The Recovery Voltage Method is based
on established knowledge: the phenomenon of the polarization of oil
paper impregnated insulation.
Many "high-voltage-working people" have made this painful experience: of
short-circuiting a high voltage capacitor (which was previously charged
with a DC voltage), and measuring a voltage more or less than zero (0)
volts and so believing that the capacitor is completely discharged, and
when touching the connectors of the capacitor receive a shock. This
shock is due to the polarization of the insulation.
There are different types of polarization. In case of moist oil paper
insulation, there is a polarization due to the water molecules contained
in the insulation. By applying a DC voltage, these molecules (which were
electrically neutral) acquire a polarity and try to drift in the
direction of the electrical field. This means, molecules are now
energized. We can short circuit and afterwards open the circuit. Some
energy is still stored in the molecules. We can measure a voltage due to
this stored energy, which is called the "recovery voltage".
By this method, insulation condition is examined by tracing the
polarization spectrum from results of the recovery voltage measurements.