Light Rail for better public transport
Metrolink Introduction > Power Supply System: updated 8 December 2011
|
Light Rail Transit Association
Light Rail for better public transport |
|
Metrolink Introduction > Power Supply System: updated 8 December 2011
|
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 3A. 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. On street running sections, which are constructed on a reinforced concrete base, the reinforcing mesh is connected at short intervals to a continuous cable back to the substations. If any stray leakage current were to occur, it would be collected in the earth mat and flow back to the substations along this cable, which is connected via a leakage return diode (which prevents current flow of the opposite polarity) to the rectifier negative.
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.
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.
These are Bury to Victoria, Deansgate–Castlefield to Cornbrook and Pomona also Trafford Bar to St. Werburgh’s Road
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.
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.
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.
Power Supply System: top of page
This page was written by Tony Williams, Manchester Area Officer, Light Rail Transit Association. Contact manwebm@lrta.org if you have any comments, ideas or suggestions about these pages.