Partial Discharge (PD) Testing Equipment

What is Partial Discharge (PD) Testing Equipment?

Partial discharge (PD) is a localized dielectric breakdown of only part of an insulation system under high voltage. In other words, a small portion of the insulating material fails without a complete arc between conductors. PD often originates at voids, cracks or interfaces within solid or liquid insulation (e.g. air bubbles in cable insulation or resin voids) when the electric field exceeds the material’s strength. There are three main PD types: internal PD (within the insulation bulk), surface PD (tracking along insulation surfaces) and corona discharge (in air/gas at sharp points). Internal PD is especially insidious because it emits no audible or visible clues until failure, whereas surface PD can produce ozone or buzzing, and corona can appear as a glow in humid air. All PD types progressively erode insulation – for example, repeated discharges form ozone and nitric acid that chemically attack polymers, leading to surface treeing and eventual breakdown. Over time, PD damage (tracking channels, mechanical cracks, moisture paths) causes complete insulation failure, short circuits, equipment damage and even fires or explosions.

PD testing and monitoring are therefore critical for high‐voltage asset reliability. Because insulation degradation underlies roughly 80–85% of switchgear failures, early PD detection provides a predictive warning that prevents catastrophic outages and saves repair costs. For example, Omicron notes that PD activity is a “reliable indicator of insulation condition” and that detecting PD early lets maintenance teams plan remediation before failure. In practice, utilities and plants use PD surveys to spot developing insulation defects (e.g. voids or tracking) so they can repair or replace components in a controlled way, avoiding unplanned downtime and ensuring safety. The remainder of this article reviews PD testing methods, sensor technologies, equipment features, and applications for modern high-voltage systems.

PD Testing Methods: Online vs. Offline

Offline (de-energized) PD testing involves applying a controlled high-voltage test to the equipment when it is taken out of service. The device under test is connected to an AC or VLF test set, which steps up the voltage (often in stages) and PD pulses are measured with couplers. For example, an offline test on a cable might use a Very-Low-Frequency (VLF) AC source; on a motor winding it might use the shop’s AC PD tester. During the test, the PD inception voltage (PDIV) is recorded (voltage at which PD starts) and the PD extinction voltage (PDEV) when discharges stop. Offline measurements follow IEC60270, which defines “apparent charge” in picocoulombs (pC) as the PD output. High-speed PD systems (often using HFCT clamps) integrate the PD current pulse to yield a charge in pC. In ideal lab conditions, this can be very sensitive (on the order of 1 pC or less), but in field settings noise and cabling raise practical thresholds (often ~5–10 pC). Offline PD testing is commonly performed during factory acceptance or commissioning for cables, switchgear, motors/generators, etc. It is the most accurate way to quantify insulation health, but requires an outage and specialized HV sources.

Online (live) PD testing and monitoring is performed while the equipment is energized or on load. Online methods do not inject extra voltage; instead they use non-intrusive sensors to pick up PD signals while the system is running. This can be done via handheld spot‐check surveys (portable detectors placed on the gear) or via permanently mounted monitors. Online techniques focus on detecting the high-frequency byproducts (RF, acoustic, optical, etc.) of PD. The main advantage is that surveys can be done without interrupting service. The downside is the higher ambient noise and the need for sensitive filtering. In practice, engineers often combine methods: e.g. routine TEV/HFCT or ultrasonic checks on switchgear or cables, supplemented by periodic offline tests during scheduled outages. Typical online monitoring tools include continuous TEV or acoustic sensors on critical assets, providing trending data so deteriorating conditions are caught early.

PD Sensor Types

Online PD detection relies on several sensor technologies, each sensitive to different PD signatures:

• Transient Earth Voltage (TEV) sensors: These are capacitive probes clamped to the grounded metal enclosure of switchgear or transformers. Internal PD discharges induce high-frequency voltage spikes on the metal surfaces, which TEV probes measure. A TEV pulse (typically output as dBmV) reflects the PD event in the enclosed space. TEV methods excel at finding internal void discharges in metal-clad equipment without opening it. They require no electrical connection (just affixed to the panel) and are insensitive to external EM noise. However, TEVs have limited reach (they stay on the metal surface) so they cannot directly detect PD deep inside solid cable insulation.

