Research Of Inverse Time Zero Sequence Current Protection With Introduction Of Speed-changing Factor

Characteristics of inverse time zero sequence current protection agree with characteristics of the load, that is relay protections will trip faults faster when fault current is larger and exerts a greater influence on the power grid. The power grid operation mode has limited impact on inverse time zero sequence current protection, which can act automatically with the power grid operation mode. To solve definite time zero sequence current protection’s selective issues, the most effective and simplest way is to achieve inverse time zero sequence current protection in the whole power grid.Firstly, the major defect of the existing inverse time zero sequence current protection in engineering application is analyzed in this paper. Setting the protection in accordance with the selective coordination requirements makes the calculation process cumbersome and complex, while all the protection in the power systems adopting the same formula and time constant makes difficult to meet the requirements of selectivity. Thus, a new inverse time zero sequence current protection with the introduction of voltage factor is proposed.Though characteristic analysis of a high-impedance ground fault, it is found that the introduction of the voltage factor will make the relay operation delay increases. Therefore the inverse time zero sequence current protection with introduction of voltage factor is amended. And both voltage factor and reactance factor are introduced. Therefore, on the base of that the protection in the power systems adopt the same formula and time constant, the revised protection scheme can not only improve the selectivity, but also has high capacity to tolerance transition resistance.Though fault feature analysis in double-circuit lines, inverse time zero sequence voltage protection based on offset voltage is put forward, which can well solve the grade trip issue.Finally, simulation results of PSCAD and MATLAB verify that compared with traditional inverse time zero sequence current protection, the revised inverse time zero sequence current protection proposed in this paper can greatly improve the selectivity and speed.