Enhancing voltage stability of grid forming power converters based on model predictive controller

Abstract

This research article proposes an advanced control strategy based on a finite control set model predictive controller (FCS-MPC) for parallel-connected voltage source inverters (VSIs) for standalone operation of AC microgrids (MGs). The AC MGs may be consisted of two or more parallel connected VSIs connected and have ability to regulate the output line to line voltages at the point of common coupling (PCC) sustaining the local power demand. It is possible to attain these functionalities using traditional and linear control approaches, but there exist several challenges like sensitivity problems associated to parametric and non-parametric variations and they are unable to handle with existing constraints in system and resulting in slow dynamic response. An imperative feature of this research study is that proportional integral (PI) as well as proportional resonant (PR) controller based VSI was designed, studied and its performance was compared with FCS-MPC based VSI. The proposed FCS-MPC-based scheme handles several challenges effectually by confirming a stable and robust operation for Gridforming (GFM) inverters of AC MGs. In this scheme, the voltage reference over the predictive horizon is tracked by formulating a cost function (CF) and droop control is used to attain power sharing among distributed generations (DGs). The operation of standalone AC MG is authenticated by extensive simulations in MATLAB/Simulink environment, demonstrating that the FCS-MPC strategy shows quick dynamic response, improved power quality as well as enhanced voltage stability. The simulation results also reveal that total harmonic distortion (THD) of output line to line voltages is 0.86% for linear loads and 0.98 % for nonlinear loads, verifying that the THD level is primarily in the acceptable range defined by the IEC as well as IEEE standards, highlighting the AC MG system’s compliance with international standards.

Keywords

• Voltage source inverters
• Grid forming converters
• Standalone operation
• Model predictive control
• AC microgrids