Differences Between Continuous Single-Phase and Online Three-Phase Power-Decoupled Converters

Second-order oscillating power is always generated by a single-phase converter. To decouple this oscillating power from its dc link requires a power decoupling circuit to operate continuously. In contrast, ac grid voltages supplying a three-phase converter are usually balanced. There will, hence, be no oscillating power, if grid currents are balanced too. However, grid voltages may become unbalanced during fault or other disturbances, which, when multiplied with balanced currents, will result in oscillating power. Power decoupling for a three-phase converter is, thus, only necessary during fault, rather than continuously. Its target should also be to improve power quality even during fault, rather than to prolong lifetimes of capacitors claimed with single-phase decoupling. Those differences cause criteria applicable to selecting a power decoupling circuit for single- and three-phase converters to be different, especially when there are seven different types of faulted voltage conditions to consider. These considerations are now studied, before presenting experimental results for verifying the effectiveness of all studied three-phase power-decoupled converters.