The Research On Differential Protection Based On Projection Transform For Transmission Line

The current differential protection, which is immune to many factors, such as, oscillation and non-phase running, etc, is widely utilized in the power system. Traditional current differential protection is severely influenced by the distributed capacitance current. Meanwhile, the imbalance current aroused by line parameter errors and CT transient noises may affect protection sensitivity; even then cause protection mal-operation. With the development of power system, how to improve differential protection dependability and sensitivity is crucial to relaying protection. Hence, a novel differential protection and algorithm based on traveling wave are proposed as well.By employing function space projection theorem, the differential equations of electric elements are transformed to be linear algebraic equations. Then, the discrete time digital equation of transmission line will be obtained. Combined with traveling wave propagation theory, the traveling wave propagation digital equation can be deduced. According to the solution of propagation equation, traveling wave propagation model will be established. At the same time, the real-time online computation method for traveling wave propagation will be presented. The crucial technique lies in how to calculate the propagating traveling waves from the other terminal on-line. The coefficients of propagation function from one terminal to another are obtained by using matrix methods. The results between ATP simulation and practical datum show that, the maximum error is less than 10% when Ts is 1ms, and the maximum error is no more than 6% when Ts is 0.1ms.A new traveling wave based phase differential protective principle is proposed by employing the characteristics of current traveling waves for the distributed parameter line model. There are fixed propagation relations between traveling waves of terminals as the line is healthy or the fault is external, however, the relationship is broken for the internal faults. The protective criterion and scheme are established by this characteristic. The principle and algorithm need not high sampling frequency, are adaptive for not only fundamental component but the transient components, as well as is not sensitive to setting parameters. The large amounts of ATP simulation tests show that the protective scheme and algorithm is simple with high reliability, security, speedy and sensitivity.This paper creatively attempts to study fast protection based on projection transform theory for power system. The tests by ATP simulations prove that novel principle is correct and efficient. These principles and theorems are approved to be the foundation of ultra high-speed protective relaying.