STUDY ON TERTIARY WINDING IN POWER TRANSFORMER WITH CASE STUDY ON THE FAULT DUE TO THIS WINDING. 1.
INTRODUCTION
Tertiary winding is an auxiliary delta winding bei being ng dist distin inct ctly ly sepa separa rate ted d from from both both prim primar ary y and and secondary winding, is provided upon the same core and kept near to the core. Initial days the use of this winding was was quite quite extens extensive ive;; but now a days days the use of this this winding is getting reduced. In this chapter it has been decide decided d (Why (Why & How) How) the the use of tertia tertiary ry is gettin getting g reduced. 2. OBJECTIVES OF TERTIARY WINDING:Why tertiary winding was used?
It is generally used in Υ / Υ Transformer to achieve the following one or more objectives 1.
2. 3. 4. 5. 6.
To red reduc ucee thir third d harm harmon onic ic con conte tent ntss of the the out out put put voltage. To provide the path for the 3 rd harmonic existing current, obtained in the core. To red reduc ucee the the zero zero seq seque uenc ncee phas phasee impe impeda danc ncee of the transformer. To perm permit it the the trans transfo form rmat atio ion n of unbal unbalan ance ced d three three phase loads. It is used used as a vol volta tage ge coil coil for for tes testi ting ng purp purpos oses es.. To sup suppl ply y an an auxi auxili liar ary y load load in in addi addittiona ionall to the the main load.
3. CONCEPTS: -
The concepts of above objectives are discussed below
1.
For the star/star connected three phase transfor transformer, mer, the voltage voltage induction induction on their phase windings become sometimes as great as 60% of the fundamental. The 3 rd harmonic voltage component operates to increase the peak value of the resultant resultant composite composite phase voltage wave, wave, thereby thereby throwi throwing ng an increa increased sed stress stress on the transf transform ormer er insulation. But for the use one of the being in delta, the circulation of 3rd harmonic currents in closed delta delta circui circuitt substa substanti ntiall ally y elimin eliminate atess harmon harmonic ic voltages.
2.
Similarl Similarly y the short short circuit circuit rd third third harmon harmonic ic curren currentt or exciti exciting ng 3 harmonic current does not affect to the external load circuit or even to the transformer, because of its closed path circulation on the closed delta tertiary winding.
3.
As 3rd harmonic harmonic currents currents are are negl neglec ecte ted d or beco become mess smal small, l, so the the zero zero sequence impedance of transformer also reduced.
4. For the case of unbalanced threethree-pha phase se load load suppli supplied ed to a trans transfor former mer with with tertia tertiary ry windi winding, ng, the unbala unbalance nce curren currentt passes passes through the tertiary winding. So the main windings are not affected much. But for the case of fault, the currents currents are so reflecte reflected d that the transform transformer er gets separated from the supply. 5. As tertiary winding is the sepa separa rate te and and dist distin inct ct wind windin ing, g, woun wound d on the the common common core core as that that of primar primary y and secondar secondary y windings, so for testing purposes, when voltage is supplied to tertiary winding, due to the induction the voltages are induced on the primary and secondary windings. Now the different comparison could be taken. 6. Due to the induction princ principl iple, e, this this windin winding g could could be easily easily used used for auxiliary load supply. Generally this winding is used for supply supplying ing the statio station n auxili auxiliari aries, es, while while for some someti time mess it is also also used used with with sync synchr hron onou ouss cond conden ense sers rs for for powe powerr fact factor or corr correc ecti tion on on transmission line. 4. COMPRO COMPROMIS MISE E WITH WITH THREE-P THREE-PHAS HASE E CORE TYPE TRANSFORMER (To what extent, the above objectives are achieved without tertiary winding)
1.
The use of core material plays the important role for obtaining 3rd harmonic component of current. Now- a- days CRGO CRGO lamina laminated ted (Cold (Cold Rolled Rolled Grain Grain Orien Oriented ted)) steels are used for the core of the transformer. The 3rd harmonic current of the exciting currents, for the cores, cores, built with this material material are very much lower as compar compared ed to those those cores cores built built with with hot rolled rolled lamina laminatio tion n or any other other mater material ial.. So, So, the use of tertiary winding can be eliminated if proper type and size of materials used for the core.
2.
