Single phase Auto-transformer:

Transcription

1 ingle phse Auto-trnsformer: B A E Fig: nput nd Output connections of single phse Auto-trnsformer The input to uto-trnsformer is given cross B nd the output of the utotrnsformer is tken cross E. f the input voltge to uto-trnsformer is given cross A, the mximum output voltge cross E is equl to input voltge. However, if we give the input voltge cross B, the mximum output voltge cn e mde greter thn the input voltge. Wttmeter: M Fig: Wttmeter onnections n wttmeter M is clled current coil nd is clled pressure coil or pressure coil. While connecting wttmeter the terminls M nd should e connected. nsted if the terminls

2 nd re connected then the current through current coil (M) will e ( ). This leds to the wrong mesurement of power consumed y lod. Hence, the wttmeter should lwys e connected s shown in the figure.

3 Experiment no. 1 PARAE OPERATON OF TWO NGE-PHAE TRANFORMER ntroduction: When the lod outgrows the cpcity of n existing trnsformer, it my e economicl to instll nother one in prllel with it rther thn replcing it with single lrger unit. Also, sometimes in new instlltion, two units in prllel, though more expensive, my e preferred over single unit for the resons of reliility- hlf the lod cn e supplied with one unit out. Further, the cost of mintining spre is less with two units in prllel. The stisfctory nd successful opertion of trnsformers connected in prllel on oth sides requires tht they fulfill the following conditions: 1. The trnsformers must e connected properly s fr s their polrities re concerned so tht the net voltge round the locl loop is zero. A wrong polrity connection results in ded short circuit. 2. The secondry sides of the two-single phse trnsformers must hve zero reltive phse displcement. 3. The trnsformer must hve the sme voltge rtings to void no-lod circulting current. ince the lekge impednce is low, even smll voltge difference cn give rise to considerle nolod circulting current nd extr 2 R loss. THEORY: et the rted voltges of the primry side of the two single phse trnsformers (trnsformer A nd trnsformer B) used in prllel opertion e nd nd let their turns rtio e N : 1 nd N : 1. As the two trnsformers re connected in prllel, y Kirchhoff s lws the voltge equtions of the trnsformers cn e written s, ( ) (1) Therefore, ( ) (2) (3) Where, nd re the impednces of the trnsformers, is the lod impednce nd is the lod current. All the quntities re referred to the primry side. Under no-lod 0, so tht the circulting current etween the two trnsformers is given y, (4)

4 Under short-circuit,, (5) On loding, (6) ustituting for in eqution (2) we get (7) imilrly, (8) f the voltge rtings of the two trnsformers re sme, i.e.,, we hve from (7) nd (8), (9) Or, i.e., pu pu (10) From eqution (9) we cn write, 2 (11) ince the voltge rtings re sme, the ka rtings of the trnsformers re, (12) From (11) nd (12) (13) As the ka distriution mong the trnsformers depends on impednce rtios, the per unit vlues my e used ut they must e djusted to the sme se ka. Ojectives: 1. To determine nd verify the polrity of the individul single-phse trnsformers.

