Last week, we discussed particle count analysis, one of the two tests that collectively are designated as particles and filming compounds analysis. This week, we are going to discuss the other analysis, which is analytical ferrography. The purpose of analytical ferrography is to determine not only how many particles in the various size ranges are present, but also what those particles are made of and what conditions may have caused them to be formed. Analytical ferrography has been used for years to test evaluate lubricating oils while looking for wear particles. The same techniques also provide valuable information when applied to insulating liquids, and especially to insulating oil from load tap changers.
Ferrography includes a microscopic examination of the insulating liquid, and the preparation of the microscope slide is the key to the value of the information that this examination yields. During slide preparation, oil flows across the slide, and the slide is subjected to a magnetic field. This facilitates separation of the particles by composition into metallic and non-metallic particles and further segregates ferrous (iron containing) metal particles from non-ferrous metal particles. The microscopic examination also gives an indication as to the conditions that caused the particles to be formed.
Ferrography yields information on the following types of particles:
- Carbon particles
- Nonferrous metals
- Ferrous metals
- Sliding wear particles
- Cutting wear particles
- Arcing spheres
- Overheated metal particles
- Insulation particles and fibers
- Dirt and debris
- Film particles or “varnish”
Arcing creates carbon particles in the oil, so some of these may be expected. As the number of carbon particles increase, or as the size of the carbon particles increase, these conditions are of more concern. Carbon particles are abrasive. When they are incorporated into the film that forms as the oil ages, the operation of the LTC prematurely wears out the contacts. This can cause overheating, accelerated contact erosion, or even failure in extreme cases.
Nonferrous metal particles in an LTC are generally conductor removed from the contact surfaces. Normal operation creates sliding wear particles and arcing spheres. These are not usually of much concern unless the number and size of the conductor metal sliding wear and arcing particles increase substantially.
In contrast, ferrous metal particles are almost always of significant concern when found in an LTC oil analysis. Iron containing (steel) components in a load tap changers are not designed to be consumable, so normal operation should not create many ferrous particles.
Cutting wear metal particles, whether ferrous or nonferrous, are of concern. These particles are formed when the edges of two metal surfaces move across each other. That generally happens only when there is something mechanically wrong with the LTC.
Overheated metal particles do not include arcing spheres. Instead, these are more irregularly shaped metal particles, caused by either sliding wear or cutting wear. After formation, these particles encounter very high heat that distorts (and may even fuse) the edges of the particles. This can happen under normal circumstance when a suspended metal particle gets in the way of an arc during operation of the device. However, if the incidence of these particles increases, this can be an indication of abnormal operation.
Other nonmetallic particles include insulation particles and fibers, dirt and debris, and film (varnish) particles. Of these, we would expect little in the way of insulation, dirt, or other debris. Film particles are almost always found. The relative size of these particles can be used diagnostically. The more advanced that filming becomes, the larger the particles of film that are noted in the analysis. As filming becomes more advanced, the operation of the device is impeded creating abnormal conditions in the LTC that are generally of concern.
Particle count distribution data and the analytical ferrography results are considered and interpreted together. If there is a clear conclusion from evaluating data that potentially hazardous conditions are occurring in the device, the preventative maintenance interval should be shortened so that the LTC may be cleaned and inspected to evaluate and correct the cause of the abnormal results. If conditions are not so clear cut, it may prove to be more advisable to monitor the LTC more closely than annual testing of the insulating liquid. In such cases, it is always a good idea to retest both the particles and filming compounds analysis and the dissolved gas analysis at the shorter interval. Each of these important tests is sensitive to a distinct group of conditions in the LTC , so monitoring and testing is much more complete when both are run.
Just as vacuum interrupter LTCs do not generate much combustible gase, these devices also do not produce significant quantities of particles. The oil in a vacuum LTC should continue to be clear of any suspended particles, or the operation of the device should be reviewed. Maintenance may be necessary.
Particles and filming compounds analysis should be performed as routine monitoring tests for the insulating liquid in LTCs along with the liquid screen, moisture content, and dissolved gas analysis. For abnormal conditions not involving either rapid aging of the oil or high moisture, closer monitoring of load tap changers should include particles and filming compounds analysis and dissolved gas analysis. When results indicate that an inspection or early maintenance should be performed, it is critical that the unit be thoroughly cleaned and inspected, while the oil is replaced or reclaimed. After cleaning and inspection, worn or damaged components should be repaired or replaced, as appropriate.
Next week’s discussion will be about performing these particle analysis methods on insulating oil samples from other types of electrical equipment.
