Output currents equalization for parallel connected three-phase PWM buck rectifiers

The three-phase PWM buck rectifier with wide output voltage range seems to be a promising solution for renewable energy sources. When the demanded nominal power is greater than single rectifier capability, the paralleling of converters is necessary. Furthermore, a proper modulation strategy allows the reduction in size of the LC filter. But in the case of parallel converters a symmetrical distribution of the power load must be guaranteed. Because of transformer-less connection it is highly probable that DC currents become unbalanced as a result of non-identical elements and input voltages distortions. Therefore, an algorithm is required to keep output currents differences inside the acceptable bandwidth. Based on the example of two parallel-connected PWM buck rectifiers, the paper describes output current balancing methods with mean values of the output voltages correction. After a theoretical background, a universal equalization algorithm is presented. Results of the tests conducted on a modular system 2×2 kVA rectifiers controlled by fixed point TMS320F2812 confirms the flexibility of the proposed solution.