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In order to ensure the high-efficient and stable operation of the MMC-HVDC transmission system, it is necessary to analyse the failure mode and failure mechanism of MMC-HVDC in detail. First, this article briefly introduced the MMC-HVDC system working principle. Then, we deeply analyse characteristic investigation under internal AC bus fault including single-phase ground fault and two-phase short circuit fault. It is proved theoretically that when asymmetrical fault occurs in AC bus, negative sequence components will appear in the system and cause double frequency fluctuations in the active power. Finally, a simulation model of modular multilevel converter DC transmission is built in the PSCAD/EMTDC electromagnetic transient simulation platform, which validates the correctness of the theoretical derivation, and provides a basis for the research of system protection strategy.

Sub-synchronous oscillation may occur when the wind-thermal-bundled system sent out via AC/DC transmission (WTBH) is integrated in power grid. Causing large-scale wind-turbine tripping, even lead the system to collapse. First, in this study the mechanism of sub-synchronous oscillation of this system is analysed. Second, the influencing factors such as the ratio of the wind-thermal-bundled installed capacity, the series compensation parameters of AC transmission line, the different operating conditions of wind power etc., are analysed, which can cause different degree impact on the sub-synchronous oscillation. Finally, the system is simulated under the Power Systems Computer Aided Design (PSCAD) environment. The results show that under the same conditions, the higher series compensation, the greater transmission capacity of the AC line, the greater risk of the sub-synchronous oscillation. Under the condition of full load, the greater proportion of installed capacity of Doubly fed Induction Generator (DFIG) is, the less risk of sub-synchronous oscillation; Under the lower wind speed condition, reducing the number of wind turbine can eliminate the risk of sub-synchronous oscillation, and the oscillation frequency vary with the number of wind turbine.

Aiming at meeting the requirement of balancing the fluctuating wind power, this study proposed an optimal control strategy for wind power hybrid energy storage system considering output power amendment. Firstly, fluctuant wind power is decomposed into low-frequency, sub-high frequency and high-frequency components. Low-frequency part is used as the expected value of wind power integration. Sub-high frequency and high-frequency components are used as the target value of output power of storage batteries and ultra-capacitor energy storage system. Secondly, a double power amendment is proposed. In the first-time power amendment, the amendment coefficient is calculated by fuzzy controller whose inputs are the target value of power and state of charge (SOC) of the energy storage system. On the other hand, complement of batteries and super-capacitors are utilised in the second amendment. Through simulation analysis, it can be indicated that the configuration of the hybrid energy storage system and the control system design are feasible. This system can be widely used in wind farm, undertaking the task of balancing the fluctuating wind power.