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principle  Of back-up protection

       The word backup protection is often heard in the power system, which is a specific noun in relay protection.The function of it is to remove the circuit breaker when the main protection or circuit breaker is refused, so as to avoid the expansion fault.back up protection is also divided into local back up and remote back up protection. local backup protection is  that reach the trip function by near power system or the protection of the circuit when the protection who protect the microcomputer refuse to trip.When the circuit breaker refused to trip, the circuit breaker failure protection will also be used as backup protection.

      there is also a special backup microcomputer protection exist  in substation and power generation. For transformer, when the  rated capacity is relatively large and need to equipped with differential protection of the main transformer, high voltage backup protection, medium voltage backup protection, low voltage backup protection and non electricity protection.the back up protection protect the main transformer high voltage side, medium and low voltage side, as well as exsit as the backup protection of the main transformer.When the transformer main protection differential protection or non electric power protection device trip but the circuit breaker refused to trip,the backup protection will issue a trip command, quickly remove the circuit fault.
      For the generator there are also with the generator differential protection, generator rotor grounding protection, and generator backup protection, when the generator differential protection and generator rotor grounding protection refused to trip ,the backup protection will trip.These are all as far back as most of the conventional microcomputer protection, there will be a current instantaneous trip protection and over current protection.This current instantaneous trip protection protection is no time limit over current protection or the time is very short, when the fault occurs, such as the phase of the short circuit, at this time the short circuit current is very large, then the quick break protection will trip.If the current instantaneous trip protection does not trip, then the over current protection will trip, the general over current protection action time is about 0.5S, it is as current instantaneous trip protection of the back up protection.
      backup protection makes the whole power plant protection more perfect,and  fill the gaps. At present market also appeared in the differential backup one microcomputer protective device, such as 9688C, 9671C. These devices is the differential and reserve in one, saving a microcomputer protection, at the same time the cost of the transformer will also be reflect
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