Research On AC And DC Voltage Stability Control Strategies For VSC-MTDC System

Voltage sourced converter based multi-terminal direct current transmission(VSC-MTDC)sysytem has the characteristics of running flexible,stability,and can achieve multi-power supply and multi-load consumption,which is beneficial to achieve the optimal allocation of China's energy resources and the integration of large-scale renewable energy.The voltage stability of the DC grid is the basis for the stable operation of the DC transmission system,and the stable control of the bus voltage at the AC side of the converter station plays an important role in the stability of the power transmission between the AC and DC systems.With the increase of integration of large-scale new energy sources and increased reliability and flexibility requirements for system operation,it causes the complication of the topology and the diversification of control objectives for VSC-MTDC system,which further leads to the complexity of AC and DC voltage control of VSC-MTDC system.Through the research on the interaction between the AC and DC voltages of the VSC-MTDC system,aiming at various application scenarios of VSC-MTDC technology,methods of improving the AC/DC voltage stability of the VSC-MTDC system are proposed and can provide theoretical support for the application of VSC-MTDC systems.The main parts of this dissertation are as follows:(1)Based on the the control mechanism of the VSC-MTDC system,an VSC transient mathematical model that accounts for the effects of DC voltage fluctuations is established.Through the research on the interaction between the AC and DC voltages of the VSC converter station,the mathematics expression and interaction influence characteristics of the voltage interaction between the AC and DC sides of the converter station are obtained.(2)Considering the developing trend of VSC-MTDC technology,the application scenarios of VSC-MTDC system atAC and DC sides is analyzed,and then the operating characteristics and control requirements of AC/DC systems are analyzed.(3)The dynamic/steady-state performance of existing multi-terminal coordinated control strategies for VSC-MTDC is analyzed from the mechanism,and then a power active balance oriented coordinating control strategy for VSC-MTDC system is proposed to realize the power active balance of the VSC-MTDC system in dynamic process and improve the dynamic control performances of both the DC voltage and transmission power of the VSC-MTDC system.(4)Based on the study on control characteristics of the bipolar VSC converter station,a multi-objective coordinating control strategy for bipolar VSC converter stations connected with large-scale new energy generation systems,weak AC grids and passive AC grids is proposed.This control strategy can not only provide stable AC voltage but also improve the flexibility of transmission power of VSC converter stations,and the voltage stability of the AC/DC side of the multi-terminal flexible grid is further improved.(5)Aiming at various application scenarios of VSC-MTDC technology,corresponding simulation systems were built in PSCAD/EMTDC and cases under different operation conditions are simulated.Simulation results verified the feasibility and effectiveness of the proposed strategy.