Parallel operation of transformers - application conditions

Energy - has long been an integral part of civilization. It is simply impossible to imagine life without it. That is why the issue of the continuity of energy supply to consumers is becoming increasingly acute. Parallel operation of transformers is one such way. However, is this method used for reservation only, and what are the requirements for the equipment?

Why do parallel transformers work?

As already mentioned above, the initially agreed work was caused by the need to improve the reliability of power supply. However, there are other, no less important features, when parallel operation of transformers is necessary. To such moments carry:

• A rapid increase in the load, which in the near future will exceed (or already exceeds) the power of one operating transformer;
• Lack of space (height, width) may not allow the installation of one large transformer, but you can arrange two small ones and include them in parallel;
• Of course, security measures play an important role, since the probability of failure of both transformers is extremely small, but it is, therefore, in addition to such an event as the parallel operation of transformers, use other methods of redundancy.

Commissioning of two transformers

The inclusion of transformers for parallel operation ensures the redundancy of particularly responsible electrical energy receivers, which means the continuity of power supply. Most substations use two or more transformers (exceptions are low-power substations of a through-type type). Work in parallel is allowed if both transformers meet a specific set of requirements, which are called "Transformer Parallel Operation Conditions":

• Groups of windings must be identical to each other;
• The transformation coefficients must be equal to each other, an insignificant deviation in the allowed normalized limits is allowed;
• The short-circuit voltage should be equal to each other, an insignificant deviation in the allowed normalized limits is allowed;
• Parallel transformers must be powered from a single network;
• The secondary cables needed to connect the transformers at a point must have approximately the same characteristics and length;
• The phase shift between the voltages of the primary and secondary windings should be the same.

Necessity of preliminary calculation

If one of the two transformers is damaged, it is not always possible to pick up absolutely the same one, which would completely correspond to the damaged by the conditions and operating modes. In these cases, the choice of the transformer is based on complex preliminary calculations, which show that the windings of both transformers will be loaded evenly, and none of them will not exceed the load capacity of each individual transformer by load.

Short-circuit voltage

The parallel operation of the transformers is possible with a relative equality of the short-circuit voltage values on the windings. If the values of the faults are different, then before switching on in the joint operation, it is necessary to first change the transformer ratio of one transformer using a special switch. In this way, it is possible to achieve compensation for the redistributed loads that arise due to differences in short-circuit voltages. Due to this discrepancy, the equalizing currents will not overload the transformer with a lower value of the short-circuit voltage.

Naturally, it is necessary to take into account the load capacities - both separately for each electrical appliance, and during such an event as the parallel operation of power transformers. At idle, however, the difference in the values of the short-circuit voltage is not affected at all, because the transformation coefficients are equalized and become the same. Under the load, the secondary voltages will be different, since unequal voltage drops can lead to the flow of equalizing currents along the windings, while in one transformer the equalizing current will be added to the main current, and the second will be subtracted. The recommended ratio of the powers of the most powerful transformer to the lowest power should be no more than three to one.

Admissible conditions

The parallel operation of three-winding, double-winding transformers on all windings is possible. It must be remembered that for electrical appliances connected in parallel, the load is distributed inversely proportional to the short-circuit voltage and is directly proportional to the power of each individual transformer. Parallel operation of the transformers, if the winding connection groups are different, is possible on all odd groups. If the switching is not coordinated, then, due to the angle of shear between the terminals of the secondary windings, a voltage appears which causes an inadmissible equalization current.