Urgent Rectifier Transformer Replacement for DLR

Enable Infrastructure (formerly BCM Construction) were instructed by Docklands Light Railway (DLR) to remove two existing rectifier transformers and install two new 2167kVA rectifier transformers at Royal Mint Street as a project variation on a design-and-build basis.  The existing transformers had failed and an immediate replacement was necessary to restore a stable supply to DLR’s busiest section of line. As the main framework contractor on three DLR jobs at the time, we were given responsibility for this important project.  

The design element of the works consisted of a proposed arrangement drawing, a cable interconnection diagram and the wiring termination schedules. The two projects were set to take place three–four weeks apart, on a seven-day programme, with the isolation and permit-to-work issued on a Thursday morning and the new rectifier transformers commissioned into service the following Thursday. The new rectifier transformers were duly commissioned into service seven days after works commenced, after a 12-hour soak period.    

“This site is key to maintaining and supporting the integrity of the railway operation. Enable Infrastructure delivered the works in two text-book operations, on programme and without compromising site or personal safety, despite being under much scrutiny due to potential operational consequences. Great performance all around!”
DLR Project Manager 

Prior to the works commencing, DLR/KAD (Keolis Amey Docklands – the networks maintainer) required an approved design, method statement and ITP (Inspection Test Plan) detailing all safety and technical aspects of the works to come. This project variation included: isolation; decommissioning; diversion of power to the generator; removal; installation; pre-commissioning; commissioning and testing of the rectifier transformers into service. Since the spec of the tertiary windings on the new transformers was lower than for the existing transformers, Enable Infrastructure were also instructed to supply and install a temporary back-up generator for essential LV supplies.  

The removal and delivery of the rectifier transformers required a road closure on Royal Mint St, and the closure of Cycle Superhighway 3: Enable Infrastructure were required to liaise with Tower Hamlets Council and Transport for London (TfL) to organise this job. It was, in fact, the first major project on DLR since the Poplar Substation fire, which meant it was a major test for the network’s resilience and was extremely time-sensitive.  

For the first rectifier transformer, KAD performed the isolation 48 hours before works began to test the network resilience and ensure that peak load times were stable without the rectifier transformer in service. Construction began on Friday morning on both occasions, following site set-up on the Thursday. Prior to the existing transformers’ removal on the Sunday, all cables were disconnected, and alarms/indication wiring tested. Existing HV cable was cut and removed from the existing transformer, and new HV cable installed.  

Once the road closure was made effective, the existing transformers were safely removed using a HIAB crane and low-loader. The new transformers were lifted into place and positioned perfectly through the use of pre-drawn markings and a laser level. Both redundant transformers were delivered to the designated storage location for refurbishing.  

Works on the newly delivered transformer began with connecting anode cables as well as indication and alarm cables, and re-establishing earth connections. New HV cable was pressure-tested before being terminated on both the transformer and the HV switchboard. All trips and alarms were tested prior to energisation of the new rectifier transformer. The energisation took place on the Wednesday with the assistance of the KAD Senior Authorised Person, and the transformers went on soak for a minimum of 12 hours. During the following shift (on Thursday), the transformers were commissioned into service and load-monitored. There were no issues in the commissioning of either transformer and both are now fully functioning.