Uneven voltage distribution across metal-oxide varistor blocks, commonly associated with tall arresters, can lead to thermal stress on blocks in the upper section.

The post Voltage Profile Across Metal Oxide Blocks of a 435 kV Arrester (Video) appeared first on INMR.

In 2009, three years after Claude’s untimely death from leukemia, INMR instituted an award in his memory to honor his achievements and keep his name and memory alive in today’s generation of T&D engineers who might otherwise never know who he was.

Past recipients of this award have included a long list of industry luminaries who earned this accolade for their remarkable contributions to the field. The last recipient (2025), for example, CIGRE President Dr. Konstantin Papailiou, had a productive and influential career in insulators prior to becoming part of CIGRE.

The recipient of this honour for 2026 is someone who shares many of Claude’s personal qualities. He is modest, despite his expertise, he is open to sharing his knowledge, despite the fact that he works for a manufacturer, and he is dedicated to the pursuit of excellence in his field, without distraction from seeking fame or attention. And, perhaps fatefully, he and Claude met in 2005 when Claude contributed what would be his final article to INMR, which covered development of RIP bushings in Sweden.

It is my profound and sincere pleasure to announce that INMR has selected Lars Jonsson of Hitachi Energy to be recipient of the Claude de Tourreil Memorial Award for 2026.

Lars has had over 35 years’ experience working with high voltage bushings, including in design, research & development, engineering solutions, applications and testing. He has also been active developing methodologies and guidelines for condition assessment, including investigation of failures of service-aged AC and DC bushings up to 1200 kV. He has authored numerous articles and conference proceedings on related subjects and since 2013 served as Convenor of the IEC Maintenance Team for bushing standards as well as Chairman of the IEC Technical Committee since 2017.

My first memory of meeting Lars was during one of the lunches at the 2013 INMR WORLD CONGRESS in Vancouver, Canada. He explained at the time that, like insulators, the field of bushings was about to undergo a profound technological shift – away from oil-impregnated (OIP) types to dry types, such as with resin-impregnated cores. To appreciate the changeover he was foreseeing, back then well over 90% of all bushings used worldwide were still OIP. In fact, this technology was so entrenched that it was hard to imagine it could ever be replaced.

Yet Lars was among the early believers in ‘dry bushing’ concepts – much like Claude was one of the first believers in composite insulator technology.

Lars and I have maintained close contact over the years, both professionally and personally. He was speaker at the 2019 INMR WORLD CONGRESS in Tucson and then served as organizer and Chair of specialized Sessions devoted to bushing technologies and service experience at INMR WORLD CONGRESSES in Berlin (2022), Bangkok (2023) and Panama City (2025). He not only helped expertly manage each of these Sessions but also contributed his own expertise in areas such as ‘Impact of Biofouling on Performance of Outdoor Bushing Insulation’.

Earlier this year, Lars showed me some of the latest developments in production of dry bushings at Hitachi Energy’s factory in Ludvika, Sweden before we travelled north to visit its greatly expanded hollow core insulator plant in Piteå.

Everyone who has met him will attest to Lars’ professionalism and dedication to neutrality and objectivity, notwithstanding that he is employed by one of the world’s largest bushing manufacturers. If Claude were alive today, he would see in Lars, a reflection of his own deeply ingrained values – pursuit of knowledge, commitment to excellence – all without distraction from seeking fame or credit.

INMR and its readers worldwide congratulate Lars for this well-deserved honour and express lasting appreciation for his decades of dedication to advancing electrical insulation in bushings.

Lars will be presented his award at the 2027 INMR WORLD CONGRESS next year in Portugal.

Marvin L. Zimmerman

The post 2026 Claude de Tourreil Memorial Award appeared first on INMR.

These natural events can be especially critical for power networks, with extensive damage to equipment that causes extended outages. In addition, cost of repair is also high while loss of energy for long durations only increases the misery of those affected.

Due to interconnection requirements and network structure, many HV and UHV substations are in high seismic areas. This implies serious challenges when it comes to design of AC and DC insulators. In addition to ground acceleration, bushings are subject to amplification by the transformers and reactors on which they are mounted. Ideally, they must be able to survive powerful seismic stresses without interruption in operation. Since it is extremely difficult to carry out seismic tests on UHV bushings to effectively verify withstand performance, use of static or modal analysis is required to demonstrate bushing capabilities versus performing real tests. These can also support the design process during development. Seismic tests performed on a shaker table can then validate prototypes and relevant simulations while providing additional information to improve design criteria and methodologies for technical verification using simulation.

This edited past contribution to INMR by experts at GE Vernova Grid Solutions in Italy reviewed seismic standards and explained that, since calculations match values measured during tests, this kind of analysis has achieved satisfactory reliability.

Hivolt Power System

China

Hitachi Energy Transformer Components and Service

Switzerland

See more suppliers of Bushings

Review of Standards

An earthquake causes ground motion of the soil. This generates a wave train characterized by different frequencies, each with its own intensity and that can be shown using a response spectrum graph (as in Fig. 1). Every type of soil is characterized by a main resonance frequency due to its composition (e.g. rocks, sand, clay, mixtures of these) as well as by other minor resonance frequencies (see Figs. 1 & 2). Based on records from past earthquakes, it has been noted that response spectra have similar shape, even with obvious different peak values and positions of peaks due to differing natural soil frequencies. In any case, all earthquakes have similar ranges in frequencies and a trapezoidal shape in the frequency domain (i.e. an increasing part, a maximum value and a decreasing part). The main differences are related to different positions of the relevant peaks, corresponding to the main soil resonance. This similarity in shape and requirement to consider all possible values of resonance of different sites has resulted in different international and national seismic standards when it comes to definition of Reference Response Spectra (RRS). This is the shape in the frequency domain of reference that seismic tests must fulfill. This RRS has an increasing section, a plateau and a decreasing section. The plateau, corresponding to the maximum acceleration value, includes all peak values recorded during past earthquakes, with the scope to consider all possible differing soil behavior. Because of this, all seismic standards have defined a similar RRS (see Fig. 5). The spectra plateau, corresponding to the maximum acceleration value, generally has a range of between 1 to 10 Hz. All spectra become flat with frequencies higher than 12 to 13 Hz at the zero period acceleration (ZPA) value.

