AC MACHINES-1 (66761) Theory

OPEN DELTA OR V-V CONNECTION

If one transformer of delta-delta connection is damaged or accidentally opened, then the defective transformer is removed, and the remaining transformer continues to work as a three phase bank. The rating of the transformer bank is reduced to 58% of that of the actual bank. This is known as the open delta or V-V delta. Thus, in open winding transformer, two transformers are used instead of three for the 3-phase operation.

Let the V_ab, V_bc and V_ca be the voltage applied to the primary winding of the transformer. The voltage induced in the transformer secondary or on winding one is V_ab. The voltage induced on the low voltage winding two is V_bc. There is no winding between points a and c. The voltage may be found by applying  KVL around a closed path made up of point a, b, and c. Thus,

Let, 

Where V_p is the magnitude of the line on the primary side.

On substituting the value of V_ab and V_bc in equation, we get

The V_ca is equal in magnitude from the secondary terminal voltage and 120º apart in time from both of them. The balanced three phase line voltage produced balanced 3-phase voltage on the secondary side.

If the three transformers are connected in delta-delta configuration and are supplying rated load and if the connection becomes V-V transformer, the current in each phase winding is increased by √3 times. The full line current flows in each of the two phase windings of the transformer. Thus the each transformer in the V-V system is overloaded by 73.2%.

It should be noticed that the load should be reduced by √3 times in case of an open delta connected transformer. Otherwise, serious overheating and breakdown of the two transformers may take place.