Cutting refrigeration costs with low harmonic drives

10 December 2013

The McCain Food’s plant at Whittlesey, Peterborough, is the UK’s largest chip factory, producing 30 tonnes of frozen chips an hour. The company is using ABB low harmonic variable-speed drives to cut its site load from 7 MW to 6 MW.

The company was looking for ways to reduce its energy consumption and costs and wanted to update the equipment in its refrigeration facility. The on-site cold store, which operates 24/7, accounted for 60% of the site electrical loading. The company also wanted to get better control of the refrigeration plant and reduce the potential impact of harmonics.Paul Derbyshire, McCain’s Electrical, Control and Automation engineer for McCain Central Engineering GB, explains the reason for using variable-speed drives to control the new plant. “We were looking for the best drive for this application and investigated several solutions from a number of manufacturers. We found that nothing gave the same performance as ABB active front-end variable-speed drives. With ABB drives, there is no interference on the line and they do not need filters to cut harmonics, as other drives do.“Although we had no known issue with harmonics on site before the project, one of our main motivators was not to generate any harmonics from the installation of such large drives. The competitors’ drives at the time all used large filters to deal with the harmonics generated by their drives.”In a conventional frequency converter, with a six-pulse diode bridge as a rectifier, the network side current is not sinusoidal and has significant harmonic content, especially fifth and seventh harmonics. Typical current distortion can range from 30 to 50% in total. In an ABB low harmonic drive, the use of the motor control platform, Direct Torque Control (DTC), together with a low pass filter, suppresses the current harmonic content, giving a distortion of less than 5%. The resulting clean sinusoidal current will therefore cause little or no distortion on the network voltage waveform.Benefits include direct connection to the network, with no complex multi-winding transformer required and no need for external filter equipment. The drives exceed the requirements of EN 61000-3-12 and IEEE519 and offer genuine unity power factor with no compensation needed.The new refrigeration plant cut the number of compressors from 20 to seven. To run the compressors, seven variable-speed drives, with ratings ranging from 350 to 650 kW were installed.Overall, the new refrigeration plant has cut the site loading by around 1 MW. “The drives also give us the ability to over speed the motor to get more refrigeration capacity, which we cannot do with traditionally controlled refrigeration plant,” said Derbyshire.

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