Voltage Unbalance Compensation with Smart Three-phase Loads

This paper describes the design, proof-of-concept simulations and laboratory test of an algorithm for controlling active front-end rectifiers to reduce voltage unbalance. Using inputs of RMS voltage, the rectifier controller allocates load unevenly on its 3 phases to compensate for voltage unbalance originating in the power supply network. Two variants of the algorithm are tested: first, using phase-neutral (P-N) voltage as input, second, using phase-phase (P-P) voltage. The control algorithm is described, and evaluated in simulations and laboratory tests. Two metrics for quantifying voltage unbalance are evaluated: one metric based on the maximum deviation of RMS P-N voltage from the average voltage and one metric based on negative sequence voltage. The tests show that controlling P-N voltage can in most cases eliminate the deviations of P-N voltage from the average voltage, but it does not reduce the negative sequence voltage. The controller that uses the P-P voltage as input eliminates the negative sequence voltage, and reduces P-N voltage deviations from the average to approximately half of their initial value. Current unbalance is reduced when the voltage unbalance is caused by asymmetrical loads, but it is increased in a scenario with unbalanced voltage sources. These results suggest that the optimal algorithm to reduce system unbalance depends on which system parameter is most important: RMS P-N voltage unbalance, negative sequence voltage, or current unbalance.