IEC 60970 Insulating liquids - Methods for counting and sizing particles
5 Comparison and limitation of the methods
Automatic particle counters using the light interruption principle are quick and easy to use, but the following points should be borne in mind:
- With some liquids it may be necessary to modify their viscosity to comply with the operating parameters of the instrument.
- It is necessary to choose a sensor head suitable for counting in the size range required. No single head can count both very small particles (<2 µm) and very large particles (>200 µm).
- The instrument records the light interruption area of the particle and from this calculates the diameter of a sphere having the equivalent area or the longer axis of a specified ellipsoid with the same area, as established by ISO 4407. When measurements are carried out with automatic particle counters, the reported sizes are expressed as μm(c) to indicate that the particle size has been calculated from the observed cross-sectional area. Particle sizes from optical microscope counting are expressed as μm. The relationship between the two units is described in ISO 4406:
• 6 μm(c) corresponds to 5 μm
• 14 μm(c) corresponds to 15 μm.
- Automatic counters give no information as to the shape of the particles, and this constitutes a limitation with respect to the recognition of fibres. Their narrow and elongated shape results in a slight light obscuration and consequently in a very small equivalent sphere diameter. The results obtained can be different from those obtained by microscope counting. When it is important to evaluate the concentration of fibres, automatic counters cannot perform this task adequately.
- When air-saturated or over-saturated liquids are shaken manually or in a shaking machine, or given high-energy ultrasonic treatment, finely dispersed micro-bubbles may be formed in the liquid. In the optical system of the automatic counter, these micro-bubbles will be counted as solid particles.
- These difficulties are avoided when using either of the optical microscopic methods. In addition, optical microscopy may give some information about the types of particles present. These methods are, however, much more time-consuming and operator dependent and may be very difficult to count particles of less than 5 µm.
6 Types and identification of particles
The origins of particles found in insulating liquids are manifold.
In new, unenergized, equipment the insulating liquid may contain cellulose fibres plus particles from the manufacturing process. These could include iron, aluminium, brass, welding cinder and sandblasting materials.
Insulating liquids in working transformers, at both normal and overload temperatures, slowly acquire soot and sludge particles. Localised overheating over 500 ºC could be a source of carbon. The carbon particles produced in the OLTC diverter may migrate by leakage, accidents or human error into the bulk fluid compartment and contaminate the full charge.
A typical source of metallic particles is from pump bearing wear, although corrosion and arcing on metallic components may also produce particles.
Cellulose lint, sand, dust and particles of varnish, plastic or rubber can also be found in the fluid of transformers in service.
A knowledge of which particle types are present in the insulating liquid can, in certain cases, help in assessing the conditions of the equipment, in diagnosing a fault or indicating a risk of failure. The most dangerous particles are the conductive ones (metals, carbon, wet fibres, etc.). Particle identification and counting have been found to be necessary procedures of condition monitoring (CIGRE brochures 157[1] and 227[2]).
Certain particles may be identified by filtering a sample through a membrane filter and examining the residue under a microscope (EN 50353). At this stage some fibres can be identified using the dispersion staining technique and a number of metals by means of spot tests or micro chemical methods. Metallic particles can be better identified and quantified by instrumental analytical methods such as atomic absorption spectroscopy (AAS), induced coupled plasma (ICP-AES), and wet chemical analysis. A detailed description of methods for the identification of particles is, however, outside the scope of this standard.
