How to Detect Insulation Aging in Transformer and Reactor Lead-Out Wires?

 

In power systems and industrial applications, transformers and reactors are critical equipment for efficient power transmission and distribution. Over time, the insulation materials of their lead-out wires inevitably age, significantly impacting the safety and reliability of the equipment. The International Electrotechnical Commission (IEC) and IEEE standards explicitly identify insulation aging as one of the primary causes of transformer and reactor failures. This article provides a detailed guide on how to scientifically and effectively detect insulation aging in lead-out wires, covering everything from fundamental principles to practical detection techniques.

Effective insulation aging detection not only prevents equipment failures but also extends service life and reduces economic losses caused by unplanned downtime. With advancements in detection technology, modern diagnostic methods can accurately assess insulation conditions without dismantling equipment, greatly facilitating maintenance. Below, we will explore the causes of insulation aging and introduce various detection methods and standards.

 

Contenido

1. Main Causes of Insulation Aging

Insulation materials undergo multiple aging mechanisms during long-term operation, primarily categorized into thermal aging, electrical aging, mechanical aging, and environmental aging. Understanding these mechanisms is essential for accurate diagnosis, as different types of aging often require distinct detection methods.

● Thermal Aging
Thermal aging is the most common form. According to the Arrhenius principle, the lifespan of insulation materials is exponentially related to temperature. IEEE Std C57.91-2011 states that for every 6°C increase in transformer winding temperature, the lifespan of insulation paper is halved. Thermal aging causes molecular chain breakage in insulation materials, producing low-molecular-weight byproducts and gradually reducing mechanical strength and dielectric performance. Notably, "hot spot" temperatures have a more pronounced impact, as these localized high-temperature areas are often the first to show signs of aging.

● Electrical Aging
Electrical aging includes phenomena like partial discharge and electrical treeing. When defects or air gaps exist in the insulation, uneven electric field distribution can lead to partial discharge in high-field-strength regions. Although the energy of such discharges is small, their long-term effects erode insulation materials and form conductive channels. Research by the International Council on Large Electric Systems (CIGRE) indicates that over 60% of high-voltage transformer failures are related to partial discharge activity.

● Mechanical Aging
Mechanical aging is caused by electromagnetic forces from short-circuit currents and thermal expansion/contraction due to temperature fluctuations. Repeated mechanical stress creates microcracks and delamination in insulation layers, accelerating other aging processes. This is particularly prominent in reactors, which endure significant electromagnetic forces.

● Environmental Aging
Environmental aging refers to the impact of external factors like moisture, oxygen, and contaminants. Moisture reduces oil breakdown voltage and promotes cellulose degradation, while oxygen leads to oil oxidation, producing acidic substances that corrode insulation materials. IEC 60599 specifies limits for moisture and oxidation byproducts in oil-immersed equipment.

Aging Type	Causas primarias	Características típicas	Most Affected Parameters
Envejecimiento termico	Long-term high temperatures	Material brittleness, darkening	Temperature, oxygen content
Envejecimiento eléctrico	High field strength, partial discharge	Carbon traces, tree-like discharge channels	Field strength, dielectric loss
Mechanical Aging	Electromagnetic forces, vibration	Layer separation, cracks	Short-circuit current, fastening force
Envejecimiento ambiental	Moisture, contaminants	Sludge formation, increased acidity	Humidity, contamination level

Table 1: Types of Insulation Aging and Key Characteristics

 

2. Key Detection Methods and Principles

Various methods exist to detect insulation aging in lead-out wires, each with specific applications and advantages. An ideal detection plan combines multiple methods to cross-validate results. Below are some of the most widely recognized and internationally accepted techniques.

● Dielectric Loss Tangent (Tanδ) Test

(1)Concepto:The dielectric loss factor (Tanδ or DF) test is one of the most classic methods for assessing insulation conditions, widely adopted by standards like IEC 60247. This test applies an AC voltage to the insulation and measures the tangent of the phase difference angle (δ) between current and voltage. For an ideal insulator, the current should lead the voltage by 90°, but due to dielectric loss, the phase difference is slightly less.

(2)Principio:Dielectric loss arises from energy dissipation caused by polarization relaxation and conductivity in alternating electric fields. Aging damages the material structure, increasing polarization loss, while aging byproducts (e.g., conductive carbides) raise conductivity loss—both reflected in higher Tanδ values. IEEE Std 286-2000 recommends that Tanδ values exceeding 0.5% for oil-paper insulation warrant attention, while values over 1% indicate severe degradation.

(3)Pruebas:Commonly performed using a Schering Bridge or modern digital dielectric loss testers at power frequency (50/60Hz) or low frequency (0.1Hz). Low-frequency tests are more sensitive to slow polarization processes, making them effective for detecting moisture and aging. Test voltages typically range from 2-10kV, depending on the equipment's rated voltage.

Nota:  Tanδ values are temperature-sensitive and should be corrected to a 20°C reference using the formula:

tanδ₂₀ = tanδₜ × e^(-α(t-20))

where α is the temperature coefficient (~0.017-0.022/°C for oil-paper insulation).

● Partial Discharge (PD) Detection

(1)Concepto:Partial discharge detection, following IEC 60270, is one of the most sensitive methods for diagnosing early-stage insulation defects. PD refers to non-penetrating discharges in localized areas of the insulation system, which, over time, degrade insulation performance.

(2)Methods: Three common techniques are:

–Electrical Method:Measures pulse currents (highest precision but requires shutdown).

–Ultrasonic Method:Detects acoustic waves from discharges (suitable for online monitoring).

–Ultra-High-Frequency (UHF) Method:Detects electromagnetic waves in the 300MHz-3GHz range (also suitable for online monitoring).

