DC Step-up Transformer and Its Control for Photovoltaic

According to the demand of photovoltaic(PV) connection with medium voltage DC grid, a PV DC step-up transformer based on power electronics is proposed, with H bridges connected in parallel at low voltage (LV) level, connected in series at medium voltage (MV) level, and is isolated by high frequency transformer. LVDC is directly boosted to MVDC with no DC-AC-DC converts. As the power flows from PV to grid is unidirectional, the H bridges at MV level are designed to be consisted of diodes to reduce the amount of IGBTs and cost of the device. Characteristics of the topology are analyzed, and formula of the equivalent duty ratio is derived. The duty ratio control is applied for PV DC step-up transformer with carrier phase shift technique. For each module, a unipolar& frequency multiplication modulation algorithm with IGBT's equal ON and OFF time is proposed to ensure the uniformity of IGBT's electrical stress and the reliability of the device. Finally, the proposed topology and control strategy is verified by the simulation on PSCAD/EMTDC platform. Results show that the voltage and current are shared equally among each sub-module, and the current fluctuation injected to network is very small. The experimental verification of the proposed modulation strategy is also carried out.