• High-Frequency Current Transformers (HFCT/RFCT): An HFCT is a clamp-on device that wraps around the ground (shield) conductor of a cable or transformer. Because PD pulses produce a short burst of current that flows to ground, the HFCT measures these pulses directly. HFCT sensors effectively convert the PD current pulse into a voltage that can be integrated into apparent charge (pC). HFCTs are especially useful for cable PD surveys: one simply clamps the HFCT on the cable’s earth end and watches for spikes. They can be left in place for continuous monitoring. Unlike TEV, HFCT readings correlate directly to PD charge, so advanced detectors display the result in pC.

• Ultrasonic/Acoustic sensors: Partial discharges emit acoustic energy at ultrasonic frequencies. Specialized ultrasonic microphones or contact transducers pick up these high-frequency sound waves (typically above 20 kHz). Handheld ultrasonic detectors (often with headphones) can “listen” for PD hiss or crackle through gear vents or on tank walls. Ultrasonic methods are very effective for detecting surface PD and corona in air or gas, because they produce sound waves that travel outside the equipment. They work well even in electrically noisy environments (since they are unaffected by EM interference). However, they require a physical air/structure path from the PD source to the sensor.

• Ultra-High-Frequency (UHF) antennas: PD in switchgear or GIS generates broadband electromagnetic energy up to the UHF band (hundreds of MHz to GHz). UHF detectors use internal or external antennas to pick up this radiated RF energy. They are commonly used in gas-insulated systems (GIS) where PD radiation can be captured by probes mounted on the enclosure. A UHF device typically reports power in dBm. UHF sensors provide localized PD detection (e.g. on cable ends, insulators, arrestors) with good sensitivity at high voltages. Unlike TEV/HFCT, UHF does not need a ground reference – it simply catches the electromagnetic “buzz” of PD in the air.

• Other methods: For offline testing, capacitive couplers and balanced bridges are used. A coupling capacitor inserted between the test object and ground lets the PD current pulse be measured by the PD analyzer (IEC 60270). Differential or balanced coupling can suppress common-mode noise (as in Omicron’s MBB1 bridge). Chemical analysis (DGA for transformers) and UV cameras for corona are also part of PD diagnostics, though not typically in portable PD testers.

Key Features and Specifications of PD Equipment

When evaluating PD testing equipment, engineers look for the following capabilities:

• Frequency Range & Bandwidth: Different sensors cover different frequency bands. Offline PD analyzers (IEC60270) typically work from tens of kHz up to a few MHz. HFCT/TEV sensors usually cover ~50 kHz–30 MHz. UHF detectors cover much higher (300 MHz–2 GHz). A good PD analyzer will allow wide-band or tunable filtering (e.g. Omicron’s MPD-600 supports narrow/medium/wideband modes). This flexibility helps isolate the PD signal from noise.

• Sensitivity (Minimum PD Level): PD testers report apparent charge in pC (or voltage in dB for TEV/ultrasound). High-end offline systems can reliably detect down to a few picocoulombs. Online detectors are typically effective in the 5–10 pC range, though this depends on noise. For example, Hioki’s ST4200 PD kit can measure charges as low as 10 pC. In practice, the limiting factor is often the environment: high background noise can mask very small PD. Manufacturers calibrate instruments (IEC60270 calibrators) so that a known pC pulse produces a known reading, ensuring traceability.

• Noise Rejection: Because PD signals are weak and high-frequency, robust interference suppression is critical. Features include high-pass/low-pass filters, digital noise gating, and galvanic isolation. Many portable PD analyzers run on battery and use optical fiber communications to avoid ground loops. Balanced (differential) inputs (e.g. Omicron’s MBB1 bridge) subtract common-mode noise. Trigger gating (e.g. ignoring 50/60 Hz spikes) and averaging can remove periodic interference. TEV and UHF methods inherently bypass some noise: TEV is immune to long-wave EMI, and ultrasound is immune to RF. Hioki’s ST4200 specifically uses high-frequency CTs to suppress inverter noise on motor test lines.

• Portability and Form Factor: PD instruments range from handheld scanners to rack-mount testers. Handheld detectors (like the HVPD PDS Insight², Megger UHF, EA UltraTEV) are lightweight, battery-powered and meant for field surveys. They have simple interfaces (touchscreen or buttons). Bench testers (Omicron MPD, Hioki ST4200) are larger, often with multiple channels, AC power input, and require external HV sources. Key specs include weight and size: for instance, the Hioki ST4200 unit is ~7 kg with dimensions akin to a small briefcase. Ruggedized cases and IP-rated enclosures are also valued for outdoor use.

