Defining Size and Location of Capacitors in an Electrical System (2)

7/27/2019 Defining Size and Location of Capacitors in an Electrical System (2)

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Def ining Size and Locat ion of Capaci tor in Electrical System (2)

Defining Size and Location of Capacitor in Electrical System

(2)

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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 fo r motor:

Placement of capacitors in dist ribution 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 Capacit or Current (In) for Standard Duty/Heavy Duty/Energy Capacitors

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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 (MaximumPermissible current) of the Detuned reactor.

2. Thermal Setting of Circuit Breaker

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

3. Magnetic Sett ing of Circuit Breaker

5 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 ) at324A

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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 thefuse 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

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In (Line Current) = 375/√3 = 216A

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

HRC Fuse Rating = 216 x 2.0 = 432A so Se lect 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 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 casewill 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, eachwith a 25 A fuse. The clea r time for a 25 A fuse @ 208.32 A is be low the published capacitor rupture curve.

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C. Size of Conductor for Capacitor Connections

Size of capacitor circuit conductors should be at least135% of the rated capacitor current in accordance with NEC

Article 460.8 (2005 Edition).

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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 siz e of capacitor bank is required toreduce reactive component (No Load Losses)of T ransformer.

Selection of capacitor fo r transformer no- load compensation

KVA Rating of the Transformer Kvar Required fo r 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

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Sizing of capacitor for motor compensation

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

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A capacitor is properly sized when its full load current rating is90% of the no-load current of the motor . This 90%rating avoids over correction and the accompanying p roblems such asovervoltages.

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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 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 currentof 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 reasonab le estimate for 3-phase motors is to take the full load amps andmultiply by 30%. Then multiply it by 90% rating figure being used to avoidovercorrection 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-loadcurrent.

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 thatMotor 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 for motor applications based on the number of motorsto have power factor correction. If only asingle motor or a small number of motors require power factor correction, the capacitor can beinstalled at each

motor such that it is switched on and off with the motor .

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Required Precaution for selecting Capacitor f or Motor:

The care should be taken in deciding theKvar rating of the capacitor in relation to the magnetizing kVA of themachine.

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

As the motor, while still in rotation a fter d isconnection from the supply, it may act as a generator by self excitation







Instal l ing a po wer capaci tor bank at the mo tor

and p roduce a voltage higher than the supply vo ltage . If the motor is switched on again before the speed has fallento about 80% of the normal running speed, the high voltage will be superimposed on the supply circuits and theremay be a risk of damaging other types of equipment.

As a general rule the correct size of capacitor for ind ividual correction of a motor should have akvar rating not

exceeding 85% of the normal No Load magnetizing KVA of the machine . If several motors connected to a singlebus and require power factor 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- transition starting is used.

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

4. If a multispeed 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 o r service entrance. Since power capacitor banks are reactive power generators, the most logical place to install themis 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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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 themotor 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.

Advantage

Larger, more cost effective capacitor banks can be installed as they supply kvar to several motors. This isrecommended for jogg ing motors, multispeed motors and reve rsing 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 g reater distance, there are higher line losses and larger voltagedrops.

Applications





Large banks of fixed kVAR with fusing on each phase.

Automatica lly 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 National ElectricalCode requirements.

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

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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 factor for an entire facility, fixed power capacitor banks are usually installed onfeeder 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 currentprotection. Also, only energized when the motor is running.

Since Kvar is loca ted 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 for lower motor current draw.





Placement of capaci tors in dist r ibut ion system

Application

Usually the best location for individual capacitors.

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Placement of capacitors in Distribution system

The location of low voltage capacitors inDistribution System effect on the mode of compensation, which may beglobal (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.

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A. Global compensation

Principle

The capacitor bank is connected to the busbars of the main LV distribution board tocompensation of reactive energy of wholeinstallation and it remains in service duringthe period of normal load.

Advantages

Reduces the tariff penalties for excessiveconsumption of kvars.

Reduces the appa rent 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 distributionboard. For this reason, the sizing of these cables and power losses in them are not improved by the g lobalmode of compensation.

The losses in the cables (I 2 R ) 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 so lution, as all the power is concentrated a t one po int and the expansioncoefficient makes it possible to op timize the capacitor banks

Makes less demands on the transformer.

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B. Compensation by sector

Principle

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

Most part of the installation System can benefits from this arrangement, mostly the feeder cables from the maindistribution Panel to each of the local distribution panel.

Advantages

Reduces the tariff penalties for excessive consumption of kvar.

Reduces the appa rent 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 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.


Remains economical

Limitations

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

For the above reason, the siz ing of these cables, and the power losses in them, are not improved bycompensation by sector

Where large changes in loads occur, there is a lways a risk of overcompensation and consequent overvoltageproblems.

Application

Compensation 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

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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 significant with respect to the declaredpower 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 thetechnical ideal configuration, as the reactive energy is produced exactly where it is needed, and ad justed tothe 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 (RI 2 ) are therefore reduced in all the lines.


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.

Application

Individual compensation should be considered when the power of motor is significant with respect to power of theinstallation.

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Common Capacitor Reactive Power Ratings

Voltage Kvar Rat ing 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 o r 3

600 5, 10, 15, 20 25, 35, 50, 60, 100 1 o r 32,400 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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Defining Size and Location of Capacitors in an Electrical System (2)