MV drives cut lifecycle costs at Sheffield pumping station 16 September 2013

The first UK installation of a new type of MV (medium voltage) ac drive has been commissioned at Yorkshire Water's Blackburn Meadows Wastewater Treatment Works, in Sheffield.

Six ACS 2000 VSDs (variable speed drives), from ABB, have replaced a dc drive system used to control a set of pumps, with motors rated at 645, 505 and 300kW.

The multi-pump arrangement is used to transfer wastewater 25 metres below ground to the head of the treatment works, and the MV drives were selected instead of LV drives, due to the long cable lengths and the potential to reduce overall project costs.

The existing dc drive system used dry-type cast-resin indoor transformers, feeding dc drives that control the dc motors.

While Yorkshire Water Services' specification for the new system would permit indoor transformers, they had to be integrated into the new variable-speed drives, and so would not be free-standing.

David Batty of ETM – a joint venture between AECOM Design Build and Galliford Try Infrasturcture – says that the ACS 2000 was selected as it can be used without an input isolation transformer. Its active front end (AFE) enables transformerless operation with reduced harmonic loadings, so allowing a direct connection to the 6.6kV line supply, he explains.

"The ACS 2000 has a lot of benefits over drives from other companies in that it is very interactive and highly modular," states Batty. "To change a component is very straightforward: you just pull out a drawer and put in a new drawer."

And he continues: "As the drive has no transformer, the heat output is significantly reduced. Also, a drive with a transformer would weigh twice as much and, because our installation is on a raised floor, we don't have as many concerns over the integrity of the floor."

Batty makes the point that direct-to-line operation combines the cost savings of a transformerless VSD system with the benefits of VSIs (voltage source inverters), including good availability and reliability, high and constant power factor and superior dynamic control performance.

And the result: "The existing system was rated at 1,750kW, whereas the new system is reduced to 1,450kW so the overall operating capacity is lower but the flow characteristics and performance are a lot greater," confirms Batty.

The overall system, including drive, motor and pump, is more efficient – not least due to lower heat losses and the use of ac motors.

"With the old system we would have had one of our engineers checking the condition of the brushes on the dc motors at least monthly," comments Richard Adamson, project engineer at Yorkshire Water

"This has now been taken away completely, resulting in just an annual visit to each drive," he continues, adding: "One of the things we were impressed with is that ABB is able to adapt the product to suit our ways of working and safe system of work. We required an additional earth switch and some Castell interlocks fitted. ABB were able to do this for us."

Brian Tinham

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