Sampling - Part 2Technical Briefs
Transformer Sampling – Part 2:
In Part 1 of this series, we discussed what information is necessary to make a correct diagnosis of the state of your electrical equipment. Just as a doctor is concerned with family history and any symptoms that may be visible, information about your electrical equipment is also critical, but is of limited use by itself. Laboratory testing is also required to get the most accurate diagnosis, but accurate and representative test results are dependent on a good and representative sample.
Most sampling from electrical equipment in the US is done in accordance with ASTM method D 923 (Standard Practices for Sampling Electrical Insulating Liquids). Two other sampling methods are IEC 60475 and IEC 60567. We will not be discussing sampling methods in this article – these sampling methods are well established – but we will discuss other issues that make for a "good" sample. These are issues that are easily overlooked or not identified in the above methods.
Safety should be your number one priority – planning for the worst case will serve you well and keep you safe. Make sure you obtain any needed permits (lockout/tagout, etc.) and permissions before you begin. Make sure you have the proper personal protective equipment and properly rated tools. NFPA standard 70E can help guide you with selecting this equipment.
Make sure you have a few extra sample bottles (they should have air tight lids) and syringes (should be ground glass). This will help save time. Also make sure you have plenty of labels and markers. It will prevent sample mix-ups if the samples are labeled as they are pulled.
Survey the area around the electrical equipment, making sure there are no electrical or tripping hazards. Also check for signs of wildlife – bees, snakes, etc. In particular, rodents, skunks, and other small animals love transformers.
Make sure that the unit is under positive pressure by following the procedure in ASTM D923 or some similar and proven method. Do not count on the pressure gauge as these can be broken or plugged. Never attempt to pull a sample if the unit is under negative pressure (vacuum). Doing so could introduce an air bubble into the equipment, which could result in equipment failure. The proper grade of nitrogen may be used to reestablish a positive pressure to the unit – use ASTM standard D 1933 Type III nitrogen.
Clean the outside of the drain valve. Material falling off the outside of the valve can very easily contaminate the sample and is often overlooked. If possible, clean the inside of the drain valve as well.
Flush the drain valve into a waste container. Over time, contaminants such as water and particulates (metal debris, paper fibers, etc.) tend to settle out of the equipment near the drain valve. A minimum of two liters (about two quarts) of oil should be flushed through the valve. Make sure that any settled contamination is no longer part of the sample. This may take two to four liters to flush out. (This extended flushing is more commonly needed with LTCs and OCBs, rather than transformers.) Often a spare sample bottle is used to collect the valve flush oil, and it is usually filled and discarded to waste several times. In this case, it is most convenient to use the last collection of flush oil (before discarding) to measure the oil sample temperature, which is needed for the lab to calculate % moisture saturation and moisture by dry weight (for oil-filled transformers only).
but planning sampling during periods of relatively steady loads and temperatures will help in obtaining representative samples. Sampling a unit soon after it was under heavy load for several days or weeks will lead to artificially high moisture results (i.e. unrepresentatively high % saturation). This can lead to a unit being deemed wet, when in reality the moisture that migrated from the paper into the oil during the heavy load has not yet come back to equilibrium (migrated back into the paper).
Ambient conditions such as rain, snow, high temperatures, wind, or very high humidity can lead to contaminated samples. Wind can carry dust that can be blown into the sample while the lid is off the sample container. High temperatures may cause contamination in the sample from perspiration.
One final thing to remember is that samples should be tested as soon as practically possible after they have been pulled. Samples that are 45 to 60 days old and are being tested for oil quality and dissolved gases are near the end of their shelf life. If they are well sealed and in the proper containers, useful and accurate data can still be obtained from them. As samples near 60 days in age, thought must be given to resampling. The exception is furans analysis and PCB testing – those tests can be performed on samples up to a year old, provided that the samples are stored under controlled conditions (shielded from heat and light).
Following established sampling methods and guidelines will help ensure that your samples, and the test results from those samples, are representative of the actual conditions inside your electrical equipment, enabling proper diagnosis to extend the life of your transformer.