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In the new era, the presence of harmonics in electrical traction used in Indian railways has become one of the major concerns in the last decade. The quality of power supply has deteriorated as the voltage and current signals are loaded with unwanted frequency components. The cause of harmonic distortion in these locomotives is due to the traction substations (TSS) of Indian Railways take two-phase power (132 or 66 kV) and convert it to single-phase 25 kV power using a step-down transformer (usually in the range of 21.6 MVA capacity). Typically, two transformers are used, with one transformer normally remaining in standby mode. One of the terminals of the secondary side of the transformer is earthed, while the other terminal feeds the substation equipment and also the overhead line. The TSS, more specifically its 25 kV supply, experiences a fluctuating traction load of 50 to 800 ampere. The power factor also varies and can reach a low value of 0.8 at higher loads. The reactive power compensation requirement for such a TSS can reach as high as 4.0 to 7.0 MVAr, depending on the minimum power factor limit set.
The second major problem of TSS is the high percentage of current harmonics, especially of lower order, and the associated total distortion Current Harmonic Distortion () and total supply voltage distortion / Voltage Harmonic Distortion (). Due to the presence of a large number of rectifiers and thyristor converters in the electric traction, the current drawn from the 25 kV supply has a high percentage of lower order current harmonics (5, 7, 11, 13......) including the 3rd order current harmonics. Since the short circuit capacity (SCC) is limited at 25 kV, the voltage distortion increases. Both the current and voltage distortions need to be addressed as per the current / prevailing standard (IEEE 519 -2014). This is the problem that is considered and addressed in detail in this paper, considering both reliability and economics of the solution.
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