Three-Phase Transformer Connections: A Comprehensive Guide with Sample Problems, Lecture notes of Electrical Engineering

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THREE PHASE

TRANSFORMER

Transformers that must handle a considerable amount of power are generally grouped together in banks for polyphase service. In three-phase systems, two or three identical transformers may be used in a bank in a bank for this purpose. There are four standard ways of connecting three- transformer bank; namely:

1. Wye – wye
2. Delta – delta
3. Delta – Wye
4. Wye – Delta For two-transformer bank,
5. Open delta
6. Scott connection

Delta – Delta Connection This arrangement is generally used in system in which the voltages are not very high and especially when continuity of service must be maintained even through one of the transformers should fail. Advantages:

1. Sinusoidal voltage at the secondary
2. Carry 58% load if one transfer is faulty in transformer bank
3. Economical for low voltage Disadvantages:
4. Not suitable for 3-phase 4-wire system
5. More insulation is require and the voltage appearing between windings and core will be equal to full line voltage in case of earth fault on one phase

Delta – Wye Connection This scheme is generally employed where it is necessary to step up the voltage. This connection become popular in distribution systems in which one can use the secondary to provide a neutral point for supplying line-to-neutral power to serve single-phase loads and ground the neutral point for safety reasons. The delta primary serves several useful functions. If the transformer secondary supplies large amounts of unbalanced loads, the delta primary winding provides a better current balance for the primary source.

Scott Connection There are two main reasons for the need to transform from three phases to two phases,

1. To give a supply to an existing two phase system from a three phase supply.
2. To supply two phase furnace transformers from a three phase source. A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase power from a three-phase source or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source.

Scott Connection

SAMPLE PROBLEMS

2. A delta-delta bank consisting of three 20 kVA, 2300/230V transformer supplies a load of 40kVA. If one transformer is removed, find the resulting V-V connection: a.) kVA load carried by each transformer b.) % rated load carried by each transformer c.) Total kVA rating of V-V bank

SAMPLE PROBLEMS

3. Three 5:1 transformers are connected in delta-wye to step-up the voltage at the beginning of a 13.2kV 3-phase transmission line. Calculate the line voltage on the high side of the transformer.

SAMPLE PROBLEMS

5. Three identical 9-MVA, 7.2 kV/4.16 kV single-phase transformers are connected in wye on the high-voltage side and delta on the low voltage side the equivalent series impedance of each transformer referred to the high-voltage side is 0.12 + j0.82 ohm/phase. The transformer supplies a balanced three-phase load of 18 MVA, 0.8 p.f. lagging at 4.16kV. Determine the line to line voltage at the high voltage terminals of the transformer.