Tertiary Winding in Power Transformers P Ramachandran,India

Transcription

1 Tertiary Winding in Power Transformers P Ramachandran,India 1. What is the function of tertiary and stabilizing tertiary windings in a Transformer? Tertiary Winding: An additional winding (third winding after primary and secondary) in a transformer that can be connected to a synchronous condenser, a reactor or an auxiliary circuit for feeding power at a voltage different from secondary. For transformers with wye-connected primary and secondary windings, it may also help a) To stabilize voltages to the neutral, when delta connected. b) To reduce the magnitude of third harmonics when delta connected. c) To control the value of the zero-sequence impedance. d) To serve load. Stabilizing Tertiary Winding A delta-connected auxiliary winding used particularly in wye-wye connected three phase transformers for the following purposes: a) To stabilize the neutral point of the fundamental frequency voltages (line to line voltages remaining same, phase voltages in individual phases varying drastically. This can happen if star neutral is not earthed.) b) To minimize third-harmonic voltage and the resultant effects on the system (over voltages can develop with resonance between zero sequence capacitance of primary/secondary system and transformer third harmonic inductance) c) To mitigate telephone influence due to third-harmonic currents and voltages ( i.e. neutral current from the winding to source neutral through the ground, inducing disturbing signals in nearby communication cables) d) To minimize the residual direct-current magneto motive force (residual magnetism) in the core 1

2 e) To decrease the zero-sequence impedance of transformers with Y-connected windings ( ie to get sufficient fault current to actuate relays during a line to ground fault and also to limit X0/X1 <3, so that during a LG fault, healthy phase voltages will not exceed the continuous power frequency voltage rating of lightning arresters) f) To reduce losses from zero sequence fluxes impinging on tank and other metal parts. Stabilizing winding is the third winding, delta connected, unloaded and normally innermost, near to core, with or without the line terminals, brought out through oil to air bushings. When terminals are not brought out, it is called a buried stabilizing winding. All the three terminals may be brought out or sometimes only one terminal will be brought out. Another alternative is to bring one corner of delta outside through two bushings and delta formed outside through a link between the two bushings. In some cases, a reactor may be connected in series inside the delta connection to get higher impedance to other windings. If only 3L fault is critical, the reactor can be outside the delta in series to line terminals. In case of a zig-zag (zn) connected secondary winding, additional stabilizing tertiary is not required as the zig-zag connection serves that purpose. But with a primary zig-zag winding, stabilizing winding will be required to deliver unbalanced secondary loads from star connected winding. If the primary or secondary winding is delta connected, there is no need for a stabilizing delta tertiary. 2. Is it a good practice to use tertiary winding for feeding local loads? It depends on the criticality of the particular transformer in the overall reliability or availability of the grid. It was a common practice to use transformer tertiary to feed reactive or capacitive load for grid control before the development of EHV line reactors and STATCOM or SVC to meet the same purpose. But it will not be wise to use the tertiary of a 1000 or 1500 MVA transformer bank to feed a station load or nearby distribution load as any fault in tertiary lines will affect the large power flow in the grid. Whenever such loading is done it is better to piggy ride the tertiary 2

3 side transformer on main transformer to avoid LL faults on the tertiary side of main transformer. 3. When stabilizing tertiary was introduced? How it is arranged in a transformer? During the first 25 years of transformer engineering, nobody thought of the need of stabilizing tertiary. During initial periods of three phase connection, it was always delta-delta connection and benefits of star connection was clear only later. But when unearthed star /earthed star (Yyn) connection was used for Transformer HV/LV of those days (instead of both primary and secondary side star winding neutrals solidly earthed, as of today) some problems cropped up in service. They were third harmonic voltages in transformer output creating line over voltages, neutral shifting creating unequal phase voltages(during single phase loading or unbalanced loading) neutral current from the third harmonic currents in the exciting current (substantial in those days) creating telephone interferences, inaction of earth fault relays due to inadequate fault current etc. To overcome these problems, a stabilizing delta tertiary was the solution found out in the first decade of last century. Stabilizing tertiary winding is normally arranged as innermost winding near to core. Sometimes it is also provided as outermost winding for economic reasons. 4. How the rating of Tertiary is decided? In case the tertiary is to be used for feeding any auxiliary load (such as station loads, capacitive or reactive loads) then the rating, voltage and impedance with other windings shall be decided accordingly. It should be remembered that in case of auto-transformers, if tertiary is to be used purely for reactive or capacitive load, it is possible to have a rating such as 100/100/ 40MVA ie when primary and secondary handles line MVA of 100 MVA, it is possible to load 40MVA on tertiary.of course, in such a case, series or common winding will get overloaded (depending on flow of power from primary to secondary or vice versa) and the design should take care of it. In 1960 s, some of the utilities used to order such 3

