Research On MMC-HVDC System Control Method Under AC System Asymmetry Faults

Modular Multilevel Converter(MMC)is a voltage source converter topology that realizes multi-level output by means of cascaded sub-modules.Due to the advantages of MMC's own modular design,low switching frequency and switching loss,and high output waveform quality,its topology is well suited for the High Voltage Direct Current(HVDC)field.MMC-HVDC is one of the development trends of the flexible HVDC transmission technology,the control strategy is the key core technology to maintain the good and stable operation of the MMC-HVDC system.The control strategy of the MMC-HVDC system under steady-state operation of the power grid has a large number of research results opened to the public,and it is relatively mature,although when the power grid occurs asymmetrically fault,domestic and foreign scholars have disclosed some control strategies,there are still some problem need to be solved.This paper studies the control strategy of MMC-HVDC system when the asymmetrical fault occurs in AC system.The main work is as follows:In order to improve the operating characteristics of the control system of MMC-HVDC system while the ac side occurred an asymmetry fault,we have researched on negative-sequence current suppression and dc voltage stabilization control strategy.For the problem of double-order inner current control system requiring rotational transform voltage and current and coupling between the d axis and the q axis,we designed second order complex filter to extract positive and negative sequence component of the grid voltage and used the idea of voltage compensation principle to design inner negative sequence voltage controller which used the extracted negative sequence component,for double-order inner current controller we had made effective improvements and simplified the structure of the control system.In addition,to achieve the control of asymmetric fault dc voltage stabilization,with the basis of the low-frequency continuous model,bridge arm power was deduced that it would have the double frequency component of negative and zero sequence fluctuations.Consequently,we designed a secondary zero sequence compensation controller.The proposed controller is simulated in the environment of PSCAD/EMTDC and simulation results show that the proposed controller can effectively inhibit the negative sequence current and stabilize DC bus voltage.In order to realize the unified control of the AC side current and the internal circulation of the MMC,we have designed the bridge arm current control strategy.There is a coupling problem between the upper and lower bridge voltage current based on the MMC bridge arm current mathematical model,we designed the feedforward compensation decoupling control method to simplify the complexity of the controller.Bridge arm current includes dc current,MMC output ac current,circulating current,in the two-phase stationary coordinate system,a controller with PI-VR1-VR2 parallel structure is used to control three types of current information,controlling the bridge arm current can realize the unified control of the ac side control and the circulation current suppression,reduce the use of the circulation additional controller,and simplify the structure of the control system.The simulation results show that the proposed bridge arm current control strategy can achieve suppression of negative sequence current and double frequency circulation and reduce the fluctuation of dc current.