The ZPA value is the asymptotic value of the seismic acceleration at which there are no amplification effects. As the frequencies become lower, the RRS values become high due to resonance effects.

Among the various standards, IEEE 693 is the best known and the RRS it defines has become an industry benchmark. Recently IEC Standards (i.e. IEC TS 61463 for bushings and IEC 62271-207 for GIS) have adopted this RRS, which is the most used worldwide and similar to other standards. While the Reference Response Spectrum indicates the response of the soil to seismic vibration, bushings are not normally directly fixed to the ground, e.g. they are mounted on transformers or reactors, part of GIS systems, or fixed on walls or roofs. Some structure is always present between ground and bushing. Moreover, this structure is never rigid but rather imparts its own contribution to ground vibration depending on geometry, material, shape, etc.

Different studies have been carried out to define the contribution of such structures, and a super elevation factor K is normally defined to be in the range of 1 to 2, depending on structure. International Standards define this amplification coefficient as normally 1.5 or 2. Other contributions are then given by the fact that the system is not perfectly linear and that there are other minor natural frequencies (e.g. a bushing has many parts that can themselves vibrate at different proper frequencies) that can increase the amplification of motion (see Fig. 5 for a better understanding). IEC 61463 Ed. 2 is used in the calculation and considers such contributions.

Another contribution is given by the damping of the bushing – a term indicating the capacity of the system to dissipate the energy with which it has been excited. The higher the damping, the lower the response of the system to seismic solicitation. For this reason, the RRS is in fact a family of curves, each corresponding to different damping (see Fig. 6).

Seismic Tests on UHV Bushings

Based on the above concepts, it is easy to understands why a bushing, which is a heavy, tall and thin apparatus made of fragile materials such as porcelain, with center of gravity at more or less half of its air side height, will be subject to huge mechanical stress during ground motion amplified by the structure to which it is mounted. This phenomenon becomes greater when the bushing, due to voltage class, is extremely high and heavy, as is the case for UHV bushings. Such bushings (i.e. voltage class ≥ 800 kV) have air side length of at least 6 to 7 m and, in most cases, have an air side insulator made of porcelain. That only further increases their significant weight. They therefore feature among the worst possible properties from the seismic point of view. To this intrinsic fact must be added the difficulty to find a mechanical laboratory able to manage a seismic test on a system that is so high and heavy. One aspect is the capability to vertically handle a test object 10 to 13 m high, plus the frame for fixing it to the shaker table. But the main problem is given by the shaking tables themselves that must synthetize the ground motion. In most cases, they are not so large as to be able to correctly reproduce the time history of the ground motion on an object with a center of gravity of many tons placed at 4 to 6 m in height. In fact, each shaking table has been designed to have a maximum overturning moment. This is the maximum moment that the system can support (in ton x m), by correctly following the seismic RRS correctly in all range of accelerations. With higher values, the table may fail to strictly follow RRS requirements in some parts of its frequency range.

It easy to understand that a UHV bushing, with an air side center of gravity placed at about 5 to 6 m and weighing some 6 tons, gives a moment of 30 to 40 ton x m – a value reachable only by the biggest shaker tables. Due to the above, in conjunction with important cost aspects, it has become common to make use of finite element methods (FEM) to simulate the system under test and then to carry out only a few real seismic tests to validate the calculations.

The bushing is modeled by FEM and a modal analysis is applied to study the dynamic properties of the system. This kind of tool identifies the dynamic response of the bushing, considered as a linear system, in each of its natural modes, when excited in the range of frequencies of the reference response spectrum. The result of this calculation is the map of vibration modes of the object (as in Fig. 7), where calculated displacements have been magnified for higher readability.

Each vibration mode is characterized by a natural frequency (i.e. result of the modal analysis). The next calculation is to excite the system with the reference earthquake response spectrum (RRS), establish the mechanical stresses generated by each mode and sum each contribution to generate a total stress map (see Figs. 8 & 9).

The model used for the FEM calculation is then validated with a real test. The stress measured during this test is compared with theoretical values obtained by exciting the system with the same input. If the results are similar, this means that the simulation has been done correctly, i.e. simplifications of the model carried out correctly, materials parameters close to real ones, mesh of the FEM built with the right complexity, exact weight of different system components, RRS correctly modeled, etc. After this validation step, the model can be used to simulate other seismic stresses, e.g. with different RRS if the bushing has to be verified for a different country with its own seismic standard or with the same RRS but another sesimic intensity. The RRS is rigidly scaled based on the Zero Period Acceleration (ZPA), which can be 0.2g, 0.3g, 0.5g, etc.

A concrete example of this process is a seismic study carried out for 1100 kV and 550 kV class bushings to be mounted on the same monophase UHV transformer for the Chinese market. These were modeled and subject to the RRS spectrum from the China GB standard. Both bushings were then tested by Chongqing University at the same seismic motion level of 0.3g, with amplification factor K=2. The modal analysis output was the map of vibration modes of the bushing (see Fig. 10). Once subjected to the RRS, as per the specification, this yielded a stress in the most critical part of the bushing, i.e. the base of the air side porcelain, close to the flange, where stress is highest. The 550 kV rated bushing was subjected to the same procedure. During the seismic test, in addition to the acceleration profile, the strain (compression and tension) on their most critical part, which is the porcelain base, was measured for both bushings. This measurement was then compared to the calculation and the results are reported in Table 1.

