Optimization-Based Fast-Frequency Estimation and Control of Low-Inertia Microgrids

• Published in: IEEE transactions on energy conversion Vol. 36; no. 2; pp. 1459 - 1468
• Main Authors: Tamrakar, Ujjwol, Copp, David A., Nguyen, Tu, Hansen, Timothy M., Tonkoski, Reinaldo
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.06.2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Algorithms; Distributed generation; Energy storage; Energy storage systems; fast-frequency support; Frequency control; Frequency deviation; Inertia; Low pass filters; Microgrids; model predictive control; Optimization; Phase locked loops; Phase locked systems; Power system dynamics; Power system stability; Predictive control; Quality of service; state estimation; Storage systems; virtual inertia
• ISSN: 0885-8969; 1558-0059
• DOI: 10.1109/TEC.2020.3040107

The lack of inertial response from non-synchronous, inverter-based generation in microgrids makes the power system vulnerable to a large rate of change of frequency (ROCOF) and frequency excursions. Energy storage systems (ESSs) can be utilized to provide fast-frequency support to prevent such large excursions in the system. However, fast-frequency support is a power-intensive application that has a significant impact on the ESS lifetime. In this paper, a framework that allows the ESS operator to provide fast-frequency support as a service is proposed. The framework maintains the desired quality-of-service (limiting the ROCOF and frequency) while taking into account the ESS lifetime and physical limits. The framework utilizes moving horizon estimation (MHE) to estimate the frequency deviation and ROCOF from noisy phase-locked loop (PLL) measurements. These estimates are employed by a model predictive control (MPC) algorithm that computes control actions by solving a finite-horizon, online optimization problem. Additionally, this approach avoids oscillatory behavior induced by delays that are common when using low-pass filters as with traditional derivative-based (virtual inertia) controllers. MATLAB/Simulink simulations on a test system from Cordova, Alaska, show the effectiveness of the MHE-MPC approach to reduce frequency deviations and ROCOF of a low-inertia microgrid.