How Are Liquid Screen, Moisture, and DGA Used to Monitor LTCs?

This article is Part 3 of a seven-part series on testing insulating liquids in load tap changers. Part 2 addressed the conditions that accelerate film formation inside an LTC. Here we will address three foundational oil tests used to monitor LTC condition between preventive maintenance intervals: the liquid screen test, moisture content analysis, and dissolved gas analysis. In Part 4, we will cover particle count distribution analysis.

Effective LTC condition monitoring between scheduled maintenance intervals depends on a coordinated set of oil tests, each sensitive to a distinct class of conditions. The liquid screen, moisture content, and dissolved gas analysis (DGA) collectively provide a broad diagnostic view of oil degradation, moisture-related risk, and thermal/electrical abnormalities within the device.

Liquid Screen Tests for LTCs

The liquid screen test package for LTCs mirrors the package used for transformers and serves similar diagnostic purposes. Neutralization number (acid number) and interfacial tension (IFT) are direct quantitative measures of oil oxidation: acid number increases and IFT decreases as the oil oxidizes. As discussed in Part 2, when these values reach ranges classified as unacceptable for LTC service, the oil has oxidized to the point where film formation accelerates substantially. D877 dielectric breakdown voltage (flat disk electrode method) is useful for detecting broad contamination categories including very high moisture and elevated particle loads. Color is used qualitatively rather than quantitatively; visual examination for color, appearance, free water, sediment, and carbonization provides more diagnostic value for LTC oil evaluation than color measurement alone. Unacceptable classification for any of these parameters indicates a need for corrective action.

Moisture Content Analysis

Moisture content is measured by coulometric Karl Fischer titration and is evaluated directly in parts per million (ppm) — a simpler and more direct assessment than the moisture-saturation and solid insulation equilibrium analysis applied to transformer oil. Moisture values at or above 60 ppm are classified as unacceptable and indicate both an elevated rate of film formation and significant risk of electrical tracking or dielectric failure within the device. Confirmed moisture content above this threshold requires maintenance action to reduce moisture to acceptable levels and to mitigate the associated hazards.

Dissolved Gas Analysis

DGA has been a cornerstone of transformer condition monitoring since the mid-1970s. Its application to LTCs is more recent but increasingly well-validated as a tool for detecting abnormal operating conditions.

Expected Gases Under Normal Operation

During normal LTC operation, transient arcing between contacts as they change position generates acetylene and hydrogen. The hydrogen, being poorly soluble in insulating oil, is typically vented through a breather to prevent buildup of explosive concentrations in the gas space. Under normal operating conditions, acetylene is the dominant dissolved combustible gas, with comparatively minor generation of other species.

Abnormal Gassing Patterns

Abnormal LTC operation produces a characteristically different dissolved gas profile. Ethylene is generated in insulating oil at temperatures exceeding approximately 300°C; significant ethylene generation in an LTC indicates that arcs are being sustained too long or that contact surfaces are overheating, shifting the ratio of ethylene to acetylene upward. Elevated ethane and methane accompany overheating of contacts, resistors, or the mechanism. Filming of the LTC mechanism causes it to operate under increased friction, generating additional heat that also contributes to abnormal methane and ethane production. Abnormal conditions associated with ethylene generation include misaligned contacts conducting below rated contact area, resistor overheating in resistive-type LTCs, and coke formation.

Standardization Challenges and Database Development

Interpretation of LTC DGA results is complicated by wide variability in gas profiles across manufacturers and even across models from the same manufacturer. Profiles that are legitimately normal can differ substantially depending on LTC design. Industry organizations including IEEE are working to establish standardized interpretation guidelines, and large diagnostic databases assembled from field results across multiple LTC models are essential inputs to that effort.

Summary:

• Liquid screen tests assess oil oxidation (acid number, IFT), gross contamination (D877, color, appearance, sediment), and dielectric integrity; unacceptable values for any parameter indicate need for corrective action.
• Moisture ≥60 ppm (coulometric Karl Fischer) is the unacceptable threshold for LTC oil — indicating accelerated filming risk and elevated probability of tracking or dielectric failure.
• DGA under normal operation produces predominantly acetylene and hydrogen; elevated ethylene relative to acetylene is the key indicator of sustained arcing or contact overheating.
• Abnormal ethane and methane generation signals overheating of contacts, resistors, or the mechanism — often related to mechanism filming or contact misalignment.
• DGA interpretation for LTCs is model-specific; normal gas profiles vary substantially across manufacturers and device types, requiring manufacturer- and model-aware diagnostic frameworks.
• Abnormal results in any of these three test categories may indicate a need for shortened maintenance intervals or immediate internal inspection.

In This Series:

• Part 1: How Does Mineral Oil Age Inside a Load Tap Changer?
• Part 2: What Causes Accelerated Film Formation in LTCs?
• Part 3: How Are Liquid Screen, Moisture, and DGA Used to Monitor LTCs?
• Part 4: What Does Particle Count Distribution Reveal About LTC Condition?
• Part 5: What Can Analytical Ferrography Reveal About LTC Particle Sources?
• Part 6: How Is Particle Count Analysis Applied to Other Oil-Filled Equipment?
• Part 7: What Standards and Case Studies Define LTC Particle Analysis?