It is evident that the results of calculations are not far from real values measured during the tests (namely 94.2%, 80.3%, 98.1% with respect to 100%) and that the calculations are conservative, giving values higher than real ones. It is also evident that differences may not be negligible due to a variety of reasons – on one hand due to the model being used with its approximations and on the other hand due to any measurement uncertainties. What is important, however, is that results provide a precise idea of the stresses involved and that, by being conservative, assure a better margin. The models can then be said to have been validated and will be used for future requests with different seismic spectra or with the same spectrum but with different ZPA.

The same approach can be followed when the bushing is equipped with a composite insulator. The modal analysis yields the vibration modes and further application of the seismic response spectrum, RRS, gives the resulting stress.

The model is then compared with a real test on a shaker table and, if the values are similar, can be said to be validated.

Verification Tests on Materials

A stress profile for different materials must be compared with the maximum allowable stress for each. While for metals it is comparatively simple to find standards or technical literature that give material limits, in the case of a porcelain or for a composite insulator this can be more difficult. There, the stress limit cannot easily be read from rupture tests on specimens and depends not only on material but also on geometry, including thickness, diameter and shape as well as on the fittings, such as material for cementing and fixing the flanges. That is why when limit values must be known with precision, it is often necessary to carry out rupture tests on real insulators, e.g. the 10 m long hollow porcelain insulator used in the 1100 kV bushing referred to above. In this case, the porcelain insulator, fixed by its base to the laboratory mechanical reference wall, was subjected to a linearly increasing bending force at its head and perpendicular to its axis until the point of rupture. By recording rupture force and taking into account weight contribution and insulator geometry, it became possible to calculate rupture stress of the material. This was then used as reference in the seismic calculations and also during the real seismic tests

ABB and PEHLA Laboratories

Germany

STRI

Sweden

See more Laboratories

Conclusions

Since the consequences of an earthquake can be disastrous for the integrity of HV and UHV electrical networks, extensive studies have been carried out in the past and are still in progress today to limit these consequences as much as possible. In depth studies of earthquakes have brought about standardization of response spectra (RRS), with similar shapes for the majority of seismic standards. Unfortunately, since bushings are mounted on frames (e.g. transformer or reactors) that amplify ground motion and being thin, tall and heavy, these are subject to high mechanical stresses – especially in the UHV voltage range. Due to the complexity and high costs in carrying out seismic tests on such bushings, much use is made of FEM simulations and modal analysis. These yield the vibration modes of the bushing, coupled with the RRS of the earthquake. The result is a mechanical stress map of the entire bushing structure.

Real seismic tests are then carried out with the goal of comparing the FEM model results with actual test results. When the model results are close to real ones, it can be concluded that the simulations are validated. These models can therefore be used for other spectra intensities or for investigating different seismic spectra. To calculate margins with respect to the maximum mechanical values that a bushing can withstand, stresses are compared with maximum allowable values of their various materials. When these values are not easily available or depend not only on material but also on geometries of the test object, mainly hollow core insulators, mechanical/rupture tests on real insulators must be done to obtain their real mechanical behavior. With these results, real margins can be evaluated to eventually improve design and increase performance in the case where values are found not to be sufficient.

The post Development of Bushings for Seismic Conditions appeared first on INMR.

Oil-impregnated paper (OIP) bushings have been applied since the start of electrification and are still widely used as a mature and reliable technology. Still, although OIP bushings fulfil all technical standards and requirements, they can be exposed to risks in service that include loss of insulation media and accelerated ageing of their paper insulation due to overheating. Under harsh service conditions, RIS bushings (RIS) with silicone rubber sheds can be applied instead to increase safety of both transformers and reactors.

This edited contribution to INMR by Alexander Doutrelepont, Bernhard Heil and T. Schnitzler at Trench Group and HSP Hochspannungsgeräte in Germany outlines the relative advantages offered by RIS.

RHM International

USA

PFISTERER

Germany

See more suppliers of Bushings

Structure & Design

The main component in a high voltage condenser bushing is the so-called active part or condenser core. In the case of RIS types, a synthetic non-woven material is the carrier for the epoxy resin in which concentric aluminum layers of individual capacitors have been precisely placed (see Fig 2). This provides the required capacitance grading.

The condenser core is housed within a composite insulator covered by silicone rubber sheds (Fig. 1). The gap between insulator and condenser core is then filled with a dry-type N2-based foam. As such, the bushing is free of hazardous fluids and gases such as mineral oil, SF₆, CO₂, etc. RIS condenser types are therefore more environmentally friendly than conventional OIP bushings and in many cases also competitive in life cycle cost versus resin-impregnated (RIP) bushings.

Electrical Function & Properties of RIS Cores

Under application of high voltage, a capacitance graded condenser bushing distributes the electric field uniformly, i.e. local electrical field stress can be reduced and spread over the full length of the bushing (Fig. 3).

One of the main advantages of an RIS condenser core is its non-hydroscopic performance which ensures no moisture or humidity ingress that might otherwise damage performance under high voltage. Paper, as used in RIP type bushings, can accumulate humidity during exposure to ambient conditions. Controlling this aspect is therefore crucial not only during manufacture but also during installation, commissioning and storage in the field.

Fig. 4, for example, compares humidity absorption of RIS and RIP condenser cores. After storage at 23°C and 50%RH, the result for paper is 6.24% while for synthetic materials it is only 0.23-0.32%.

