What Can Analytical Ferrography Reveal About LTC Particle Sources?

This article is Part 5 of a seven-part series on testing insulating liquids in load tap changers. Part 4 addressed particle count distribution analysis as a quantitative tool for evaluating LTC condition. Here we will address analytical ferrography — the complementary analysis that characterizes particle composition and origins, providing diagnostic information that particle counting alone cannot supply. In Part 6, we will cover particle analysis methods applied to other oil-filled equipment types.

Particle count distribution quantifies suspended particles in LTC oil by size; analytical ferrography identifies what those particles are, where they came from, and what operational conditions produced them. Together, they constitute the full particles and filming compounds analysis and provide a substantially more complete picture of LTC condition than either test alone.

Principle and Slide Preparation

Ferrography involves microscopic examination of a prepared slide derived from the insulating oil sample. During slide preparation, oil flows across the slide surface under the influence of a magnetic field. This field-assisted flow separates particles by composition: metallic particles are segregated from non-metallic particles, and ferrous (iron-containing) particles are further differentiated from non-ferrous metallic species. The resulting spatial distribution of particles on the slide allows the microscopist to examine each class of particles for morphology, size, and condition — all of which carry diagnostic significance.

Particle Types and Diagnostic Significance

Carbon Particles

Carbon particles are an expected byproduct of normal arc quenching during tap changes. Their presence is not inherently abnormal; however, increasing counts or sizes of carbon particles are of concern because carbon is abrasive and, when incorporated into contact film, accelerates contact wear that can lead to overheating or failure in severe cases.

Non-Ferrous Metal Particles

Non-ferrous metal particles in LTC oil typically originate from conductor material removed from contact surfaces. Normal operation generates sliding wear particles and arcing spheres from contact metal. These are generally not a primary concern unless their numbers or sizes increase substantially, which would indicate accelerated contact wear.

Ferrous Metal Particles

Ferrous (iron-containing) particles are almost always diagnostically significant in LTC oil. Steel components within an LTC are not designed to be consumable, and their presence in the oil indicates abnormal mechanical conditions. Any meaningful quantity of ferrous particles warrants investigation.

Cutting Wear Particles

Cutting wear particles — whether ferrous or non-ferrous — form when the edges of two metal surfaces move across each other under abnormal loading conditions. Their presence is a reliable indicator of a mechanical fault within the LTC.

Overheated Metal Particles

Overheated metal particles differ from arcing spheres in their irregular morphology; they are produced by sliding or cutting wear events followed by exposure to extreme heat that distorts or fuses particle edges. Under normal circumstances, a suspended metal particle intercepting an arc can produce this morphology. Increasing incidence of overheated metal particles indicates abnormal thermal conditions.

Non-Metallic Particles: Film and Debris

Film (varnish) particles are nearly always present in LTC oil and are expected. Their diagnostic value lies in their size: as filming becomes more advanced, the film particles identified by ferrography become larger, correlating directly with impaired LTC operation. Insulation particles, fibers, and external debris are not expected in significant quantities and, when present, require investigation.

Integrating Ferrography With Particle Count Data

Particle count distribution and analytical ferrography results are evaluated together. Where both analyses indicate potentially hazardous conditions, the preventive maintenance interval should be shortened to allow cleaning, inspection, and remediation before failure can occur. Where findings are equivocal, a shortened retesting interval — repeating both particles and filming compounds analysis and DGA simultaneously — is the appropriate response. Both tests are sensitive to distinct subsets of LTC conditions, and concurrent testing maximizes diagnostic completeness.

Ferrography for Vacuum Interrupter LTCs

Vacuum interrupter LTCs do not generate significant quantities of particles during normal operation, as arcing occurs within the vacuum bottles rather than in the oil. LTC oil from vacuum interrupter devices should remain substantially free of suspended particles. Any meaningful particle presence in a vacuum LTC should prompt review of device operation and potential maintenance.

Summary:

• Analytical ferrography characterizes particle composition (carbon, non-ferrous metal, ferrous metal, film, debris) and morphology to identify sources and conditions of formation.
• Ferrous particles are almost always abnormal in LTC oil; steel components are not consumable, and their presence indicates a mechanical fault.
• Cutting wear particles indicate abnormal metal-on-metal contact and are a reliable mechanical fault indicator regardless of ferrous or non-ferrous composition.
• Film particle size is directly correlated with filming severity; larger film particles indicate more advanced film development and degraded LTC operation.
• Ferrography and particle count distribution should be interpreted together; each is sensitive to different conditions, and concurrent testing provides the most complete diagnostic picture.
• Vacuum LTC oil should be essentially particle-free; any particle presence warrants operational review.

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?