(3)Parámetros clave:

–Apparent discharge magnitude (q, in pC).

–Discharge repetition rate (n).

–Discharge inception voltage (Vi) and extinction voltage (Ve).

 

International standards typically limit apparent discharge to ≤10pC at 1.5 times the rated phase voltage. Advanced PD localization techniques combine electrical and ultrasonic signals to pinpoint discharge sources with centimeter-level accuracy, ideal for large transformers and reactors.

● Dissolved Gas Analysis (DGA)

(1)Concepto: For oil-immersed equipment, DGA (based on IEC 60599 and IEEE Std C57.104) is highly effective for monitoring insulation aging. Insulation materials decompose under thermal and electrical stress, producing characteristic gases whose composition and concentration reveal internal insulation conditions.

(2)Key Gases:H₂, CH₄, C₂H₂, C₂H₄, C₂H₆, CO, CO₂, O₂, and N₂. Different aging processes produce distinct gas combinations:

–Partial discharge: H₂ and CH₄.

–Oil overheating (<300°C): CH₄ and C₂H₄.

–Oil high-temperature overheating (>700°C): C₂H₂.

–Solid insulation overheating: CO and CO₂.

 

(3)Critical Ratios:

–Methane ratio (CH₄/H₂).

–Acetylene ratio (C₂H₂/C₂H₄).

–Ethylene ratio (C₂H₄/C₂H₆).

 

Tipo de falla	Gases primarios	Gases secundarios	Rango de relación típico
Descargo parcial	H₂, CH₄	C₂H₂, CO	CH₄/H₂ >0.1
Low-Temp Oil Overheating	CH₄, C₂H₆	H₂, C₂H₄	C₂H₄/C₂H₆ 1-3
High-Temp Oil Overheating	C₂H₄, H₂	CH₄, C₂H₆	C₂H₂/C₂H₄ <0.1
Descarga de arco	C₂H₂, H₂	CH₄, C₂H₄	C₂H₂/C₂H₄ >3

Table 2: DGA Fault Identification Standards (IEC 60599)

Modern DGA techniques include online monitoring using gas chromatography or photoacoustic spectroscopy, providing real-time gas concentration tracking—especially useful for reactors with high field strengths.

 

3. Analysis and Lifetime Assessment

After collecting detection data, scientifically analyzing and assessing insulation conditions is crucial for reliable operation. Internationally, multi-parameter results are compared with historical data, peer equipment data, and standard limits, using mathematical models to predict remaining lifespan.

● Multi-Parameter Diagnosis

Single indicators often fail to fully reflect insulation conditions. A comprehensive evaluation might include:

(1)Elevated Tanδ without increased PD suggests uniform aging.

(2)Normal Tanδ but high PD indicates localized defects.

(3)High CO/CO₂ ratios in DGA point to solid insulation aging.

CIGRE recommends a "scoring system" for comprehensive diagnosis, assigning points based on deviations from baseline values. Advanced systems use fuzzy logic or neural networks to process multi-dimensional data for a holistic assessment.

● Lifetime Prediction Models

Lifetime prediction relies on thermal aging models and cumulative damage theory. The classic Montsinger equation describes the temperature-lifespan relationship:
                                  L = Ae^(-Bθ)
where L is lifespan, θ is temperature, and A/B are material constants.

 

A more precise model from IEC 60076-7 accounts for multiple stresses:
          L = L₀ × 2^[(θ₀-θ)/6] × (1/PD)^n × (1/M)^p
where PD is partial discharge intensity, M is mechanical stress, and n/p are empirical exponents.

 

In practice, "relative lifetime" compares current remaining lifespan to initial lifespan. For example, when Tanδ triples or the degree of polymerization (DP) falls below 200, insulation is considered end-of-life.

 

4. Preventive Maintenance and New Technologies

Scientific preventive maintenance based on detection results can significantly extend equipment lifespan. Internationally, condition-based maintenance (CBM) strategies have been shown to extend transformer/reactor lifespans by over 30% while reducing sudden failures by 60%.

● Targeted Maintenance

Tailor measures to aging type and severity:

(1)Uniform thermal aging:Optimize cooling or reduce load.

(2)Descarga parcial:Locate and repair discharge sources (e.g., vacuum oil filling).

(3)Moisture-induced aging:Dry with vacuum oil circulation and replace desiccants.

(4)Severe aging:Plan replacement or retirement.

 

For reactors, regular checks of fasteners and insulation support integrity are critical due to high field strengths. Standards recommend full mechanical inspections every 3-5 years or after severe short circuits.

 

● Advanced Monitoring

Emerging technologies like IoT and AI are revolutionizing insulation monitoring:

(1)Distributed Temperature Sensing (DTS): Real-time temperature mapping.

(2)High-Frequency Current Transformers (HFCT): Detect nanosecond PD pulses.

(3)Quantum Dot Sensors: Monitor oil moisture and acidity online.

(4)Digital Twin Technology: Virtual real-time equipment mirroring.

Combining these with traditional methods creates a comprehensive monitoring system. For example, one manufacturer reported extending fault from 72 hours to over 30 days using smart monitoring.

 

En resumen

Detecting insulation aging in lead-out wires is a systematic task requiring expertise and specialized equipment. Effective detection should adhere to:

(1)Multi-method integration: Combine Tanδ, PD, DGA, etc., for cross-validation.

(2)Trend analysis: Focus on parameter trends over time, not single data points.

(3)International standards: Follow IEC/IEEE guidelines for scientific judgment.

(4)Embrace new technologies: Smart monitoring enhances efficiency and accuracy.

By applying these methods and staying updated on aging trends and detection technologies, operators can optimize maintenance, ensuring power system reliability and cost-effectiveness.

 

 

Contáctenos

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