Calculation of Voltage Unbalance in Transmission Systems Due to AC Railway Operation: A Simplified Methodology Using Network Admittance Matrix

Modern AC railway is well known for introducing large asymmetrical loads which can lead to considerable levels of voltage unbalance in power transmission systems. Unbalance assessment is often limited to a single traction substation contribution and only at the connection point. However, as unbalance propagates from the railway feeding nodes through the electric network, interactions between different traction loads can be significant, and therefore should not be neglected. Three-phase power flow calculations implemented by power systems software can be used for this kind of studies. As an alternative, this paper proposes a simplified but efficient methodology for calculating voltage unbalance through the entire system due to one or multiple ac traction loads. The main idea behind this approach is the simplification of the power flow problem into a system of linear equations which can be solved using the admittance matrix of the network. The good accuracy and the straightforward application of the suggested method can make it particularly useful in planning activities of railway operators.