Three Phase Transformer

Large scale generation of electric power is usually 3-phase at generated voltage of 13.2 kV or somewhat higher. Transmission is generally accomplished at higher voltages of 110, 132, 275 and 750 kV for that 3 phase transformers are necessary to step up the generated voltage. Most of the consumer, the distribution voltages are still reduced to utilization voltages of 440,220 or 110 volts. Here three phase step down transformer is used. It is economical to use three phase transformer for transmission and utilization purpose.

Three phase transformer construction is similar to single phase transformer as shown in Figure 1.44. Operation of three phase transformer is similar to single phase transformer. Three phase supply is given to primary winding. Flux induced in the core. This flux is linked with secondary winding. Depending upon the Number of turns in the secondary winding voltage should be stepped up or stepped down. The connection may be star or delta.

Core Type Three Phase Transformer

Three phase transformer also categorized as core type and shell type like single phase transformer.

Three phase core type transformer is shown in Figure 1.45. Magnetic circuits of 3 phases are interlinked. Each limb carries the fluxes of more than one phase.

Shell Type Three Phase Transformer

In this type each limb carries one primary and one secondary winding as shown in Figure 1.46. Windings are mounted on the central limb. Magnetic circuits of three phases are more independent than in core type transformer.

Three Phase Transformer Connections

1. Star-Star (Y-Y) Connection

2. Delta-Delta (A-A) Connection

3. Star-Delta (Y-A) Connection

4. Delta-Star (A-Y) Connection

5. Open-Delta (or) V - V Connection

6. Scott Connection (or) T - T Connection.

Star - Star Connection

Figure 1.47 shows star-star connection and vector group. This is most economical for small current rating, high voltage transformers because the phase voltage is 1/√3 times the line voltage.

Number of turns per phase and quantity of insulation required is minimum. There is a phase shift of 30° between phase voltage and line voltage on both primary and secondary where as line voltages in both primary and secondary are in phase with each other as shown in the phasor diagram. This connection works satisfactorily only if the load is balanced.

With unbalanced load to neutral, the neutral points shifts there by making the three line to neutral voltage unequal.

Ratio of voltages on the primary and secondary side is equal to the transformation ratio of each transformer. The star-star connection works well for balanced load. If the load is unbalanced neutral shifts. To prevent this, star point of the primary is required to connect to the star point of generator.

Let V_L1, be the voltage on the primary side. The phase voltage on the primary side is:

V_ph1 = V_L1 / √3

If 'k' is the turns ratio, the phase voltage on the secondary is given by:

Advantages:

1. Less number of turns and less quantity of insulation is required because

V_ph =V_L / √3.

I_ph= I_L, current through the winding is high. The windings must have a large cross-section and must be mechanically strong so that they can bear heavy load and short-circuit.

3. Suitable for three phase four wire system.

Disadvantages:

1. The neutral point shifts due to unbalanced load.

2. Third harmonic present in the alternator voltage may appear in secondary and cause distortion of secondary voltage.

Delta-Delta Connection (A-A Connection)

This setup is generally used in systems, which carry large current on low voltages, where continuity of service must be maintained even though one of the phases develops fault. Delta-Delta connection shown in Figure 1.48 and 1.49.

V_LI → Line voltage primary

V₁₂ → Line voltage secondary

V_ph1 → Phase voltage primary

V_ph2 → Phase voltage secondary

Advantages:

1. Output voltage to be sinusoidal, it is necessary that the magnetizing current of transformer must contain 3^rd harmonic component.

2. During unbalanced load condition, no neutral shifting.

3. If one transformer become disabled, the system can continue to operate in open delta or in V-V at capacity of 58% and not 66.7% of nominal value.

Star-Delta (Y-A)

The main use of this connection is at the substation end of the transmission line where the voltage is to be stepped down.

From the phasor diagram Figure 1.50. 30° phase difference between the primary side and the secondary allows flow of third harmonics.

Advantages:

i. Primary is star connected, few turns are required in primary, which is economical for power transformers.

ii. To avoid distortion, primary neutral should be earthed.

iii. Possible to handle, unbalanced load.

Disadvantages:

i. It is not possible to make parallel with star-star and delta-delta transformers. Because secondary voltage is not in phase with primary.

