An Isolated Unidirectional DC Transformer With Active Current Reshaping for the Photovoltaic Plant DC Collection System

Unidirectional isolated dc converters are employed in the large-scale PV dc collection systems to connect subfields with the medium-voltage dc (MVdc) grid. As the dc grid voltage in PV systems varies within a narrow range, the single active bridge (SAB) topology is more competitive over partial-power (PP) converters due to its simpler structure and control. Notably, the SAB’s input-side IGBTs operate under hard switching mode. In contrast, the LLC converter can realize the quasi-zero-current switching (QZCS) operation to improve the system’s efficiency at the cost of a large resonant capacitor. However, the LLC’s main circuit must sustain a relatively high current stress caused by the corresponding sinusoidal waveform. Thus, this article proposes an LLC-derived novel dcX that inherits the same zero-voltage switching (ZVS) and QZCS functions, and replaces the large resonant capacitors with an auxiliary converter. The adopted auxiliary converter reshapes the main circuit current into a trapezoidal waveform, leading to a smaller current stress than the LLC and SAB counterparts. Validated by a 1.5 kV/15 kV/1 MW simulation, results show 5.9% and 4.36% reductions in power losses and volume compared with the LLC, and 38.01% and 9.17% compared with the SAB. A 75 V/300 V/1.5 kW scale-down prototype is also developed, proving the proposed corresponding energy balance control.