Advanced Control of Modularized Bridge Rectifier Solid-State Transformers in MVAC-LVDC Applications

Solid-state transformers (SSTs) enable the interfacing of medium-voltage (MV) ac grids to low-voltage (LV) dc loads with high power density and advanced power flow control over traditional approaches based on low-frequency transformers (LFTs). The modularized bridge rectifier (mBR) is a promising, yet understudied, topology which has been recently proposed for highpower electric vehicle (EV) charging applications. The mBR is essentially composed of a three-phase diode rectifier with isolated dc-dc converters connected in parallel to the series-connected diodes, thus forming six branches; the LV outputs of the isolated dc-dc converters are connected in parallel to the LVdc port. In this paper, the mBR SST and its operating principle are first recalled along with an in-depth analysis of the state-of-the-art branch-oriented current control method, which, however, cannot adequately handle the inherent coupling between the branches. Thus, a novel ΣΔ-Vector modeling and control method is proposed, which is based on the Clarke transform and a subsequent ΣΔ decoupling to treat the sums (Σ) and differences (Δ) of the branch quantities separately. The two corresponding equivalent circuits contain only the four effectively available degrees of freedom the mBR topology offers, and hence facilitate a clear analysis and a straightforward, decoupled controller design. Finally, the novel ΣΔ-Vector control method is validated by means of circuit simulations of a 1-MW, 10-kV case study system, and found to show better dynamic performance and robustness than the conventional branch-oriented control method.