bush bar protection-lecture notes

The protection scheme for a power system should cover the whole system against all probable types of fault. Unrestricted forms of line protection, such as overcurrent and distance systems, meet this requirement, although faults in the busbar zone are cleared only after some time delay. But if unit protection is applied to feeders and plant, the busbars are not inherently protected. Busbars have often been left without specific protection, for one or more of the following reasons:
a. the busbars and switchgear have a high degree of
reliability, to the point of being regarded as
intrinsically safe
b. it was feared that accidental operation of busbar
protection might cause widespread dislocation of
the power system, which, if not quickly cleared,
would cause more loss than would the very
infrequent actual bus faults
c. it was hoped that system protection or back-up
protection would provide sufficient bus protection
if needed
It is true that the risk of a fault occurring on modern metal-clad gear is very small, but it cannot be entirely ignored. However, the damage resulting from one uncleared fault, because of the concentration of fault MVA, may be very extensive indeed, up to the complete loss of the station by fire. Serious damage to or destruction of the installation would probably result in widespread and prolonged supply interruption. Finally, system protection will frequently not provide the cover required. Such protection may be good enough for small distribution substations, but not for important stations. Even if distance protection is applied to all feeders, the busbar will lie in the second zone of all the distance protections, so a bus fault will be cleared relatively slowly, and the resultant duration of the voltage dip imposed on the rest of the system may not be tolerable.

The lecture notes