Secondary Frequency Stochastic Optimal Control in Independent Microgrids with Virtual Synchronous Generator-Controlled Energy Storage Systems

Abstract

With the increasing proportion of renewable energy in microgrids (MGs), its stochastic fluctuation of output power has posed challenges to system safety and operation, especially frequency stability. Virtual synchronous generator (VSG) technology, as one effective method, was used to smoothen frequency fluctuation and improve the system's dynamic performance, which can simulate the inertia and damping of the traditional synchronous generator. This study outlines the integration of VSG-controlled energy storage systems (ESSs) and traditional synchronous generators so they jointly participate in secondary frequency regulation in an independent MG. Firstly, a new uncertain state-space model for secondary frequency control is established, considering the measurement noises and modelling error. Then, an improved linear quadratic Gaussian (LQG) controller is designed based on stochastic optimal control theory, in which the dynamic performance index weighting matrices are optimized by combining loop transfer recovery (LTR) technology and the distribution estimation algorithm. On the issue of secondary frequency devices' output power allocation, the dynamic participation factors based on the ESS's current state of charge (SOC) are proposed to prevent the batteries' overcharging and overdischarging problems. The energy storage devices' service life can be prolonged and OPEX (operational expenditure) decreased. Multiple experimental scenarios with real parameters of MGs are employed to evaluate the performance of the proposed algorithm. The results show that, compared with the lead-compensated-proportional-integral-derivative (LC-PID) control and robust µ-control algorithms, the proposed stochastic optimal control method has a faster dynamic response and is more robust, and the fluctuations from renewable energy and power loads can be smoothened more effectively.

Fractional-Order Model Predictive Frequency Control of an Islanded Microgrid

“…When a microgrid is islanded, an optimal controller should be designed to minimize the frequency deviations. In recent years, some research works have been reported related to the optimal frequency control issue of an islanded microgrid [7][8][9][10], but most works have focused on proportional-integral-derivative (PID) control method and its different kinds of improved versions based on intelligent optimization algorithms.…”

Efficient Economic and Resilience-Based Optimization for Disaster Recovery Management of Critical Infrastructures

“…Similarly, N1 c M,t in Equation (14) denotes the binary parameter for N-1 contingency state of the transmission line between the MV and LV level. The N1 c M,t = 0 forces the transmission flows to be zero when the transmission line is in contingency state and otherwise, N1 c M,t = 1 for normal operations.…”

“…A smart energy management system to optimize the operation of microgrid was presented by Chen et al [13] where the optimization problem of ESS, economic load dispatch and the optimization of DER was simplified into a single-objective problem. Yang et al [14] proposed a novel stochastic optimal control strategy focusing in independent microgrids based on the integration of ESSs and traditional synchronous generators. The approach was based on an improved design of linear quadratic Gaussian controller and the consideration of the ESS's current state of charge (SOC) that prevented the batteries' overcharging and overdischarging problems, thus adaptively regulating the power output to minimize the frequency fluctuations.…”

Adaptive Command-Filtered Backstepping Control for Virtual Synchronous Generators

“…By introducing the rotor swing equation, the virtual inertia and primary frequency regulation instructions were conceived to simulate the external characteristics of the SG using this concept. The VSG can increase the inertia of the inverter and improve the system output response [6][7][8][9]. Although the VSG is equivalent to the traditional synchronous generator in terms of output characteristics, it is still difficult to maintain system stability under high transient conditions.…”