Abstract

High Voltage Direct Current (HVDC) transmission systems traditionally use PI controllers with fixed gains. These controllers operate satisfactorily within a small operating range because of the non-linearity and uncertainty of the plant. However, when AC systems are weak, the HVDC systems are prone to repetitive commutation failures under disturbances, which may lead to the instability and collapse of the DC link. This thesis is a contribution to the study of these problems. In order to make the HVDC systems have optimal control over a wider operating range, an Incremental Fuzzy Gain Scheduling Proportional and Integral Controller (IFGSPIC) is proposed for the current control of the rectifier. For this study using the EMTP RV simulation package, a typical HVDC system connected to a weak AC system is modeled with the detailed representation of the converter, converter transformer and AC/DC filters etc. The DC current error and its derivative are taken as two parameters necessary to adapt the proportional (P) and integral (I) gains of the current controller based on fuzzy reasoning.