Design and Application Research of Synchronous Temporary Block Function between Valve Groups on Modular Multilevel Converter Ultra High Voltage Direct Current System

Abstract

Compared with the half controlled power electronic devices such as thyristor, the over-current ability of fully controlled power electronic devices such as insulated gate bipolar transistor (IGBT) is weak. In order to ensure that bridge arm current does not exceed the bearing range of modular multilevel converter (MMC) switching devices under AC fault ride through and other scenarios, MMC high voltage direct current (HVDC) system projects are equipped with temporary block (TB) function, IGBTs are protected by suspending the trigger pulse before bridge arm current reaches the maximum safe current of the switch. The structure of high side and low side valve group (VG) in series are adopted in ultra high voltage direct current (UHVDC) system. For line commuted converter (LCC) UHVDC system, the converter is current source type, During normal operation, there is no need to control the voltage balance between high side and low side VG, and after one VG is disturbed by commutation failure, the impact on another VG is relatively small. For MMC-UHVDC system, the converter is voltage source type, during normal operation, the voltage balance between high side and low side VG should be controlled. When a transient disturbance such as AC fault occurs, if one VG is temporary blocked due to bridge arm over-current, and another VG is not temporary blocked or the time of temporary blocked is later, the VG blocked first may tripped due to sub-module (SM) overall over-voltage or bridge arm over-current, and the scope of the accident is expanded, which is not conducive to the flexible and reliable operation of MMC-UHVDC. Therefore, the impact of non synchronous temporary block of MMC-UHVDC VGs is firstly analyzed in this paper, and then synchronous temporary block (STB) function between VGs is proposed, and the hardware interface scheme, STB signal sending and receiving logic, enable conditions and other aspects between VGs are elaborated. Finally, taking kunliulong project, the world’s first multi terminal Hybrid (MTH) UHVDC project, as the application background, a real-time simulation (RTS) test system for control and protection (C&P) system is established. The enable conditions of the STB function are verified. The response and dynamic characteristics of the system under abnormal measurement of bridge arm current, AC fault and VG short circuit fault when enabling or disabling STB function are compared. The simulation results show that the reliability of MMC-UHVDC can be improved by enabling STB function.