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Home / Technical Articles / Principles of Transformers in Parallel Connection (part 2)
Principles of Transformers in Parallel Connection (part 2)
Principles of Transformers in Parallel Connection (part 2)

Continued from first part – Principles of Transformers in Parallel Connection (1)

Other necessary condition for parallel operation

  1. All parallel units must be supplied from the same network.
  2. Secondary cabling from the transformers to the point of paralling has approximately equal length and characteristics.
  3. Voltage difference between corresponding phase must not exceed 0.4%
  4.  When the transformers are operated in parallel, the fault current would be very high on the secondary side. Supposing percentage impedance of one transformer is say 6.25 %, the short circuit MVA would be 25.6 MVA and short circuit current would be 35 kA.
  5. If the transformers are of same rating and same percentage impedance, then the downstream short circuit current would be 3 times (since 3 transformers are in Parallel) approximately 105 kA. This means all the devices like ACBs, MCCBs, switch boards should withstand the short-circuit current of 105 kA. This is the maximum current. This current will get reduced depending on the location of the switch boards, cables and cable length etc. However this aspect has to be taken into consideration.
  6. There should be Directional relays on the secondary side of the transformers.
  7. The percent impedance of one transformer must be between 92.5% and 107.5% of the other. Otherwise, circulating currents between the two transformers would be excessive.

Summary of Parallel Operation of Transformer

Transformer Parallel Connection TypesEqual LoadingUnequal LoadingOverloading CurrentCirculating CurrentRecomm. connection
Equal Impedance & Ratio , Same KVAYesNoNoNoYes
Equal Impedance & Ratio But different KVANoYesNoNoYes
Unequal Impedance But Same Ratio & KVANoYesYes


Unequal Impedance & KVA But Same RatioNoYesYesNoNo
Unequal Impedance & Ratio But Same  KVAYesNoYesYesNo
Unequal Impedance & Ratio & different  KVANoNoYesYesNo

The combinations that will operate in parallel

Following Vector group of Transformer will operate in parallel.

Operative Parallel Operation
1∆∆∆∆ or Yy
2YyYy or ∆∆
3∆y∆y or Y∆
4Y∆Y∆ or ∆y


  • Single-phase transformers can be connected to form 3-phase transformer banks for 3-phase Power systems.
  • Four common methods of connecting three transformers for 3-phase circuits are Δ-Δ, Y-Y, Y-Δ, and Δ-Y connections.
  • An advantage of Δ-Δ connection is that if one of the transformers fails or is removed from the circuit, the remaining two can operate in the open-Δ or V connection. This way, the bank still delivers 3-phase currents and voltages in their correct phase relationship. However, the capacity of the bank is reduced to 57.7 % (1 3) of its original value.
  • In the Y-Y connection, only 57.7% of the line voltage is applied to each winding but full line current flows in each winding. The Y-Y connection is rarely used.
  • The Δ-Y connection is used for stepping up voltages since the voltage is increased by the transformer ratio multiplied by 3.

The combinations that will not operate in parallel

Following Vector group of Transformer will not operate in parallel:

Inoperative Parallel Operation

To check Synchronization of Transformers

Synchronization of transformer can be checked by either of following steps:

Checked by synchronizing relay and synchronous scope. If Secondary of Transformer is not LT Then we must use check synchronizing relay and Commission the system properly. After connecting relay. Relay must be charges with only 1 supply and check that relay is functioning properly.

Synchronizing should be checked of both the supply voltages. This can be checked directly with millimeter between L1 phases of transformer 1 and L1 phase of transformer 2. Then L2 phase of transformer 1 and L2 phase of transformer 2. Then L3 phase of transformer 1 and L3 phase of transformer 2. In all the cases MultiMate should show 0 voltages theoretically. These checks must be done at synchronizing breakers only. We have to also check that breaker out going terminals are connected in such a way that L1 terminals of both the Breakers comes to same Main Bus bar of panel. Same for L2 and L3.

Best way to check synchronization on LT is charge complete panel with 1 source up to outgoing terminals of another incoming breaker terminal. Then just measure Voltage difference on incoming and outgoing terminals of Incoming Breaker. It should be near to 0.

To check circulating current Synchronize both the transformer without outgoing load. Then check current. It will give you circulating current.

Advantages of Transformer Parallel Operation

1) Maximize electrical system efficiency:

Generally electrical power transformer gives the maximum efficiency at full load. If we run numbers of transformers in parallel, we can switch on only those transformers which will give the total demand by running nearer to its full load rating for that time. When load increases we can switch no one by one other transformer connected in parallel to fulfil the total demand. In this way we can run the system with maximum efficiency.

2) Maximize electrical system availability:

If numbers of transformers run in parallel we can take shutdown any one of them for maintenance purpose. Other parallel transformers in system will serve the load without total interruption of power.

3) Maximize power system reliability:

If any one of the transformers run in parallel, is tripped due to fault other parallel transformers is the system will share the load hence power supply may not be interrupted if the shared loads do not make other transformers over loaded.

4) Maximize electrical system flexibility:

There is a chance of increasing or decreasing future demand of power system. If it is predicted that power demand will be increased in future, there must be a provision of connecting transformers in system in parallel to fulfil the extra demand because it is not economical from business point of view to install a bigger rated single transformer by forecasting the increased future demand as it is unnecessary investment of money.

Again if future demand is decreased, transformers running in parallel can be removed from system to balance the capital investment and its return.

