By Michel Belanger
Detection techniques serve as a warning to developing abnormalities in a transformer or one of its components. Detection techniques are comprised of parametric measurements and visual inspection. If a parametric measurement or visual inspection finds an abnormal condition, we either proceed with other tests which can better specify the abnormality, or we carry out a detailed inspection. In minor cases, we plan for a shut down of the unit. In serious cases we will remove the equipment from service without delay.
The parametric measurements most often used are the current, the voltage, the internal pressure of the tank, the oil level, the oil and winding temperature, gas in oil analysis, and winding power factor, to name a few. The least frequently used measurements include the load tap changer acoustic vibration, acoustic surveillance of partial discharge, etc.
The use of one or other of the detection techniques is justified by the nature of the transformer, its utilization, the cost to replace it and the redundancy of the electrical system. In general, the more unique and costly the transformer, the more elaborate the detection system.
The frequency of the input of information, whether it be for the parametric measures or others, is dictated by the speed of the development of the abnormality which we wish to detect. For example, follow-up of the concentration of antioxydant in the oil requires a sample every five years because the consumption rate of antioxydant is relatively slow. In comparison, the follow-up of the current by differential relay requires that the current be sampled 6000 times per second, because an internal defect could degenerate in a few milliseconds. So we can see that the spectrum of frequency of input is quite varied.
The cost of retrieving information is getting cheaper, thanks to applied research, technology development and the spread of communication networks. As a result, operators have access to an increasingly large volume of information that is more accurate, allowing them to make quicker and more accurate decisions and providing them with new areas of decision making never before available.
Tests
A whole range of tests exists for diagnosing the state of a transformer. It is neither economical, nor technically justifiable to apply all the tests available.
Let us consider that there are three categories of tests: the acceptance tests, the routine tests and the tests after a breakdown.
The acceptance tests are used to confirm that the unit was received in good condition from the manufacturer. If these tests are done at the factory, they should be available for later comparison, since this data will serve as a reference, especially following tests after startup.
Routine tests are those carried out periodically in order to monitor the condition of the transformer. All the tests are not carried out at the same time. By necessity certain tests are performed more often such as those for dissolved gases.
Among the most usual routine tests are:
On-line monitoring devices are available to measure temperature, level, gas in oil, humidity in paper, and vibration signature of load tap changer.
When there is a breakdown, tests are used to determine the cause of the shut down. We can carry out an analysis of dissolved gases in oil by taking gases at the accumulation relay to carrying out an analysis of the oil. An external visual inspection could reveal some signs. Sometimes it will be necessary to carry out an internal inspection.
Frequency of transformer tests
The main objective of a transformer verification is to obtain necessary information in order to:
How often tests are carried out is a decision made following the analysis of historical data and the importance of the equipment. If there is no historic data and we are looking at new or newly repaired equipment, we have to adopt a tight sampling rate - say a couple of weeks to a couple of months between each sampling - and when we are convinced of the integrity of the unit we reduce the rate. If however, we are faced with a unit with a history of being problem-free, then a normalized (yearly) sampling is sufficient. The importance of a specific transformer in the electrical distribution influences the rate of sampling. If the equipment is essential or operates close to its nominal capacity, it would be judicious to carry out a tighter follow-up.
The latent fault follow-up of the interior of the tank is mainly determined by past analysis of dissolved gases and the utilization of the unit. If the level and the rate of change of the concentration of gases are low, an annual sampling is generally sufficient. The rate should be augmented if the gas levels increase, if a problem is suspected, or if the equipment is operating under more demanding conditions.
It is important to mention that the concentration of dissolved gases depends on the conditions under which the transformer is operating. Consequently, the load variations and the operating temperature will lead to a variation in dissolved gas concentrations. So, when an annual sampling campaign is undertaken, it should be done at the same time each year and preferably done after the maximum loading period of the transformer, so that sampling is done under the same conditions from one year to the next. This allows a fault condition to be more accurately noticed. This method of testing is recommended as fault conditions are more accurately detected immediately after a period of elevated stress. Waiting may allow the dissolved gases to diffuse and become less detectable.
The frequency of inspections is determined by the usage and importance of the unit. Generally, this comes down to: - a daily or weekly inspection - a semi-annual or annual inspection - a triennial or quinquennial inspection A daily or weekly inspection allows for the detection of any operational abnormalities, as well as any increase in the temperature of the oil or a number of operational abnormalities in the load tap changer.
A semi-annual or annual inspection is best suited for sampling and analyzing of insulating oil. This allows for accurate tracking of changes to the oil and helps track latent faults. The silica gel is checked and replaced if necessary.
A triennial or quinquennial inspection allows for a complete inspection of the unit. This inspection includes testing the windings, inspection of the bushings, and allows for inspection of the components and accessories that are only accessible when the transformer is shut down.
The above mentioned frequencies of inspection are suggested for units functioning in an industrial environment having: little or no dust, a relatively constant load, and good power quality.
Power quality is rather difficult to evaluate. However, we can perceive the parameters which degrade the quality of the sinewave. These parameters are:
Frequencies of Inspections and Detection Tests
Inspections and tests allow for evaluation of the transformer's components and condition by a non-destructive and non invasive method. Tests on the insulating oil allow for evaluation of the chemical properties, as well as any by-products and contaminants. By contaminants, we mean all foreign products of insulating oil.
In Table 2 you will find recommended frequencies of tests applicable to transformers and the necessary information to implement a maintenance program. You should adapt the suggested frequencies to your own operating conditions.
Implementation details
Two measurements should be taken after shop repairs, major field work or start up of a unit - measurement just before energizing and measurements following energizing or loading. In this way it is possible to detect all internal faults at an early stage
The minimum waiting time for sampling, after adding oil, depends on the temperature of the equipment (oil viscosity). However a common practice is to have a minimum waiting period of 12 hours before returning into service any transformer whose repairs involve a change to the insulating oil. The waiting period is also applicable to oil sampling to allow the oil to blend. The waiting time before sampling could be reduced by half if a forced pumping system is in place and has been working a minimum of two hours. In this way the mixing of the oil is more efficient and reduces the stagnation zones, thus rendering a more representative sampling.
When sampling a sealed transformer, one must always consult the pressure indicator. This shows whether the tank is in negative or positive pressure with respect to the atmosphere. A sealed transformer is in negative pressure during cold periods or when lightly loaded. A transformer, where the internal pressure is negative, should not be sampled unless adequate measures are taken to avoid the risk of introducing air bubbles and compromising the insulation of the windings. The pressure indicator also shows the presence of leaks. If the pressure always indicates zero, you can suspect a leak. Leaks should be avoided to stop the introduction of water and oxygen into the tank. Water and oxygen accelerate insulation aging. You can also detect a leak from DGA by looking at the nitrogen to oxygen ratio. If a leak exists, this ratio would tend toward the atmosphereic ratio of 78 per cent / 21 per cent or 3.7.
Michel Belanger is the owner of SEIDEL Inc. He is an electrical power systems consultant and designer of diagnosis systems for the industry.
ET