The Control Strategy Of VSC-HVDC In Hybrid Multi-Infeed HVDC System

Due to numerous advantages of VSC-HVDC technology, such as commutation accurate, adjusting the active power and reactive power independently, supply to passivity system, and so on, VSC-HVDC system can perfectly solves serious problems of traditional HVDC system. Therefore VSC-HVDC technology has become a major research subject in HVDC technology.As the model and control method of the VSC-HVDC system is implemented basically in the thesis, an adaptive current limiter is presented to propose the conventional control strategy of VSC-HVDC in hybrid multi-infeed HVDC system.This thesis first introduces the development status of HVDC, including several kinds of HVDC, then inrouduces the VSC-HVDC technology and hybrid multi-infeed HVDC system.In construction and working principle of VSC-HVDC part, this thesis focuses on the constriction of VSC-HVDC system, the functions of each parts, the topology of the VSC converter and connection mode of the system.In the mathematical model of VSC-HVDC part, this thesisi establishes the VSC-HVDC mathematical models in the three-phase stationary coordinate and the synchronous rotating coordinate respectively as the converter of VSC-HVDC is a strong coupling, time-varying nonlinear system.In the control strategy research of VSC-HVDC part, this thesis first designs the conventional double closed-loop control method of VSC-HVDC in the synchronous rotating coordinate, including designing the PLL, the inner and the outer loop controller. Secondly this thesis uses MATLAB simulation platform to simulate three kinds of situations--the VSC-HVDC system normal operation, active power of the system reversing and grid fault at receiving end VSC. Then the thesis analysis the simulation result. The control result turns out to be efficacious and available from the simulation result. However the simulation result also exposes the drawback of the conventional control method in maintaining busbar voltage because of pre-setting the current limits.In the hybrid multi-infeed HVDC system research part, this thesis builds a hybrid multi-infeed HVDC system which contains a traditional HVDC link and a VSC-HVDC link. The thesis deduces the effective short-circuit ratio of the hybrid multi-infeed HVDC system to evaluate the stability of the system, and then analysis the impact of pre-setting current limits to the system. In allusion to the disadvantage of the conventional control method, this thesis put forward an adaptive current limiter which can change the current limits dynamically. Finally the simulation of the proposed the control method is established in this thesis through the MATLAB simulation platform.