What Standards and Case Studies Define LTC Particle Analysis?

This article is Part 7 of a seven-part series on testing insulating liquids in load tap changers. Part 6 addressed particle count distribution analysis applied to oil-filled circuit breakers, switches, and transformers. Here we will address the current standards landscape governing particle count distribution and analytical ferrography, and examine two case studies where particles and filming compounds analysis was pivotal in diagnosing active problems in newly installed equipment.

The technical framework for particle count analysis in electrical insulating liquids is defined across several active standards from ASTM, ISO, and IEC, though unified industry guidance for evaluation and maintenance thresholds — particularly for in-service equipment — remains incomplete. Field experience and laboratory databases continue to fill the gaps while standardization efforts proceed.

Applicable Standards and Guidance

The primary standards governing particle count analysis in insulating liquids include:

• ASTM D6786 – Standard Test Method for Particle Count in Mineral Insulating Oil Using Automatic Optical Particle Counters — the test method used for particle count distribution analysis, incorporating reference to ISO rating methods.
• ISO 4406 – Hydraulic fluid power – Fluids – Method for Coding the Level of Contamination by Solid Particles — provides the three-value contamination rating system applied to particle count results.
• ISO 11171 – Hydraulic fluid power – Calibration of Automatic Particle Counters for Liquids — the calibration standard referenced in ASTM D6786.
• IEC 60970 – Methods for Counting and Sizing Particles in Insulating Liquids — the IEC equivalent test method.
• IEC 60422 – Supervision and Maintenance Guide for Mineral Insulating Oils in Electrical Equipment — contains an informative annex (Annex B) with descriptions of particle levels in power transformer oil, sourced from CIGRE Brochure 157.

Unit and Interval Differences Across Standards

Practitioners comparing results across standards should be aware of key differences in reporting conventions. The IEC 60422 Annex B uses particles per 100 mL as the unit of measure, whereas ASTM D6786 reports particles per mL. Size interval designations also differ: IEC uses approximately 5 µm and 15 µm intervals, while ASTM uses >4 µm and >14 µm. These differences require normalization before cross-standard comparisons are valid.

CONTAMINATION

IEC 60422 Annex B contamination chart showing Normal, Marginal, and High particle level descriptions with ISO code values for ≥5 µm and ≥15 µm size ranges, in particles per 100 mL.

TYPICAL AND ACCEPTABLE DISTRIBUTION

SDMyers’ typical and acceptable particle count distribution table for in-service transformer oil, adjusted for ASTM D6786 units and size intervals.

CONTAMINATION – Typical & Acceptable for Transformers

Status of IEEE Standardization Efforts

An effort within IEEE to establish technical guidance for particle count values in new oil, newly installed oil, and newly energized transformers was initiated but did not reach consensus. Transformer manufacturers maintain internal guidance values, but these have not been validated through a formal standards body approval process.

Case Study: Remanufactured Reactor

A remanufactured reactor showed no anomalies in pre-energization or one-month post-energization oil testing. Beginning in the latter half of year one and continuing through year two, liquid power factor values at both 25°C and 100°C showed erratic and distinctly upward trends that exceeded the variability attributable to test method precision. Particles and filming compounds analysis revealed increasing particle count distribution values across most size ranges and the presence of non-metallic particles with morphology consistent with varnish but of an unusual and initially unexplained shape.

Collaborative investigation with the owner and remanufacturer identified the particle source as a resin used during the remanufacturing process that was being dissolved by the insulating oil. The dissolved resin was elevating liquid power factor and generating the observed abnormal particles. A field processing strategy was developed to remove the particles and dissolved resin from the oil, followed by intensified monitoring using liquid power factor and particles and filming compounds analysis. After a second field processing event, the condition stabilized. Subsequent routine monitoring has confirmed stability over multiple subsequent years of service.

Case Study: New Power Transformers at a Power Station

Several large new transformers at a generating station under construction showed low D1816 dielectric breakdown voltage values in pre-installation oil testing. The transformer manufacturer’s recommended maximum particle count values for newly installed units were also exceeded. The units were reprocessed in the field, and D1816 and particle count values recovered to acceptable levels immediately following reprocessing — but then degraded again, indicating an active ongoing condition rather than a resolved contamination event.

Expanded testing including particles and filming compounds analysis — replacing particle count distribution alone — and liquid power factor analysis (recommended by the transformer manufacturer) was added to the monitoring program. High and increasing 100°C liquid power factor values developed across all units, alongside continued low D1816 and rising particle count values. Analytical ferrography identified the probable source of the large, increasing non-metallic particles as the solid insulating materials used in these units. The manufacturer and owner agreed on a reprocessing and monitoring protocol. Following recommended reprocessing, all units entered service with intensive first-year monitoring. No continuing issues have been identified.

Conclusions From the Case Studies

In both cases, multiple oil test parameters pointed toward an active problem, but the conclusions from those parameters were supported — and ultimately resolved — only by the characterization of particle composition provided by analytical ferrography. The ability to identify particle origin was decisive in developing effective remediation strategies and confirming resolution of the conditions.

Summary:

• ASTM D6786, ISO 4406, ISO 11171, IEC 60970, and IEC 60422 constitute the current standards framework for particle count analysis in insulating liquids; unified evaluation guidance for in-service equipment remains incomplete.
• Unit and size interval differences between IEC and ASTM standards require normalization before cross-standard result comparisons are valid.
• IEEE standardization efforts for particle count thresholds in new and newly energized transformers did not reach consensus; transformer manufacturer internal guidance is not yet formally validated.
• Case Study 1: Analytical ferrography identified a remanufacturing resin as the source of abnormal particles and elevated liquid power factor in a remanufactured reactor; two field processing events were required to stabilize the condition.
• Case Study 2: Ferrography identified solid insulation materials as the probable particle source in new transformers with persistent low D1816 and rising power factor; reprocessing and intensive monitoring resolved the condition.
• Across both cases, particle characterization via ferrography was the decisive diagnostic step — supporting conclusions from other test parameters and enabling effective, targeted remediation.

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?