Instrument Transformer Partial Discharge Detection: Principles, Methods, and Standards

Partial discharge detection is one of the most sensitive and widely adopted diagnostic techniques for evaluating the insulation integrity of instrument transformers, including current transformers, voltage transformers, and capacitor voltage transformers. This article explains the physical principles of partial discharge, the applicable IEC and IEEE standards, laboratory and field testing methods, acceptance criteria, and the practical applications of instrument transformer partial discharge detection in factory routine testing and condition-based maintenance programs.

Technical Overview

Partial discharge (PD) is a localized dielectric breakdown that occurs within, or on the surface of, an insulation system when the local electric field exceeds the ionization threshold of a gas-filled cavity, an interface, or a sharp protrusion, without completely bridging the live and earthed electrodes. Although a single discharge event involves only a tiny transferred charge, typically measured in picocoulombs (pC), repetitive PD activity progressively erodes dielectric material through ionic bombardment, chemical attack, and local heating, producing electrical trees, carbonized tracking channels, and gas evolution that ultimately lead to complete dielectric failure.

Instrument transformers, comprising current transformers (CTs), inductive voltage transformers (VTs/PTs), and capacitor voltage transformers (CVTs), are especially vulnerable to PD-related deterioration. These devices compress a large number of insulation barriers, grading shields, and winding layers into a compact geometry, so even small manufacturing defects such as voids in cast resin, incomplete oil impregnation, moisture in cellulose paper, conductive particle contamination, or poorly finished electrode edges can concentrate the electric stress to unsafe levels. Because instrument transformers are usually installed directly on busbars or feeders without a dedicated circuit-breaking device between them and the system, an internal insulation failure can escalate into a violent, explosive rupture that endangers personnel and adjacent equipment.

For these reasons, partial discharge detection is mandated as a routine factory test for instrument transformers rated above approximately 3.6 kV under the IEC 61869 series, and it is a core element of type testing under IEC 61869-1 and IEEE C57.13. In service, PD measurement serves as an early-warning indicator that enables utilities to intervene before catastrophic failure, making it a cornerstone of asset condition assessment and predictive maintenance strategies for substation fleets.

Testing Samples

Current Transformers (CTs)

Current transformers in bar-primary, wound-primary, window-type, and top-core (hairpin or eyebolt) configurations are tested with their primary terminals short-circuited and the HV potential applied between primary and the shorted, grounded secondary windings and core. Common PD sources include gas voids in epoxy resin castings, delamination at resin–paper interfaces, insufficient drying of oil-paper insulation, and burrs on electrostatic grading shields.

Voltage Transformers (VTs / PTs)

Inductive voltage transformers feature multi-layer windings with substantial axial and radial field gradients along the high-voltage winding. PD testing is performed by energizing the primary winding at power frequency with secondaries shorted and grounded. Typical defects detectable by PD include interlayer voids in enameled-wire windings, weak points in winding-end stress control, trapped gas in impregnated paper, and surface contamination on bushing terminations.

Capacitor Voltage Transformers (CVTs)

CVTs rely on a capacitive divider stack of oil-impregnated paper–film elements, in which even minute partial discharges inside individual capacitor elements degrade the dielectric and can shift the divider ratio, affecting both metering accuracy and protection reliability. PD testing is applied to the complete divider column and, where feasible, to individual capacitor sections.

Combined Units and Gas-Insulated Designs

Combined CT/VT units, switchgear-embedded transformers, and SF6 gas-insulated instrument transformers are tested with the same fundamental approach, but interpretation must account for the different propagation characteristics of PD pulses in gas dielectrics and the possible use of ultra-high-frequency sensors integrated into the enclosure.

Testing Items

A complete partial discharge evaluation of an instrument transformer normally covers the following measured quantities:

• Apparent charge magnitude (q, in pC) at the specified test voltage, which is the contractual acceptance parameter in routine tests.
• Partial discharge inception voltage (Ui), the applied voltage at which repetitive PD pulses first exceed a defined magnitude.
• Partial discharge extinction voltage (Ue), the voltage at which PD activity ceases when the voltage is reduced; a value above the maximum continuous operating voltage indicates stable insulation behavior.
• Phase-resolved PD pattern (PRPD), pulse amplitude distribution, and pulse repetition rate, which support defect-type identification such as internal voids, surface discharge, or corona.
• Background noise level and circuit calibration verification, confirming that the measurement chain satisfies the sensitivity requirements of IEC 60270.
• Correlation with complementary diagnostics, including dielectric dissipation factor (tan delta), insulation resistance, capacitance measurement, and dissolved gas analysis (DGA) for oil-immersed units.

Testing Methods

Conventional AC Partial Discharge Measurement (IEC 60270 Pulse Current Method)

The reference method for laboratory and factory testing is defined in IEC 60270. A corona-free high-voltage source energizes the test object while a coupling capacitor (Ck) in series with a coupling device (measuring impedance) extracts the discharge current pulses, which are quantified by a calibrated PD analyzer over a bandwidth of approximately 40 kHz to 1 MHz. Direct (series and parallel) coupling circuits are used for most instrument transformers, while balanced (bridge-type) circuits suppress common-mode interference for high-sensitivity measurements. Before testing, a calibrator injects a known charge (q0) across the test object terminals so that the reading in pC can be traced to a physical quantity. To achieve reliable readings at the 10 pC level, tests are preferably performed in a shielded laboratory with filtered supply circuits, corona-free terminations, and background noise held far below the specified limit, ideally no more than 25 percent of it.

