WHAT IS A PARTIAL DISCHARGE?
A partial discharge (PD) is, as the name implies, a partial failure of an insulator. This means that electrical charges will move across the insulator either sporadically or, more often, regularly. If the failure progresses, it may lead to a complete insulator break- down. The presence of partial discharges can therefore be used to predict catastrophic system failures and preemptively fix these
problems through planned maintenance actions.
Partial discharges may occur across any type of insulator: solid, air, gas, vacuum, or liquid. There are several different types of partial discharges, depending on the type of insulator as well as other factors.
When the sound source in focus is a possible partial discharge and show a so-called phase-resolved partial discharge (PRPD) pattern calculated from that sound signal.
You can press the PRPD to toggle between a small view and a large view of the pattern. For on-device AI-assisted analysis and severity assessment of the detected PD, take a snapshot of it, set the distance, voltage, and component where the PD was detected and click “PD analysis”. A classification of the PD type will be shown, together with the estimated severity of the PD, as well as a description and a recommendation of the detected issue for immediate, in-the-field decision support.
The Si2 from FLIR we use features AI-assisted on-device analysis and severity assessment for in-the-field decision support.
SURFACE DISCHARGE
Partial discharges across an insulator surface may begin to occur due to contamination or damage to or defects in the insulator surface. So-called dry-band arcing may also take place when different regions of the insulator are wet while others are dry. Surface discharges will rapidly cause especially organic insulator materials to deteriorate, and may easily lead to flashover across the whole insulator [1]. Any sign of surface discharges should therefore be thoroughly inspected and the need for action assessed. The presence of surface discharges depends highly on environmental conditions, such as humidity and temperature [2], and this should be taken into account when planning for inspections. The term tracking may also be used when talking about surface discharges.
TYPICAL LOCATIONS • All different types of insulators and bushings • Cable terminations and joints
PROBLEMS AND RISKS • May lead to insulator breakdown and outages
INSPECTION SCHEDULE • Regular inspections, for example once every year • Shorter intervals once potentially severe partial discharges have been found
SUGGESTED ACTIONS • Risk assessment of any detected partial discharges • Has the partial discharge progressed over time? • What are the risks associated with an insulator breakdown? • Cleaning or replacement of critical components • Quick replacement of faulty cable terminations and joints PRPD PATTERN Discharges on the surface or inside components have PRPD patterns characterized by two clusters of somewhat symmetric shape and size. Surface discharges may, however, have a moderate amplitude difference between the two clusters. The clusters often have a triangular or “hill-like” shape. Example of the PRPD pattern of a surface discharge. Example of the PRPD pattern of a surface discharge.
FLOATING DISCHARGE
Floating discharges take place between two components or parts of a medium or high-voltage system. Floating discharges have a highly symmetrical PRPD pattern, with an equal number of discrete discharges during each half cycle of the voltage. A floating discharge takes place between a conductor and a metal object at a floating potential. Discharges take place when the potential difference between the two objects grows large enough to induce sparking. Floating discharges may be a result of bad contact between different components, for example, due to oxidized or contaminated contact surfaces [1]. In some cases, floating discharges are harmless, but they may also be a sign of faults in design or installation, as well as damaged components. Depending on the location of floating discharges, they may lead to more severe problems over time. When found, this type of discharge should always be inspected more closely.
TYPICAL LOCATIONS • Clamps of busbar support insulators • Ungrounded or poorly grounded components
PROBLEMS AND RISKS • May indicate damaged components or faulty design or installation
INSPECTION SCHEDULE • Inspection when commissioning new substations, power lines, and equipment • Regular inspections, for example once every year.
SUGGESTED ACTIONS • Risk assessment of any detected partial discharges • Is the partial discharge due to bad contact between components? Is this a problem in this specific case? • Does the partial discharge indicate design or installation faults or damaged components? • Repair or replacement of faulty designs and installations as well as damaged components
CORONA DISCHARGE
Corona discharges typically produce highly asymmetrical PRPD patterns. A weak corona has discharges only during the negative half cycle of the voltage. A stronger corona may cause discharges also during the positive half cycle. These discharges have a larger amplitude than during the negative half cycle, but the cluster is often not as wide. Corona is a partial discharge into the air from a sharp point at high potential. Corona discharges take place when the strength of the electric field is large enough to ionize the air. Typically, the corona is observed during the negative half cycle of the voltage (so-called negative corona). If the electric field strength is high enough, the corona can be observed also during the positive half cycle (so-called positive corona). A positive corona contains fewer discharges per half-cycle than a negative corona, but the amplitude is larger . In some cases, corona may also be observed at overstressed points at ground potential . In most cases, corona is harmless. Corona does, however, cause power loss, electromagnetic interference, and audible noise, which might be a problem in some cases. Often the most severe problem associated with corona is that it produces ozone and corrosive chemical compounds that damage nearby organic materials, such as polymeric insulators .
TYPICAL LOCATIONS • Sharp points, edges, and corners of conductors • Arcing horns • Broken strands on power lines
PROBLEMS AND RISKS • Power loss • Electromagnetic interference • Audible noise • Deteriorates nearby polymeric insulating materials
INSPECTION SCHEDULE • Inspection when commissioning new substations, power lines, and equipment • Regular inspections, for example once every two years
SUGGESTED ACTIONS • Risk assessment of any detected partial discharges • Is electromagnetic interference or audible noise a problem? • Should broken strands be repaired? • Are there insulators nearby that might be damaged by the corona? • Add missing corona rings near polymeric insulators • Repair power lines with broken strand.
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