Fast direct voltage controller for VSC transmission

This paper develops a controller that enables good stability and fast responses of voltage source converter (VSC) transmission. The design is challenging because the uncontrolled system is multivariable having two possible rectifier inputs and many possible rectifier outputs and similar inputs and outputs on the inverter side. It is determined that the fastest possible controller is inverter DC voltage. An additional controller loop is introduced to further improve the controller performance. Although the additional stabilising controller reduces the percentage overshoot it has negative affect at higher frequencies and this is improved by introducing an additional transfer function zero. The fast controller ensures that the system is stable but steady state error is eliminated by the introduction of two slower controllers: rectifier DC power and inverter AC voltage. The fast VSC transmission controller is tested in MATLAB and also in PSCAD to confirm operation in non-linear environment. It is concluded that this design achieves very fast DC voltage control with minimum overshooting and in this way components´ overvoltages will be minimized.