Control of Voltage Source Converters for Voltage Dip Mitigation in Shunt and Series Configurations

Licentiate thesis, 2004

Custom Power is the application of power electronics to improve the quality of power distribution for sensitive industrial plants. Industries reporting production stops due to voltage disturbances, like short interruptions and voltage dips, include paper mills, semiconductors facilities and other industries with fully automated production. Voltage dips are sudden drops in voltage, with duration between half a cycle and some seconds, mostly caused by clearing of short-circuit faults in the power system. Power electronic converters connected in shunt or series with the grid and equipped with energy storage can provide protection of sensitive processes against voltage dips. This thesis focuses on the control of Voltage Source Converter (VSC) connected in series or in shunt with the grid for mitigation of voltage dips. In both configurations, the core of the controller is the current controller. Since most voltage dips are unbalanced and have short duration, decisive factors for successful compensation are high speed of response and capability of handling unbalances. Three different algorithms for current control of VSC under unbalanced voltage conditions are presented and analyzed in detail. These algorithms include time delay compensation and reference voltage limitation with feedback, to improve the current control during overmodulation. Stability analysis of all three controllers is carried out. Different methods for sequence component detection are presented and compared, based on their transient performance. Simulations and experimental results on a 69-kVA prototype are presented and discussed for all three controllers. For use in a series-connected VSC, the three current controllers are completed with an outer voltage loop, thus realizing three cascade controllers that are presented and analyzed in detail. For all three controllers, stability analysis and simulation results with balanced and unbalanced voltage dips are also presented. The three controllers have been tested on a prototype series compensator and experimental results are presented in this thesis. Design and testing of the protection system for the series-connected VSC is also described in detail. Experimental results with non-linear load (diode rectifier) are shown. Finally, a similar cascade controller for voltage dip compensation with shunt-connected VSC is presented and tested via simulations. To improve the performance, a modified configuration including an LCL-filter between the VSC and the grid is proposed. It is also shown that by adding an active damping term in the inner current controller, undesired resonances between the filter components can be overcome and satisfactory mitigation of voltage dips can be achieved. A drawback is the high amount of injected current required if the grid is strong at the point of connection of the VSC.