DIAGNOSTICS

Bushings are a critical part of the transform- er, and their failures often have destructive consequences for the transformer, with a high risk of fire

ABSTRACT Bushings are highly stressed parts Diagnostics of HV of power transformers and their failures can lead to a transformer breakdown. The diagnostics and predictive surveillance of bushings bushings through is essential for uninterrupted oper- ation of transformers. Diagnostic tools applied include electrical tests, oil sampling and thermography and, to a minor extent, oil analysis for oil-impregnated bush- ings. This paper gives a short review of bushing types and related appli- analysis cations, including procedures for in field inspection and oil sampling. Dissolved Gas Analysis (DGA) issues Experience with GSU transformers in bushings’ mineral oil are briefly discussed and diagnostic criteria for 1. Introduction DGA interpretation are given, com- Bushings are extremely stressed equip­ ment, subject to a strong electric field in­ paring the experience of the authors Bushings are essential accessories of tensity with great difference of electrical with the available literature and stan- powe­r transformers, circuit breakers potential and close distances. dards. and other electrical equipment, making the connection between high voltage A significant percentage of all transformer KEYWORDS wind­ings and inlet or outlet conductors. failures is caused by defective bushings They allow the high voltage conductor and such failures can destroy a transform­ mineral oil, SF6, bushings, transform- to pass through the tank walls, which are er. A recent survey by CIGRE [1] conclu­ er, DGA, diagnosis grounded.­ ded that 30 % of all bushing faults resulted

142 TRANSFORMERS MAGAZINE | Special Edition: Bushings, 2017 Riccardo ACTIS, Riccardo MAINA, Vander TUMIATTI in transformer fire and 10 % in burst or er dissipation factors than OIP and RIP transformer tank, during outage periods, explosion. bushings. Moreover, since RBP cores are when oil temperature is low, the oil com­ not gas tight, they cannot be used in appli­ partment may be at negative pressure. A Making a systematic diagnostic surveil­ cations involving SF6 apparatus. loss of tightness of the sealing gaskets may lance of HV bushing is essential for safe induce moisture and air entering from operation of transformers. Therefore, For the above mentioned reasons, and the atmosphere. On the other hand, if the even if it is complex and quite expensive, because of the relatively low acquisition sealing gaskets between SF6 compartment many big utilities and power generation cost, OIP bushings still dominate today’s and filling oil are corrupted, an increas­ companies include monitoring of aging market. Here, the capacitor core is im­ ing amount of SF6 gas may enter into the and degradation of bushing in their main­ pregnated with transformer grade min­ bushing’s oil. The insulation properties tenance policy. eral oil and placed inside a housing made of the liquid insulation are not affected of porcelain or a composite insulator to by the presence of SF6, but an excessive The most widely applied diagnostic and avoid moisture ingress. compression of the bellow may occur, predictive tools are thermography and resulting in an unexpected trip of the oil electrical tests, such as measurement of Among the OIP bushing population, two maximum level switch. capacity, dielectric losses, power factor categories of equipment may be identified: (tan delta) and partial discharges. Other models of oil-SF6 bushings, from • Oil-air type bushing, where the capaci­ different manufacturers, can be found in Oil analysis is not a common practice; tor core impregnated with mineral oil is service, such as CTkg, CTg, CTzk, Fig. 3, nevertheless, it has been recognized as a sealed but in contact with dry air or ni­ etc. They have expansion bellows of an­ reliable tool for detecting aging and bush­ trogen blanket, Fig. 1. The HV terminal cient technology, where the filling oil of ing degradation at an early stage and, con­ is surrounded by open air the bushing core is always kept at positive sequently, for driving the corrective and pressure, by the transformer oil hydro­ preventive maintenance actions in a very • Oil-SF6 type, where the capacitor core static pressure or by an inner spring. effective way. is sealed, being completely filled with mineral insulating oil, and the HV ter­ 3. Visual inspection and oil Since 1998, a big nuclear power gen­ minal is surrounded by SF6 gas eration company in France has been sampling performing a visual inspection and oil In both cases, the capacitor core remains A regular inspection of the external state sampling and analysis of HV bushings of in a closed liquid environment throughout of bushings is the first method of collec­ their GSU power transformers: more than its lifetime and there may be occasional ting information about their health. Pre­ 700 pieces of equipment are systematically problems of leakage around gaskets. sence of rust, oil leakages or bleedings, controlled, with about 17 % of them being as shown in Fig. 4, as well as ruptures or under enforced surveillance, and 5 % The oil-SF6 type may also be affected by t­races of discharges on the porcelain sur­ replaced before experiencing a failure. leakages of SF6 gas, which have a higher face, moisture or dust into the capacitive pressure (up to 2 bar), into the oil com­ plug are all evidence of aging easy to ob­ 2. Technology of bushings partment, resulting in an excessive in­ serve and useful to initiate a condition crease of the internal pressure and the based maintenance action. Bushings for systems having a voltage auto­matic disconnection of the trans­ over 36 kV are capacitor type, designed former by electrical contact trip. Oil sampling from oil-air bushings is not a to reduce maximum field stress and op­ critical or hard operation; it may be easily timize field distribution in both axial and The SOT bushing, Fig. 2, is largely used in done from the top of the bushing using a radial directions when passing through a the gas insulated terminal of GSU trans­ syringe and a plugging hose. Oil sampling ground­ed transformer enclosure. formers in French nuclear power plants. from a sealed type bushing, such as oil-SF6 type bushings, can be done using the oil HV bushings are typically composed of a The capacitor core of this bushing is filled filling and/or drainage valves. The temp­ core, made from a conductor wounded as with oil under vacuum and sealed to pre­ erature of the oil and the expansion quota a fuse by several layers of paper, alternated vent air or moisture inlet. An expansion of the bellow should be measured before with thin aluminium sheets, and of an ex­ bellow allows for changes in oil volume and after sampling. When necessary, a ternal housing of porcelain or polymeric under temperature variations, with two top-up with pre-treated unused mineral material. electrical switches operating an alarm if oil should be performed. oil volume reaches the maximum or min­ Three main technologies for capacitor imum level. Being completely isolated Oil sampling from and topping-up of oil- bushings have evolved over the years: the from the gas compartment and from the gas bushings are critical operations which Resin-bonded Paper (RBP) type, the Oil- impregnated Paper (OIP) type and the Resin-impregnated Paper (RIP) type. Even if not a common practice, oil analysis Due to their manufacturing process, RBP has been recognized as a reliable tool for bushings are not guaranteed free from partial discharges and tend to show high­ early detection of bushing degradation www.transformers-magazine.com 143 DIAGNOSTICS

