Partial Discharge Testing: Principle, Procedure, Equipment and IEC 60270
Introduction to Partial Discharge Testing
Partial Discharge (PD) Testing is an important diagnostic and quality-control technique used in High Voltage Engineering to evaluate the insulation condition of electrical equipment.
Partial discharge is a localized electrical discharge that partially bridges the insulation between conductors or within an insulation system. It does not completely bridge the insulation between conductors.
Partial discharge activity can occur due to insulation defects, air voids, sharp electrode edges, contamination and electrical field concentration.
If partial discharge continues over a long period, it may gradually deteriorate insulation and contribute to electrical equipment failure.
Therefore, Partial Discharge Testing plays an important role in transformer manufacturing, cable testing, switchgear manufacturing, capacitor testing and high-voltage research laboratories.
In this article, we will explain the working principle, types, test setup, measurement procedure, equipment, IEC standards and industrial applications of Partial Discharge Testing.
1. What Is Partial Discharge?
Partial discharge is a localized electrical discharge that partially bridges the insulation between conductors or occurs within an insulating material.
Unlike a complete electrical breakdown, partial discharge does not completely short-circuit the insulation system.
It generally occurs in localized regions where the electric field exceeds the withstand capability of the local insulation.
Common causes include:
Air voids inside solid insulation.
Gas bubbles in liquid insulation.
Surface contamination.
Sharp edges and electrode irregularities.
Improper insulation manufacturing.
Insulation ageing.
Electrical stress concentration.
2. Why Is Partial Discharge Testing Important?
Partial discharge testing helps engineers identify and evaluate localized insulation defects.
Major benefits include:
Detecting insulation defects.
Evaluating insulation quality.
Supporting product quality assurance.
Assessing insulation behaviour under electrical stress.
Supporting condition monitoring.
Improving electrical equipment reliability.
Reducing the risk of unexpected insulation failure.
PD measurements should be interpreted with the equipment design, test conditions and applicable acceptance criteria in mind.
3. Types of Partial Discharge
Partial discharge can be classified according to its physical location and discharge mechanism.
A. Internal Partial Discharge
Internal PD occurs inside solid or liquid insulation systems, such as in cavities or voids.
Examples:
Air voids in epoxy insulation.
Gas cavities in solid dielectric materials.
Defects within insulation structures.
B. Surface Partial Discharge
Surface PD occurs along the surface of insulating materials.
It may be associated with contamination, moisture and high electrical field stress.
C. Corona Discharge
Corona is a gas discharge occurring around conductors where the local electric field is sufficiently high to ionize the surrounding gas.
It may occur around sharp edges, terminals and poorly designed electrode arrangements.
Corona and other gas-discharge phenomena can contribute to measured PD activity, depending on the measurement arrangement.
4. Working Principle of Partial Discharge Measurement
Partial discharge produces short-duration electrical pulses.
These pulses can be detected using an appropriate measuring circuit.
In conventional electrical PD measurement, a coupling capacitor and measuring impedance are connected in a suitable test arrangement.
When a discharge occurs, a transient current pulse flows through the measuring circuit.
The PD measuring instrument processes the signal and determines the apparent charge.
Important PD Parameters
1. Apparent Charge
Apparent charge is the charge that, if injected instantaneously at the terminals of the test object in a specified measurement circuit, would produce the same instrument response as the partial discharge pulse.
It is generally expressed in picocoulombs (pC).
2. Partial Discharge Inception Voltage (PDIV)
PD inception voltage is the voltage at which detectable PD activity begins under the specified test conditions and detection sensitivity.
3. Partial Discharge Extinction Voltage (PDEV)
PD extinction voltage is the voltage at which detectable PD activity ceases during a specified decreasing-voltage procedure.
4. Phase-Resolved Partial Discharge Pattern (PRPD)
PRPD patterns show the distribution of PD pulses with respect to the phase angle of the applied AC voltage.
They help engineers study discharge characteristics and distinguish possible PD sources.
