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DISTRIBUTION On-line monitoring of oil dielectric breakdown strength by Tim Cargol, Weidemann-ACTI, USA This paper discusses the current methods of dielectric breakdown strength testing and reports on a new non-destructive method. Oil dielectric breakdown strength is a valuable indicator of the insulation condition in virtually all oil filled equipment. Many damaging phenomena such as moisture ingress, particulates, burning, overheating and carbonization result in changes in oil dielectric breakdown strength. By monitoring for changes in breakdown strength, many problems with oil filled equipment can be detected and repaired early, before they become costly failures. Oil is used in high voltage apparatus for two primary purposes, to act as a coolant and to electrically insulate the high voltage components. Under stress and over time, oil can lose its ability to electrically insulate. A dielectric breakdown strength test directly measures the oil’s ability to electrically insulate. The test electrically stresses the oil to the point of failure and records those conditions. By applying a high voltage stress to the oil, such tests simulate the stresses the oil is under in actual use. In equipment with electrical contacts, such as on-load tap changers, circuit breakers, and regulators the oil has an additional function -to quench the arcs that are produced under the normal operation of these devices. The by-products of these switching arcs degrade the oil’s breakdown strength, and it is the dielectric breakdown strength of the oil that determines the oil’s ability to quench the arc. Thus, in normal operation of contacting equipment, the oil is a wearing component that must be checked and maintained for optimal performance. While the oil in high voltage equipment is seldom allowed to deteriorate to the point where it directly causes a failure, monitoring the condition of the oil and maintaining its quality can make a significant difference in the overall health of the equipment. Unusual changes in the dielectric breakdown strength can be indicative of serious problems such as overheating conductors or moisture ingress. Continued use of lower quality oil can have a damaging effect on the cellulose insulation, especially if the oil has a high moisture concentration [1]. As a transformer’s cellulose insulation is not replaceable without an overhaul, and the oil can be easily cleaned and dried on site, it can certainly be considered worthwhile to maintain the oil in the best condition, lest it contribute to any premature aging or wear. Existing test methods There are several existing methods for the measurement of dielectric breakdown strength such as the ASTM D877, ASTM D1816 and IEC 60156 methods. While these methods can often be performed on-site with portable equipment and are valuable laboratory tests, they suffer from poor repeatability, and due to their destructive nature, cannot be used on-line. Advancements have been made in controlling the destructive energy released by these test devices [2], but not to the level or cost that would be appropriate for on-line use. Like the arcs produced in contacting equipment, the arcs produced in the test instruments degrade the breakdown strength of the oil. Since the dielectric breakdown strength is the quantity under measurement, this limits the number of successive tests that can be run on a given sample. In the ASTM methods, five test shots are performed on a given sample before it must be discarded. With only five samples, it can be difficult to get a statistically valid representation of the oil and, in fact, the ASTM standards allow a range of 92% of the mean over the five test shots to be considered valid [3], [4]. With several types of chemical and physical particulate contaminants commonly present, oil can be an inhomogeneous media. Temperature variations can locally influence the relative saturation of moisture; turbulence of flow and proximity to sources of pollution can influence the type and concentration of particulate contaminants. Thus, it can be difficult to get a representative sample of the oil’s true condition with the small sample sizes that are used in existing test instruments and the limited number of specimens that can be drawn from the finite size of the oil compartments. One can imagine trying to taste a bowl of soup with a spoon that will not hold all of the ingredients. The inhomogeneity of the oil, combined with the fact that the number of test shots that can energize - May 2006 - Page 22 be administered is limited further reduces the capability of existing test methods. It is not surprising that while the importance of dielectric breakdown strength is recognized, less faith is placed in the ability of the standard test methods to measure it accurately. New developments The key to both improving the accuracy of laboratory test methods and enabling on-line testing is the reduction of energy dissipated during the breakdown of the oil. If the test shots do not damage the oil, more test shots can be performed and a more accurate statistical sample can be developed. Traditionally, existing test methods used a rather bulky but simple scheme to generate the high voltage necessary to break down the oil. These devices consist of basically a variable autotransformer that is used to raise the voltage in the test cell until a breakdown occurs, at which point a relay shuts off current to the transformer. With all the energy stored in the magnetics and capacitance of the transformer, the energy that is released into the test cell after the relay shuts off (assuming the relay works in a timely fashion) can be some tens of joules. Many of the less expensive dielectric breakdown test sets available today still rely on the variable autotransformer approach. Recently, however, efforts have been made to reduce the energy dissipated during a breakdown even with the use of resonant test sets. These test sets limit the stored