ELEC5205 Chap.7 Partial Discharge Measurement
Partial Discharge Measurement
Measure an equivalent terminal effect
Partial discharge is local activity that does not bridge the full insulation path. The hidden charge movement cannot be observed directly at equipment terminals, so calibration defines apparent charge as the injected terminal charge that creates the same measured voltage change.
Filtering suppresses the large power-frequency component, while phase-resolved patterns, repetition and multiple sensors support a qualified diagnosis of void, surface or corona activity.
Calibrate the complete measurement path
A known charge pulse is injected through a calibration capacitance using a known voltage step.
Its response travels through the test-object capacitance, coupling network, measuring impedance, filter, cable and instrument. The ratio between known injected charge and displayed amplitude becomes the scale for later events. Because changing gain, bandwidth, cable length or coupling configuration changes that path, calibration belongs to the assembled setup rather than to the instrument name alone.
Record settings with the result.
Keep apparent charge and defect charge distinct
The terminal measurement cannot reconstruct the exact microscopic charge displaced inside an inaccessible void. Apparent charge is an equivalent input defined by the same terminal voltage change.
It is useful because it supports repeatable comparison under a stated circuit and standard method, not because it reveals every internal dimension.
A strong interpretation reports polarity, magnitude, repetition, phase position and noise controls, then labels the proposed source as a hypothesis rather than a directly observed cavity.
Read patterns through mechanism and alternatives
Internal void discharge, surface activity and external corona can occupy different phase regions and show different pulse populations, but pattern recognition is not unique.
Supply distortion, electromagnetic interference, multiple simultaneous sources and changing inception voltage can alter the display. Compare channels, move or shield a sensor, change voltage deliberately, and use acoustic or ultra-high-frequency evidence when appropriate.
A diagnosis becomes stronger when an alternative predicts a different response to one of those controlled changes.
Connect detection to condition decisions
Trend is meaningful only when voltage, temperature, sensor position, bandwidth and calibration remain comparable. A larger apparent-charge value can reflect a changed coupling path as well as a changed defect.
Report inception and extinction behaviour where available, distinguish sporadic interference from phase-related repetition, and relate the evidence to insulation type and operating stress. The final recommendation may be repeat measurement, localisation, increased monitoring or outage inspection; its urgency should be proportional to repeatable evidence, not to one striking pulse.
What this chapter covers
- 01
Apparent Charge
- 02
Calibration Pulse
- 03
Coupling Network
- 04
Phase Pattern
- 05
UHF Detection
- 06
Acoustic Location
Worked example · free
Scale a calibrated pulse
- 2Calculate calibration charge from capacitance times voltage step.
- 2Scale by the unchanged pulse-amplitude ratio.
- 2Interpret the result as apparent terminal charge.
Key terms
- Apparent Charge
- Apparent Charge is the terminal-injected charge that would create the same measured voltage response as a discharge event.
- Phase-Resolved Pattern
- Phase-Resolved Pattern maps discharge magnitude and occurrence against AC phase over many cycles.
- Inception Voltage
- Inception Voltage is the applied voltage at which repeatable partial-discharge activity first appears under the test conditions.
Partial Discharge Measurement FAQ
Why is calibration required for partial-discharge measurement?
The measuring circuit, filter, amplifier and test-object capacitance transform the fast discharge pulse. A known injected charge establishes the relationship between displayed response and apparent charge for that exact measurement chain.
What does phase position reveal?
It shows when the local field exceeds inception and when activity extinguishes on each AC half-cycle. Polarity symmetry, phase bands and repetition can support a mechanism hypothesis, but they remain sensitive to environment and interference.
Which sensors can complement electrical detection?
Acoustic sensors can help locate a source, ultraviolet imaging can show exposed corona, UHF sensing suits enclosed resonant structures, and current transformers detect pulses in earth paths. Access and interference determine which method adds useful evidence.
Exam move
Write the apparent-charge definition without using the phrase actual defect charge. Recalculate one pF-times-volt calibration and explain why the amplitude ratio requires unchanged settings. Compare void and point-corona phase patterns, then name one complementary sensor that could test the proposed location.
Build a measurement record containing injection capacitance, voltage step, gain, bandwidth, coupling path, sensor position and applied voltage. Practise separating observation from inference: first describe magnitude, polarity, phase and repetition, then propose a source and an alternative. End each diagnosis with one controlled change whose response would distinguish equipment discharge from interference.
Preserve raw traces and operating conditions so a later reader can challenge the classification instead of receiving only the final defect label.
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