Har­monic fil­ters for VSD mo­tors

DEMM Engineering & Manufacturing - - CONTENTS -

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.”

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