What Causes Accelerated Film Formation in LTCs?

This article is Part 2 of a seven-part series on testing insulating liquids in load tap changers. Part 1 addressed how mineral insulating oil ages inside an LTC through polymerization and film formation. Here we will address the specific conditions that cause filming to advance at an accelerated rate. In Part 3, we will cover liquid screen tests, moisture content analysis, and dissolved gas analysis used to monitor LTC condition between planned maintenance intervals.

Film formation in an LTC proceeds as a natural consequence of oil aging; however, certain conditions significantly accelerate that process. Accelerated filming is operationally dangerous because it can produce mechanically and electrically hazardous conditions well before the end of a normal preventive maintenance interval. Understanding the drivers of accelerated filming enables earlier detection and more targeted corrective action.

Oxidized Oil as a Filming Accelerant

Although oxidation is not the primary aging concern for LTC oil, heavily oxidized oil forms film at a substantially faster rate than clean, unoxidized oil. The industry has historically applied less restrictive oxidation thresholds to LTC oil than to transformer oil, in part because the absence of solid cellulosic insulation reduces the risk of damage from oxidation byproducts. However, operational experience with particle and filming compound testing supports applying the same acid number and interfacial tension (IFT) threshold ranges used for in-service transformer oil to LTC oil as well — with the important distinction that recommendations are interpreted differently. For LTC oil, unacceptable acid number values (greater than 0.10 mg KOH/g) or IFT values below 28 mN/m indicate that oil replacement or reclamation is warranted.

Elevated Moisture Content

Elevated moisture content in LTC oil accelerates the rate of film formation on contacts and the mechanism, analogous to the effect of moisture on oxidative aging in transformer oil — though the LTC-specific relationship is less thoroughly documented in published literature. Field experience with particles and filming compound analysis, corroborated by observations during LTC preventive maintenance inspections, confirms a direct correlation between moisture content and filming severity. Moisture values at or above 60 ppm are classified as unacceptable and correspond to conditions under which runaway filming is expected. At these levels, moisture also introduces unacceptable dielectric risk, including the potential for moisture tracking and dielectric failure within the device.

High or Oversized Particle Loads

During normal LTC operation, film is continuously deposited on and wiped from contact surfaces, and arcing generates carbon particles. These particles remain suspended in the oil until they are incorporated into newly forming film. When particle counts are elevated or particle sizes are unusually large, film forms more rapidly. Metallic and carbon particles are particularly problematic: they are substantially more abrasive than the non-metallic particles typically found in LTC oil, and their incorporation into film on contact surfaces accelerates contact erosion. Their incorporation into film on mechanism components increases mechanical resistance and impedes efficient tap changing.

Incompatible Cleaning Compounds

The use of incompatible solvents during LTC maintenance is a frequently identified and often underappreciated source of accelerated filming. Only hot mineral oil dielectric fluid is appropriate for cleaning LTC contacts and mechanisms. The use of chlorinated solvents, aromatic solvents, brake cleaners, lacquer thinners, or similar materials may facilitate film removal during the current maintenance event but will substantially accelerate film formation thereafter. Films produced in the presence of incompatible compounds also tend to be harder to remove in subsequent maintenance cycles. When an LTC exhibits an unusually severe or persistent filming tendency, targeted diagnostic oil testing can often identify incompatible compound residues as the root cause.

Summary:

• Oxidized oil forms film faster: unacceptable acid number (>0.10 mg KOH/g) or IFT (<28 mN/m) values indicate elevated filming risk and are thresholds for oil replacement or reclamation in LTCs.
• Moisture ≥60 ppm is unacceptable: at this level, accelerated filming and elevated risk of moisture tracking and dielectric failure are both expected.
• High particle counts and large particles increase the rate of film formation; metallic and carbon particles are especially abrasive and harmful when incorporated into contact or mechanism film.
• Incompatible cleaning solvents are a common cause of abnormally severe filming; only hot mineral oil dielectric fluid should be used to clean LTC contacts and mechanisms.
• Accelerated filming can compromise LTC operation — through sustained arcing, contact overheating, or mechanism binding — before the end of a normal PM interval, making early detection critical.

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?