Partial discharge detection is changing how utilities and asset managers evaluate the condition of electrical equipment. In Part 1 of this series, we examined the origins of partial discharge in transformers and its impact on insulation health. Understanding these fundamentals is only the first step; the real challenge for asset operators lies in detecting partial discharge early, before localized defects escalate into irreversible insulation failure or unplanned outages.

In this next blog series, we shift the focus to partial discharge monitoring techniques. We will further explore both traditional and advanced techniques of partial discharge detection, describing how they facilitate early fault detection, improve insulation assessment, and enhance overall system reliability and safety.

 Techniques for Partial Discharge Detection in Transformers

Effective partial discharge monitoring is based on advanced diagnostic methods that capture discharge signals in various domains. There are several diagnostic methods for real-time partial discharge detection. Each method provides unique insights about discharge patterns, location, and severity. Integrating partial discharge monitoring into transformers helps utilities identify partial discharge activity

The following sections outline widely used techniques for partial discharge detection in transformers:

1. Electrical Detection

Electrical detection is one of the most widely used techniques for partial discharge detection in transformers. This method captures the high-frequency current pulses generated during a partial discharge event. These pulses are measured in the form of apparent charge, usually expressed in picocoulombs (pC) or nanocoulombs (nC). The method has been standardized by IEEE and IEC, and CIGRE has reviewed its application in power transformers. In practice, partial discharge signals are determined using external coupling capacitors or bushing taps across a calibrated impedance. This method is most effective for offline testing, such as factory acceptance or pre-commissioning inspections. However, it can also be adapted for online monitoring when bushing taps are available.

Beyond detection, the real value lies in interpreting the data. Partial discharge pulse amplitude, its trend over time, and phase-resolved partial discharge (PRPD) patterns are strong indicators of insulation degradation. Comprehensive databases of PRPD patterns have been developed for various discharge modes and high-voltage components, enabling engineers to match test results with known patterns and identify the source of insulation defects. This makes traditional partial discharge measurement an effective method, explaining why it continues to be widely applied in both factory acceptance and on-site transformer testing.

2. Acoustic Detection

Acoustic methods are a common approach for partial discharge detection in transformers. Partial discharge produces acoustic waves that are detectable by using specific sensors. In the case of transformers, this is typically done using ultrasonic detectors and piezoelectric sensors placed on the transformer tank, with a magnetic holder used to capture acoustic emission signals.  These sensors detect ultrasonic signal ranges, and the values are usually expressed in decibels (dB) or millivolts(mV).

The Acoustic Emission (AE) method is widely used due to its immunity to electromagnetic interference, operation under normal conditions, and relatively low cost. It is also capable of detecting multiple sources of partial discharge, making it applicable to field use. The strategic arrangement of sensors on the asset surface enables real-time monitoring of PD activity, allowing for the detection of issues before they become critical. This method offers excellent performance among detection equipment, providing a wider coverage area and an improved signal-to-noise ratio.

3. Chemical Detection

Partial discharge detection using chemical methods in transformers is carried out through Dissolved Gas Analysis (DGA), a common technique used to detect gases emitted during the degradation of transformer oil and cellulose insulation. Oil degradation generally produces hydrogen(H₂), methane (CH₄), ethane(C₂H₆), ethylene(C₂H₄), and acetylene (C₂H₂), while breakdown in cellulose insulation predominantly yields carbon monoxide (CO) and carbon dioxide (CO2).

Traditionally, DGA is performed offline employing techniques such as gas chromatography or pump systems based on air circulation, although these are time-consuming. Specifically, the pump system detects hydrogen based on air-circulation-related effects. To overcome such limitations, online gas detection methodologies have been introduced, including hydrogen sensors, photoacoustic spectroscopy (PAS), fiber Bragg grating (FBG) sensors, oil-immersed probes, and membrane-based technologies.

Monitoring the concentration and change in gases over time helps determine insulation faults and partial discharge activity, which in turn helps evaluate the overall internal condition of the transformer.

4. Electromagnetic Detection

The electromagnetic (EM) technique are widely used for partial discharge detection in transformers. They detects partial discharge through the measurement of ultra-high frequency (UHF) signals that are produced during discharge activity. This technique utilizes antennas such as conical, spiral, and Vivaldi designs as sensors.

UHF detection is popular due to its resistance to low-frequency interference, which minimizes noise from transformer construction with denoising techniques, and eliminates corona-related disturbances. However, it can still be affected by switching activity and radio interference, which must be carefully filtered out.

In addition to UHF antennas, other transformer sensors, such as Rogowski coils, high-frequency current transformers (HFCT), and radio-frequency current transformers (RFCT), are utilized. With advanced signal processing, the electromagnetic method not only indicates the presence of partial discharge but also assists with localizing multiple partial discharge sources and identifying their characteristics.

5. Transient Earth Voltage (TEV) Detection

The TEV is a non-intrusive method for partial discharge detection in transformers and other high-voltage equipment. When partial discharge occurs inside the insulation, it produces high-frequency electromagnetic signals that travel along the metal surface of the equipment. The signals, which are known as TEV pulses, result from rapid voltage fluctuations due to the discharge.

Specialized TEV sensors are used to capture such pulses. Mounted on or close to the equipment surface, they convert the electromagnetic activity into measurable voltage signals for analysis. Through examination of parameters like amplitude and frequency, operators can identify the occurrence, severity, and probable source of PD.

TEV detection is highly sensitive and effective for identifying insulation defects at an early stage. It enables continuous, real-time monitoring without interrupting operations.

Why is Partial Discharge Detection Essential in Transformers?

Implementing partial discharge monitoring provides utilities and operators with key benefits:

• Early fault detection – Continuous monitoring allows asset managers to identify insulation defects in transformers before they develop into major failures.
• Asset life extension – Predicting partial discharge sources at an early stage helps prolong the transformer lifespan, minimizing premature replacements.
• Quality assurance – Partial discharge testing identifies insulation defects, helping ensure the quality and reliability of transformer materials.
• Ensures compliance – Routine partial discharge detection enables industries to adhere to safety standards and maintenance protocols.
• Operational safety and reliability – Partial discharge monitoring supports proactive maintenance, reducing downtime and safeguarding assets and personnel.

Enhancing Transformer Reliability with Partial Discharge Detection

From traditional methods to advanced non-intrusive techniques, such as acoustic, TEV, and UHF methods, partial discharge detection provides asset managers and operators with actionable insights into insulation performance under electrical stress.

Real-time monitoring enables a transition from reactive to predictive maintenance, maximizing asset health, reducing operational risk, and facilitating a resilient and reliable power grid. As the power industry moves toward digitalization, partial discharge detection within transformers is no longer a choice—it is necessary for extending transformer lifespan and protecting critical assets.