ABB Relion 670 Series Applications Manual page 212

Section 8
Current protection
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.
8.15.2.1 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:
206
© Copyright 2017 ABB. All rights reserved
1MRK 511 401-UUS F
GUID-9E537AFB-BB23-4431-AC98-D4827A608F81 v2
Bay control REC670
Application manual