Last week, we discussed some of the tests that are routinely performed on insulating liquid from load tap changers – liquid screen, moisture content, and dissolved gas analysis. Routine testing for the particles found in the oil also provides critically important information. Particles and filming compounds analysis includes two separate analyses – particle count distribution and analytical ferrography. This week, we are going to talk specifically about particle count distribution for load tap changer insulating liquid samples.
Particle counting – both total count and the size distribution – is performed according to ASTM Standard Method D6786. The automatic counters view a test specimen of insulating liquid optically. They count and assign standard sizes to a representative group of particles. The total number of each size in a given volume of oil is calculated, and the values are reported as size distribution in “particles per milliliter” for each size range. Older or non-standard methods may have reported particles per 10 mL or 100 mL, and this must be considered when evaluating past history.
Particle count distribution results are reported graphically, like this example:
The above graphic is from the analysis of oil samples from six load tap changers. Below the TC identifications and sample draw dates, each set of results is characterized by a rating of the distribution according to ISO Method 4406. The first (largest) value in the rating is based on the number of particles per milliliter larger than 4 μm (“μm” is listed as “microns” in the data table). Similarly, the middle value is based on the number of particles per milliliter larger than 6 μm, and the last (smallest) value is based on the number of particles per milliliter larger than 14 μm. Although developed primarily for applications involving lubricating oils, these ISO ratings can be easily adapted for insulating oil applications and are discussed in the ASTM standard method.
The far right column in the graphic indicates what a typical particle count distribution for a load tap changer should be that we would consider being acceptable for an arc-in-oil device. Typically, if the values for > 6 microns and > 14 microns are within the appropriate typical ranges, elevated or high values for > 4 microns are usually of little concern. However, high values for the larger size ranges, > 23 microns and > 50 microns, are generally of much more concern. In addition to this base outline, changes since the last analysis, and trends upward, in particular, also affect interpretation of particle count data.
The most significant source of particles suspended in the insulating oil of an LTC is the filming of the oil. The film accumulates over time as deposits on the contact surfaces. Under normal operation of the LTC, this deposited material is “wiped off” and sent into the oil as small particles. These small particles typically stay suspended in the oil until they are reincorporated into new film as it forms in the device.
Another significant source of particles in an LTC is carbon formation in the oil due to the natural arcing that occurs as the contacts change position. Carbon particles are also incorporated into film as the insulating oil ages in the device. During normal operation of an LTC, these small film and carbon particles will be noted primarily in the >4 microns and > 6 microns values and will generally not cause those values to become excessively high. As filming becomes more advanced, or if the LTC operates much more frequently than usual, values for these two smaller ranges increase. More significantly, much larger values for the larger size ranges are also noted when filming becomes more advanced.
Industry standards for evaluating particle count distribution results for load tap changer insulating liquids are not very well established. Our laboratory uses our own experience and database to evaluate particle count results from insulating oil in electrical equipment. Form many years these results were compared to actual LTC servicing provided in the field. As noted above, we have established ranges that we consider to be typical and acceptable.
Beyond this, we categorize increasing values for particle count in the various size ranges as elevated, high, and very high. There is also a category – “extremely high” – that is rarely used. This is typically for the larger size ranges and can represent a hazard of failure of the device due to serious problems such as binding.
Particle count distribution provides valuable information concerning the condition of a load tap changer, particularly the rate at which film is both forming and subsequently being disrupted during operation of the device. There is also a second powerful tool for characterizing particles found in a load tap changer – analytical ferrography – that describes the composition and sources of the particles found. We will discuss that analysis in next week’s article.
