Analysis Of The Risk Of Subsynchronous Oscillation In Two DC Converter Station System

Abstract:The grid is gradually developing into AC/DC hybrid power system. AC-DC hybrid transmission brings great benefits, it will also bring subsynchronous oscillation at the same time. Therefore, the study of subsynchronous oscillation caused by HVDC system is of great significance. With the development of HVDC transmission technology, there may be two or more of the DC transmission line in the same area. There is a case that units have a strong electrical contact with two or more converter stations simultaneously, then factors affecting unit sub-synchronous oscillation risk is more complex, so the research of subsynchronous oscillation in two DC-DC converter station system is particularly important.An equivalence approach is put forward in this paper for analyzing Sub-synchronous oscillations (SSO) of identical generators operating in parallel. This method is based on the principle that the similar matrices have the identical eigenvalues. By constructing an appropriate transformation matrix, the state matrix of identical N-machine systems can be similar to a block diagonal matrix. Therefore, the SSO analysis of N-machine systems can be conducted based on N single-machine systems of two types. Moreover, the modeling method of the two types of single-machine systems is presented. By this equivalent method, the dimension of multi-machine systems can be reduced efficiently, while no detail of the system’s eigenvalue information is lost. The theoretical analysis and calculated results show the validity and the practicability of the full equivalent simplification method.Complex torque coefficient method is used to analyze thermal power unit sub-synchronous oscillations risk caused by two DC Converter Station System. The damping characteristics of two DC Converter Station System is studied. It analyzes the electrical damping characteristics of AC/DC parallel operation and DC islanded operation, and summarizes the influence of electrical parameters to electrical damping characteristics.Finally, this paper proposes a equivalent simplified method for two DC converter station system sub-synchronous oscillations modeling. This method is mainly used in the network structure of two mutually symmetrical DC converter system. Electrical damping characteristics remain the same by this method.