Transient Earth Voltage (TEV) Partial Discharge Detection: Principles, Methods, and Applications

Transient Earth Voltage (TEV) partial discharge detection is a non-invasive, on-line diagnostic technique used to identify insulation defects in energized medium- and high-voltage electrical equipment. By measuring electromagnetic signals induced on grounded metal enclosures by internal partial discharge activity, TEV testing provides early warning of developing insulation faults in switchgear, cable terminations, and transformers without any service interruption. This article explains the underlying principles, applicable standards, testing procedures, result interpretation, and typical applications of TEV partial discharge detection.

Technical Overview

Partial discharge (PD) is a localized dielectric breakdown that occurs within or on the surface of electrical insulation without completely bridging the conductors. PD activity is widely recognized as both a symptom and an accelerating cause of insulation degradation, and it is one of the most reliable early indicators of impending failure in medium-voltage (MV) and high-voltage (HV) equipment.

Transient Earth Voltage (TEV) detection is an unconventional PD measurement method based on the electromagnetic energy radiated by internal discharge events. When a partial discharge occurs inside a metal-enclosed apparatus such as MV switchgear, the discharge current generates an electromagnetic wave with frequency content typically in the range of 3 MHz to 100 MHz. This energy couples capacitively to the grounded metal enclosure, and transient voltage pulses—with nanosecond-scale rise times—leak to the outer surface of the enclosure at discontinuities such as panel seams, gaskets, cable entry glands, and bolted joints. These pulses are known as transient earth voltages.

A TEV detector uses a capacitive coupling sensor pressed against the exterior surface of the enclosure to capture these voltage pulses. The instrument quantifies the discharge activity by measuring the amplitude (usually expressed in decibels referenced to one millivolt, dBmV) and the pulse count per power frequency cycle. Compared with conventional PD measurement performed according to IEC 60270, TEV detection offers the significant advantage of being completely non-intrusive: measurements can be carried out quickly and safely on live equipment, making it ideal for routine condition screening and trend monitoring.

TEV detection is generally supported by the following reference frameworks and standards organizations:

• IEC 60270 – High-voltage test techniques: Partial discharge measurements (conventional reference method)
• IEC TS 62478 – Measurement of partial discharges by electromagnetic and acoustic methods (non-conventional techniques)
• IEEE 1434 – IEEE Trial-Use Guide to the Measurement of Partial Discharges in Rotating Machinery
• GB/T 7354 and DL/T 417 – Chinese national and power-industry standards covering on-site partial discharge measurement of power equipment

Testing Samples

Medium-Voltage Metal-Clad Switchgear and Ring Main Units

Metal-clad switchgear, ring main units (RMUs), and compact secondary substations are the primary application for TEV detection. Internal defects such as insulation surface contamination, moisture in cable boxes, defective stress cones, and loose or floating metallic components generate strong TEV signals that can be detected on the panel surface.

Gas-Insulated Switchgear (GIS) and Bus Ducts

TEV measurement provides a rapid screening method for GIS compartments and bus duct enclosures, where free metallic particles, floating shield potentials, and spacer surface defects produce measurable electromagnetic emissions.

Cable Terminations and Joints

Termination boxes and joints located within switchgear panels can be screened for poor workmanship, water ingress, and insulation aging, which are among the most common causes of MV cable system failures.

Distribution Transformers and Reactors

PD originating inside oil-filled or dry-type transformer tanks induces TEV signals on the tank walls, allowing a preliminary assessment of winding, bushing, and insulation condition without de-energizing the unit.

Other Enclosed Apparatus

Instrument transformers, vacuum and SF6 circuit breakers, surge arresters, busbar supports, and insulating spacers installed within grounded enclosures can also be evaluated using TEV techniques.

Testing Items

A standard TEV partial discharge detection campaign typically records and evaluates the following parameters:

• TEV amplitude in dBmV, measured across the sensor bandwidth (typically 3–100 MHz)
• Pulse count or discharge repetition rate, usually expressed as pulses per power frequency cycle
• Background noise level measured at a reference location away from energized equipment
• Relative severity classification of each measurement point (low, moderate, high, or severe)
• Phase-resolved TEV patterns, where supported by the instrument, to distinguish PD from noise sources
• Inter-panel comparison values used to localize the defective compartment
• Time-difference-of-arrival data from dual-sensor measurements for defect localization
• Historical trend data for periodic or continuous monitoring comparisons

Testing Methods

Instrumentation and Sensors

TEV instruments consist of a handheld or permanently installed detector coupled to a capacitive sensor. The sensor is held firmly against a bare metal point on the enclosure surface, and the instrument samples the incoming pulses, filters out frequencies outside the detection band, and computes amplitude and pulse-count statistics over a defined integration window (commonly two seconds, corresponding to 100 or 120 power cycles).

