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 CB, Fuse and Conductor of Capacitor Bank

A. Thermal and Magnetic setting of a Circuit breaker

1. Size of Circuit Breaker

1.3 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 Breaker

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


3. Magnetic Setting 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) at 324A

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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 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

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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 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.

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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 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 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 TransformerKvar Required for compensation
Up to and including 315 KVA5% of KVA Transformer Rating
315 to 1000 KVA6% of KVA Transformer Rating
Above 1000 KVA8% 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 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.

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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 current 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 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.

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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.

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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.

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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 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

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 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.

Advantage

Larger, 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

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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 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.

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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 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.

Application

Usually the best location for individual capacitors.

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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

Placement of capacitors in distribution system


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A. Global compensation

Principle

The 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.

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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 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.

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). 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.

Application

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

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Common Capacitor Reactive Power Ratings

VoltageKvar RatingNumber of Phases
2165, 7.5, 131/3, 20, 251 or 3
2402.5, 5, 7.5,10, 25, 20, 25, 501 or 3
4805, 10, 15, 20 25, 35, 50, 60, 1001 or 3
6005, 10, 15, 20 25, 35, 50, 60, 1001 or 3
2,40050, 100, 150, 2001
2,77050, 100, 150, 2001
7,20050, 100, 150, 200,300,4001
12,47050, 100, 150, 200,300,4001
13,80050, 100, 150, 200,300,400

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author-pic

jiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.



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