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The windings and core are crucial components for transmitting and transforming electromagnetic energy in a transformer. Accurate assessment, detection, and rectification of core faults are essential to ensure normal transformer operation. One of the most common and dangerous issues is transformer core multiple grounding fault, which can cause localized overheating, oil decomposition, and even catastrophic failure. This guide explains the hazards, causes, detection methods, and repair procedures for this critical problem.
During normal transformer operation, the core must have only one grounding point. An alternating magnetic field exists around the windings, and parasitic capacitance creates a floating potential on the core. To eliminate intermittent spark discharges that would otherwise degrade the oil and insulation, we connect the core to the shell with a single grounding point.
However, when two or more grounding points appear on the core or other metal components, they form a closed loop. This closed circuit allows circulating current to flow, causing:
Therefore, a single grounding point is mandatory – multiple grounding points are a serious fault that requires immediate attention.
The main causes include the following:
| Cause Category | Specific Examples |
|---|---|
| Poor construction techniques | Improperly designed grounding strips; short circuits due to design flaws |
| Accessories & external factors | Foreign objects; transport stabilizer pins left in place |
| Manufacturing defects | Core burrs, rust, welding slag; rough core finishing |
| Foreign objects | Metal debris left inside the tank (screwdrivers, copper wires) |
| Insulation degradation | Moisture, sludge deposition, or insulation damage in clamps and pads |
| Mechanical wear | Pump shaft bearing wear releasing metal powder into the tank |
| Poor maintenance | Failure to perform periodic inspections as scheduled |
Common faults include the following six types:
We can detect core faults using three primary methods – each suited to different operational conditions.
For transformers with externally led core grounding lines, we use the clamp ammeter method for accurate, non‑stop testing.
Procedure:
We analyze oil samples for dissolved gases. The gas composition reveals the nature of the fault:
| Gas Signature | Indication |
|---|---|
| Total hydrocarbons significantly increased; methane + ethylene dominate; CO/CO₂ unchanged | Bare metal overheating – likely core multiple grounding or inter‑lamination insulation damage |
| Acetylene appears in total hydrocarbons | Intermittent, unstable core multiple grounding fault |
Therefore, regular chromatographic analysis helps us track core condition without shutting down the transformer.
We use a 2500‑volt megohmmeter to measure resistance between the core and the shell.
| Measured Resistance | Interpretation | Action |
|---|---|---|
| ≥ 200 MΩ | Good core insulation | No action needed |
| 200 – 400 Ω | High‑resistance grounding points | Treatment required |
| 1000 Ω (with small current) | Minor issue | Continue operation with enhanced monitoring |
| 1 – 2 Ω | Metal grounding points | Immediate treatment required |
Based on the fault type and severity, we apply the following treatment methods:
For transformers with external grounding lines, we can connect a resistor in series on the grounding circuit to limit circulating current. This is only an emergency measure – not a permanent solution.
When metal foreign objects cause the fault, a hood inspection typically reveals the problem. We open the transformer cover and visually inspect for debris, then remove it.
For grounding faults caused by core burrs or accumulated metal powder, the following methods have shown good results:
These methods burn off or displace the conductive particles without requiring full disassembly.
During overhaul, we ensure the core meets the following quality standards:
| Item | Quality Requirement |
|---|---|
| Core flatness | Flat, no peeling of insulation paint, tight laminations |
| Side silicon steel sheets | Not raised or wavy |
| Core surfaces | Free of oil and impurities; no short circuits or overlaps between laminations |
| Joint gaps | Meet design requirements |
| Upper/lower clamps & yoke insulation | Good insulation between core and all clamp components |
| Steel pressure plate | No closed loop; single grounding point; uniform gap with core |
| Insulation pressure plate | Intact, no damage/cracks, properly fastened |
| Through‑core bolts | Securely fastened; insulation resistance consistent with previous tests |
| Oil passages | Unobstructed; pads not loose or blocked; neatly arranged |
| Single grounding point | One copper grounding piece (0.5mm thick, ≥30mm wide), inserted into 3‑4 core levels, exposed part wrapped in insulation |
| Overall grounding | Good insulation and reliable grounding; no closed loops |
| Fault Type | Detection Method | Action |
|---|---|---|
| Metal grounding (1‑2 Ω) | Insulation resistance | Immediate treatment (hood inspection or electrical impact) |
| High‑resistance grounding (200‑400 Ω) | Insulation resistance | Schedule treatment |
| Intermittent discharge | Chromatography (acetylene present) | Enhanced monitoring, plan outage |
| Bare metal overheating | Chromatography (hydrocarbons high) | Investigate core grounding or lamination insulation |
Contact a Xinghe representative today to learn more about our Transformer.
