Fault Analysis And Load Recovery Strategy Reasearch On AC/DC Hybrid Distribution Network

With the rapid development of power system,electronic technology has also made great progress.Controllable thyristor,IGBT and other power electronic devices have also been widely used.With the mature of the converter technology,DC Transmission technology research and application is also widely carried out around the world.In the application of the long-distance and high-capacity electric transmission,new DC transmission technology has played a siginificant role.In the application of the distribution grid,on account of the function of control of DC system,AC/DC hybrid distribution network with VSC-HVDC can better cope with the impact of shock and suppress short circuit current effectively in comparison with the tradditonal distribution network system.It is important to study the mutual influence of AC side and DC side fault parameters and the reconstruction strategy of AC/DC hybrid distribution network in fault recovery when the flexible DC transmission system fails.The main research on VSC-HVDC system and AC/DC hybrid power distribution system of this paper are as follows:(1)The structure and principle of VSC-HVDC are studied,and a mathematical model is established to describe the operating state of the system.The model of the two-phase ??coordinate is deduced by the mathematical model under the three-phase stationary coordinate,and the model under the synchronous dq rotation coordinate is obtained.On this basis,the mathematical model and expression of AC side under normal operating conditions and fault conditions are studied.(2)The transient process of the system after DC side fault is analyzed.The process is divided into three stages:DC capacitor discharge,commutation bridge natural commutation and diode conduction.On the basis of the transient process analysis,the current limiting circuit which suppresses the short circuit current of the DC side fault is designed by changing the idea of the line parameters.(3)By establishing the converter function model of the converter and the steady-state response model of the DC control system and modeling it,the equivalent model of the fault on DC system for the AC side and DC side is calculated.The relationship between the voltage and current components of the DC side injection AC system and the fundamental component of the voltage and current on the converter bus under the asymmetric operating condition is calculated by the alternating iteration method of the AC side and the DC side.(4)Genetic Algorithm is adopted to proceed the research on network reconfiguration strategy of AC/DC hybrid distribution network in fault recovery process.The network topology and line power flow constraints are considered in the process of coding the action sequence of the switch,in order to study the process of the DC line's latchout as the fault and the process of resumption of DC re-investment and analyzes the influence of DG capacity and DC line control strategy on the network reconstruction scheme.