Analysis and Control of DC-link Oscillations of Voltage Source Inverters during Unbalanced Grid Faults

The increasing amount of renewable energy sources in power systems requires more stringent rules for the grid connected power converters. Especially the Fault Ride-Through during unbalanced grid faults is critical for the converter and its control. These unbalanced grid faults result in power oscillations, which propagate to the DC-link, leading to double-fundamental-frequency oscillations of the DC-link voltage, affecting the aging of the capacitors and performance of the converter control. Hence, limiting these oscillations is crucial for the reliability and performance of the overall inverter system. To understand the reason why these oscillations occur, this paper derives an analytical model of the DC-link voltage oscillations based on the Instantaneous Power Theory for multiphase AC systems. The model is used to identify the ratio of positive to negative sequence currents as suitable mean to determine the DC-link oscillations during unbalanced grid faults. A control strategy is presented, which sufficiently rejects the DC-link oscillations for all power operation points and fault types. Recent grid codes demand maximum current injection of the inverter to improve the grid voltage support in most applications. This makes it necessary to implement a suitable Peak-Current Limitation. This limitation must guarantee the rejection of DC-link voltage oscillations even in current limitation mode. As opposed to conventional approaches, a positive and negative sequence current vector limitation is proposed. This sufficiently limits the maximum output current and maintains the AC and DC-power oscillation characteristics of the inverter. In summary, the DC-link oscillations are analytically described. The proposed control strategy with a Peak Current Limitation completely rejects the double-fundamental frequency DC-link oscillations during all fault types and operation points. These results are validated in a numerical simulation and an experimental setup under various fault scenarios and operation points.