Constant power constant voltage startup procedures for VSC-LCC based hybrid pseudo bipolar HVDC system

Hybrid bipolar HVDC technology is a new research hotspot for the grid-connected of new energy power generation system. In order to reduce the undesired impacts and damages on the HB-HVDC system and the connected AC system during the startup procedures, appropriate startup control strategy should be dedicatedly designed. There are three steps in the startup procedures. Firstly, the VSC and LCC are both locked, the inverter parallel diodes of the VSC are used for charging the capacitance in the DC side by uncontrolled rectifier. Secondly, when the dc voltage achieves 0.52pu, the VSC converter is unlocked, the controlled variable of active power is DC voltage and the controlled variable of passive power is passive power with the instruction value of 1.0pu. The DC voltage will continue to rise. Thirdly, when the DC voltage achieves to the instruction value, the switch in parallel with the current limiting resistor is closed and the resistor exits, the voltage will keep stability. Then, the LCC inverter is unlocked with the constant DC voltage control, the voltage instruction is 1.0pu. At the same time, the VSC converter uses active power as the controlled variable of active power with the instruction value increasing from zero to 1.0pu. The entire startup procedures are complete. The steady-state control strategy and start-up strategy are proposed based on the deduced quasi-steady state mathematic model of the hybrid pseudo bipolar HVDC system. The hybrid pseudo bipolar HVDC simulation system is built in PSCAD/EMTDC, and the effectiveness of the proposed steady-state control strategy and start-up strategy are illustrated. The entire startup process is smooth and stable, and the control strategy of the hybrid pseudo bipolar HVDC system is perfect. After startup, the system tracks target reference accurately and operates stably.