TRANSFORMERS 387 



to consumer's circuits, shall be tested with 10,000 volts from 

 primary to core and secondary combined. The secondary winding 

 shall be tested with twice normal voltage plus 1000 volts. 



Under certain- conditions it is permissible to test transformers 

 by inducing the required voltage in their windings, in place of 

 using a separate testing transformer. By " required voltage " is 

 meant a voltage such that the line end of the windings shall receive 

 a test to ground equal to that required by the above general rules. 



Transformers with " graded " insulation shall be so marked, 

 and shall be tested by inducing the required test voltage in the 

 transformer and connecting the successive line leads to ground. 

 The term " graded " is used to indicate the employment of less 

 insulation towards the end of the windings where the insulation 

 stresses are low, i.e., towards the ground, and more insulation at 

 the high-potential ends. Such transformers usually have the wind- 

 ing grounded within the tank and all transformers so connected 

 shall be tested by induced voltage. 



Until the adoption of the sphere gap as a method of voltage 

 measurement, transformers were generally tested by the use of 

 the needle gap. This resulted in more or less inconsistent tests, 

 due mainly to the effect of the variation in humidity and also to 

 some extent due to temperature, barometric pressure and corona. 

 Accordingly, when needle gaps were used for voltage measure- 

 ments, the actually applied voltage depended upon the particular 

 season of the year and the atmospheric conditions at that time, 

 and this naturally resulted in that in many instances the trans- 

 former tested did not receive the required voltage. With the 

 adoption of the sphere gap the variation in the applied voltage is 

 eliminated and by insisting upon this method of measurement it is 

 safe to assume that the full potential is actually applied. This is, 

 of course, of great importance with very high voltage transformers. 



Taps. It is customary to provide the high-voltage transformer 

 windings with taps for four 2 per cent steps below the normal 

 operating voltage so as to compensate for voltage drop in the line. 

 Fig. 226 illustrates this point, the diagram representing a single- 

 phase system for the sake of simplicity. 



For the step-up transformers in the generating station it is 

 obvious that taps are not required, but they are sometimes pro- 

 vided for the sake of uniformity with the step-down transformers. 

 Thus, with a 10 per cent voltage drop in the line, the conductors 



