Parallel Operations of Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE) PDF Download

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 below 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
Fig: Two single-phase transformers in parallel.Fig: Two single-phase transformers in parallel.
By KCL,
IA + IB = IL
BY KVL,

Parallel Operations of Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Parallel Operations of Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
By solving the above two equations, we get
Parallel Operations of Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
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.
Circulating currents have the following undesirable effects:

  • They increase the copper loss.
  • They overload one transformer and reduce the permissible load KVA.

Conditions for parallel operation of Single-Phase transformers:

Necessary conditions

  1. The transformers must have the same polarities.
  2. The transformers should have equal turn ratios.

Desirable conditions

  • The voltages at full load across transformers internal impedance should be equal.
  • The ratios of their winding resistances to reactances should be equal for both transformers. This condition ensures that both transformers operate at the same power factor, thus sharing active power and reactive voltamperes according to their ratings.

Three-phase transformers in parallel
The conditions for proper parallel operation of single-phase transformers are as follows:

  • The polarities of the transformers should be the same.
  • Identical primary and secondary voltage ratings.
  • Impedances inversely proportional to the kVA ratings.
  • Identical X/R ratios in the transformer impedances.

The condition for the parallel operation of single phase and three phase transformer is the same but with the following additions:

  1. The phase sequence of the transformers must be identical.
  2. The primary and secondary voltages of all the transformers connected in parallel must have the same phase shift.
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FAQs on Parallel Operations of Transformers - Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

1. What is parallel operation of transformers in electrical engineering?
Ans. Parallel operation of transformers refers to the connection of multiple transformers together in a parallel configuration to meet the power demand of a system. This arrangement allows for increased power capacity, improved reliability, and load sharing between the transformers.
2. What are the advantages of parallel operation of transformers?
Ans. The advantages of parallel operation of transformers include: - Increased power capacity: By connecting transformers in parallel, the overall power capacity of the system can be expanded. - Load sharing: The load is distributed among the transformers, preventing overloading of a single unit. - Redundancy: If one transformer fails, the remaining transformers can still provide power to the system. - Improved efficiency: Parallel operation allows for better utilization of the transformers, leading to improved overall system efficiency.
3. How are transformers connected in parallel?
Ans. Transformers are connected in parallel by connecting their primary windings together and their secondary windings together. This is achieved by ensuring that the polarities of the transformers are matched and that the voltages and phase angles are identical. Additionally, parallel transformers should have similar impedance and voltage ratios to ensure equal load sharing.
4. What precautions should be taken when operating transformers in parallel?
Ans. When operating transformers in parallel, the following precautions should be taken: - Matching parameters: The transformers should have similar impedance and voltage ratios to ensure equal load sharing. - Polarity and phase angle: The polarities and phase angles of the transformers should match to avoid circulating currents. - Synchronization: The transformers should be synchronized before parallel operation to prevent voltage and frequency fluctuations. - Protection: Adequate protection devices should be installed to detect faults and prevent any damage to the transformers.
5. Can transformers with different ratings be operated in parallel?
Ans. It is generally not recommended to operate transformers with significantly different ratings in parallel. Transformers in parallel should have similar ratings to ensure load sharing and prevent unequal voltage distribution. However, if transformers with different ratings need to be operated in parallel, additional measures such as impedance matching devices or voltage regulators may be required to maintain proper operation.
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