Minimization of Low Frequency Current Oscillation in Resonant Link of a Solid State Transformer by Passive Filters

Solid State Transformer (SST) is expected to be a key component in future smart grid systems, which integrates MVAC grid with LV DC/AC grid. For a three stage SST, resonant converter is a potential candidate as the DC-DC stage due to high conversion efficiency. However, it exhibits an oscillation of 100Hz (double line frequency) in the high frequency (HF) link current, which increases device current stress and also increases conduction loss. This paper proposes an analytical model which determines the magnitude of the double line frequency oscillation in terms of input line current, input power factor, modulation index of AC-DC stage, and DC bus capacitors. The obvious solution to minimize the double line frequency oscillation is to keep a high value of capacitor at the MVDC bus. However, it reduces the power density of the system. Therefore, this paper proposes an improved solution with a combination of DC-link inductor and capacitor at LVDC bus, which reduces the double line frequency current oscillation in HF link without the need of high MVDC bus capacitors. The design is verified in a laboratory prototype of 8 kVA, 1.65kV/300V solid state transformer.