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- powe r 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 wind ings 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 ground ed. 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 oil­SF6 bushings, from • Oil­air 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 • Oil­SF6 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 oil­SF6 type may also be affected by t races 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 auto matic disconnection of the trans­ over 36 kV are capacitor type, designed former by electrical contact trip. Oil sampling from oil­air 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 ground ed transformer enclosure. formers in French nuclear power plants. from a sealed type bushing, such as oil­SF6 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 top­up with pre­treated 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 topping­up of oil­ bushings have evolved over the years: the from the gas compartment and from the gas bushings are critical operations which Resin­bonded Paper (RBP) type, the Oil­ impregnated Paper (OIP) type and the Resin­impregnated 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 oil­gas 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 top­up 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 gas­solid 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.