High-voltage DC cables are important equipment for cross-sea long-distance power transmission and new energy grid integration. DC cables with high voltage levels and current capacity are still in the research and development stage.In order to promote the research and development of high-voltage DC cables and test the long-term operation performance, this paper relies on a domestic ±500 kV flexible dc grid project to establish a DC cable comprehensive test station.The DC line of the domestic ±500 kV flexible dc grid project mainly adopts the overhead transmission line scheme. The test station is located at the single-pole outlet of one of the converter stations, and the trial DC cable runs in parallel with the overhead line.The operation control and protection coordination are complex, the reliability requirements are extremely high, and there is no engineering experience to follow.In this paper, aiming at the connection mode of parallel switching operation between 500k V DC cable test section and overhead line, the DC cable monitoring and input and exit control strategy, fault protection strategy and overload operation control protection strategy in the test station are proposed for the first time, and are verified by simulation experiments, it can realize reliable operation of DC cable access to flexible DC grid.

Power supply structure has undergone tremendous changes with the increasing proportion of renewable energy in the power system. The high permeability, randomness and uncertainty of the output of renewable energy cause problems increasingly prominent, such as demand of power system peak-shaving resources on both power supply and load side. Firstly, this paper summarizes temporal and spatial distribution characteristics of renewable energy, and explains the peak-shaving problem of power system with high proportion of renewable energy. Then, the equipment, characteristics and development trend of various peak-shaving energy storage types are summarized. The paper also elaborates the peak-shaving modes of the demand side in the power system, including demand response construction, power market and auxiliary peak-shaving service market, and enhancing power transmission capacity. Finally, the future research prospects of peak-shaving of power system with high proportion of renewable energy are discussed and prospected.

With the large-scale access of new energy, the power grid presents the characteristics of strong DC system and weak AC system. When commutation failure occurs in HVDC transmission system, transient overvoltage will appear in weak sending end system. Since STATCOM responds to the change of grid voltage through the command of control link, the lag of its control link will lead to the enhancement of transient overvoltage by STATCOM. Firstly, in view of this phenomenon, this paper analyzes the generation mechanism of AC transient overvoltage in weak sending end system under commutation failure. Secondly, by analyzing the frequency response characteristics, theoretical derivation of STATCOM response delay time is given. Finally, an optimization strategy based on damped switching is proposed to suppress STATCOM’s increasing effect on overvoltage response, and the effectiveness of the optimization strategy is verified by simulation. This work is supported by National Natural Science Foundation of China (No. 51277066) and State Grid Corporation of China Research Program (No. 5102-201956301A-0-0-00).

As an inherent drawback of LCC-UHVDC, commutation failure often causes dramatic reactive power fluctuation in the AC power grid both on the rectifier side and the inverter side. The application of bypass pair is one of the important control strategies of DC system protection. When a fault occurs in the HVDC system, the switch of bypass pair after the protection action of the inverter side will help to achieve the purpose of stopping the HVDC system quickly and isolating the fault. At present, the research of bypass pair is mainly focused on the application under DC fault cases, whereas the application of that in commutation failure is lacking. According to the analysis of the impact of bypass control on the voltage characteristics on the rectifier side and the inverter side, this paper demonstrates the contribution of reasonable bypass control on improving the voltage characteristics of UHVDC transmission line after commutation failure. And a bypass pair control scheme is proposed, which takes the commutation failure in inverter valves and the reduction of AC bus voltage as the initialization criterion to dynamically adjust DC current. Then, the bypass pair will quit as the system recovering. The effectiveness of the proposed bypass pair control scheme on isolating AC and DC systems and reliving the reactive power and the voltage fluctuation during the transient process of AC system fault is verified in PSCAD/EMTDC.