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Providing capacitive reactive compensation with shunt capacitor banks" alt="Providing capacitive reactive compensation with shunt capacitor banks
Providing capacitive reactive compensation with shunt capacitor banks” alt=”Providing capacitive reactive compensation with shunt capacitor banks

Few words about shunt capacitor banks

Shunt capacitor banks are mainly installed to provide capacitive reactive compensationpower factor correction. The use of SCBs has increased because they are relatively inexpensive, easy and quick to install and can be deployed virtually anywhere in the network.

Its installation has other beneficial effects on the system such is: improvement of the voltage at the load, better voltage regulation (if they were adequately designed), reduction of losses and reduction or postponement of investments in transmission.

The main disadvantage of shunt capacitor banks is that its reactive power output is proportional to the square of the voltage and consequently when the voltage is low and the system need them most, they are the least efficient.

Capacitor unit

Capacitor unit
Capacitor unit

The capacitor unit is the building block of a shunt capacitor bank. The capacitor unit is made up of individual capacitor elements, arranged in parallel/series connected groups within a steel enclosure. Each capacitor unit is provided with a discharge resistor that reduces the unit residual voltage to 50V in 5 minutes.

Capacitor units are available in variable voltages and sizes.

Capacitors are intended to be operated at or below their rated voltage and frequency as they are very sensitive to these values; the reactive power generated by a capacitor is proportional to both voltage and frequency (kVAr=2πfv2).

The IEEE Std 18-1992 and Std 1036-1992 specify the standard ratings of the capacitors designed for shunt connection to ac systems and also provide application guidelines.

These standards stipulate that:

  1. Capacitor units should be capable of continuous operation up to 110% of rated terminal rms voltage and a crest voltage not exceeding 1.2 x √2 of rated rms voltage, including harmonics but excluding transients. The capacitor should also be able to carry 135% of nominal current.
  2. Capacitors units should not give less than 100% nor more than 115% of rated reactive power at rated sinusoidal voltage and frequency.
  3. Capacitor units should be suitable for continuous operation at up to 135%of rated reactive power caused by the combined effects of:
    1. Voltage in excess of the nameplate rating at fundamental frequency, but not over 110% of rated rms voltage.
    2. Harmonic voltages superimposed on the fundamental frequency.
    3. Reactive power manufacturing tolerance of up to 115% of rated reactive power.

Capacitor bank configuration

The use of fuses for protecting the capacitor units and its location (inside the capacitor unit on each element or outside the unit) is an important subject in the design of capacitor banks. They also affect the failure mode of the capacitor unit and influence the design of the bank protection.

  1. Externally fused capacitor unit/bank
  2. Internally fused capacitor unit/bank
  3. Fuse-less shunt capacitor banks

1. Externally Fused Capacitor Unit/Bank

An individual fuse, externally mounted between the capacitor unit and the capacitor bank fuse bus, typically protects each capacitor unit. A failure of a capacitor element welds the foils together and short circuits the other capacitor elements connected in parallel in the same group.

Externally Fused Capacitor Unit/Bank
Externally Fused Capacitor Unit/Bank

The remaining capacitor elements in the unit remain in service with a higher voltage across them than before the failure and an increased capacitor unit current. If a second element fails the process repeats itself resulting in an even higher voltage for the remaining elements.

Successive failures within the same unit will make the fuse to operate, disconnecting the capacitor unit and indicating the failed one.

Externally fused Shunt Capacitor Banks are configured using one or more series groups of parallel-connected capacitor units per phase. The available unbalance signal level decreases as the number of series groups of capacitors is increased or as the number of capacitor units in parallel per series group is increased.

However, the kiloVAr rating of the individual capacitor unit may need to be smaller because a minimum of parallel units are required to allow the bank to remain in service with one fuse or unit out.

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2. Internally Fused Capacitor Unit/Bank

Each capacitor element is fused inside the capacitor unit. The fuse is a simple piece of wire enough to limit the current and encapsulated in a wrapper able to withstand the heat produced by the arc. Upon a capacitor element failure, the fuse removes the affected element only.

Internally Fused Capacitor Unit/Bank
Internally Fused Capacitor Unit/Bank

The other elements, connected in parallel in the same group, remain in service but with a slightly higher voltage across them.

In general, banks employing internally fuses capacitor units are configured with few capacitor units in parallel and more series groups of units than are used in banks employing externally fused capacitor units. The capacitor units are normally large because a complete unit is not expected to fail.

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3. Fuse-less Shunt Capacitor Banks

The capacitor units for fuse-less capacitor banks are identical to those for externally fused described above. To form a bank, capacitor units are connected in series strings between phase and neutral.

Fuse-less Shunt Capacitor Banks
Fuse-less Shunt Capacitor Banks

The protection is based on the capacitor elements (within the unit) failing in a shorted mode, short-circuiting the group.

When the capacitor element fails it welds and the capacitor unit remains in service. The voltage across the failed capacitor element is then shared among all the remaining capacitor element groups in the series.

EXAMPLE //

For example, if there are 7 capacitor units in series and each unit has 10 element groups in series then there are a total of 70 element groups in series.

If one capacitor element fails, the element is shortened and the voltage on the remaining elements is 70/69 or about a 1.5% increase in the voltage. The capacitor bank continues in service; however successive failures of elements will lead to the removal of the bank.

Manufacturer’s experience is that for modern capacitor units all element failures result in strong gas-free welded short circuits on the elements.

Damaged Internally Fused Capacitor Units
Damaged Internally Fused Capacitor Units

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References:
  • Substation design / Application guide – V AYADURAI BSC, C.Eng, FIEE Engineering Expert
  • Shunt Capacitor Bank Fundamentals and Protection – Gustavo Brunello, Dr. Bogdan Kasztenny and Craig Wester (GE Multilin, Canada)

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

4 Comments


  1. Ming Qi
    May 26, 2015

    The statement “If one capacitor element fails, the element is shortened and the voltage on the remaining elements is 70/69 or about a 1.5% increase in the voltage.” is wrong. The right answer is that if one capacitor element fails, the element is shortened and the voltage on the remaining elements is 7/6 or about a 16.7% increase in the voltage.


  2. akash Kumar Sharma
    Dec 15, 2014

    It’s a really good to hear about machinery topics in this website.


  3. Anand
    Dec 10, 2014

    Required newsletter for latest update


  4. Ahmad Azab
    Dec 05, 2014

    good particle
    i think these capacitors should be near the factory not in the power station to minimize losses in transmission line.
    but i still think that shunt capacitor is a better choice than series one ,Although it is more expensive for the same reactive power , but placing it in the ground level ,failure causing only voltage reduction , gradual increase in the voltage and low current is all advantages in shunt capacitors.

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