UPS for rail signalling application

International Railway Journal, March, 2003

A 25kV traction supply Uninterruptible Power Supply (UPS) can provide an alternative to a diesel generator as a source of secure and stable ac power for signalling in situations where both overhead traction and 400V three-phase power supplies are available.

DIESEL generators are often used to back-up the regional electricity company's (REC) supply, but their capital and maintenance costs are high. The alternative source of supply from any 25kV overhead catenary anywhere in the world is often viewed as a "dirty" ac supply source and therefore unacceptable for signalling applications.

However, Chloride Power Protection, in a joint development with the engineering group of Network Rail, Britain, has designed its Traction Supply UPS that can accept the dirty ac supply from the 25kV overhead line and therefore eliminate the need for an expensive back-up generator. More than 20 systems have been built for use on Network Rail and Britain's Channel Tunnel Rail Link. They are also in use with French National Railways (SNCF) in the Paris area, and are applicable to any 25kV ac electrification system anywhere in the world.

Network Rail is a substantial user of diesel generators to provide alternative ac power to enable the continued operation of signalling systems in the event of failures in the incoming supply. The generator is normally designed to start automatically in the event of failure of the mains and in key locations UPS systems are also fitted. The UPS ensures continuity of signalling power during the time that it takes to detect the loss of mains power and to start up the diesel generator.

Until now, the characteristics of the overhead supply have precluded its use for UPS. Measurements show that not only is the waveform often very distorted with frequent high energy spikes, transients, high harmonic content and multiple "zero crossovers" but there are also regular voltage excursions as wide as 17kV to 29kV. Such an ac supply is certainly not suitable for solid state interlocking (SSI) based signalling that needs a very stable and clean ac supply source.

The new Chloride UPS provides an ac output that is both sinusoidal and stable to [plus or minus]1% of the required supply voltage with less than 5% total harmonic distortion, even with the constant electrical "noise" of the traction supply and with its wide voltage variations.

The Traction Supply UPS is designed to accept its supply from either the traction supply or the 400V REC supply. The rectifier/charger is equipped with an automatic changeover arrangement to enable it to be supplied by the REC supply as an alternative, thus enabling the UPS to continue to operate indefinitely until the 25kV supply is restored.

The two inputs to the rectifier of the UPS could be designed to accept different supply voltage inputs anywhere in the world. Many locations have a 650V single phase or 800V traction-derived supply available and this can easily be used in conjunction with the 400V phase-to-phase voltage available from the REC incoming supply.

Furthermore, either of the two supplies can be designated as the preferred source with reversion to the alternative supply in an emergency. The UPS output is unaffected by transfers between incoming supplies.

It can be seen from Figure 1 that there is no possibility that the system load can be supplied directly from the 25kV traction supply at any time. This ensures that there is no chance of the dirty 25kV-derived supply from ever being connected to the critical signalling supply load. This would not be the case if a series-connected UPS were to be used (Figure 2).

In the event of a fault on the UPS inverter output, or in the event of a serious overload, the UPS static switch simply transfers the signalling load to the REC supply without a break in power. This transfer is made seamlessly without any break in the electrical supply to the system. As soon as inverter operation is restored or the load returns within limits, the system recovers automatically and reverts to normal operation again by transferring the system load back to the UPS-derived output without any break in supply to the load.

The trial site at Whitmore near Crewe has proved the effectiveness of the 25kV Traction Supply UPS. Despite large ac voltage and waveform disturbances on the 25kV supply due to its location on a busy main line, with all types of rail traffic passing in both directions on a regular basis, tests have shown that the system output is "exceptionally stable at all times".

The original trial system was based on a single-phase ac output but the system can also be designed for 400V three-phase output. The three-phase version retains the same single-phase rectifier/charger deriving its supply from the 25kV supply, but uses a three-phase inverter and static switch to convert the dc/battery power to three-phase ac output power.

When the load power requirements exceed 15 to 20kVA it is normally preferable to draw balanced three-phase power from the REC supply. In this case the UPS is equipped with two fully rated rectifier/chargers (Figure 3). This arrangement enables the UPS to draw power from both the 25kV traction-derived supply and the three-phase REC supply. The UPS can operate indefinitely from either or both supplies and the failure of one will have no effect on the operation of the UPS.