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 its high conversion efficiency. However, it exhibits an oscillation of 100 Hz (double line frequency) in the high frequency (HF) link current, which increases device current stress and conduction loss. This paper proposes an analytical model which determines the magnitude of this oscillation at different power factor operations. The obvious solution to minimize this oscillation is to keep a high value of capacitor at each MVDC bus, but it reduces the power density of the system. Therefore, this paper proposes an improved solution with a combination of inductor and capacitor at the LVDC bus. The proposed filter reduces the RMS value of the HF link current by 29% at unity power factor operation. The dynamics of the proposed filter is also included in the control system design by deriving a dynamic equivalent circuit in this paper. The design is verified in a laboratory prototype of 8 kVA, 1.65 kV/300 V solid state transformer.