Special Transformers-1 -Auto-Transformers-Part 1

Power Engineers will be interested to know, why and where auto-transformers are used in power system and how they differ from standard two winding transformers. Let us see how standards define an auto-transformer.

IEC- Auto-transformer - A transformer in which at least two windings have a common part [IEC 60050-421:1990, 421-01-11]. NOTE Where there is a need to express that a transformer is not auto-connected, use is made of terms such as separate winding transformer, or double-wound transformer (see IEC 60050-421:1990, 421-01-13) (Clause 3.1.2 of IEC 60076-1 -2011 Power Transformers-General)

IEEE- Autotransformer: A transformer in which at least two windings have a common section (Clause 3.23 of IEEE C57.80-2012  Standard Terminology for Power and Distribution Transformers)

Scientists discovered the electromagnetic induction with auto-connection very early, just five years after Faraday discovered in 1831 electromagnetic induction, with two separate windings on a common iron core. In 1836, J.C. Page from USA (Washington DC) established that sparks can be produced across the tapping of a solenoid by interrupting its current circuit in remaining part of the solenoid. Referring to Fig1, when DC supply was made or interrupted across A-B, spark was noticed across C-D indicating voltage induction in winding B-D. Connection point B made the galvanic connection between the two windings.

To understand the principle of auto-transformers, please see fig 2. A 600 KVA 15/10 kV two winding and auto-transformer are shown. With two winding unit, there are two windings of 600 KVA individual rating while auto-transformer require only two 200 KVA windings, but delivers 600 KVA at line ends. So, the auto transformer of 600 KVA will be of size of a 200 KVA two winding transformer.

Main advantages of auto-transformers, compared to a two-winding transformer, are higher efficiency (lower losses), lower size and sometimes lower prices. When we compare a 100 MVA 220/132 kV auto-transformer with 100 MVA two winding transformer, the equivalent rating of auto transformer will be 220-132/220 = 0.4 times that of a two winding rating. It means the size and losses of 100 MVA 220/132 kV auto unit will be that of a 40 MVA 220/132 kV two winding transformer. It also means that in auto-transformer of above example, only 40 MVA will be transferred through core to the other circuit by transformer action, but balance 60 MVA will be simply jumping over to the other circuit through the common galvanic connection point. Someone may ask why only 60 MVA and not 80 MVA pass on. Here comes the transformer rule- ampere turns of series and common windings must be equal and opposite. This voltage ratio- (HV- LV) /HV -is termed as the co-ratio of auto-transformer, indicating the equivalent rating of unit. In reality, the auto-transformer size will be more than that of a 40 MVA transformer due to the following:

1)   With all other aspects  remaining the same, the percentage impedance of auto-transformer will be the co-ratio times the impedance of two winding unit. ie natural impedance of 100 MVA auto-transformer will be 4 % if the impedance of two winding unit is 10 %. Since 4 % is a very low impedance for 100 MVA, designer will put more turns to increase impedance to limit the over current on external faults. This way auto-transformers are normally copper machines (more copper and less core steel, resulting in a lower core / copper ratio)

2)   Tap winding for voltage regulation and tertiary winding ( stabilizing or loaded) increase the equivalent size of auto-transformer. Even though stabilizing tertiary of auto units need to be only one third of equivalent MVA, in many countries one third of line MVA is provided for additional short circuit strength for tertiary. Once a utility ordered a unit combining a Generator transformer and interconnecting auto-transformer, with the tertiary receiving power from generator.

3)   While star connected two winding units are with single neutral end tap changer, auto-transformers require three-line end tap changers, one in each phase. With line end tap-changers, the tap-changer will  always be at high potential (220 or 132 kV) and utmost care is to be taken in the maintenance and monitoring of diverter switch oil. Any flash over to ground from the diverter switch part will be a low impedance fault as the entire grid will be collapsing in to the arc, without passing through transformer impedance. Such high energy arc can generate huge volumes of carbon gases that will burst the top of tap changer and gases catch fire, starting a transformer fire. Hence some utilities avoid tap-changers and stabilizing tertiary in such EHV auto-transformers. In auto-transformers also, neutral end tap-changers can be opted instead of line end tap-changer. In fact, before 1960s, when line end OLTCs were developed, that was the only option. With neutral end taps in auto connected windings,  flux in the core will be changing with tap-changing (variable flux regulation) making the unit still costlier. But when voltage ratio is around 1:3, neutral end tap-changer for LV variation (for HV variation for the same tap range, flux variation will be too high) can be economical than line end tap-changing. In many countries, neutral end tap-changers can be found with 400/132 kV auto-transformers. UHV auto-transformers (800 & 1100 kV) are always with neutral end tap changing. MR once developed a 400 kV line end OLTC, but transformer designers refused to use, considering the difficulties of managing 400 kV tap leads to OLTC, inside a transformer.

Due to above reasons, you may find a 100 MVA 220/132 kV auto and two winding transformers may cost the same price, but still the auto unit will have the benefit of higher efficiency. Auto-transformers are widely used for interconnecting power grids when voltage ratio is less than 1:3. Hence, we will see auto-transformers with voltage ratio of 400/220 kV or 400/132 kV but not 400/33 kV. Of course, 400/33 KV unit also can be made with auto connection, but the savings will be negligible. Today there are auto transformer banks up to maximum size of 1000 MVA 400/220 kV, 2500 MVA 765/400 kV and 4500 MVA 1100/ 500 kV.

Another applications of auto-transformers are in Aluminum smelters (to continuously vary input voltage of 132-400 kV by 10-100 %  to rectifier transformers  using special OLTC with tap numbers more than 100), Railway traction, and as boosters to compensate high  voltage regulation.

Transformer engineers who want an in-depth knowledge on auto-transformers may refer to the following seminal papers.

1939- J.E Clem, Equivalent circuit impedance of regulating transformers, AIEE Transactions Vol58, Pages 871-873,1939.

1954-O.T. Farry, Auto-Transformers for Power System, AIEE Transactions-Part III -B, Vol 73, December 1954, Pages 1486-1499

1956-B.A. Cogbill, A zero sequence equivalent circuit of auto-transformer connections which yields neutral shift, AIEE Transactions, December 1956, Pages 1228-1232

1958-Eric T.B.Gross & J.C.Pohlman, Rating of auto-transformers for system interconnections, AIEE Feb 1958, Pages 1236-1244

1961-O.T. Farry, Tertiary windings in auto-transformers, AIEE Transactions, April 1961,Pages 78-82

1967-G.W. Alexander &William J.McNutt, EHV Applications of Auto-Transformers, IEEE Transactions Vol. PAS 86 No8, Aug 1967,Pages 995-1000.

1973- Dr B Heller, Methods and means of voltage regulation of large auto- transformers, presented at 1971 CIGRE colloquium and published in Electra No 29 (Pages11-29) 1973

1975-H.B. Margollis, J.D.M.Phelps, A.A.Carlomagno, A.J.McElroy, Experience with part-winding resonance in EHV Auto-Transformers -Diagnosis and corrective measures, IEEE PAS 94 No.4,Pages 1294-1300