Estimation of electrical traction load harmonic impact on distribution networks

The paper deals with the time varying aspect of harmonics by facing the problem of harmonic impact estimation of DC electrified trains in operation in subway systems. To this aim, the paper illustrates the results of a detailed harmonic survey performed on such a system operating in an Italian city. The system provides railway lines supplied at 1,500 V DC through several AC-DC converters. The power supply of the system is provided at medium voltage (20 kV) from five different connections to the public distribution network. Fourteen AC-DC converter electrical substations (ESS) provide the total demand of the system of about 20 MW. In the paper, the problem of identifying the typical daily patterns of harmonic distortion due to the time-variant nonlinear traction load has been assessed. The results thus obtained are finally illustrated and discussed in due detail.