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 for coil and insulation condition.

Unequal insulation resistance between coils and each phase.

Possible casing damage.

Check insulation resistance between each phase lead and ground.

No-Load Test

High no-load loss and current value.

Short circuit may occur due to improper assembly of iron core screws or yoke, or grounding piece issues.

Check grounding condition and turn-to-turn short circuits. Use a 1000V megger to measure insulation resistance of iron screws and inspect clamp insulation. If PAC/PAB = PAC4PBC ≤ 25%, it indicates a turn-to-turn short circuit.

Very high no-load loss.

Poor insulation between laminations in the iron core.

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

Large no-load current.

Insufficient silicon steel sheets leading to poor iron core joint assembly.

Observe the iron core joint and measure the cross-section of the core.

Short Circuit Test

High impedance voltage.

Poor connection between components.

Measure DC resistance in sections.

Excessive short-circuit loss.

Break in parallel wire, incorrect transposition, or smaller wire cross-section.

Short the low-voltage side. When high-voltage is connected in Y, apply pressure on AB, BC, and CA lines respectively, and perform three short-circuit tests. Compare results. For Δ connection, short one phase at a time.

Coil Connection Group Measurement

Results do not match any standard connection group.

One coil in a phase has the opposite direction.

Use the connection group measurement method to identify the incorrectly wound coil.