Evaluating the impact of increasing system fault currents on protection

Every year the capacity of power systems is increasing, meaning that short circuit current levels are continuously going up. The ratings of instrument transformers at many existing substations that were designed many years ago are now becoming underrated with present fault current levels. Such a situation is obviously concerning to protection engineers who strive to ensure security and dependability of the protection is not jeopardized. Besides risks of trivial CT saturation, which affects the security of the protection system, the concern is if a particular relay is capable of processing such high current, while maintaining adequate accuracy to ensure protection security and dependability. This paper first reviews the causes of increasing short circuit current level in power systems. Some historic and present fault current levels are described. Then, the impacts of increasing short current on protective relays are investigated in detail, in terms of signal processing algorithms, relay clamping level and CT saturation. A thought analysis is presented to examine the security and dependability of a specified transformer differential protection system. Some evaluation techniques and methods are introduced for the purpose of generalized analysis.