A manufacturer s view of bushing reliability, testing and analysis. Lars Jonsson Håkan Rudegard

Transcription

1 A manufacturer s view of bushing reliability, testing and analysis By Lars Jonsson Håkan Rudegard 1

2 A manufacturer s view of bushing reliability, testing and analysis Lars Jonsson Håkan Rudegard ABB Sweden 1. Introduction Capacitance graded bushings facilitate electric stress control through the insertion of floating equalizer screens made of aluminum or other conducting materials. The capacitor core in which the screens are located decreases the field gradient and distributes the field along the length of the insulator. The screens are positioned coaxially resulting in an optimal balance between external and internal puncture strength; see Figure 1. Capacitor cores are generally impregnated with transformer-grade mineral oil and placed inside an insulating envelope of oil and porcelain, which prevents the bushing oil from mixing with the transformer oil. Systems incorporating this design are called Oil Impregnated Paper (OIP) bushings. In a Resin Impregnated Paper (RIP) bushing, the oil is replaced by a curable epoxy resin to form a solid capacitor core. The latest addition to the dry concept is Resin Impregnated Synthetics (RIS), where the paper web is replaced by a polymeric fiber. The outer insulation is made from an insulation material, ceramic or polymeric, and is usually of the anti-fog type with alternating long and short sheds to produce a long creepage distance with good self-cleaning properties. 2. Some methods used in condition assessment and failure analysis The vast majority of all bushings perform well throughout their service lives. However failures do occur and some of the more common mechanisms are: loss of ground connection, electrical and thermal stress that exceeds the design limits, mechanical stress, ingression of moisture, contamination, and ageing. Factors such as improper sealing systems and ground connections, over-voltages of various types, overloads, vandalism, excessive line pull, seismic events, handling errors and improperly selected ratings are often involved. When failures occur, it is essential to carry out proper root-cause analyses and possibly condition assessments of the installed base to prevent further incidents. The following is a review of some of the most commonly used methods in condition assessments and failure investigations. Depending on the results and type of investigation, additional methods may be used, e.g. various types of microscopy and chemical analyses. The most important is a systematic approach to observations, data collection, event classifications and knowledge of the bushing design and production methods. 2.1 Dielectric Frequency Response (DFR) analysis The DFR test is a measurement of the dielectric properties (i.e. capacitance and dissipation factor) as a function of frequency, while the standard dissipation factor test is only performed at 50 or 60 Hz. DFR has a lot to offer when conducting failure analysis, e.g. detection of contact problems and partial breakdown inside the capacitor core which can be seen from the high frequency spectrum. For field assessment the low frequency spectrum offers a better moisture valuation compared to 50/60 Hz testing alone, in particular for spare bushings. This is illustrated by an example Condition assessment of spare bushings The heat generated from the dielectric losses has to be dissipated for all operational conditions. An uncontrolled temperature increase and eventually a breakdown will otherwise occur. This is assured by a proper design in combination with a stable production process. It is still recommended that a dissipation factor test be carried out before bushings are put into service. The dissipation factor has a temperature dependency that increases with the moisture content; see Figure 1. It is for this reason both more revealing and relevant to measure the dissipation factor at operating temperature. This is obviously not possible for spare bushings for example, and DFR may provide an alternative method thanks to its high sensitivity in the low frequency spectrum. 2

3 Figure 1 Dissipation factor at 50 Hz as a function of temperature. The curves diverge more at operating temperature the higher the moisture content. It can also be noted that the dissipation factor for 90 C at 50 Hz corresponds to 20 C at 0.08 to 0.2 Hz. DFR could thus provide a method to simulate operational conditions. Further testing and evaluation are necessary however to confirm stability under field conditions. DFR provides a valuable tool for condition assessment and failure analyses together with other techniques and proper engineering judgment. 90ºC Figure 3 The graph shows the dissipation factor over the main insulation C 1 on one 52 kv ABB GSA dry-insulated bushing at two different temperatures over a frequency range. 2.2 High voltage testing The main advantage of carrying out a full power frequency withstand voltage test on a service bushing is to analyze the level of partial discharges at withstand voltage. The stability of the dissipation factor at different voltage may also give indication of the status of the electrical connections inside the bushing. Trouble-free high voltage testing of larger bushings requires equipment and experience usually only available at the manufacturer s site; see Figure 4. 20ºC Figure 2 The graph shows the dissipation factor over the main insulation C 1 on one 52 kv ABB GOB oil-insulated bushing at two different temperatures over a frequency range. Figure 4 Bushing being set up for power frequency withstand voltage test.