4 auto-transformers. (Kerala State Electricity Board 31.5 MVA and 50 MVA 110/66 kv auto-transformers) Another point to be considered in three winding transformers with different MVA ratings for secondary and tertiary is the selection of impedances. For proper load sharing, the impedances shall be selected such that Z2/Z3 =P3/P2 where P2 &P3 are secondary and tertiary MVA, primary MVA= P2+P3. Z2 &Z3 are per unit impedances of secondary and tertiary branches of T- equivalent impedance star circuit. Primary to tertiary impedance = Z1+Z3, Primary to secondary =Z1+ Z2, secondary to primary =Z2+Z3. It is also important to select impedance for tertiary such that for all types of fault, the fault current in tertiary or secondary should not be excessive (normally less than 6-10 times the rated current of winding in large transformers and times in small transformers) 150 MVA bank 220/66/11 kv three winding transformers (both imported and Indian) that repeatedly failed at Bhakra switchyard during 1970 s 80 s was due to this anomaly in selecting inter winding impedances. Short circuit requirements (ie current in tertiary winding) during a LG fault either in HV or LV, decide the conductor size and arrangement of stabilizing winding. Continuous rating of stabilizing tertiary (for which cooling is to be provided) depends on the extent of unbalanced loading seen by the transformer. The first part, generally requires 1/3 of line MVA. In auto-transformers, generally 35 % of line MVA or 1/3 of the electromagnetic MVA (ie {(HV-LV)/HV} voltages x Line MVA) is usually introduced with the intention to achieve adequate short circuit current withstand strength during ground faults in the HV or LV sides. But In India, stabilizing tertiary winding capacity of auto-transformers is generally 1/3 of line MVA and not of self (electromagnetic) MVA. This is done to get extra short circuit withstand capability for tertiary winding. Voltage rating of stabilizing tertiary can be left to manufacturer to decide. Usually it is 11KV up to 100 MVA Transformer rating and above this, 33 kv to make robust winding and to limit the current to reasonable figures. But the BIL of the tertiary and line bushings shall be sufficient 4

5 to withstand the transferred surges from nearby high voltage windings. In cases, where 132 kv winding is near to tertiary winding (220/132/11kV Transformers) 170 kv BIL (33kV) is provided for tertiary and in case of 400/220/33 kv transformers 250 kv BIL (52kV) is provided. 5. Problems with Stabilizing Winding: In India, we were providing stabilizing tertiary winding in star/star connected power transformers. In 1960s, size of transformers was quite low (eg 20 MVA 132/66 kv auto, 4MVA 66/11 kv, 8 MVA 132/11 KV etc) and the size of the tertiary winding in such units was small, 1/3 of line MVA with winding width less than 20 mm. This slender winding was prone to mechanical failure (buckling to core or toppling telescopically, leading to a turn to turn fault) from fault current flow in tertiary, during line to ground (LG) faults on primary or secondary lines. This type of winding failures increased when system fault level in grid went up. With consequent system impedance reduction, fault currents increased, affecting adversely the tertiary winding. In some places, where the problem became acute, utilities opened the tertiary at one corner, bringing two terminals through separate bushings, earthing one terminal and keeping the other floating. This, in effect, was removing the stabilizing winding as no current could flow in tertiary winding and thus transformer failures could be prevented. In new transformers, CBIP recommended to eliminate stabilizing tertiary winding in Ynyn connected transformers up to 20 MVA in 1973 and later extended the limit to 50 MVA (1975) and 100 MVA (1987) First Indian made 100 MVA 220/132/11 kv auto transformers were manufactured in early 1970 s and some of the units soon failed in service. The 11 kv stabilizing tertiary was provided with 12 kv rated, 95 kv BIL bushings. One corner of the delta was kept earthed and other terminals were provided with 12 KV LAs. Even though the tertiary windings with stood the transferred surges during factory impulse tests, the bushings were not seeing voltages as they were earthed during testing. So during service, due to transferred surges, the tertiary bushing flashed over to earth 5

