What are the types of transformer partial discharges? Interpretation of internal discharges, discharges along the surface and corona discharges
Date: May 21, 2026 14:12:02
- Internal discharge: Occurring within air gaps or impurities in insulating materials, this is the most common type of partial discharge. The discharge is distributed across both the positive and negative halves of the cycle, and the PRPD spectrum exhibits a symmetrical elliptical distribution.
- Surface discharge: This occurs along the interface between the insulating material and the oil and is more harmful than internal discharge, as the discharge path continuously erodes the insulating surface, leaving carbonized marks.
- corona discharge: Discharge caused by the concentration of electric fields at metal tips or sharp corners, occurring near the peak of the power-frequency voltage, serves as an indicator of electric field design flaws.
- Corona discharge: Occurs on metal components that are not properly grounded; the discharge energy is high, posing the greatest hazard; the characteristic spectrum features a strong signal concentrated within a specific phase range.
1. The Four Basic Types of Partial Discharge
| Discharge Type | Location | Hazard Level | PRPD Spectral Characteristics | Typical Causes |
|---|---|---|---|---|
| Internal discharge | Air gaps within insulating materials | moderate | Symmetrically distributed across half a cycle, with moderate amplitude | Manufacturing defects, insulation aging |
| Surface discharge | The Interface Between Insulating Materials and Oil | mid-to-high | Wide distribution range and large phase span | Moisture in the insulation, surface contamination |
| corona discharge | Metal tips or sharp edges | relatively low | Concentrated near the voltage peak, with a lower amplitude | Structural design flaws |
| Corona discharge | Ungrounded metal parts | your (honorific) | Concentrated in a specific phase range, with high amplitude | Poor grounding, loose components |
2. Detailed Characteristics of Various Types of Discharges
2.1 Internal Discharge—The Most Common Form of Partial Discharge
Internal discharge occurs within microscopic air gaps or around impurities within the insulating material. Since the dielectric constant of gas is lower than that of the surrounding insulating material, the electric field concentrates at the air gaps; when the electric field strength exceeds the breakdown voltage of the air gap, discharge occurs. Internal discharge is characterized by: generally low discharge levels, slow development, and symmetrical occurrence during both positive and negative half-cycles. Prolonged internal discharge causes the air gap to gradually expand, potentially penetrating the entire insulation layer.
2.2 Surface Discharge—Erosive Discharge on Insulating Surfaces
Surface discharge develops along the surface of insulating materials and is an advanced form of internal discharge. When the carbonized path created by internal discharge extends to the surface of the insulation, it transforms into surface discharge. Surface discharge causes far greater damage to insulating paperboard than internal discharge—the high temperatures of the discharge channel leave permanent carbonized marks (dendritic discharge marks) on the insulating surface, which continuously reduce the insulation strength.
2.3 Corona Discharge—Lessons from Electric Field Design
Corona discharge occurs at sharp tips or edges on the surface of metal conductors because the electric field is highly concentrated at these locations. While the energy of the corona discharge itself is not high and poses little direct threat to insulation in the short term, it serves as an important indicator of a design flaw—suggesting that there is a problem with electric field concentration inside the transformer. Over time, corona discharge also generates ozone and nitrogen oxides, accelerating the aging of insulating oil and insulating materials.
2.4 Floating Discharge—The Most Dangerous Form of Partial Discharge
When metal components inside a transformer (such as clamps, shielding rings, bolts, etc.) are not properly grounded due to loosening or installation errors, these components become floating potential bodies. Under the influence of an alternating electric field, discharges occur between the floating components and adjacent grounded parts. The energy level of floating discharges is significantly higher than that of the other three types, and they tend to rapidly develop into arc discharges, constituting an emergency situation that requires immediate shutdown and resolution.
3. How can discharge types be distinguished based on their characteristics?
3.1 Characteristics of Phase Distribution
There are significant differences in the phase distribution of various discharge types across the power-frequency cycle. Internal discharges are distributed across the first and third quadrants of both the positive and negative half-cycles; corona discharges are concentrated near the voltage peak; and suspension discharges are concentrated within a specific phase range. Phase distribution is the primary basis for determining discharge types in PRPD spectra.
3.2 Amplitude and Repetition Rate Features
Corona discharges have low amplitude but high repetition rates—they occur near the peak of each power-frequency cycle. Stray discharges have high amplitude but potentially lower repetition rates—depending on the time constants of the charged and discharged particles. The amplitude and repetition rate of internal discharges fall between the two and evolve gradually over time.
3.3 Characteristics of Trend Evolution
Internal and surface discharges typically develop gradually, with a trend toward a gradual increase in discharge amplitude or frequency. Floating discharges may occur suddenly (due to mechanical loosening or transport shocks), and should be treated with the utmost caution once they occur. Corona discharges are highly sensitive to voltage fluctuations and may be more pronounced during periods of high-voltage operation.
4. Frequently Asked Questions FAQ
4.1 Q: How can I quickly distinguish between internal discharge and corona discharge?
A: Look at the phase distribution in the PRPD spectrum. Corona discharge is concentrated near the peaks of the power-frequency voltage (around 90° and 270°), with asymmetry between the positive and negative half-cycles. Internal discharge is distributed relatively evenly across the positive and negative half-cycles. This is the most intuitive way to distinguish between them.
4.2 Q: How long does it typically take for surface discharge to develop into breakdown?
A: There is no fixed timeline; it depends on the intensity of the discharge, the type of insulating material, and the operating environment. Under continuous, high-intensity surface discharge, insulating paperboard may progress from localized carbonization to complete breakdown within a few months to one or two years. Therefore, once it is confirmed that surface discharge is progressing, maintenance should not be delayed.
4.3 Q: Is it easy to confuse floating discharge and internal discharge based on the signal?
Answer: It is not easy to confuse the two. The amplitude of suspension discharges is typically much greater than that of internal discharges, and they are concentrated within a specific phase range. If a signal with high amplitude and a narrow phase distribution appears on a PRPD spectrum, the first diagnosis should be a suspension discharge.
4.4 Q: Does corona discharge generally require treatment?
Answer: Minor corona discharge can be tolerated for a short period, but it should be documented and included in the maintenance schedule. If the intensity of the corona discharge continues to increase or is accompanied by other types of discharge, it must be investigated as soon as possible. Corona discharge can accelerate the degradation of insulating oil, and if left unaddressed for an extended period, it can compromise the overall insulation performance of the transformer.
4.5 Q: If acetylene is detected in oil chromatography and discharges are observed during partial discharge monitoring, what is the sequence of these events?
Answer: Partial discharge monitoring detects discharge signals earlier than oil chromatography. By the time acetylene appears in the oil, the discharge has already reached a certain energy level. Therefore, partial discharge monitoring provides an earlier warning window for discharge-related faults than oil chromatography.
5. Summary
Understanding the types and characteristics of partial discharges is essential for accurately interpreting partial discharge monitoring data. Different types of discharges require different response strategies—corona discharges can be addressed through scheduled maintenance, surface discharges require close monitoring, and floating discharges should be addressed immediately. Partial discharge monitoring systems automatically identify discharge types using PRPD spectra, providing operations and maintenance personnel with accurate basis for judgment.
Disclaimer: The content of this article is for technical exchanges and reference only, and does not constitute any form of procurement commitment or contract offer. Product technical parameters, configuration programs and prices are subject to the actual signed contracts and technical agreements. The technical data and cases involved in this article are from public information and engineering practice, if updated without notice.
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