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About Partial Discharge

PD Training Course




PD Activity 1 PD Activity 2 PD Activity 3 PD Activity 4
Click image to enlarge
PD Activity 1
PD Activity 2
PD Activity 3
PD Activity 4


Why Measure PD (Partial Discharge) Activity?

EA Technology are experts on the causes and effects of Partial Discharge activity – and on techniques for detecting, locating, measuring, recording and analysing it, to make electrical plant more efficient, reliable and safer.

Partial Discharge Definition

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

Partial Discharges emit energy as:

  • Electromagnetic emissions, in the form of radio waves, light and heat
  • Acoustic emissions, in the audible and ultrasonic ranges
  • Ozone and nitrous oxide gases

Partial Discharge Causes

PD activity 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 causes include:

  • Voids within solid insulation
  • Contamination by particles on the surface of insulating material
  • Irregularities (e.g. sharp points) on the surface of insulating material
  • Gas bubbles in liquid insulation
  • Floating particles in gas insulation
  • Discharges around an electrode in gas (corona activity)
  • Mechanical failure or damage to insulation materials

PD activity is also affected by environmental factors, including temperature, atmospheric pressure and humidity.

Partial Discharge Effects


PD Effects 1 PD Effects 3 PD Effects 4 PD Effects 5 PD Effects 6
Click image to enlarge
PD Effects 1
PD Effects 3
PD Effects 4
PD Effects 5
PD Effects 6

Partial Discharge activity provides clear evidence that an asset is deteriorating in a way that is likely to lead to failure.

Once Partial Discharge activity is present, even if it is intermittent, it will ALWAYS increase. For example, Partial Discharge activity in voids in insulation causes erosion, which leads to increased Partial Discharge activity.

The process of deterioration can propagate and develop, until the insulation is unable to withstand the electrical stress, leading to flashover. Where protection systems are in place, this will cause them to trip out, causing outages.

Partial Discharge Health & Safety Issues

Where protection systems are not fitted or are ineffective, the failure of HV/MV assets is often sudden and catastrophic, producing major damage, injury or death. Sudden failures of live assets often release large amounts of energy, leading to explosions and fires.

Undetected Partial Discharge activity can also be hazardous to personnel, who are exposed to the danger of electrocution by assets which are unexpectedly energised.

Partial Discharge Fault Detection

Detection of PD-related faults is based on the principle of determining whether PD activity it is at a critical level.

Low levels of Partial Discharge activity are often present in assets at levels which may be considered acceptable i.e. non-critical, or unlikely to develop into failures in the short to medium term.

Faults are detected by testing to establish whether PD activity exceeds critical thresholds:

Threshold 1

PD activity is not present or at sub-critical levels. Results provide assurance that PD-related faults are unlikely to be developing at present.

Threshold 2

PD activity is at a level which indicates that a fault is developing and requires further investigation or monitoring.

Threshold 3

Partial Discharge activity is at a level which indicates that a fault is at an advanced stage of development. The fault is likely to cause imminent failure – and the asset may be energised to an extent that is a danger to personnel. This level of PD activity indicates the need for immediate investigation and intervention.

Partial Discharge Fault Prediction & Prevention

The prediction and prevention of faults is based on the principle that, once present, PD activity inevitably increases.

Measuring and plotting changes in PD activity make it possible to predict with a high degree of accuracy when a fault is likely to develop into a failure. Managers can therefore take appropriate and timely action to rectify faults, as they develop and before they lead to outages.

The ability to predict the way in which PD related faults develop is greatly enhanced by knowledge of how specific assets behave, once the process of deterioration has begun. For example:

To obtain the most accurate predictions of failure requires:

  • Specialised PD measurement instruments
  • The knowledge to interpret readings
  • An extensive database containing details of how specific assets deteriorate over time, in particular applications and environments

Partial Discharge Condition Data Gathering

Accurate measurement of PD activity is the key to understanding the true condition of assets.

PD measurement is a useful technique for assessing the condition of individual assets, but it is of far greater value when applied to groups of assets – for example a substation or an entire network.

Condition data based on PD measurement may be applied in several ways:

1: First Pass Partial Discharge Survey

Basic PD detection and measurement instruments are used to indicate the presence and severity of PD activity in multiple assets. This is often a valuable basis for more intensive surveys and data gathering.

2: Detailed Partial Discharge Survey

More sophisticated instruments are employed to locate precisely, measure and record PD activity in multiple assets. The data collected from such surveys are interpreted by experts to provide valuable management reports.

This is the foundation of true condition based asset management, providing the information needed to develop registers of assets, including:
  • Current condition and ‘health indices’ for each asset
  • Timeline predictions of likely failure
  • The ability to prioritise and schedule maintenance and replacement

3: Ongoing Partial Discharge Monitoring

PD activity inevitable changes over time and as a result of changes in environmental conditions, including temperature and humidity.

PD monitoring systems are employed, either temporarily or permanently, to provide the most detailed assessments of the condition of assets – particularly those where there are histories of problems, or where failure would have unacceptable consequences. Again, expert interpretation of the data gathered provides the most valuable management information.

4: Partial Discharge Alarm Systems

New generations of PD monitors have been developed to act as permanent alarm systems, especially for mission critical assets such as strategically important substations. These continuously check for critical PD activity, providing instant alerts when faults begin to develop, before they lead to failures.


For more details please contact: 0800 028 7243 or email sales@eatechnology.com

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