6 creating a LL fault as one of the terminals was in earthed condition. Transformers, where manufacturer provided a higher voltage bushing (33kV) for tertiary, worked without any problem proving the need to provide a higher voltage bushing for the tertiary. Wherever lower voltage rated bushings were provided at tertiary, surge arresters and surge absorbers were required at tertiary terminals to reduce the transferred surges within the withstand value of 12 kv bushings. Initially 15 kv class, LA and capacitors were provided. That too failed and finally 18 kv class protective devices were selected. The thumb rule for selecting protective devices for such unearthed system is as below. Continuous power frequency withstand voltage of LA = 1.1x nominal rated line to line voltage.so rated voltage of LA shall be still higher (18 kv and 54 kv for 11 & 33 kv tertiaries) 6. Why stabilizing tertiary is not required now? Stabilizing tertiary is no longer required in 3 phase 3 limbed transformers due to the following reasons. Surprising point is that even in the first edition of J&P Transformer book (1925) it was clearly mentioned that stabilizing tertiary is not required in 3 phase 3 limbed core type transformers. But it seems engineers missed this recommendation. Today situation is much better than those days due to technological improvements and generally, stabilizing tertiary winding can be avoided in YNyn connected 3 phase 3 limbed transformers, irrespective of voltage, MVA ratings and connections (auto or two winding). 1) Our grid supply systems are solidly grounded ones and increase in zero sequence impedance in Ynyn transformers due to elimination of tertiary is not appreciable as can be seen from below table. It should be remembered that effective zero sequence impedance depends not only on transformer construction, but also on system neutral grounding conditions. In YNyn connection with solidly grounded systems on both sides, the fault current reduction due to tertiary elimination is not appreciable to affect the protection scheme. 6

7 Type of Core construction 3 Phase 3 limbed & 1Phase 2 limbed 3 phase 5 limbed & 1 Phase 3 Limbed With Stabilizing Delta 80 % of ( Z1+ Z2/Z3) from HV or 90% of (Z2+Z1/Z3) from LV ( Z1+ Z2/Z3) from HV or (Z2+Z1/Z3) from LV Without stabilizing delta 80% of Z 12 from HV 90% of Z12 from LV Z12 Table: Zero Sequence Impedance of Transformers-with and without tertiary (Ref.Table 1 of IEC :1997 Application Guide- 1,2,3 refers to positive sequence impedance of HV,LV,TV circuits 12 refers to positive sequence impedance between HV-LV windings ) 2) The exciting current of power transformers has come down from 5% in early days to less than 0.5 % of full load current, due to better grades of CRGO and superior joints in core laying. Hence the third harmonic content has also come down in neutral current. Neutral ground current is no longer a problem with telecommunication circuits where technology has taken care of interferences by eliminating ground return paths and using fiber optic cables. 7. Benefits of eliminating stabilizing Tertiary winding: - Reduction in cost by 5-10 %. In case both tertiary and OLTC are removed in 315 MVA or 500 MVA 3 phase auto-transformers, the cost saving will be nearly %. - Reduction in overall losses by 2-5 % - Higher reliability, as tertiary is always a weak link from short circuit current withstand capability as it is difficult to provide sufficient axial and radial strength due to smaller width of the winding, especially for transformer ratings below 50MVA. 7

