How to detect partial discharge?

What is Partial Discharge?

First, a quick recap: Partial Discharge is a localized electrical discharge that only partially bridges the insulation between conductors. It can occur within cavities in solid insulation, along the surface of insulation, or in gas bubbles in liquid insulation. If left unchecked, PD progressively degrades insulation, leading to ultimate failure.

1. Fundamental Principles of PD Detection

All detection methods rely on measuring the physical phenomena that PD activity generates:

Electrical Pulses: The primary effect is a rapid, nanosecond-scale current pulse.

Electromagnetic Emissions: The current pulse radiates electromagnetic energy, including radio frequency (RF) and high-frequency signals.

Acoustic Emissions: The discharge creates a tiny shockwave, an audible or ultrasonic "click."

Chemical By-products: PD decomposes insulation materials, releasing specific gases (like Ozone) and creating chemical changes.

2. Primary Detection Methods

Here are the main methods, categorized by the phenomenon they detect.

A. Electrical Detection Methods (The Gold Standard)

This is the most direct and quantitative method, following the IEC 60270 standard.

How it works: It measures the apparent charge (in picoCoulombs, pC) of the current pulses caused by PD. A coupling capacitor and a detection impedance form a circuit to capture these high-frequency pulses.

Setup:

A coupling capacitor is connected in parallel to the test object.

A measurement impedance (Quadripole) is connected in series with the capacitor.

A specialized PD detector measures the pulses, filters out noise, and displays the results.

Key Output:

PD Magnitude (pC): The apparent charge of the discharge.

Phase-Resolved Partial Discharge (PRPD) Pattern: A plot showing discharge magnitude and number versus the phase angle of the AC power cycle. This pattern is like a "fingerprint" that helps identify the type of PD (e.g., internal, surface, or corona).

Pros:

Highly sensitive and quantitative.

Provides the most accurate measurement of discharge severity.

PRPD analysis is excellent for fault diagnosis.

Cons:

Requires a direct electrical connection to the high-voltage apparatus.

Often requires taking the asset offline (offline testing).

Susceptible to electrical interference/noise.

B. Acoustic / Ultrasonic Emission (AE) Sensing

This is a very popular non-intrusive method, especially for live equipment.

How it works: It uses ultrasonic sensors (or acoustic emission sensors) to detect the high-frequency sound waves (typically 20 kHz to 300 kHz) produced by the PD. Since human hearing tops out around 20 kHz, these are "ultrasonic."

Setup:

Sensors are placed on the surface of the equipment (e.g., transformer tank, switchgear cabinet).

Portable ultrasonic "guns" are used for general scanning.

Multiple fixed sensors can be used for permanent monitoring and to triangulate the exact location of the PD source.

Key Output:

Ultrasonic dB level: The intensity of the sound.

"Hiss" or "Crackle" sound: Many devices have headphones to hear the heterodyned (down-mixed) ultrasonic signal.

Pros:

Excellent for pinpointing the physical location of the PD.

Can be used on live, energized equipment (online testing).

Immune to electrical interference.

Cons:

Sound is easily attenuated and blocked by solid barriers (e.g., inside a transformer tank).

Not as effective for quantifying the severity (pC) of the discharge.

Background acoustic noise can be an issue.

C. High-Frequency Current Transformer (HFCT / RFCT) Sensing

This is one of the most common methods for online monitoring of cables, switchgear, and transformers.

How it works: A clamp-on HFCT sensor is placed around the earth (ground) wire or cable sheath. It acts as a current transformer tuned to high frequencies (typically 100 kHz to 50 MHz), detecting the RF current pulses from PD that flow to ground.

Setup: The clamp is simply placed around the conductor. No direct electrical connection to high voltage is needed.

Key Output:

PD pulse magnitude and phase.

Can generate PRPD patterns for analysis.

Pros:

Non-intrusive and easy to install on live equipment.

Good sensitivity and provides phase-resolved data.

Excellent for cable and switchgear monitoring.

Cons:

Sensitivity depends on placement and the integrity of the earth connection.

Can be affected by radio frequency interference (RFI).

D. Transient Earth Voltage (TEV) Sensing

Widely used for testing metal-clad switchgear.

How it works: When PD occurs inside metal-clad switchgear, the current pulses travel along the inner metal surfaces. At gaps or joints (like doors), these pulses couple to the outer surface, creating a Transient Earth Voltage. A TEV probe measures this voltage on the outside of the metal casing.

Setup: A handheld meter with a capacitive coupling plate is placed against the metal surface of the switchgear.

Key Output: TEV magnitude in millivolts (mV).

Pros:

Very quick and simple for condition screening of switchgear.

Non-intrusive online testing.

Cons:

Provides a relative measure, not an absolute pC value.

Calibration and interpretation can be manufacturer-specific.

Mainly applicable to metal-clad equipment.

E. Ultra-High-Frequency (UHF) Sensing

The premier method for online monitoring of power transformers and Gas-Insulated Switchgear (GIS).

How it works: PD events emit electromagnetic waves in the ultra-high-frequency range (300 MHz to 3 GHz). UHF sensors (internal or external) are antennas that detect these signals.

Setup:

GIS: Sensors are installed through dielectric windows or couplers in the GIS tank.

Transformers: Sensors can be installed in drain valves or dedicated ports.

Key Output:

UHF signal amplitude.

PRPD patterns for advanced diagnosis.

Pros:

Extremely sensitive and immune to lower-frequency external noise.

Excellent for online, permanent monitoring of critical assets.

Can locate the source using time-of-flight differences between multiple sensors.

Cons:

Requires specialized, often expensive, equipment.

Calibration to pC is very difficult.

Installation in existing equipment can be challenging.

F. Chemical / Gas Detection

Dissolved Gas Analysis (DGA): For oil-filled transformers, PD produces specific gases like Hydrogen (H₂) and Methane (CH₄). DGA of the oil can indicate PD activity.

Ozone Detection: PD in air produces ozone, which can sometimes be smelled or detected with sensors.

3. A Practical Step-by-Step Guide for a Basic PD Survey

For a technician starting with switchgear or a substation, a common approach is:

Planning: Review equipment single-line diagrams and historical data. Identify potential PD hotspots.

Initial Screening (TEV & Ultrasonic):

Use a TEV meter to scan metal-clad switchgear panels. Record mV levels on all accessible surfaces.

Simultaneously, use an Ultrasonic Gun to listen for discharges around bushings, cable terminations, and vents.

Data Analysis & Triangulation:

If high readings are found, use the ultrasonic sensor to precisely locate the source of the "clicking" sound. Move the sensor to find the point of loudest intensity.

Follow-up / Detailed Investigation (if needed):

If a serious source is suspected, more advanced methods may be required.

Offline Testing: Perform an IEC 60270 standard electrical test during a planned outage to quantify the PD level in pC.

Online Monitoring: Install HFCT sensors on the relevant earth leads for continuous monitoring and PRPD analysis.

Summary Table of Methods