• Data Logging & Connectivity: Modern PD equipment includes built-in memory and PC software. It should log waveforms, PRPD patterns (phase-resolved PD), and trend data. Features like timestamping, asset tagging (e.g. barcode scanner in HVPD Insight²), and automated report generation are important for end users. Many instruments allow USB/SD export or wireless transfer to a PC/Cloud. Sophisticated systems (e.g. EA’s UltraTEV Plus²) integrate with cloud platforms to compare results over time. Also look for compatibility with asset management systems: for example, standardized CSV/XML export or REST APIs.

• Display and User Interface: A clear display (often color touchscreen) that can show instantaneous values, 3D PRPD histograms, trends, and alarms is crucial. Some tools guide users step-by-step (guided mode in UltraTEV) to ensure consistent survey procedure. Graphical output (waveforms, bar charts of PD count) helps in quick assessment. Dual-channel inputs (e.g. Megger’s UHF PD detector) allow comparing two sensors or two phases simultaneously.

• Integration with HV Sources: Offline PD testers must interface with HV generators (AC/VLF/test sets). Some kits (like Hioki’s ST4200) can automatically control the test voltage supply and synchronize measurement. Others require manual connection. Ensure the equipment supports the voltage range you need (e.g. up to 35 kV or more, depending on your system).

In summary, choose PD test gear that covers the frequency and sensitivity needed for your asset, has strong EMI filtering, is rugged enough for the environment, and outputs data in a format that fits your maintenance workflow.

Applications of PD Testing

PD testing applies to nearly all high-voltage apparatus. Typical applications include:

• Metal-Clad Switchgear (AIS): Partial discharge can occur in cable terminations, bus bars, insulators or epoxy components. Online TEV/ultrasonic detectors are commonly used to survey energized switchgear cubicles. Offline PD testing is done during manufacturing or retrofits (e.g. applying test voltage to individual phases of a switchgear cell).

• Gas-Insulated Switchgear (GIS): UHF PD detection is ideal for GIS, since the SF₆ environment and metal housing channel UHF waves. Dedicated UHF couplers or through-wall antennas pick up PD in GIS transformers, isolators, and busbars. Offline factory testing of GIS typically uses UHF sensors as well as capacitive couplers.

• Cables and Accessories (MV/HV): XLPE cables often have manufacturing voids or termination defects. Online PD surveys on live cables use HFCT clamps on sheath grounds (with the cable energized, PD pulses travel to earth). Ultrasound can also detect PD at cable terminations. Offline, every factory-assembled cable undergoes an IEC60270 test (with PD couplers at both ends) to ensure no inception up to the design voltage.

• Power Transformers: PD can originate in winding voids or on core and insulation surfaces. TEV probes on transformer tank walls detect discharges inside. Acoustic sensors on the tank can pick up PD “buzz”. UHF sensors mounted in the bushings or cavity are used in specialized tests. Offline, transformer windings are tested for PD and dielectric loss before commissioning.

• Bushings and Insulators: Capacitive PD couplers are often built into bushings to continuously monitor PD activity. For stand-alone tests, an insulating coupling capacitor is connected to the bushing terminal and ground to measure PD during HV testing.

• Rotating Machines (Motors/Generators): PD often occurs in stator windings. Offline AC PD tests (IEC60034‑27) apply voltage between phase leads (Hioki ST4200 is designed for this). Surge (impulse) PD tests (IEC61934) apply an impulse to coils (Hioki supports this too). Online, HFCT clamps on the machine’s star point or lead cables can detect PD, and ultrasound is sometimes used. Special motor-insulation testers from Megger or Omicron can also measure PD in stator windings.

• Ancillary Equipment: Arresters, bushing capacitors, current transformers and other insulators may be surveyed with UHF or acoustic detectors. Overhead line insulators often get corona surveys via UV cameras rather than traditional PD equipment.

Each application may use a different combination of sensors. For example, internal PD in switchgear is best caught with TEV and HFCT, while external corona on outdoor insulators may only be visible with UV. Table below summarizes common sensor use-cases:

Leading PD Test Equipment (Brands and Models)

A number of specialized companies provide PD testing tools. Below is a comparison of prominent solutions:

• Omicron MPD Series (MPD 600/800) – High-end offline PD measurement systems for factory or site testing. These multi-channel analyzers comply with IEC60270 and include fiber-optic isolation. They offer extremely low-noise digital filtering, and even UHF extensions (Omicron’s UHF 620) for multi-band PD detection. The MPD-800 (latest model) delivers “highly sensitive” measurements and automated reporting to detect and localize defects. Omicron also provides accessories like calibrated coupling capacitors and balanced bridges to suppress interference. (Omicron does not sell handheld online PD scanners; its focus is lab/field test units and online monitoring solutions separately.)

