Editing Partial discharge (section)

==Discharge mechanism==
[[File:Partial discharge.svg|thumb|A partial discharge within solid insulation. When a spark jumps the gap within the gas-filled void, a small current flows in the conductors, attenuated by the voltage divider network Cx, Cy, Cz in parallel with the bulk capacitance Cb]]

PD usually begins within voids, cracks, or inclusions within a solid [[dielectric]], at [[Conductor (material)|conductor]]-dielectric interfaces within solid or liquid dielectrics, or in bubbles within liquid [[dielectric]]s.  Since PDs are limited to only a portion of the insulation, the discharges only partially bridge the distance between [[electrode]]s. PD can also occur along the boundary between different insulating materials.

Partial discharges within an insulating material are usually initiated within gas-filled voids within the dielectric. Because the [[dielectric constant]] of the void is considerably less than the surrounding dielectric, the [[electric field]] across the void is significantly higher than that across an equivalent distance of dielectric. If the voltage stress across the void is increased above the [[Corona discharge|corona]] inception voltage (CIV) for the gas within the void, PD activity will start within the void.

PD can also occur along the surface of solid insulating materials if the surface tangential electric field is high enough to cause a breakdown along the insulator surface. This phenomenon commonly manifests itself on overhead line insulators, particularly on contaminated insulators during days of high humidity. Overhead lines  use air as their insulation medium.

===PD equivalent circuit===
The equivalent circuit of a dielectric incorporating a cavity can be modeled as a capacitive [[voltage divider]] in parallel with another [[capacitor]]. The upper capacitor of the divider represents the parallel combination of the capacitances in series with the void and the lower capacitor represents the capacitance of the void. The parallel capacitor represents the remaining unvoided capacitance of the sample.

===Partial discharge currents===
Whenever partial discharge is initiated, high frequency transient current pulses will appear and persist for nanoseconds to a microsecond, then disappear and reappear repeatedly as the voltage sinewave goes through the [[zero crossing]]. The PD happens near the peak voltage both positive and negative. PD pulses are easy to measure using the high frequency current transducer (HFCT) method. The [[current transducer]] is clamped around the case ground of the component being tested.  The severity of the PD is measured by measuring the burst interval between the end of a burst and the beginning of the next burst.  As the insulation breakdown worsens, the burst interval will shorten due to the breakdown happening at lower voltages. This burst interval will continue to shorten until a critical 2 millisecond point is reached. At this 2&nbsp;ms point, the discharge is very close to the zero crossing and will fail with a full blown discharge and major failure.  The HFCT method needs to be used because of the small magnitude and short duration of these PD events. The HFCT method is done while the component being tested stays energized and loaded.  It is completely non-intrusive. Another method of measuring these currents is to put a small current-measuring [[resistor]] in series with the sample and then view the generated voltage on an [[oscilloscope]] via a matched [[coaxial]] cable.

When PD, [[arcing]] or sparking occurs, electromagnetic waves propagate away from the fault site in all directions which contact the transformer tank and travel to earth (ground cable) where the HFCT is located to capture any EMI or EMP within the transformer, breaker, PT, CT, HV Cable, MCSG, LTC, LA, generator, large hv motors, etc. Detection of the high-frequency pulses will identify the existence of partial discharge, arcing or sparking. After PD or arcing is detected, the next step is to locate the fault area. Using the acoustic emission method (AE), 4 or more AE sensors are placed on the transformer shell where the AE and HFCT wavedata is collected at the same time. Bandpass filtering is used to eliminate interference from system noises.

Partial discharge degradation can be caused by various factors, including inadequate stress regulation, or the presence of voids or delamination in the ground wall insulation. These issues can eventually result in machine failure.<ref>{{cite web |title=Motor Monitoring Solutions and Sensors {{!}} Monitra |url=https://www.monitra.co.uk/motor-monitoring-solution |website=www.monitra.co.uk |language=en-gb}}</ref>