What is Shunt Capacitor Bank? 4 SCB Configurations

A shunt capacitor bank is an array of multiple capacitor units combined in series and parallel connections to meet overall system needs. These units are commonly housed in a metallic frame, where each level is referred to as a block.

Shunt capacitor banks (SCB) are mainly installed in substations to provide capacitive reactive compensation/ power factor correction.

The use of SCB has increased because they are relatively inexpensive, easy and quick to install and can be deployed virtually anywhere in the network.

The role of SCBs increased recently in the light of blackout prevention activities, and increasing penetration of distributed generation, wind farms in particular, which add generation without addressing the problem of reactive power support.

Moreover, capacitor banks are valuable assets that must be available for the daily demands of system operation and must provide reliable operation through abnormal power system scenarios.

Benefits of Installing Shunt Capacitor Bank:

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
• reduction or postponement of investments in transmission.

Disadvantage of SCB:

The main disadvantage of SCB 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.

The 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.

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).

Protection engineering for shunt capacitor banks requires knowledge of the capabilities and limitations of the capacitor unit and associated electrical equipment including individual capacitor unit, bank switching devices, fuses, location and type of voltage and current instrument transformers.

Capacitor 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.

There are generally four types of the capacitor unit designs to consider.

1. Externally Fused
2. Internally Fused
3. Fuseless Shunt Capacitor Banks
4. Unfused Shunt Capacitor Banks

1. Externally Fused Capacitor Banks

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 SCBs 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 number of parallel units are required to allow the bank to remain in service with one fuse or unit out.

2. Internally Fused Capacitor Banks

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.

Figure 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.

3. Fuseless Capacitor Bank

Fuseless Capacitor Bank designs are typically the most prevalent designs in modern day. The capacitor units for fuseless capacitor banks are connected in series strings between phase and neutral, as shown in the figure. The higher the voltage for the bank, the more capacitor elements in series.

The expected failure of the capacitor unit element is a short circuit, where the remaining capacitor elements will absorb the additional voltage.

For example, if 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 the string. If one capacitor element fails, this element is shorted and the voltage across the remaining elements is 48/47 of the previous value, or about 2% higher. The capacitor bank remains in service; however, successive failures of elements would aggravate the problem and eventually lead to the removal of the bank.

The fuseless design is usually applied for applications at or above 34.5kV where each string has more than 10 elements in series to ensure the remaining elements do not exceed 110% rating if an element in the string shorts.

4. Unfused Capacitors

Contrary to the fuseless 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 34.5kV, 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.