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Transformer Condition Monitoring Sethuraman Muthukarupp

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  Transformer Condition Monitoring Condition Monitoring Unit, Asset Maintenance, Transmission Division Page 1 of 8 TRANSFORMER CONDITION MONITORING Sethuraman Muthukaruppan Senior Engineer (Condition Monitoring) Asset Maintenance TNB Transmission ABSTRACT One of the major problems facing the power system managers is the ability to determine the expected life of the power transformer. Due to the complexity of the power transformer design, operational and maintenance policies, it is impossible to accurately predict the expected life of a transformer. Consequently, power system operators have to depend on various condition monitoring and condition assessment techniques available to determine plant condition and remnant life. This paper describes the various condition monitoring and condition assessment methods available to provide plant managers with the information necessary to make decisions on the future of the  power transformers. Effective transformer condition monitoring and condition assessment  programs allow us to accurately plan and adopt strategies for refurbishment or replacement of  power transformers. KEYWORDS  Power Transformer, Condition Monitoring, High Voltage, Electrical Testing, On-site Testing, Insulating Oil Analysis, Insulation System 1.0 INTRODUCTION The life of a power transformer is primarily governed by the life of the insulation system. The insulation system is required to provide electrical insulation between the various current carrying components of the transformer and to provide mechanical support to the windings. The insulation system consists of the paper insulation and insulating oil. This paper will describe the various tests  performed on transformers by the condition monitoring unit. These tests can be divided into two main sections which are: electrical tests and insulating oil analysis. 2.0 ELECTRICAL TESTS Preventive maintenance testing of in-service transformers has the primary objective of monitoring conditions in the insulation and evaluating the useful life still available in the transformer tested.  Transformer Condition Monitoring Condition Monitoring Unit, Asset Maintenance, Transmission Division Page 2 of 8 2.1 Winding Resistance The winding resistance is measured in the field to identify shorted turns (although this is better identified in the ratio test), poor joints, high resistance connections or contacts and open circuits. The resistance is measured on all taps of a tapped winding to ensure that the OLTC dose not open circuit during the tap changing operation. 2.2 Insulation Resistance and Polarisation Index The perfect dielectric can be represented, at power frequencies, as a lumped prefect capacitance. The application of a direct electric field to this capacitance will results in a charging current flowing for a short time giving the capacitor sufficient charge to support a voltage of V= Q/C. The time taken for the capacitance to achieve this equilibrium will  be determined by the supply source resistance. In practical dielectrics the charging current does not cease after this short period but decreases gradually to a minimum value. The taken for this minimum value to be reached depends upon the dielectric and can range from seconds to days. The insulation resistance is defined from Ohm’s Law as the ratio of the applied voltage to this residual current. In practical applications the charging current can consist of volume and surface currents. Therefore the insulation resistance measurement of plant will be affected by the condition of the insulation itself and the cleanliness of the insulation surfaces. This effect can be allowed for in some plant types by the use of guarding electrodes. The time dependency of the insulation resistance can result from electronic and ionic conductivity, dipole orientation (dielectric absorption), and space charge polarisation. As the charging current time constants are affected by the  presence of impurities, the time taken for the leakage current to settle down can be used as an insulation condition indicator. The ratio of the insulation resistance value take ten minutes after application of the measurement voltage to that taken one minute after voltage application is known as the Polarisation Index. Generally insulation in good (dry) condition has a PI greater than 1.2. 2.3 Winding and Bushing Power Factor One of the major tests performed in the field is the measurement of the winding and bushing capacitance and power factor. Capacitance measurements of each of the windings to ground and between windings is performed to provide an indication of the condition of the winding insulation and some indication of the structural integrity of the windings. Similar measurements are performed on the bushings to provide an indication of the condition of the insulation in the condenser bushing and of the power factor test points. As described above, the perfect dielectric can be represented as a lumped perfect capacitance. The charging current flowing in the capacitance when an AC filed is applied should lead the applied voltage by 90 ° . In practical insulating systems losses (caused by conduction and polarisation currents) cause the current to lead the voltage by less than 90 ° . The complement of the angel between the voltage and current vectors is called the dielectric loss of the angle δ  or DLA. The tangent of this angle, tan δ , provides an indication of the losses in the insulation and is known as the POWER FACTOR or DIELECTRIC DISSIPATION FACTOR (DDF). The power factor of the windings and the bushings is usually measured in the field as a condition assessment tool. The  power factor can give an indication of the moisture content of the paper and oil in the transformer and the bushings. Major deterioration of the insulation will also be detected. 2.4 Low Voltage Excitation Current Test The low voltage excitation test is performed to identify shorted turns or severe core damage. This method is a natural extension of the power factor test and makes use of the same equipment. The test results of a three-phase core form transformer will give a pattern of two similar currents and one lower current. This is usually the H2 phase of the  Transformer Condition Monitoring Condition Monitoring Unit, Asset Maintenance, Transmission Division Page 3 of 8 transformer as the magnetic reluctance of this phase is lower than the other two phases resulting in a lower excitation current value. 2.5 Transformer Turns Ratio The ratio of the transformer is normally measured at commissioning or after major refurbishment. The test is also  performed to identify incipient faults or after a transformer fault trip to identify shorted turns. A turns ratio measurement can show that a fault exists but does not determine the exact location of the fault. 