Test Voltage Sequence and Acceptance Criteria

Per IEC 61869-1, the routine PD test applies a defined voltage program to the transformer: the voltage is raised to the specified level, commonly referenced to the maximum system voltage Um, and the apparent charge is read after a stabilization period. For most rated voltage levels the acceptance limit for new liquid-immersed and cast-resin instrument transformers is an apparent charge not exceeding 10 pC at the specified measuring voltage, for example approximately 28 kV for a device with Um = 40.5 kV measured at 1.2 Um/√3. Type tests additionally include a brief pre-stress at an elevated multiple of Um/√3 to demonstrate that any discharges extinguish at operating stress. The exact voltage sequence and limits must always be confirmed against the current edition of IEC 61869-1, the relevant product part of the 61869 series, IEEE C57.13, or the purchase specification.

Ultra-High-Frequency (UHF) Detection

UHF sensors capture electromagnetic radiation in the 300 MHz to 3 GHz range emitted by internal PD events. Because corona and most external radio interference lie below this band, UHF detection offers excellent noise immunity and is well suited to gas-insulated switchgear with dielectric windows, as well as to permanently installed online monitoring of critical instrument transformers. Sensitivity is verified through pulse-injection checks rather than direct pC calibration, so UHF results are typically interpreted comparatively and by pattern recognition.

Acoustic Emission (AE) Detection

Piezoelectric sensors mounted on the transformer tank or housing detect acoustic waves in the 20 kHz to 200 kHz range generated by discharge activity. The non-invasive nature of AE testing makes it valuable for locating PD sources inside oil-filled units and for confirming electrical findings, although strong attenuation in resin, paper, and flanged structures limits its standalone sensitivity.

High-Frequency Current Transformer (HFCT) and Transient Earth Voltage (TEV) Techniques

For online diagnostics without an outage, an HFCT can be clamped onto the transformer grounding lead or capacitive tap to capture high-frequency PD currents, while TEV sensors detect voltage transients induced on the enclosure by internal discharges and surface tracking. These techniques enable periodic screening surveys and permanent condition monitoring, with anomalies triggering more detailed offline investigation.

Supplementary Chemical and Thermal Indicators

In oil-immersed instrument transformers, dissolved gas analysis revealing elevated hydrogen and acetylene provides chemical evidence of PD activity, and infrared thermography can expose external surface tracking. These indicators complement electrical measurements and improve diagnostic confidence in combined assessment protocols.

Applications

Instrument transformer partial discharge detection is applied across the entire asset life cycle:

• Factory routine testing of every new CT, VT, and CVT prior to shipment, in accordance with IEC 61869 or IEEE C57.13.
• Type testing and design qualification of new insulation systems, materials, and manufacturing processes.
• Commissioning and post-transport acceptance testing to verify that handling and storage have not degraded insulation quality.
• Periodic offline diagnostics during planned outages as part of time-based or condition-based maintenance programs.
• Continuous online monitoring of instrument transformers in critical substations, generation plants, and high-consequence industrial installations.
• Forensic root-cause analysis after failures, where PD evidence helps distinguish insulation defects from overvoltage or thermal causes.
• Ageing studies and remaining-life assessments supporting repair-versus-replace and fleet prioritization decisions.

FAQ

Why is partial discharge detection so important for instrument transformers?

Partial discharge is both a symptom and an accelerator of insulation degradation. Detecting PD early allows defects to be identified long before dielectric breakdown occurs, preventing unplanned outages, protection maloperation, metering errors, and the safety hazards associated with explosive failures of oil-filled transformers.

What is the typical acceptance criterion for a new instrument transformer?

Under IEC 61869-1, the routine test requirement for most rated voltage classes is an apparent charge not exceeding 10 pC at the specified measuring voltage, which is commonly 1.2 Um/√3. Project specifications and regional standards such as the GB/T 20840 series or IEEE C57.13 should always be checked for the applicable limit and voltage sequence.

Can partial discharge testing be performed with the transformer in service?

Yes. UHF sensors, HFCT clamps on grounding connections, TEV detectors, acoustic sensors, and dissolved gas analysis all support online detection without de-energizing the equipment. These methods are ideal for screening and trending; confirmation and precise quantification are usually performed during an outage using the IEC 60270 method.

What kinds of defects can partial discharge detection reveal?

Typical detectable defects include gas voids and porosity in cast resin, delamination between insulation layers, moisture and incomplete impregnation in oil-paper systems, conductive particle contamination, sharp points on electrodes or shields, floating potential components, and surface tracking on bushings and housings.

How is measurement accuracy ensured and interference controlled?

The measurement chain is calibrated by injecting a known charge into the test circuit, and circuits are verified for correct reading in pC. Interference is suppressed through shielded enclosures, power-line filters, corona-free terminations, balanced measuring circuits, time-synchronized gating, and modern digital noise-separation tools based on pulse waveform and phase-resolved statistics.

How frequently should field partial discharge testing be carried out?

Common utility practice combines a commissioning baseline measurement with periodic offline diagnostics every three to six years, adjusted according to equipment criticality, service duty, and family failure history. Transformers in strategic locations are increasingly covered by continuous online PD monitoring with automated alarm thresholds.