8 8. Do we need a tertiary winding in EHV auto-transformers? As stabilizing tertiary can be avoided in two winding star/star connected transformers, it can be eliminated in 3 phase 3 limbed auto transformers also. Five limbed core construction is used in large transformers to limit the transport height of the heaviest part of transformer. With three phase transformers, having three limbed core (standard up to 200 MVA), the zero sequence flux from each phase will close the path through gap between winding and tank or impinge the tank walls (as there is no magnetic path for the flux in three limbs to pass through yoke and side limbs) and create circulating currents, forming a virtual tertiary winding. Hence the increase in zero sequence impedance due to elimination of tertiary winding is not appreciable in such transformers. Some of the Indian utilities have already in service 160 MVA 220/132 kv auto transformers without tertiary winding. There are utilities abroad, having MVA 3 phase 400/220 kv and MVA bank auto-transformers without stabilizing tertiary winding. No serious operational problems are reported. We can refer to recommendations by some transformer experts in this matter. (1965,M.Christoffel) Christoffel from Brown Boveri wrote QUOTE The circumstances in auto-transformers are very similar. Certainly it is possible to dispense with the balancing winding (ie stabilizing winding as per our current terminology) in many cases, attention having been drawn to this some years ago ( R.J. Kayser : Interconnecting Auto Transformers, CIGRE 1960, Report 119) In a more recent publication (R.F. Brower: 345 kv development in a metropolitan area CIGRE1964, Report No.314) though no reasons are given, the surprising conclusion is given that when studying the coupling between a 345 kv and a 138 kv system using auto-transformers, a balancing winding gives rise to more drawbacks than advantages. A prerequisite condition, though, is that two systems have small zero sequence reactances, independent of one another. For transformers with star connected main windings (separate or auto connected) used for interconnecting systems, it seems perfectly feasible that a balanced winding may be dispensed with under certain system conditions. UNQUOTE 8

9 Bernard Hochart (1982-B. Hochart) from Alstom, France wrote in his book, QUOTE The addition of a tertiary winding of power 1/3 P, where P is the through put power, increases the cost of transformer by about 10 percent. For an auto-transformer, depending on the voltages, this increase can be up to 50 percent. A tertiary winding, therefore, should be considered only if it is absolutely necessary.usually a transformer whose power does not exceed some tens of MVA, with a three legged core, has no need of a tertiary winding in delta. Autotransformers have operated satisfactorily like this for a number of years in North America and Europe. UNQUOTE 9. What is the operational experience of transformers without stabilizing winding? (a) India In 1960s, Madras State Electricity Board (TNEB) ordered a few 220/110 kv 3 phase Auto-Transformers on GE Canada. Specifications did not clearly call for stabilizing winding, considering that it is an absolute part of the transformer. But the supplier executed strictly as per specifications and did not provide tertiary winding. This was found out during investigation for the cause of frequent cable fault noticed by P &T in their telephone cables near to Salem substation. The transformer was replaced by a unit with tertiary stabilizing winding and then the problem disappeared. (Source: MSEB engineer 1970 s) Star/star connected 3 phase transformers without tertiary were ordered first time in the country by UPSEB in early 1970 s. These were 12.5 or 20MVA, 132 kv units and by 1973 there were at least 20 units in service. Initially some apprehensions were raised when operators noted heating on tank outside surface, probably during unbalanced loading in three phases. CBIP working group on Transformers decided in their meeting dated , that in star/star connected transformers with earthed neutrals, delta connected stabilizing winding need not be provided for transformers up to and including 20 MVA. It was also noted that savings for non provision of tertiary winding could be as much as 10 percent of that of the transformer. In the working group meeting of &08, it was decided to extend the range for elimination of delta tertiary up to and including 50 MVA. 9

