For the past few weeks, we have discussed particles and filming compounds analysis, incorporating both particle count distribution analysis and analytical ferrography, applying the discussion mainly to insulating liquids from load tap changers. This week, we are going to discuss applying these analyses to other equipment types and insulating oil applications. We will start by discussing particle count distribution analysis for in-service oil-filled circuit breakers, switches, and transformers.
Aside from the load tap changer applications for particles and filming compounds analysis, the most frequent use of particle count distribution analysis in our laboratory is for insulating liquid samples from oil-filled circuit breakers and switches. For these devices, the concern is that these units generate increasing numbers of particles as the oil ages in service. Further, these types of equipment may fail mechanically due to excessive numbers of particles, particularly very large particles.
We use the same “typical and acceptable” distribution for OCBs and switches as we use for LTCs. The following is the same graphic we presented a couple of weeks ago:
In interpreting the particle count results, we generally are not as concerned with the upward trending of the values for the size ranges. However, we monitor the particle count distribution much closer when the values for > 6 microns exceed 10,000 particles/mL. Timely maintenance is recommended when this value reaches 50,000 particles/mL or when the larger size ranges reach ten times the typical and acceptable values. Extremely high values for small particles and very high values for larger size ranges greatly increase the hazard of failure due to binding.
Moisture content and dissolved gas analysis are generally considered to be optional tests for oil-filled circuit breakers and for switches. Routine, annual testing of insulating liquid should include particle count distribution and the liquid screen tests which give much more valuable results.
Switches and OCBs that operate frequently should also be subject to regular preventative maintenance including periodic oil service and internal inspection and cleaning. An optional use of dissolved gas analysis for this equipment is to better define whether these devices are operating frequently enough so that this sort of PM program is recommended. Also, because of mechanical failure modes for OCBs and switches, acceptable results for oil tests are no guarantee that the equipment will continue to perform adequately. Further, some manufacturers for oil-filled switches have recommended removal of their devices from service and replacement by devices using alternate technologies.
As is the case with vacuum interrupter load tap changers, switches, where the circuit is made and broken inside a vacuum interrupter, use oil as a cooling medium, but dissolved gases and particles are generally not created in the insulating liquid as a result of the arcing within the vacuum bottles. In these cases, particle count distribution values are usually extremely low, and dissolved fault gases are not generated. Any presence of particles or dissolved fault gases in such a unit will generate a recommendation for investigation.
Particle count distribution analysis is generally not run as a routine test for in-service transformers. Transformers will develop lower levels of particles than other electrical equipment, and these levels are typically not much of issue during operations. However, particle count distribution analysis is frequently run on in-service transformers where there are questionable or unacceptable values for dielectric breakdown voltage. In particular, if the D1816 method using VDE spherical electrodes has been performed and shows values that do not meet the guidelines listed in IEEE Standard C57.106, Guide for Acceptance and Maintenance of Insulating Oil in Equipment, particle count distribution analysis can be used as a diagnostic tool to help identify the conditions causing the poor D1816 values. Dielectric breakdown voltage results using this method can be depressed by moisture in the oil, oxidation and aging, and some forms of contamination, including particles in the oil, all of which may be considered to be detrimental to the continued health of the unit. D1816 values may also be depressed by dissolved gases in the oil. For units that have either a static or a continuous inert gas blanket, the values may be depressed to result in dielectric breakdown voltage values below the recommended guidelines even if all of the other conditions listed above (moisture, aging, and contamination) are within acceptable ranges.
When low D1816 results are obtained during routine oil tests, the other results (liquid power factor, moisture, acid number, interfacial tension, etc.) are cross-referenced to help diagnose the cause. If review of the other results does not completely identify the cause of the values that are of concern, higher than normal particle content of the oil may be suspected. To confirm this, the appropriate recommendation may be to include a particle count distribution analysis at the next routine sampling interval.
There is no established standard for particle count distribution values for in-service insulating liquids in transformers. We use the following as a typical and acceptable distribution based on our database of values and experience (compared to values for other equipment types discussed previously):
||Typical & Acceptable
|Typical & Acceptable
(LTCs, OBCs, Switches)
|> 4 microns
|> 8 microns
|> 14 microns
|> 23 microns
|> 50 microns
Next week’s article will tie up the discussion of these particle analysis methods by talking about current efforts to establish a standard for particle count values in new transformers and a couple of cases where we have used these analyses to diagnose and identify causes of apparent problems in newly installed units.