2.6 Tap Changer Dynamic Resistance Measurement The dynamic resistance measurement detects carbonized spots and weak contacts in the mechanism of a tap changer. The advantage of this diagnostics is that not only end positions of the tap changer contacts can be checked but the complete stroke of contact movement while changing between taps. This also allows one to diagnose the diverter switch in the tap changer mechanism. 2.7 New Condition Monitoring Tools Moisture, in conjunction with the other factors, acts on the paper insulation reducing the paper’s strength and volume. This reduces the paper’s, and the transformer’s, ability to perform its function. Therefore two news tests have been introduced to determine the moisture content in the cellulose accurately and movement of the winding structure respectively. 2.7.1 Recovery Voltage Measurement (RVM) One of the critical measures of transformer condition is the moisture content of the paper insulation. It is well known that an increase in the paper moisture content will result in a corresponding increase in the transformer ageing rate. One method of determining the moisture content of paper is to use equilibrium diagrams that relate oil/water content, sample temperature and paper moisture content. However, as transformers in the field are rarely in equilibrium, this method has varying degrees of accuracy. A second method of determining the paper moisture content is to drain oil and take an a paper sample from the insulation. This method is more accurate, but costly and exposes the transformer to the atmosphere and the possibility of moisture ingress. RVM provides an indication of the paper moisture content without the drawbacks of the above two methods. RVM is non-intrusive and has proven to be accurate when compared to known paper moisture contents in oil test cells. Moisture and the decay products from insulation degradation are polar in nature. When an electric field is applied to a dielectric containing polar contaminants, the polar products become aligned with the electric field. If the levels of these contaminants increase, the time required for the dipoles to align with the applied field is reduced. This is equivalent to a reduction in the system time constant. The RVM determines the equivalent paper moisture content by measuring the time constant of the insulation system. Instrumentation software calculates the equivalent paper moisture content from the system time constant and temperature. 2.7.2 Frequency Response Analysis (FRA) The conventional techniques of ratio, resistance, DDF and even HV testing are often unable to detect winding deformation, except in the most serious of cases. Any changes in the spatial position of the winding structrue will result in relative changes to the internal inductive and capacitive network of the winding structure which produce changes in the frequency response of the transformer. FRA measures the frequency response of the transformer windings up to 10 MHz. This method involves injecting a low voltage signal of varying frequency into each end of the winding and measuring the response at the other end of the winding. The transformer under test is always disconnected from adjacent equipment. This is done to eliminate the effect of connecting equipment although it is reported that short lengths of busbar are not usually a problem. Winding movement is more likely to occur in older, aged transformers that have reduced winding clamping pressure. This is  Transformer Condition Monitoring Condition Monitoring Unit, Asset Maintenance, Transmission Division Page 4 of 8  particularly true when the transformer is placed under a high mechanical load such as experienced during fault conditions. The advantage of obtaining a baseline signature of the transformer is that future tests will be able to determine the extent of any winding distortion that occurs after the measurement has been taken. This test is particularly valuable as a  baseline reference for a new transformer prior to placing in service as well as for older transformer after re-refurbishment. A spectrum analyzer is used to excite, monitor and record the response from the transformer. A software program downloads the recorded data to a PC for analysis. Results for each phase are then plotted against frequency. Each winding is tested separately. To ensure repeatable measurements, all other windings in the transformer are left floating. The test tap position selected to ensure that the maximum amount of winding is included in each measurement. 3.0 INSULATING OIL ANALYSIS A regular program of oil testing is recommended to monitor for changes in oil quality. Specialized tests are also performed that identify specific compounds in the oil and helps determine whether fault conditions exist inside the unit. The recommended battery of tests include the following: -   Liquid power factor at 25 o  and 100 o  C -   Dielectric breakdown strength -   Moisture -    Neutralization number(Acidity) -   Interfacial tension -   Color/Visual Examination -   Sludge/Sediment -   Inhibitor -   Dissolved gas analysis -   Dissolved metal analysis -   Furanic compounds 3.1 Liquid Power Factor The IEC standard method for this test is IEC 247. This involves measuring the power loss through a thin film of the liquid being testing. Water, contamination, and the decay products of oil oxidation tend to increase the power factor of the oil. New oil has very low power factor values – much less than 0.1% at 25 o  C and 1.0% at 90 o  C. As the oil ages and moisture accumulates, or if the unit is contaminated, the liquid power factor tends to increase. This increase in liquid power factor is a direct indication that materials harmful to the paper and to the continued operation of the transformer are  building up. Many transformer owners make the mistake of having this test run at only one temperature. While the 90 o  C test is more sensitive, both temperatures need to be used. The relationship between the 25 o  and 90 o  values can help in making a diagnosis as to whether the problem is moisture, oxidation, or contamination. 3.2 Dielectric Breakdown Strength The dielectric breakdown voltage is a measure of the ability of oil to withstand electric stress. Dry and clean oil exhibit an inherently high breakdown voltage. Free water and solid particles, the latter particularly in combination with high levels of dissolved water, tend to migrate to regions of high electric stress and reduce the breakdown voltage dramatically. The measurement of breakdown voltage, therefore, serves primarily to indicate the presence of

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Sep 22, 2019
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