Partial discharge is an electrical discharge that does not bridge the space between two conducting electrodes, typically as a result of a localised breakdown in cable insulation in high-voltage systems. Due to the discharge not spanning the entirety of the insulation, thus not being electrode to electrode, the discharge is referred to as ‘partial’ in nature.

Partial discharge can be classified according to three primary types: internal partial discharge, occurring inside insulation; surface partial discharge, tracking across insulation; and corona partial discharge, from a sharp electrode into gas. Each kind of partial discharge is likely to manifest itself differently, however the effects are always harmful if left unchecked, degrading insulation’s protection over time. In most cases, insulation will eventually become unable to withstand the high voltage load, resulting in failure and the possibility of dangerous arc flashes. It has been observed that up to 80% of sub-station failures can be traced back to partial discharge, meaning that regular and diligent inspections are required to reduce instances of this common issue.

Places where partial discharge can occur include:

  • MV & HV cables, terminations and underground vaults
  • Indoor metal clad switchgear cubicles
  • Indoor and outdoor insulators
  • Transformers
  • Transformer cable boxes

Partial discharge emits energy in a variety of forms including electromagnetic, gas and acoustic, allowing several ways to detect it when present. While one common approach is through the use of a Transient Earth Voltage (TEV) detector, which is able to identify instances of surface and internal discharge caused by the breakdown of insulation material, another more popular method is with the use of acoustic imaging cameras. Due to the aforementioned emission of audible and ultrasonic sound, acoustic imaging cameras allow the user to locate and visualise the source of the sounds emitted during partial discharge using an array of precise microphones. These sensitive devices are then able to translate the sounds into visual format and display the information on screen for the user.

The ability to visualise sound on screen makes this approach one the easiest ways to identify instances of partial discharge, as large areas can be scanned from a distance. Even in loud environments, most acoustic imaging cameras are capable of filtering out other sounds, focusing on detecting those associated with just partial discharge.

One such device that can be used to detect partial discharge is the FLIR Si124. With 124 integrated microphones, this product is available in 3 versions, 2 of which are capable of detecting partial discharge from a distance of up to 130m away. The Si124 can also classify partial discharge according to type and allows for export to external devices for analysis. You can read more about the FLIR Si124 here.