Sliding Mode Control for Power Electronic Converters in Transmission andDistribution Grids- Applied to Three-Phase LCL-Filter Grid Coupling and Series Converteror UPFC

With the introduction of wide-bandgap semiconductors, significantly higher switching frequencies are feasible for power electronic applications; computa- tional power to process the control algorithms is however not increasing with the same pace. Sliding mode controllers are known to have low computational demands and high robustness. Unlike continuous control methods, a sliding mode controller produces discrete control outputs, similar to the desired discrete switching behaviour of power electronics. This difference allows sliding mode control to combine two control levels, the external control level - where the application control goals are achieved, and the internal control - where the switching states of the power electronic converter are decided. We investigate if these controllers are able to reach state-of-the-art performance in power electronic applications in the distribution and transmission grid. Specifically, we investigate sliding mode control of three-phase LCL-filter grid connections and of a series converter of Unified Power Flow Controllers. In literature, the design of an LCL-filter for power electronic applications is based on recursive calculations and/or experience, with no guarantee of optimality. We adapt an analytic design method for analog filters in signal transmission to a grid-connection filter interpretation. Based on inductor design rules, we expand the method to incorporate the inherent impedances of power components as compared to the discrete components it was designed for. This allows us to optimise the controller analytically to any function of the filter components. We design an LCL-filter optimised for total life-cycle cost including the projected incurred power losses. Compared to other design methods, our resulting LCL- filter is significantly cheaper in the projected life cycle. Previous work did not yet design a sliding mode controller for a three-phase LCL-filter grid connection. Based on the dynamic model of an LCL-filter grid connection, we develop an external and internal sliding mode controller. The design method and tuning of the sliding mode controller is based on a single- phase model. Using a detailed three-phase model including the power-electronic switches we simulate the designed sliding mode controller. We compare the implementation of a three-phase abc and an equivalent two-phase αβ version. The two-phase implementation reduces competing switching decisions between the three phases, creating a more performant control under equal conditions. Previous work concerning the power flow control with a UPFC focusses on steady-state models of the control problem which results in lower dynamic performance. Complex cross-coupling compensations and tuning mechanisms fail to respond optimally to reference changes and create averse affects during unbalanced conditions. The internal power electronic aspects of the control issue are generally considered out of scope. Using dynamic power flow models and the instantaneous derivations, we isolate the key instantaneous system dynamics and develop a Dynamic Inverse Model Controller as well as a Direct Power Controller. The Inverse Model Controller is an continuous external controller combined with a sliding mode internal controller, the direct power controller is a combined external and internal sliding mode controller. Based on a detailed model of a UPFC equipped with a multi-level inverter, including power-electronic switches, we simulate the developed control systems under balanced and unbalanced conditions, while comparing their performance to continuous controllers from literature. Both developed controllers outperform the literature in balanced as well as unbalanced conditions, the Direct Power Controller having the best performance in all cases. In a scaled laboratory model with a multi-level inverter, we demonstrate the Direct Power Controller in balanced conditions. The congruence of the experimental and simulated results convinces us that we fully comprehend the control problem and demonstrate the performance of our developed controllers adequately. With this work we demonstrate that sliding mode control is an adequate control method for power-electronic applications. The design methodology explained in this work is easily adaptable to other converter topologies and various other applications. The totality of the work spans several types of control problems and demonstrates adequate solutions to each of them, easily transportable to other applications.

Publication year:2015

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