Disadvantages of Transformer Parallel Operation

  • Increasing short-circuit currents that increase necessary breaker capacity.
  • The risk of circulating currents running from one transformer to another Transformer. Circulating currents that diminish load capability and increased losses.
  • The bus ratings could be too high.
  • Paralleling transformers reduces the transformer impedance significantly, i.e. the parallel transformers may have very low impedance, which creates the high short circuit currents.
    Therefore, some current limiters are needed, e.g. reactors, fuses, high impedance buses, etc
  • The control and protection of three units in parallel is more complex.
  • It is not a common practice in this industry, since Main-tie-Main is very common in this industry.


 Loading considerations for paralleling transformers are simple unless kVA, percent impedances, or ratios are different. When paralleled transformer turn ratios and percent impedances are the same, equal load division will exist on each transformer. When paralleled transformer kVA ratings are the same, but the percent impedances are different, then unequal load division will occur.

The same is true for unequal percent impedances and unequal kVA. Circulating currents only exist if the turn ratios do not match on each transformer. The magnitude of the circulating currents will also depend on the X/R ratios of the transformers.

Delta-delta to delta-wye transformer paralleling should not be attempted.

  • Say, M.G. The performance and design of alternating current machines.
  • Application Guide, Loading of Transformer, Nashville, TN, USA.
  • Toro, V.D. Principles of electrical engineering.
  • Stevenson, W.D. Elements of power system analysis.
  • MIT Press, Magnetic circuits and transformers, John Wiley and Sons.

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More Information

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. Sashi Rajan
    Jul 11, 2023

    Good Morning,
    Before paralleling 2 dyn11 individually loaded transformers i got the following measurements;
    A-A = 250V
    B-B = 250V
    C-C = 250V
    A-B = 465v
    B-A = 465 v
    C-A = 465 v
    would you please advise what would be the problem
    with thanks

  2. Musa Ali Egeh
    Dec 14, 2022

    We have 6000kva transformer with two primary windings and one Secondary winding,
    my question is: is it possible to supply the two primary windings from two different unsynchronized sources?.

  3. Jeffrey Echiemunor
    Jun 05, 2018

    Kindly help me with this solution.
    I built a synchronizing panel for two transformers of 2.5MVA, 11KV/415V each; when we powered the two transformers, the output voltage of each was 401V phase to phase (average) for both transformers.
    when the first transformer was connected to load, the following readings were recorded by the DSE8610 synchronizing module:
    Voltage ph = 216V
    Voltage LL = 373V
    Current = 2330A
    Power = 1367KW
    R Power = 548KVar
    Pf = 0.94lg

    This were recorded when one transformer was connected to the loads.

    At this point, the synchronizing module refused to engage the second transformer because the parameters are as follows:

    Voltage L-L = 404V

    How do I resolve this?

    Thank you.

  4. charles benwari
    Nov 29, 2016

    please we intend to synchronize the output of a 2188kVA, 11kV gas generator (star) and a 349kVA, 415V gas generator to be stepped up using a 750kVA step-down transformer (11/0.4kV) in the reverse, hence the output will be 11kV, delta. how do we synchronize a generator star output and transformer delta output on a common bus?

  5. Ericson Pasion
    Oct 09, 2015

    Sir I need your help regarding to this matter.
    I have a three 50 kva single phase transformer connected in parallel and another three 25 kva single phase transformer connected also in parallel. This transformer are both connected in distribution utilities or grid. My question is can I connect this two parallelled transformer to a thesame busbar and supply a load with a total of 190 kva?! What I want to do is to synchronize this so that I won’t upgrade anymore my existings transformers. It is possible?! thank you and I hope you can help me to this problem.

  6. Parnesh
    Aug 27, 2015

    Dear Sir

    Hi, how are you?
    I have a situation where i have to use 2 transformers of 750kVA in parallel with 2 ACB’s. The ACB’s load side is interconnected and the requirement for both the ACB’s is to trip together in case of any fault situation. Even if there is a phase failure in one of the Transformer. The ACB’s to come back on when the supply normalizes.

    The ACB consist of: under voltage trip, shunt trip and charging motor.

    Please can you advise me on how to design a control circuit for this operation.

    Would be very grateful for your help.

    Thank You

    Parnesh Kumar
    +679 9987 203

  7. Hasan Heydari
    May 10, 2015

    Dear sir
    I hope every thing is going perfect for you.
    Please help me by answering below question.
    we have 2 transformer and we want parallel them. one of them is 0.4/20 KV and other one is 11/20KV. both of them are step up. also the secondary voltage for them are similar.
    i would like to know can we parallel them or not?
    with best regard
    H. Heydari

  8. anirban
    Oct 14, 2014

    we have 2nos generator Trf.One Yd1 and another Yd11. can these two be synchronised and run in parallel at 400kv bus

  9. Nathan
    Aug 13, 2014

    We had two 11kV/433V, 1.5MVA transformers operating in parallel. One was 6.75% impedance and the other 9%. This situation was temporary but the transformers operated fine with the loading of the 433V board less than 1MVA. What ill-effects might we have seen with continued operation? Load flow, short circuit and arc flash studies were within acceptable limits for this configuration.

  10. Ankur saxena
    May 16, 2014

    i am Ankur Saxena a fresher. i just want to ask u a question related to transformer
    my question is what are element used when we are connecting parallel two transformer ?
    this question is asked in an interview .but i don’t know he answer.

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