The control of VSC-HVDC and its use for large industrial power systems

Licentiatavhandling, 2003

With the recent developments in semiconductors and control equipment, Voltage Source Converter based High Voltage direct Current (VSC-HVDC) has become feasible. Due to the use of VSC technology and Pulse Width Modulation (PWM), it has a number of potential advantages: short circuit current reduction; rapid, independent control of the active and reactive power, etc. With such very favorable advantages VSC-HVDC will likely be part of future transmission and distribution systems which supply industrial systems with a high load density, high reliability and quality requirements, and high costs associated with production stoppages. The thesis deals with the control of VSC-HVDC, the use of VSC-HVDC in a passive industrial system and the system design with di®erent dc voltage levels. The objective of the work is to assess the potential and limitation of the use of dc distribution in industrial power systems. A model of a VSC based dc link using PWM Technology and IGBT semiconductors is designed. A mathematical model of the control system based on the relationships between voltage and current is described for the VSC connected to the transformer secondary side. A control system is developed combining an inner current loop controller which is divided into positive current controller and negative current controller and a number of outer controllers. Different control strategies are studied and corresponding dynamic performance under step changes and different types of faults is investigated in PSCAD/EMTDC simulation package. The simulation results verify that the model can fulfill bi-directional power transfers, ac system voltage adjustment and fast response control and that the system has good transient and steady state performance. VSC-HVDC is investigated for its ability to supply a passive industrial system. In this thesis the comparison of a pure ac supplied distribution system and a dc supplied distribution system is performed based on balanced and unbalanced faults on the grid side and motor starting on the load side. The influence of the current limitation on the performance is studied. It is shown that VSC-HVDC applied to industrial systems is able to mitigate voltage dips. it is also shown that the rating of the dc link significantly influences its ability to mitigate voltage dips. The possibilities of multi-level dc networks and a mixed ac/dc system are investigated. The control of the dc/dc converter is developed for this. Again the rating of the converters has a significant effect on the performance of the system during faults and motor starting. A discussion is started on the relation between converter rating and industrial power-system design.