A Micro Grid Voltage Harmonic Suppression Strategy Based On H∞ Repetitive Control

With the vigorous development and utilization of new energy sources,in order to reduce the difficulty of power system dispatching and improve the reliability of power systems,distributed power sources mostly appear in the form of microgrids.There are a lot of nonlinear loads in the microgrid.When the microgrid is operating in an island,there is no large power grid to provide voltage support,and the nonlinear load harmonic current flows through the equivalent impedance and line impedance of the inverter,resulting in abnormal voltage at point of common coupling(PCC point),which affects the efficient and stable operation of microgrid.Therefore,this paper proposes a control strategy to improve the voltage quality of the PCC points in islanding microgrid by combining H∞ repetitive control,droop control and virtual impedance technology.Firstly,the output voltage control loop based on H ∞ repetitive control is designed to reshape the equivalent output impedance of the inverter as a small harmonic impedance.In the design,based on the controlled object model with active damping feedback of the filter inductor current,the reduced-order design method of the compensator in H ∞ repetitive control is studied.By reasonably configuring the damping coefficient,the good resonance suppression of LC filter can be achieved and the digital realization of the controller can be simplified.Compared with the passive damping method,it can reduce the system loss and increase the design flexibility of the controller.The introduction of adjustable active damping coefficient well reconciles the contradiction between the realization of H ∞ repetitive controller and the performance loss of the controller.Simulation and experiment prove that the active damping of the filter inductor current is superior to the passive damping method of the filter capacitor series resistance in reducing the output voltage THD of the H∞ repetitive control system.The FFT analysis of the output harmonic voltage under different load conditions verifies that the proposed design method of reducing the order of the compensator hardly leads to the performance degradation of the controller.Compared with PI controller,the proposed strategy can effectively reduce the harmonic distortion of inverter output voltage under nonlinear load.Then,in view of the situation that the traditional droop control strategy has uneven power distribution due to the difference in line impedance,an improved droop control strategy based on virtual impedance is proposed.The virtual impedance algorithm superimposes the harmonic voltage reference signal on the inverter fundamental wave output reference voltage,so that the impedance between the inverter and the common grid connection point is dominated by the designed virtual impedance,thereby reducing each inverter to the common The equivalent impedance difference between grid-connected points improves the power sharing accuracy.H∞ repetitive control exhibits a high open-loop gain at low harmonic frequencies,thereby enabling accurate tracking of the generated virtual harmonic voltage reference signal and ensuring accurate simulation of virtual impedance.Finally,combined with the designed H ∞ repetitive controller,virtual impedance technology and droop control,a control strategy for parallel operation of inverters that can improve the voltage quality of the PCC point of the low-voltage microgrid and achieve highprecision load power distribution is proposed to achieve While reducing the THD of the common point voltage of the microgrid,the parallel units can share the load current well,and the effectiveness of the proposed strategy is verified based on the simulation of the built microgrid model.