Research On Operation And Control Of Zhuhai Three Terminal Flexible DC Distribution Network

The flexible DC distribution network has the characteristics of flexible control and strong networking ability,which can be used as a new solution to solve the problem of urban power supply in the future.In December 2018,the world’s largest multi terminal AC/DC hybrid flexible distribution network project was successfully put into operation in Tangjiawan,Zhuhai,Guangdong.The project realizes the flexible interconnection of regional medium voltage AC distribution network through three MMC converter stations,and it plays a leading role in improving the distribution network structure and building the future energy Internet.The structural characteristics and control strategy of flexible direct distribution network are essentially different from that of traditional AC distribution network.Based on the structure and function of flexible DC distribution network,this paper focuses on the analysis of the control strategy and operation mode of Zhuhai three terminal flexible DC distribution network.Firstly,this paper introduces the working principle of MMC converter valve,DC transformer and DC circuit breaker.Then the paper analyzes the special network structure of the three terminal flexible DC distribution network and puts forward four main functions that the flexible DC distribution network can achieve.Then,combined with the control method of flexible DC converter station,the paper proposes the hierarchical control logic,analyzes the physical configuration of converter station control system,and designs station level controller and valve level controller according to the requirements of hierarchical control system.The simulation model of Jishan1 converter station is built on the MATLAB/Simulink to test its power control ability and voltage control ability and to verify the effectiveness of the hierarchical control system.After that,Aiming at the shortcomings of the redundant control strategy used in the converter station,a redundant control scheme based on external power auxiliary charging is proposed.The scheme can greatly improve the input voltage of the redundant module,reduce the oscillation amplitude and stability time of the system when the redundant module is put into operation,and the effect of the scheme is verified by simulation comparison.Finally,the paper studies the coordinated operation mode of three terminal flexible DC distribution network based on master-slave control strategy and analyzes the functions of flexible DC distribution network under nine typical operation modes.Aiming at the problem of unstable power distribution in emergency power support mode,the paper proposes a control strategy based on the feedback of active power at passive side,which can balance the output power of converter station according to requirements.Aiming at the problem that the master-slave control will fail in the case of communication system failure,the voltage margin control strategy can be introduced into Jishan 2 converter station to improve the reliability of flexible DC distribution network,and the voltage margin controller of Jishan 2 converter station is designed.The paper builds a three terminal coordinated control simulation model of Zhuhai flexible DC distribution network on MATLAB/Simulink,and verifies its functions in three aspects: load balancing of transformers,regional power flow reversal and emergency power support.The unsteady state of power allocation in emergency power support mode is simulated and analyzed,and the effectiveness of the proposed active power feedback control strategy based on passive side is verified.The voltage margin controller model of jishan-2converter station is also built and simulated to verify the role of voltage margin control in improving the reliability of flexible direct distribution network.