Qualification of RIS Bushings

Although the only component of an RIS condenser bushing that differs from an RIP type is the synthetic carrier for the epoxy resin and aluminum layers, an extensive qualification program is nonetheless necessary. This is to ensure that all the established benefits of an RIP condenser bushing are maintained and even improved. For example, Fig. 5 depicts a thermal cycle test of RIS bushings with dynamic mechanical loading and continuous electrical load. A set-up inside a climate chamber enabled 30,000 load cycles of 2500 N with ambient temperature that varied between -60°C and +50°C (see Fig. 6). During the test, the bushings were stressed continuously with 80% Up. At each temperature peak, key parameters were measured including capacitance, dissipation factor and partial discharge. The bushings passed without failure.

As a result, RIS condenser bushings are confirmed to cover the full range of both, low ambient temperature up to -60°C and seismic requirement such as RIP condenser bushings. Comparative accelerated aging tests of RIP and RIS condenser cores were caried out in the HSP long-term test laboratory and confirmed that the lifetime of RIS condenser bushings exceeds the lifetime of RIP condenser bushings.

Since the first RIS condenser bushings were installed and commissioned in 2014, more than 2000 RIS condenser bushings are in operation to date with more than 50 million accumulated operating hours and a failure rate of 0%.

HVDC Condenser Bushings in RIS Technology

Specific equipment is needed or HVDC grids and inter-connections such as oil-immersed shunt reactors or power converter transformers and condenser bushings are required to connect such equipment. In this type of application, the DC electrical stress leads to resistive grading in addition to the capacitive grading of AC electrical stress. Therefore, design and testing of HVDC condenser bushings is elaborated and defined by specific international standards.

To assess the suitability of RIS technology for these types of HVDC applications, development and extensive testing was performed on a 545 kV HVDC converter transformer bushing. To verify design and testing, individual test sequences were performed in parallel with both RIS and RIP technologies. All applicable type and design tests were successfully completed in 2022. Fig. 7, for example, shows the set-up for the even wetting DC voltage test.

Life Cycle Assessment

Environmental impact and use of eco-friendly components and equipment for more sustainable transmission and distribution have become increasingly important for TSOs and other electricity supply utilities. A validated method to determine CO2 impact of different alternatives is key.

Such assessments calculate carbon footprint for each product category – from initial sourcing to manufacture, delivery, installation, commissioning, operation, removal from service and disposal.

Fig. 8 compares carbon footprint for RIP and RIS condenser bushings under the following base line scenario: total lifetime of 40 years and anticipated temperature based on an average load of 70% as in-service operating conditions. RIS condenser bushings have a 45.8% lower carbon footprint than RIP types. Moreover, operating losses are significantly lower as well.

Ambient Performance of Silicone Insulators

Another important aspect for users is the outdoor performance of alternative types of condenser-graded bushings. Weather events such as heavy wetting and pollution exposure among others can reduce a bushing’s insulation properties. These days, state-of-the art bushing designs are silicone insulated, offering easier handling due to less weight and better outdoor performance.

Due to its intrinsic hydrophobicity, silicone rubber has a water repellent surface that prevents complete wetting. Accumulation of conductive pollution layers is thereby reduced and so is risk of flashovers. Moreover, this hydrophobic effect will also transfer over time to the surface pollution layer. High pressure washing of silicone rubber insulated bushings is not recommended and should be avoided. Most established suppliers offer their own guidelines when it comes to how best to perform cleaning.

Conclusions

RIS types are often the ideal condenser bushing technology for application in AC as well as in DC transmission and distribution grids. This is because utilization of a synthetic non-woven as the carrier material for the epoxy resin is the most advanced from the viewpoint of applying components and manufacturing materials. Compered to OIP or RIP styles, dry RIS condenser bushings offer better service performance as well as key advantages when it comes to installation, commissioning and storage, including:

i) no paper and no insulation fluids such as mineral oil or ester fluids;
ii) reduced ageing due to elimination of organic materials;
iii) higher resistance to ambient humidity -> non-hygroscopic material;
iv) lower dissipation factor compared to RIP types and no PD activity;
v) homogenous material characteristics, i.e. no risk of voids;
vi) lower carbon footprint.

Bibliography
[1] Publication IEC 60137 “Insulated Bushings for alternating voltages above
1 000 V”, 2017
[2] Publication IEEE Standard C57.19.00-2004 “General requirements and test procedures for power apparatus bushings”, 2004
[3] A. Del Rio, N. Lange, M. Mohamad, “Synthetic Dry Bushings for Application in HV Power Transformers & Reactors”, INMR World Congress, October 20th – 23rd 2019, Tuscon, Arizona, USA
[4] Publication IEC/IEEE 65700-19-03 “Standard for Bushings for DC application”, 2014
[5] J. M. Seifert, W. Hubl “Hydrophobicity effect of silicone housed composite insulators and its transfer to pollution layers – Design and environmental parameters influencing the hydrophobic surface behaviour”, Iraklion Symposium, April 26th – 27th 2001, Kreta, Greece

The post Selecting Condenser Bushings for Today’s AC & DC Grids appeared first on INMR.

Silicone composite insulators show exceptional properties but are vulnerable to natural degradation processes under standard operating conditions over time. Ageing is a gradual process that results in loss of function and eventual breakdown. Repeated breakage and reformation of bonding can lead to changes in a material’s appearance and performance.

The post Multiscale Surface Characterization of Composite Polymer Insulator Surfaces (Video) appeared first on INMR.

Years ago, T&D specialist Raouf Znaidi met staff at the Ministry of Electricity & Water (MEW). In this edited past contribution to INMR, he reported on how maintenance and technical staff implemented countermeasures to enhance reliability in a service environment dominated by desert, industrial and marine pollution.

PPC Insulators

Austria

EB Rebosio SRL, A Gruppo Bonomi Company

Italy

See more suppliers of Silicones & RTV Silicone Coatings

The MEW network comprises some 600 substations that connect some 10,000 circuit-km of overhead lines operating at 400 kV, 300 kV, 132 kV and 33 kV. This network runs along 500 km of shoreline and also crosses a flat sandy desert to supply regional desalination plants as well as petroleum refineries and petrochemical industries. MEW produces, transmits and distributes electricity to serve the needs of an expanding urban population with one of the world’s highest per capita energy demands while also benefiting from among the lowest electricity prices.