10 By1982, PSEB had 130 transformers, MPEB 158 units, BBMB 147 units of capacity up to 50 MVA without tertiary winding, working without any reported problems. (1984-CBIP Technical Report 38, Study on the causes of failure of tertiary windings of power transformers and its protection, Pages 61) CBIP, in the revised edition of Transformer Manual of 1987, eliminated tertiary winding in YNyn connected 3 phase 3 limbed transformers up to 100 MVA. But many utilities procured 3 phase 3 limbed transformers up to 160 MVA without tertiary winding (e.g. PGCIL 220/132 kv Auto-Transformers) Reliance Jamnagar refinery has few 174 MVA 220/33 kv Transformers without tertiary but with 3 phase 5 limbed core construction, commissioned in Some of the power stations in India have 255 MVA (3x85 MVA) 765/132 kv YNyn transformers without tertiary for feeding the auxiliary supplies. (b) Outside India: Several two winding and auto-transformers, star /star connected without tertiary ( MVA kv 3 phase) were working in US without problem during 1950 s.(1959, B.A.Cogbill, Page 970) In 1973, April, US Department of Interior informed CWPC, New Delhi that they have 13 numbers of Auto Transformer banks without tertiary, working without trouble since These were 250MVA 230/115 kv 3 phase Core type, 600 MVA Bank 345/230kV Shell Type, 1200 MVA Bank 525/241.5 kv Core Type. (1984, CBIP) In April 1973, US Department of Interior, Bureau of Reclamation informed CWPC that they have a 1200 MVA 525/241.5 kv Auto Transformer bank without tertiary operating since December,1971 at Grant Coulee. They omitted tertiary in this bank because their requirements were adequately met by other grounded transformers already installed (1984, CBIP) In 1980, AEP,USA had 22 numbers 525/241 kv three phase auto-transformer banks of rating 900 to 1600 MVA (first bank commissioned in 1967) out of which 7 banks were without tertiary.(1981,p.l.bellaschi) 10

11 In Germany tertiary winding is invariably provided as their system is not solidly earthed one. Tertiary is called Ausgleichswickung meaning compensation or equalizer winding. 10. Probable issues when tertiary stabilizing winding is eliminated. (a) 3 phase 3 limbed core or single phase 2 limbed core Normally no problem is seen with the elimination of tertiary, with effectively earthed system where X0/X1 <- 3 and R1/R3 <- 1. During single phase loading or unbalanced secondary loading, a zero sequence current flows through neutral, with a zero sequence magnetic flux flowing from core and coils to tank walls. Sometimes this can produce intense to moderate local heating of tank.eg. Transformer supplied by two lines under fault conditions. (2012, P. Penabad Duran) (b) 3 phase 5 limbed core, single phase 3 limbed bank The zero sequence impedance of transformer may go up increasing the X0/X1 of the system. The impact of this is as below: - During an LG fault on secondary, the voltages at healthy phases will go up. It should not go above the maximum power frequency continuous with stand voltage of the lightning arresters. - When tertiary is eliminated Xo (Zero sequence reactance) will go up (slightly in case of 3 limbed 3 phase) and can be substantial in case of 5 limbed core and three phase banks. This can create a situation of Xo/X1 of the system more than 3, not acceptable for effectively earthed system. If it is more than 3, the voltage level on healthy phases during an LG fault will go up, increasing the LA voltage rating, thereby reducing the protective margin with winding BIL. - The increased Zero sequence reactance of transformer become equal to the zero sequence capacitance of the lines, thereby initiating series resonance and over voltages. Such a situation was reported from BPA, USA in A 1000 MVA 525/241 kv auto-bank was working without tertiary. Tertiary was kept open and earthed. While de-energizing the unit (by opening 525 kv first and after 30 11

