Modeling and Analysis of Zero-Sequence Current in Power Electronics Mission Profile Emulator for Photovoltaic Application

With the widespread integration of photovoltaic (PV) power generation into power grids, the reliability of PV converters is becoming crucial, and there is an urgent need for comprehensive testing before PV converters are put into field operation. Due to complex mission profiles, significant challenges have been experienced to reliability testing. A mission profile emulation (MPE) system, involving a three-level neutral-point-clamped (NPC) converter and a two-level converter, is proposed as a promising method for new approaches of converter testing, offering programmable mission profiles, improved testing efficiency and reduced testing costs. Typically, to reduce power loss, DC links of two converters are connected to achieve power circulation, which inevitably creates zero-sequence current, as can be classified as high-frequency zero-sequence ripples and low-order zero-sequence harmonics, and will further result in current distortions. However, in terms of MPE system with different converter topologies and mismatched carriers, mechanism of zero-sequence current excitation remains unclear, and potential sampling distortions and control errors have seldomly been studied. In this paper, the mathematical model of zero-sequence current in MPE system with different converter topologies and mismatched carriers is established. Impacts of zero-sequence current, including waveform distortions, sampling distortions and control errors, are also discussed in details.