Partial Discharge (PD) is an electrical phenomenon which causes insulation to deteriorate and frequently is the reason for breakdown of an insulation system resulting in failure of the equipment.

Partial Discharge can be described as an electrical pulse or discharge in a gas-filled void or on a dielectric surface of a solid or liquid insulation system. This pulse or discharge would partially bridge phase to ground insulation or phase to phase insulation in an electrical apparatus, thus causing the electrical equipment to fail.

Partial Discharges can occur for a number of different reasons. For example, PD can occur when high voltage structures have sharp projections; internal discharge can occur in voids and contact noise can occur if the ground connection to a bushing is poor.

Partial discharge in high voltage metal-clad switchgear can result in catastrophic failure of the individual plant and in some cases, of the substation (HV) plant. Conventional Partial Discharge Equipment is very expensive and difficult to use, and requires the equipment under test to be de-energised.

Through the use of modern techniques, it is now possible to detect and locate Partial Discharge with the equipment ON LINE, without the need to make direct connections to high voltage terminations, and hence, no necessity to shut down essential equipment.

By utilizing the Partial Discharge Locator Model PDL1 and complemented by the Ultraprobe UE2000 a comprehensive partial discharge testing service can be achieved.

The Partial Discharge Locator, PDL1 makes use of the Transient Earth Voltage (TEV) measurement technique, operating within a bandwidth of 15 to 60MHz, to detect and locate the PD source.

In the case of the Ultraprobe UE2000, it operates from 20KHz to 100KHz. Both systems are non-invasive diagnostic techniques which have proven their worth in a lot of Electric Utilities like the CEGB of UK, PUB of Singapore, New Zealand Supply Authorities, etc.

The advantage of both these methods is that it is unnecessary to shutdown and de-energize the equipment under test.

The two methods of measurement can be described as follows :-

• Partial Discharge Locator PDL1
  How Transient Earth Voltage Measurement Works
  If a Partial Discharge occurs in the phase to earth insulation of an item of high voltage plant such as a metal-clad switchboard or a cable termination, a small quantity of electrical charge is transferred capacitively from the high voltage conductor system to the earthed metal-cladding. The quantity of charge transferred is very small and is normally measured in pico-coulombs. The transfer occurs typically, in a few nanoseconds.
  
  When the partial discharge occurs, electromagnetic waves propagate away from the discharge site. Due to the skin effect, the transient voltages on the inside of the metalwork cannot be directly detected outside the switchgear. However, at an opening in the metal cladding such as the gasketted joints, the electromagnetic wave can propagate out into free space. The wave then generates a transient earth voltage on the metal surface. Hence, the technique is called TEV for Transient Earth Voltage.
  
  
  

• Ultrasound UE2000
  How Ultrasound Electrical Detection Works
  Arcing, tracking and corona all produce some form of ionization which disturbs the air molecules around it. The Ultraprobe 2000 (Ultrasonic equipment) detects the high frequency noise produced by this effect and translates it, via heterodyning, down into the audible ranges. The specific sound quality of each type of emission is heard in headphones while the intensity of the signal is observed on a meter. Normally, electrical equipment should be silent; although some may produce a constant 50-cycle hum or some steady mechanical noises. These should not be confused with the erratic, sizzling, frying, uneven and popping sounds of an electrical discharge.
  
  Besides the subjective mode of detection by sound recognition, the Spectra Plus software (Fast Fourier Transform software) is utilized to make the test more quantitive and the results can also be used as a baseline for similar or future reference.
  
  
  
• Possible Defects which may be detected from Partial Discharge Testing
  

  Some examples of causes of Partial Discharge resulting in failure are :-

  • Incorrect usage & installation of cable termination kits

    e.g.	-	Poor cutbacks on cable terminations & splices
    	-	No stress control tubes or insufficient length installed
    	-	Insufficient clearance between cores in a cross-core configuration

  • Air voids created by inconsistent heat shrinkage of sleevings

  • Air voids within cable insulation caused by manufacturing defects

  • Sharp edges, or nicks left on conductors

  • Insufficient clearance between conductors and surrounding insulation

  • No angle boots installed to cover bare high voltage conductors in VT boxes  

  • Strong odor of Ozone, presence of whitish to bluish green powder due to formation of nitrous acid from ionization

  • Colour fading and dark tracks on insulation near conductors, usually with a "treeing" pattern

  Records by Power Utilities indicate that the main power outages in the electrical grid are caused by defective cable terminations and joints..
  
  
  

• Remedial Action
  Upon detection of possible defect, the equipment must be shutdown and the potential fault area has to be carefully inspected visually to assess the seriousness of the problem. In most instances the affected components need to be replaced.