• Megger – Megger’s PD portfolio spans both testing and monitoring. The Megger UHF PD Detector is a handheld online survey tool for MV/HV substations. It has dual channels (compare two UHF antennas or two phases), a large color touchscreen, and displays PRPD patterns to distinguish defects from noise. Megger also markets ICMcompact (a single-channel, IEC60270 PD analyzer for offline tests) and GISmonitor (parallel multi-channel on-line monitors for GIS). In essence, Megger provides the hardware for in-service surveys (UHF/TEV detectors) and laboratory PD diagnostics (IEC PD detectors).

• EA Technology (UltraTEV Plus²) – EA’s UltraTEV Plus² is a leading handheld TEV/Ultrasonic detector. It combines built-in TEV and airborne ultrasonic sensors in one unit. Designed for switchgear inspection, it features an intuitive guided workflow and cloud connectivity for data management. The instrument automatically compares readings to thresholds, assists in locating PD sources, and synchronizes data with EA’s Managed Surveys platform for trend analysis. In short, UltraTEV offers a user-friendly PD survey experience optimized for preventive maintenance of switchgear.

• HVPD (Monitra) Insight² – Formerly HVPD, now part of Monitra, the PDS Insight² is a handheld online PD detector focused on ease of use. It includes built-in TEV and acoustic sensors, plus an optional HFCT clamp (Kit‑2 adds the HFCT). The Insight² reports peak PD level, counts, and cumulative activity, and even features a barcode scanner to tag assets. Its software (OLPD Manager) allows users to log results, trend PD over time, and benchmark against criteria. The unit is color-coded (green/yellow/red) for PD severity. It is widely used for MV cables, switchgear, and transformers – essentially any in-service asset where a quick PD survey is needed.

• Hioki ST4200 PD Kit – The Hioki ST4200 is an offline PD test kit for motors and coils. It is unique in that it is built to handle the high-frequency noise of inverter-driven motor tests. The ST4200 kit includes high-frequency current transformers (HFCTs) for PD detection, enabling “noise-resistant” measurements even on motor production lines. The instrument supports both AC PD testing (compliant with IEC60270 and IEC60034-27-1) and surge PD testing (IEC61934). Key specs include an applied-voltage frequency range of 45 Hz–1.1 kHz, and a charge measurement range of 10–500 pC (for typical motor capacitances). The ST4200 runs on mains AC and provides a graphical display of PD pulses, Q vs phase, PDIV etc. Its strengths are high sensitivity (low pC), immunity to radio-frequency interference, and automatic HV control (it interfaces with Hioki HV test sets). The downside is that it is not a portable field instrument – it is meant for benchtop testing in a lab or factory.

Each of these systems has strengths and ideal use cases: Omicron’s MPD units excel in precision offline testing of large equipment (cables, transformers, stators) with full IEC traceability; Megger’s handhelds and monitors suit field surveys and OEM testing; EA Tech’s UltraTEV is tailored for fast, guided inspections of switchgear; HVPD/Monitra’s Insight² is a versatile online survey tool for substations; and Hioki’s kit is specialized for motor winding diagnostics. Table 1 summarizes key attributes of representative PD instruments:

(Units: pC = picocoulombs of charge; dBmV = decibels relative to 1 mV; dBµV = decibels relative to 1 µV.)

Hioki ST4200 PD Kit: Capabilities and Limitations

The Hioki ST4200 deserves special mention since it represents a modern approach to PD testing in motor applications. Its dual-mode capability lets a technician perform both AC withstand tests and surge tests on the same machine. In AC PD mode, it follows IEC60270 by using band-pass filters and measures apparent charge (Q) of discharges. It can measure very low-level PD (as low as 10 pC) and handle motor winding capacitances from 200 pF up to 10 nF. In surge PD mode, it samples at 200 MS/s and detects PD pulses during impulse testing, reporting peak discharge (Qpk) and inception voltages.

Key features of the ST4200 include: battery-powered HFCT sensors (for noise isolation); graph displays of PD amplitude and phase; and data logging to SD/USB. It also provides a differential measurement mode (MBB1) that subtracts two channels to reject noise. In terms of noise immunity, the ST4200 can tolerate up to 10 V/m of RF interference with less than 50 pC error. It interfaces with Hioki HV sources (e.g. HiTester 3153) to automate voltage ramping.

