SREEKAVITHAENGINEERIG COLLEGEEEE Department – EM-II LAB

Experiment No: 9 III/IV EEE (I Semester) Page 1 of 4

PARALLEL OPERATION OF TWO SINGLE PHASE TRANSFORMERS

Aim : - To perform parallel operation of two single phase transformer & observe the load sharing between them.

Apparatus: -

S.No.

Meter Range Type Quantity

1. Voltmeter 0-300V MI 22. Ammeter 0-20A MI 23. Ammeter 0-10A MI 24. Variac 1-Φ, 260 V/15A 1

5. Load 1-Φ, 230V/13AResistiv

e1

Name Plate Details: - 1-Φ Transformer.

Voltage : 230/230 VoltsPhase : Single.KVA : 3KVA.Full Load Current : 13 Amps.

Circuit Diagram: -

Theory: -

Lab In Charge HOD, EEE

SREEKAVITHAENGINEERIG COLLEGEEEE Department – EM-II LAB

Experiment No: 9 III/IV EEE (I Semester) Page 2 of 4

In order to operate two or more transformers connected in parallel (on the primary as well as secondary sides) they must satisfy the following conditions:

When two transformers are to be operated in parallel the following conditions are to be fulfilled.1. Their transformation ratios must be equal.2. The voltage rating of both the transformers must be same.3. The equivalent impedance of each transformer must be

inversely proportional to its current ratings.4. The ratio of equivalent resistance to equivalent reactance

should be same for each transformer.5. Percentage impedance of the two transformers should be

equal.Of these conditions, (1) is easily comprehended;

condition (2) is essential. There is some latitude possible with conditions (3) and (4). If condition (3) is not exactly satisfied i.e. the two transformers have slightly different transformation or voltage ratios, even then parallel are possible. But due to inequality of induced e.m.f’s in secondaries, there will be even on no-load, some circulating current between them when secondary terminals are connected in parallel.

If condition (4) is not exactly satisfied i.e. impedance triangles are not identical in shape and size, parallel operation will still be possible, but the power factors at which the two transformers operate will be different from the power factor of the common load.

It should be noted that the impedances of the two transformers may differ in magnitude and in quality. It is worthwhile to distinguish between the percentage and numerical value of impedance. For parallel operation, the regulation must be the same, this condition being enforced by the very fact of their being connected in parallel. It means that the currents carried by the two transformers are proportional to these ratings and their percentage impedances are identical.

If the quality of the two percentage impedances is different , then this will result in divergence of phase angle of the two currents, with the result that one transformer will be operating with a higher and the other with a lower power factor than that of the combined load.

Lab In Charge HOD, EEE

SREEKAVITHAENGINEERIG COLLEGEEEE Department – EM-II LAB

Experiment No: 9 III/IV EEE (I Semester) Page 3 of 4

Procedure: -1. Connect the two single-phase transformers in parallel as

shown in the circuit diagram.2. Apply the rated voltage on the primary side using 1-Φ

variac and note the load readings of all meters.3. The load on the transformers is increased step by step

until the transformers are fully loaded. Measure the readings of meters for each step and tabulate the results.

4. Verify that the readings of the load currents of the two transformers (A & B) are equal or not and plot graphs of IA

versus IL & IB versus IL.

Observation: -

S.No.Primary Voltage

Vp

Terminal Voltage

VT

Supply Current IS

T/R A Circuit

IA

T/R B Circuit

IA

Load Curren

t IL

Model Graphs: -

Precautions: -

1. Remove the entire load on the transformers before switching off the supply.

Lab In Charge HOD, EEE

IL

IAO

IL

IBO

SREEKAVITHAENGINEERIG COLLEGEEEE Department – EM-II LAB

Experiment No: 9 III/IV EEE (I Semester) Page 4 of 4

2. Take care, such that the load current does not exceed the transformers rated current.

3. The condition for parallel operation should be strictly established before transformers are operated

4. Connections should not be loose.5. Meter Readings should be taken without parallax

error.

Result: -

Performed the parallel operation of two single-phase transformers and verified that the load is equally shared by the transformers.

Lab In Charge HOD, EEE