What do you mean by transformer impedance

Principles of transformers connected in parallel (part 2)

Principles of transformers connected in parallel (part 2)

Continuation of the first part - principles of transformers in parallel connection (1)

Another necessary condition for parallel operation

  1. All parallel units must be supplied from the same network.
  2. The secondary cabling from the transformers to the parallel point has approximately the same length and properties.
  3. The voltage difference between the corresponding phases must not exceed 0.4%
  4. If the transformers are operated in parallel, the fault current on the secondary side would be very high. For example, suppose the percent impedance of a transformer is 6.25%, the short circuit MVA would be 25.6 MVA, and the short circuit current would be 35 kA.
  5. If the transformers have the same power and the same percentage impedance, then the downstream short-circuit current would be 3 times (since 3 transformers are connected in parallel) about 105 kA. This means that all devices such as ACBs, MCCBs and switchboards should withstand the short circuit current of 105 kA. This is the maximum current. This current will be reduced depending on the location of the switch boards, cables and cable length, etc. However, this aspect must be taken into account.
  6. Directional relays should be available 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 transformer. Otherwise the circulating currents between the two transformers would be too high.

Summary of the parallel operation of the transformer

Transformer parallel connection typesSame loadUneven loadingOverload currentCirculating flowReceive connection
Same impedance and ratio, same KVAYesNoNoNoYes
Same impedance & ratio but different KVANoYesNoNoYes
Unequal impedance with the same ratio and KVANoYesYes


Unequal impedance and KVA, but the same ratioNoYesYesNoNo
Unequal impedance and ratio but the same KVAYesNoYesYesNo
Unequal impedance & ratio & different KVANoNoYesYesNo

The combinations that work in parallel

The following Vector group from Transformer will work in parallel.

Operational 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 the Δ-Δ, Y-Y, Y-Δ, and Δ-Y connections.
  • One advantage of the Δ-Δ connection is that if the transformers fail or are removed from the circuit, the remaining two can operate in the open Δ or V connection. This way the bank will still deliver 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.
  • With the Y-Y connection, only 57.7% of the line voltage is applied to each winding, but the full line current flows in each winding. The Y-Y connection is rarely used.
  • The Δ-Y connection is used to increase voltages because the voltage is increased by the transformer ratio multiplied by 3.

The combinations that don't work in parallel

The following Transformer Vector group does not work in parallel:

Inoperative parallel operation

How to check the synchronization of transformers

The synchronization of the transformer can be checked with one of the following steps:

Checked by synchronizing the relay and the synchronous area. If the transformer is secondary not LT Then we need to check the synchronization relay and get the system up and running properly. After connecting the relay. The relay only needs to be charged with a supply voltage and check that the relay is working properly.

The synchronization should be checked by both supply voltages. This can be checked directly with millimeters between the 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 cases MultiMate should theoretically display 0 voltages. These checks only need to be performed when synchronizing circuit breakers. We also need to check that the outgoing breaker terminals are connected so that the L1 terminals of the two breakers go to the same main busbar on the panel. Same for L2 and L3.

The best way to check synchronization under LT is to charge full panel with 1 source up to outgoing terminals of another incoming breaker. Then simply measure the voltage difference at the incoming and outgoing terminals of the incoming breaker. It should be close to 0.

How to check the circulating current Synchronize the transformer with no outgoing load. Then check current. It gives you circulatory electricity.

Advantages of parallel operation of the transformer

1) Maximizing the electrical system efficiency:

In general, electrical transformer gives the maximum efficiency at full load. By running the number of transformers in parallel, you can only turn on the transformers that make up the total need by going closer to full load for that time. If the load increases, we cannot connect another transformer in parallel to meet the overall requirement. This way we can operate the system with maximum efficiency.

2) Maximizing the electrical system availability:

With multiple transformers running in parallel, we can shut down each one for maintenance. Other Parallel transformers In the system, the load is served without a complete interruption of the power supply.

3) Maximizing Power System Reliability:

If one of the transformers is running in parallel, some other fault will trip Parallel transformers If the system shares the load, the power supply may not be interrupted if the shared loads do not overload other transformers.

4) Maximizing the flexibility of the electrical system:

There is a possibility to increase or decrease future demand for electricity system. If the energy demand is forecast to increase in the future, transformers connected in parallel must be connected to meet the additional demand as it is not economical from a business perspective to install a single transformer with a higher rating by predicting the increased system future demand as it is an unnecessary investment of money.

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

Disadvantages of using Transformer in parallel

  • Increased short-circuit currents, which increase the required switching capacity.
  • The risk of circulating currents flowing from one transformer to another transformer. Circulating currents that reduce the load capacity and increase the losses.
  • The bus ratings could be too high.
  • Parallel transformers reduce the transformer impedance significantly, i. H. The parallel transformers can have very low impedance, which creates 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.
  • This is not common in this industry as main-tie-main is very common in this industry.


Parallelizing Transformers Considerations are simple unless KVA, percent impedances, or ratios are different. If the turns ratios of the parallel transformers and the percentage impedances are the same, the load distribution on each transformer is the same. If the kVA values ​​of the parallel transformer are the same, but the impedances are different in percent, an unequal load sharing occurs.

The same applies to unequal impedances and unequal kVA. Circulation currents only exist if the turns ratios do not match on every transformer. The size of the circulating currents also depends on the X / R ratios of the transformers.

No attempt should be made to connect delta-delta-delta star transformers in parallel.

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  • Application Guide, Transformer Loading, Nashville, TN, USA.
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  • MIT Press, Magnetic Circuits and Transformers, John Wiley and Sons.