THE TRANSFORMER 307 



The Testing Transformer. All apparatus must be tested 

 for insulation before it is put in use. The voltage used in 

 this test is specified at perhaps twice the normal voltage 

 of the piece of apparatus being tested. Whenever a high- 

 voltage power transformer, for example, is to be tested 

 there must be available for making the test, a transformer 

 which can generate about twice that of the transformer 

 being tested. Also there is required a high voltage for 

 making corona loss tests, tests on the dielectric strength of 



FIG. 198. A Current Transformer for Mounting in the Station Bus- 

 bars. Westinghouse Electric and Mfg. Co. 



air, etc. So it is necessary to build transformers (for test 

 purposes only) that can generate between 500,000 and 

 1,000,000 volts. These test transformers do not have 

 much capacity but they are very bulky because of the space 

 needed for the separation of the coils to give proper insulation. 

 78. Polyphase Transformers. In transforming the volt- 

 age of a two- or three-phase power system it is possible 

 to use two or three single-phase transformers, or one poly- 

 phase transformer may be used. A two-phase transformer 

 has no commercial importance but three-phase transformers 



