The polarity of each phase of a three phase transformer is obtained in similar way as that in case of single phase transformers. The voltage vector diagram is then constructed.
Consider a bank of three single phase transformers or a single unit of three phase transformer whose polarity is previously checked and is found to be additive. These transformers are connected in delta-delta fashion. This is shown in thew Fig. 1.
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| Fig. 1 Three single phase transformers of additive polarity connected delta delta with 180o angular displacement |
The corresponding vector diagram for this connection is shown in the Fig. 2(a). It is represented with clock method in the Fig. 2(b). Its symbol is Dd6.
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| Fig. 2 (a) Voltage vecto9r diagram with clockwise phase rotation (b) Clock method representation |
The voltage vectors H1 - H2, H2 - H3 and H3 - H1 represents the induced voltages of the three phase transformers R, Y and B are displaced from each other by 120o. As the polarity is additive the low voltage vector of R i.e. nothing but vector x1 - x2 is opposing in direction to H1 - H2. Similar is the case with other two phases. The angular displacement between the high voltage and low voltage windings is the angle between the lines H1 - N and x1 - n where N and n are neutral points of voltage vector diagram. Here this angle is x1.
The low voltage vector diagram is constructed by drawing x1 - x2 opposite in direction to H1 - H2 . x2 - x3 in opposite direction to H2 - H3 while x3 - x1 in opposite direction to that of H3 - H1. This forms a closed delta. If the connections are not made properly then voltage diagram will not be closed to form a delta.
Related articles :
- Leblling of Transformer Terminals
- Three Phase Transformer Phasor Groups
- Parallel Operation of Three Phase Transformers
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