However, the Hioki kit is not a general-purpose on-line PD detector. Its limitations include: it requires the equipment to be de-energized; it is focused on motor/generator stators (stator capacitance must be within its range); and it relies on HFCT clamps around coil leads – it cannot “listen” with ultrasound or catch UHF waves. In other words, the ST4200 excels at quantifying very small PD charges in a controlled test environment, particularly where electrical noise (from inverters or the grid) would blind a traditional UHF antenna or ultrasonic sensor. Its ideal use case is manufacturing or maintenance of large electric motors and generators.

FAQ

Q: What is the minimum detectable PD level? Modern PD test systems can sense very low discharge charges. In calibrated IEC60270 systems, 1 pC (10⁻¹² C) is a common lower calibration point. In practice, high-quality laboratory setups may resolve sub-picocoulomb pulses, but in field conditions typical on-line detectors reliably measure down to a few picocoulombs. The exact threshold depends on the device’s noise floor. For example, the Hioki ST4200 can detect PD pulses from ~10 pC upward. In noisy environments, practical detectability is often ~5–10 pC and above. Note that TEV and UHF methods report in voltage (dBmV), so their “sensitivity” is characterized differently.

Q: Can I use PD testing on energized systems? Yes – but only with on-line methods. TEV, HFCT, UHF and acoustic sensors allow live PD surveying without interrupting power. Handheld detectors (e.g. UltraTEV, HVPD Insight, Megger UHF) are explicitly designed for energized inspections. However, offline PD testing (IEC60270 high-voltage PD tests) requires the asset to be de-energized so that a controlled test voltage can be applied. In summary: on-line PD testing is for in-service monitoring (no outage needed), whereas offline PD testing is done during planned outages or factory acceptance with the equipment isolated from the grid.

Q: How often should PD testing be done? There is no one-size-fits-all answer; frequency depends on asset criticality, age, and operating environment. As a rule of thumb, utilities often conduct PD surveys on distribution switchgear every 1–2 years, and more frequently for deteriorating or mission-critical gear. For example, historical practice for primary switchboards was TEV testing every 2 years plus extended monitoring campaigns every few years. Many organizations now supplement periodic surveys with continuous PD monitoring. If trend data is available, PD “hot spots” can be caught well in advance. In any case, it is wise to test often enough that the potential-to-functional (P–F) window for failure is covered: for severe defects this might mean annual checks, whereas stable assets might go several years between surveys. As a guideline, incorporate PD testing into each maintenance cycle (e.g. at least annually) or use permanent monitors for real-time alerts.

Q: What’s the difference between TEV and HFCT detection? The key difference is what each sensor measures. A TEV sensor senses the high-frequency voltage induced on the enclosure of grounded HV equipment by a PD event. It outputs a voltage signal (usually displayed in dBmV). TEV is effectively a voltage probe on the grounded panel. In contrast, an HFCT (High-Frequency Current Transformer) clamps around a grounding conductor and measures the current pulse of the PD as it flows to earth. An HFCT produces a waveform from which the PD charge can be calculated (in pC). In practice, TEV and HFCT are complementary: TEV is excellent for metal-clad switchgear (no wiring needed), whereas HFCT is standard for cable PD (it quantifies charge directly). Note that TEV readings (dBmV) cannot be directly converted to pC without calibration on the specific asset, whereas HFCT readings are inherently charge-based.

Q: How does Hioki’s PD kit differ from UHF or acoustic systems? The Hioki ST4200 kit is a contact-based, offline PD analyzer specifically for motors. It uses high-frequency CT couplers and an IEC-standard band-pass method to measure discharge charge. In contrast, UHF detectors (like Megger’s) are remote sensors that pick up RF radiation from PD (typically used in GIS or cable terminations). And acoustic/ultrasonic detectors simply listen for the PD’s sound waves (used for surface/corona PD). The Hioki kit does not have a UHF antenna or microphone – it only senses PD electrically via the CT. Its strength is in detecting tiny PD charges in stator windings under test conditions, even with inverter noise present. Meanwhile, UHF and ultrasonic systems are used in different scenarios (e.g. surveying a live substation or switchgear) and are not suitable for precision winding tests. In summary, Hioki’s PD kit is an offline, high-resolution current-based tester (for motors), whereas UHF and acoustic are online, non-contact PD monitors for other apparatus.