Studies On Hybrid HVDC System And Its Control Strategies Used For Grid Integration Of Wind Power

To foster the sustainable development of economics and environment in China, it is necessary to make sure that there are sufficient and reliable power supplies. The coal-fired power dominates China’s electric power structure, on one hand, the fossil fuels consumed by thermal power are mainly limited reserved resources; one the other hand, the burning process of fossil fuels will also cause damage to our ecological environment. So it is imperative that more renewable energy, which is represented by wind power, should be integrated into the grid in a reasonable and well planned way, and the energy structure should be adjusted. In terms of wind power integration, there are two access modes, i.e., integrating wind power into ac grid or dc grid. Compared to ac grid, integrating the wind power into dc grid has the advantages that it can achieve asynchronous interconnection and does not change the short circuit capacities of connected ac grid system. Among the current HVDC transmission technologies, VSC-HVDC and LCC-HVDC have different features and different suitable application situations. This article discusses the technical problems of how to integrate large capacity renewable energy through hybrid multi-terminal HVDC system which contains both VSC stations and LCC stations, does corresponding researches focused on the control strategy of each station in the hybrid multi-terminal HVDC grid and the coordination of the whole hybrid multi-terminal HVDC system. The main research contents in this article include:1) The article points out synchronverter’s shortcomings and makes corresponding improvements so that the improved synchronverter is able to limit its current output. When disturbances occur in the system, the improved synchronverter can ensure the connected grid’s power angle stability and voltage stability.2) The article analyses and compares some general coordination and control strategies of multi-terminal HVDC grid, which are master slave control, voltage margin control and voltage droop control. For the hybrid multi-terminal HVDC system which contains two LCC stations as well as several VSC stations and integrate large capacity wind power, a brand new hybrid multi-terminal HVDC control and coordination strategy--active voltage feedback control--is proposed, which features that it does not rely on high speed communication system.3) Models of the improved synchronverters are built on PSCAD/EMTDC platform. First, simulation results show that not only cannot traditional synchronverter regulate the frequency with no error, but also the overcurrent phenomena will probably occur under fault conditions, and then the power electronic components will be damaged or the VSC station may be tripped out. Second, simulation is used to validate that the improved synchronverter can achieve secondary frequency control, and limit its current output under fault conditions.4) Adopting the proposed active voltage feedback control, a hybrid multi-terminal HVDC system, which contains two LCC stations and three VSC stations, is built in PSCAD/EMTDC software package. Thereafter time-domain simulation is used to test and verify system’s steady state performance during normal operation, and transient state performance under abnormal conditions. Some key parameters in the control parts are also analyzed to show their influence on system’s operation state.