Major power transformers are filled with a fluid that serves several purposes. The fluid acts as a dielectric media, an insulator, and as a heat transfer agent. The most common type of fluid used in transformers is of a mineral oil origin. Other types that are not as common include the askerals and silicone types. This brief discussion will be limited to those units that utilize the mineral oil type fluids.
During normal use there is usually a slow degradation of the mineral oil to yield certain gases that collect in the oil. However, when there is an electrical fault within the transformer, gases are generated at a much more rapid rate. There are three major types of electrical faults that differ in their severity. The least severe is a partial discharge or corona, localized hot spots are next in severity, and the most severe is arcing. There are typically nine fault gases that are analyzed and each of the above faults generates certain key gases and a distribution pattern of these gases. Thus by determining the various gases present and their amounts, one can infer the nature of the fault giving rise to them.
There are a couple of different types of transformers; those that have a gas blanket above the oil and those that are completely oil filled. For those units that have a gas blanket one could analyze a sample of gas from the gas blanket. However there are some disadvantages to this method. Firstly the electrical faults usually occur beneath the surface of the oil and the gases that are produced must saturate the oil, diffuse to the surface, and then collect in the gas blanket. These processes introduce a time factor into the early detection of a fault that is undesirable. Secondly gas blanket analysis is not applicable to those units that are completely oil filled. On the other hand dissolved gas analysis is applicable to all types of transformers and gives the earliest possible detection of the fault gases. This procedure only requires proper collection of a sample of the transformer oil, extracting the gases from the oil via a high vacuum apparatus, and then analyzing the extracted gases for their identities and amounts.
Dissolved gas analysis is probably the most widely used preventative maintenance technique in use today to monitor the operation of transformers. Properly used it can be a powerful tool in a well disciplined maintenance program. Depending on the location of a transformer and the nature of its usage, an appropriate dissolved gas analysis schedule can be set up. The more critical the unit is the more frequently it should be sampled. When an adverse situation is detected the sampling frequency should also be increased. This latter philosophy allows one to determine how rapidly the gases are being generated and thus how serious the problem might be so that proper action can be taken before the unit suffers additional damage. It is also quite important to maintain a history of each unit so that one can determine if any gases are residual ones from a previous fault or are they due to a newly developing situation.
If a fault is detected in a transformer there are other tests that might be recommended that can help to locate the site of the fault within the unit. The more information that can be obtained before repairing a unit, the less down time will be required. Working with laboratory personnel that are familiar with the various methods of interpreting dissolved gas analysis in conjunction with other tests and the history of the unit are essential to properly utilize this technique in a preventative maintenance program.