Frequency control of networked microgrids with voltage constraints

Abstract

Due to environmental and economic concerns, small distributed generators (DGs) powered by renewables become increasingly popular. However, the growing penetration of renewable-powered DGs also brings us challenges. Due to their low inertia and fast changes in power outputs, the system frequency may easily deviate from nominal values and large frequency excursions may occur, even when the frequency control reserve is sufficient. The concept of microgrid (MG) has been proposed as an effective way to incorporate more renewables. Moreover, MGs can reduce dependence on long transmission lines and can integrate various distributed units such as synchronous generators, renewable-powered DGs, and energy storage devices (ESDs). However, designing a proper control scheme to coordinate different units in an MG to stabilize the system frequency is still a big challenge. Moreover, voltage limits must be considered when designing the frequency control scheme due to the coupling of active and reactive power in MGs. Additionally, with the development of MGs, the number of MGs is expected to grow. By connecting nearby MGs together, a larger networked MG system (NMS) will form. Through interactive support among MGs, the NMS shows better system reliability and flexibility compared to an isolated MG. But it also brings us the challenge of MG coordination. This thesis focuses on the design of the frequency control scheme for NMS that aims to coordinate the controllable units within voltage limits. Firstly, by utilizing the characteristic of voltage-sensitive loads, a novel dual-mode voltage-based frequency controller (VFC) is developed. The proposed controller has two operating modes: a frequency restoration mode to restore the frequency immediately upon the frequency deviation from the nominal value being beyond a certain limit, and a voltage restoration mode to restore the voltage after the frequency is recovered to a satisfactory range. A proper switching logic is designed to enable the mode switching. Therefore, the controller can reserve enough voltage margin to stabilize the frequency under consecutive disturbances, as well as to mitigate any negative impact on loads. To achieve a more comprehensive control that takes system dynamics and constraints into account and is adaptive to the changing system conditions, the NMS frequency control problem is formulated into a model-predictive-control (MPC) problem, and decentralized and centralized MPC-based frequency control schemes are proposed. For better scalability and privacy, a distributed MPCbased frequency controller is proposed, where the optimal solution of the MPC problem is obtained via cooperative optimization among neighboring MGs by applying the alternating direction method of multipliers algorithm. Finally, a fully distributed control scheme designed for inverter-based NMS with loads, renewable-powered DGs and ESDs is proposed. The frequency regulation problem is formulated as an MPC problem that aims to maintain the supply-demand balance within each microgrid and net tie-line power between connected microgrids at minimal regulation costs. The effciency of ESDs, constraints on bus voltages and energy of ESDs are also considered. The problem is solved by a distributed projection-based algorithm via peer-to-peer communication between neighboring buses.