Which type of fusing is used may depend on can manufacturer or utility preference and
previous experience.
Because the SCBs are built from the individual capacitor units the overall connections
may vary. Typically used SCB configurations are:
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.
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
Section 3 1MRK504116-UUS C
IED application
496
Application manual
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