12 minutes 230 kv CB opened. In between combustible gases collected in one of the single phase transformers due to over voltages from resonance (1971- P.L.Bellaschi) (c) A transformer without tertiary can operate in parallel with units having tertiaries. Without a tertiary winding of medium voltage rating, the manufacturer may not have a source voltage to do no-load and induced overvoltage tests esp in case of EHV auto Transformers with LV voltage of 220 kv and above. In such cases any other transformer available in production line is to be used to step up source voltage and the losses of this transformer should be deducted from total loss to get loss of tested transformer. Those who are manufacturing EHV shunt reactors will have reactor testing transformer that can be used as HV source for transformer testing. When tertiary is eliminated, HV winding will be coming near to core, and electrostatic core shield need to be provided as in case of reactors. 11. Protection of Stabilizing Tertiary Winding When stabilizing winding is buried -Terminals are not brought out- manufacturer has to take responsibility of insulating the terminals of the delta winding inside the tank and to take care of the transferred surges from primary and secondary windings. Usually in such cases one terminal of delta will be earthed inside the tank, the grounding terminal being accessible from outside through any inspection hand hole. A better solution is to bring the terminals of the delta outside through two bushings and short and earth on tank top or brought down to earth by bus bars on insulator porcelains. Advantage of this is the availability of terminal for testing the tertiary winding and possibility of isolating the tertiary by opening the delta and grounding one end and keeping the other terminal open or connected to lightning arrester. In this condition no current will flow through tertiary during fault conditions, a condition beneficial when the mechanical with stand strength of tertiary is in doubt. When all terminals of tertiary are brought out, one terminal shall be kept grounded.this is done to avoid developing high potential in tertiary Other 12

13 terminals can be kept as such in case the bushings used are one step higher voltage rating (eg for 11 kv tertiary, 33 kv bushings and for 33kV, 52 kv bushings.) But if the bushings are of same voltage class as winding, 110 percent lightning arresters shall be provided at the ungrounded terminals. When all the terminals are to be kept floating due to occasional loading etc, the winding has to be kept stabilized with respect to ground, This can be achieved by several ways (a) using 3 phase 5 limbed electromagnetic PT (with an overvoltage factor of 1.9 for 8 hours) connected to tertiary bus (in star neutral grounded) with a broken delta secondary available for connecting to ground detection relay or loading with a resistor load. Watts dissipated in the resistor shall be equal or more than the reactive kvar due to the total capacitance to ground of the tertiary circuit when one phase of the circuit is grounded. Ground detection relay can be energized by the voltage drop across the resistor (b) by using 3 numbers single phase PTS with ultimate connections as in (a) (c) Using three single phase distribution transformers with grounded primary star and secondary broken delta connection, all others as in (a) Typical values for a 300 MVA 345/138/3.8 kv 5kVA distribution transformers, 15.4 ohm 17.8 A resistor with 15 KV arresters. (1967 Alexander) 13

14 12. Recommendations: 1) It is possible to eliminate tertiary stabilizing winding in all 3 phase 3 limbed core type transformers irrespective of voltage, MVA rating or connections (auto or two winding).other countries have experience of eliminating tertiary even in single phase auto -transformer banks and 3 phase 5 limbed transformers. But in such cases careful study will be required to see that enhanced zero sequence impedance does not create system problems. As per current technology, three phase 3 limbed construction is possible up to 200 MVA two winding transformers and up to 500 MVA in auto connection but without tap changer in later case. 2) Provide one step higher voltage bushings for tertiary and stabilizing tertiary. Then no special surge protection is required at tertiary terminals. In case of 3 phase transformers, one corner of stabilizing tertiary winding may be kept earthed in service, avoiding chances of earth fault from other two terminals. In case of banks where extensive delta forming bus connections are present, provide neutral earthing by means of single phase PTs with 1.9 voltage factor. One of the PT secondary with 110/3 V rating may be used for broken delta connection for grounding detection. With one step higher rated voltage and corresponding BIL for the tertiary winding and bushings, no extra protection by means of LA or surge absorbers are required. 3) Select the inter winding tertiary impedances in consultation with manufacturer, so as to limit fault current in tertiary within reasonable limits. This impedance matching will be required also for proper load sharing between secondary and tertiary, when both are loaded simultaneously, with a common primary input. There is scope for reducing the stabilizing tertiary MVA, based on actual fault current in tertiary depending on tertiary impedance. 14

