Scb Protection - ABB REC670 Applications Manual

1MRK 511 401-UEN A
8.15.2.1
Bay control REC670 2.2 IEC
Application manual
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 directly earthed ,
earthed via impedance or isolated from earth. Which type of SCB earthing is used
depends on voltage level, used circuit breaker, utility preference and previous
experience. Many utilities have standard system earthing principle to earth 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 earthing 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:
Section 8
Current protection
GUID-9E537AFB-BB23-4431-AC98-D4827A608F81 v2
217