Test: Three-Phase Transformer Connections - Electrical Engineering (EE) MCQ

Delta-Delta or Delta-Delta or ∆ − ∆ Connection:

This connection is economical for large, low-voltage transformers in which the insulation problem is not so urgent, because it increases the number of turns/phase.

The transformer connections and voltage triangles are shown in Fig.

• The ratio of transformation between primary and secondary line voltage is exactly the same as that of each transformer.
• Further, the secondary voltage triangle abc occupies the same relative position as the primary voltage triangle ABC i.e. there is no angular displacement between the two.
• Moreover, there is no internal phase shift between phase and line voltages on either side as was the case in Y − Y connection.
   

This connection has the following advantages:

• In order that the output voltage is sinusoidal, it is necessary that the magnetizing current of the transformer must contain a third harmonic component. In this case, the third harmonic component of the magnetizing current can flow in the ∆-connected transformer primaries without flowing in the line wires. The three phases are 120° apart which is 3 × 120 = 360° with respect to the third harmonic, hence it merely circulates in the ∆. Therefore, the flux is sinusoidal which results in sinusoidal voltages.
• No difficulty is experienced from unbalanced loading as was the case in Y − Y connection. The three-phase voltages remain practically constant regardless of load imbalance.
• If one transformer becomes disabled, the system can continue to operate in open-delta or in V − V although with reduced available capacity. The reduced capacity is 58% and not 66.7% of the normal value.

Delta connection in 3ϕ transformer:

• In the delta connection, the line voltage is equal to the phase voltage.
• The line current is equal to √3 times of phase current.
• The delta-connected 3ϕ transformer can be used for both balanced and unbalanced loading.
• In the delta connection, the third harmonic component flows rounds the closed-loop of the delta.
• Hence, the third harmonic components are co-phased and hence cancel out in the line. 
• So, the third harmonic does not appear in the output voltage wave.

Concept:

For delta connection:

Line voltage = Phase voltage

Line current = √3 × Phase current

For star connection:

Line voltage = √3 × Phase voltage

Line current =  Phase current

Calculation:

Given single phase transformer data 127 V / 13.2 kV

A three-phase transformer connection in star delta using three single-phase transformers is as shown

Primary line voltage V_L1 = √3V_ph1 = √3 × 127 = 220 V

Secondary line voltage V_L2 = V_ph2 = 13.2 kV

The line-to-line voltage ratio for the three-phase transformer is 220 / 13.2 kV

Concept:

In large voltage transformer, which are connected as Y-Y, need to connect a tertiary winding for removing third harmonic.

And connection like as below diagram

So it is converted as 3 winding Y-∆-Y transformer.

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.

The secondary winding of the Distribution transformer is always connected in a star connection. This is done because:

• A distribution transformer is used to step down (reduce) the high voltage to the low voltage for supplying the consumer’s load. The high-to-low voltage conversion is possible only by star connection.
• The distribution of power to the domestic household is done in phase voltage. Hence, a phase with neutral is required which is possible only in a star connection.
• The delta connection does not provide neutral.

Star Connection:

In a star connection:

where, V_p = Phase Voltage

V_L = Line Voltage

Delta Connection:

In delta connection:

V_L = V_p

Neutral is absent in delta connection

Star-Delta transformer:

• Consider a 3-phase star-delta transformer with primary side Y connected (star connected) and secondary side with delta connected as shown below

• The polarity markings are indicated on each phase.
• The dots on the winding indicate the terminals which are positive at the same time on the undotted terminals.
• Phase on star side – A, B, C
• Phase on delta side – a, b, c
• The labeling is indicated to the diagram corresponding to +90° connections in which the positive sequences on the delta side are lead by 90° corresponding to the star side.
• Thus the line current flows through the phase and A.
• If we label delta as b → a, c → b, and a → c. There we will get standard Yd1, –30° connection
• If polarities on the delta side are also reversed, we get standard Yd11, 30° connection as shown in the figure given below.
• Thus we can get a phase difference of 30° between input and output of three-phase transformer by using a star-delta connection.

Important Points:

The phase shift in different connections of transformer are given below.

• Large unbalanced and balanced loads can be handled satisfactorily by a delta to star transformer.
• The star-delta & delta-star connection has no problem with third harmonic components due to circulating currents in the delta.
• It is also more stable to unbalanced loads since the delta connection partially redistributes any occurred imbalance.

Concept:

• Star-star type of transformer is rarely used due to problems with unbalanced loads.
• It is economical for small high voltage transformers as the number of turns per phase and the amount of insulation required is less.

Applications of different three-phase transformers:

• Delta – star connection type three-phase transformer is used for both large and low voltage rating transformers.
• Delta – star transformer is used at the generator side to step up the voltage levels
• Star – delta transformer is used at the load side of distribution systems to step down the voltage levels.
• Star – star connection transformers are used for small, high voltage transformers.
• Delta – delta connection transformers are used for large, low voltage transformers.