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

Defining Size and Location of Capacitor in ElectricalSystem (1)

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jiguparmar

Content

Type of Capacitor Bank as per Its Application:

Type of Capacitor as per Construction

Selecting Size of Capacitor Bank

Selection of Capacitor as per Non Liner Load

Configuration of Capacitor:

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 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 day

Reactive 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

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2. Automatic type capacitor banks

The 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.

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Types of APFC – Automatic Power Factor Correct ion

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 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 Disadvantages

Individualcapacitors

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

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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 f or f luctuating load.

2. Non linear up to 20%.

3. Suitable f or APFC Panel.

4. Harmonic f iltering

3. LT Capacitor

Application:

Suitable f or f luctuating load.

Non linear up to 20%.

Suitable f or APFC Panel & Harmonic f ilter 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 Voltage Minimum rating of capacitor bank

3.3 KV , 6.6KV 75 Kvar

11 KV 200 Kvar

22 KV 400 Kvar

33 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.

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Selection of Capacitor as per Non Liner Load

For 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

<=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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Conf iguration 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.

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1. Star-Solidly Grounded

Init ial cost of the bank may be lower since the neutral does not have to be insulated f rom 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 f ault path which may require revision tothe existing system ground protection scheme.

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)

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2. Star-Ungrounded

Industrial 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.

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3. Delta-connected Banks

Delta-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.

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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. Thevoltage rating of the capacitor is usually the same as or a litt le higher than the system voltage.

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Series Connection

This 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.

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De fining Size and Lo catio n o f Cap acito r in Ele ctrical Syste m (2)

Defining Size and Location of Capacitor in ElectricalSystem (2)

electrical-engineering-portal.com /def ining-size-and- location-of -capacitor- in-electrical-system-2

jiguparmar

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

Content

1. If no- load current is known

2. If the no load current is not known

Placement of power capacitor bank for motor:

Placement of capacitors in distribution system:

Common capacitor reactive power ratings

Size of CB, Fuse and Conductor of Capacitor Bank

A. Thermal and Magnetic sett ing of a Circuit breaker

1. Size of Circuit Breaker

1.3 to 1.5 x Capacitor Current (In) f or Standard Duty/Heavy Duty/Energy Capacitors

1.31×In f or Heavy Duty/Energy Capacitors with 5.6% Detuned Reactor (Tuning Factor 4.3)

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

1.12×In f or 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 Sett ing of Circuit Breaker

1.5x Capacitor Current (In) f or Standard Duty/Heavy Duty/Energy Capacitors

3. Magnetic Sett ing of Circuit Breaker

5 to 10 x Capacitor Current (In) f or 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 (ShortCircuit) at 324A

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B. Fuse Select ion

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

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

1.35×In f or Heavy Duty/Energy Capacitors with 5.7% Detuned Reactor (Tuning Factor 4.3)

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

1.15×In f or 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 f use 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

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Problems with Fusing of Small Ungrounded Banks

Example: 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 f ails, 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 f ault (3 x 69.44 A = 208.32 A ). Thecapacitor case will rupture long bef ore the f use clears the f ault.

The solution is using smaller units with individual fusing. Consider 5 No’s of 100 kVAR capacitors perphase, each with a 25 A f use. The clear t ime f or a 25 A f use @ 208.32 A is below the published capacitorrupture curve.

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C. Size of Conductor for Capacitor Connections

Size of capacitor circuit conductors should be at least 135% of the rated capacitor current in accordancewith NEC Article 460.8 (2005 Edition).

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Size of capacitor for Transformer No-Load compensation

Fixed compensation

The transf ormer works on the principle of Mutual Induction. The transf ormer will consume reactive powerf or magnetizing purpose. Following size of capacitor bank is required to reduce reactive component (NoLoad Losses) of Transf ormer.

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 Transf ormer Rating

315 to 1000 KVA 6% of KVA Transf ormer Rating

Above 1000 KVA 8% of KVA Transf ormer Rating

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Sizing of capacitor for motor compensation

The capacitor provides a local source of reactive current. With respect to inductive motor load, this reactivepower is the magnetizing or “no load current“ which the motor requires to operate.

A capacitor is properly sized when its f ull 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.

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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 multiplyby 0.90 (90%).

Example:

Size a capacitor f or a 100HP, 460V 3-phase motor which has a f ull load current of 124 amps and a no- loadcurrent of 37 amps.

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

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2. If the no load current is not known

If the no- load current is unknown, a reasonable estimate f or 3-phase motors is to take the f ull load ampsand multiply by 30%. Then multiply it by 90% rating f igure being used to avoid overcorrection andovervoltages.

Example:

Size a capacitor f or a 75HP, 460V 3-phase motor which has a f ull load current of 92 amps and an unknownno-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.

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Placement of Power Capacitor Bank for Motor

Capacitors installed f or motor applications based on the number of motors to have power factorcorrection. If only a single motor or a small number of motors require power f actor correction, thecapacitor can be installed at each motor such that it is switched on and off with the motor.

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Required Precaution for select ing Capacitor for Motor:

The care should be taken in deciding the Kvar rating of the capacitor in relation to the magnetizing kVA ofthe machine.

