Simplified control model for HVDC Classic Master of Science Thesis

In this Master’s project, a simplified control model for HVDC classic has been built in the EMTDC simulation program with PSCAD v.4.2 interface. The control functions in the simplified control are based on functions from ABB Power Systems. A number of cases have been simulated to evaluate the simplified control model, which demand correct function during earth fault and load disturbances. The main functions in the simplified control are the Voltage Dependent Current Order Limiter (VDCOL), Current Order Amplifier (CCA), as well as functions that are acting on the upper and lower limits of the CCA. The VDCOL will reduce the current order at direct voltage reduction. This will avoid voltage instabilities during and after AC disturbances. It will also ease the stresses on the valves and speed up the recovery after disconnection of the earth fault. The CCA is principally a proportional-integral controller, which give the current control loop proper dynamics. For an inverter, it will decrease the firing angle and for a rectifier it will increase the firing angle. Furthermore, the CCA controller may also be used for controlling the DC voltage to a constant value. Through simulation it has been shown that during single-phase ground fault disturbances between the converter transformers and the rectifier, the current and the voltage curve shapes are practically the same, independent of which model is used in the surge arresters that are connected in parallel with each thyristor valve. The current and the voltage curve shapes in the surge arrestors connected to the neutral line deviate in some cases. However, the shape of the DC voltage when the system is disturbed is practically the same when the simplified control model is used compared to the detailed model. This is probably the best result that can expected with a simplified control. Unfortunately the simplified model cannot control the DC voltage when there is a disturbance in the AC network. Preface This Master’s thesis constitutes the final of my Master of Science programme in Electrical Engineering at the Royal Institute of Technology. With this preface, I would like to acknowledge those who have assisted me and contributed to this work. I especially would like to thank all at ABB Power Systems that participated in the study for all their support during this work. Furthermore, I would like to thank my supervisor at KTH, Professor Stefan Östlund for his input on this work. Table of