Shunt Capacitor Bank Fundamentals and Design

Posted on December 22, 2017 in Capacitors, Substation by With 0 Comment
Shunt Capacitor Bank Fundamentals and Design

Shunt Capacitor Bank Fundamentals and Design (Credit: www.becker-mining.com)

Shunt capacitor banks are used to improve the quality of the electrical supply and the efficient operation of the power system. Shunt capacitor banks are relatively inexpensive and can be easily installed anywhere on the network. Shunt capacitor banks are mainly installed to provide capacitive reactive compensation/power factor correction. Its installation has other beneficial effects on the system such as: 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 Bank benefits to:

  • Increase power transmission capability
  • Improve system stability
  • Reduce system losses
  • Improve voltage profile on the lines
  • Optimize power flow between parallel lines

The Capacitor Unit and Bank Configurations

The Capacitor Unit

The capacitor Unit

Figure 1 – The capacitor Unit

The capacitor unit, Fig. 1, 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. The internal discharge device is a resistor that reduces the unit residual voltage to 50V or less in 5 min. Capacitor units are available in a variety of voltage ratings (240 V to 24940V) and sizes (2.5 kvar to about 1000 kvar).

Capacitor unit capabilities

Relay protection of shunt capacitor banks requires some knowledge of the capabilities and limitations of the capacitor unit and associated electrical equipment including: individual capacitor unit, bank switching devices, fuses, voltage and current sensing devices.

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 of them (kVar~2π f V2). 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:

  • 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.
  • Capacitors units should not give less than 100% nor more than 115% of rated reactive power at rated sinusoidal voltage and frequency.
  • 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.

Bank Configurations

The use of fuses for protecting the capacitor units and it location (inside the capacitor unit on each element or outside the unit) is an important subject in the design of Shunt capacitor banks. They also affect the failure mode of the capacitor unit and influence the design of the bank protection. Depending on the application any of the following configurations are suitable for shunt capacitor banks:

a) Externally Fused

An individual fuse, externally mounted between the capacitor unit and the capacitor bank fuse bus, typically protects each capacitor unit. The capacitor unit can be designed for a relatively high voltage because the external fuse is capable of interrupting a high-voltage fault. Use of capacitors with the highest possible voltage rating will result in a capacitive bank with the fewest number of series groups.

A failure of a capacitor element welds the foils together and short circuits the other capacitor elements connected in parallel in the same group. The remaining capacitor elements in the unit remain in service with a higher voltage across them than before the failure and an increased in 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 bank and capacitor unit

Figure 2 – Externally fused shunt capacitor bank and capacitor unit

Externally fused Shunt capacitor banks are configured using one or more series groups of parallel-connected capacitor units per phase (Fig. 2). 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 number of parallel units are required to allow the bank to remain in service with one fuse or unit out.

b) Internally Fused

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

Internally fused shunt capacitor bank and capacitor unit

Figure 3 – Internally fused shunt capacitor bank and capacitor unit

Fig. 3 illustrates a typical capacitor bank utilizing internally fused capacitor units. In general, banks employing internally fused capacitor units are configured with fewer 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.

c) 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, shown in Fig. 4.

Fuseless shunt capacitor bank and series string

Figure 4 – Fuseless shunt capacitor bank and series string

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. For example, is there are 6 capacitor units in series and each unit has 8 element groups in series there is a total of 48 element groups in series. If one capacitor element fails, the element is shortened and the voltage on the remaining elements is 48/47 or about a 2% increase in the voltage. The capacitor bank continues in service; however, successive failures of elements will lead to the removal of the bank.

The fuse less design is not usually applied for system voltages less than about 34.5 kV. The reason is that there shall be more than 10 elements in series so that the bank does not have to be removed from service for the failure of one element because the voltage across the remaining elements would increase by a factor of about E (E – 1), where E is the number of elements in the string. The discharge energy is small because no capacitor units are connected directly in parallel. Another advantage of fuse less banks is that the unbalance protection does not have to be delayed to coordinate with the fuses.

d) Unfused Shunt Capacitor Banks

Contrary to the fuse less configuration, where the units are connected in series, the unfused shunt capacitor bank uses a series/parallel connection of the capacitor units. The unfused approach would normally be used on banks below 34.5 kV, where series strings of capacitor units are not practical, or on higher voltage banks with modest parallel energy. This design does not require as many capacitor units in parallel as an externally fused bank.

Capacitor Bank Design

The protection of shunt capacitor banks requires understanding the basics of capacitor bank design and capacitor unit connections. Shunt capacitors banks are arrangements of series/paralleled connected units. Capacitor units connected in paralleled make up a group and series connected groups form a single-phase capacitor bank. As a general rule, the minimum number of units connected in parallel is such that isolation of one capacitor unit in a group should not cause a voltage unbalance sufficient to place more than 110% of rated voltage on the remaining capacitors of the group. Equally, the minimum number of series connected groups is that in which the complete bypass of the group does not subject the others remaining in service to a permanent overvoltage of more than 110%.

The maximum number of capacitor units that may be placed in parallel per group is governed by a different consideration. When a capacitor bank unit fails, other capacitors in the same parallel group contain some amount of charge. This charge will drain off as a high frequency transient current that flows through the failed capacitor unit and its fuse. The fuse holder and the failed capacitor unit should withstand this discharge transient.
The discharge transient from a large number of paralleled capacitors can be severe enough to rupture the failed capacitor unit or the expulsion fuse holder, which may result in damage to adjacent units or cause a major bus fault within the bank.

