How to Detect Damaged Wire Insulation?

—A Guide to Preventing Winding Short Circuits

Winding short circuits in transformers and reactors are a common cause of equipment failure, and damaged insulation is often a precursor to such faults. Effectively detecting wire insulation damage and implementing preventive measures are critical for the maintenance of electrical equipment. This article provides a detailed guide on insulation damage detection methods, preventive measures, and international standards to help you reduce the risk of winding short circuits and extend equipment lifespan.

Content

1. Why Does Damaged Wire Insulation Lead to Winding Short Circuits?

Transformer windings are made of conductive materials (such as copper or aluminum) and are coated with insulation to prevent inter-turn or inter-layer short circuits. Common insulation materials include Nomex® paper, epoxy resin, or polyimide film, which offer high-temperature resistance, chemical corrosion resistance, and high dielectric strength.

When insulation is damaged, exposed conductors can cause:

(1)Partial Discharge (PD): Ionization occurs at weak insulation points under high electric fields, leading to further insulation deterioration over time.

(2)Inter-Turn Short Circuits: Direct contact between adjacent conductors due to insulation failure creates a low-impedance loop, causing localized overheating or even burnout.

(3)Ground Faults: If conductors come into contact with the core or enclosure, grounding faults may occur, triggering equipment shutdowns.

Therefore, regular inspection of insulation condition is essential.

2. How to Detect Damaged Wire Insulation?

● Visual Inspection

(1)Applicable Scenario: During equipment power outages.

(2)Method:

– Use high-intensity light sources and magnifiers to check for cracks, peeling, or discoloration on the winding surface.

– Inspect insulation paper or film for mechanical damage (e.g., scratches, folds).

(3)Advantages: Simple, intuitive, and low-cost.

(4)Limitations: Only detects visible external damage; internal defects remain undetected.

● Insulation Resistance Test (IR Test)

(1)Standards:IEC 60076 / IEEE 43

(2)Method: Apply a 500V or 1000V DC voltage between the winding and ground using a megohmmeter (Megger) to measure insulation resistance (unit: MΩ).

(3)Abnormal Indicators:

-Insulation resistance below standards or a >50% drop compared to historical data indicates deterioration.

-Absorption ratio (60s/15s resistance) <1.3 suggests moisture ingress or localized damage.

● Partial Discharge Detection

(1)Principle: Micro-discharges at insulation damage points emit high-frequency electromagnetic waves and ultrasonic signals.

(2)Equipment:

-High-frequency current transformer (HFCT)

-Ultrasonic sensor (AE Sensor)

(3)Standard:IEC 60270 (PD testing requirements)

(4)Judgment Criteria:

-PD magnitude <10 pC (new equipment)

-PD magnitude >100 pC (potential severe defects)

(5)Advantages:Enables online monitoring and precise fault location.

● Frequency Domain Spectroscopy (FDS)

(1)Principle:Measures the dielectric response of insulation materials across frequencies (0.001Hz–1kHz) to assess aging.

(2)Formula:

   Where:

tanδ: Dielectric loss factor

ε'': Loss dielectric constant

ε': Storage dielectric constant

(3)Interpretation:

– A rising tanδ with frequency indicates moisture or degradation.

– A >20% deviation from baseline data warrants further inspection.

3. How to Prevent Insulation Damage?

● Material Selection: Use High-Stability Insulation Materials

The primary preventive measure is selecting high-performance insulation materials compliant with international standards. Premium materials like DuPont Nomex® aramid paper (220°C rating) and polyimide film (>300°C rating) resist thermal and chemical degradation, delaying insulation aging. These materials should meet UL 1446 and IEC 60505 certifications. For harsh environments (e.g., offshore wind transformers), fluororubber-coated insulation can combat salt spray corrosion.

● Temperature Control: Optimize Cooling and Real-Time Monitoring

Controlling operating temperatures is critical to preventing thermal aging. Per Arrhenius' law, insulation degradation accelerates exponentially with temperature. For example:

(1)Oil-immersed transformers: Top oil temperature ≤95°C (IEEE C57.91).

(2)Dry-type transformers: Winding temperature ≤155°C (IEC 60076-11).

Measures:

(1)Install PT100 or fiber-optic temperature sensors for real-time hotspot monitoring.

(2)Enhance cooling systems (e.g., forced oil circulation or air cooling).

(3)Clean radiators regularly to maintain efficiency.

(4)For overload-prone transformers, deploy smart thermal systems with predictive models.

● Mechanical Protection: Minimize Stress and Vibration Damage

Mechanical stress—from short circuits or transportation—can damage insulation. Mitigation strategies include:

(1)Structural Design: Use epoxy-impregnated fiberglass spacers and axial clamps (IEC 60076-5) to withstand electromagnetic forces.

(2)Process Optimization: Apply vacuum pressure impregnation (VPI) to boost mechanical strength by 30% (per IEC 61378).

(3)Transport/Installation: Use vibration accelerometers (<3g threshold) and anti-shock pads to prevent resonance damage.

● Technology Integration and Standards Compliance

(1)Material-Temperature Synergy: High temperatures reduce mechanical strength; thus, heat-resistant materials must pair with cooling measures.

(2)Quantitative Standards:Short-circuit withstand tests (IEC 60076-5) require no visible post-fault deformation.

(3)Smart Monitoring: IoT-based multi-parameter analysis (vibration + temperature + PD) enables early warnings.

This multi-layered approach can systematically prevent insulation damage, extending equipment life by 10+ years.

International Standards and Best Practices

In Summary

Detecting insulation damage requires a combination of methods—from visual checks to PD monitoring—for comprehensive risk coverage. Preventive measures like premium materials, temperature control, and mechanical protection are equally vital. Adhering to IEC and IEEE standards maximizes reliability and lifespan.

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