7/27/2019 Zigzag Trafo for Harmonic Reduction

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APPLICATION OFZIGZAG TRANSFORMERSFO R REDUCING HARMONICS INTH ENEUTRALCONDUCTOROF LOW VOLTAGE DISTRIBUTION SYSTEM

PREM P . KHERAMember, IEEE3M CompanyF a c i l i t i e s E n g i n e e r i n g

S t . P a u l , MN 55133, USAP.O. BOX 33331, Bldg. 42-7W-03

A b s t r a c t - A novel appl ic a t i on of z igzag t ransfor mersf o r c i n g h armonics in the neut ra l conductors of athr ee phase , 208/120V dis t r ibu t io n sys tem i s p resen ted .Thi rd harmonic cur ren ts add u p in the neut ra l conductorof th e d i s t r ib u t i on system feeding non-1 ine ar loadssuch as persona l computers and e lec t ro n ic o ff i cemachines with switch mode power sup pl ies . Others o l u t i o n s t o t h i s p ro bl em i n c l u d e d e r a t i n g t h e s i z e o fthe d i s t r i bu t io n t ransformer by 50%. However,a p p l i c a t i o n o f t h e z i g z ag t r a n s f o r m e r e l i m i n a t e s t h en ee d f o r t h i s d e r a t i n g .b e f o r e a nd a f t e r t h e i n s t a l l a t i o n o f a z ig za g t r a n s -former a re presen ted .

In the past few years switch mode power supplies (SMPS)have become the power conversion technology used inpersona l computers and e le c t r on ic o ff ic e machinesd u eto th e i r h igh e f f ic i enc y and low co s t compared wi ththe previous ly used 1 ne ar reg ula tor power supp l ies .One problem wi th SMPS i s t ha t i t s non l in ear i t i es g iver i s e t o harmonics.120V dev ices with SMPS a r e powered from a th re e phase208/120V di st r i bu ti on system, t he amount of harmonicsgenera ted a r e h igh enough to cause overhea t ing of t hed i s t r i b u t i o n t r a n s f o r m e r. In a d d i t i o n , t h e n e u t r a lconductor suff e rs f rom overhea t ing because the th i rdharmonic cur r en ts f rom the th r ee phases do not cance lwi th in t he conductor.

One poss i b le so lu t io n to overcome the hea t ing e ff ec tof h a rm o ni c c u r r e n t s i s t o d e r a t e t h e e x i s t i n gd i s t r i b u t i o n t r a n sf o r m e r t o a s low a s50% of i t s r a t i n g .Another so l u t io n i s to add a z igzag t ransformer in th ed i s t r i b u t i o n s y st em .u se d i n t h e p a s t f o r c r e a t i n g a n e u t r a l , t h e r eb yc o n v e r t i n g a t h r e e p ha se , t h r e e w i r e d i s t r i b u t i o nsystem to a th r ee phase , four wi re system. Theschematic diagram of th e zigzag transf ormer i s showni n f i g u r e 1 and the vector diagram in f i g u r e 2 .

Measurements of current draw

When a l ar ge number of si ng le phase

The zigzag transformer has been

. - - PHASE AT IL-FOR BRANCH CIRCUITPANEL FEEDINQNON-LINEAR LOADz o n O B v.ROM

DlSTRlBUTlONTRANSFORMER '-\

B-tI+puEPHASE CFIG 2 VECTOR DIAGRAM

ZIGZAG WSTALLATKW

The appl ica t ion presen ted here does no t involve thec r e a t i o n o f a n e u t r a l b u t r a t h e r u s e s t h e z i g z a gt r a n s f o r m e r n e u t r a l t o s h a r e t h e l oa d n e u t r a l c u r r e n t .The amount of th is sha ring dependson the impedanceof t h e a l t e r n a t e c u r r e n t p a t h s.

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As shown in fi g u re 3 , three 5KVA, 120 X 240 -120/240V, dry type transformers were wired in a zigzagf a s h i o n . A 2 0 - t u r n Y 8 i n c h d i a m e te r c o i l i n a i rwaschosen t o in t rod uce an impedance in the ne ut r a l ,thusa l l ow i n g t h e c u r r e n t t o s p l i t i n t o tw o p a t h s , o n e t ot h e d i s t r i b u t i o n t r a n sf o r m e r , t h e o t h e r t o t h e z ig za gt r a n s f o r m e r. Current measurements were done usingt r u e RMS meter. Before the z igzag t ransformeri n s t a l l a t i o n , t h e p h as e l o ad c u r r e n t s w er e a v e ra g e da t 89A. Excluding the load unbalance on the thr eephases, t he neu tral harmonic cu rr en t was 102A. Aft ert h e i n s t a l l a t i o n , t h e c u r re n t i n t h e d i s t r i b u ti o ntran sfor me r neu tral was 54 Amps, a reduction of 47%.Also, th e tr ue power fac to r was measured to be 91%a n d t h e d i s p l a c e m e n t f a c t o r 96%.

The intr odu ctio n of th e 20-turn coi l impedance in then e u t r a l c o n d u ct o r lo w er ed t h e f a u l t c u r r e n t on al i n e t o n eu t ra l f a u l t , b u t d i d n ot a f f e c t t h ei n s t a n t a n e o u s t r i p p i n g of t h e c i r c u i t b r e a k e r i n t h eevent o f a fa u l t . The co i l impedance was ca lcu l a te dt o be 0.0112 Ohms. More turns could be added in thec o i l t o f u r t h e r r e du c e t h e c u r r e n t i n t h ed i s t r i b u t i o n t r a n s f o r m e r n e u t r a l a nd n o t a f f e c t t h ei n s t a n t a n e o u s t r i p p i n g of t h e c i r c u i t b r ea k e r.

Ref erenc ks

Computer Business Equipment ManufacturersA s s o c i a t ion 's Info rm ati on L et te r, December 1988.

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Un iver si ty of Wisconsin cours e on "E ffe cts ofNon-1 ine ar Loads on the Power Distr ibution System"April 5 - 6 , 1989.

90/CH 2935-5/90/0000-1092$01.0001990 IEEE

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