To minimize the probability of failure of the expulsion fuse holder, or rupture of the capacitor case, or both, the standards impose a limit to the total maximum energy stored in a paralleled connected group to 4659 kVar. In order not to violate this limit, more capacitor groups of a lower voltage rating connected in series with fewer units in parallel per group may be a suitable solution. However, this may reduce the sensitivity of the unbalance detection scheme. Splitting the bank into 2 sections as a double Y may be the preferred solution and may allow for better unbalance detection scheme. Another possibility is the use of current limiting fuses.

The optimum connection for a Shunt capacitor banks depends on the best utilization of the available voltage ratings of capacitor units, fusing, and protective relaying. Virtually all substation banks are connected wye. Distribution capacitor banks, however, may be connected wye or delta. Some banks use an H configuration on each of the phases with a current transformer in the connecting branch to detect the unbalance.

Grounded Wye-Connected Banks

Grounded Wye Shunt Capacitor Banks

Figure 5 – Grounded Wye Shunt Capacitor Banks

Grounded wye capacitor banks are composed of series and parallel-connected capacitor units per phase and provide  low impedance path to ground. Fig. 5 shows typical bank arrangements.

Advantages of the grounded capacitor banks include:

  • Its low-impedance path to ground provides inherent self-protection for lightning surge currents and give some protection from surge voltages. Banks can be operated without surge arresters taking advantage of the capability of the capacitors to absorb the surge.
  • Offer a low impedance path for high frequency currents and so they can be used as filters in systems with high harmonic content. However, caution shall be taken to avoid resonance between the Shunt capacitor banks and the system.
  • Reduced transient recovery voltages for circuit breakers and other switching equipment.

Some drawbacks for grounded wye Shunt capacitor banks are:

  • Increased interference on telecom circuits due to harmonic circulation.
  • Circulation of inrush currents and harmonics may cause misoperations and/or over operation on protective relays and fuses.
  • Phase series reactors are required to reduce voltages appearing on the CT secondary due to the effect of high frequency, high amplitude currents.

Multiple Units in Series Phase to Ground – Double Wye

When a capacitor bank becomes too large, making the parallel energy of a series group too great (above 4650 kvar) for the capacitor units or fuses, the bank may be split into two wye sections.

The characteristics of the grounded double wye are similar to a grounded single wye bank. The two neutrals should be directly connected with a single connection to ground.

The double Wye design allows a secure and faster unbalance protection with a simple uncompensated relay because any system zero sequence component affects both wyes equally, but a failed capacitor unit will appear as un unbalanced in the neutral. Time coordination may be required to allow a fuse, in or on a failed capacitor unit, to blow. If it is a fuse less design, the time delay may be set short because no fuse coordination is required. If the current through the string exceeds the continuous current capability of the capacitor unit, more strings shall be added in parallel.

Ungrounded Wye-Connected Banks

Ungrounded Wye Shunt Capacitor Banks

Figure 6 – Ungrounded Wye Shunt Capacitor Banks

Typical bank arrangements of ungrounded Wye Shunt capacitor banks are shown in Fig. 6. Ungrounded wye banks do not permit zero sequence currents, third harmonic currents, or large capacitor discharge currents during system ground faults to flow. (Phase-to-phase faults may still occur and will result in large discharge currents). Other advantage is that over voltages appearing at the CT secondaries are not as high as in the case of grounded banks. However, the neutral should be insulated for full line voltage because it is momentarily at phase potential when the bank is switched or when one capacitor unit fails in a bank configured with a single group of units. For banks above 15kV this may be expensive.

a) Multiple Units in Series Phase to Neutral – Single Wye

Capacitor units with external fuses, internal fuses, or no fuses (fuse less or unfused design) can be used to make up the bank. For unbalance protection schemes that are sensitive to system voltage unbalance, either the unbalance protection time delay shall be set long enough for the line protections to clears the system ground faults or the capacitor bank may be allowed to trip off for a system ground fault.

b) Multiple units in series phase to neutral-double wye

When a capacitor bank becomes too large for the maximum 4650 kvar per group the bank may be split into two wye sections. When the two neutrals are ungrounded, the bank has some of the characteristics of the ungrounded single-wye bank. These two neutrals may be tied together through a current transformer or a voltage transformer. As for any ungrounded why bank, the neutral instrument transformers should be insulated from ground for full line-to-ground voltage, as should the phase terminals.

Delta-connected Banks

Delta-connected banks are generally used only at distributions voltages and are configured with a single series group of capacitors rated at line-to-line voltage. With only one series group of units no overvoltage occurs across the remaining capacitor units from the isolation of a faulted capacitor unit.

H Configuration

Some larger banks use an H configuration in each phase with a current transformer connected between the two legs to compare the current down each leg. As long as all capacitors are normal, no current will flow through the current transformer. If a capacitor fuse operates, some current will flow through the current transformer. This bridge connection can be very sensitive. This arrangement is used on large banks with many capacitor units in parallel.

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Hello, I'm Kalpesh, An Electrical Engineer and the founder of Substation System.

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