The power system in Kuwait serves a typical load profile dedicated mainly to extensive air conditioning and water desalination. The former systems account for nearly 70% of peak load and over 45% of total electricity consumption. As annual demand has continued to grow over past years, MEW faced challenges in assuring consistent quality service to users. MEW’s electrical sector therefore implemented a plan to improve key performance indicators of its transmission and distribution systems. The end goal was to keep the Kuwaiti power network among the most reliable and with one of the lowest outage rates – despite being exposed to severe pollution.

Among the initiatives in this regard was one focused on predictive and preventive maintenance as well as remedial measures. The cornerstone of these measures was a program to apply RTV silicone coatings to improve pollution performance of porcelain long rods. This program proved particularly notable since it was the first in the region to feature live replacement using a new generation modified heavy Zoom line along with high-performance techniques.

Outdoor Insulation on MEW O/H Network

MEW’s overhead system is insulated mainly using anti-fog type porcelain long rods. This applies to the entire 400 and 300 kV network and some 98% of the 132 and 33 kV systems, the balance of which is porcelain cap & pin strings

To mitigate impact from deposition of contaminants as well as heavy wind loads, almost all lines in Kuwait are over-dimensioned. This has led to unified specific creepage distances of as high as 65 and 63 mm/kV (according to the latest IEC 60815/TS) and to 320 kN and 230 kN minimum failing loads. Coupling size for 400 and 300 kV tension & suspension insulators are 24 mm and 20 mm respectively. Tables 1 and 2 summarize circuit lengths as well as the insulation and technical characteristics of long rod insulators used on each system.

Maintenance Strategy

Pollution has not been the only factor affecting MEW’s transmission lines. For example, the network near the Al-Zour Power Station in the south and the southern network in general have suffered dense fog, sandstorms and heavy winds. In fact, a combination of these led to a major blackout a decade ago and provided an impetus to resolve perceived weaknesses in overhead line insulation.

Events such as this influenced subsequent decisions in regard to the most effective global maintenance strategy that includes pollution countermeasures, reinforcement of lines, periodic inspection, live washing and even dead line cleaning. For example, to reduce incidence of network outages following the blackout, at least 24,000 towers were live washed annually while in 2017 some 900 towers were inspected using infrared cameras as part of the targeted preventive and predictive maintenance program.

RTV Coating Program

There has been growing worldwide interest in application of RTV silicone coatings to mitigate pollution-related flashovers of ceramic insulators that suffer poor pollution performance due to inappropriate selection or insufficient self-cleaning. Such a solution is usually based on a range of criteria, including effectiveness in suppressing leakage current, maintaining hydrophobicity as well as satisfactory adhesion and minimizing arc degradation or surface damage due to discharges. In fact, studies have demonstrated that there can be significant differences between different RTV coatings available on the market in terms of their long-term performance, especially after ageing in a harsh desert environment.

Given this, special effort was devoted to selecting the most suitable coating type and formulation to be applied in Kuwait, including the most reliable primer as bonding agent. MEW’s implementation team then looked to this solution to minimize pollution related leakage current while optimizing the costly and time-consuming remedial measures used in the past. Testing then confirmed that the RTV formulation selected by MEW showed exceptional adhesion properties and was virtually impossible to remove from coated surfaces after curing.

The solution of applying RTV coatings combines the reliable mechanical properties of porcelain with the excellent pollution withstand and leakage current suppression of silicone rubber. At the same time, it was decided that taking down and coating existing long rod insulators with RTV silicone would be more cost effective than the alternative of a wholesale change in insulator technology. This solution was also felt relatively easy to implement inside a specialized coating plant, which included a curing area with controlled temperature and humidity.

In the end, construction of a specialized facility for RTV coating in Kuwait was the result of collaboration between stakeholders. The air-conditioned workshop consisted of separate areas that together allowed for a finished coated product that met all technical specifications and requirements. One area was reserved for insulator cleaning & silicone preparation. A second was dedicated to coating application, accomplished using conventional air spray gun equipment and a novel machine with rotary axes modified to hold up to three long rod insulators at once. This set-up allowed optimizing application time and costs such that at least 40 long rod insulators could be coated each day by a team of trained workers. A third area saw ambient curing of coated insulators while a fourth focused on quality control and packing of insulators such that they would be protected during transport and ready for installation on site.

Since dust can have adverse impact on adhesion of RTV coatings, cleaning and preparation of insulators is of paramount importance. All incoming long rod insulators were therefore washed carefully by pressurized water and then cleaned by hand and dried. A first thin layer of primer was then applied, consisting of a bonding solution of reactive siloxanes and silanes that adhere firmly to porcelain under the controlled ambient temperature and humidity. Final coating quality required a consistent primer layer of less than 10 µm, with no trapped air. This was followed by a drying time of between one to two hours at ambient temperature.

Then, no more than 5 hours later, two or more successive RTV silicone layers were sprayed on so as to achieve average coating thickness in the range of 250/300 microns, as specified for the MEW electrical network. As the solvent mixed with the RTV material evaporates, moisture in the air triggers vulcanization that forms a solid rubber coating. The speed at which this process takes place depends on the type of RTV silicone and solvent as well as on ambient temperature and humidity inside the workshop. The curing process for coated insulators must take place under consistent, well-controlled environmental conditions.

Quality engineers and MEW supervisors carried out daily quality control inspections of coating thickness on each cured long rod insulator using pre-calibrated ultrasonic thickness gauges. Results from at least five different measurements taken at different locations on top and from the under-rib shed surfaces showed typical thickness variations of between 125 and 322 microns.

