Industrial power system protection against transmission system blackouts

This paper discusses the implementation aspects of a protection scheme against transmission system blackouts that can be used for industrial customers that have onsite generation. Recognizing the complex patterns that characterize the cascaded events leading to power system blackouts, the scheme employs a combination of underfrequency and undervoltage relays. This combination does not use interlocking arrangements but requires a protection coordination study taking into account voltage and frequency variation patterns that may be similar during normal operation with those that occur in blackout situations. The discrimination features of the scheme, required for avoiding unwanted separation from the transmission system are discussed. Two set-ups of the separation scheme are presented assuming different contractual arrangements between the industrial customer and the transmission network operator concerning the frequency support in case of system stressed operation. The relay settings for both set-ups are indicated together with simulation results from several transient stability studies that demonstrate the suitability and effectiveness of the proposed schemes.