Zero sequence impedance, sometimes plays the role for the detection of earth fault by relays. Now a days it has become become the common common practi practice, ce, that that all such such transfor transformers mers are operated operated with both sides, sides, neutral neutral solidly solidly grounded. grounded. So zero sequence impendanc impendancee value dose not become high enough and taken equal to the order of positive positive sequence impendance impendance.. In practice this zero sequence should be present in the trans transfor former mer for detect detection ion of earth earth fault fault.. So use tertiary winding for the reduction of zero sequence impendance can be eliminated.
3.
In a three phase phase core type transfor transformer, mer, which are generall generally y used now-a-days has magnetic coupling between the
windings. So 3rd harmonic residual flux is greatly reduce reduced. d. Also Also use of neutra neutrall being being apprec appreciab iably ly stabil stabilize ized, d, provid provides es high high reluc reluctan tance ce path path for the harmonic fluxes. So the use of tertiary winding does not become become necess necessar ary y for threethree-pha phase se core core type type transformer. 4. Testing of the transfor transformer mer can also also be done done in genera generall by applying the testing supply to one of the winding and comparing the results on the other winding’ so tertiary winding is not compulsory for testing the transform transformer. er. But availabi availability lity of tertiary tertiary winding winding brings the testing procedure easy. 5. Use of tertiary winding provides auxiliary load along with main load. This practice of providing auxiliary load can also be obtained by using separate tapings from the the same same wind windin ing g alon along g with with main main load load.. But But disadvantages are there, that by this technique, the number contact points and tapings become more, for which question of abnormal short circuit may occur due to number of joints. 6. Now-a-days the protection relays are also used being equipped with harmonic restraint circuit. So at the time of energisat energisation ion of transfor transformer mer on idle condition, condition, the effects of harmonic currents are easily eliminated by the relay without use of tertiary winding. 5. EFFECT OF FLOATING TERTIARY WINDING 5.1 Definition of floating: The terminal is called floating when it is 1) Isolated from ground OR 2) connected to ground through high impendence. Few examples Terminals connected to distribution of • small power transformer. Few meters of cable either isolated or • connected with high impendance. 5.2 EFFECTS:For For the the tran transf sfor orme mer, r, twotwo-ty type pe s of indu induct ctio ions ns generally takes place between windings
1.
Electroma gnetic induction:- This induction is responsible for the development of certain voltage, on one winding due to supply voltage on the other winding, on the principle of mutual induction. Electrosta 2. tic induction:induction:- This induction is determined by the relati relative ve distri distribut bution ion of capaci capacitan tance, ce, among among the windings and ground. When When the the lowe lowerr rate rated d volt voltag agee wind windin ing g is floated, floated, the electrost electrostatic atic induction induction becomes becomes hazardous hazardous alon along g with with elec electr trom omag agne neti ticc indu induct ctio ion. n. Beca Becaus usee electrostatic electrostatic induction is determined not by turn ratio, but
by by the the rela relati tive ve dist distri ribu buti tion on of capa capaci cita tanc ncee amon among g wind windin ings gs and and grou ground nd,, and and ofte often n the the capa capaci cita tanc ncee distri distribut bution ion leads leads to higher higher potent potential ialss in the lower lower voltage windings than turn ratio would have produced.
5.3SEVERITY OF ELECTROSTATIC INDUCTION It becomes severe for the following conditions 1. Lightning: - Lightning on the high voltage lines are the source of maximum elec electr tros osta tati ticc indu induct ctio ion n from from the the high high volt voltag agee winding to a floating lower voltage winding. Due to rapid rise of these potential, floating terminals may be raised too much higher potential than average of the winding.
2.
Non-identical Non-identical Switching. Switching. - The contacts of circuit breaker may not close or open simultaneously. It has certain time gaps among the problems or adjustment. When it results with consid considera erable ble differ differenc encee among among the contac contacts, ts, the abnormal conditions are developed. • Swit Switch chin ing g in/o in/out ut with with single pole. • Swit Switch chin ing g in/o in/out ut with with double poles. For these abnormal conditions, the closed pole with close winding will result high voltage and due to electr electroma omagne gnetic tic and electr electrost ostati aticc induct induction ion,, the float floating ing windin windings gs may be affec affected ted with with higher higher potential.
3.
