A power transformer insulation system consists of solid (paper) and liquid (insulating oil) insulation. Their insulation strength degrades over a period of time depending on transformer loading and the surrounding environment condition. The characteristics of by-product generation and changes in properties provide a good indication of the insulation condition of the transformer. Sudden increases or drastic changes in these by-products provide substantial warning to engineers for necessary actions needed to avoid any disastrous consequences. The techniques in monitoring these phenomena are mainly governed by the standards published by ASTM and IEC. The diagnostic methods of insulation degradation are categorized into electrical and chemical analysis.
Electrical Diagnostic Techniques
Electrical diagnostic techniques for condition monitoring or for studying the degradation of electrical insulation in power transformers are relatively new. Partial discharge (PD) is an electrical phenomenon that occurs inside a transformer and the magnitude of its discharge could cause progressive deterioration and lead to insulation failure. There are a number of PD measurement guidelines available in ASTM. One of them is ASTM D1868 (test method for detection and measurement of partial discharge pulses in evaluation of insulation system). It is a specific guideline for the detection and measurement of PD (corona) pulses at the terminals of an insulation system under an applied test voltage, including the determination of PD (corona) inception and extinction voltages as the test voltage is raised and lowered. It is also used for determining quantities such as apparent charge and pulse repetition rate together with such integrated quantities as average current, quadratic rate and power, which are essential in the design and inspection of molded, laminated, and composite insulation of power transformers. Because the presence of partial discharges (corona) in an apparently solid insulation may be an indication of the existence of internal cavities, which may lead to disastrous consequences.
Dielectric breakdown voltage measurement of insulating oil is another electrical diagnostic method that is commonly used for transformer insulation assessment. ASTM D1816 is the standard method that has been used in diagnostic and laboratory investigations of the dielectric breakdown strength of oil in insulating system. Dielectric breakdown voltage measures the ability of the liquid to withstand electric stress without failure. Low breakdown voltage indicates the presence of contaminating agents such as water, dirt, cellulosic fibers, or conducting particles in liquids. There are other electrical diagnostic methods that test the dissipation factor (or power factor) in new electrical insulating liquids and in-service transformer oil.
Saha had provided a significant contribution in reviewing the modern electrical diagnostic techniques for insulation condition assessment in transformers. In recent years new diagnostic methods have been promoted, complementary to the classical PD, dielectric breakdown voltage and dissipation factor measurement. Only some of these methods are covered briefly in this section, amongst which are time domain polarization and frequency domain polarization measurement as published in. Insulation assessment by time domain polarization measurement is achieved by calculating the response function of a return voltage from a dielectric that is charged with DC voltage for a long period of time. Osvath et al stated that the smaller the time taken to reach the return voltage peak value, the worse is the insulation condition of the transformer. On the other hand, frequency domain analysis uses AC voltage and measuring on the dissipation factor as a function of the frequency of the AC voltage. Gafvert et al. compared the result from time domain and frequency domain measurements. He reported that the former one is useful but sensitive to systematic errors while the latter is the better for fieldwork.
The report published by CIGRE Task Force 15.01.09 highlightes the importance and problems in the interpretation of results from return voltage, time and frequency domain polarization measurement. It stated that the influence of oil gap (condition of oil conductivity) has a significant impact on the dielectric response which should be taken into consideration while attempting to diagnose the moisture level of solid insulation.
Chemical Diagnostic Techniques
Chemical diagnostic techniques have important roles in the current trend of power transformer insulation assessment. C57-106 of IEEE highlights the significance of oil characteristics in affecting the overall reliability of the transformer performance. It also outlines the standard test methods that should be used in detecting these by-products, such as moisture analysis by ASTM D1533, dissolved gas analysis (DGA) by ASTM D3612, and furan derivatives measurement by high performance-liquid chromatography (HPLC) in ASTM D5837. In addition, tensile strength of the paper is monitored by using degree of polymerization (DP) measurement in ASTM D4243.
Electrical characteristics of new or used insulating oil are the major concern in insulation strength monitoring. Its characteristic are affected by excessive water content. High level of water content renders the insulating oil unsuitable for use in a power transformer due to the deterioration of its dielectric strength - such as lower dielectric breakdown voltage. The use of ASTM D1533 that utilizes the method of Karl Fischer titration techniques is the current practice of assessing moisture level in transformer oil.
DGA by ASTM D3612 is accepted worldwide as another technique for the detection of incipient insulation faults. Insulating oil and oil-immersed insulation materials decompose under the influence of thermal and electrical stresses, and therefore, generate gaseous decomposition products of varying composition which dissolve in the oil. The amount of each individual is used to indicate the type and degree of the abnormality.
The gases of interest are:
1. Hydrocarbon gases and hydrogen: CH4, C2H6, C2H4, C2H2, C3H8, C4H10 and H2
2. Carbon oxides: carbon monoxide (CO) and carbon dioxide (CO2)
Emsley et al discovered that a healthy transformer has less than 0.05ml of hydrocarbon gases per 100ml of oil. Detailed analysis on interpretation of gas-in-oil analysis using IEC 60599 and IEC TC 10 databases is available in. Generally, a surge of these gases is normally caused by:
1. Corona or partial discharge
2. Thermal heating
3. Arcing
Current techniques of insulating paper strength diagnostic are mainly by DP measurement using ASTM D4243 and furan analysis by HPLC using ASTM D5837. Transformer paper is a sheet of material made from organic cellulose, which consists of 90% wood cellulose, 7% lignin and 3% hemicelluloses. In ASTM D4243, the quality of the cellulose is measured by determining the average viscometric DP of the new or aged transformer paper. DP measurement is also defined as the measurement of the average number of glucose monomer units available in the cellulosic paper. Due to the impracticability of collecting paper samples from an operating power transformer, furan analysis by HPLC using ASTM D5837 is an alternative for solid insulation assessement in power transformers. Furanic compounds are oil soluble derivatives from the degradation of cellulosic materials used in the insulation systems. By using HPLC equipped with a suitable analytical column and UV detector, a high concentration of furan measurement indicates serious cellulose degradatin caused by aging or incipient fault conditions. Also, the result from this test can be considered complementary to the test result from DGA.