5. Partial Discharge Testing Setup
A conventional IEC 60270 partial discharge measuring system generally includes the following components:
High Voltage AC Test Transformer
Coupling Capacitor
Measuring Impedance (Quadrupole)
Partial Discharge Detector
Test Object
Calibration Pulse Generator
Measuring and Connecting Cables
Shielded Test Environment
Function of Each Component
High Voltage AC Test Transformer: Generates the required AC test voltage for the test object.
Coupling Capacitor: Provides a suitable coupling path for the high-frequency PD measurement signal.
Measuring Impedance: Converts the discharge current pulses into measurable electrical signals.
Partial Discharge Detector: Detects, processes and displays PD measurement signals.
Calibration Pulse Generator: Injects a known calibration charge to establish the relationship between instrument response and apparent charge.
Test Object: The electrical equipment or insulation specimen under investigation.
The exact circuit configuration depends on the test object, measuring system and applicable test procedure.
6. Partial Discharge Testing Procedure
The following is a general procedure for conventional AC PD measurement. The actual test voltage, sequence, duration and acceptance criteria must follow the applicable product standard and test specification.
Step 1: Visual Inspection
Inspect the test object for:
Physical damage.
Surface contamination.
Improper connections.
Visible insulation defects.
Loose terminals.
Step 2: Prepare the Test Area
Ensure the test area is properly arranged.
Verify grounding connections.
Establish safety barriers and interlocks.
Keep unauthorized personnel away.
Follow the approved HV laboratory safety procedure.
Step 3: Connect the Test Circuit
Connect the HV source, coupling capacitor, measuring impedance and PD detector according to the approved test circuit.
Step 4: Calibration of the Measuring System
Use a suitable calibrator to inject a known charge into the test object terminals or specified calibration point.
The calibration establishes the relationship between the detector response and the apparent charge in picocoulombs.
Calibration should be performed for the actual test arrangement as required by the applicable procedure.
Step 5: Apply High Voltage
Increase the test voltage according to the specified voltage sequence.
Observe and record the PD activity at the required voltage levels.
Step 6: Measure Partial Discharge
Record the required parameters, which may include:
Applied voltage.
Apparent charge in pC.
PD inception voltage.
PD extinction voltage.
Phase-resolved PD pattern.
Background noise level.
Test frequency.
Test duration.
Step 7: Analyze the Results
Evaluate the recorded PD activity according to the applicable product standard, manufacturer's specification and agreed acceptance criteria.
Step 8: Discharge and Ground the Test Object
After completing the test, reduce the voltage safely, isolate the supply, discharge the test object and apply grounding according to the approved safety procedure.
7. Partial Discharge Testing in Different Electrical Equipment
A. Power Transformer
PD measurement is important for evaluating insulation quality in power transformers.
It can help identify possible internal insulation defects, including voids, contamination and other localized discharge sources.
IEC 60270 and IEC 60076-3 are important references, together with the applicable transformer test specification.
B. High Voltage Cables
PD measurement can help identify insulation defects and localized discharge activity in cable systems.
It is useful in suitable factory testing and diagnostic applications.
C. Gas-Insulated Switchgear (GIS)
PD measurement can help identify insulation defects in GIS equipment.
Depending on the measurement technique, conventional electrical or non-conventional methods may be used.
D. Capacitors
PD-related testing and dielectric evaluation can be relevant to suitable capacitor insulation systems according to their product requirements.
E. Bushings
Partial discharge measurement can be used to assess suitable bushing insulation systems during specified factory tests and investigations.
8. Conventional and Non-Conventional PD Measurement Methods
Partial discharge activity can be detected using different measurement technologies.
| Measurement method | Basic principle |
|---|---|
| IEC 60270 Electrical Method | Measures electrical PD pulses and apparent charge |
| HFCT Method | Detects high-frequency current pulses |
| UHF Method | Detects ultra-high-frequency electromagnetic signals |
| Acoustic Method | Detects acoustic signals generated by discharge activity |
| TEV Method | Detects transient earth voltage signals on suitable metal enclosures |
Important: These methods are not directly interchangeable. IEC 60270 apparent charge measurements and non-conventional sensor outputs have different measurement principles and units.