Figure 1. Oil-air bushing Figure 2. Oil-SF6 bushing SOT model Figure 3. View of a CTzk bushing model require skilled and trained personnel who For oil-gas bushings the DGA shall in­ prove the sensitivity of quantifying CO are equipped with proper tools and oper­ clude the measurement of SF6 dissolved in and CO2. SF6 can react with the nickel- ating a reliable and proven technical prac­ the mineral oil. Other tests such as parti­ based catalyst of the methanizer and con­ tice, Fig. 5. The air entering the bushing cles, metals, furanic compounds or corro­ sequently be converted to nickel sulfide core during sampling operation or during sive sulphur are considered complement­ and get trapped. This leads to a partial or oil top-up after sampling may generate ary, addressed to specific investigation complete loss of SF6, and a progressive partial discharges or insulation break­ purposes. reduction of the methanizer efficiency, down, thus it should be strictly avoided. resulting in underestimated values of car­ 4.1 DGA in bushing’s oil bon oxides, even in the following samples 4. Oil analysis which are not affected by the presence of In the power transformer application, SF6. Moreover, SF6 cannot be easily sepa­ A typical monitoring plan, based on oil DGA is the main diagnostic tool which rated from other gases (i.e. ethylene and analysis, generally includes a few basic provides information about oil and paper acetylene) with the commonly used chroma­ tests, which require low oil volume, but thermal degradation and presence of par­ tographic columns, e.g. PLOT Q type provide key information about aging or tial discharges and other electrical faults. (Agilent) or similar. PLOT type column degradation of the bushing core. The measurement of SF6 concentration (Porous Layer Open Tubular) belongs to in the insulating liquid is a key parame­ the gas-solid GC (gas chromatographic) The recommended tests are: ter for monitoring gasket tightness. SF6 is columns, and usually provides good sepa­ not among the gases detected by the IEC ration of very volatile solutes (gases — C1- • Dissolved Gases Analysis (DGA), ac­ 60567 [2] analysis method. There are two C3 hydrocarbons). The Q-type stationary cording to IEC 60567 [1] main problems which are encountered phase is a porous polymer. when a traditional DGA method is ap­ • Water content, according to IEC 60814 [3] plied to SF6 detection. First, this method, Different approaches have been used as well as ASTM D3612 [5], requires the to detect SF6 during the DGA test, with • Dielectric Dissipation Factor (DDF) of use of an in-line methanizer to convert most of them requiring some modifica­ the oil, according to IEC 60247 [4] carbon oxides in methane in order to im­ tion to the ordinary DGA procedure. For example, gas chromatographic columns that allow a better separation of SF6 from Bushings are sealed equipment where hydrocarbons have been tested, as well as shin-carbon type columns (Restek — gases accumulate during the life, and gas high surface area carbon molecular sieve) concentrations and ratios are different from which allow a good separation of SF6, but require longer analysis runtime. Alumina those in transformers type columns are also used.

