Quantcast High-Voltage Circuit Breakers

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Electrical Distribution Systems CIRCUIT BREAKERS High-Voltage Circuit Breakers High-voltage  circuit  breakers  (including  breakers  rated  at  intermediate  voltage)  are  used  for service on circuits with voltage ratings higher than 600 volts.  Standard voltage ratings for these circuit breakers are from 4,160 to 765,000 volts and three-phase interrupting ratings of 50,000 to 50,000,000 kVA. In  the  early  stages  of  electrical  system  development,  the  major  portion  of  high-voltage  circuit breakers  were  oil  circuit  breakers.    However,  magnetic  and  compressed-air  type  air  circuit breakers have been developed and are in use today. The magnetic air circuit breaker is rated up to 750,000 kVA at 13,800 volts.  This type of circuit breaker interrupts in air between two separable contacts with the aid of magnetic blowout coils. As   the   current-carrying   contacts   separate   during   a   fault   condition,   the   arc   is   drawn   out horizontally and transferred to a set of arcing contacts.  Simultaneously, the blowout coil provides a magnetic field to draw the arc upward into the arc chutes.   The arc, aided by the blowout coil magnetic field and thermal effects, accelerates upward into the arc chute, where it is elongated and divided into many small segments. The construction of this type of circuit breaker is similar to that of a large air circuit breaker used for low-voltage applications, except that they are all electrically operated. Compressed-air circuit breakers, or air-blast circuit breakers, depend on a stream of compressed air  directed  toward  the  separable  contacts  of  the  breaker  to  interrupt  the  arc  formed  when  the breaker  is  opened.    Air-blast  circuit  breakers  have  recently  been  developed  for  use  in  extra high-voltage applications with standard ratings up to 765,000 volts. Oil circuit breakers (OCBs) are circuit breakers that have their contacts immersed in oil.  Current interruption takes place in oil which cools the arc developed and thereby quenches the arc.  The poles of small oil circuit breakers can be placed in one oil tank; however, the large high-voltage circuit breakers have each pole in a separate oil tank.   The oil tanks in oil circuit breakers are normally sealed.  The electrical connections between the contacts and external circuits are made through porcelain bushings. Circuit Breaker Control As  we  have  discussed,  circuit  breakers  may  be  remotely  operated.    In  order  to  operate  the breakers from a remote location, there must be an electrical control circuit incorporated.  Figure 7 shows a simple control circuit for a remotely-operated breaker. Control power is supplied by an AC source and then rectified to DC.  The major components of a simple control circuit are:  the rectifier unit, the closing relay, the closing coil, the tripping coil, the auxiliary contacts, and the circuit breaker control switch. Rev. 0 Page 11 ES-15


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