Online Impedance Measurement of a Modular Multilevel Converter

This paper presents an online method to obtain the frequency-dependent impedance of a device under test based on the perturbation voltage approach. For this purpose, the system is perturbed for defined frequencies over time. The impedance at perturbation frequency is calculated at the same time as the voltage and current measurements are taken using the definition of Fourier series. Thus, the immediate response of the device under test and the factors influencing the measurement can be directly investigated. The method is applied on an average value model of a modular multilevel converter controlled by a cascaded vector control. It is shown that outer control loops such as active power and DC voltage control influence the frequency-dependent impedance of the converter at lower frequencies in positive and negative sequence and should therefore not be neglected for harmonic stability studies.