What is Partial Discharge?

Partial discharge (PD) refers to a localised dielectric breakdown in an insulating material under high voltage stress that does not completely bridge the space between two conductors. PD is not a full flashover but indicates a weak spot, which if left unaddressed, can progressively deteriorate the insulation and ultimately lead to a catastrophic failure of the equipment. Read more here.

Historical Background and Development

The study of partial discharge dates back to the early 20th century, but it was in the mid-1900s that researchers began to formally quantify and study PD in HV systems. The development of standards like IEC 60270, and countless case studies have made PD testing a routine and critical aspect of electrical maintenance.

How PD Arises

Defects introduced during manufacture or installation, environmental contamination and moisture ingress, and mechanical/thermal cycling resulting in aging all reduce dielectric strength over time. Overvoltage events and switching duty can accelerate the problem.

PD mechanisms typically fall into three types:

• Internal PD - occurs within voids or inclusions and is often considered the most serious due to its concealed nature.
• Surface PD - develops along insulation surfaces, especially where moisture or contamination provides a tracking path.
• Corona - forms in gas/air under high field stress; it’s less destructive but still signals design or ageing issues. Corona is sometimes considered a form of noise which can mask other PD types during testing/monitoring.

Understanding which mechanism is present guides the urgency and type of remediation.

How Monitra Detects and Interprets PD

PD events generate distinct electrical, electromagnetic, and acoustic signatures. The selection of a specific sensor is contingent upon the characteristics of the asset under inspection, the method of access, and the ambient noise environment. To ensure robust analysis, data acquisition is synchronised, allowing for the confident comparison and trending of results. In addition, Phase Resolved Partial Discharge (PRPD) allows PD type to be identified, facilitating the understanding of the required remedial works.

• HV Coupling Capacitors (HVCCs) are used both on-line and off-line set-ups. Testing is conducted as per the IEC 60270 for the likes of factory/commissioning contexts. They provide calibrated, phase-resolved data (PRPD) and pulse metrics suitable for acceptance to specification or baseline characterisation, in addition to trending and comparisons.
• High-Frequency Current Transformers (HFCTs) measure PD currents on cable screens/earths and switchgear earthing conductors, typically during on-line testing/monitoring. We also deploy high current design HFCTs on rotating machines (motors and generators) to detect stator insulation activity under standard running conditions.
• Transient Earth Voltage (TEV) sensors are applied on metal-clad switchgear to measure internal activity, sometimes referred to as ‘Local PD’ from electromagnetic signals. Synchronous acquisition Time-of-flight measurements from multiple TEVs can provide localisation on switchgear lineups.
• Ultrasonic / Acoustic Emission (airborne and contact) is used to detect surface activity on terminations, bushings and within switchgear compartments, often via direct line of sight, and for contact AE on oil-filled transformers or gas-insulated switchgear (GIS).
• UHF sensors are the preferred method for GIS with pre-installed internal sensors being utilised, or retrofit types attached to insulation barriers and/or inspection windows.
• Bushing tap sensors detect PD signals from the transformer windings and bushings, through the capacitance of the bushing.

Acquisition is time-synchronised across channels. Analysis combines PRPD patterns, pulse polarity and width, spectral content and repetition statistics with operating context (load, temperature, humidity, etc). Machine learning automatic classification assists with noise rejection and mechanism tagging; our engineers review borderline cases, confirm PD, provide localisation and recommendations for future works from repeat testing to trend and compare, to directing remedial works based off PD type and similar experience. Thresholds and severity bands are asset-specific and referenced to baseline measurements, noise floor and applicable standards or guidelines.