Transformer: Assessing the Technical Condition
There are several thousand medium power transformers being operated in Poland. More than half of these are over 30 years old. Despite this, most of these units are in relatively good condition (in terms of insulation) and can be used for another 10-20 years. However, the one condition which needs to be fulfilled is thorough renovation and modernization.
Risk assessment of further operation and evaluation of planned modernization profitability of transformers should be individually estimated for each unit. In this case, complex assessment of its technical condition is of great importance. Based on this, a range of renovation and life expectancy is formed which depends on a given load of transformer and its installation point in the power system. These elements have fundamental importance for the solutions described.
Thorough assessment of the technical condition of transformers is also an essential issue in case of rationalization of operating costs. Considering the strategic points of the power system allows transformers to be grouped in accordance with the impact on system reliabilitythe reliability. Identification of units which have an essential importance to the power system enables the economic analysis of operation, modernization or purchase plan.
The main elements of reliable and complex assessment of the technical condition of transformers, apart from typical periodic inspections and tests, are:
diagnostics transformer condition based on modern methods of oil analysis
diagnostics of moisture content in the insulation
diagnostics of windidings and bushings
diagnostics of tap changers
Diagnostics of transformer oil
The main advantage of advanced oil tests is the possibility of early identification of detrimental physical and chemical processes occurring in transformers. The evaluation of causes and consequences of partial or complete discharges and thermal deterioration is possible by means of dissolved gas analysis in oil (DGA). Determination of physicochemical parameters of insulating oil and its furans content (especially 2FAL) allows for the estimation of oil and cellulose ageing level. It should be stated that very important stages of this analysis for the quality of the result is the method of sample collecting and its transport to the laboratory. Improper sample preparation leads to a large number of errors in analysis with the consequence of incorrect interpretation. Another important issue is the preparation and calibration of available chromatographs for the analysis of transformer oil.
Determining the level of moisture content in the insulation
An important factor influencing decisions on prolonging transformer operation is moisture content level in windings and insulating boards. Water content in cellulose higher than 2.5% results in significant intensification of deterioration processes which leads to loss of mechanical properties and causes the risk of the "bubble-effect" during short circuit and overload.
The level of moisture content in solid insulation of transformers can be found directly from various measurements of polarization phenomena. The methods which have gained practical significance are Recovery Voltage Measurements (RVM), measurements of tgδ frequency characteristics, Frequency Domain Spectroscopy (FDS) and Polarization and Depolarization Currents measurements (PDC).
Theoretically, all results of the previously mentioned test methods should be similar. However, practical experiences show that, depending on the method of measurement, moisture content results are diverse. Therefore, in order to obtain correct diagnosis, it is necessary to use at least two test methods. Energo-Complex has elaborated a method of joint RVM and PDC measurement, applied for standard tests for water content in cellulose. This type of measurement significantly increases accuracy of water content determination in transformer insulation, whilst retaining test time at an acceptable level.
Detection of winding deformation
A common problem during the operation of older units is the loosening and deformation of windings. These problems arise from effects of dynamical forces during short circuits. Winding deformation not always results in immediate and fatal damage of insulation, but significantly increases the risk of failure during subsequent short circuit or voltage surge.
The diagnosis of winding deformation is based on comparing frequency characteristics of impedance recorded on the same winding at various time intervals at adjacent phases of the same transformer or on similar units. Changes in winding geometry result in differences in the frequency response characteristic. Energo-Complex is the first company in Poland which started conducting such measurements and boasts a database of over one hundred medium and high power transformers.
The application of modern research methods and analysis of previous operation and parameters of unit simplifies decision-making processes in respect of operation and investment. Diagnostics, if used properly, help in detecting defects, avoiding serious failure in ordinary operation, but also improves economical effects by prolonging transformer operation at acceptable levels of the risk of failure. Assessment of the technical condition can be helpful for identification of units, which can operate for a 60% longer period than their lifetime defined by producers.
The algorithm of the decision-making process and estimated cost of modernization is presented below, for the example of a 30 year old, 110/15 kV, 25 MVA transformer, operated at low load without any serious breakdowns.
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