What is partial discharge test in transformer?

1. What is Partial Discharge (PD)?

To understand the test, you must first understand the phenomenon it detects.

Definition: Partial Discharge is a localized electrical discharge that only partially bridges the insulation between conductors. It can occur in voids (gas pockets) within solid insulation, at the boundaries between different insulating materials, or in bubbles within liquid insulation (oil).

The Problem it Causes: Although each individual PD is a very small spark, it erodes the insulation over time. The energy from these discharges breaks down the chemical bonds in the insulating material (paper, oil, resin) through:

Thermal Stress: Heat from the spark.

Chemical Degradation: Producing ozone and nitrous acids (in air) or hydrogen and methane (in oil).

Physical Erosion: Literally burning away tiny pits in the material.
This slow, cumulative damage progressively weakens the insulation until it can no longer withstand the electrical stress, leading to a complete puncture (breakdown) and a catastrophic transformer failure.

2. Why is the PD Test So Important?

Early Fault Detection: It is one of the most sensitive tests to detect incipient (developing) faults in the insulation system at a very early stage, often years before a failure.

Prevents Catastrophic Failures: By identifying weak insulation early, maintenance or repair can be scheduled proactively, avoiding unplanned outages, expensive repairs, and potential safety hazards.

Quality Assurance: It is a mandatory routine test for new transformers before they leave the factory, ensuring they meet the highest quality standards and are free from major manufacturing defects.

Assesses Condition During Service: For transformers in the field, periodic PD testing is a key part of a condition-based maintenance program, helping to determine remaining life and the need for oil processing or drying-out.

3. How Does a Partial Discharge Test Work?

The test is performed by applying a voltage higher than the transformer's normal operating voltage to its windings, but lower than its power-frequency withstand test voltage. The goal is to stress the insulation enough to excite any existing defects without causing new damage.

The core principle is that when a PD occurs, it generates three measurable phenomena almost simultaneously:

Electrical Pulses: A very fast, small-current pulse (nanoseconds to microseconds) flows in the test circuit.

Electromagnetic Waves: The pulse emits electromagnetic energy, including radio frequency (RF) signals.

Acoustic Waves: The tiny spark creates an ultrasonic "click" or sound wave within the transformer tank.

The test setup involves:

A High-Voltage Source: To energize the transformer.

Coupling Capacitor: Provides a low-impedance path for the PD pulses to be measured.

PD Detector/Measurement System: A sophisticated instrument that captures, filters, and analyzes the pulses.

Sensors: These can be:

Electrical Sensors: To measure the current pulses directly.

High-Frequency Current Transformers (HFCT): Clamped around the grounding connection to pick up the RF signals.

Ultra-High Frequency (UHF) Sensors: Antennas placed inside the transformer (through oil valves) or externally to detect electromagnetic waves in the GHz range. This is very popular as it is immune to external electrical noise.

Acoustic Emission (AE) Sensors: Microphones placed on the outside of the transformer tank to "listen" for the ultrasonic sounds of the PD. This is excellent for locating the source of the PD.

4. Key Measurements and Interpretation

The PD detector doesn't just detect the presence of a discharge; it quantifies it with several key parameters:

Apparent Charge (q): The most important quantity. It is expressed in PicoCoulombs (pC). Since the actual discharge is inside the insulation and cannot be measured directly, the "apparent charge" is a calibrated measure of the charge that, if injected instantaneously between the transformer terminals, would have the same effect as the internal PD. Lower values are better.

PD Inception Voltage (PDIV): The voltage at which PDs first start to occur as the test voltage is raised.

PD Extinction Voltage (PDEV): The voltage at which PDs cease as the test voltage is lowered.

Interpretation: Engineers don't just look for the presence of PD; they analyze the pattern, magnitude (in pC), and phase relationship of the pulses. Different types of defects (voids, surface discharges, floating components) produce unique "fingerprints" on the voltage waveform.

Standards and Acceptable Limits

International standards (like IEC 60270) define the test procedures and acceptable PD levels. For a new transformer, the typical acceptance criterion is that the PD level, under a specified test voltage, should not exceed ****

100 pC to 500 pC

, with 100 pC being a common and very strict limit for high-voltage equipment. For transformers in service, higher levels might be tolerated with a plan for monitoring and intervention.