On Control of Grid-connected Voltage Source Converters - Mitigation of Voltage Dips and Subsynchronous Resonances

Doctoral thesis, 2007

Custom Power and Flexible AC Transmission Systems (FACTS) denote the application of power electronics in distribution and transmission networks, respectively. Custom Power is the application of power electronics to improve the quality of power distribution for sensitive industrial plants. Power electronic converters connected in shunt or series with the grid and equipped with energy storage can provide protection of sensitive processes against voltage disturbances, like short interruptions and voltage dips. The first part of 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 control system is the current controller. Here, the deadbeat current controller for grid-connected VSC is presented and analyzed in detail. The controller includes time delay compensation and reference voltage limitation with feedback, to improve the current control during overmodulation. Improvements for proper control of the current under unbalanced conditions of the grid voltage are investigated. For use in a series-connected VSC, the deadbeat current controller is completed with an outer voltage loop, thus realizing a cascade controller that is presented and analyzed in detail. Further, a similar cascade controller for voltage dip compensation using shunt-connected VSC is investigated. In both configurations, it is shown that control of the negative-sequence component of the injected voltage is needed for a proper mitigation of unbalanced voltage dips. FACTS is instead the application of power electronics at transmission level. In transmission systems, other control objectives are more important than voltage dip compensation. Controllable series compensation is used for e.g. power flow control, stability improvement, and damping of power oscillations. Traditional non-controllable series compensation based on series capacitors can create problems due to unwanted resonance with the rest of the power system. A specific problem that often arises in conjunction with series capacitors is subsynchronous resonance (SSR), which can lead to damage of generator shafts. In this case, a series-connected VSC, similar to the one used for the distribution system and here called Static Synchronous Series Compensator (SSSC), could be used as a dedicated device for SSR mitigation. In the second part of this thesis, a novel control strategy for the SSSC for SSR mitigation is investigated and analyzed. It is shown that, by injecting only a subsynchronous voltage into the power system, SSR mitigation is achieved by increasing the network damping only at those frequencies that are of danger for the generator-shaft system. This will allow to provide SSR damping with very low voltage injection, leading to a cost-effective alternative to the existing solutions.