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

As the motor, while still in rotation af ter disconnection f rom the supply, it may act as a generator by selfexcitation and produce a voltage higher than the supply voltage. If the motor is switched on again bef orethe speed has f allen to about 80% of the normal running speed, the high voltage will be superimposed onthe supply circuits and there may be a risk of damaging other types of equipment.

As a general rule the correct size of capacitor f or individual correction of a motor should have a kvarrating not exceeding 85% of the normal No Load magnetizing KVA of the machine. If several motorsconnected to a single bus and require power f actor correction, install the capacitor(s) at the bus.

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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-transit ion starting is used.

3. If the motor is subject to repetit ive 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 f eeder,load or service entrance. Since power capacitor banks are reactive power generators, the most logical placeto 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.

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Install ing a p o we r cap acito r b ank at the mo to r

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Location 1 (The line side of thestarter)

Install between the upstream circuitbreaker and the contactor.

This location should be used f or the motorloads with high inertia, where disconnecting the motor with the power capacitor bank can turn the motorinto a self excited generator, motors that are jogged, plugged or reversed, motors that start f requently,multi-speed motors, starters that disconnect and reconnect capacitor units during cycling and starters withopen transit ion.

Advantage

Larger, more cost ef f ective capacitor banks can be installed as they supply kvar to several motors. This isrecommended f or jogging motors, multispeed motors and reversing applications.

Disadvantages

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

Since reactive current must be carried a greater distance, there are higher line losses and largervoltage drops.

Applicat ions

Large banks of f ixed kVAR with f using on each phase.

Automatically switched banks

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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 NationalElectrical Code requirements.

With this option the overload relay can be set f or nameplate f ull load current of motor. Otherwise thesame as Option 1.

No extra switch or f uses required.

Contactor serves as capacitor disconnect.

Change overload relays to compensate f or reduced motor current.

Too much Kvar can damage motors.

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

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

Applicat ion:

Usually the best location f or individual capacitors.

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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 newinstallations in which the overloads can be sized in accordance with reduced current draw.

When correcting the power f actor f or an entire f acility, f ixed power capacitor banks are usuallyinstalled on f eeder circuits or at the service entrance.

Fixed power capacitor banks should only be used when the f acility’s load is f airly constant. When apower capacitor bank is connected to a f eeder or service entrance a circuit breaker or a f useddisconnect 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 of f with the motors, eliminating the need f or separate switching devices orover 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 systemcapacity is maximized.

Disadvantages

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

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

Applicat ion

Usually the best location f or individual capacitors.

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Placement of capacitors in Distribution system

The location of low voltage capacitors in Distribution System ef f ect on the mode of compensation, whichmay be global (one location for the entire installation), by sectors (section-by-section), at load level, or somecombination of the last two.

In principle, the ideal compensation is applied at a point of consumption and at the level required atany instant.

Place me nt o f cap acito rs in d is trib utio n syste m

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A. Global compensation

Principle

The capacitor bank is connected to thebus bars of the main LV distributionboard to compensation of reactiveenergy of whole installation and itremains in service during the period ofnormal load.

Advantages

Reduces the tarif f penalties f or excessive consumption of kvars.

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

Relieves Reactive energy of Transf ormer , which is then able to accept more load if necessary

Limitat ion

Reactive current still f lows in all conductors of cables leaving (i.e. downstream of) the main LVdistribution board. For this reason, the sizing of these cables and power losses in them are notimproved by the global mode of compensation.

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

Applicat ion

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 theexpansion coef f icient makes it possible to optimize the capacitor banks

Makes less demands on the transf ormer.

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B. Compensation by sector

Principle

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

Most part of the installation System can benef its f rom this arrangement, mostly the f eeder cables f rom themain distribution Panel to each of the local distribution panel.

Advantages

Reduces the tarif f penalties f or 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 additionalcapacity f or 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 transf ormer.

Remains economical

Limitat ions

Reactive current still f lows 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 bycompensation by sector

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

Applicat ion

Compensation by sector is recommended when the installation is extensive, and where the load/timepatterns dif f er f rom one part of the installation to another.

This conf iguration is convenient f or a very widespread f actory Area, with workshops having dif f erent loadf actors

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C. Individual compensation

Principle

Capacitors are connected directly to the terminals of inductive circuit (Near to motors). Individualcompensation should be considered when the power of the motor is signif icant with respect to thedeclared 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 (transf ormer) may also be benef icial.

Directly at the Load terminals Ex. Motors, a Steady load gives maximum benef it to Users.

The capacitor bank is connected right at the inductive load terminals (especially large motors). Thisconf iguration is well adapted when the load power is signif icant compared to the subscribed power.This is the technical ideal conf iguration, as the reactive energy is produced exactly where it isneeded, and adjusted to the demand.

Advantages

Reduces the tarif f penalties f or 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 thepoint where it is consumed. Heat losses (RI2) are theref ore reduced in all the lines.

Makes less demands on the transf ormer.

Limitat ions

Signif icant reactive currents no longer exist in the installation.

Not recommended f or Electronics Drives.

Most costly solution due to the high number of installations.

The f act that the expansion coef f icient is not incorporated.

Applicat ion

Individual compensation should be considered when the power of motor is signif icant with respect to powerof the installation.

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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

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