Principles of Transformers in Parallel Connection (part 2)

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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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About Author


Jignesh Parmar

jiguparmar - Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.