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

Ting Yang,Zhaoxia Wang,Yajian Zhang,Haibo Pen

doi:10.3390/en11092388

Ting Yang, Zhaoxia Wang + Show 2 more

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https://doi.org/10.3390/en11092388

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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.

Highlights

Energies such as photovoltaic (PV) and wind power are widely applied as the main sources for independent microgrids (MGs), since they enjoy advantages such as cleanliness and renewability [1,2,3].the intermittent and stochastic active power output may have a detrimental effect on system frequency stability [4]
The reasons can be summarized as follows: (i) a mismatch between power load demand and power generation may be exacerbated by the superposition of the renewable/load fluctuation, which may lead to frequency deviation problems or even over-limit [5]; (ii) the application of large-scale renewables in independent MGs is equivalent to decreasing the proportion of traditional synchronous generators
To solve the proposed uncertain state-space model of secondary frequency frequency regulation in independent MGs, a novel secondary controller is designed based on the regulation in independent MGs, a novel secondary controller is designed based on the linear quadratic Gaussian (LQG) method

Summary

Introduction

Energies such as photovoltaic (PV) and wind power are widely applied as the main sources for independent microgrids (MGs), since they enjoy advantages such as cleanliness and renewability [1,2,3]. The intermittent and stochastic active power output may have a detrimental effect on system frequency stability [4]. The frequency oscillation will be intensified when power mismatching occurs in independent MGs. the flexible access/exit of renewable energy and loads may change the structure and the operating parameters of the MG system. The flexible access/exit of renewable energy and loads may change the structure and the operating parameters of the MG system All these difficulties have resulted in higher requirements of the robustness and dynamic performance of the frequency control system [8]

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Results

Conclusion