Research On Commutation Failure Mitigation Control Strategy In HVDC Systems

The world's largest AC/DC hybrid power grid with the the highest voltage level has been established in China.Commutation failure in high-voltage direct current(HVDC)systems is one of the key challenges for power grid.The features of “DC systems are stronger than AC systems and multi-HVDC system feed-in”will be more and more apparent in China with the following Ultra-HVDC projects put into operation one after another.The increasing of AC-DC coupling degree will lead to more frequent and more complex commutation failures,and the scope of their influence will extend from local to the whole power grid.Therefore,it is of great significance for power grid security to enhance the commutation failure immunity of HVDC.Commutation failure is affected by various factors and occurs under different conditions.Besides,improper control interactions may lead to subsequent commutation failures after the first commutation failure,which will further expand the fault influence.Reasonable converter control is important for commutation failure mitigation under complex power system disturbance conditions.Therefore,this thesis focuses on the commutation failure mitigation control in HVDC systems.The control strategies to mitigate the first commutation failure and subsequent commutation failure are studied respectively,so as to improve the ability of power system to resist commutation failures.The main work of this thesis is summarized as follows:(1)An equivalent method of HVDC rectifier side considering converter control is proposed to analyze the coupling law among the influencing factors.By using R-C series branch to equivalent to PI control and linearizing the external characteristics of the rectifier,more accurate DC current response were obtained.The commutation condition will be more severe after considering the coupling effect of the commutation failure factors.In particular,the non-ideal controlled characteristics of DC current will significantly increase the risk of commutation failure.The reason for subsequent commutation failures is given on the basis of the interaction process of inverter control loops after the first commutation failure.It is found that the extinction angle is out of control for a period after the control mode switching,although the inverter side is under constant extinction angle control mode.This raises the risk of subsequent commutation failures.(2)The HVDC converter only commutates 12 times in each power frequency cycle.That is,the commutation processes is relatively discrete in time.This leads to serious control lag and is not conducive to the suppression of first commutation failure.Aiming at this problem,a pseudo commutation process based commutation failure mitigation method is proposed.The proposed pseudo commutation process realizes the continuous extension of the discrete commutation process on the time axis,so that the extinction Angle and the commutation overlap Angle can be solved in real time with the sampling frequency.On this basis,a pseudo firing angle prediction algorithm is presented,which is able to adjust the firing angle order timely after faults due to its real-time performance.Simulation results indicate that the proposed strategy improves the commutation failure immunity of HVDC by coordinating with existing control methods.(3)Focusing on the commutation failure caused by commutation voltage distortion,an extinction angle control strategy based on the perspective commutation area drop is proposed.The presented perspective commutation area drop index is calculated considering both the commutation voltage waveform and the commutation mechanism.Therefore,the upcoming commutation failure risk is able to be evaluated dynamically.The proposed extinction angle control takes the backward shift of commutation end instant and the maximum zero-crossing point shift of commutation voltage in to account,thus ensuring a sufficient commutation margin.Simulation results verifies the performance of the proposed strategy in commutation failure mitigation during both AC faults and voltage distortion.(4)Aiming at subsequent commutation failures caused by improper control interactions of the inverter side,a constant extinction angle deviation control is proposed to prevent subsequent commutation failures.The automatic adjustment of the extinction angle control reference after the first commutation failure is realized by calculating the extinction angle deviation target and designing the control starting logic.Therefore,the uncontrolled status of extinction angle can be avoided during the system recovery.Simulation results show the superiority of the proposed control strategy in preventing subsequent commutation failures.(5)On the basis of the above of inverter control optimization,a subsequent commutation failure prevention strategy based on rectifier side current limiting control is proposed.This method quantifies the severity of inverter AC faults using the difference between the requirement and availability of the commutation volt-time area.The pseudo commutation area insufficient index fundamentally establishes the coupling relationship between the effects of DC current and commutation voltage on commutation process.Furthermore,the index is introduced into the voltage dependent current order limiter(VDCOL)to improve the fault recovery performance of the HVDC system.Simulation results show that the proposed strategy has better ability to mitigate subsequent commutation failures compared with the traditional VDCOL control.Focusing on commutation failure mitigation control,this thesis has formed a framework,from mechanism analysis of commutation failure,to mitigation of the first commutation failure,to prevention of subsequent commutation failures.Theoretical research and simulation results show that the proposed strategies is helpful to improve the commutation failure immunity of HVDC systems,so as to ensure the safe operation of power grid.This work provides theoretical basis for the optimization design of HVDC control and protection system.