Testing and Treatment Methods for Transformer Core Faults

1. Testing Methods for Transformer Core Faults

The general testing methods for transformer core faults are as follows:
(1)Clamp Ammeter Method (On-line Measurement)

Applicable to transformers with external core grounding leads, enabling accurate non-stop testing for multi-point grounding faults.

Operation: Regularly measure the grounding lead current annually (should be below 100 mA). If the current exceeds this value, enhance monitoring.

Judgment Criteria:

If the grounding current exceeds 1 A or significantly increases compared to the initial value (measured continuously after commissioning), it may indicate low-resistance or metallic grounding faults, requiring urgent handling.

A large initial resistance suggests high magnetic leakage in the transformer; slight subsequent changes indicate no fault.

(2)Chromatographic Analysis (Live Oil Sampling)

Method: Sample oil for chromatographic analysis.

Judgment Criteria:
- 显著增加的总烃 (Total Hydrocarbons) with methane and ethylene as main components, while carbon monoxide and carbon dioxide remain unchanged, indicate bare metal overheating (possibly multi-point grounding or damaged insulation between silicon steel sheets).

The appearance of acetylene in total hydrocarbons suggests unstable, intermittent multi-point grounding.

(3)Insulation Resistance Method (Off-line Testing)

Tool: 2500V megohmmeter to measure resistance between core and casing.

Judgment Criteria:

Insulation resistance ≥ 200 MΩ: Good core insulation.

Zero resistance (or continuity): Use an ohmmeter for further measurement:

200–400 Ω: High-resistance grounding fault, requiring treatment.

1000 Ω: Low grounding current; can continue operation with regular on-line monitoring (e.g., clamp ammeter, oil chromatography) and handle when anomalies occur.

1–2 Ω: Metallic grounding fault, mandatory treatment.

2. Treatment Methods for Transformer Core Multi-point Grounding

Common treatment methods for core multi-point grounding:
(1)Series Resistance in Grounding Loop: For cores with external grounding leads, connect a resistor in series to limit the grounding current (temporary emergency measure).
(2)Debris Removal via Tank Lifting: For faults caused by metal foreign objects, inspect by lifting the tank to locate and remove debris.
(3)Treatment for Faults Caused by Core Burrs or Metal Powder Accumulation:
1)Capacitor Discharge Impact Method
2)AC Arc Discharge Method
3)High-Current Impact Method (e.g., Welding Machine)

3. Quality Standards for Power Transformer Core Maintenance

Core Structure:

The core should be flat, with intact insulating paint, tightly stacked laminations, and no warped or wavy silicon steel sheets at the edges.

Surfaces should be free of oil stains and impurities, with no short circuits or overlaps between laminations, and joint gaps meeting requirements.

Insulation Between Core and Components:

Maintain good insulation between the core and upper/lower clamping pieces, square irons, pressure plates, and bottom plates.

Steel Pressure Plates:

Ensure uniform gaps between steel pressure plates and the core.

Insulating pressure plates should be intact, crack-free, and properly fastened.

Steel Plate Grounding:

Steel pressure plates must not form a closed loop and should be grounded at one point.

Insulation Resistance Testing:

After disconnecting the connection plates between the upper clamping piece and core, and between the steel pressure plate and upper clamping piece, measure insulation resistance. Values should not significantly differ from historical data.

Bolt Fastening:

Bolts should be tight. Positive/negative pressure nails and lock nuts on clamping pieces should be free of looseness, with good contact to insulating washers and no discharge burn marks. Negative pressure nails should maintain sufficient distance from the upper clamping piece.

Through-Core Bolts:

Fastened tightly, with insulation resistance consistent with historical records.

Oil Paths:

Oil channels should be unobstructed, with no fallen or blocked spacers, and neatly arranged.

Core Grounding:

Only single-point grounding is allowed. The grounding strip should be a 0.5 mm thick, ≥30 mm wide copper strip inserted 3–4 layers into the core (insertion depth ≥80 mm for large transformers). The exposed part must be insulated to prevent core short circuits.

Mechanical Strength and Insulation:

Components should be firmly fixed, have sufficient mechanical strength, good insulation, and not form loops or contact the core.

Grounding Reliability:

Ensure good insulation and reliable grounding.

Note: Adhering to these standards during maintenance can effectively prevent core faults and ensure transformer safety and stability.