Controlled discharging of Inductive HV Equipment

HV equipment need to be discharged prior to any maintenance work.  The discharge is usually done with an earthing rod after verifying that there is no voltage present on them with an non-contact voltage detector.

In the case of inductive equipment such as transformers and motors with high inductance value in their windings, a controlled discharge needs to be carried out.  Inductive components such as windings in transformers and motors have high inductance.  Sudden discharge of these windings will create a high discharge current and a rapid change in the flux which will result in a high voltage pulse (according to Lenz' Law). This can damage the winding insulation.

The controlled discharge is carried out with a discharge rod with a resistor in series.  The resistor used is a special non linear resistor which has a reverse temperature coefficient which means that as the temperature increases the resistance falls.  When the discharge is started, the resistance is high. As current flows through the resistor, the temperature of the resistor rises and its resistance falls.  Thus the current flow is increased.

The resistor ensures that the discharge is gradual.

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