Secondary Frequency Regulation in Islanded Microgrids

In the modern power system, microgrids (MGs) are considered as a controllable element of the smart grids. MGs may have either renewable or non-renewable generating units. In islanded MGs, an active power imbalance between generation and load demand can be established cause of the stochastic variations of renewable units output power. This will affect the MG frequency hence the stable operation of the system. In this paper, a secondary frequency regulation framework for islanded MG based on Linear Quadratic Control (LQR) and Model Predictive Control (MPC), has been proposed. The secondary frequency control aims to cancel the steady state error following the primary control is applied to MG generators. Methods using traditional controllers face difficulty in tuning and calculating their parameters when applied to multiple-input, multiple-output MIMO systems. Moreover, constraints cannot be applied in LQR controllers, unlike MPC, which allows for constraints to be included within its algorithm. The linear model of the MG was first extracted. As some of system's state variables are difficult to measure physically, so Kalman state estimator was used. The proposed MG consists of a set of loads, a diesel generator, a solar generator, batteries and supercapacitors. Performance comparison, when load variations have been applied, showed that MPC satisfies the constraints compared to LQR while LQR presents a fast settling time and lower overshoot.