Back to Back Test for Efficiency of Transformer

Back to Back Test for Efficiency of Transformer

Q. Explain with neat sketch back to back test for efficiency of transformer.

Connection-circuit diagram

Primary of auxiliary transformer

A and B an the two identical Y - Y connected transformers. One more transformer is needed called as an auxiliary transformer (C). This is also a Y connected transformer. Supply is fed from main bus-bars to transformer A and B. Primary of auxiliary transformer is connected to an auxiliary bus shown in the Figure (A) This test is aimed to work - out temperature rise of big - capacity transformers.

The secondary's of A and B transformers are connected in phase-opposition ( i.e. a₁ to a_1; b₁ to b_1; and c₁  to c₁ connected). When no voltage is introduced from auxiliary transformers and A and B switched on, the secondary will have zero voltage due to equal and opposite secondary voltages of A and B. Primary winding receive their rated voltage from the main bus bar. Due to the rated voltage supplied to primaries No load losses at normal flux-density take place.

 Auxiliary transformer's primary receive variable voltage from auxiliary bus-bars. This voltage is so adjusted that both A and B transformers primary and secondary take their full load currents. These current produce full load copper losses. Full load currents can be indicated on the Ammeters on primary /secondary sides of A and B transformers.

Condition of circulating full load current is maintained till final steady state temperature is achieved. These losses appear in the form of heat and temperature rises. Supply is switched off and immediately the value of winding resistances per phase are measured (say R₂ ). The resistances prior  to commence the test were measured at room temperature t_1. That is noted as R_1. Final temperature (t₂) and (t₂ - t₁) is worked out by using following formula. R₂/R₁ = 234.5 + t₂ / 234.5 + t₁

If voltmeters, ammeters and watt-meters are connected then losses can be measured. From these losses efficiency can be determined. Figure B shows, how ammeter, voltmeter and wattmeter can be connected.

How ammeter, voltmeter and wattmeters can connected

This test is economical and suitable mostly for big capacity transformers in which case, direct loading test cannot be performed. In the testing place or laboratory, that much load facility may not be available.