Online monitoring system for microwater in transformer oil: technology, data and diagnostics

In the insulation system of a power transformer, insulating oil and insulating paper (cellulose) form the oil-paper composite insulation. Moisture (H₂O) is one of the most hazardous impurities in this system, and accurate monitoring of its content is a core aspect of evaluating the health of transformers, preventing insulation breakdown accidents, and extending the service life of the equipment. The on-line monitoring system for microwater in transformer oil is the key preventive maintenance (PdM) technology to realize this goal.

Part I: Hazardous mechanism of moisture on oil-paper insulation system

Moisture damages the transformer insulation system in many ways, with the core hazards being accelerated insulation aging and reduced insulation strength.

1. Accelerated aging of solid insulation (insulation paper): Water is the main catalyst for cellulose hydrolysis reactions. Each cellulose molecular chain break consumes one water molecule. The higher the moisture content, the faster the rate of hydrolysis reaction, resulting in a sharp decrease in the degree of polymerization (DP) of the insulating paper, a decrease in mechanical strength, and ultimately brittleness and failure.

2. Reduces the breakdown voltage of the insulating oil: The water dissolved in the oil has a small effect on the breakdown voltage, but when the water reaches saturation and exists in the form of free water (tiny water droplets), the breakdown voltage of the oil will drop significantly. Under the action of the electric field, the water droplets will be polarized and elongated, and it is very easy to form a conductive path, triggering the breakdown.

3. Induced Partial Discharge (PD): Moisture reduces the starting discharge voltage of insulation materials. Especially at the oil-paper interface and in the tiny air gaps of the insulation, the presence of moisture significantly increases the risk of partial discharges.

4. "Bubble effect" risk: When the transformer load is suddenly increased or the temperature rises sharply, the water in the insulating paper evaporates rapidly but cannot be dissolved into the oil in time, resulting in the formation of gas bubbles on the winding surface. The dielectric strength of the gas is much lower than that of the oil, and these bubbles are highly susceptible to breakdown under the operating electric field, which may lead to catastrophic turn-to-turn or layer-to-layer short circuits.

Part II: Sources of moisture inside the transformer

• Manufacturing residuals: Incomplete vacuum drying treatment of the transformer body, resulting in initial moisture remaining in the insulating material.

• External intrusion: Poor sealing (e.g., deterioration of bushings, gaskets), moisture absorption in the breather, and substandard oil injection or filtration processes result in moisture in the air entering the interior of the transformer.

• Internally generated: Insulating paper in the normal aging process, its cellulose molecular chain breakage will by-product into water molecules. This is a self-accelerating aging process.

Part III: Working Principle and Composition of Online Monitoring System

The microwater online monitoring system realizes the continuous measurement of the moisture content in the insulating oil by installing the sensing probe in the transformer oil circuit.

1. Sensing and Measurement Unit (Core):

   • Technical Principles: ubiquitousCapacitive Thin-Film Sensors (Thin-Film Sensors). At the heart of the sensor is a polymer film whose dielectric constant changes with the number of water molecules absorbed, causing a change in the capacitance value. By measuring the change in capacitance value, the water parameters in the oil can be accurately calculated.

   • Mounting method: It is usually mounted on the oil drain or backup valve of the transformer body via a ball valve, with the probe immersed directly in the oil. In order to ensure representative measurements, it is preferable to install it in a location that reflects the main oil circulation (e.g. cooler inlet and outlet lines).

2. Data Acquisition and Transmission Unit:

   • Converts the capacitive signal output from the sensor into a standardized digital or analog signal.

   • Built-in temperature sensor to temperature compensate moisture measurements to provide more accurate readings.

   • Often integrated with the sensing probe in an integrated structure.

3. Communication and display unit:

   • Provide local digital display function, easy for site inspectors to read the data.

   • The data is transmitted to the substation back-office monitoring system (SCADA) or a dedicated data server via RS-485 (Modbus protocol) or 4-20mA analog output.

Part IV: Interpretation of key monitoring indicators: ppm vs. water activity (aw)

Online monitoring systems typically provide two core metrics and it is critical to understand the difference between them.

• Absolute moisture content (ppm - Parts Per Million).

  • Definition: A mass ratio that indicates how many milligrams of water are contained in each kilogram of insulating oil (mg/kg).

  • Limitations: It's aHighly temperature dependentParameters. Under the condition that the total amount of moisture remains unchanged, when the oil temperature rises, the solubility of the oil for water increases, and the moisture in the insulating paper will migrate to the oil, resulting in an increase in the ppm value of the oil; on the contrary, when the oil temperature is lowered, the ppm value of the oil decreases. Therefore, the simple ppm value cannot accurately reflect the degree of moisture in solid insulation.

• Water Activity (aw - Water Activity) or Relative Saturation (%RS).

  • Definition: aw = P / P₀, where P is the vapor pressure of water in the oil and P₀ is the saturation vapor pressure of pure water at the same temperature. It indicates the tendency or "liveliness" of the water in the oil to escape, ranging from 0 (completely dry) to 1 (saturated).%RS = aw × 100%.

  • Advantage: The water activity isEquilibrium State Parameters Characterizing Moisture in Oil-Paper Insulation SystemsIn the case of oil-paper moisture equilibrium, the water activity of the oil is equal to that of the paper. At oil-paper moisture equilibrium, the moisture activity of the oil is equal to that of the paper. aw is a direct reflection of the "wetting" pressure of the moisture on the solid insulation and is independent of temperature changes.Therefore, water activity (aw) is a more fundamental and reliable indicator to determine whether solid insulation is exposed to moisture.

Part V: Typical technical parameters

Part VI: Frequently Asked Questions (FAQ)

Q1: How can online monitoring data be used to determine transformer insulation status?
Answer: Should focus onWater activity (aw)respond in singingData trendsThe

• aw < 0.2: Insulation is dry and in good condition.

• 0.2 ≤ aw < 0.4: Insulation is slightly damp and needs attention, combined with oil chromatography data.

• aw ≥ 0.4: The insulation is heavily damped and there is a risk of a "bubble effect", which should be planned for drying.

• Trend Analysis: A steady aw value over a long period of time indicates a good seal. If the aw value shows a constant, slow upward trend, it may indicate that the insulation is deteriorating or that there is slow external moisture ingress. If the aw value fluctuates dramatically with temperature, it may also indicate that the solid insulation contains a high level of moisture.

Q2: Do I need to have a power outage to install an online microwater monitoring device?
Answer: Usually not required. The unit is designed for installation under pressure. By using a special high-pressure ball valve, it can be safely installed on the valve of an operating transformer. The entire installation process does not affect the normal operation of the transformer.

Q3: Why is there a difference between the ppm value of online monitoring and the ppm value of laboratory assay?
Answer: Differences mainly stem fromDifferent temperatures during sampling and measurement. The temperature of the oil sample has been changed to room temperature prior to laboratory assay, whereas online monitoring is measured at real-time oil temperature. Since the ppm value is sensitive to temperature, the two values are usually not comparable. Moisture activity (aw), on the other hand, is very little affected by temperature at equilibrium, so online aw values should be in better agreement with laboratory aw values.

Q4: Can the system completely replace traditional offline oil testing?
Answer: Not a complete substitute, but complementary to each other. Online monitoring providesContinuous data and trendsThis is not possible with offline testing and is critical for timely detection of anomalies and trend analysis. In contrast, periodic oil testing in the laboratory (e.g., oil chromatography analysis DGA, breakdown voltage testing, etc.) provides more comprehensive information on oil quality. The best practice is to combine online monitoring data with periodic laboratory results to form a comprehensive, three-dimensional assessment of the transformer's condition.