Integrated isolation and voltage balancing link of 3-phase 3-level PWM rectifier and inverter systems

For a 3-phase pulse width modulated high-bandwidth AC voltage source, this paper presents a series resonant DC-DC converter (SRC) with a high-frequency transformer operated by only two half-bridges interconnecting a 3-phase 3-level rectifier and 3-phase plus neutral conductor 3-level inverter stage. On the primary side, one terminal of the transformer is connected through the resonant capacitor and inductor to one bridge-leg output and the other terminal is connected to the DC-link midpoint. On the secondary side, both terminals directly connect to the second bridge-leg output and the DC-link midpoint. With the proposed SRC, the galvanic isolation and the balancing of the capacitor voltages of the inverter-side split DC-link can be achieved also for an unequal loading of the DC-link capacitors. The AC source needs to supply not only passive symmetrical 3-phase loads, but also passive or active single-phase, 2-phase, DC and asymmetrical 3-phase loads. Hence, the unequal loading of the DC-link capacitors can be generated, for example, in case a DC load is connected to the 4-wire inverter stage. The operation principle of the SRC is described in detail for an unequal loading of the DC-link capacitors. Moreover, design guidelines for the suggested SRC are derived and finally the theoretical analysis is successfully verified by measurements conducted on a 1 kW proof-of-concept SRC prototype.