Introduction Of NDT Technology
Electrical Partial Discharge Locating And Analyzing (Capacitive EMF Approach)
What is it?
Partial Discharge (PD) is an electrical phenomenon, which causes insulation deterioration and frequently is the reason for breakdown (failure) of an insulation system. Partial Discharge can be described as an electrical pulse or discharge in a gas-filled void or on a dielectric surface of a solid or liquid insulation system.
This pulse or discharge partially bridges phase to ground insulation or phase to phase insulation in an electrical apparatus. Partial Discharges can occur for a number of different reasons. For example, PD can occur when high voltage structures have sharp projections, internal discharge can occur in voids and contact noise can occur if the ground connection to a bushing is poor.
The Capacitive EMF Detection Partial Discharge Analyzing makes use of the Transient Earth Voltage (TEV) measurement technique, operating with a bandwidth from 15 to 60MHz, to detect and locate the PD source.
How does it works?
If a Partial Discharge occurs in the phase to earth insulation of an item of high voltage plant such as a metal-clad switchboard or a cable termination, a small quantity of electrical charge is transferred capacitively from the high voltage conductor system to the earthed metal cladding. The quantity of charge transferred is very small and is normally measured in pico-coulombs. The transfer occurs typically, in a few nanoseconds.
When the partial discharge occurs electromagnetic waves propagate away from the discharge site. Due to the skin effect, the transient voltages on the inside of the metalwork cannot be directly detected outside the switchgear. However, at an opening in the metal cladding such as the gasket joints in Figure 1, the electromagnetic wave can propagate out into free space. The wave then generates a transient earth voltage on the metal surface. Hence, the technique is called TEV for Transient Earth Voltage.
Capacitive EMF Detection Partial Discharge Analyzing measures the peak voltage of the detected transient and not it’s integral with respect to time. It does not, therefore, measure the charge transfer directly. The readout on the Capacitive EMF Detection Partial Discharge Analyzing is in decibel (dB) and the relationship it and the actual transient voltage measured is given in the table below:
Correlation table between dB readings on Capacitive EMF Detection Partial Discharge Analyzing instrument and the actual Trigger Voltage (Vtrig) readings in milliVolts
Mathematical Expression = 20 X LOG (Vtrig) dB
The Capacitive EMF Detection Partial Discharge Analyzing is primarily suited for comparative measurements and it will show changes in the discharge activity of individual items of a plant with time and enable deterioration to be detected.