Reduction of Phase Unbalance in Low-Voltage Urban Distribution Networks through Optimal Rephasing

Abstract

Relevance: Uneven allocation of single-phase consumers in low-voltage urban networks causes phase-current unbalance, increased fundamental-frequency neutral current, unequal bus voltages and additional conductor losses. Rephasing can improve the operating state without new compensating equipment, but both the number of switching operations and feeder losses should be considered.

Aim: to reach a prescribed phase active-power imbalance limit with the minimum number of rephasings and then select, within that minimum-switch layer, the configuration with the lowest active-power loss in a 0.4 kV                     three-phase four-wire feeder.

Methods: a 230/400 V five-section radial feeder with 15 equivalent single-phase load groups was studied. The minimum number of rephasings was determined by complete combinatorial enumeration. All admissible               configurations in the minimum layer were then evaluated by nonlinear three-phase four-wire power-flow               calculation. Sensitivity to the imbalance limit, loading level and neutral resistance was also examined.

Results: for the 5% imbalance target the minimum number of rephasings is three, with 57 admissible                               configurations. The L1: A→C, L7: A→B, L8: B→C configuration gives the minimum active-power loss of 5.3051 kW, a fundamental neutral current of 7.29 A and a maximum negative-sequence voltage factor of 0.237%. Relative to the initial 7.5850 kW, active-power loss is reduced by 30.06%.