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Design and type of transformer

  • Design and type of transformer

    The purpose of transformers is to transfer electrical energy from systems of one voltage

    U1 to systems of another voltage U2.

    Transformers can be differentiated according to their manner of operation (Fig):

    1. Power transformers, the windings of which are in parallel with the associated systems. The systems are electrically independent. The transfer of power is solely by induction.
    2. Autotransformers, the windings of which are connected in line (series winding RW and parallel winding PW). The throughput power SD is transferred partly by conduction and partly by induction.
    3. Booster transformers; their windings are electrically independent, one winding being connected in series with one system in order to alter its voltage. The other winding is connected in parallel with its associated system (excitation winding EW). The additional power SZ is transferred purely inductively.



    Different types of transformers according to their manner of operation: a) Power transformer, b) Autotransformer, RW Series winding, PW Parallel winding, c) Booster transformer, EW Excitation winding, RW Series winding.


    The following distinctions are made according to applications:

    1. Transformers for the supply of power DIN EN 60076-1 (VDE 0532 Part 101), such as distribution or main transformers, machine transformers and system-tie transformers,
    2. Industrial transformers,  such  as  welding  transformers,  furnace  transformers, starting transformers and converter transformers,
    3. Transformers for traction systems,
    4. Special transformers, e.g. for testing, protection and control purposes.

    Three-phase  distribution  transformers  are  covered  by  standards  DIN  42500 (    HD 428.151) and DIN 42523 (    HD 538.151).


    Transformers are divided into the following categories:


    1. Class A: dry-type transformers (e.g. cast-resin transformers)

    Core  and  windings  are not  contained in an insulating liquid. Heat losses are dissipated direct to the ambient air, hence large surface area and low current density.

    Up to approximately 20000 kVA and a maximum of 36 kV.

    ABB resin-encapsulated transformers of the RESIBLOC type are characterized by extremely high mechanical resistance of the windings because of fibre-glass- reinforced resin insulation and a very high resistance to fluctuations in temperature.


    1. Class 0: oil-immersed transformers

    Core and windings are contained in mineral oil or similarly flammable synthetic liquid with a fire point ≤ 300 °C which is simultaneously a coolant and insulating medium.


    1. Class K

    Core and windings are contained in a synthetic liquid having a fire point > 300 °C which is also a coolant and insulating medium. In construction, they are much like oil-immersed transformers.



    Vector groups and connections

    The vector group denotes the way in which the windings are connected and the phase position of their respective voltage vectors. It consists of letters identifying the configuration of the phase windings and a number indicating the phase angle between the voltages of the windings.

    With three-phase a.c. the winding connections are categorized as follows:

    1. a) Delta (D, d)
    2. b) Star (Y, y)
    3. c) Interconnected star (Z, z)
    4. d) Open (III, iii)

    Capital letters relate to the high-voltage windings, lower-case letters to the medium and low-voltage windings. The vector group begins with the capital letter. In the case of more than one winding with the same rated voltage, the capital letter is assigned to the winding with the highest rated power; if the power ratings are the same, to the winding which comes first in the order of connections listed above. If the neutral of a winding in star or interconnected star is brought out, the letter symbols are YN or ZN, or yn or zn, respectively.

    To identify the phase angle, the vector of the high-voltage winding is taken as a reference. The number, multiplied by 30° denotes the angle by which the vector of the LV winding lags that of the HV winding. With multi-winding transformers, the vector of the HV winding remains the reference; the symbol for this winding comes first, the other symbols follow in descending order according to the winding’s rated voltages.


    For a transformer with three power windings (HV windings 220 kV in neutral connection with brought-out neutral, MV winding 110 kV in neutral connection with brought-out neutral, and LV winding 10 kV in delta connection), if the vectors of the neutral voltage of HV and MV winding are in phase and the vector of the neutral voltage of the LVwinding lags behind them by 5 · 30 = 150°, the identifying symbols are:

    YN, yn 0, d 5.


    Preferred connections

    Yyn 0     for distribution transformers. The neutral point can be loaded continuously with up to 10 % of the rated current, or with up to 25 % of the rated current for a maximum of 1.5 hours. Example: for connecting arc suppression coils.

    YNyn 0  with  compensating  winding,  used  for  large  system-tie  transformers.  The neutral point can be loaded continuously with the rated current.

    YNd 5    intended for machine and main transformers in large power stations and transformer stations. The neutral point can be loaded with the rated current. Arc suppression coils can be connected (delta winding dimensioned for the machine voltage).

    Yzn 5     for distribution transformers, used up to approx. 250 kVA for local distribution systems. The neutral point can be loaded with the rated current.

    Dyn 5     for distribution transformers above approx. 315 kVA, for local and industrial distribution systems. The neutral point can be loaded with the rated current.


    Reference: ABB


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