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Views: 0 Author: Site Editor Publish Time: 2025-09-30 Origin: Site
Partial discharge (PD) in oil-immersed power transformers remains a globally recognized challenge in the transformer industry. Numerous manufacturers have suffered significant losses due to PD-related failures.
PD exceedances may occur during factory testing, third-party inspections, or at customer sites. Locating PD sources is often like "finding a needle in a haystack," leading to rework lasting days or even months, causing substantial quality losses for manufacturers or end-users.
Therefore, scientifically diagnosing and rapidly identifying the causes of excessive PD is critical.
While no official definition exists, the author defines PD as:
[Discharge occurring at localized positions within a transformer that has not yet caused immediate insulation breakdown or flashover.]
PD scenarios vary widely but share a common essence:
[Structural, material, or manufacturing defects in the insulation system cause localized electric field distortion exceeding the dielectric strength at that point, resulting in repetitive, micro-scale, non-penetrating ionization breakdown.]
In short, the nature of PD lies in localized electric field concentration exceeding the PD inception field strength.
Based on PD mechanisms, any factor causing excessive localized electric fields may trigger PD exceedances.
3.1 PD Locations
PD may originate from:
Bushings
OLTC/DETC tap changers
Leads
Windings
Grounding components
Insulation surfaces/internal defects
Transformer oil
Most vulnerable sites: Air voids in solid insulation or gas bubbles in oil.
Reason: Under voltage stress, electric field intensity is inversely proportional to dielectric constant (ε).
Paper insulation ε ≈ 4.4
Air voids ε ≈ 2.0
→ Air voids experience ≈2.2× higher field strength.
With low breakdown strength (AC ≈2kV/mm), voids/bubbles become weak points for PD initiation.
3.2 PD Types
Common PD types in oil-immersed transformers:
Gas bubble discharge
Moisture-induced discharge (damp insulation)
Sharp electrode discharge (high-voltage/ground electrode tips)
Floating potential discharge
Wedge-shaped oil gap discharge
Discharge from metallic/contaminant particles
Adhesive defects (excessive/poor-quality glue in clamping plates/end rings)
Key Insight:
PD exceedances are rarely design-related (≈0.5% probability).
95%+ stem from material, process, or manufacturing defects.
Rationale: When overvoltages (LI, LIC, SI, LTAC) are converted to equivalent 1-min power-frequency withstand voltage (DIL conversion), all exceed PD test voltage (IVPD). Main/longitudinal insulation is designed for the highest overvoltage scenario.
| No. | PD Type | Location | Mechanism | Common Cases |
1 | Sharp Electrode Discharge | Clamping parts, tank, rising bushing, lead crimping terminals | Small curvature radius → high charge density → extreme field concentration | Unshielded bolts near HV electrodes; sharp edges on magnetic shielding |
| 2 | Gas Bubble/Void Discharge | Bubbles in oil / voids in solid insulation | Low dielectric constant (ε≈1) → high field stress + low breakdown strength (2kV/mm) | Incomplete vacuum; fast oil filling; excessive/poor adhesive in end rings/equalizing spheres |
| 3 | Moisture-Induced Discharge | Windings, core insulation, leads | Moisture reduces dielectric strength by 60-70% | Inadequate core drying; overexposure to ambient air during assembly |
| 4 | Floating Potential Discharge | Pressboard, lead supports, magnetic shunts | Charge accumulation → sudden discharge pulse | Ungrounded magnetic shielding; poorly connected electrostatic rings |
| 5 | Contaminant Discharge | Water/fibers/metal particles in oil | Field distortion + water increases field stress 2.9× | Inadequate oil filtration; contaminated core; moisture ingress |
Understanding common PD types, mechanisms, locations, and case studies is essential for targeted troubleshooting.
Combined with transformer connection principles, structural design, PD waveform characteristics, polarity localization, and diagnostic tests, this knowledge enables rapid root-cause identification and minimizes quality losses.
