The Fig. A below shows the Surge Test waveform of a coil of 13.3KV HT Motor and it has been highlighted with numbers to explain the important stages & charachteristics of the waveform.

Fig. A

HIGHLIGHTED NO. DESCRIPTION OF WAVEFORM 1. The charging of the energy storage capacitor, is charge to the peak (Zero to Peak ) value of + ve half cycle of 50 Hz Mains frequency as shown in figure above for 10 milli sec. 2. Injection of Surge (impulse ) energy into the coils (impedance) under test. 3. The Impulse (surge) injection circuit is disconnected. i.e. the storage capacitor is disconnected from the coil (impedance) under test. 4. The energy which is injected into the coil under test, induced the feed back energy or we can say the voltage is generated in the coils. As you can see in the fig, the amplitude of the voltage peak is higher than the surge voltage injected in the coils. The other way , we can say the loss is negligible or Q of the coil is good. So the IInd peak is Q-factor of the coil. 5. This voltage so induced charges the winding capacitance i.e. the Stray capacitance between the winding & core, as shown in fig. above. 6. The coil has three parameters such as: Inductance L of the coil Stray winding capacitance The resistance of the coil The coil will begin to oscillate with its natural frequency i.e. f = 1/2π √LC Hz. These oscillations are the deciding factor in determining the quality of coil. 7. Voltage Induced decay oscillations of the coil.