What is PD? Partial Discharge
A Partial Discharge is an electrical discharge or spark that bridges a small portion of the insulation between two conducting electrodes.
Partial Discharge activity can occur at any point in the insulation system, where the electric field strength exceeds the breakdown strength of that portion of the insulating material.
PD activity can occur at any point in an insulation system
Partial Discharges emit energy as:
- Electromagnetic emissions, in the form of radio waves, light and heat
- Acoustic emissions, in the audible and ultrasonic ranges
- Ozone and oxide of nitrogen gases
Partial Discharge & Transient Earth Voltage (TEV) emissions
When Partial Discharge activity occurs within high voltage switchgear insulation, it generates electromagnetic waves in the radio frequency range. The signal can travel through insulating materials or components, though the signal attenuation increases with each surface or medium that it traverses. Essentially, the majority of the electromagnetic pulses produced by these Partial Discharge ‘sparks’ are conducted away by the surrounding metalwork, but a small proportion impinge onto the inner surface of the casing. These small charges (between 0.1mV to 1V each) escape through joints in the metalwork, or gaskets on gas insulated switches, and pass as a raised voltage across the outside surface of the switch to earth. These pulses of charge were first discovered at EA Technology in the 1970s by Dr John Reeves, who named them Transient Earth Voltages (TEVs). He established that these TEV signals are directly proportional to the condition of the insulation for all switchgear of the same type, measured at the same point.
The discovery of TEVs produced a powerful technique for non-invasively checking the condition of switches of the same type and manufacture, in that TEV readings are measured in dBmV and the instruments are used comparatively. From a practical perspective, Partial Discharge activity may be detected non-intrusively by placing a probe on the outside of the switchgear whilst the switchgear is in service.