Effects of unbalanced loads in a low-voltage network on flow distribution in a medium-voltage network

Abstract

We address the problem of improving the calculation accuracy of power flow in a medium-voltage distribution network based on the measurements of smart meters installed on the secondary side of 6(10)/0.4 kV transformers. In order to account for the effect of unbalanced loads in the low-voltage network on power flow in the medium-voltage network, three-phase three-wire lines were reduced to a single-line option. This enabled the use of symmetric mode calculation programs for the asymmetric mode. The loads in the medium-voltage network were determined by adding power losses in transformer windings and core to the loads measured on the secondary side of transformers. The calculation of winding power losses using the methods of phase coordinates and symmetrical components involves determination of currents in the windings of each phase according to 48 sections of load capacity and voltage module measurements, performed by the smart meter during the day. The correctness of expressions for calculating power losses in transformer windings is confirmed by the equality of total losses in phase coordinates and symmetrical components. The negative sequence power losses in transformer windings were found to be close to zero, while zero sequence losses are significantly lower than the positive sequence losses for almost all transformers with a double star-zero winding connection scheme, regardless of the load factor and rated power. The conducted studies confirmed the possibility and effectiveness of using smart meter measurements for determining loads and calculating power flow in the medium-voltage network. This conclusion was illustrated using an actual distribution network with 26 transformers. Future research should aim to clarify the mathematical models of transformers in the joint calculation of medium- and low-voltage distribution networks.