Backup power helps keep gas under pressure

An Alcad product story

Edited by the Engineeringtalk editorial team Jun 4, 2004

Transco has upgraded the DC power backup system for its Scunthorpe gas compressor station by installing two 110V rechargeable nickel-cadmium batteries and a thyristor-controlled rectifier power unit.

Transco, the UK's gas pipeline operator, has upgraded the DC power backup system for its Scunthorpe gas compressor station by installing a combination of two 110V Alcad rechargeable nickel-cadmium batteries and a Benning Thyrotronic thyristor-controlled rectifier power unit.

The two new batteries, comprising a total of 182 cells, have replaced the existing Alcad batteries which gave Transco over 30 years of reliable service and even at the end of their life could still hold 80% of their original capacity.

Scunthorpe is one of Transco's 24 UK compressor station sites which boost gas pressure up to 85bar to increase transmission capacity and move gas through the pipeline.

It is powered by an industrial jet engine fuelled by natural gas taken from the pipeline.

When Transco decided to replace the existing DC backup power system at Scunthorpe it drew up a detailed system specification for the protection of: motor control centre switchgear; standby power supplies; communications and instrumentation systems; fire and gas detection systems; telemetry and alarm systems.

The capability to monitor all parameters and alarms via local indicators for the above systems was also a key requirement.

In addition, the power supply system also had to provide the necessary charging characteristics for the replacement Alcad nickel-cadmium batteries that were being installed.

The Benning Thyrotronic series of thyristor-controlled rectifiers for stationary battery systems, which is combined with a microprocessor controlled monitoring and control system, was selected as the optimum solution.

The Thyrotronic is designed to operate with industrial nickel-cadmium batteries and provides a float charge to maintain them in a fully charged state as well as supplying power for the electrical loads.

In the event of mains power failure the batteries will then supply power for up to 24 hours, and when power is restored the Thyrotronic switches from float to a boost charge to restore the battery charge as quickly as possible.

The main advantages of nickel-cadmium technology for power backup applications include its long and reliable service life - in excess of 25 years in many applications - with low maintenance requirements and no risk of the 'sudden death' failure which can affect lead acid batteries.

At Scunthorpe, Alcad has supplied two 110V batteries together with the battery stands.

One battery comprises 92 HC34P cells and provides a nominal capacity of 34Ah to support high current demands over short periods - 120A for 5s and 10A for 5min.

The other battery comprises 90 MB550P cells and has a nominal capacity of 550Ah to support the electrical load for longer periods - up to 24 hours - as well as handling mixed loads which involve a mixture of high and low discharge rates.

The batteries were supplied and installed by Alcad's UK distribution company, Industrial Batteries, which also organised the collection and recycling of the old nickel-cadmium batteries through the Alcad Bat-Re-Back scheme.