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Metrolink, Power Supply System

A description of the Metrolink power supply, at 750V dc, with overhead line equipment

Metrolink substations take power from the local electricity distribution system, where the incoming supply voltage is either 11kV or 6.6kV ac. This is converted to 750V dc to feed the overhead line.

Eleven substations were provided for phase 1. One supplies Queens Road Depot itself. A further ten located at Altrincham, Timperley, Dane Road, Trafford Bar, Deansgate–Castlefield, Victoria, Woodlands Road, Prestwich, Radcliffe, and Bury feed the line. The transformer rectifiers were, at first, rated at 600kW but space was provided so that they can be up rated to 1500kW.

For the Eccles line new power supply substations were provided at Cornbrook, Broadway and Eccles. An additional sub station was built at Stretford to re–enforce the traction supplies to the Altrincham Line. A further new substation was built at Piccadilly, it came into use on 11 August 2010 for Phase 3 extensions.

Additional power supplies are required for Phase 3. Trafford Bar substation has been considerably modified to provide power to the new Trafford depot and the South Manchester Line.

Prestwich and Woodlands Road substations have both been upgraded to 1200kW. A new substation has been built at Bowker Vale on the Bury line. This work was completed in August 2011. These upgrades will allow a possible 9 double trams per hour along Bury line, compared with 3 previously.

A system of inter–tripping of dc circuit breakers is employed between substations such that in the event of a fault on the line, all feeds to that section of the line are automatically broken. The SCADA system (System Control and Data Acquisition) at the control centre continually monitors system parameters (currents, voltages) to ensure correct operation.

The power system is designed to minimise electrolytic corrosion of public services by stray currents. The rails provide the return path to the substations for vehicle traction current, and are insulated from earth (apart from in the depot where the rails are earthed for safety reasons). In addition, the rails are bonded together at regular intervals, and bonded or welded at joints, to provide as low a resistance path as possible for the return current. At the substations the rails are connected to the rectifier negative.

When street running sections of the Altrincham via Manchester to Bury lines and later the Eccles extension were built with a reinforced concrete base; the reinforcing mesh was connected at short intervals to a continuous cable back to the substations. The theory was that any stray leakage current would be collected in the earth mat and flow back to the substations along this cable and via a leakage return diode to the rectifier negative. However this is flawed in practice, as the voltages which drive stray currents go both positive and negative in normal tram operation.

Current practice is not to provide any permanent connections between the negative bus–bar and earth, leaving the rails “floating” relative to earth. There is a finite resistance between rails and earth, the rails will adopt an average potential close to that of the earth. Where the diodes have been removed from some of the earlier tramways, there has been a demonstrable improvement in stray current performance.

Overhead line equipment

The 750V overhead line equipment is an unearthed system with either double insulation (consisting of two physically separate insulators between live and earth) each rated for the full system voltage, or reinforced insulation insulators with electrical characteristics which are far in excess of the system requirements.

When the Altrincham via Manchester to Bury and Eccles lines were built; inbound and outbound lines had separate feeds from the substations, like the main line railway uses.

Phase three overhead line equipment is cross bonded between inbound and outbound lines at intervals, there are also parallel feeders in the line side ducts. Each section has a single feed from its substation.

City Centre and Eccles line

In the city centre and from Pomona to Eccles there are both reserved and shared running sections. A fixed–termination trolley wire system is used. The twin wires each have a ‘cottage loaf’ cross–sectional area of 120sq.mm and they are suspended at a nominal 6.05 metre above the track. On the second cross city route a single contact wire is used.

Segregated Sections; electrification for phases one and two

These are Bury to Victoria, Deansgate–Castlefield to Cornbrook and Pomona

An auto–tensioned simple catenary system is used. This has twin catenary wires and a single contact wire for each track. Each of these three wires has a cross–sectional area of 120sq.mm. An arrangement of weights and pullies at both ends of a section keeps the wires under constant tension.

Segregated Sections; electrification for phase three

Trafford Bar to St. Werburgh’s Road and some other sections have twin catenary wires. They are sections with heavy traffic or a long distance between substations. There are also some segregated sections that have a trolley wire system with a single contact wire.

Cornbrook Junction to Altrincham

First electrified at 1500V dc in 1931, compaired with 25kV ac electrification, heavier catenary and contact wires were required. They were supported by portal structures. A fixed–termination arrangement was used for the overhead line equipment (OHLE).

With the original MSJA trains and OHLE reaching the end of their working lives, British Rail converted the line to 25kV AC in 1971. Most of the existing portal structures were re–used. The catenary and contact wires, insulators and other components were replaced with 25kV equipment. A fixed–termination arrangement was used but different section lengths required different termination points.

On conversion to Metrolink, the 25kV OHLE was retained. For each track an additional heavier cable was suspended from the portal structures and electrically bonded to the 25kV catenary and contact wire. Subsequently a second heavier cable has been added and bonded to the first.

During the August 2009 upgrades blockade, the old 25kV OHLE was removed. New auto–tensioned catenary and contact wires have been installed. The two previously installed additional cables have been retained and bonded to the new OHLE.

Depots

Vehicle operation within the Depot is of low speed and hence requires relatively low power. A fixed–termination trolley wire system with a single contact wire is used.

A short length of the Chorlton line from the junction to Ayres Road, with visibly different gradients and the connection to Trafford Depot, also has a fixed–termination single trolley wires.


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This page was written by Tony Williams. Contact manwebm@lrta.org if you have any comments, ideas or suggestions about these pages.