Parallel-Connected Voltage Source Converters With a DC Common Mode and an AC Differential Mode PWM Filter

Two 3-phase converters can be connected in parallel using a range of inductor types. Coupled inductors used together with interleaved pulsewidth modulated (PWM) techniques can produce a high-quality multilevel PWM line output voltage. These inductors can be designed to have a high inductance between the two converter outputs to control circulating currents, combined with a low per-phase output inductance. Since the 3-phase PWM voltage between the two converters consists of two components, a differential mode and common mode, two inductor types can be used to separately filter the circulating currents produced by these two components. The inductor types presented places one coupled inductor between the 3-phase output terminals of the two converters, a
3-limb CI
, and a second coupled inductor located between the two converter dc input terminals, a dc coupled inductor or
dc choke
. The
3-limb CI
filters circulating currents due to the differential mode PWM voltage and the
dc choke
filters circulating currents due to the common mode PWM voltage. The
dc choke
uses four windings in contrast to the six required of an equivalent common mode 3-phase ac choke. The two converters are operated with a filter capacitor across their dc input terminals. The 3-phase output of the two parallel converters is located at the center tap of the
3-limb CI
windings in each phase and a very low effective per-phase output inductance is obtained. Consequently, the converter structure can supply very high-frequency fundamental output voltages with high-quality multilevel PWM line voltages. The performance of the coupled inductors presented is compared with a conventional coupled inductor and designs show that they can be 13% smaller with 16% lower losses. The parallel converter structure presented is suitable for high-power high-speed electric drives with applications in aerospace and electric vehicles where lightweight magnetics is required. Simulations and experimental results are presented for an 8 kW (300 Vdc, 208 Vac/22A) prototype operating with a fundamental frequency of 60 Hz –1.1 kHz.