Parallel Operation of Transformers

          The following conditions must be strictly observed in order that 3-phase transformers may operate in parallel.

(a)  The secondaries must have the same phase sequence or the same phase rotation.

(b)  All corresponding secondary line voltages must be in phase.

(c)  The same inherent phase angle difference between primary and secondary terminals.

(d)  Same polarity.

(e)  The secondaries must give the same magnitude of line voltages.

In addition, it is desirable that:

(f)   The impedances of each transformer, referred to its own rating should be the same, i.e. each transformer should have the same percentage or per unit resistance and reactance.

         If conditions (a) to (e) are not complied with, the secondaries will simply short-circuit one another and no output will be possible.

If condition (f) is not complied with, the transformers will not share the total load in proportion to their ratings and one transformer will become over-loaded before the total output reaches the sum of the individual ratings. It is difficult to ensure that transformers in parallel have identical per unit impedance and this affects the load sharing.

          Three Winding Power Transformers

          An example of an EHV substation having three different voltages is a 330KV substation with voltages at 330KV, 132KV and 11KV.

          A comparison is now made as whether to have two winding transformers of 330/132KV and 132/11KV or three winding transformers of 330/132/11KV in an EHV substation with three voltages.  The 11KV load in such a substation is to meet the local loads around the substation and also for the requirements of the station auxiliary supplies.  This load may be around 10 to 15MVA.

The two schemes are shown by single line diagrams as follows: 

Comparing scheme (B) with scheme (A) we have the following merits and demerits

       Merits

(a)  The number of transformers, circuit breakers, CT’s, isolators and control panels is reduced to a minimum.  There is therefore a considerable saving in the cost of equipment required.

(b)  There is considerable saving in the cost of civil engineering and structural works because of the fewer equipment.

(c)  The layout is simple and occupies less space because of the fewer equipment

and operation is also simple.

(d)  There is saving in energy because of the reduced transformation losses.

(e)  Besides, it is inevitable to provide a third winding in a star-star connected power transformer.  This third winding in such transformers is also called a `Stabilizing Winding' or ‘Tertiary Winding’.  This winding is connected in a closed delta to provide a circulating path for the third harmonic voltages and zero sequence currents or ground fault currents.

It is pertinent to note here that a star-star connection is almost always resorted to in the case of EHV transformers of 132KV and above such as in 330/132KV transformers.  The reason being that the cost of such a transformer is cheaper because the windings need be insulated for only 1/Ö3 times of the line voltage instead of for the full line voltage of Ö3 times the star voltage with a delta winding. Such a closed delta winding can be made use of for the third voltage, without the necessity of having a separate transformer.

Demerits

(a)  The main disadvantage is the increased fault level at 11KV because the voltage is directly transformed from 330 to 11KV.  Hence 11KV switchgear of adequately higher rupturing capacity will have to be installed.

The cost of such switchgear may be much more than that of such switchgear installed in the secondary of a 132/11KV transformer.

(b)  Since the third winding is a closed delta, an artificial neutral has to be necessarily

created by the use of earthing transformers.  This is a disadvantage as it adds to the initial cost.