5 2. To find the impednce of the single phse trnsformers y short circuit test. 3. To study prllel opertion of (the ove) two single phse trnsformers nd oserve the lod shring etween them. Determintion of Polrity of ingle Phse Trnsformer: 1. Mrk the primry winding terminls of one single phse terminls s A 1 nd A 2. The choice of terminl for mrking is ritrry. imilrly mrk secondry winding terminls s 1 nd onnect the terminls of the two windings s shown in Fig Also connect the primry winding terminls to n utotrnsformer through doule pole doule throw (DPDT) switch (Fig. 1). 4. hoose pproprite vlue of the Fuse wire in the DPDT. 5. Mke sure tht the utotrnsformer is in the zero voltge position. Now switch on the power supply nd throw on the DPT switch. 6. Grdully increse the utotrnsformer voltge (up to the rted primry voltge) nd mesure the voltge etween terminls A 2 (primry) nd 2 (secondry) from voltmeter. uppose the rted voltge rtio of the trnsformer is 1 : 2. n cse terminl A 1 nd 1 re of the sme polrity, the reding of the voltmeter will e ( 1-2 ), otherwise the reding of the voltmeter will e ( 1 2 ). imilr polrity terminls cn e mrked with the sme sign ( or - ). 7. Once done with the polrity determintion, reduce the utotrnsformer voltge to 0 nd open the DPT switch. Turn off the power supply. 8. Determine the polrity of the other trnsformer following the sme procedure. From Auto Trns former DPDT A1 Primry ` 1 econdry A2 2 erifiction of the otined polrity: Fig. 1: ircuit connection for polrity test 1. Mke the connection s shown in Figure 4 (A single floting voltmeter cn serve s 1, 2 nd 3). 2. witch on the power supply to the utotrnsformer nd follow the sme procedure s efore while grdully incresing the utotrnsformer output voltge keeping 1 closed. Note keep ll other switches open. The Wttmeter connections should e mde s ws explined erlier in the mnul.

6 3. Oserve the reding of 2. f it s non-zero (i.e. nything other thn very smll voltge difference) then the polrity connections for ny one of the Trnsformer s should e reversed. 4. n cse polrity reversl is to e mde switch off the system following the procedure mentioned ove, nd chnge the connections of ny one trnsformer (i.e. the connections t the secondry end). Prllel Opertion of Two ingle Phse Trnsformers: 1. Hving done the polrity check next close the switch For the lod choose gng rheostt of pproprite power rting (f the trnsformers re perfectly mtched or re ner perfectly mtched, s is the requirement for zero inter trnsformer circultion current, the power shring will e in ccordnce with the respective power rtings of the two trnsformers). 3. n cse there re two secondry windings nd one is eing used or the output voltge eing tpped is percentge of the mximum ville then the KA rting of the trnsformers during usge will e less thn tht given on Nme Plte. f either of the previously mentioned conditions hppens to e true, determine the correct KA rting of usge. Bsed on it determine the mximum rted current tht shll flow in the primry nd the secondry. 4. Now increse the lod in steps of 0.5 (the minimum step of the gng rheostt) till either A1 or A2 reds the mximum permissile current (clculted in step 3). At ech step oserve the reding of 3. s it less thn the no lod voltge? f so then cn you reson it? Does the mximum rted current through either T1 or T2 (in which ever it hppens to rrive erlier) flow eyond the rted power of the lod (tht is the sum of the power rting of usge of T1 nd T2)? f so then cn you reson it? 5. Note the redings in following tulr form. tep (od) A1 W1 A2 W2 A3 3 Tle 1: od test with two trnsformers in prllel 6. Note the multipliction fctor of the Wttmeter. Do not go eyond the rted current. 7. witch off the system s explined ove. 8. Bsed on 3 s reding t mximum rted current, determine the mximum power t the output t full lod. Determine the theoreticl output voltge t full lod sed on the impednce s otined from the short circuit test of the two trnsformers. 9. Also determine the corresponding voltge regultion. hort ircuit Test of ingle Phse Trnsformer: 1. Mke the connections s shown in figure. The secondry will e short circuited. Mke sure the secondry is the ow voltge side (why?). 2. Determine the rted current which would flow in the primry (the high voltge side) depending on the rted KA ccording to trnsformer usge. 3. lose the DPT switch nd grdully increse the Autotrnsformer voltge till A1 reds the rted current.

7 4. Note down the redings of 1, A1 nd W1. Note down the multipliction fctor of the Wttmeter nd proceed to clculte the short circuit prmeters of the single phse Trnsformer. 5. Repet the ove procedure for the other trnsformer. M W1 A1 N 1 :1 1 ` 1 H Fig2: ircuit connections for short-circuit test 1 N 1 1 N 1 :1 1 2 N 2 :1 N 2 2 Fig3: Equivlent ircuit

8 U P P Y 1 1 M M W1 W2 A1 A2 T1 N 1 :1 ` N 2 : A3 O A D 3 T2 Fig4: onnection for lod test with two trnsformers in prllel