Sliding Mode Control Enabled Hybrid Energy Storage System for Islanded DC Microgrids with Pulsing Loads

This paper aims to improve the control of Hybrid Energy Storage Systems (HESS) within an islanded DC microgrid with pulsing power loads. While the PV power generation unit operates as the main power source, a combination of battery and supercapacitor is incorporated to efficiently fulfill the excess power demand based on different loading conditions. In order to ensure the proper battery discharge rate, the conventional low pass filtering approach for decoupling the average and transient HESS current components is replaced with a single rate limiter. By the elimination of low pass filter (LPF), the challenges for non-systematic selection of cut-off frequency is rectified as more tangible factors, such as battery discharge rate, can be incorporated for decoupling purposes. In addition, the associated undesirable phase lag and enhanced nonlinear effect on the current responses caused by the low-order LPFs are avoided. To ensure proper tracking of the rate-limited current setpoints and avoid common windup issues associated with PI controllers in the presence of system nonlinearities and uncertainties, the Sliding Mode Control (SMC) is employed for the battery current regulation. In order to address the sluggish response of linear controller to the fast transients frequently observed on the supercapacitor current setpoints, the same sliding surface structure with modified tuning is utilized. For the supercapacitor cyclic charging instances, an on-the-fly charging approach is deployed, which unlike the previously reported schemes ensures uninterruptible charging in the presence of frequent load transients such as pulsing power loads. To verify the effectiveness of the proposed scheme, the real-time simulation studies using Typhoon HIL-402 is performed.