Enhanced Control of Superconducting Magnetic Energy Storage Integrated Upqc for Power Quality Improvement in Ev Charging Station

Abstract

Distribution-grid connected electric vehicle charging stations draw nonlinear current, which causes power quality issues including harmonic distortion, DC-link fluctuation etc. Recent literature found that a unified power quality conditioner with superconducting magnetic energy storage (UPQC-SMES) can alleviate charging induced power quality issues. However, the performance improvement can be greatly enhanced by suitably designed control strategy. In this paper, an effective control approach is proposed for the UPQC-SMES system. In the proposed method, an all-pass filter based quasi type-1 phase-locked loop is proposed, which can robustly detect voltage sag/swell for the control of UPQCs series converter. A robust sequence estimator based on exponential forgetting factor cost function is developed for nonlinear load current sequence estimation to be used inside UPQCs parallel converter control system. Finally, a nonlinear proportional-integral controller is applied to make the system fast responsive with lower DC-link ripple compared to a conventional linear proportional-integral controller. The combined use of SMES and nonlinear proportional-integral controller can significantly eliminate the voltage fluctuation in DC-link and reduce the size and cost of the DC-link capacitor, making the system more efficient. To enable grid-friendly charging operation, the proposed control strategy considers harmonic currents elimination, reactive power compensation, power factor improvement, restoring sags and swells of the grid voltage and mitigation of DC-link voltage fluctuations. Comparative verification results show that the total harmonic distortion by the proposed method always remained between 1.0 similar to 1.5% irrespective of the test cases, while the conventional approach could contain the distortion to less than 5% in some cases only.