LIGHT rail vehicles require a stabilised low voltage DC power supply for on-board equipment. This auxiliary supply must not be affected by fluctuations in the traction power supply, and it must also be isolated from the medium voltage potential to earth. Typically, this will nominally be 600 to 750V DC, but the actual line voltage can fluctuate between 400 and 1000V.

Transtechnik’s Combiverter uses static power conversion devices operating at more than 20kHz, which allows the size of the magnetic components to be reduced. The input stage normally consists of two half-bridges with an IGBT in each arm, and the mid-points connected to the primary of a transformer, which isolates the auxiliary supply from the line voltage. Fluctuations in the line voltage are compensated by adjusting the IGBT switching pulses.

One of the half-bridges has load-reducing capacitors connected in parallel with the IGBTs, allowing the latter to be turned off during periods of light load. The capacitors then maintain the mid-point voltage across the transformer primary. As the output load increases, the control signals for the IGBTs in this bridge are extended in time, using pulse-width modulation. All the IGBTs are switched only when either the voltage across them or the current through them is effectively zero. This feature reduces high-frequency interference, and also raises efficiency by reducing switching losses.

Series connection of several stages on the input side allows these converters to be fed from 1·5 kV and even 3 kV DC traction supplies. Where the low-voltage output required is in excess of 30 kVA, several stages can be connected in parallel.

Transtechnik GmbH, Germany

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