Page 18 - Basic PD Theory

Basic PD Theory

Contamination, Phase to Phase discharges). End-arm vibration, water leaks and penetration of the insulation by a foreign object
do not develop voids and thus these latter two mechanisms are not often detected by PD testing.

2.4 Electrical Discharges
Once a void is created within the bulk or on the surface of insulation, a potential difference will build * Partial discharges are
across it. The magnitude of this voltage will depend upon the applied voltage, the capacitance of the small electrical sparks
insulation and the gas in the void. Other issues besides the gap length and applied voltage that can within insulation or on
affect the electric stress in a void are diameter, winding temperature, internal gas and pressure, and the an insulation surface.
nature of the surface in the void.
A discharge can only occur across a void when the electric stress (V/mm) exceeds the electrical breakdown point for the gas
based on Paschen’s law. In general, the product of the gap separation and the gas pressure establishes the voltage necessary to
lead to a discharge, i.e., breakdown voltage [1]. In air at standard temperature/pressure (STP), the electrical breakdown voltage
point is:
Eair = V air/d air = 3 kV/mm (at STP)
Thus, the magnitude of the discharge is proportional to the volume of the air void, and the larger the volume, the bigger the
discharge.

2.4.1 Capacitive Model
The voltage that develops across the gap (or void) is modeled in Figure 9.

Figure 9: Electrical Stress in a Void between the Coil and the Core

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