Normally, the parallel connection of transformers is used to supply the growth in energy demand in an electrical system; although it is also used. In addition, it can be used as a power backup for eventual moments of peak consumption, for system interruption in case of failure or maintenance of one of the equipment.
The parallel connection of transformers is a very common connection today and allows multiple benefits to the electrical system. This connection is made with two or more transformers, in single-phase, three-phase and polyphase systems.
In order to be able to make a parallel connection of two or more transformers, it must be verified that the potential difference of the homologous terminals in each of the phases in the different transformers to be connected, both in the primary and in the secondary, must be zero at any instant.
There are 5 basic considerations that 2 or more transformers must meet, so that they can be connected in parallel:
a. Equality of primary and secondary tensions.
b. Same relationship of transformation.
c. Equality in the internal voltage drop, both resistive and inductive.
d. Equal design frequencies.
e. The relative polarities must be the same in all transformers for any instant.
It is essential to carry out the parallel transformers the exact fulfillment of the last of the aforementioned points; the other conditions do not always have to be met 100%. However, an attempt should be made to meet these conditions as accurately as possible.
Basic considerations
Equality of primary and secondary stresses
This first condition can accept small variations (+/-5% and depending on the design of the transformer). That is to say, it can be accepted that a transformer that is not designed for a primary nominal voltage can be connected to it, this will make it work at a different point of the induction curve of the core; and will result in the rated vacuum current, no-load losses, audible noise and insertion current being modified from their original design conditions.
You should consult with the equipment manufacturer about this action and analyze the possibility of carrying it out.
Equality of Transformation Relationship
This condition is of fundamental compliance.
A small difference in the transformer ratios can be admitted in the connections, of the maximum order of 0.5%, which is noted in the different transformer manufacturing standards. When the difference in the transformation ratio of the different transformers is greater than 0.5%, it is not advisable to connect them in parallel.
Failure to comply with this equality will result in a circulation current in the secondary transformers, a situation that will be present at all times when the transformers themselves are connected.
The condition of equality of internal falls for the different transformers
This condition is closely related to the comparison of the UCC (Short-circuit voltages). However, the comparison must be made not only with the UCC, but with its resistive and inductive components (Ur and Ux).
In other words, this point establishes that the external characteristic of the transformers due to the load must be as similar as possible to each other.
This condition is the one that can accept the greatest deviation.
It is suggested that the different internal impedances of the equipment do not have differences greater than 10%.
The greater the equality between the different percentage impedances (UCC) of transformers, the more equitable the load sharing of the different transformers will be when the secondary system requires energy.
When in parallel, there are transformers with different UCC,; The power demand of the load will cause the transformer(s) with higher internal impedance to be charged percentage-wise in a lower way than the transformer with lower relative internal impedance.
In this situation, one of the transformers will reach the limit of its capacity before the other transformers and will cause the parallel assembly to deliver less power than expected, since once this transformer reaches the load limit, it will not be able to continue charging the system and the other transformers will be below the nominal load level. Therefore, in this load condition, the system will not be able to deliver the maximum available capacity, and one of the teams will always be working more demanding than the rest.
This situation of uneven loads can also occur when the equipment has the same percentage impedance (UCC), but that its resistive components (Ur) and reactive (U)x) are different,. So they will charge unevenly, depending in this case on the charging power factor.
Equal Frequency Design
This is a fundamental condition; since, except in very exceptional cases, the connection of a transformer of a different design frequency to that of the grid to be connected cannot be accepted.
Relative polarities must be the same
As stated above, the last of the established conditions does not accept difference. In other words, if the relative polarity between the terminals to be connected is not the same at any time, the parallel operation should not be carried out.
That they have the same relative polarity implies that each phase of the secondary to be connected has the same time offset with respect to its relative primary.
As a phase shift, the rotation order of the phasors and polarity are intimately linked to each other, it is a sine qua nom condition to make a parallel connection, to verify their equality, because otherwise at a certain moment potential differences would appear between homonymous terminals, producing a short circuit.
The connection group of the transformers determines the offset between primary and secondary, and only transformers with the same connection group can be connected in parallel, provided that the connection of the terminals with the same designation is respected.
In three-phase systems, connection groups with the same hourly offset index can also be connected in parallel, which are summarized in the table below:
However, each of these particular cases must be analyzed very well since they do not always meet the other conditions and are usually loaded differently.
Power ratio between the transformers of the parallel system
It is not advisable to parallel transformers whose power differs too much. Ratios of up to 1:3 and in extreme cases 1:5 are recommended.
The reason for this is that, as power increases, transformers tend to have their internal impedance more and more inductive, which means that the percentage loads of the transformers are different for the different load power factors. In which case, as we have already expressed above, the transformers are loaded differently percentage-wise, one of them reaching the load limit and limiting the available power of the system.
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