ABB Relion 670 Series Applications Manual page 219

1MRK 505 370-UUS Rev. K
1. Delta-connected banks (generally used only at distribution voltages)
2. Single wye-connected banks
3. Double wye-connected banks
4. H-configuration, where each phase is connected in a bridge
Additionally, the SCB star point, when available, can be either grounded , grounded via impedance or
isolated from ground. Which type of SCB grounding is used depends on voltage level, used circuit
breaker, utility preference and previous experience. Many utilities have standard system grounding
principle to ground neutrals of SCB above 100 kV.
Switching of SCB will produce transients in power system. The transient inrush current during SCB
energizing typically has high frequency components and can reach peak current values, which are
multiples of SCB rating. Opening of capacitor bank circuit breaker may produce step recovery voltages
across open CB contact, which can consequently cause restrikes upon the first interruption of capacitive
current. In modern power system the synchronized CB closing/opening may be utilized in such a manner
that transients caused by SCB switching are avoided.
7.10.2.1 SCB protection
IED protection of shunt capacitor banks requires an understanding of the capabilities and limitations of
the individual capacitor units and associated electrical equipment. Different types of shunt capacitor bank
fusing, configuration or grounding may affect the IED selection for the protection scheme. Availability and
placement of CTs and VTs can be additional limiting factor during protection scheme design.
SCB protection schemes are provided in order to detect and clear faults within the capacitor bank itself or
in the connected leads to the substation busbar. Bank protection may include items such as a means to
disconnect a faulted capacitor unit or capacitor element(s), a means to initiate a shutdown of the bank in
case of faults that may lead to a catastrophic failure and alarms to indicate unbalance within the bank.
Capacitor bank outages and failures are often caused by accidental contact by animals. Vermin,
monkeys, birds, may use the SCB as a resting place or a landing site. When the animal touches the HV
live parts this can result in a flash-over, can rapture or a cascading failures that might cause extensive
damages, fire or even total destruction of the whole SCB, unless the bank is sufficiently fitted with
protection IEDs.
In addition, to fault conditions SCB can be exposed to different types of abnormal operating conditions. In
accordance with IEC and ANSI standards capacitors shall be capable of continuous operation under
contingency system and bank conditions, provided the following limitations are not exceeded:
1. Capacitor units should be capable of continuous operation including harmonics, but excluding
transients, to 110% of rated IED root-mean-square (RMS) voltage and a crest voltage not exceeding
of rated RMS voltage. The capacitor should also be able to carry 135% of nominal current. The
voltage capability of any series element of a capacitor unit shall be considered to be its share of the
total capacitor unit voltage capability.
2. Capacitor units should not give less than 100% nor more than 110% of rated reactive power at rated
sinusoidal voltage and frequency, measured at a uniform case and internal temperature of 25°C.
3. Capacitor units mounted in multiple rows and tiers should be designed for continuous operation for a
24h average temperature of 40 °C during the hottest day, or −40 °C during the coldest day expected
at the location.
4. Capacitor units should be suitable for continuous operation at up to 135% of rated reactive power
caused by the combined effects of:
Busbar protection REB670
Application manual
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Section 7
Current protection
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