A transmission line differential protection algorithm immune to high degree of CT saturation

Current transformers are vital to the performance of most of the power system protection relays. In particular, their performance during faults is very critical for the operation of differential protection relays. Designing an optimum current transformer that is economical and does not affect the performance of the protection relay has been a challenge to power system protection engineers. Although a current transformer designed not to saturate for the worst fault will ensure proper operation of the relay, it can often result in very high cost and size. Thus it is important to study the performance of current transformers during fault transients and to design protection relay algorithms that would accommodate a certain degree of current transformer saturation.;The focus of this research is to study the performance of various line differential protection algorithms during CT saturation and to develop a new algorithm with better performance and lower current transformer requirement. This research includes a detailed study of the protection class CT and techniques used in commercial algorithms to handle CT saturation. Then a new line current differential protection algorithm is developed. Unlike the usual protection algorithms which rely on the value of input phasors after the transient response of the digital filter, the proposed current differential protection algorithm adopts a new approach of using the transient response of the full cycle DFT filter to confirm the zone of fault. This approach provides a very fast decision, thus accommodating CT saturation as early as half cycle after fault inception. This fast decision making capability also allows implementing a truly adaptive biased differential characteristic, thereby achieving the difficult protection objective of providing high security performance without compromising on dependability.;This novel approach adopted in the proposed algorithm opens up a totally new dimension in differential protection algorithm design. This approach creates the possibility of achieving sub-cycle operation even with the conventional one cycle DFT filter based phasor estimation. Though the proposed algorithm is developed and tested for line current differential application, this technique can be adapted for use in other differential protection applications such as, transformer, reactor, bus-bar, motor, etc.;Key words. Power system protection, protection relay, protection algorithm, line current differential protection, transmission line, stability, sensitivity, dual biased differential characteristics, alpha plane characteristics, current transformer, phasor estimation, DFT, CT saturation, transient dimensioning factor, CT dimensioning, knee point voltage, CT saturation detector.