Summary:
Dissolved gas analysis (DGA) has come a long way in recent years. To guarantee the consistency of the results, several methods and procedures must be followed. DGA helps the industry by giving laboratories the ability to check their methods and guarantee results, results that can be relied upon.
Dissolved gas analysis (DGA) has been an industry standard for the detection and determination of faults in transformers for more than 30 years. Developed in the late 1960s, DGA has been recognised worldwide as the leading tool for preventing catastrophic failures in power transformers. Taking an oil sample is only the first step in the process of assessing the operating condition of these large pieces of equipment. Oil is used as a coolant and an insulant in large transformers. It could be called the blood of a transformer. Just as a doctor performs a blood test to determine your health, the DGA engineer has the means to determine the health of transformers. One question keeps being asked: the analysis is carried out, but are the results valid?
Dissolved gas analysis has been accepted as the industry standard for determining incipient faults in transformers. Provided the method is followed correctly, the results obtained should give the information needed to make an informed decision about the operating condition of a transformer. However, the equipment needed to carry out DGA monitoring is now much cheaper and easier to operate, and more laboratories have started to offer this service. This is good for the industry, but at the same time it can be damaging, since the results from these new laboratories may not offer the same reliability as those from long-established laboratories.
There are three accepted DGA standards, along with a new one that has gained a great deal of recognition in recent years. All four methods require an oil sample. The sample is handled in such a way that the gases found in the sample are removed or extracted. The gases are separated by means of a gas chromatograph (GC). The GC is a precise analytical instrument comprising an oven, a number of 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 held at a constant temperature in the oven, which helps to separate the gases. When the separated gases leave the columns, they enter the detector flow, which has the ability to quantify the gases. The GC is easy to calibrate, given the oven temperature, the carrier gas flows and the detector sensitivity. DGA analysis has another step, which is the extraction of the gases. This is the part of the analysis where most errors can occur and a calibration standard is required.
Developed in the late 1960s, the vacuum extraction method was the first ASTM standard accepted for the analysis of gases dissolved in oil, DGA. It essentially extracts the gases from the oil. By introducing the oil into a vacuum, the gases dissolved in the oil are released so that they can be collected and then injected into a GC. There are some problems with this method: one is the high vacuum system and the second is the mercury involved. Because of the efficiency of the extraction process, only a small amount of sample is needed. In this case, a 30 cc syringe of oil is all that is required. Once the oil is exposed to the vacuum, the gases that are released are then isolated. By using mercury as a piston, the gases are compressed and brought to atmospheric pressure. These can then be injected into a gas chromatograph. As long as the apparatus works correctly, it is a safe system. But as stated earlier, together with the high vacuum it is a concern and the system must be handled with care. Mercury can make many people uncomfortable, but it is completely isolated and the technician does not need to worry about coming into contact with it. In addition, it is in its elemental state and therefore much safer to work with.
A note on samples before we proceed: the analysis can only be as good as the sample obtained. Proper sampling procedures must be followed. Taking an oil sample from a transformer can be a simple operation, but you must always make sure that the drain valve is properly flushed. This is an area of stagnant oil that will not be part of the oil flow circulation in the transformer. Make sure that enough oil flows through the valve to remove the stagnant oil. If this valve is at the bottom of the transformer there may be a large amount of sediment or free water. Again, make sure the valve is properly flushed. The sample containers must be rinsed as well. This can be done with the oil that is being drained from the transformer. Once the drain valve has been flushed, use a little more oil from the transformer to rinse the sample containers. In Canada and the USA the container of choice is a glass syringe. It is easy to work with and they can be transported easily.
The Stripping Method (the separation method) was accepted by ASTM during the 1990s. This method avoids the use of high vacuum degassing apparatus and mercury. The oil samples are injected directly into the instrument, where a flow of nitrogen comes into contact with the oil. The nitrogen forces the other gases out and they enter the flow into the gas chromatograph. This sounds like a much easier method to follow, but there are problems with this method: one is the efficiency of the extraction, and another is that the machine is much more complicated.
To make sure it is working correctly, an oil standard must be used to ensure that the extraction efficiency is the best possible. A calibration gas must be used to calibrate the gas chromatograph.
Headspace has recently been accepted by the ASTM standard as a method for determining dissolved gas in transformer oil. As with The Stripping Method, (the separation method) a high vacuum system is not needed.
The oil samples obtained are placed in vials and the vials are first purged with argon gas. Once the oil is placed in the vials, a blanket of argon is maintained above the oil. Exact volumes of oil and gas must be maintained. The vials are shaken for a period of time, allowing the gas that is dissolved in the oil to escape into the gas blanket. This gas blanket, or Headspace, is then introduced into the gas chromatograph where the dissolved gas analysis is performed. The problems inherent in this method are that the oil volume has to be precise, the temperature of the shaking bath has to be kept at the optimum temperature and the pressure must be kept constant.
