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TRANSFORMER OIL TESTING
Trusting The Results Of Dissolved Gas Analysis
By William Morse
Dissolved gas analysis (DGA) has been an industry standard for the detection and determination of faults in transformers for over 30 years. Developed in the late 1960s, DGA has been recognized worldwide as the main tool to prevent catastrophic failures of power transformers. Taking an oil sample is just the first step in the evaluation process of the operating status of these large pieces of equipment. Oil is used as a coolant and an insulator in large transformers. It can be called the blood of a transformer. Just like a doctor analyzing your blood to determine your health, DGA provides the engineer the means to determine the health of transformers. One caveat remains - the analysis is performed, but are the results valid?
Dissolved Gas Analysis has been accepted as the industry standard for the determination of incipient faults in transformers. Provided the methods are followed properly, the results obtained from this service should give the engineer the information needed to make an informed decision on the operating status of a functioning transformer. However, since the equipment required to perform DGA is now much cheaper and easier to operate, many more labs have begun offering this service. This is good for the industry but at the same time can be harmful since the results from these new labs may not be up to the reliability of labs that have been performing this type of analysis since its inception.
There are three accepted DGA standards along with one new one that has gained much recognition over the past few years. All four methods require an oil sample. The sample is then manipulated in such a way to remove or extract the gases found within the sample. The gases are then separated using a gas chromatograph (GC). The GC is a precise analytical instrument that comprises an oven, some columns and one, two or three detectors. The gas extracted from the samples is injected into the GC where the columns separate the gases. The columns are kept at a constant temperature by the oven, which helps in the separation of the gases. When the separated gases exit the columns they flow into the detector, which has the ability to quantify the gases. A GC is easy to calibrate providing the oven temperature, carrier gas flows and detector sensitivity remain constant. With today's machines this is really not a problem. DGA has another step, which is the extraction of the gases. This is the part of the analysis where many errors may occur and a calibration standard is required.
Developed back in the late 1960s the vacuum extraction method was the first accepted ASTM standard for DGA. It essentially sucked the gases right from the oil. By introducing the oil to the vacuum, the gases dissolved in the oil are liberated where they can be collected and then injected into a GC. This method was developed in our labs at Morgan Schaffer. There are a few problems with this method; one being the high vacuum system and the second being the Mercury involved. A typical high vacuum degassing apparatus is shown in Figure 1. Due to the efficiency of the extraction process only a small amount of sample is required. In this case a 30 cc syringe of oil is all that is needed. Once the oil is exposed to the vacuum, the gases are released where they are isolated. By using the mercury as a piston the gases are compressed and brought to atmospheric pressure. They can then be injected into a gas chromatograph. As long as the apparatus is operated properly it is a safe system. But as stated earlier, working with the high vacuum is a concern and the system should be manipulated with care. The mercury can make many people uncomfortable but it is completely isolated and the technician need not worry about coming into contact with it. As well, it is in its elemental state and therefore much safer to work with.
One note about sampling before proceeding; the analysis can only be as good as the sample obtained. Proper sampling procedures must be followed. Sampling oil from a transformer may be a simple operation but always ensure that the drain valve is flushed. This is an area of stagnant oil that will not be part of the oil circulation flow within the transformer. Make sure that a sufficient amount of oil is allowed to flow through the valve to remove the stagnant oil. If this valve is on the bottom of the transformer there could be a lot of sediment or free water present. Again, make sure that the valve is well flushed. The sampling containers must be rinsed as well. This can be done with the oil that is being flushed from the transformer. Once the drain valve is flushed, use a bit more oil from the transformer to rinse the sampling containers. In Canada and the US the container of choice is a glass syringe. It is easy to work with and can be shipped easily.
The Stripping Method was accepted by ASTM during the 1990s. This method avoids the use of the high vacuum degassing apparatus and the Mercury. Samples of oil are injected directly into the instrument where a flow of nitrogen gas is allowed to bubble through the oil. The nitrogen forces the other gases to come out of solution where they are allowed to flow into a gas chromatograph. This sounds like a much easier method to follow but there are problems with this method as well; one being the extraction efficiency, another being that the machine is much more complicated.
To be sure of proper operation, an oil standard must be used to ensure that the extraction efficiency is at its best. A calibrating gas should be used as well to calibrate the gas chromatograph. One of these instruments can be seen in Figure 2.
Headspace has recently been accepted by ASTM as an approved method for the determination of dissolved gas in transformer oil. As with the Stripping Method, a high vacuum system is not required (see Figure 3). Obtained oil samples are put into vials and the vials are first purged with argon gas. Once the oil is put into the vials, a blanket of argon is kept above the oil. Precise volumes of both the oil and gas must be maintained. The vials are agitated for a period of time, allowing the gas that is dissolved in the oil to escape into the blanket gas. This blanket gas or Headspace is then put into the gas chromatograph where the analysis is made. The problems that are inherent with this method are that the volume of oil must be precise, the temperature of the agitation bath must be kept at the optimal temperature and the pressure must be kept constant.