Delta-Star Connection

This connection is generally employed where it is necessary to step up the voltage. It is used at the beginning of high tension transmission system. The connection is shown in Figure 1.51. Neutral of the secondary is grounded for providing 3 phase 4 wire service. It can be used to serve both 3 phase power equipment and single phase lighting circuits as well. (1φ,3φ loads).

From phasor diagram it can be seen that there is a 30° phase difference between the primary and secondary line voltages.

Advantages:

i. 3φ 4 wire system is possible, since secondary having neutral.

ii. No distortion due to third harmonics.

iii. Cost of insulation was reduced.

Disadvantages:

i. Affected by unbalanced load.

Open Delta or V - V Connection

If one of the transformers of a ∆ - ∆ is removed and 3 phase supply is connected to the primary as shown in Figure 1.52. Three equal 3 phase voltage will be available at the secondary terminals on no load. This is called open delta or V - V connections. If the load increases, in future the open delta can be closed to increase the rating.

The 3 phase load which can be carried without exceeding the ratings of the transformer is 57.7 per cent of the original load rather than the expected 66.7%. Overload may be carried temporarily, but some provision must be made to reduce the load if over heating and consequent break down of the remaining two transformers is to be avoided.

Disadvantages

1. The average power factor at which the V bank operates is less than that of the load. This power factor is actually 86.6% of the balanced load power factor.

2. Secondary terminal voltages tend to become unbalanced to a great extent when the load is increased, this happens even when the load is perfectly balanced.

Scott Connection or T-T Connection

This is another method of transformation of 3 phase power from one voltage to another by using two transformers. Charles F Scott proposed it which is shown in Figure 1.53. If requires two transformers on each side instead of three transformers and accomplishes three phase to three phase transformations. The transformer which is a horizontal member of the connection having centre taps both on primary and secondary is know as the main transformer. The other transformer of primary and secondary whose one end is connected to the main transformer has a 0.866 tap and it is called the teaser transformer. Three phase supply is given to the other end of the teaser and the two ends of the main transformer.

Phasor diagram of Scott Connection Transformer

The line voltages of the 3-phase system V_AB, V_BC and V_CA which are balanced are shown in the figure 1.54. The same voltage is shown as a closed equilateral triangle. The figure below shows the primary windings of the main and the teaser transformer.

The D divides the primary BC of the main transformers into two halves and hence the number of turns in portion BD = the number of turns in portion dC = T_p/2. The voltage V_BD and V_DC are equal, and they are in phase with V_BC

The teaser transformer has the primary voltage rating that is √3/2 or 0.866 of the voltage ratings of the main transformer. Voltage V_AD is applied to the primary of the teaser transformer and therefore the secondary of the voltage V_2t of the teaser transformer will lead to the secondary terminal voltage V_2m of the main transformer by 90° as shown in the Figure below.

Then, for keeping the voltage per turn same in the primary of the main transformer and the primary of the teaser transformer, the number of turns in the primary of the teaser transformer should be equal to (√3/2) T_p.

Thus, the secondaries of both transformers should have equal voltage ratings. The V_2t and V_2m are equal in magnitude and 90° apart in time; they result in the balanced 2-phase system.

Position of Neutral Point N

The primary of the two transformers may have a four wire connection to a 3- phase supply if the tapping N is provided on the primary of the teaser transformer such that the voltage across AN = V_AN phase voltage = V₁ / √3.

The equation above shows that the neutral point N divides the primary of the teaser transformer in ration,

AN : ND = 2 : 1

Applications of Scott Connection

The following are the applications of the Scott-T connection.

1. 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.

2. 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.

3. 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.

The Scott-T connection permits conversions of a 3-phase system to a two-phase system and vice versa. But since 2-phase generators are not available, the converters from two phases to three phases are not used in practice.

Example 1.20:

A 3 phase transformer is used to step down the voltage of a 3 phase, 11 kV feeder line, per phase turn ratio is 10. For a primary line current of 25 A. Calculate the secondary line voltage, line current and kVA for the following connections. (i) Star-Delta (ii) Delta-Star.

Solution:

(a) Star-Delta

(a) Delta - Star

APPLICATIONS OF THREE PHASE TRANSFORMER

i. Power Station Generator

ii. Machine Transformer

iii. Network Transformer

iv. Distribution Transformer

v. Substation Transformer