Partial Discharge is "A localized electrical discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor" ---- quoted from IEC60270.
When the Voltage stress exceeds the breakdown strength of that portion of the insulating material, a Partial Discharge begins and continues to deteriorate that insulation.
When Partial discharges occur, various physical and chemical changes may happen, which produce emissions that we can detect, localize, and characterize to provide the information needed to prevent insulation failures of Medium and High Voltage electrical equipment.
PD occurs in Solid, Liquid or Gaseous insulating mediums. PD can also occur in the form of Corona, Surface Tracking, or floating electrodes (metal to metal discharge) causing degradation of the insulation. Once PD begins it will always get worse. 80% of electrical fires are caused by insulation breakdown or failure and PD is the very first indication that insulation is breaking down. NFPA 70B states that insulation breakdown is the number one cause of electrical failures.
Partial Discharge Mechanisms
Corona - discharge to air
Floating Electrode - metal to metal
Particle discharge - conductive particles contaminate insulation medium
Voids - gaps in solid insulation or gas bubbles in oil
Surface discharge - tracking over outside of insulators/bushings
Bi-products of PD Activity
Light - sometimes visible
Heat - infrared
Gasses - nitrous oxide, ozone other gasses with odor
Acoustic - audible and inaudible ultrasonic
Electromagnetic - transmitted UHF or induced TEV and HFCT
Chemical - Verdigris -nitrous oxide +moisture=nitric acid
Acoustic Emissions (AE)
a. Airborne - ultrasonic microphone 40kHz center frequency
b. Contact Probe - broad range 20kHz -300Khz (PD in oil or SF6 gas)
a. UHF Ultra-High Frequency 300MHz-1.5GHz
b. TEV Transient Earth Voltage 3MHz-100MHz
c. HFCT High Frequency Current Transformer 500kHz-50MHz
1. Void Discharge:
Manufacturer’s defects in solid insulation. Found in Cables, Bushings, GIS Junction insulation. Highly destructive to insulation. Voids typically continue to grow until failure. If a void PD is discovered, the insulator should be replaced.
Sensors: UHF, HFCT, AE Contact Probe, TEV
2. Corona Discharge:
Corona is a discharge to air from the sharp surface of a conductor. Corona is typically not a problem besides the sound and the radio frequency emission. Fortunately the corona signal has a many characterists that are much different from other forms of PD. Corona disturbances do not interefere with other PD measurements.
Sensors: AE Ultrasonic HFCT
3. Surface Discharge:
Discharge along the surface of insulation can be very destructive. Also known as "surface tracking". Usually cause by contamination or weathering of insulator surface. It is different from corona because it tends to track to grounded metal. Corona discharges to air. Corona conditions can evolve into surface PD as they become more severe. This can happen on any MV and HV equipment. Happens when strength of insulation breaks down in high humidity environments. or poor maintenance of equipment can lead to this phenomenon. Moistures Intrusion is also common cause of surface PD.
Sensors: AE Ultrasonic and Contact Probe, UHF, TEV (low magnitude)
4. Floating Discharge:
Occurs when exposed load carrying conductor is exposed to another conductive surface of different potential not connected to said conductor. Types: Metal to Insulation OR Metal to Metal. Caused by manf. defect, non-grounded price of metal within the field. Floating Discharge is the most common type of PD. Often caused by human interaction. IE conductors not positioned properly. Foreign or loose object inside of insulation.
Sensors: UHF, AE Ultrasonic and Contact Probe, TEV, HFCT
5. Particle Discharge:
Occurs in GIS (SF6 gas) and oil insulated transformers. Caused when conductive particles are left inside system. Allows PD to jump from particle to particle. Purifying and processing the oil or SF6 is recommended.
Sensors: UHF, AE Contact Probe, HFCT