Video Tutorial Masterclass - PD Partial Discharge & Electrical Cable Breakdown

By Chris Dodds on 2nd February, 2016

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 Video Tutorial Masterclass - PD Partial Discharge & Electrical Cable Breakdown

What’s a Partial Discharge?

A Partial Discharge (PD) is an electrical discharge or spark that bridges a small portion of the insulation between two conducting electrodes.

PD can occur at any point in the insulation system, where the electric field strength exceeds the breakdown strength of that portion of the insulating material.

PD can occur in voids within solid insulation, across the surface of insulating material due to contaminants or irregularities, within gas bubbles in liquid insulation or around an electrode in gas (corona activity).

On-Line Partial Discharge Testing 27.6 kV XLPE Cable (By Kinetrics)

On-Line Partial Discharge Testing 27.6 kV XLPE Cable (by Kinetrics)

Learn more from EA Technology's excellent 13 part video tutorial course.

Watch T&D YouTube :

  1. Introduction
  2. What is Partial Discharge?
  3. Causes of Partial Discharge
  4. Effects of Partial Discharge
  5. Partial Discharge Fault Detection
  6. Partial Discharge Activity Measurement
  7. Partial Discharge Activity Investigation
  8. Partial Discharge Activity Monitoring
  9. Partial Discharge Activity Alarms
  10. Partial Discharge Activity Data
  11. Partial Discharge Activity Support
  12. Partial Discharge Services
  13. Partial Discharge Demonstration 

 

Partial Discharge (PD) Measurement Instruments for High Voltage Cables

SEBA KMT oscillating wave test system (OWTS) is used to identify, evaluate and locate partial discharge (PD) faults in HV high voltage cable insulation, cable joints and cable terminations.

The HV Partial Discharge unit generates the attenuated oscillating frequency up to the peak voltage of the system. An embedded controller for the partial discharge (PD) operation and the digital signal processing and storage from two 100 MHz AD converter systems is integrated in the cylindrical housing. The communication to the user interface of the notebook is done by W-LAN. Optional optical data link is possible. The OWTS partial discharge instrument can also be coupled with a VLF System.

 

Pfisterer

 


Introduction to Partial Discharge (PD)
Even in times of deregulation, the underground cable networks of the power utility and DNO's are one of the most important assets. This fact is often not considered the way it should be in the planning of investment budgets i.e. incident-oriented HV cable network maintenance leads step-by-step to exhaustion of the available safety reserves. Since the 1980's, a large number of XLPE first generation high voltage cables have shown "water-treeing" and have been exchanged or renovated for the most part. It is possible to determine the condition of the power cables with the known dielectric diagnosis methods. Much older paper oil filled cables are inconspicuous regarding their operating performance.

Pic : "Water-Treeing" cable with a forest of water trees, which can be seen without dissecting the insulation. Water trees require both moisture and an electric field in the insulation to devleop. If the cable is an older design made and installed before or about 1980, and has extensive water tree growth, thet the root cause of cable failure is simply that the cable has reached its normal end-of- life.

Water Treeing Cables

Pic : Partial Discharge Mapping - On-line Partial Discharge Mapping was carried out here for Dunedin Electricity Ltd. Prior to re-configuring a 47 year old paper-lead standby sub transmission circuit, confirmation was required that it could handle increased loading. By carrying out an on-line survey, PD Engineers were able to determine that no PD was present in this circuit, which meant an old asset could continue to be utilised and handle the load. Result :  Deferment of $1.1million worth of planned capital works.

Partial Discharge Mapping


The recording, location and evalulation of partial discharge (PD) inside the cable insulation (including within high voltage cable joints and terminations) offers the possibility of an early diagnosis of cable network failure. In order to be able to carry out an evalutation of the risk factor of partial discharge defects, the applied voltage for a partial discharge (PD) diagnosis should be within the range of the operating frequency, because the typical PD parameters, such as inception and extinction voltage, partial discharge level and partial discharge pattern then correspond to the relevant values under operating conditions.

 

Partial Discharge Faults in High Voltage Cable Systems 
Partial discharge is understood to be the partial electrical breakdown of an insulation system, i.e. only a limited section of the total cable insulation section is bypassed. Inside cables, partial discharge defects are generally ionisable, gas-filled voids, which either developed already during the production of the cable insulation, were caused by mechanical damage or are present inside the high voltage cable joints or terminations. In addition, thermal degradation processes inside cable joints with improperly performed cable jointing workmanship can also lead to partial discharge (PD) inception.

Pic : Electrical Cable Breakdown Caused By Partial Discharge By Jim Cahill (Medium and High Voltage Electrical Equipment Testing and Monitoring)

Partial Discharge - Electrical Cable Breakdown

However, partial discharges can also occur when the electrical strength of the respective insulation material is locally surpassed by the stressing field strength - e.g. by large water trees or uneven semi-conducting layers on the inner conductor of a PE or XLPE cable.

Inside paper insulated cables (PILC), joints and terminations a completely different partial discharge behaviour is observed. In the laminated and impregnated paper insulation, partial discharge can occur in dry areas, but may disappear again due to the mass migration in the case of thermal load changes. In the same way, carbonisations have the effect of conductive bridges due to the reacting partial discharge so that the potential differences are "short-circuited" and the PD discontinued.

Due to the barrier effect of the laminated paper cable insulation, the propogation of partial discharge channels is largely impeded so that PILC cables with PD in the insulation may be safe for operation even for many years - similar partial discharge behaviour can be found on oil-filled cable joints. The operation of high voltage drained cable terminations, however is directly threatened, because self-healing by a continuous flow of mass or cable oil cannot occur.

From a multitude of partial discharge (PD) measurements on high voltage PE, XLPE and PILC cables, there is knowledge and experience for the evaluation of the threat to the respective insulation systems by partial discharge.

 

Further Reading:

 

Invitation

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Category:  HV Cable Jointing & Terminating

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