Modelling, control design and analysis of VSC based HVDC transmission systems

The development of power semiconductors, specially IGBTs has led to the small power HVDC transmission based on voltage source converters (VSCs). The VSC based HVDC transmission system mainly consists of two converter stations connected by a dc cable. This paper presents the modelling and control design of VSC based HVDC which uses twelve pulse three level converter topology. The reactive current injected by individual VSCs can be maintained constant or controlled to regulate converter bus voltage constant. While one VSC regulates the dc bus voltage the other controls the power flow in the dc link. Each VSC can have up to 4 controllers depending on the operating mode. The controller structure adopted for power controller is of PID type and all other controllers are of PI type. Each operating mode requires proper tuning of controller gains in order to achieve satisfactory system performance. This paper discusses a systematic approach for parameter optimization in selecting controller gains of VSC based HVDC. The analysis of VSC based HVDC is carried using both D-Q model (neglecting harmonics in the output voltages of VSC) and three phase detailed model of VSC using switching functions. While the eigenvalue analysis and controller design is based on the D-Q model, the transient simulation considers both models. The analysis considers different operating modes of the converters.