Characteristic Analysis and Calculation of Frequencies of Voltages in Out-of-Step Oscillation Power System and a Frequency-Based Out-of-Step Protection

During the out-of-step oscillation process, the frequency of voltage at every point in power system changes continuously. In this paper, the voltage frequency variation regulation is analyzed in an equivalent two-terminal system whose two-terminal potential amplitudes are not equal and all electrical component impedance angles are different. It finds the bus voltage frequencies at different sides of out-of-step center own different characteristics. In order to use the frequency variation characteristics to construct the criterion of power system out-of-step protection, a fast frequency calculation algorithm is proposed, in which the frequency is calculated by toggling different formulas at different sampling points and the calculation accuracy is improved by the aid of coordinate rotation digital computer (CORDIC) algorithm. The algorithm is easy to implement and measurement errors can keep under a low level. Finally, a novel frequency based out-of-step protection with wide area information is described. It can judge the occurrence of out-of-step oscillation in the first out-of-step oscillation cycle and determine the electrical component where the out-of-step center locates, which lays the foundation for the determination of out-of-step splitting section for complex power systems. The test results with IEEE39 system in PSCAD verify the correctness of the proposed protection.