energy available during a breakdown event and can very quickly detect a breakdown and de-energize the test set. Such sets are capable of limiting the energy dissipated during a breakdown to a mere 20 mJ [2]. Unfortunately, this advanced capability comes at the price of complexity and cost, making these test sets excellent laboratory devices, but not yet suitable for on-line use. Beginning in the late 1980s work was begun at MIT to investigate new methods and devices for monitoring transformers [5]. Among the topics explored in this program were new methods to measure the dielectric breakdown strength of oil. Development continued on dielectric breakdown methods DISTRIBUTION Number of Sample D-877 Value Average breakdown time to events breakdown EastSt+carb 25 kV 20/20 127 ns E36St 32 kV 20/20 141 ns Southport 33 kV 20/20 209 ns SaltdomeT1 36 kV 17/20 183 ns AddisT2 40 kV 18/20 212 ns Clean-barrel 53 kV 0/20 >300 ns Table 1: Sample laboratory NDBD Data [10]. 20 test shots per sample, 300 ns pulse. Fig. 1: Graphical depiction of the ASTM/IEC methods of slowly raising an ac test voltage and the NDBD method of applying a high speed pulse through the 1990s and resulted in the nondestructive breakdown (NDBD) test. The non-destructive breakdown test Two factors influence the amount of energy that is dissipated in a breakdown event during a breakdown test: first, the speed at which the test voltage can be shut off, and secondly, the amount of energy stored in the system that will be dissipated after the voltage is shut off. The NDBD test addresses both of these issues by effectively miniaturizing the test both in instrument size and in time. When the voltage is raised in the existing test methods to cause a dielectric breakdown, the breakdown event itself is extremely fast relative to the voltage ramp. Relative to the 50 Hz or 60 Hz sine wave of the test instrument, a typical breakdown event with a breakdown transition time of only a few nano-seconds [6] appears to transition from a static or DC voltage. In the NDBD test, only the portion of the sine wave - under conditions that are known to cause a breakdown - are applied to the test cell. Thus, the NDBD test applies a very short DC pulse to the test cell which has a geometry and voltage stress that will cause a breakdown. The measurement that the NDBD test then performs is the time it takes for the oil to break down under these conditions. Whereas the existing methods measure the voltage of breakdown as their output variable, the NDBD test measures the time to breakdown (Fig. 1). With a proper understanding of the test conditions, the two can be quite agreeable [7]. In a typical NDBD test scheme, the oil is placed in a test cell consisting of a needle to plane gap. The needle is pulsed negative to a fixed voltage between 20kV and 30 kV depending on the sensitivity desired. Research has shown that a negatively pulsed needle to plane gap provides the best sensitivity [8], [9]. The spacing between the needle and plane is somewhere between 0,15 mm and 0,30 mm, and pulsed for a duration between 300 ns and 500 ns. The gap spacing and pulse width are normally fixed, but can be changed to bring out a greater sensitivity or to accommodate the equipment available. By using a very short pulse, the total energy stored in the system is very small. Even when a breakdown event occurs at the beginning of the pulse, the remainder of the pulse width is less than 500 ns, which is much faster than any existing method can switch off. Depending on the impedances used to generate the pulse, the overall energy available to be dissipated in the test cell can be less than 1 µJ [10]. The energy released in the NDBD test is so small that even after many thousands of test shots, no measurable dissolved gas or change in dielectric strength can be detected [9]. The output of a NDBD test is the time it takes for the oil to breakdown under the pulse. This value will be anywhere from 0% to 100% of the width of the pulse (300 ns or 500 ns). If the oil is very clean, it may not break down even under the full pulse width. Thus, the NDBD test method has an upper limit on the quality of oil it can measure, much like the upper limit on the test voltage that can be applied through existing instruments. Changes to gap spacing and pulse voltage can increase or decrease this limit as needed. In addition to measuring the time to breakdown, the number of breakdown events versus non-breakdown test shots can be used as a simpler means to classify measurements. Laboratory analysis [10] of several oil samples revealed a strikingly good correlation between the ASTM D877 breakdown strength and the NDBD energize - May 2006 - Page 24 number of breakdown events and time to breakdown (Table 1). By not damaging the oil under test, the NDBD method can perform many more shots than the five limited by existing methods. Typically a NDBD test regimen will perform between 20 and 32 test shots per sample. With this larger number of shots a smaller standard deviation can be developed with the large range of data values possible from the oil’s inhomogeneous consistency and stochastic characteristics. Bringing the NDBD test to the field Taking the NDBD test method out of the laborator y and into a field deployable monitor required some careful consideration to both the durability and cost of such a sophisticated instrument. Additionally, a remote monitor must not be able to fail in a way that could accidentally result in damage to the oil. The key to assuring this safety, durability, and reasonable cost is in how the pulse is generated. Creating and controlling a fixed width high voltage pulse can be a technically complex endeavor. However, as long as only a fixed width is required, a transmission line pulse generator is an elegant solution. In a transmission line pulse generator, a piece of coaxial cable transmission line is charged up to the pulse voltage and then discharged through a switch or trigger device. In this scheme, the pulse