For the past few weeks, we have discussed particles and filming compounds analysis. This week, we will talk about some standards activities concerning these analyses and a couple of real life cases that were unusual applications for particles and filming compounds analysis.
Particles count distribution analysis and analytical ferrography have been widely used in electrical insulating liquids over the past several years. S. D. Myers began offering particle count distribution analysis in 1999 and particles and filming compounds analysis in 2002. Although there have been some sporadic efforts by commercial laboratories and owners of large transformers to establish some industry standards for technical guidance regarding the use of these analytical methods, there has been nothing firmly established.
ASTM International has a standard test method for particle count distribution analysis entitled D6786 – Standard Test Method for Particle Count in Mineral Insulating Oil Using Automatic Optical Particle Counters. The International Standards Organization has two applicable standards, ISO 4406 – Hydraulic fluid power – Fluids – Method for coding the level of contamination by solid particles – and ISO 11171 – Hydraulic fluid power – Calibration of automatic particle counters for liquids. ISO 4406 includes the rating system we use as part of our evaluation of particle count distribution results. Both of the ISO standards are referenced in the ASTM standard method that we use. IEC also has a standard method, 60970 – Methods for counting and sizing particles in insulating liquids.
IEC has some guidance in standard 60422 – Supervision and maintenance guide for mineral insulating oils in electrical equipment. There is a table including a description of particle levels encountered in power transformer insulating oil in Annex B (informative). This standard is descriptive of levels found and is not considered guidance for purposes of recommending maintenance. The source of the values in this standard is CIGRE Brochure 157 – Effect of Particles on Transformer Dielectric Strength. Annex B suggests using ISO 4406 to measure particles and report the results. This standard uses different size intervals from those that we currently use when determining particle count distribution using ASTM D6786. (The interval in the IEC document stated as 5 μm (microns) or greater corresponds to the interval stated in the ASTM method as > 6 microns, while the interval in the IEC document stated as 15 μm (microns) or greater corresponds to the interval we state as > 14 microns.) Further, the unit of measurement in the IEC document is particles per 100 mL, while the standard unit in the ASTM method is particles per mL.
The following description and values come from information taken from Annex B (note that the ISO Code has only two values, and that only values for 5 microns and greater and 15 microns and greater are listed):
|
“Low Contamination”
(maximum values) |
“Normal Contamination”
(maximum values) |
“High Contamination”
(values exceeding) |
ISO Code |
10/7 |
15/12 |
17/14 |
5 microns
and larger |
1,000 particles
per 100 mL |
32,000 particles
per 100 mL |
130,000 particles
per 100 mL |
15 microns
and larger |
130 particles
per 100 mL |
4,000 particles
per 100 mL |
16,000 particles
per 100 mL |
Values in between “Normal” and “High” are listed as “Marginal”. Low contamination levels are typically considered to apply to new transformer oil, installed properly in new transformers.
Values we would consider to be “Typical and Acceptable” for transformers (adjusting the values from last week’s article for the difference in units and size intervals):
|
Typical and Acceptable (Transformers) |
ISO Code |
13/9 |
5 microns and larger |
15,000 particles per 100 mL |
15 microns and larger |
300 particles per 100 mL |
The actual values as we report are as noted in last week’s article:
|
Typical and Acceptable (Transformers) |
ISO Code |
17/13/9 |
> 4 microns |
1,500 |
> 6 microns |
150 |
> 14 microns |
3 |
> 23 microns |
2 |
> 50 microns |
1 |
Currently, there is an effort underway in IEEE to establish some technical guidance for particle count distribution values in new oil, newly installed oil, and in newly energized transformers. Transformer manufacturers frequently have internal guidance for such values currently, but these have not been subject to the approval process of a consensus organization such as IEEE.
As with in-service load tap changers, an owner of a new transformer may sometimes needs to have a more complete idea about what is causing particle formation than a simple particle count distribution analysis can provide. We would like to outline a couple of cases where particles and filming compounds analysis helped us to identify the underlying cause of some difficulties transformer owners were having.
The first case involved a remanufactured reactor. Upon installing the newly remanufactured reactor, there were no issues noted with the oil test results either before or one month after energizing the unit. Later, very high liquid power factor values were noted both for the 25 oC and the 100 oC values. During the latter half of the first year and for the entire second year of service, these values were more erratic than the precision of the test method could account for, but showed a distinct upward trend. Particles and filming compounds analysis indicated increasing particle count distribution values for most size ranges and the presence of non-metallic particles closely resembling varnish particles. The shapes of the particles were unusual and could not immediately be explained. In working with the owner and with the remanufacturer of the reactor, we were able to identify the particle origin as being a resin that was used during the remanufacture of the equipment and that was being dissolved by the insulating oil, resulting in increased liquid power factor and abnormal particle generation. Based on this identification, a strategy was developed to process the oil in the field to remove the particles and the dissolved resin followed by closer monitoring using oil tests including the liquid power factor and particles and filming compounds analysis. After the first field project, the liquid power factor had not stabilized and the particle generation continued. After a second field project, the situation had stabilized. Subsequent routine monitoring has indicated that the solution continues to hold after the reactor has been in service for several years since the field processing.
The second case concerned several large, new transformers being installed at a power station that was being constructed. Upon installing the new transformers, low D1816 dielectric values were noted in the oil testing prior to installation. Also, the new equipment manufacturer had recommended maximum values for particle count distribution values in newly installed transformers prior to installation, and these units exceed those values. Based on the manufacturer’s recommendation, the units were reprocessed, and had good D1816 and particle count distribution values immediately after the reprocessing. However, these results soon degraded, indicating that there was a probable active condition. We expanded the testing to include particles and filming compounds analysis instead of just particle count distribution, and we added liquid power factor analysis to the oil monitoring which was recommended by the transformer manufacturer. High and increasing 100 oC power factor values were noted in all the transformers in addition to the low D1816 values and increasing particle count distribution values. Upon sharing the results of the analytical ferrography with the manufacturer, we were able to identify a probable source of increasing numbers of large, non-metallic particles in the solid insulating materials. The owner and manufacturer agreed upon procedures to reprocess and monitor the oil to remove the particles and to identify whether the problem were continuing or had stabilized. This was accomplished and the transformers were eventually put in service with close monitoring for the first years of service. There are not continuing issues.
In both of these cases, there were other test results indicating potential problems. The conclusions from those other tests were supported by the particle count distribution analyses. Characterizing the particles that were being formed by using particles and filming compounds analysis turned out to be critical for eventually solving the problem to the satisfaction of the owners and their suppliers.
Next week, we will introduce a new topic. See you then.
