Research On Control And AC Side Fault Protection Strategy Of MMC-HVDC System

With the rapid development of China’s economy,the society’s demand for electrical energy is generally increasing.In order to meet the power demand in some areas of our country,it is necessary to transmit power across regions.When power is transmitted over a long distance and in a wide range,AC transmission has a greater loss,so HVDC power transmission technology has become a better choice.Modular multilevel converter based HVDC(MMC-HVDC)has many advantages.Because of its flexible operation and high reliability,it has been widely used in island interconnection,submarine cable power supply and other fields.This thesis mainly studies the control and AC side fault protection strategy of MMC-HVDC system.Firstly,in order to make the MMC-HVDC system run stably,this thesis uses the steady-state control strategy in static coordinate system.This strategy avoids the coupling process of the traditional method in the inner loop current control,improves the response speed of the system,and makes the control of the system more simple.Secondly,the circulating current will be caused by the unbalanced voltage of the bridge arm during the operation of MMC.The circulating current will distort the current waveform of the bridge arm and increase the loss of the system.Therefore,circulation suppression is needed.There are higher even harmonic components in circulation,of which the second h armonic is the highest,so the main suppression of circulation is to suppress the second harmonic component.Aiming at the circulating current component in the bridge arm,this thesis proposes a circulation suppression strategy based on quasi PIR control,which can eliminate the second harmonic component in the circulation and achieve better effect in the circulation suppression.At the same time,this strategy does not require the decoupling process required by traditional control,and improves the response speed of the system.The simulation results on PSCAD show that the proposed control strategy can effectively suppress the circulating current.Finally,a fault protection control strategy is proposed for the asymmetric fault on AC side of MMC-HVDC system.The strategy can effectively eliminate the negative sequence current in case of ground fault and reduce the harm of fault current to the system.On the basis of restraining the negative sequence current,this thesis analyzes the instantaneous value of MMC bridge arm power,and finds that there is a zero sequence component of double frequency in the bridge arm power,which will cause power fluctuation.Therefore,this thesis proposes a method of zero sequence voltage compensation to reduce the fluctuation when the fault occurs,which makes up for the shortcomings of traditional methods and improves the stability of the system.The proposed method is simulated and verified on the PSCAD platform,and the results show that the strategy is effective and feasible.