4 2.3 Low voltage testing For most practical purposes a test voltage of some hundred volts is sufficient when measuring the level of the dissipation factor and capacitance over the main insulation C 1 ; see Figure 5. Bushings need to be dry and clean to avoid incorrect test results and the results must be corrected for temperature and compared to the power frequency withstand voltage test stated on the rating plate. Provided this is done correctly there is very good comparability between the results from a test bridge of this type and the results from a full high voltage factory test described above. The dissipation factor and capacitance between the test tap and flange (C 2 ) should never be used as a diagnostic tool since it is strongly influenced by the way the bushings are mounted onto the transformer and other test conditions. 2.4 Oil analysis As insulating materials break down from thermal or electrical stress, byproducts are formed. The dissolved gases are detectable in low concentrations, which makes dissolved gas-in-oil analysis (DGA) a standard analysis in most failure investigations and sometimes also for condition assessment. For certain types of problems, such as insulation degradation caused by transient stress, it is the only reliable method for early detection. IEC provides a very useful guide for interpretation of the results and actions. Moisture analysis of the oil may provide an early indication of leakage, but has a more limited value for bushings compared to transformers. The reason is that the ratio between paper and free oil in a bushing is very different. The moisture contained in the small amount of free oil has therefore very little influence on the dissipation factor for the complete bushing. The particle content in the oil is also irrelevant in bushings compared to transformers. The reason is that bushings have a capacitive controlled, uniform voltage distribution. This means that the maximum stress in the oil is very low and almost never close to the breakdown strength of the oil, even if it is contaminated with particles. Figure 5 Definition of C 1 and C Some design considerations In recent years it has become common in the utility industry to specify a certain dissipation factor as a quality requirement for high voltage bushings. This is a questionable approach and may lead to improper priorities among the manufacturers, as well as a false sense of security among the utilities. It is crucial that bushings are designed in such a way that moisture is prevented from penetrating, and that grounding is carried out in a robust manner that is a stable over time. This is accomplished by: Using a robust sealing system together with a professionally designed compressing system that provides an adequate tightening pressure under all service conditions. Using a well-proven system for joining together porcelain when necessary. Verifying the tightness of all parts individually, as well as the complete system during final testing. Selecting design elements that reduce the number of potential leakage points, e.g. magnetic oil level indicators without any through-shaft connections. Reducing the number of design elements and thereby the number of potential leakage points. 4

5 Using a robust and proven ground connection system The insulation life is optimized based on the electrical requirements set by the standards. Verification is based on the principle of Design Insulation Level; a high electrical stress during a short duration can be translated into a lower stress over an extended period. The thermal stress limit is based on the requirements set by the standards and well-established models for aging. Bushings naturally require full thermal stability at their highest voltage while considering both the ohmic losses and the capacitive losses. Sufficient convective cooling inside OIP bushings has to be ensured under all service conditions so that the insulation service life is not shortened; see Figure 6. Figure 6 Heat dissipation through convective cooling at the top of an ABB GOE oil-insulated bushing. 4. Some special considerations for dry bushings Test results clearly show that the RIP technology has several advantages in the event of failure incidents; please refer to article TechCon 2013 Asia-Pacific. It is worth pointing out however, that handling and storage of RIP bushings must be conducted with care in order to prevent ingress of moisture. Water uptake leads to increased dielectric losses, and in the field this means that the bushings may need to be dried before assembly, consequently leading to delays. The following is a rough guide for drying of RIP bushings based on practical experience with ABB GSA and GSB bushings. Unused bushings that have a dissipation factor over the main insulation C 1 elevated by more than 10% from the value given on the rating plate must be dried. Drying is by exposure to hot air until the rating plate value is reached. The necessary drying time depends on the amount of moisture and its distribution inside the capacitor core. Drying under vacuum accelerates the process only to a limited extent. 1. Place the bushing in a convection oven at a temperature of approximately 90ºC for about one week with the test tap cover removed. Make sure that the bushing does not rest on the silicone sheds or is exposed to direct heat radiation. If a vacuum vessel is used, maintain the ambient pressure until the entire bushing has reached the desired temperature. For ABB GSA bushings this time is 24 hours, and for GSB bushings it is 72 hours. 2. After one week, remove the bushing from the oven and allow it cool without using forced cooling. For GSA this time is 24 hours, and for GSB 72 hours. Remember to protect the bushing from moisture during cooling. 3. Carry out a dissipation factor test. The bushing is sufficiently dry if the dissipation factor over C 1 is within ±10% of the rating plate value, and the dissipation factor over C 2 is less than 3% (absolute value) 4. If both threshold values have not been reached, repeat the process. It is our experience that reaching the threshold for dissipation factor over C 2 is often decisive for the drying time. 5. Summary and conclusions This article has reviewed certain key aspects related to design and testing of bushings. Among the subjects covered were the use of Dielectric Frequency Response (DFR) analysis, as well as traditional electrical testing in laboratories. Some of the most important conclusions are: Dielectric frequency analysis provides better status information about spare bushings in the field compared to traditional methods. The reason being that high moisture content is easier to see in a bushing at room temperature with a frequency spectrum. A well-designed bushing takes into account the total losses of the bushing, i.e. the ohmic and the dielectric losses. The total losses for which a bushing is designed may never be exceeded during its service life. Stable dielectric losses over a bushing s service life are ensured by a well-designed compression system in combination with a good sealing system. Some recommendations for drying of RIP bushings are outlined based on practical experience. 5

6 Biography Håkan Rudegard has worked with high voltage bushings at ASEA/ABB since His professional experience includes production engineering, testing and workshop management. He is presently working as a senior engineer at the Technical Lead Centre in Ludvika (Sweden), responsible for technical support for ABBs bushing factories throughout the world. Lars Jonsson has worked with transformer components and their applications for twenty-five years and his experience includes design, product development and field investigation of bushings. Mr. Jonsson is the convenor of IEC SC 36/JMT5, responsible for the bushing standards IEC and IEC Lars Jonsson has a Master of Science degree from the Luleå University of Technology, Sweden. 6