On-Site Testing Procedure

Testing is performed on energized equipment under normal service conditions. A typical procedure includes the following steps:

• Verify safety clearances and apply appropriate personal protective equipment and live-work procedures
• Measure and record the environmental and background TEV level to establish the noise floor
• Select measurement points at panel seams, door edges, and positions adjacent to bushings, cable boxes, and busbar sections
• Record TEV amplitude and pulse count at each point, repeating measurements to confirm reproducibility
• Compare readings between adjacent panels and against previous survey results
• Where elevated readings are found, perform dual-sensor time-of-flight measurements to localize the discharge source
• Supplement with ultrasonic, UHF, or high-frequency current transformer (HFCT) measurements where confirmation is required

Severity Classification and Interpretation

Interpretation combines absolute amplitude levels with relative comparisons. Although exact thresholds depend on instrument calibration and the applicable guidance, commonly used screening criteria are as follows:

• Readings close to the background noise level: insulation condition generally considered acceptable
• Moderately elevated amplitude with a low pulse count: monitoring recommended, with shortened re-test intervals
• High amplitude combined with a high pulse count: detailed diagnosis required, potentially including de-energized conventional PD testing per IEC 60270
• Readings significantly above all neighboring panels: localized source suspected in that compartment

Because TEV signal magnitude depends on discharge type, propagation path, and enclosure construction, trend analysis over time and comparison between identical equipment are often more diagnostic than a single absolute reading.

Complementary and Confirmatory Techniques

TEV detection is most powerful when combined with other condition assessment methods, including ultrasonic (acoustic emission) detection for surface discharge and corona, UHF detection for GIS, HFCT measurement on cable earth connections, infrared thermography for loose connections, and laboratory-based conventional PD testing carried out by accredited third-party laboratories on suspected defective components.

Applications

TEV partial discharge detection is applied across the full life cycle of electrical assets:

• Condition-based and predictive maintenance programs for MV distribution networks
• Acceptance testing and pre-energization checks of newly installed or refurbished switchgear
• Periodic on-line screening of substations in utilities, industrial plants, and infrastructure facilities
• Reliability assurance for critical facilities such as data centers, hospitals, airports, and rail transit power systems
• Wind farm collector systems and photovoltaic plants, where switching transients and cable defects are prevalent
• Petrochemical and offshore installations where unplanned outages carry high safety and economic consequences
• Continuous or periodic monitoring within asset health indexing and remaining-life assessment programs
• Post-failure forensic investigation of adjacent equipment following a switchgear fault event

By identifying insulation defects months or even years before catastrophic failure, TEV-based programs materially reduce unplanned outage risk, arc-flash hazards, and total cost of ownership.

FAQ

Is TEV testing performed on energized equipment?

Yes. TEV detection is a non-intrusive, live-line technique. The sensor is placed on the outside of the grounded metal enclosure, so no outage, no electrical connection, and no interruption to plant operation is required.

What TEV amplitude indicates a problem?

There is no universal threshold, as readings depend on equipment design, sensor bandwidth, and instrument calibration. In general, amplitudes near the background noise level are acceptable, moderately elevated values call for intensified monitoring, and high amplitudes accompanied by high pulse counts warrant detailed diagnosis. Consistent interpretation criteria, trend analysis, and inter-panel comparison should always be applied.

Can TEV detect all types of partial discharge?

TEV is highly sensitive to internal discharges within metal-enclosed equipment, floating potential defects, and surface discharge activity. Sensitivity decreases for deeply shielded defects and for open-air corona, which is better detected with ultrasonic instruments. Combining TEV with complementary methods provides the most complete assessment.

What are common sources of false readings?

Electromagnetic interference from variable frequency drives, switching operations, lighting inverters, and external radio transmitters can produce signals resembling PD. Good practice—including background measurement, phase-pattern analysis, repetition-rate checks, and confirmation with a second technique—minimizes misdiagnosis.

How often should TEV detection be carried out?

For most MV assets, an initial survey followed by periodic testing at 6- to 12-month intervals is typical. Critical or aging equipment, or units showing elevated readings, should be re-tested at shorter intervals or upgraded to permanent continuous monitoring.

Who should perform TEV partial discharge detection?

Testing should be carried out by trained and certified engineers using calibrated instruments, either by in-house condition monitoring teams or by accredited third-party laboratories specializing in electrical diagnostics, with findings interpreted against applicable IEC, IEEE, and GB/DL standards.