Harmonic filters for VSD motors
Large users of electricity often generate energy-sapping harmonics that compromise the performance of the network on which they depend.
Fonterra’s Darfield plant could have been a culprit – if not for an innovative harmonic filtering solution supplied by Schneider Electric.
The Darfield facility, located near Christchurch, comprises of two dryers capable of producing 45.5 tonnes of milk powder an hour.
The site’s overall electrical load is around 11MW, much of which is used by electric motors driving plant machinery.
The majority of motors at the facility are controlled by VSDs (variable speed drives). There are more that 400 VSDs at the site, including eight 710kW models.
VSDs are essential for controlling AC motors precisely to the process requirements, while significantly reducing the energy consumption, especially for pumps and fans. As a side effect, they also represent a non-linear load, contributing to harmonic distortion of the electrical supply.
Harmonics cause “transmission losses” across New Zealand’s national grid, creating waste by generating excessive heat.
Eliminating harmonics not only greatly improves power quality and the grid’s efficiency, it also helps to contain the cost of electricity.
Network operators are now insisting that large customers install filtering technology to cut harmonics to acceptable limits – a regulatory regime that was applied to Fonterra for the first time at Darfield.
The entire Darfield plant, says Fonterra’s Glenn Sullivan (group manager, electrical engineering), uses Schneider Electric technology almost exclusively – including the switchgear, switchboards, VSDs, and its harmonic filters.
Darfield was built in two stages. Dryer 1, the smaller dryer (15 tonnes/hour), was completed in August 2012, and Dryer 2 in 2013.
The core harmonic strategy involved fitting active harmonic filters to all the main transformers, along with standard passive filters.
Using this combination not only significantly reduced the capital expenditure cost for filtering technology, but also cut losses by some 20 kW/hour, saving an estimated $10,000 in annual power charges.
Twice the size of Dryer 1, it could reasonably be expected that a conventional harmonic filtering solution for Dryer 2 would double that cost. But the Fonterra/Schneider Electric designers developed a much more costeffective option using phase-shifting transformers.
“After modelling and running various simulations of the entire plant (Dryer 1 and Dryer 2 in combination),” says Glenn. “We established that by using phaseshift transformers for Dryer 2, we could redeploy a number of the Dryer 1 filters for the new installation. Remarkably, the nett effect was a better distortion signature than for Dryer 1 alone.”
Despite the slightly more expensive phase-shift transformers and the cost of relocating the filters from Dryer 1, the capital expenditure for Dryer 2’s harmonic mitigation came in at less than 10 per cent of the Dryer 1 spend. Dryer 2 uses nine transformers.
“Harmonics can be trimmed by smarter, improved designs for our plants,” says Glenn. “And while design alone won’t entirely eliminate the issue, it can reduce harmonics to a point where the filtering solutions won’t be as expensive. Darfield is an excellent example.
“By meeting Orion’s regulatory requirements Darfield’s harmonicsbusting strategy has contributed in a small way to the deferral of NZs next transmission line upgrade.”