What is Partial Discharge?
Partial Discharge (PD) is a dielectric break down of electrical insulation under medium or high voltage stress. Once initiated PD causes a progressive breakdown of insulating materials which can result in catastrophic failure.
The electrical phenomenon is known as partial discharge because the electrical discharges only partially bridge the gap between two conductors.
PD often begins within cracks or gas-filled voids in solid insulation or within gaseous bubbles in liquid insulation. The gas has a much lower dielectric constant than the surrounding material which leads to a substantial increase in the electric field within the void or bubble. A significant increase results in electrical breakdown and PD events.
PD can also occur at the interface between conductor and insulation or along the surface of solid insulating materials.
PD is often detected in older equipment where insulation has deteriorated with age or with increased thermal stresses. In some cases it may also occur in newer equipment which is defective or has been installed incorrectly, such as replacement of HV cables.
Over time, reoccurring PD events cause irreversible mechanical and chemical damage to the insulation. Chemical changes at the initial PD site increase the conductivity of the previously unaffected areas of insulation, worsening the electrical stress at these regions and increasing deterioration of the insulating material.
Repetitive PD events can also instigate numerous channels or ‘electrical trees’ within solid insulating material, weakening the insulation and ultimately causing a complete failure.
Online PD Monitoring
When PD occurs high frequency transient currents are produced, persisting from nano-seconds to up to a micro-second. A high speed wideband oscilloscope can be used to capture these waveforms via Transient Earth Voltage (TEV) sensors and High Frequency Current Transformers (HFCT). Acoustic detectors can also be used in some applications.
TEV sensors are a capacitive coupler that can be magnetically attached to the earthed metal-clad equipment under test. The sensor is able to detect any high frequency discharges emanating from within the equipment. To help pinpoint the location of the discharge site Time of Flight (TOF) measurements can be made by using multiple TEV’s. From analysis of the discharge pulses it is possible to calculate which TEV is first “seeing” the discharge and hence which TEV is closest to the site of the discharge.
Split core HFCT’s are placed around either the insulated core of a cable or its earth strap to measure any discharge currents in the cable. If PD is found on a cable it is possible to inject a pulse to determine the exact location of the discharge on the cable – this is known as cable mapping. The HFCT’s can also be used to measure PD in motors and generators by placing the HFCT around the cable core of the machine.
Once recorded the data must be analysed to determine the location, type, size and frequency of the discharges. Not all PD is detrimental and in some instances the PD may exist throughout the equipment’s life without failure, however, other discharges are extremely serious. It is therefore essential that the data is correctly classified. The software used by LIVE HV is able to classify the PD based on the size and shape of the recorded pulses and determine its severity by analysing the magnitude and regularity of the discharges.
Recommendations can be made from the recorded PD data – ranging from no intervention, to monitoring the PD for an extended period of time (days to weeks) or taking immediate remedial action.