Line to ground ground fault on isolated isolated system system. A line line to grou ground nd fault ult on an isolated 3Phase system shifts the neutral, and the average average potential of the high voltage voltage windings windings and therefor thereforee induces induces correspon corresponding ding abnormal abnormal electro electro static potential in a floating winding. Someti time mess sync synchr hron onou ouss cond conden ense serr is NOTE TE:: - Some connected connected to Tertiar Tertiary/Te y/Termin rminals als of a Transfor Transformer mer for improvement of Power Factor and winding is taken as third third windi winding. ng. In such such cases, cases, the design design of tertia tertiary ry wind windin ing g depe depend ndss upon upon the the fact factor orss like like syst system em impendance, fault levels, and fault feedings from one end or both ends. 5.4 REMEDY EDY INDUCTION
OF
THE
ELECTRO TROSTATIC TIC
The use of LA (Lightning Arrestor) is the best solution solution for the remedy of the Electrostat Electrostatic ic effect effect on Tertiary winding. The rating of such LA depends upon the the type type of wind windin ing g and and seve severi rity ty of the the effe effect ct of Electrost Electrostatic atic Induction Induction on the winding. winding. However However the general calculation is derived from the following formula
Rating of LA = (Phase Voltage Of winding x √2 x 0.9 /√3) But for some cases direct solid earthing to one of the winding is chosen to avoid the Electrostatic effect on Tertiary winding.
5.5 Vector Diagram Terti Tertiar ary y windi windings ngs are genera generally lly of delta delta connec connected ted system. For loading tertiary all the three terminals are exposed to out side, where loads can be connected. As discussed above that for the case of non-loading of the same one of the terminals has to be directly connected to solid earth or LA is to be connected to all the three winding. The vector diagrams are drawn below for clear understanding of the earthing connection. 6.CAUSES OF FAILURE: As descri described bed earli earlier, er, that that when when • transf transform ormer er windin winding g is subjec subjected ted to light lightnin ning g / switch impulse, transient voltages are produced due to electrostatic and electromagnetic induction. And for the case of floating tertiary, the transferred surge voltages voltages may exceed exceed the basic impulse impulse insulatio insulation n level and damage the insulation provided for it. • The The faults faults like like phasephase-toto-pha phase se faulting, results severe electrostatic induced voltage on tertia tertiary ry windin winding. g. This This become becomess seriou serious, s, for avai availa labi bili lity ty of high high gene genera rati tion on capa capaci city ty near near terminals terminals of the winding and low % impendanc impendancee between LV and tertiary winding. For For the the case case of shor shortt circ circui uitt • condition different forces act on the windings of the transformer. These are mechanical force, radial and axial axial tensi tensile le forces forces.. the tertia tertiary ry windin winding g being being present near to the core and surrounded by other windings, suffer severe mechanical stresses and gets failed. • Some Someti time mess due due to inad inadeq equa uate te clearance among the tertiary bushings outside tank, this winding fails. Some Some cases cases insuff insuffici icient ent energy energy • dissipati dissipation on capabilit capability y lighting lighting arrestor arrestorss such used, used, causes the failure of tertiary winding. 7. ADVANTAGES OF ELIMINATION
Terti Tertiar ary y windin winding g is a potent potential ial weak spot in the transformer, as it is kept near to the core, being surrounded by high voltage winding. So, by eliminating this winding, the possible weak spot could be eliminated. With ith elim elimin inat atio ion n of tert tertia iary ry • windin winding, g, the the faults faults due to light lightnin ning, g, switc switchin hing g surges etc. could be avoided. The physical size of the • transform transformer er could could be reduced as compared compared to the MVA rating, because the portion, portion, where tertiary tertiary wind windin ing g was was kept kept coul could d be repl replac aced ed by stro strong ng insulation, according to MVA rating. The cost of transformer could be • saved in comparison. So, from the above discussion, it got concluded that the tertiary winding should be eliminated Note: - Central Board of Irrigation and Power (CBIP) and Standardiz Standardizatio ation n and Optimizat Optimization ion Sub-Commi Sub-Committee ttee (SOS (SOSC) C) unde underr Deve Develo lopm pmen entt Coun Counci cill for for Heav Heavyy Electrical Industry constituted by Government of India •
have recommended that the delta connected stabilized windi windings ngs should should be elimi eliminat nated ed up to transf transform ormer er of rating 50MVA. 8
