Against the stability of the multi-infeed HVDC (MIDC) transmission system, voltage source converter based HVDC (VSC-HVDC) is applied to MIDC systems. The new systems can reduce probability of commutation failure and improve the stability of the MIDC systems. A physical model of a multi-infeed HVDC and VSC-HVDC system is developed, and its mathematical model is analyzed. The rectangular coordinate form of VSC's power transmission equations is deduced through coordinate conversion. Based on the inverse system method for multivariable nonlinear control, the nonlinear controllers of the multi-infeed HVDC and VSC-HVDC system are designed. Simulation results by PSCAD/EMTDC show that the designed nonlinear controllers improve effectively the voltage stability of the multi-infeed HVDC and VSC-HVDC system under small or large disturbances, such as short circuit fault, reduce the probability of commutation failure in the inverter of HVDC systems and increase the operation reliability of HVDC systems
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