Study on the Operation Area of the 100Hz Protection in an HVDC System

Abstract

The 100Hz protection function in an HVDC system is a backup protection for the ac system. The misoperation of the 100Hz protection has severe effect on the safe and stable operation of the power system. Based on the proposed sequence switching-function model of the HVDC converter, and combined with the short circuit calculation program for the ac system, a practical method of calculation and Identification the operation area in the ac system for the 100Hz protection is present in this paper. The asymmetric switching characteristic of the converter, involving the switching instant deviation and unbalanced commutation overlaps, and effects of the ac background harmonic to the dc side harmonic are considered in this method. The method is verified to be accurate and have great engineering practicability by simulation using the PSCAD/EMTDC software and the 500kV ac power grid in the year 2008. Tianguang HVDC system is caused by operation of the 100Hz protection during the ac fault as the long time delay of the ac system protection(2). Therefore, the area, in which the ac fault occur and can result in operation of the 100Hz protection, must be firstly calculated and indentified so the improvement measure can be performed to prevent misoperations of the 100Hz protection. At present, most of the analyses of protection operation characteristic of the HVDC system are based on digital simulation and dynamic simulation test using the PSCAD/EMTDC and RTDS software. However, in the large scale ac/dc hybird system, as the limitation of the simulation model scale and the calculation time, using time domain simulation software to check every fault locations is infeasible(2). By assuming the 100Hz component of the dc current can be estimated only by the negative voltage of fundamental frequency of converter bus, a method to indentify the operation area of 100Hz protection is present in (2) by a batch calculation to the fault locations in the ac system using the short circuit calculation program. The method greatly improves the calculation efficiency, However, the harmonic interactions between the ac and dc system during the ac fault, such as the effects of the positive voltage, background harmonics and the converter asymmetric switching to the 100Hz current, are not considered in the method as its assumption. In this paper, the switching-function based converter model framed in sequence components is present by considering dynamic characteristics of the converter, such as the switching instants deviation and the unbalanced commutation overlaps. The model can be applied in the analyses in normal and fault conditions. Furthermore, using this model and the ac system short circuit calculation program, 100Hz current in the dc side can be calculated accurately. Therefore, a practical method of quantitative calculation the operation area in the ac system for the 100Hz protection is proposed. This method can be applied in large scale ac/dc hybrid system, reveals the mechanism of harmonic interaction between the ac and dc system by algebra analytic, and establishes the theoretical basis for further studies on the control and protection coordination between the ac and dc system.