OPEN ENDED
LAB
ELECTRICAL MACHINES
(EE-246)
SECTION B, BATCH 2021
GROUP MEMBERS:
 SABRINA KHAN (EL-054)
 M. AYAN ADIL (EL-068)
 AYESHA WAQAR (EL-085)
 SYEDA MICHELLE SAJJAD
(EL-087)
 MAHEEN WASIM (EL-081)
OBJECTIVE:
To perform the parallel operation of two transformers of different KVA ratings.
PARALLEL CONNECTION OF TRANSFORMERS:
When we connect the primary windings of two transformers to a common supply voltage and the
secondary windings of both the transformers to a common load, this type of connection of transformer is
said to be the parallel operation of transformers.
REASONS FOR PARALLEL OPERATION:
The reasons for operating the transformers in parallel are as follows:
1. This is an economical method because a single large transformer is uneconomical for large load.
2. If the transformers are connected in parallel, we require extra load then we can expand the system
by adding more transformers in the future.
3. Parallel operation reduces the space capacity of the substation when we connect transformers of
standard size.
4. The parallel connection maximizes the electrical power system availability as we can shut down
any system for maintenance without affecting other systems performance.
Single-phase transformers in parallel:
The diagram drawn shows the circuit diagram of two transformers A and B connected in parallel.
Let,
a1 = turns ratio of transformer A
a2 = turns ratio of transformer B
ZA = equivalent impedance of transformer A referred to the secondary side.
ZB = equivalent impedance of transformer B referred to the secondary side.
ZL = load impedance across the secondary side.
IA = current supplied to the load by the secondary of transformer A.
IB = current supplied to the load by the secondary of transformer B.
VL= load secondary voltage.
IL= load current
NECESSARY EQUATIONS:
By KCL,
IA+IB=IL
BY KVL,
By solving the above two equations, we get
Each of these currents has two components; the first component represents the transformer's share of the
load current and the second component is a circulating current in the secondary windings.
Conditions for Parallel Operation of Transformers
When two or more transformers run in parallel, they must satisfy the following conditions for satisfactory
performance. These are the conditions for parallel operation of transformers.
1. Same voltage ratio of transformer.
2. Same percentage impedance.
3. Same polarity.
4. Same phase sequence.
OBSERVATION:
Supplied voltage=221V
When there is no load connected to the circuit:
TRANSFORMER 1
TRANSFORMER 2
PARALLEL COMBINATION
When secondary is short-circuited:
TRANSFORMER 1
TRANSFORMER 2
PARALLEL COMBINATION
TRANSFORMER 2
PARALLEL COMBINATION
When load is connected:
TRANSFORMER 1
Purpose of Parallel Operation
Parallel operation of transformers is necessary to achieve the following objectives:
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Increased power capacity: By combining the output of multiple transformers, the overall power
capacity can be increased to meet higher load demands.
Redundancy and reliability: Operating transformers in parallel provides backup in case of failure
of one transformer, ensuring continuous power supply.
Load sharing: Parallel operation allows for sharing the load between multiple transformers,
preventing overloading of any single transformer.
 Requirements for Parallel Operation
To ensure successful parallel operation, the following requirements must be met:
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Same voltage rating: The transformers must have the same primary and secondary
voltage ratings.
Similar impedance: The transformers should have similar impedance values to ensure
balanced load sharing.
Proper polarity: The polarities of the transformers must be aligned correctly to avoid
phase or voltage imbalances.
Transformer Information
Transformer A:
- Rated KVA: KA
- Rated primary voltage: VA
- Rated secondary voltage: VA'
- Rated primary current: IA
- Rated secondary current: IA'
Transformer B:
- Rated KVA: KB
- Rated primary voltage: VB
- Rated secondary voltage: VB'
- Rated primary current: IB
- Rated secondary current: IB'
Polarity Test:
Perform a polarity test on the transformers to ensure their polarities are aligned correctly. Mark the
polarities of each transformer with dots. The polarity test can be conducted using a simple voltage source
and a voltmeter. Connect the primary winding of each transformer to the voltage source and measure the
voltages across the secondary windings using the voltmeter. The polarities should match, and the voltages
should be in phase.
Open-Circuit and Short-Circuit Tests
Perform open-circuit and short-circuit tests on both transformers to determine their percentage
impedance. The percentage impedance is a crucial parameter for load sharing in parallel operation.
 Open-Circuit Test:
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Connect the primary winding of one transformer to a variable AC voltage source.
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Keep the secondary winding open.
Adjust the primary voltage to the rated voltage while measuring the primary current
(IAoc and IBoc) and the primary voltage (VAoc and VBoc).
Calculate the open-circuit voltage (VA'oc and VB'oc) as a ratio of the measured primary
and secondary voltages.
Determine the percentage impedance using the following formula for each transformer:
Percentage Impedance = [(VAoc - VA'oc) / VA'oc] * 100 and [(VBoc - VB'oc) / VB'oc]
* 100.
 Short-Circuit Test:
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Connect the primary winding of one transformer to a variable AC voltage source.
Short-circuit the secondary winding.
Adjust the primary voltage to a low value (below the rated voltage) while measuring the
primary current (IAsc and IBsc).
Calculate the short-circuit impedance using the following formula for each transformer:
Short-Circuit Impedance = (VAoc / IAoc) and (VBoc / IBoc).
Conclusion
In conclusion, the parallel operation of transformers allows for increased power capacity, redundancy, and
load sharing. The experimental setup and procedures described in this report provide a systematic
approach to achieving parallel operation. Through proper testing, measurements, and calculations, the
load sharing between transformers can be verified. The challenges and deviations encountered during the
process help identify factors that affect the accuracy of results.