Power Quality Improvement in ac Railway Substations: The concept of chopper-controlled impedance.

The 25-kV/50-Hz ac single-phase supply is a widely used railway system in France with a length of 9,698 km. Overhead lines are supplied by substations drawing power from two phases of a three-phase utility. They behave as nonlinear and time-varying loads and represent one of the most important sources of voltage unbalance for the electricity-transmission network. In the case of weak networks, railway operators are required to install compensation systems in substations to satisfy utility regulations and to avoid penalties regarding voltage unbalance and reactive power consumption. The limits are established by the energy provider with a view to guaranteeing an acceptable power quality to other customers.