Grid-Supporting Battery Energy Storage Systems in Islanded Microgrids: A Data-Driven Control Approach

Islanded microgrids have low real and reactive power generation capacity and low synchronous inertia. This makes them susceptible to large frequency and voltage deviations caused by switching of large loads or equipment failures. These events deteriorate power quality and can cause frequency or voltage collapse. Grid-supporting lithium-ion battery energy storage units are a possible solution to this problem as they are able to respond quickly to changes of their real and reactive power set-points. In this paper, a decentralized grid-supporting inverter control structure for lithium-ion battery energy-storage units is proposed. A fixed-structure controller manipulates the energy storage unit's real and reactive power set-points, enabling fast response to voltage and frequency fluctuations of the microgrid. To tune the controller, no parametric model of the microgrid is required. Only the frequency-domain data, computed from a signal injection approach, are used to minimize H1 performance criteria. The performance of the proposed controller is verified through realtime software-in-the-loop simulation on a detailed microgrid model, where it is compared with a conventional inverse-droop controller. It is found that the proposed controller can effectively minimize the impact of a wide range of disturbances on the microgrid's voltage and frequency.

Frequency-Domain Control Design in Power Systems

Christoph Manuel Kammer

The scope of this thesis encompasses two main subjects: fixed-structure data-driven control design on one side, and control design in power systems on the other. The overall goal is to identify challenging and relevant problems in power systems, to express them as rigorous specifications from the viewpoint of control systems, and to solve them by developing and applying advanced methods in robust control. This work aims to combine expertise from both fields to open up a holistic perspective and bridge the gap between control and power systems. First, the derivation of a novel fixed-structure, data-driven frequency-domain control design method for multivariable systems is described. A key feature of the method is that only the frequency response of the plant is required for the design, and no parametric model is required. The designed controllers are fully parametrized in terms of matrix polynomial functions and can take a centralized, decentralized or distributed structure. The controller performance is formulated as H_2 and H_infinity constraints on any loop transfer function. A convex formulation of the optimization problem is derived, and it is shown that the solution converges to a locally optimal solution of the original problem. The versatility of the design method is demonstrated in various simulation examples, as well as in experiments on two electromechanical setups. Next, a frequency-domain modeling approach for power grids is discussed. A model based on dynamic phasors is developed that represents the electromagnetic and electromechanic dynamics of lines, inverters, synchronous machines and constant power loads. It also offers a modular structure that makes it straightforward to combine white-, grey- and blackbox models in a single framework. Then, the control design method and dynamic phasor model are applied in two relevant power systems case studies. First, the design of a decentralized current controller for parallel, grid-connected voltage source inverters in a typical distribution grid is considered. It is shown how performance specifications can be formulated as frequency-domain constraints in order to attenuate the resonances introduced by the output filters and coupling effects, and to provide robustness against model uncertainties and grid topology changes. The controllers for all VSIs are designed in a single step, and stability and performance is guaranteed by design. Furthermore, an approach for plug-and-play control design is presented. The results are validated in numerical simulation as well as in power-hardware-in-the-loop experiments. The second study concerns the design of a distributed controller that combines primary and secondary frequency and voltage control for an islanded, meshed low-voltage grid with any number of voltage source inverters and synchronous generators in a single framework. No assumption on the R/X-ratio of the lines is made, and it is shown how advanced control specifications such as proportional active power sharing, zero frequency steady-state error and decoupling can be formulated as constraints on the norm of weighted sensitivity functions. Furthermore, the communication delays of the distributed controller are considered exactly during the design. The controller is implemented in numerical simulation, and results show significantly improved performance as compared to the classical hierarchical structure.

EPFL2018

Frequency-Domain Control Design in Power Systems

Christoph Manuel Kammer

EPFL2018