Capacitor Bank: Learn the Purpose, Cost, and Lead Time to Procure

Purpose of capacitor bank

Capacitors consume active power and release reactive power. They also present a low impedance to harmonics; in other words, they attract harmonic frequencies. Thus, they are installed in substations to:

• Correct power factor, consequently minimizes voltage drop at the load end, by injecting reactive volt-amp a.k.a. VARs.

• Minimize voltage fluctuations, typically generated near industrial facilities, using harmonic filter banks – improving power quality in the process.
• Capacitors are an integral piece of Flexible AC Transmission (FACT) device. Some of the FACT devices are:
  1. Static Var Compensator: Used to provide reactive power support (either consume VARS using inductors or release VARs using capacitors – each device plugged into the circuit using thyristor valves) and to filter out harmonics
  2. Series capacitors on high voltage transmission line: Used to maximize power flow on the line by consuming lagging VARs, improve the steady-state and dynamic stability of the power system (by preventing the generators from operating at the maximum allowable power angle. Learn more)

Cost of capacitor bank

Cost depends on the MVAR rating and whether it is an indoor or outdoor unit.

• 12kV 4MVAR capacitor in a metal-clad switchgear: ~$70,000
• 34.5kV 10.8MVAR capacitor in a metal-clad switchgear: ~$170,000
• 69kV 14.4MVAR capacitor fuseless outdoor type: ~$75,000
• 138kV 65MVAR capacitor fuseless outdoor type: ~$180,000
• 230kV 100MVAR capacitor fuseless outdoor type: ~$250,000
• 345kV 150MVAR capacitor fuseless outdoor type: ~$400,000
• 500kV 230MVAR capacitor fuseless outdoor type: ~$700,000

Lead time to procure capacitor bank

~25 weeks

Information on cost and lead time is for your general knowledge only. Contact vendor with your equipment specifications for actual figures.

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Substation Major Equipment

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An industrial facility is plagued with voltage flicker (due to harmonics) and poor power factor at the service entrance. Which device would you install to fix both issues?

Install capacitor bank

Voltage regulator

Harmonic filter bank

Cap bank fixes the poor power factor only. It does provide a low impedance path for harmonics but it is not tuned to filter them out. A regulator does not remove the harmonics and neither does it fix the power factor. A filter bank addresses both - it is made up of inductors and capacitors therefore tunable and also capable of releasing VARs. (Note: one manifestation of harmonics is voltage flicker). Learn more.

Cap bank fixes the poor power factor only. It does provide a low impedance path for harmonics but it is not tuned to filter them out. A regulator does not remove the harmonics and neither does it fix the power factor. A filter bank addresses both - it is made up of inductors and capacitors therefore tunable and also capable of releasing VARs. (Note: one manifestation of harmonics is voltage flicker). Learn more.

A transformer develops an internal fault. Which device would you install on the transformer high-side to interrupt fault current?

Disconnect switch

Circuit switcher

Circuit breaker

Either a breaker or a circuit switcher

Both the breaker and the switcher have an interrupting chamber that can safely interrupt the fault current. A switch does not. A switch will draw an arc, that is damaging and a safety hazard. Learn more about switches here and circuit switchers here.

Both the breaker and the switcher have an interrupting chamber that can safely interrupt the fault current. A switch does not. A switch will draw an arc, that is damaging and a safety hazard. Learn more about switches here and circuit switchers here.

Which combination of devices would you use to implement power line carrier?

Wave trap and line tuner

Wave trap, capacitor voltage transformer and line tuner

Wavetrap, capacitor coupled voltage transformer, and line tuner

You need a device that allows a certain frequency to pass (CCVT and line tuner) and a device that stops it (wavetrap). Note a capacitor voltage transformer does not come with coupling accessories as such it does not work for transmitting carrier frequencies.

Click here for more detail on wavetrap and here for CCVT.

You need a device that allows a certain frequency to pass (CCVT and line tuner) and a device that stops it (wavetrap). Note a capacitor voltage transformer does not come with coupling accessories as such it does not work for transmitting carrier frequencies.

Click here for more detail on wavetrap and here for CCVT.

You are building an interconnect station, tying wind farm generation to the power grid. At the substation, the sources are tied through a tie-breaker. Which device would you install to make sure the sources are synchronized?

Current transformer

Power transformer

Ideal transformer

Voltage transformer

A synchronization check is done using a relay. It needs low voltage input (from both sides), obtained using a voltage transformer a.k.a. instrument transformer. Learn more.

A synchronization check is done using a relay. It needs low voltage input (from both sides), obtained using a voltage transformer a.k.a. instrument transformer. Learn more.

What's the purpose of a conventional power transformer?

Transform power, voltage, and current

Transform voltage and current

Transform voltage only

Transforms voltage and current only. Power remains the same on both sides. Learn more.

Transforms voltage and current only. Power remains the same on both sides. Learn more.

In a distribution substation, the available ground-fault current on the low side of the step-down transformer is excessive, over-dutying the switching equipment. What is your cost-effective solution to reducing the fault current?

Add inductor in the transformer neutral

Buy new transformer with higher impedance (%Z)

Add series inductors

Add current limiting fuses in series on the switchgear

All methods mentioned help with limiting the fault current. The cheapest though is a neutral inductor, especially when mounted directly on the transformer. Learn more.

Current limiting fuses directly on the transformer secondary are rare. Mostly because of the continuous current requirement (1000amps or more) - fuses are rarely fabricated this large. This was a trap answer.

All methods mentioned help with limiting the fault current. The cheapest though is a neutral inductor, especially when mounted directly on the transformer. Learn more.

Current limiting fuses directly on the transformer secondary are rare. Mostly because of the continuous current requirement (1000amps or more) - fuses are rarely fabricated this large. This was a trap answer.

For which event should a surge arrester operate? Check all that apply.

Switching surge

Lightning strike

Phase-to-ground short-circuit

Temporary overvoltage

Surge arresters are specified to discharge lightning strikes and switching surges. The equipment/system insulation can handle temporary overvoltage. Learn more.

Surge arresters are specified to discharge lightning strikes and switching surges. The equipment/system insulation can handle temporary overvoltage. Learn more.

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