Special attention was devoted to protecting the RTV silicone coating with bubble sheet protectors and wooden packaging before storage and transport to the job site by a truck equipped with an adapted control crane. The entire process – from insulator preparation to coating to curing to inspection to packing and transport for live replacement – was closely supervised.

Live Insulator Replacement

MEW devoted months to mobilizing large human and material resources as well as equipment specially adapted for live line work. The goal was to complete one of the most ambitious network maintenance projects in the region, namely live replacement of existing 300 kV and 400 kV porcelain long rod insulators by the same type but with locally-applied RTV coatings. Since all HV insulator strings in Kuwait are standardized and interchangeable, the option chosen to optimize installation time was to remove existing long rod insulators and simply replace these by coated insulators brought in from the local workshop.

To accomplish this, two latest generation modified Zoom lines were brought in. The first crane was dedicated to horizontally (tension towers) or vertically (suspension towers) holding 2 to 4 long rod insulators strings through an insulated stick extension. The second Zoom line, with insulated basket for live work, served to hold workers.

Equipment capabilities included an 80-ton lifting capacity and an up to 70 m height, along with insulated telescopic length for up to 500 kV. Together with high performance work techniques, this allowed live replacement of four existing long rod strings by four locally coated RTV strings in less than 90 minutes for suspension towers and less than 3 hours for tension towers.

As of the time of this visit and report in 2018, over 40% of the pre-existing porcelain long rods on 400 kV lines and 33% of those on 300 kV lines had already been replaced with locally produced RTV-coated versions. This high-speed live replacement technique therefore serves as a model for other utilities in the region and elsewhere as part of their OHL maintenance programs.

The post Coating Line Insulators to Prevent Pollution Flashover appeared first on INMR.

 阅读本篇文章的中文版

Since the early 1990s there has been growing application of composite-housed HV apparatus at substations. This has particularly been the case for instrument transformers where the safe failure mode of a silicone housing compared to porcelain is seen as a crucial benefit. Of course, expected performance and service life of the polymeric housings, compared to porcelain, is also a concern to users. That is why any evidence of premature ageing is important, as is what remedial measures are available should this be discovered.

Past inspection of composite-insulated instrument transformers at substations operated by CEPS – the Czech Transmission System Operator – revealed varying degrees of ageing of housings. Degradation typically took the form of a hard surface layer that had become either hydrophilic or offered only poor hydrophobicity. There were also cases of more extreme degradation marked by spontaneous development of cracks on housing trunks as well as flaking of the surface.

This article, contributed by EGU HV Laboratory in Prague as well as CEPS and Lapp Insulators, discussed application of an RTV silicone coating as a remedial measure to restore the condition of degraded polymeric housings.

Guangzhou MPC Power International Co. Ltd.

China

PPC Insulators

Austria

See more suppliers of Silicones & RTV Silicone Coatings

Instrument transformers equipped with composite housings have been installed at 245 kV and 420 kV substations in the Czech Republic since 1992. These transformers, all supplied by one manufacturer, have been operating in a temperate middle European climate where annual sun radiation ranges from 1300 to 1800 hours duration and ambient temperatures can drop to -30°C. All substations where these units have been put into service have low pollution exposure (according to IEC 60815-1) and pH values of local rain range between 4.2 and 4.9.

Regular inspection of polymeric housings on all instrument transformers installed at CEPS substations has revealed that degree of degradation does not always correlate with years in service. For example, it was found that some instrument transformers with only 5 years’ service suffered high housing degradation while similar transformers with a 20-year service history showed no degradation. Moreover, degree of degradation of housings at the same substation and with the same number of years of service differed from one transformer to the next. This suggests that the composition of the polymeric insulating material was not the same.

In some cases, degradation along the length of the housing was uniform (e.g. distributed evenly along the core as well as on the top and bottom of sheds). In other cases, much degradation was evident on the top of sheds but little on the bottom; or degradation was higher on the south-facing side of the transformer than on the shaded side – suggesting some influence of UV. Finally, there was often no difference in surface degradation between the line and grounded ends of the transformer housing, suggesting that it was not caused by corona discharges.

Investigating Degraded Housings

The problem of degradation of polymeric housings on instrument transformers is still under investigation. But there are already some results based on measuring thickness of the degraded layer using Fourier Transform Infrared Spectroscopy (FTIR) and thermal gravimetric analysis as well as by studying the influence of acidic solutions and UV radiation. Partial results include:

• Maximum thickness of the degraded surface layer was up to 250 μm;

• Hydrolytic degradation of siloxane chains takes place in the surface layer under typical service conditions. This process is connected with reduction in molecular weight and results in loss of physical properties such as strain and flexibility as well as formation of a non-flexible, brittle layer on the surface. While the degradation process starts and progresses from the surface, the inner portion of the housing remains unaffected from a certain depth;

• Thermal stability of degraded samples after years of service falls within acceptable limits;

• Acidic solutions and UV radiation seem to be factors linked to degradation;

• Degradation can occur along the entire surface of the housing. This suggests it could be influenced by the specific composition of individual batches of the silicone rubber material used in manufacturing the insulator or by slight variations in the production process.

Based on all the above, it is reasonable to assume that, whenever the degraded surface of the housing remains unbroken by cracks, degradation reaches a certain thickness that then the surface is protected against further degradation. The degraded housing either becomes hydrophilic or is only barely hydrophobic and such insulators have to be considered as offering no hydrophobicity transfer properties (i.e. no capability to reduce surface conductance and leakage current activity).

If a degraded insulator has sufficient creepage distance, even though hydrophilic, such insulators can still provide the required service performance. This assumption has been confirmed during inspection by the fact that no erosion or tracking was recorded on housings having degraded surfaces. Moreover, three transformer housings with degradation were wetted under voltage and corona camera inspection showed no partial or corona discharges along their surfaces.

