A79-Types of Transformers-Part 5-Based on Circuits and Connection

In the first part of this series, types of transformers were listed, based on application, connections, insulation system used, installation location or construction details. Parts 2-4 covered various types of transformers based on application. Now we will see various types of transformers based on connections and circuits.

A55-Types of Transformers- Part 1

https://www.dhirubhai.net/pulse/a55-types-transformers-p-ramachandran

A 65 -Types of Transformers - Part 2

https://www.dhirubhai.net/pulse/65-types-transformers-part-2-p-ramachandran

A 66 -Types of Transformers - Part 3 | LinkedIn

A 67-Types of Transformers-Part 4 | LinkedIn

1. Single-Phase Transformers

In the initial stages of AC power distribution, only single-phase power was employed till polyphase AC transmission was proposed around 1890 by Nikola Tesla (Westinghouse, USA), Galileo Ferraris (Italy), Jonas Wenstrom (ASEA, Sweden) and DoliVo-Dobrovolsky (AEG, Germany).?After three-phase AC power became the standard, Europe went ahead with three-phase transformers while the US preferred to use single-phase units that were connected to get three-phase connections. The advantages claimed were less costly spare transformers, flexibility etc., even though the option had the disadvantages of more cost, less efficiency with more footprint at substations. ?This preference for three-phase banks from single-phase units continued in the US till around 1940.

The largest single-phase transformers in service today are 1500 MVA 1050/550 kV Auto-transformers with OLTC in China. These were with five limbed cores with three wound limbs in parallel. Hitachi supplied 700 MVA 420 kV single-phase Generator Transformers to EdF, France in 2022. In the case of HV DC transformers, 621 MVA?Transformers were used at the Pacific Intertie system in the USA. These HV DC single-phase transformers, made by ABB, were of a unique construction, with a six-limbed core and four wound core limbs in parallel.??ABB had also supplied 608 MVA HV DC single-phase transformers to China for their 1100 kV HV DC Transmission system. (commissioned in 2018)

Single-phase transformer primary may be designed suitable for connection between two of the three-phase lines (eg distribution transformers /traction trackside supply transformers) or between phase line and neutral (to form three-phase banks). It may be in the neutral earthing circuit between the neutrals of generators/transformers to the ground.

When single-phase units are used for three-phase banks ( eg: large rating generator transformers) it is usual to specify close tolerance for the percentage impedance of units in the lot, usually +- 2 %. In the case of auto-transformer banks with delta tertiary, it is essential that all the impedances (H-M, H-T, L-T) shall be the same for the single phase units. Otherwise, there can be circulating currents in the closed delta tertiary.

LV Transformers

Single-phase LV transformers are widely used in control and monitoring circuits. These are micro or mini, low-voltage transformers, used as step-down or step-up transformers in numerous appliances and applications.

MV Transformers

A neutral grounding transformer is a single-phase unit, the primary connected in series with the earthing link from equipment (star-connected generator or transformer) neutral to earth with the secondary shorted or through a resistor. The purpose is to insert a reactance or resistor in the zero-sequence circuit to reduce the fault current in the event of an L-G fault. By using an HV-LV transformer, a small resistor at the secondary will contribute to a high resistance in the primary circuit.?

Single-phase distribution transformers (insulating liquid filled or dry type) are used with the primary connected either line to neutral or across two lines of three-phase lines.

Another popular use of single-phase transformers in power transmission is for supplying power for 25 kV AC traction overhead lines (IEC 62695-2014; EN50329 Traction Transformers for fixed lines) In India, when introduced in the 1950s traction transformer ratings were 10-12.5 MVA that increased to 30 to 40 MVA, with the primary connected across two lines of 33-220 kV three phase power and single-phase secondary supply at 25 kV.

HV Transformers

When the three-phase transformer size or weight exceeds the limitations of the transport route to the site, single-phase construction is adopted to reduce the transport weight. Then of course total cost of the three-phase bank will be more than a three-phase transformer. When planners are confronted with various options, the following table will be useful for decision-making.

P MVA ?3-phase Transformer?????????????? ?-?100 %

2X P/2 MVA 3-phase Transformers??????? ? -120-130 %

3x P/3 MVA single phase Transformers? ?-125-140 %

4X P/3 MVA Single phase Transformers ?-160-175 %

It should also be remembered that apart from cost, no-load losses also will be more for a three-phase bank compared to a three-phase transformer.

? ?UHV AC transformers at 800 kV and 1100 kV voltage levels are always single-phase transformers due to the difficulty in providing enormous air clearances ( 8 metres for 800 kV 2050 BIL) between the phase-to-phase bushings on transformer tank. In the early years of 800 kV transmission, some units were made as three-phase units but in later years single phase units were universally adopted.

Large HV DC Transformers are always single-phase units with the HV DC bushings projecting into the valve halls. These are designed and installed in such a way that in case of any single-phase unit, the failed unit can be rolled out and the spare unit can be moved in its position with minimum delay. Generator transformer banks in power stations are also arranged in this way. In the case of EHV or UHV auto-transformer banks substation layout is done such a way that the spare unit can be immediately switched in to three phase bank with a disconnected failed unit remaining in its place.

Core Construction

??Nowadays very large single-phase generator transformers are made in shell type core construction. Of course, core-type construction also can be used for such ratings as was done in the 1970s for supplies to EdF.

Core-type construction may use different options as shown in the sketch:

(A)??For very small rated distribution transformers

(B)?Standard for trackside supply transformers. Since the wound limb MVA rating is only half, a better design from a short circuit withstands strength angle. This was the standard for single-phase units in the early years of transformer engineering. In 1970 when my company got some orders for single phase 48 & 60 MVA 220 kV single phase GSUs, we made a design as per (B) as per company standard. But we also gave an alternate design as per (C) being more economical. During vetting, Hitachi agreed to try as per (C) to our pleasant surprise.

(C)?This is the popular one today and GSUs with?270 MVA wound limb capacity have been short-circuit tested in India. Yoke and side limbs are 50 % area of the limb.

(D)?This is used for very large ratings. To reduce the overall core height and thereby transport height, this type of four-limbed core is used. When India made the first 200 MVA single-phase GSU in 1980, I used this core construction.

(E)?This is a variation of (D) to get reduced no-load losses. The yoke is 60 % and the side limbs are 40 % limb area.

(F)?This construction is for extra large units. The three wound limbs are in parallel. Chinese 4500 MVA 1050 kV Auto-Transformer bank used this construction. Each wound limb was of 500 MVA rating.

Post Script: It is two years since I started this Newsletter- “All about Transformers.” ?When I started, I never knew so many power engineers will be interested in this “dry” topic. Thank you, readers, and your enthusiasm every time pushed me out of my lazy slumber to write again!

Wish you all a very happy and prosperous 2023!!