15 13. References: F.O. Blackwell, Y or Delta Connection, AIEE July, 1903 Pages F.C. Green, Problems in the operation of Transformers, AIEE February 1911, Pages R.C.Clinker, Harmonic and Currents in Y and delta connected Transformers, AIEE May 1914, Pages: L.N. Robinson, Phenomena accompanying transmission with some types of star transformer connections, AIEE, September 1915, Pages J.F.Peters,Tertiary Windings in Transformers, their effects on short circuit current, Electric Journal,November, J.F.Peters;M.E.Skinner, Transformers for interconnecting high voltage transmission systems for feeding synchronous condensers from a tertiary winding, AIEE June,1921,Pages J.Mini; L.J.Moore; R.Wilkins, Performance of Auto-Transformers with tertiaries under short circuit conditions, Pacific Coast Convention of AIEE, Delmonte, CA, October,1923,Pages O.T.Farry, Auto-Transformers for Power Systems, AIEE Transactions pt- IIIB (Power5 Apparatus and Systems) Vol 73, December, 1954,Pages Eric T.B.Gross, Rating of Auto-Transformers having three windings, AIEE Transactions, Power apparatus and systems, December, 1957, Pages Eric.T.B. Gross, Rating of Auto-Transformers for system Interconnection, AIEE Transactions, Power Apparatus and systems, February, 1958, Pages B.A. Cogbill Are stabilizing windings necessary in all Y-connected Transformers, AIEE Transactions, Power Apparatus and Systems, Part III, October,1959 Pages O.T.Farry, Tertiary windings in Auto-Transformers, AIEE, Transactions, April,

16 T.Smith; W.Smith Star/Star Transformers without delta tertiary, Electrical Review, July, E.T.Norton, Specifying Tertiary windings, Allis Chalmers Electrical Review, Doust of Tertiaries Value questioned at IEE discussion Pages M.Christoffel, Zero Sequence reactances of Transformers and Reactors, Brown Boveri Review, Nov /December 1965, Pages GE Publication GET-3388B The Whys of Wyes- The behavior of transformer connections, Pages 20 (Revision of A.Boyajian, B. A.Cogbill - GE Publication GEA The Whys of the Ys.) G.W Alexander; W.J.McNutt, EHV Applications of Auto-Transformers, IEEEPAS-86 No8, August 1967, Pages P.L.Bellaschi - Ferroresonance Discussion of the C.L.Dawes Paper-Doble International Conference Paper No B E.Lekatsas, P. Margaritidis; J. Petrakis- Reconsideration of the insulation co-ordination for the tertiary system connected to the 400/150/30 kv Auto- Transformers of the Greek system after the experience of some insulation failures, CIGRE, 1980, Paper P.L.Bellaschi, Tertiaries in large Power Transformer Banks The problems they present (a Case Study) Doble International Conference papers Paper & Discussions a~d B. Hochart, Power Transformer Handbook, English Edition, 1987 Pages CBIP Technical Report No.38, Study on the causes of failure of tertiary windings of Power Transformers and its protection, Pages IEC Specifications for Power Transformers-Part 8 Application Guide L.Bergonzi etc, Power Transmission Reliability: Technical and Economic issues relating to the short circuit Performance of Power Transformers, Paper , CIGRE Session

17 Abhay Agrawal, 400 kv Transformer Specifications in India-Proposals for Economy and Dependability, CBIP International Conference on Transformers, Delhi P. Penabad -Duran etc, Transformer Tertiary Stabilizing Windings Part 1: Apparent Power Rating, Proc.of XX International Conference on Electrical Machines (ICEM) Vol.No.Pp , Sept, P. Penabad-Duran etc, Transformer Tertiary Stabilizing Windings Part 2: Overheating Hazard on Tank walls, Proc. of XX International Conference on Electrical Machines (ICEM) Vol.No.Pp , Sept, P. Penabad-Duran etc, Performance Evaluation of Tertiary and Stabilizing Windings: Calculation Strategies for Their Apparent Power Rating, Advanced Research Workshop on Transformers, ARWtr 2013,Baiona, Spain IEEE Standard-PC57.158D7A- Draft Guide for the application of tertiary and stabilizing windings in Power Transformers, Pages 66 17