Mechanism and Solution of Inaccurate Perturbation Output of DC-AC Type IMU

Impedance measurement unit (IMU) with injection of broadband perturbations is widely used to obtain grid and converter impedance. The accuracy of measured impedance and the power quality of the measured system rely on the amplitude control of the injected perturbations. However, the accuracy of output perturbation of IMU is lack of attention especially around the fundamental frequency of the measured system. In this brief, a three-phase DC-AC type IMU is introduced and the mechanism and solution of inaccurate perturbation output of the IMU is study. First, the DC-AC type IMU with PR current controllers, DC voltage controller and phase locked loop (PLL) is introduced. Next, the mechanism of inaccurate perturbation output is analysis, which is caused by the introduction of DC voltage ripple. Then, improved DC voltage control solutions with notch filter or delay filter are proposed to achieve the accurate control of single-frequency and multi-frequency perturbation current respectively. Finally, control hardware-in-the-loop experiment results are given to demonstrate and verify the effectiveness and feasibility of the proposed method.