width is determined by the length of the transmission line and thus, it cannot fail in a way that could cause additional energy to flow into the test cell. The only modes of failure for the coaxial cable involve it becoming shorter and so it can never produce a pulse that is too long. In an actual NDBD test instrument such as the Weidmann Centurion, careful attention must be paid to avoiding any stray capacitances DISTRIBUTION that can unnecessarily store energy that could be dissipated in the test cell during breakdown. In addition to the inherent energy limitation of the NDBD test method, the Centurion incorporates special circuitry to reduce the current available to flow thorough the test cell. This special circuitry reduces the energy dissipated to virtually immeasurable levels, below 1 µJ. This is not only necessary to protect the oil from any possible damage, but also to ensure the longevity of the device since any electrode erosion cannot be repaired once the device is installed. Field experience In the fall of 2001 the first prototype of a NDBD based dielectric breakdown strength monitor was installed on the on load tap changer of a 40 MVA transformer. Much more recent data has been collected from this and other installations, but the data from the first few months of operation has proven the most interesting and dynamic seen thus far in dielectric breakdown strength monitoring. This particular transformer had suffered a leak that allowed rainwater to enter the OLTC compartment and saturate the oil with moisture. In response to an indication of low dielectric strength from the monitoring device, oil samples were taken in early August of 2001 and laboratory testing indicated an ASTM D-877 breakdown voltage of 13.9 kV and a moisture content of 85 ppm. Later in August, repairs were made and the oil in the LTC compartment was replaced with new oil. As can be seen in Fig. 2, the relative dielectric strength (a number representing the normalized time to breakdown) rebounded with the change in oil. As can also be seen in Fig. 2, the day-to-day and week-to-week dielectric strength of this installation continued to oscillate somewhat. This particular transformer experienced large swings in load current which roughly correlated with the changes in dielectric strength. It is believed that the small changes in dielectric strength were due to moisture migrating in and out of the cellulose insulation with the loading induced changes in temperature. Fig. 2: Plot of the relative dielectric strength, a normalized average of the time to breakdown, from the prototype monitor non-destructive breakdown testing in the field, and further engineering development has led to an inexpensive monitor implementing the NDBD technology. The ability to continuously and accurately measure oil dielectric strength on a live apparatus makes way for a whole new level of apparatus care and maintenance. The liquid insulation of all oil filled apparatus can be knowledgably maintained in its best condition, preventing any unnecessary wear or aging; and in contacting equipment online measurements of dielectric strength can be used to indicate the need for necessary preventative maintenance. As additional dielectric strength monitors are deployed and laborator y research is continued, the exciting field of nondestructive breakdown testing can only be expected to grow. References [1] [2] Conclusion The non-destructive method of dielectric breakdown strength testing promises to be a great improvement over the existing methods. By not damaging the oil sample under test, the new method can repeat its diagnostic many more times than the existing methods and produce much greater repeatability and accuracy. This new NDBD method also enables the dielectric strength to be measured on-line. Prototype experience has demonstrated the efficacy of [3] Fabre, J., Pichon, A., “Deteriorating Processes and Products of Paper in Oil. Application to Transformers.”, Paper 137, CIGRE Session 1960, Paris, June 15-20, 1960 Mathis, H.-J., Baur, M., Blank, R., Woschitz, R., and Überfall, T., “New Ways of Disruptive Discharge Recognition in Insulation Testing Devices,” Electrical Insulating Materials: International Issues, ASTM STP 1376, M.M. Hirschler, Ed., American Society for Testing and Materials, West Conshohocken, PA, 1999 American Society for Testing and Materials. “D877-02e1 Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes” ASTM International, West Conshohocken, PA, 2005 energize - May 2006 - Page 25 [4] American Society for Testing and Materials. “ D 1 8 1 6 - 0 4 S t a n d a r d Te s t M e t h o d for Dielectric Breakdown Voltage of Insulating Oils of Petroleum Origin Using VDE Electrodes” ASTM International, West Conshohocken, PA, 2005 [5] Kirtley, J.L., Jr. Hagman, W.H. Lesieutre, B.C. Boyd, M.J. Warren, E.P. Chou, H.P. Tabors, R.D. “Monitoring the health of power transformers”, IEEE Computer Applications in Power, Volume: 9 , Issue: 1, pp. 18-23, Jan. 1996 [6] Grzesik, R. G., Utreja, L. R., “The Charge Carrier Velocity Model of the Spark Gap”, Conference Record, 7th International Pulsed Power Conference, pp. 522-526, 1989 [7] Cooke, C. M., Hagman, W., “NonDestructive Breakdown Test for Insulating Oil” EPRI Substation Equipment Diagnostics Conference III, New Orleans, Nov. 1994. [8] Mazzetti, C., Pompili, W., Forster, E. O., “Study of the Time to Breakdown Under Impulse Conditions” Proceedings, Second International Conference on Properties and Applications of Dielectric Materials, pp. 67-69, 1988. [9] Cooke, C. M., Hagman, W. H., Final Technical Report, A Non-Destructive Breakdown Measurement for Oil Dielectric Strength Testing, Laboratory for Electromagnetic and Electronic Systems and Electric Utility Program, M.I.T., Cambridge, MA, April 1994 [10] Cargol, T., A Non-Destructive Transformer Oil Tester, Massachusetts Institute of Technology, Cambridge, MA, June 2000 Contact Dave Prentice, Whiteleys - a division of Powertech Caldus, Tel (011) 614-2231, [email protected]