| VSC-HVDC technology based on voltage source converter can not only realize large-scale and large-capacity power transmission,but also realize active and reactive power decoupling control.It does not need reactive power compensation,inter-station communication,fast power flow reversal,small area and a series of advantages.Therefore,high-voltage DC power grid develops very rapidly and has been widely used in power system.Compared with AC power grid,DC power grid can reduce the intermediate links of new energy generation system,frequency conversion control load and DC load connected to power grid,and improve energy conversion efficiency.DC distribution network also has the advantages of low line cost,low transmission loss and low charging capacitance of DC cable.Under the background of current world energy demand,DC power grid will be widely used in power system.There are no reactive power and frequency control problems in DC power grid,but small signal stability problems are easily caused by the large number of fast response converters in DC power grid.Scholars at home and abroad have done a lot of research on the stability of mathematical model and small signal model of DC power grid.There is a class of converters in DC power grid that need constant active power control to track dispatching instructions.However,in the small signal stability analysis of DC power grid,AC/DC converter is usually treated as a linear proportional amplifier or inertia link.This approximate processing will be fixed.The DC-side admittance model of power-controlled converter has great influence on the accuracy of small signal stability analysis of DC power grid.For this reason,this paper does not do approximate treatment First of all,considering the voltage fluctuation on the DC side,the relationship between the converter port voltage and the modulation signal function is established.Then,the DC side admittance model of the converter is established,and compared with the amplitude-frequency and phase-frequency characteristics of the admittance established by the traditional method.Then,it is applied to the small signal stability analysis of the simple DC power grid.Finally,the electromagnetic transient simulation model of VSC-HVDC system is built by PSCAD/EMTDC simulation software,and the admittance calculation model is built by MATLABD.The results show that the DC-side admittance model derived in this paper can better fit the measurement admittance in low frequency band,while the admittance model derived by traditional method has some deviation in low frequency band and measurement admittance,so it is judged that the DC power grid is small.In signal stability,the admittance model deduced by this method can accurately determine the small signal stability of DC system.However,the traditional admittance model deduced by this method is prone to misjudgement,which affects the determination of small signal stability of DC system. |
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