A much different situation could arise, however, in the case of spontaneous development of cracks in the degraded layer on the housing’s trunk. Due to the internal tension in the polymeric material at this location, any cracks will remain open and degradation processes can continue. A similar situation can occur in the case of extensive flaking on the surface, where relatively large areas of virgin material become exposed to environmental conditions. The most dangerous situation would be if cracks reached all the way to the equipment’s FRP tube core, with resulting possible moisture ingress and contamination of its internal oil or gas insulation. In such cases, prompt measures to prevent further degradation would have to be considered.

Types of Degradation

Surface degradation can be divided into two categories in regard to its potential impact on performance and reliability of the apparatus:

• Type A Degradation

This is degradation without spontaneous development of cracks and where any cracks become visible only upon bending the insulation.

• Type B Degradation

Here, degradation is marked by spontaneous development of cracks (e.g. on the insulator housing’s trunk) and/or extensive flaking of the degraded surface layer.

Type A Degradation

Examples of different degrees of Type A degradation are shown in Fig. 2.

Type A degradation does not constitute an immediate threat to service life and reliability of the transformer and therefore it is sufficient to inspect any apparatus with this form of degradation every 4 years.

Type B Degradation

Examples of different degrees of Type B degradation are shown in Fig. 3.

Samples of silicone rubber were cut from insulator housings with cracks on the trunk and subjected to scanning electron microscope (SEM/EDX) analysis (see Fig. 4).

Type B degradation constitutes a potential risk to service performance and therefore apparatus with this type of degradation should be inspected every year. At the same time, remedial measures were investigated to improve the surface condition of housings with such degradation.

Results of Maintenance Inspections

Instrument transformers with polymeric insulator housings that are currently in service at CEPS substations were manufactured between 1991 and 2007. In total, 300 such units were subjected to inspection with some inspected repeatedly to monitor the progress of degradation. The proportion of insulator housings found to have experienced degradation is shown in Fig. 5.

The main years of manufacture of apparatus with Type B degradation are 1993, 1995, 1999 and 2000. Fig. 6 shows the incidence and distribution of degradation, by type, at the substation with the highest number of instrument transformers.

The dates of manufacture of the instrument transformers at this substation along with their respective types of degradation are outlined in Table 1.

Measures to Restore Service Performance

The degradation on transformer housings inspected up to now has not yet reached a level that immediately threatens the reliability of their operation. However, continuing Type B degradation, marked by development of deeper cracks on the trunk of the insulator housing, could become dangerous.

As one possible way to improve the surface condition of housings with Type B degradation, application of an RTV silicone coating was tested on a sample cut from the shed of a degraded transformer housing. Half the sample was coated with RTV silicone while the other half was left uncoated. The sample was then subjected to a 1000-hour accelerated ageing test in a xenon chamber with UV radiation of 0.47 W/m2, 340 nm and with dry and wet cycle duration of 102 min and 18 min respectively. Fig. 7 shows that no further degradation was apparent on the coated portion of the sample after the test.

Moreover, adhesion of the RTV silicone coating to the sample was the same as before the test. While the portion of sample with the RTV silicone coating has again become hydrophobic, the part without coating is hydrophilic.

Experimental Coating of Polymeric Housings in Service

Experimental coating with RTV silicone was performed first on a 220 kV combined (voltage & current) instrument transformer with degraded housing. For this first application, a procedure was used that allows for monitoring and evaluation of:

• effectiveness and durability of the coating;

• influence of electric field;

• progression of degradation on the surface, both with and without the coating; and

• necessity to apply a primer before coating.

The RTV silicone coating was therefore applied to only two-thirds of the housing surface: the upper one-third at the line end and the lower third at the ground end. The middle section was left without coating. Moreover, half the circumference of each coated section had a primer applied before the entire circumference was coated.

Coating Procedure

1. Surface Cleaning:
Before coating, the surface of housing was cleaned with a hard brush to remove any unstuck parts of the degraded surface layer. Dust was removed with a wet cloth. The surface was then washed with a water/soap solution sprayed onto the surface. After drying, the rest of the surface contamination was cleaned with acetone.

Application of Primer:
After the acetone evaporated, a primer was sprayed onto the selected parts of the surface. The goal was to find out if there is a difference in adhesion of an RTV coating to an aged housing surface with or without primer.

Coating:
RTV silicone material was applied to the housing surface with airless spraying equipment. In total, there were 3 layers of silicone material such that between each layer application there was more than 30 minutes of drying time for solvent evaporation.

RTV Coating 220 kV Transformer Housing
RTV coating of an instrument transformer was performed in August 2014. The state of surface degradation of its housing before coating is shown in Fig. 8.

A hard degraded layer on the top and bottom surfaces of sheds was found along the entire insulator housing. On some parts, cracking and minor flaking of the degraded surface layer had occurred and the entire housing was hydrophilic.

Fig. 10 shows the difference between parts of the housing with and without coating, while Fig. 11 reveals the state of the surface and the hydrophobicity of the coated housing portion.

Inspection of Instrument Transformer with RTV Coating

The first inspection of the instrument transformer with the RTV silicone coating was performed in June 2015 (i.e. 10 months after coating). The condition of the housing surface is shown in Fig. 12.

The results of inspection showed that the layer of RTV silicone coatings on both sections (upper and lower) is without any damages and traces of surface degradation. Surface hydrophobicity of the instrument transformer housing parts with coating is excelent (HC from 1 to 3 according to IEC/TS 62073, cl. 6. The surface of the middle section without coating is totally hydrophilic. The adhesion of the RTV silicone coating on the degraded housing surface is excelent and there is no detectable difference in adhesion on the surface with and without primer (see Fig. 13).

