DC-Link Voltage Dynamics of Three-Level Four-Wire Grid-Connected Converters during Harmonics Injection Operations

As the amount of decentralized electric power generation is steadily growing, not only the direction of power flow, but also the overall behavior of the grid changes. This means that harmonic voltages in the supply grid become more problematic. To deal with this problem, inverters of decentralized renewable energy resources can help to mitigate these harmonic grid voltages by injecting harmonic currents. However, this so-called active filter functionality leads to additional stress of the dc-link capacitor due to inevitable oscillations of the dc-link current and voltage. Hence, this article for the first time presents an analytical derivation of these dc-link oscillations caused by a harmonic current injection into the grid. The analysis is presented for two-level and three-level inverters with split dc-link in a four-wire configuration. Simulation and experimental measurement results show the validity of the proposed analytical calculations. Furthermore, the impact of these dc-link harmonics on the capacitor lifetime is discussed. An exemplary scenario with a 10-KW photovoltaic inverter mitigating a 5th-harmonic grid voltage shows a possible lifetime reduction of the dc-link capacitor of up to 52% due to the oscillations in the dc-link voltage and current. Thus, this article aims to raise awareness of converter designers regarding this lifetime reduction, so that they take these oscillations into account during the design process and avoid unexpected early failure of the dc-link capacitor.