A new approach to reduce AC grid for coupled HVDC systems

Abstract

When the dynamic performance study of the HVDC system are carried out or the over voltage characteristics is analysed, electromagnetic transient simulation is needed. The reduction of the AC system is inevitable since the computation burden is huge taking the whole ac grid as well as detailed HVDC models into account. Because the task is to maintain the dynamic response of AC system in a short term and focuses inside the HVDC transmission system, reasonable reduction of the ac grid can be done to simplify this problem without effecting the analysis conclusion. However, the existing approaches to reduce large ac grid aims to analyse the transient stability under asymmetric faults. In these methods, the remaining ac grid is still too large considering full HVDC models which is usually represented by simplified ones. In order to solve the specific aforementioned problem, multi-port Thevenin equivalent is adopted to simplify the original large power grid. By the transient stability simulation and short-circuit calculation, the node impedance matrix can be obtained. According to the admittance matrix obtained by the impedance matrix, the structure of the equivalent system as well as the impedance of the source are determined. The power flow of the equivalent system approximates that of the original system based on the quadratic programming. A UHVDC hierarchically connecting to girds of two voltage levels i.e. 500 kV and 1000 kV is taken as an example, which verifies the validity of the proposed method. Given the relatively small scale of the reduced system, further study on the full HVDC system can be done under computational constraints.