Additionally, the SCB star point, when available, can be either directly earthed , earthed viaimpedance or isolated from earth. Which type of SCB earthing is used depends on voltagelevel, used circuit breaker, utility preference and previous experience. Many utilities havestandard system earthing principle to earth neutrals of SCB above 100 kV.Switching of SCB will produce transients in power system. The transient inrush current duringSCB 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 steprecovery voltages across open CB contact, which can consequently cause restrikes upon thefirst 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.7.15.2.1 SCB protection GUID-9E537AFB-BB23-4431-AC98-D4827A608F81 v2IED protection of shunt capacitor banks requires an understanding of the capabilities andlimitations of the individual capacitor units and associated electrical equipment. Differenttypes of shunt capacitor bank fusing, configuration or earthing may affect the IED selectionfor the protection scheme. Availability and placement of CTs and VTs can be additional limitingfactor during protection scheme design.SCB protection schemes are provided in order to detect and clear faults within the capacitorbank itself or in the connected leads to the substation busbar. Bank protection may includeitems such as a means to disconnect a faulted capacitor unit or capacitor element(s), a meansto initiate a shutdown of the bank in case of faults that may lead to a catastrophic failure andalarms 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 animaltouches the HV live parts this can result in a flash-over, can rapture or a cascading failures thatmight cause extensive damages, fire or even total destruction of the whole SCB, unless thebank is sufficiently fitted with protection IEDs.In addition, to fault conditions SCB can be exposed to different types of abnormal operatingconditions. In accordance with IEC and ANSI standards capacitors shall be capable ofcontinuous operation under contingency system and bank conditions, provided the followinglimitations are not exceeded:1. Capacitor units should be capable of continuous operation including harmonics, butexcluding transients, to 110% of rated IED root-mean-square (RMS) voltage and a crestvoltage not exceeding of rated RMS voltage. The capacitor should also be able to carry135% of nominal current. The voltage capability of any series element of a capacitor unitshall 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 reactivepower at rated sinusoidal voltage and frequency, measured at a uniform case and internaltemperature of 25°C.3. Capacitor units mounted in multiple rows and tiers should be designed for continuousoperation for a 24h average temperature of 40 °C during the hottest day, or −40 °C duringthe coldest day expected at the location.4. Capacitor units should be suitable for continuous operation at up to 135% of ratedreactive power caused by the combined effects of:• Voltage in excess of the nameplate rating at fundamental frequency, but not over110% of rated RMS voltage• Harmonic voltages superimposed on the fundamental frequency• Reactive power manufacturing tolerance of up to 115% of rated reactive power5. Capacitor units rated above 600 V shall have an internal discharge device to reduce theresidual voltage to 50 V or less in 5 or 10 minutes (depending on national standard).Note that capacitor units designed for special applications can exceed these ratings.Section 7 1MRK 511 358-UEN ACurrent protection166Application manual