9. Important IEC Standards for Partial Discharge Testing
The following standards are useful references for electrical engineers:
IEC 60270: High-voltage test techniques – Charge-based measurement of partial discharges.
IEC 60076-3: Power transformers – Insulation levels, dielectric tests and external clearances.
IEC 60060-1: High-voltage test techniques – General definitions and test requirements.
IEC 60060-2: High-voltage test techniques – Measuring systems.
IEC 62271 Series: High-voltage switchgear and controlgear.
Always check the current applicable edition and the specific product standard before performing a test.
10. Factors Affecting Partial Discharge Measurement
PD measurement results may be influenced by several factors:
Background electrical noise.
Electromagnetic interference.
Grounding arrangement.
Test circuit configuration.
Measuring system sensitivity.
Coupling capacitor characteristics.
Test object capacitance.
Calibration arrangement.
Test voltage and frequency.
Environmental conditions.
A suitable shielded test environment and a properly calibrated measuring system are important for reliable results.
11. Partial Discharge Testing Equipment
| Equipment | Function |
|---|---|
| AC HV Test Transformer | Generates AC test voltage |
| Coupling Capacitor | Couples PD measurement signals |
| Measuring Impedance | Converts current pulses into voltage signals |
| PD Detector | Detects and evaluates PD signals |
| Calibration Generator | Injects known calibration charge |
| Oscilloscope | Displays signal waveforms |
| HFCT Sensor | Detects high-frequency current pulses |
| UHF Sensor | Detects electromagnetic PD signals |
| Shielded Enclosure | Reduces external electromagnetic interference |
12. Skills Required for a Partial Discharge Testing Engineer
An electrical engineer working in a high-voltage testing laboratory should develop knowledge of:
High Voltage Engineering fundamentals.
Partial discharge measurement principles.
IEC 60270 requirements.
High voltage test circuit connections.
Calibration of PD measuring systems.
Electrical noise identification.
PRPD pattern interpretation.
Test report preparation.
Electrical safety and laboratory procedures.
These skills are valuable in transformer manufacturing, capacitor testing, switchgear manufacturing, cable testing and R&D laboratories.
13. Frequently Asked Questions
Q1. What is Partial Discharge Testing?
It is a high-voltage measurement technique used to detect and evaluate localized electrical discharge activity in insulation systems.
Q2. Which IEC standard is used for partial discharge measurement?
IEC 60270 is the principal reference for conventional charge-based PD measurement.
Q3. What is apparent charge in PD testing?
It is the equivalent charge that produces the same measuring system response as the actual discharge pulse, expressed in pC.
Q4. What is the function of a coupling capacitor?
It provides a suitable coupling path for detecting PD pulses in a conventional electrical measuring circuit.
Q5. What is a PD calibrator?
A calibration generator injects a known charge into the measuring circuit to calibrate the PD measurement system.
Q6. What is PD inception voltage?
It is the voltage at which detectable PD activity begins under specified measurement conditions.
Q7. What is the difference between PDIV and PDEV?
PDIV is the voltage at which detectable PD activity starts during voltage increase. PDEV is the voltage at which detectable activity ceases during voltage reduction.
Q8. Can partial discharge testing detect insulation defects in transformers?
Yes. It can help identify and evaluate localized insulation discharge activity, although interpretation and diagnosis require suitable expertise.
Conclusion
Partial Discharge Testing is an important technique in High Voltage Engineering for evaluating electrical insulation and identifying localized discharge activity.
Understanding the PD principle, measurement circuit, calibration, apparent charge, PDIV, PDEV and PRPD patterns helps electrical engineers develop practical knowledge for high-voltage testing laboratories.
Whether you are a student, fresher or experienced electrical engineer, learning partial discharge measurement can strengthen your technical knowledge and professional skills.
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