Control of Voltage Source Converter for Distributed Generation Applications during Voltage Disturbances

Rapport, 2003

Voltage Source Converters (VSC’s) are known to be sensitive to voltage disturbances, e.g. voltage dips. As a result of the drop in voltage, they can trip due to overcurrent or, in case of unbalanced dips, also because of fluctuations of the DC link voltage. As the amount of distributed generation with power electronic interface in the grid grows, it becomes unacceptable to loose generating units every time a disturbance occurs. Keeping the VSC on line during unbalanced voltage dips becomes thus a very critical issue. The necessary increase in VSC rating to ride through dips can be obtained if the currents do not increase much during the fault, and also a smaller DC link capacitor can be used if the DC link voltage can be kept close to its reference during faults. To accomplish this, the VSC controller must ensure robust current control and DC link voltage regulation. An option is to use a Dual Vector Current Controller (DVCC) with two separate current controllers for positive and negative sequence components. In this work, the performance of VSC with DVCC is examined under all possible voltage dips that can occur at the VSC terminals due to faults. Two methods for generating the current references in order to suppress the oscillating powers due to the grid voltage unbalance are implemented and compared through simulations. Voltage dips with phase angle jump are also considered. Design equations for the VSC are given, which allow calculating the required current rating of the converter switches to ride through different voltage dips in the grid. The effect of changing the input power of the DG system is also examined.