To be completely sure of the results, an oil standard should be run though the instrument to ensure extraction efficiency, just as in the Stripping Method.
The Shake TestŪ method (Figure 4) is a new method that has not yet been accepted by ASTM, but the principles are similar to Headspace. Oil samples are obtained using a Shake TestŪ syringe. The oil sample is larger than the other methods but the equipment needed to perform the analysis is much simpler to operate. The Shake TestŪ syringe is a 100cc syringe. To extract the gases, the technician mixes the oil in the syringe with a fixed quantity of CO2 free air. This takes approximately one minute. Then the syringe is attached to a portable GC where the analysis of the gases is done.
This method allows the lab to be moved to the field should an emergency arise, since all that is needed is the GC, which is portable, a laptop computer and the syringe. The complete analysis can be performed on-site in less than five minutes. The specially designed and calibrated syringes are all that is needed to extract the gases. The calibrating gas that is provided with the instrument is composed to calibrate the GC and the calibration levels of the gases found in the calibrating gas are designed to work specifically with these syringes. As long as the technician follows the Shake TestŪ procedure, an oil standard is not required. However, to be completely sure, an oil standard will confirm this method just as in the other methods.
Developed in our labs at Morgan Schaffer, the True North oil standard is now available to help labs calibrate their equipment. It can also be used by transformer owners to ensure that their lab is performing DGA properly. The need for an oil standard was observed when some of our customers sent oil samples taken from their transformers at the same time, to different labs. The results were questioned since they did not match nor were they within the expected standard variation. Every few years ASTM would set up a round robin for many of the main labs in North America. They would send to the labs, samples of oil standards that they had produced. Once the results were in, it showed that more work was needed to make sure that the labs were following methods and calibration procedures properly. One key to unlocking this problem was to make available to the industry with a certified DGA oil standard.
True North was a project that was two years in the making. It sought to create a standard that was stable, that could be easily shipped around the world and inexpensive; not only to make but cost effective enough for the labs to use on a daily or weekly basis. The first hurdle was how to make the standard. Obtaining known quantities of the different fault gases to dissolve in the oil was no easy task. Morgan Schaffer started first with new Voltesso 35, one of the transformer insulating oils that is common in Canada. Using a specially designed degassing apparatus, virtually all gas was removed from the oil. The difficult part was how to get the gas into the oil. The oil was then put into containers that allowed for expansion and contraction. This enabled the gas to go into the oil at atmospheric pressure. It is well known that transformer oil will generate or lose some fault gas when it is exposed to UV light. Therefore, the containers had to be kept out of the light while the oil absorbed the gas. They also had to keep it out of the light during storage. Storage was another issue. If during storage the standard became unstable and gas levels in the oil changed due to reactions, this of course would be unacceptable. It was found that at a level of 100 ppm in oil, the standard did remain stable for a period of months. To get True North to our customers, all that was required was to transfer the oil into syringes and ship them out. To guarantee the results of the standard the oil must be analyzed within four weeks after reception.
True North was sent out to a number of labs in Canada. The labs involved were performing DGA according to the accepted ASTM standard D-3612. These labs followed one of the three methods: vacuum extraction, stripping or headspace. The results obtained were not very comforting. In some cases, greater than 90 per cent variation in the expected results were sent back. This showed that procedures were not being followed and calibrations were not done properly. Calibration of the gas chromatograph is a simple procedure but seeing that the extraction of the gas from the oil is the primary step in the analysis, a verification of the extraction should be done as well. This is where True North will help labs standardize their equipment and the industry as a whole will benefit. The results from a dissolved gas analysis are used to make important and expensive decisions. If the results are unreliable; costly, incorrect decisions can and will be made.
The latest advance for the detection of faults in transformers is the development of on-line systems that continuously monitor transformers for incipient faults. The Calisto Hydrogen and Water monitor is designed to continuously monitor transformers for the generation of Hydrogen and Water. Hydrogen is developed in all transformer faults and is a key gas that can be used to warn of developing faults. Water damages the solid insulation of transformers and must be kept at a minimum. By monitoring these two components there is an extra insurance level provided to the operation of the equipment. One nice property of Calisto is that calibration is not required. Therefore, a True North standard is not needed.
Other on-line systems do require calibration. Fuel cell technology has been used for many years but calibration is still required in the field. The transformer nursing unit and the on-line transformer GC are new developments that will provide a complete DGA for questionable transformers. But these are very expensive monitors and are only used on very critical units. However, these devices require continuous calibration. An oil standard would be beneficial to ensure their operation. In conclusion, dissolved gas analysis has come a long way over the years. To ensure the consistency of the results; methods and procedures must be followed. The True North DGA standard will assist the industry by giving the labs the ability to check their methods and ensure reliable results.
William Morse is the Director of Sales and Marketing with Morgan Schaffer Systems Inc. ET
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