To be completely certain of the results, an oil standard must be run through the instrument to guarantee the extraction efficiency, just as in the Stripping Method. The Shake Test ® method is a new method that has not yet been accepted by ASTM, but the principles are similar to those of Headspace. The oil samples are obtained using a Shake Test ® syringe. The oil sample is larger than in the other methods, but the equipment needed to perform the analysis is much simpler to handle. The Shake Test ® syringe is a 100cc syringe. To extract the gases, the technician mixes the oil in the syringe with a fixed amount of CO2-free air. This takes approximately one minute. The syringe is then attached to a portable GC where the analysis of the gases is performed.
This method allows the laboratory to be taken into the field in an emergency, 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 calibration gas supplied with the instrument is formulated to calibrate the GC, and the calibration levels of the gases found in the calibration gas are designed to work specifically with these syringes. As long as the technician follows the Shake Test ® procedure, an oil standard is not necessary. However, to be completely certain, an oil standard will confirm this method just as in the other methods.
Developed in our laboratories at Morgan Schaffer, the True North oil standard is now available to help laboratories calibrate their equipment. It can also be used by transformer owners to make sure that their laboratory is performing DGA analysis correctly. The need for an oil standard was noted when some of our customers sent oil samples taken at the same time from their transformers to different laboratories. The results were called into question, since they did not match and were not even within the anticipated level of variation. Every few years ASTM used to send a round robin to many of the leading laboratories in North America. They would send the laboratories samples of the oil standards they had produced. Once the results were received, it was shown that even more effort was needed to make sure that the laboratories were following the proper calibration methods and procedures. One of the keys to unlocking this problem was to make a certified DGA oil standard available to the industry.
True North was a project that took two years to create. The aim was to create a standard that was stable, that could be transported easily all over the world and that was inexpensive; not only to produce but cost-effective enough for laboratories to use it daily or weekly. The first hurdle was how to produce the standard. Obtaining known quantities of the different fault gases to dissolve in oil was no easy task. Morgan Schaffer started first with new Voltesso 35, one of the common transformer insulating oils in Canada. Using a purpose-designed degassing apparatus, practically all the gas was extracted from the oil. The difficult part was how to get the gas into the oil. The oil was then placed in containers that allowed expansion and contraction. This allowed the gas to enter the oil at atmospheric pressure. It is well known that transformer oil will generate or lose some of the fault gas when it is exposed to ultraviolet light. Therefore, the containers had to be kept away from light while the oil absorbed the gas. They also had to be kept away from light during storage. Storage was another issue. If, during storage, the standard became unstable and the gas levels in the oil changed because of reactions, this would of course be unacceptable. It was found that at a level of 100 ppm in the oil, the standard did not remain stable over a period of months. To deliver True North to our customers, all that was needed was to transfer the oil into syringes and transport them that way. To guarantee the results of the standard, the oil must be analysed within four weeks of receipt.
True North was sent to a number of laboratories in Canada. The laboratories that took part were performing DGA analysis in accordance with the accepted ASTM D-3612 standard. These laboratories followed one of the three methods: vacuum extraction, the stripping method and headspace. The results obtained were not very reassuring. In some cases, a variation in the anticipated results of more than 90 per cent was returned. This showed that the procedures were not being followed and that the calibrations were not being performed correctly. Calibration of the gas chromatograph is a simple procedure, but given that the extraction of the gas from the oil is the primary step in the analysis, a verification of the extraction must be carried out as well. This is where True North will help laboratories to standardise their equipment and the industry as a whole will benefit. The results of a dissolved gas analysis are used to make important and costly decisions. If the results are not reliable, incorrect and costly decisions could be, and will be, made.
The latest advance in the detection of faults in transformers is the development of online systems that continuously monitor transformers for incipient faults. The hydrogen monitor Calisto is designed to monitor transformers continuously for the generation of hydrogen and water. Hydrogen develops in all transformer faults and is a key gas that can be used to warn of developing defects. Water damages the solid insulation of transformers and must be kept to a minimum. Monitoring these two components adds an extra level of assurance about the operation of the equipment. A convenient property of Calisto is that no calibration is necessary. Therefore, a True North standard is not required. Other online systems do require calibration. Fuel cell technology has been used for many years, but field calibration is still required. The transformer nursing unit and the online transformer GC are the new developments that will offer full DGA for questionable transformers. However, this transformer monitoring equipment is very expensive and is only used on very critical units. These devices do, however, require continuous calibration. An oil standard would be beneficial in guaranteeing their operation. In conclusion, dissolved gas analysis has come a long way in recent years. To guarantee the consistency of the results, several methods and procedures must be followed. The True North DGA standard will help the industry by giving laboratories the ability to check their methods and guarantee reliable results.