Advancements in Moisture Measurement for Power Transformers: The Shift to Sensor-Based Monitoring
- Augusto Moser
- Apr 7
- 5 min read
Updated: Apr 27
Power transformers are critical components in electrical grids, and their reliability heavily depends on the condition of their insulation systems, particularly the moisture (water) content within oil-paper insulation. Excessive moisture and temperature can accelerate aging, reduce dielectric strength, and lead to bubble formation, all of which compromise transformer performance and longevity (refer to table 1). Traditionally, moisture assessment has relied on manual oil sampling followed by Karl Fischer titration and the application of equilibrium diagrams. However, a groundbreaking study by Maik Koch, Diagnostic Application of Moisture Equilibrium for Power Transformers, highlights the limitations of this method and advocates for a more effective approach using sensor-based moisture saturation measurements.
Table 1 - Life of Paper and Water Content
Water Content of Paper (% by weight) | Life of Paper at 80 °C (years) | Life of Paper at 90 °C (years) |
0.5 | 485 | 151 |
1.0 | 243 | 76 |
2.0 | 121 | 38 |
4.0 | 61 | 19 |
8.0 | 30 | 8 |
Source: Doble
Definitions
Water content or water relative to weight (W) is the water mass in a material related to its weight. For moisture in cellulose, the measure is a percentage of weight (%) and for moisture in oil, it is parts per million of weight (ppm).
Saturation water content (Ws) is the maximum amount of water a material, such as oil or cellulose, can hold at a specific temperature and pressure before free water (e.g., droplets in oil or excess moisture in solids) begins to form. It represents the material's full capacity to absorb or dissolve water under those conditions.
Water saturation or relative saturation (RS) is the ratio of the actual water content (W) in a material (like oil or cellulose) to its saturation water content (Ws) at a given temperature and pressure, expressed as a percentage. A value of 100% indicates water saturation (droplets in oil) whereas zero indicates the total absence of water molecules.
Relative humidity (RH) is defined as the ratio of partial water vapor pressure (p) in a gaseous mixture of air and water to saturation water vapor pressure (ps) at a given temperature and pressure.
Moisture sorption isotherms relate moisture relative to weight (W) of a material to the relative humidity (RH) of the surrounding air under equilibrium conditions at a specific temperature.
The Limitations of Traditional Methods
The conventional technique involves three steps: sampling oil under service conditions, measuring water content via Karl Fischer titration¹, and deriving moisture content in paper using equilibrium diagrams. While widely used, this method is fraught with challenges. Errors arise during sampling, transportation, and titration processes, while equilibrium conditions are rarely achieved due to temperature variations and long time constants in transformers. Additionally, the accuracy diminishes in low-moisture regions, and existing equilibrium diagrams—often based on outdated data or extrapolations—fail to account for material aging. Aging significantly alters the moisture adsorption capacity of cellulose (e.g., Kraft paper and pressboard) and the solubility of water in oil, leading to overestimations of moisture content. For instance, a moisture content of 20 ppm in oil might suggest 2.8% moisture in new paper, but only 1.5% in aged pressboard (figure 1), underscoring the need for updated diagnostic tools.
Fig. 1. Moisture equilibrium diagrams with the influence of aging (new oil and new Kraft paper, aged oil with thermally degraded Kraft paper, and aged oil with aged pressboard).
Introducing Sensor-Based Moisture Saturation Measurement
The study proposes a transformative shift to measuring relative moisture saturation using capacitive sensors, both in oil and cellulose, offering a more reliable and practical alternative.
"The method is based on the thermodynamic law, that moisture relative to saturation is the same for materials in contact with each other under equilibrium conditions."
By using moisture saturation isotherms²—developed from extensive measurements of new and aged cellulose (pressboard, Kraft paper, and thermally upgraded paper) and oils (mineral and vegetable types)—the approach eliminates many of the uncertainties associated with traditional methods. Key advantages include:
Negligible Impact of Aging and Oil Type: Unlike moisture content (ppm), relative saturation accounts for changes in oil solubility due to aging or the use of vegetable oils, which can hold 20-50 times more water than mineral oils.
Reduced Temperature Dependence: Saturation-based graphs are less affected by temperature fluctuations compared to moisture content-based diagrams.
Elimination of Sampling Errors: Online sensor deployment avoids errors from sampling, transportation, and titration, with capacitive probes offering high accuracy (±0.2% relative humidity at room temperature, ±2% at elevated temperatures).
Online Monitoring Capability: Sensors integrated into monitoring systems can provide continuous data, with long-term averages compensating for load cycle variations, ensuring a stable moisture assessment.
Direct Link to Deterioration: Moisture saturation better correlates with destructive effects (e.g., dielectric breakdown, cellulose hydrolysis, and bubble formation) than weight-based moisture content, as it reflects available, active water molecules.
Practical Implementation and Benefits
The study demonstrates practical application through an online monitoring system equipped with a capacitive probe installed in the hot oil flow near the transformer windings or top oil, installed on an oil filling valve or an ancillary valve—where aging and moisture effects are most pronounced (figure 2). For example, a probe measuring 10% relative saturation at 60°C corresponds to approximately 3% moisture content in new Kraft paper and 2.8% in thermally degraded paper (figure 3). This setup provides a reliable average moisture level, balancing daily load cycles over time.
Fig. 2. Temperature distribution in a large transformer according to IEC 60354.
Fig. 3. Determination of water content in new and thermally degraded Kraft paper via moisture sorption isotherms.
Conclusion
The transition from manual oil sampling, water content in oil (ppm), to sensor-based measurement, moisture relative saturation (RS%), represents a significant advancement in transformer diagnostics. This method offers improved accuracy, real-time monitoring, and a direct correlation to moisture-related degradation, making it ideal for condition-based maintenance. As transformers age and operate under varying conditions, adopting this technology ensures better reliability and longevity, addressing the critical need for effective moisture measurement and management for power transformers.
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¹ "Karl Fischer titration is a method in analytical chemistry that determines trace amounts of water in a sample using volumetric or coulometric titration. The german chemist Karl Fischer (1901-1958) introduced this titration method in 1935. Titration basically means to add a reagent of known concentration (titre) to a unknown substance until the concentrations are balanced."
² As isotermas de adsorção de umidade mostram quanta água um material (como celulose) absorve do ambiente (ar ou óleo) em diferentes níveis relativos de umidade, sob condições de equilíbrio em uma temperatura específica.
References
Koch, Maik. Diagnostic Application of Moisture Equilibrium for Power Transformers, IEEE Transactions on Power Delivery (Vol. 25, No. 4, October 2010).
Koch, Maik. Reliability and Improvements of Water Titration by the Karl Fischer Technique (2007).
