Research On VSC-HVDC Control Strategy For Improving AC System Voltage Stability

Voltage stability is an important aspect of power system stability. As the power grid scale expands, the voltage level rises, and the transmission distance increases, the influence of voltage stability problems on power system is getting greater. Since the1970s, several significant voltage collapse incidents have occurred at home and abroad, and therefore ensuring the voltage stability is very important for the safety and stability of power grid operation. Voltage source converter based HVDC (VSC-HVDC) has the capability of independent and quick regulation of active and reactive power and can provide reactive power support to the grid voltage, and also it doesn’t need the power grid to provide commutation voltage so that VSC-HVDC can connect to weak AC system or passive networks. All these features of VSC-HVDC bring benefits for enhancing the power system voltage stability. This article conducts research on VSC-HVDC system-level control strategy for improving AC system static voltage stability and dynamic voltage stability. The main research contents and conclusions are as follows.The influence of VSC-HVDC on AC system static voltage stability has been studied based on PV and QV analysis:The steady-state power output characteristic of VSC has been discussed. The PV and QV curves characteristics of VSC-HVDC has been analyzed in contrast with AC transmission line. The influence of VSC-HVDC active power and reactive regulation on AC system voltage static stability in AC/DC systems has been studied. An active and reactive power allocation strategy of VSC-HVDC based on QV curves has been proposed to improve the crucial bus reactive power margin.Linear dynamic equations for small signal stability analys has been obtained based on VSC-HVDC transient stability model. The transfer function between VSC-HVDC reactive power setting and bus voltage has been obtained through system identification from frequency response. A VSC additional reactive power controller for suppressing voltage fluctuation has been designed based on pole-zero configuration technology.Simulation has been accomplished in power system analisis program PSS/E:in voltage stability analysis equivalent system example, the reactive power support capacity of VSC-HVDC to improve system voltage level and load bus voltage robustness. The VSC-HVDC active power setting has been obtained through power allocation strategy based on QV curves, and simulation result has verified the effectivity of the power allocation strategy to increase the reactive power margin of load bus and to improve system voltages. In AC/DC parallel system with single machine case, the parameters of VSC additional reactive power controller have been selected, and dynamic simulation result has verified that VSC with the additional reactive power controller can suppress AC voltage fluctuation effectively. In Dalian urban power networks case, the influence of VSC-HVDC in different operation modes on system power flow distribution and transient voltage characteristics after AC system faults has been analyzed. The effectivity of VSC-HVDC on improving real AC grid voltage level and accelerating the voltage recovery speed after faults has been verified.