste m (1)

Content

Type of Capacitor Bank as per Its Application1. Fixed type capacitor banksThe reactive power supplied by the f ixed capacitor bank is constant irrespective of any variations in thepower f actor and the load of the receivers. These capacitor banks are switched on either manually (circuitbreaker / 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 f actor also varies as a f unction of the load requirements so it is dif f icult to maintain aconsistent power f actor by use of Fixed Compensation i.e. fixed capacitors.Fixed Capacitor may provide leading power f actor under light load conditions, Due to this result inovervoltages, saturation of transf ormers, mal-operation of diesel generating sets, penalties byelectric supply authorit ies.

Applicat ion:

Where the load f actor is reasonably constant.Electrical installations with constant load operating 24 hours a dayReactive compensation of transf ormers.Individual compensation of motors.Where the kvar rating of the capacitors is less than, or equal to 15% of the supply transf ormerrating, a f ixed value of compensation is appropriate.Size of Fixed Capacitor bank Qc 15% kVA transformer

Go to Content

2. Automatic type capacitor banksThe reactive power supplied by the capacitor bank can be adjusted according to variations in the powerf actor 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 of f of all or part of the capacitor bank is controlled by anintegrated power factor controller.

The equipment is applied at points in an installation where the active-power or reactive power variationsare relatively large, f or example:

At the bus bars of a main distribution switch-board,At the terminals of a heavily- loaded f eeder cable.

Where the kvar rating of the capacitors is less than, or equal to 15% of the supply transf ormer rating, afixed 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 f luctuating loads, f ast and highlyrepetit ive connection of capacitors is necessary, and static switches must be used.

Go to Content

Types of APFC Automatic Power Factor Correct ionAutomatic Power Factor correction equipment is divided into three major categories:

1. Standard = Capacitor + Fuse + Contactor + Controller2. De tuned = Capacitor + De tuning Reactor + Fuse + Contactor + Controller3. Filtered = Capacitor + Filter Reactor + Fuse + Contactor + Controller.

Advantages:

Consistently high power f actor under f luctuating loads.Prevention of leading power f actor.Eliminate power f actor penalty.Lower energy consumption by reducing losses.Continuously sense and monitor load.Automatically switch on/of f relevant capacitors steps f or consistent power f actor.Ensures easy user interf ace.Automatically variation, without manual intervention, the compensation to suit the load requirements.

Applicat ion:

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 DisadvantagesIndividualcapacitors

Most technically ef f icient, most f lexible Higher installation & maintenance cost

Fixed bank Most economical, f ewer installations Less f lexible, requires switches and/orcircuit breakers

Automaticbank

Best f or variable loads, prevents over voltages,low installation cost

Higher equipment cost

Combination Most practical f or larger numbers of motors Least f lexible

Go to Content

Type of Capacitor as per Construction1. Standard duty CapacitorConstruction: 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-dutyConstruction: Rectangular and Cylindrical (Resin filled / Resin coated-Dry/oil/gas)

Application:

1. Suitable f or f luctuating load.2. Non linear up to 20%.3. Suitable f or APFC Panel.4. Harmonic f iltering

3. LT CapacitorApplication:

Suitable f or f luctuating load.Non linear up to 20%.Suitable f or APFC Panel & Harmonic f ilter application.

Go to Content

Selecting Size of Capacitor BankThe 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 bank3.3 KV , 6.6KV 75 Kvar11 KV 200 Kvar22 KV 400 Kvar33 KV 600 Kvar

Unit sizes lower than above is not practical and economical to manuf acture.

When capacitors are connected directly across motors it must be ensured that the rated current of thecapacitor bank should not exceed 90% of the no- load current of the motor to avoid self -excitation of themotor and also over compensation.

Precaution must be taken to ensure the live parts of the equipment to be compensated should not behandled f or 10 minutes (in case of HT equipment) af ter disconnection of supply.

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

Go to Content

Selection of Capacitor as per Non Liner LoadFor power Factor correction it is need to f irst decide which type of capacitor is used.

Selection of Capacitor is depending upon many f actor i.e. operating lif e, Number of Operation, Peak Inrushcurrent withstand capacity.

For selection of Capacitor we have to calculate Total Non-Liner Load like: UPS, Rectif ier, Arc/InductionFurnace, AC/DC Drives, Computer, CFL Blubs, and CNC Machines.

Calculation of Non liner Load, Example: Transf ormer Rating 1MVA,Non Liner Load 100KVA% of non Liner Load = (Non Liner Load/Transf ormer Capacity) x100 = (100/1000) x100=10%.According to Non Linear Load Select Capacitor as per Following Table.

% Non Liner Load Type of Capacitor

Typically not applied to ungrounded systems. When applied to resistance-grounded systems,dif f iculty in coordination between capacitor f uses and upstream ground protection relays (considercoordination of 40 A fuses with a 400 A grounded system).Application: Typical f or smaller installations (since auxiliary equipment is not required)

Go to Content

2. Star-UngroundedIndustrial and commercial capacitor banks are normally connected ungrounded Star, with paralleledunits 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 theremaining units when one is removed f rom the circuit.

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

In industrial or commercial power systems the capacitors are not grounded f or a variety of reasons.Industrial systems are of ten resistance grounded. A grounded Star connection on the capacitor bankwould provide a path for zero sequence currents and the possibility of a f alse operation of ground faultrelays.

Also, the protective relay scheme would be sensit ive to system line-to-ground voltage Unbalance, whichcould also result in f alse relay tripping.

Application: In Industrial and Commercial.

Go to Content

3. Delta-connected BanksDelta-connected banks are generally used only at distributions voltages and are conf igured with a Singleseries group of capacitors rated at line-to- line voltage. With only one series group of units no overvoltageoccurs across the remaining capacitor units f rom the isolation of a f aulted capacitor unit.

Theref ore, unbalance detection is not required f or protection and they are not treated f urther in this paper.

Application: In Distribution System.

Go to Content

Effect of series and Parallel Connection of capacitorParallel ConnectionThis is the most popular method of connection. The capacitor is connected in parallel to the unit. Thevoltage rating of the capacitor is usually the same as or a litt le higher than the system voltage.

Go to Content

Series ConnectionThis method of connection is not much common. Even though the voltage regulation is much high in thismethod,

It has many disadvantages.

One is that because of the series connection, in a short circuit condition the capacitor should be able towithstand the high current. The other is that due to the series connection due to the inductivity of the linethere can be a resonance occurring at a certain capacitive value.

This will lead to very low impedance and may cause very high currents to f low through the lines.

Go to Content

Defining Size and Location of Capacitor in Electrical System (1)ContentType of Capacitor Bank as per Its Application1. Fixed type capacitor banksDisadvantages:Application:

2. Automatic type capacitor banksTypes of APFC Automatic Power Factor CorrectionAdvantages:Application:

Type of Capacitor as per Construction1. Standard duty Capacitor2. Heavy-duty3. LT Capacitor

Selecting Size of Capacitor BankSelection of Capacitor as per Non Liner LoadConfiguration of Capacitor1. Star-Solidly Grounded2. Star-Ungrounded3. Delta-connected Banks

Effect of series and Parallel Connection of capacitorParallel ConnectionSeries Connection