Why the Transformers are tested at double the rated frequency which never happens practically?

               DVDF (Double voltage and double frequency) test is popular test conducted on the Transformer before it is commissioned. In this test double the rated voltage and double the rated frequency is applied to the Transformer under test and observed for its withstand capability usually for 1 min. Double voltage is applied to test the Transformer withstand capability for higher voltages which are sometimes occur in the form of lightning surges and faults on the transformer. But, practically, no transformer is subjected to double the rated frequency as the frequency variations are not allowed beyond 0.5% by the power supply grid controlling authority. In such a case, is it essential to test the transformer at 200% i.e. double the rated frequency conditions?

                When alternating electrical source is applied to the primary winding of the transformer, it draws magnetizing current which produces alternating flux in the core of the transformer. This flux links both primary and secondary windings and due its alternating nature EMF is induced across both windings and Erms can be deduced by the equation.

                                              Erms= 4.44ᴓmfN volts

                                           Where      Erms is RMS voltage induced

                                                             ᴓm is maximum flux linked

                                                             f is the operating frequency

                                                             N is the number of turns in the winding.

               As per the above equation, when double the rated voltage is applied while testing the transformer, without doubling the frequency, the maximum amount of flux linked will also be doubled as the number of turns is always constant for a particular design. This causes the abnormal heating of core of the Transformer under test and the magnetizing properties of the core are disturbed permanently. Hence, to avoid this abnormal heating of the Transformer due to increase in flux, applied frequency will also be doubled along with applied voltage to test the high voltage with stand capability of the Transformer. Thus the test is named as Double Voltage Double Frequency test.

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