Study Of Operation And Control Of The HVDC-VSC Transmission System

In the past, HVDC links have been used almost exclusively to transmit huge power over long distance. HVDC Light is a new transmission technology based on voltage sourced converters and insulated gate bipolar transistors that extend the economical power range of HVDC transmissions down to just a few megawatts, besides being a cost-competitive alternative to conventional AC transmission and local generation.Firstly, the mathematical model of 3-phase voltage sourced converter is established using state space average method. Theoretical analysis on the DC and AC side characteristics of the 3-phase voltage sourced converter is conducted. The applicability of the space vector pulse-width modulation in the 3-phase voltage sourced converters is demonstrated. It is instructive for the main circuit design of the 3-phase voltage sourced converters.Next, according to the steady-state mathematical model of a HVDC-VSC system which is connected to active networks on both sides, the control strategy of HVDC-VSC system are studied for two typical control manners and under asymmetrical conditions of AC system. The corresponding decoupled relationship between three controlling variables and two controlled variables in the voltage sourced converter is determined. An asymmetrical steady-state model controller for H.DC-VSC system is proposed and its effectiveness is proved.Finally, Matlab/Simulink simulation software is used for the controller testing. Through the testing, the parameters of the asymmetrical controller are determined. The simulation results under steady state conditions show the sufficient response speed and accuracy of the asymmetrical controller model. And the effectiveness of the proposed control strategy under asymmetrical operating conditions of the AC system is also confirmed by the simulation testing.