Moisture Measurement Troubleshooting With SF6 Analyzers

Gas-insulated equipment, particularly in high-voltage applications, plays a critical role in the transmission and distribution of electrical power. These systems often use sulfur hexafluoride (SF6) gas as an insulating medium due to its excellent electrical insulation properties. However, even small amounts of moisture can compromise the integrity of SF6 insulation, leading to reduced equipment reliability and costly maintenance. In this article, we’ll delve into the importance of moisture measurements in gas-insulated equipment, explore common sensor technologies, understand dew point, and discuss common troubleshooting techniques for moisture sensors.

Why Test for Moisture in Gas-Insulated Equipment:

1. Maintaining Dielectric Strength: Moisture in SF6 insulation can lower its dielectric strength, increasing the risk of electrical breakdowns and equipment failure. Regular moisture testing ensures that insulation remains effective.
2. Preventing Corrosion: Moisture can lead to the formation of corrosive by-products, damaging the internal components of gas-insulated equipment over time. Accurate moisture measurements help identify and mitigate this risk.
3. Extending Equipment Lifespan: Early detection and management of moisture-related issues can extend the operational life of gas-insulated systems, reducing replacement and maintenance costs.

Common Sensors Used:

1. Aluminum Oxide Sensors: Aluminum oxide sensors are widely used for moisture measurement in SF6 gas. They operate on the principle of capacitance change due to moisture absorption. These sensors offer good accuracy and reliability, making them a popular choice in the industry.
2. Polymer-based Sensors: Polymer-based sensors utilize changes in electrical resistance or capacitance to measure moisture levels. They are known for their robustness and ability to withstand harsh environmental conditions. Our FLEX SF6 analyzer utilizes this sensor
3. Chilled Mirror Hygrometer: Chilled mirror hygrometers are precise instruments that directly measure the dew point temperature. They are highly accurate and suitable for applications requiring precise moisture measurements.

Understanding Dew Point and Moisture Concentration:

Dew point is the temperature at which air becomes saturated with moisture, leading to the formation of dew or condensation. In moisture measurement, the dew point temperature is a crucial indicator because it represents the point at which moisture begins to condense from a gas. This temperature can be correlated with the moisture concentration in ppmv (parts per million by volume). Understanding this relationship is essential for interpreting moisture measurements accurately.

Here’s a handy calculator for converting dew point to PPMv

Troubleshooting High Moisture Readings:

When a moisture sensor reads higher than expected, several factors may be at play:

1. Ensure that the couplings and test hoses in use are thoroughly wiped to remove any lingering moisture.
2. Ensure the sensor module screws are tightened down using the torque wrench provided.
3. Verify that no rubber components are present in the testing equipment, such as regulator diaphragms or testing hoses.
4. Apply pressure to the inlet hose and employ a leak detector to inspect for any potential leaks. Leaks can lead to elevated moisture readings.
5. Execute the “Dry System” function on the analyzer, followed by conducting supplementary tests.
6. Regulate and attach a Nitrogen bottle to the analyzer’s inlet. Run a series of tests to desiccate the internal contents of the device. Reconnect to the previous gas source and perform a test

Checking Sensors With Calibrated Gas

Using calibrated gases to periodically check that your moisture sensors are operating within their specified range is a great sanity check for using analyzers. Keep in mind that the calibrated gas that you use will also have a tolerance given that is in ± PPMv. See the below chart for a better explanation.