144 TRANSFORMERS MAGAZINE | Special Edition: Bushings, 2017 The presence of the methanizer remains The authors’ experience suggests that some to be the main issue. SF6 should not pass through this device in order to avoid of the values, commonly used as a reference irreversible retain. While this can be resolved by eliminating the methanizer, for the diagnos-tics of faults in bushings, Fig. 6, it also implies that CO and CO2 are not realistic and seem to require revision sensitivity will be severely jeopardized. The use of detectors different from FID (Flame Ionization Detector) and TCD (Thermal Conductivity Detector) is a possible many gases can be present in much higher consequence is that a healthy apparatus solution to avoid the methanizer, such as concentrations, even in absence of faulty can easily show ethane levels up to PDD (Pulsed Discharges Detector), Figs. conditions. 250-300 µl/l (ppm by volume). The same 7 and 8. can be said of hydrogen. This increasing It must be considered that bushings are tendency of oils to form stray gassing is Other approaches use a multiple-valve sealed equipment, and thus gases have no currently under investigation by CIGRE, switching to bypass the methanizer dur­ way to escape from the oil (as happens in e.g. within the D1.70 WG. ing the chromatographic run, while SF6 is transformers through the open conser­ being eluted. vator). Gases accumulation during the So, the normal values reported in the life of the bushing may result in CO/CO2 currently available standards seem However, the above mentioned ap­ ratios higher than those expected in power to require a revision, based on actual proaches are not standardized and would transformer oils. Very often carbon monox­ values observed in the tested bushings require an effort from normalization bo­ ide is present at a concentration of the same population. According to the experience dies (IEC and others) to produce standard order of magnitude of carbon dioxide;­ of the authors, and relying on a methods for SF6 detection. this behavior, which in power transform­ers population of 1,400 bushings regularly leads to abnormal paper degradation con­ surveyed (which includes bushings 5. Typical reference values ditions, is quite typical in bushings. In fact, from other generation plants), 90 % of CO is a very volatile gas which escapes the population values were estimated as for diagnostic purposes quite rapidly from the oil in open systems reported in Table 2. Typical DGA values in bushings are re­ (such as power transformers), lead­ing to ported in two IEC documents: IEC 60599 a lower CO/CO2 ratio. The same carbon Finally, how to distinguish a problematic [5] (table A.10) and IEC TR 61464 [6], oxides’ rate of generation in bush­ings bushing from a healthy one? The answer which is shown in Table 1. Both tables c­auses an apparent enrichment in CO. relies more on gas increase rates than on converge giving the same reference values, This means that typical values of the two concentrations. corresponding to the 90th percentile of the carbon oxides should be higher. population. Other gases may accumulate in the These values are commonly used as a ref­ bushing oil during the time. Ethane erence for the diagnostics of faults in bush­ is one of them. Especially in the last ings, but the experience of the authors decade when mineral oils with a relevant suggests that some of them are not real­ stray gassing formation have entered istic. More than 10 years of regular DGA the market, ethane is formed and survey of HV bushings indicates that accumulated in the oil of bushings. The

Figure 4. Rusty and leaking valve on CTG bushing Figure 5. Oil sampling from SOT bushing www.transformers-magazine.com 145 DIAGNOSTICS

Table 1. Normal values for dissolved gases in bushings (IEC TR 61464 [6]) Concentrations Type of gas µl gas/l oil

Hydrogen (H2) 140

Methane (CH4) 40

Ethylene (C2H4) 30

Ethane (C2H6) 70

Acetylene (C2H2) 2 Carbon monoxide (CO) 1,000

Carbon dioxide (CO2) 3,400

Table 2. Typical 90 % values for dissolved gases (expressed in µl/l) in bushings (author’s enquiry) GAS 90 % typical value

Figure 6. Example of SF6 detection in DGA, TCD detector, with a switching valve to exclude the H2 184 methanizer CH4 98

C2H6 104

C2H4 22

C2H2 0 CO 902

CO2 1,663

Gases accumulation along the time is q­uite slow, and can be observed in the trend analysis as a smooth-sloped curve. Fault-related gassing (typically hydro­ gen increase) can be identified as a rapid and sharp formation. To appreciate the difference, regular DGA survey is strict­ ly necessary. Gas formation rates are not easy to calculate from robust statistical populations because of the relatively low testing frequency. In the authors’ statis­ tical enquiry, it was possible to calculate typical values, but calculating the increase Figure 7. Example of SF6 detection in DGA, PDD detector rate requires a larger amount of data. As a rule of thumb, it is possible to consider a gas formation rate as abnormal if the slope of the gas evolution curve changes sharply compared to the historical data.

In the absence of fault symptoms, regular DGA testing frequency can be applied (each 12 to 36 months), but in case of evidence of abnormal gas formation, the controls should be run more frequently, even weekly or daily in worst cases.

