Test items and methods for transformer fault inspection - Database & Sql Blog Articles

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Test Results

Cause of Issue

Inspection Method

Insulation Resistance Measurement (using a 1000-2500 V megohmmeter)

Coil-to-Coil

Coil-to-Ground

Insulation resistance is zero

A short circuit exists between the coil and ground or between coils.

Disassembly Check: Inspect coil and insulation condition

Unequal insulation resistance between coils and each phase

Potential casing damage may be present

Check insulation resistance of each phase lead to ground

No-Load Test

High no-load loss and current values observed

Iron core screws or yoke screws may have a short circuit with the core, or improper grounding can cause a short circuit

Inspect grounding condition and check for turn-to-turn short circuits. Use a 1000V megger to measure insulation resistance of the iron screw and check clamp insulation. When a phase is shorted, PAC/PAB = PAC4PBC ≤ 25%. If not, there's a turn-to-turn short circuit

Very low no-load loss

Poor insulation between laminations in the core

Use DC voltage and current method to measure insulation resistance between laminations

Large no-load current

Inadequate silicon steel sheets or poor core joint assembly

Observe core joints and measure cross-section of the core

Short-Circuit Test

High impedance voltage

Poor electrical connections

Measure DC resistance in sections

Excessive short-circuit loss

Parallel wire breakage, incorrect transposition, or smaller wire cross-section

Short the low-voltage side. For high-voltage Y connection, apply pressure on AB, BC, and CA lines respectively for three tests. Compare results. For high-voltage delta connection, short one phase at a time

Coil Connection Group Measurement

Result does not match any standard connection group

One coil in a phase has the opposite direction

Use connection group measurement method to identify and correct the faulty coil