Partial Discharge Testing: Need, Testing Methodologies and Addressing Defects

Studies show that over 85% of disruptive failures in high voltage (HV) and medium voltage (MV) equipment are Partial Discharge related.

Partial Discharge (PD) activity provides evidence  of deterioration in assets, which can propagate and increase, until the insulation is no longer able to withstand the electrical stress therefore leading to flash-over and finally equipment failure. Therefore, early detection of PD is crucial to maintenance of electrical infrastructure.

SCOPE T&M is a leading player in the Indian Electrical T&M industry and we are presently on a mission to become Experts at PD Testing. In service of this mission, today we will share our knowledge on basics of Partial Discharge followed by various projects and case studies in future blog posts.

Figure 1: This Photo by Unknown Author is licensed under CC BY-SA

Understanding Partial Discharge

IEC 60270 defines Partial Discharge as “Localized electrical discharges that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor”.

Typically, PD is a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. Generally, such discharges appear as pulses having a duration much less than 1µs. This is therefore not a permanent phenomenon – PD can occur in regular interval or it may not.

Energy emitted by Partial Discharge is distributed in the surrounding area in various forms such as electromagnetic signal, acoustic and light signal (corona/spark), heat, or gases such as Nitrous oxide, Ozone, Carbon Monoxide, Carbon Dioxide, etc. This emitted energy is measured by PD Testing kits to understand severity of PD at a given location.

Types of Partial Discharge

  • Floating Electrode / Floating Potential: Occurs when there is an ungrounded conductor within the electric field between conductor and ground (or between phases). This is the most common type of PD. Generally: UHF Antenna, TEV Sensor and HFCT are useful for detection of this type of PD.
  • Particle Discharge / Floating Particle: Conductive particles can contaminate fluid dielectric, such as oil or SF6 gas. Particle PD is most common in SF6 gas compartments. UHF sensors are preferable for this type of PD. Therefore, analysis is done in an ultrasonic pulse timing mode.
  • Void Discharge or Internal OD : Generally occurring in solid dielectrics, it is typically a manufacturing defect caused by non-detection of tiny micro voids during manufacturing and subsequent testing. These voids can be detected by ultrasonic contact method, HFCT or, UHF Antenna.
  • Corona Discharge: While there is diversity of thought on whether corona is a type of PD, it is an electrical phenomenon which cannot be avoided. Furthermore, corona can evolve into surface PD owing to increased severity and breakdown of strength of insulation in high humidity environments or due to poor maintenance.
  • Surface Tracking: Usually caused by dirty, contaminated, or aged insulator surfaces. Easy to detect with UHF Antenna, Airborne ultrasonic probe, TEV sensor or, HFCT

PD Testing Technologies by Asset Type

The table below describes the test method used for a particular asset types in the power system:

Power TransformerYesYesYesYes
Instrument TransformerYesYesYesYes

PD Analyzer: Portable PD Diagnostic System

SCOPE marketed PD Analyzer is a state-of-the-art ‘all in one’ solution for testing and analysing partial discharges in the insulation of HV transformers, cables, GIS and electric machines. The PD Analyzer helps utilities diagnose their insulation state and find any type of defects very effectively.

The PD Analyser is most useful for condition estimation of high-voltage insulation. This multipurpose device is designed for:

  • Partial discharge measuring in high-voltage insulation at a high noise level
  • Fast detecting of the defects in different high-voltage equipment and identifying how dangerous/severe they are.


For information on SCOPE marketed Partial Discharge Test Kits, please visit or write to us as

For more detailed training on Partial Discharge, please register for our knowledge sharing webinar “Substation Saturdays” at

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