Welcome to your guide on 3-Phase transformers – what are they, what different types exist, how are they connected, what are their advantages and disadvantages and how do you protect them… We’re answering all these question in our NEW series. Don’t miss out, SUBSCRIBE to receive these newsletters straight to your inbox!
What are Transformers?
A transformer is an electrical device that consists of two or more coils of wire that transfers electrical energy by means of a changing magnetic field. A voltage transformer is a static electro-magnetic device that works on the principle of Faraday’s law of induction by converting electrical energy from one magnitude to another. It performs this function by linking together two or more electrical circuits by a common oscillating magnetic field, which is produced by the transformer itself. The 3-phase transformer consists of three sets of primary and secondary windings that have each set wound around one leg of an iron core assembly.
The below figure is a representation of a 3-Phase Transformer:
How do Transformers work?
AC voltages and currents are used in our daily lives. These are easily generated at a convenient voltage of 20 kV and are then transformed to a much higher voltage of various magnitudes. These voltage levels can either be 132 kV, 275 kV, 400 kV or 765 kV, which are then transmitted across the country. The voltage is transformed to a greater magnitude because a higher voltage will result in a lower current required for the same amount of power. This reduces the I2*R losses along the networked grid of cables. These high magnitude AC voltages are then transformed down to 11 kV for local distribution and then further reduced to 400 V or 230 V for the end user.
How are 3-Phase Transformers Connected?
The primary and secondary windings of the transformer will either be connected in ? (delta) or Y (wye)/star configurations to form a complete unit. A star connection is one in which the 3-phase windings are joined together at a common point. This common point is known as the star point or neutral point. A delta connection is a connection where the ends of a three-phase winding are joined together. This means that the finishing end of one phase will be connected to the starting end of the other phase thus obtaining a closed circuit.
The diagram below illustrates Star and Delta connections of a Transformer.
The following are combinations in which a 3-phase transformer can be connected in:
- Y – Y (Star – Star)
- Y – ? (Star – Delta)
- ? – Y (Delta – Star)
- ? – ? (Delta – Delta)
Star Connected Transformers
Star connected transformers, which are used in high-voltage operations, have the advantage of reducing the voltage on an individual transformer. This reduces the number of turns required and leads to an increase in the size of the conductors, thereby making the coil windings easier and cheaper to insulate as opposed to delta transformers.
Delta Connected Transformers
The advantage of using a delta-delta connected transformer is that if one transformer in a group of three should experience a fault or become disabled, then the two remaining transformers will continue to deliver 3-phase power with a capacity that is equal to approximately two thirds of the original output from the transformer unit.
In Part 2 of this series, we tackle the different types of 3-Phase Transformers, and compare oil- cooled to air-cooled transformers.