Partial Discharge in Oil-Immersed Transformers: Nature and Common Causes of Excessive PD Levels

01 Introduction

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.

02 Definition and Nature

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.

03 Primary Causes

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

04 Outlook

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.