A novel criterion based on the comparison of components of the phase voltage and the differential current for converter transformer protection in hvdc

Summary

With the construction of HVDC in China, the capacity of the converter transformer as the core component of the DC converter is increasing, and its operating characteristics are particularly noteworthy. To improve the reliability of the differential protection of the converter transformers, a comprehensive main protection scheme for converter transformer is proposed. Firstly, according to the characteristic of transformer core, there should be a time interval between the sudden change of phase voltage and the emergence of differential current, which can be utilized to distinguish between the external and the internal faults. A new time-interval based relay criterion for converter transformer differential protection is proposed to identify transformer fault type by virtue of highly accurate time interval. Then, a novel preset filter algorithm based on discriminants is proposed to determine the time interval, which focuses on the superimposed component in time domain. Next, a comprehensive protection scheme is designed on the basis of time-difference based method, together with second harmonic restraint criterion and cross-country fault identification criterion. The main criterion will reach two results, internal fault and non-internal fault. For the first result, the protection will trip and the judgment process will finish. For the other result, the judgment may go to one of two sub-criteria according to the residual voltage level. If the residual voltage is relatively low, it should be determined that this scenario results from a through fault. Otherwise, this scenario should result from the converter transformer energization. Theoretical analysis and simulation test results show that the proposed scheme can deal with a variety of complicated scenarios, and can guarantee the high-speed response to the internal fault.

Additional informations

Keywords

differential relay, time difference, superimposed component, filter algorithm, comprehensive protection scheme