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Classification of Transformers Family TECHNOLOGY BASICS ABSTRACT Transformers are used in the electrical Classification of networks everywhere: in power plants, substations, industrial plants, buil- dings, data centres, railway vehicles, Transformers Family ships, wind turbines, in the electronic devices, the underground, and even 1. Introduction undersea. The focus of this article is on transformers applied in the trans- ransformers basically perform a of transformers in the most systematic way mission of energy, usually called pow- very simple function: they increase rather than elaborating on each type. er transformers. Due to very versatile Tor decrease voltage and current for requirements and restrictions in the the electric energy transmission. It is pre- The most important international orga- numerous applications, ranging from cisely stated what a transformer is in the nisations with focus on such transfor- a subsea transformer to a wind turbine International Electrotechnical Vocabula- mers are IEC1 through E14, its technical transformer, a small distribution trans- ry, Chapter 421: Power transformers and committee for the world standards; IEEE2 former to a large phase shifter trans- reactors [1]: through the Transformers Committee former, it is very difficult to organise a mainly for the American standards and structured overview of the transformer “A static piece of apparatus with two or CIGRE3 through the Study Committee types. Also, different companies sup- mo re windings which, by electromag- A2 Transformers which mainly produ- ply different markets and each have netic ind uction, transforms a system ces technical brochures and guidelines their own classification of the trans- of alterna ting voltage and current into on many subjects. Main standards for formers, which makes the transformer another system of voltage and current the transformers in question are the IEC family even more difficult to organise. usually of different values and at the 60076 [2] and the IEEE C57 [3] series. This paper will attempt to provide a re- same frequency for the purpose of trans- latively common point of view on most mitting electrical power.“ 2. Classification of of those transformers types. transformers family The focus of this article is on the transfor- mers which enable transmission of energy As mentioned above, transformers per- Keywords: in the electrical grid, while all other types, form a very simple function and they can such as the instrument transformers (i.e. vol- have many applications. Transformers are classification, distribution transformers, tage and current transformers) and the au- 1 IEC - International Electrotechnical Commission power transformers, reactors, transfor- dio transformers, etc., are excluded. The aim 2 IEEE - Institute of Electrical and Electronics Engineers mers is to provide an overview of different types 3 CIGRE - Council on Large Electric Systems 26 TRANSFORMERS MAGAZINE | Volume 1, Issue 1 Mladen BANOVIC, Jean SANCHEZ Transformers exist for more than a century and they can be manufactured and used very differently according to customers needs. must be produced as single-phase trans- Despite not being a perfect one, perhaps formers. Another reason for using a sin- the most practical classification used by gle-phase unit rather than a three-phase the industry is the one according to the unit, is the possibility of having a fourth transformer application. According to this identical unit as a spare. Despite its simp- approach, transformers can be roughly licity and clarity, this type of classification classified as: does not help in classification of the whole - power transformers transformers family. - distribution transformers - reactors Classification according to basic techno- logy of a transformer design and manu- This classification could, however, raise facturing is also often used. There are two some questions. There are no obvious tech- main technologies for designing and ma- nical reasons for classifying a transformer nufacturing the transformers: as a distribution transformer rather than - core type a power transformer but it is widely used used in every power plant, all grid subs- - shell type in practice so it is helpful. The term “distri- tations, buildings, in the industry, the un- bution transformer” is somewhat used in Classification of derground installations, wind turbines, In a shell-formed transformer, the pri- the IEC 60076, while it is commonly used on platforms, marine vessels, under the mary and secondary windings are quite in IEEE C57. Some companies define dis- sea, etc. Due to peculiarities of all the- “flat” and are positioned on one leg sur- tribution transformers as the power trans- Transformers Family se applications, many different types of rounded by the core. In a core-formed formers below 10 MVA. The 2.5 or 5 MVA transformers have been developed in the transformer, cylindrical windings are limits are also used instead of 10 MVA. course of history. To simplify the over- like “coils” and cover the core legs. How- view of many transformer types, it is use- ever, this classification is also limited in The classification above is even more dubi- ful to have some kind of systematic clas- the large portfolio of either of those two ous with regards to reactors, because they sification. However, this is not easy to do transformer types. are not transformers at all but are usually because there are many ways of doing it. grouped with transformers because they The transformer types could be classified Transformers can be classified according share most of the technology with power according to their power rating, voltage, to the insulating/cooling fluid in: transformers and they are designed and current, weight, type of cooling etc., but - liquid-filled transformers produced in the transformer factories. It such approach would have a limited ap- - gas-filled transformers (mainly with SF6) took me some time at the beginning of plicability. - dry-type transformers my carrier to distinguish a reactor from a transformer and I believe that others had Probably the simplest and the clearest As the dry-type, and particularly gas- a similar experience. transformer classification is according to filled/insulated transformers have limited the number of phases in: applications in a large power system, this The classification by application will be - single-phase transformers classification is also not perfect. used in this article. - three-phase transformers In a three-phase system, the single-phase units are used in a bank of three trans- formers linked together. A single three- Since transformers have been in existance, phase transformer costs approximately many different types have been produced and 15% less and occupies less space than one unit of three single-phase transformers consequently their classification is quite chal- within the same tank. However, due to limitations during the manufacturing lenging! and mainly transportation, particularly of large units, the transformers sometimes www.transformers-magazine.com 27 TECHNOLOGY BASICS 3. Power Transformers Medium Power Transformers - step-down transformers (MPT) - phase-shifting transformers (PST) Power transformers cover the population This group includes transformers with a - HVDC converter transformers of the largest transformer units by means power range from 60 to 200 MVA (or 40 - transformers for industrial applications of power and voltage ratings. Manufactu- to 250 MVA), and a high voltage of up to - traction transformers red units range from 1500 MVA, and up around 275 kV. - mobile transformers to 1785 kV. Several large phase shifting - test transformers transformers consisting of two linked units have been manufactured with a 3.1 Generator step-up transformers combined capacity of 2750 MVA. (GSU) are essential components of the power plants linking the plant generator There are several different classifications to the transmission network. Built as three of power transformers according to their single-phase or three-phase units in the power and voltage ratings or size, and/or core or shell technology, they transform according to the application. voltage from the generator voltage level up to the suitable transmission voltage le- Classification according to size vel, which may go up to 800 kV nowadays. Classification according to size is a bit am- biguous because different companies use GSU transformers usually have delta- different power and voltage range for par- Figure 2: MPT [4] connected Low Voltage (LV) windings ticular types. This discrepancy may be due (energised by the generator), and star to the fact that manufacturers divide their Small Power Transformers (SPT) connected High Voltage (HV) windings portfolio according to the market they sup- Transformers from roughly 10 to 60 MVA (connected to the transmission lines). The ply so, in their specific case, other classifica- and a maximum service voltage of 170 kV connection of such transformers is mainly tions are considered pointless. Utilities can belong to this group. Other limits used are YNd. also have different fleets; therefore a certain from 5 to 40 MVA and up to 145 kV. classification can be better for each utility. They often operate continuously at full The ranges mentioned here do not render load facing variations in voltage due to presumption of general validity. changes of the load or the network re- quirement for reactive power. High rated Large Power Transformers (LPT) currents, particularly in the larger units, This group covers the largest units in the require a good
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