DGA on bushings may require some special analytical solutions when SF6 can contam-

Figure 8. Example of SF6 detection in DGA (separation from ethylene), PDD detector inate the oil

146 TRANSFORMERS MAGAZINE | Special Edition: Bushings, 2017 As a rule of thumb, a gas formation rate can tric dissipation factor (tan d) and d.c. be considered as abnormal if the slope of resistivity, 2004 the gas evolution curve changes sharply [5] ASTM D3612 – 02, Standard Test Method for Analysis of Gases Dissolved compared to the historical data in Electrical Insulating Oil by Gas Chro- matography, 2009

Once an abnormal condition is found ment - Sampling of gases and of oil for [6] IEC 60599, Mineral oil-filled electri- (based on the increase rate of gases), key analysis of free and dissolved gases – cal equipment in service - Guidance on ratios allow identification of the fault type. Guidance, 2011 the interpretation of dissolved and free The key ratio criteria are very similar to gases analysis, 2015 the ones usually applied in power trans­ [3] IEC 60814, Insulating liquids - Oil- former diagnostics. The main difference impregnated paper and pressboard - [7] IEC TR 61464:1998, Insulated is that the H2/CH4 ratio linked to partial Determination of water by automatic bushings - Guide for the inter­pre­ta­ discharges is quite higher than the one coulometric Karl Fischer titration, 1997 tion of dissolved gas analysis (DGA) applied to power transformers: 13 vs. 10. in bushings where oil is the This is another consequence of the fact [4] IEC 60247, Insulating liquids - Mea- impregnating medium of the main that bushings are sealed units, and hydro­ surement of relative permittivity, dielec- insulation, 1998 gen cannot escape once formed.

Partial discharges are the most frequent­ ly observed type of fault in bushings. Of Authors course, strong stray gassing can simulate Riccardo Actis holds a degree in mechanical engineering PD, and the hydrogen formation should obtained from Polytechnic University of Turin in 1995. always be confirmed to be in positive Today he is the Head Manager of the On-Site Service and fast evolution to link it with a real Department at Sea Marconi, with 15 years of experience PD phenomenon; and electrical PD mea­ at the organization and follow-up on-site work activities surement should be performed as a final gained in Italy, France, Spain and other EU countries. Riccardo has a deep knowledge of condition based confirmation. Localized overheating (that maintenance, as well as treatment and decontamination generates ethylene as the main gas) is quite techniques for mineral insulating oils. He participates in standardization rare. C2H2 formation above 2 µl/l is always working groups of IEC TC10 , CEI TC14, and CIGRE. an alarming symptom, and often leads to the replacement of the unit. Riccardo Maina has been working for Sea Marconi Technologies since 2001. As the Laboratory Manager at Conclusions Sea Marconi he has developed new analytical methods for oil analysis, diagnostics and decisional algorithms for Bushings are a critical part of the trans­ test interpretation, software for data mining and LCA. former, and their failures often have de­ Riccardo is a member of the Italian IEC TC10 National structive consequences for the transform­ Committee since 2002, and is actively participating in er, with a high risk of fire. Their regular several WGs and MTs of IEC TC10 and Cigre SC D1 inspection, by means of electrical on-line and A2, dealing with analytical methods (particles, DGA, acidity, additives, testing (continuous) and oil sampling (an­ metals), maintenance guides, and guides for interpretation and diagnostics nual), is of a paramount importance to (oil degradation, DGA, thermal life of cellulose). He is the author of many ensure transformer reliability. DGA is a papers on diagnostics, transformers LCA, and corrosive sulphur. In 2008, he powerful tool for predictive maintenance, was awarded the IEC 1906 Award. but its interpretation requires certain ex­ Vander Tumiatti is the founder and owner of Sea perience and a sufficient background in Marconi Technologies, established in 1968 in Torino, real cases in order to correctly estimate Italy. Within the company, which is involved in research, the typical values and, most of all, the gas technologies, products and services for energy & en- rates of increase that can be associated to vironment, he has developed BAT & BEP Sustainable a faulty condition. DGA on bushings may Solutions for Life Cycle Management (LCM) of insulting require some special analytical solutions liquids and transformers focalized on inventory, control, when SF6 can contaminate the oil. diagnosis, decontamination, depolarization (DBDS, TCS, Polar Compounds) end dehalogenation /detoxification (PCBs/POPs). Vander References has more than 40 international patents and is the author of many international technical and scientific publications. He has been the Assistant Secretary of [1] CIGRE Brochure 642, Transformer IEC TC10 since 2000. He is also a member of several international groups, Reliability Survey, 2015 with major participation in technical normative activities (CEN, IEC, CIGRE, IEEE). In 2009, he was awarded the IEC 1906 Award. [2] IEC 60567, Oil-filled electrical equip- www.transformers-magazine.com 147