Which among the following is a type of three phase transformer connect...
Three-Phase Transformer Connections:
A three-phase transformer can be built by suitably connecting a bank of three
single-phase transformers or by one three-phase transformer.
The primary or secondary windings may be connected in either star (Y) or delta (D) arrangement.
The four most common connections are:
(i) Y-Y
(ii) Δ-Δ
(iii) Y-Δ
(iv) Δ-Y
(i) Y-Y Connection:
In the Y-Y connection shown in Fig. below, 57.7% (or 1/ √3 ) of the line voltage is impressed upon each winding but full line current flows in each winding.
Power circuits supplied from a Y-Y bank often create serious disturbances in communication circuits in their immediate vicinity. Because of this and other disadvantages, the Y-Y connection is rarely used.
(ii) Δ-Δ Connection:
The Δ-Δ connection is shown in Fig. below. It is often used for moderate voltages.
An advantage of this connection is that if one transformer gets damaged or is removed from service, the remaining two can be operated in what is known as the open-delta or V-V connection.
(iii) Y-Δ Connection:
The Y-Δ connection shown in Fig. below is suitable for stepping down a high voltage.
In this case, the primaries are designed for 57.7% of the high-tension line voltages.
(iv) Δ-Y Connection:
The Δ-Y connection shown in Fig. below is commonly used for stepping up to a high voltage.
Other Special Three Phase Connection:
(i) Scott Connection:
- The Scott-T Connection is the method of connecting two single-phase transformers to perform the 3-phase to 2-phase conversion and vice-versa.
- Scott connection of transformer is also known as T – T connection.
- The two transformers are connected electrically but not magnetically.
- The Scott-T connection is used in an electric furnace installation where it is desired to operate two single-phase together and draw the balanced load from the three-phase supply.
- It is used to supply the single phase loads such as electric train which are so scheduled as to keep the load on the three phase system as nearly as possible.
- The Scott-T connection is used to link a 3-phase system with a two–phase system with the flow of power in either direction.
(ii) Open-Delta Connection:
- An open delta connection transformer uses two single-phase transformers to provide a three-phase supply to the load.
- An open delta connection system is also called a V-V system.
- Open delta connection systems are usually only used in emergency conditions, when one of the transformer in Delta-Delta system is damaged, and removed from service.
- Efficiency for the operation of open delta connection is low as compared to delta-delta systems which are used during standard operations.
(iii) Zig-Zag Transformer:
- If a neutral point is required which otherwise is not available (e.g., delta connection, bus bar points, etc.), a zig-zag transformer is used.
- These transformers do not have secondary winding.
- Each limb of the transformer has two identical windings wound differentially such that under normal conditions the total flux in each limb is negligibly small and, therefore, the transformer draws very little magnetizing current.
- The grounding transformers are of short time rating usually 10 seconds to 1 minute.
- Therefore, the sizes of such transformers are small as compared to the power transformers of the same ratings.