An online measurement method for common-mode impedance in three-phase grid-connected converters

Three-phase converters are common in modern power systems in interfacing of renewable sources and for improving power quality. A three-phase converter, whether operating as an inverter or active front-end, produces a varying common-mode voltage when conventional sinusoidal PWM is used. Frequency components of the common-mode voltage are located at the switching frequency which ranges usually from few kilohertz to tens of kilohertz and its multiples. The high-frequency AC component produces common-mode current which depends on the magnitude of common-mode impedance seen by the power converter. The common-mode current is usually mitigated by using common-mode chokes, complex passive filter topologies or active methods. The design of these filters is not straight-forward since the common-mode impedance is often unknown and may exhibit resonances. Therefore, an accurate method for measuring common-mode impedance is needed to enable deterministic filter design. Moreover, the common-mode impedance should be measured online, i.e., in the normal operating environment of the converter. In this paper an online method for measuring the common-mode impedance is proposed. The method can be implemented by using a converter which is able to achieve high switching frequency, such as an inverter based on SiC-switches.