RTV Coating 400 kV Instrument Transformer Housing

Coating a degraded 400 kV transformer housing was performed in July 2015. Here, the entire housing (upper and bottom parts of the sheds and trunk) had uniform degradation and was hydrophilic. Spontaneous development of cracks as well as flaking of the degraded surface layer were investigated and it was discovered that the flexibility of sheds had been reduced (e.g. cracks occurred even with minor bending). The extent of surface degradation is shown in Fig. 14.

Coating the housing was performed from a scaffold built around the transformer. To shield the work area from wind, the scaffold was protected using plastic tarps (see Fig. 15).

The coating procedure was basically the same as used for the 220 kV transformer but this time without any application of a primer. The state of the surface and degree of hydrophobicity of the coated housing are shown in Fig. 16.

Conclusions

Degradation of the surface of polymeric housings on instrument transformers had been observed in the Czech Republic during routine maintenance inspection. The degradation typically took the form of a hard surface layer up to 250 μm thick. The surface with this hard layer had become either hydrophilic or much less hydrophobic than when new.

Inspection results indicated that the degree of degradation was not dependent on duration of service. Moreover, degradation of the same type of apparatus from the same producer and the having the same length of service was different from one transformer to the next – even at the same substation with the same environmental conditions. This degradation had to be caused by some combination of environmental conditions (e.g. pollution, UV, etc.) and variations in the properties of the polymeric housing material (e.g. composition of different batches) or changes in the manufacturing process.

In most cases the hard surface layer cracks only when the sheds are subjected to bending. Moreover, whenever the thick, hard degraded surface layer occurs more or less uniformly along the entire top and bottom surfaces of sheds, they lose elasticity and even small bending leads to cracking.

If the housing has sufficient creepage distance, service performance and reliability of apparatus with surface degradation (but without spontaneous cracking) is not immediately affected. Spontaneous development of cracks (mostly on the housing trunk) and/or flaking of the degraded surface layer were observed on only a few transformer housings. Under such degradation, performance of the apparatus may decrease and measures to restore reliability should be considered.

RTV silicone coating of polymeric housings with a high degree of degradation (i.e. marked by spontaneous development of cracks on the insulator trunk) is one measure to increase service reliability. The coating can be sprayed directly onto the clean degraded housing, without previous application of a primer. The RTV material seems to fill the surface cracks and restores hydrophobicity of the previously degraded polymeric housing. It cannot, however, restore elasticity to the sheds.

STRI

Sweden

CATU Test Laboratory

FRANCE

See more Laboratories

The post Applying RTV Silicone to Restore Degraded Polymeric Housings appeared first on INMR.

Leer artículo en español

Finding ideal profiles for ceramic insulators has proven challenging, as demonstrated by the numerous different designs developed over the past 100 years. All aimed to achieve some optimal combination of pollution flashover performance and self-cleaning properties. As a result of this evolution, there is now a broad range of different profiles of disc insulators in service on transmission networks worldwide. Whatever preferences have emerged for one design over others have been due to factors such as local climate, topography and tradition.

By contrast, the main factors affecting shed profile in the case of composite insulators have traditionally been related to production. Depending on manufacturing technology, most sheds are smooth and not overly complex so that they can easily be removed from the mold cavity. Variations that do exist tend to be in shed size, spacing and angle of inclination. Yet there has still not been sufficient research on how shed profile impacts pollution performance of these types of insulators. Some experts believe specific creepage distance plays the decisive role while others disagree.

This edited past contribution to INMR by retired Professor, Guan Zhicheng at Tsinghua University, Shenzhen Campus, presented findings comparing pollution flashover performance of different composite insulator shed designs.

EMCO Industries

Pakistan

Wenzhou Yikun Electric

China

See more suppliers of Silicones & RTV Silicone Coatings

Past co-operation between Tsinghua University and the China Southern Power Grid aimed to evaluate shed profiles for composite insulators in terms of relative pollution flashover performance. Supported by local manufacturers, this research assessed 36 different shed profiles. All were classified into 4 main categories, depending on number of different shed diameters within each repeating unit:

A. one large, one small;
B. one large, two small;
C. one large, one medium, two small;
D. one large, one medium, four small.

Fig. 1 below, for example, depicts a design with alternating single large and single small sheds.

Fig. 2 shows the DC pollution flashover voltage obtained for these 36 different shed design categories using the solid layer pollution method and with test voltage applied by the constant ‘up and down’ approach. Values shown on the vertical axis represent a proportionate comparison of flashover voltage of each different design versus that of the best-performing geometry, i.e. alternating one large and one small shed (#22). These findings confirm that shed geometry can have profound impact on pollution flashover performance of composite insulators, e.g. by up to 22% for units with the same insulation distance.

As part of the same research, a total of 19 specimens of alternating shed design with different shed spacing and radii were compared in terms of relative pollution flashover performance. As seen in Fig. 3, all 5 designs with different radii have superior flashover performance when shed spacing is 100 mm. Moreover, it is not necessarily true that the bigger the diameter, the higher the flashover voltage. Rather, there is an optimal value and flashover voltage was found to be highest with large/small shed radii of 90 mm and 66 mm.

Finally, Fig. 4 shows the impact of average radius of large and small shed diameters on pollution flashover behavior given 3 different types of spacing of the large sheds. As can be seen, flashover voltage was highest at a spacing of 80 mm.

Other factors, apart from only pollution flashover performance, also have to taken into account when deciding on ideal shed profile for composite insulators in any new line project. These include protection against flashover due to bird streamers and against ice bridging.

Laboratory Center of Koncar Electrical Engineering Institute

Croatia

Powertech Labs Inc.

Canada

See more Laboratories

The post Influence of Shed Profile on Pollution Performance of Composite Insulators appeared first on INMR.