Predictive Voltage Control for Grid-Forming Power Converters with Virtual Output Impedance

The ever-increasing expansion of decentralized generation units based on renewable energy sources led to a paradigm shift in the operation of electrical power grids. Grid-forming and voltage-impressing control of grid-tied power converters are anticipated to guarantee stable grid operation. This paper presents a predictive voltage controller with a virtual impedance control loop to operate grid-tied power converters as voltage sources with defined (virtual) output impedance. The prominent finite-control-set model predictive control approach is implemented for the inner voltage control loop to achieve high dynamic and robust behavior. The outer virtual impedance loop is realized with a state feedback approach. To compensate inner voltage drops across the virtual impedance, another voltage reference regulation loop is introduced. The methodology and the respective design and implementation aspects are discussed on the example of a three-level voltage source inverter with LCL filter. To demonstrate the effectiveness of the proposed control scheme particularly during short-circuits, simulations are carried out. Further experiments on a dedicated converter test bench validate its dynamic and steady state control performance.