Florent has an affiliation with INMR going back 10 years, when he was a first-time delegate at the 2017 INMR WORLD CONGRESS in Sitges, Spain. Then an expert and business development specialist with the Surge Arrester Div. at Siemens (now Siemens Energy), he has since built an impressive career in the high voltage industry. After working with Tridelta Meidensha in Germany, he founded his own consultancy, Metarresters, which provides support services and solutions in the fields of lightning performance, surge arrester technology, and insulation coordination.

Florent has served as Session Chair (Surge Arresters) at past INMR WORLD CONGRESSES in Bangkok (2023) and Panama City (2025). In this regard he brings to his newly expanded role valuable experience from organizing those highly successful Sessions, while also interfacing with experts from across the globe.

As in the past, the 2027 INMR WORLD CONGRESS will feature Sessions devoted to key topics in electrical insulation and surge protection, including:

• MATERIALS & DEVELOPMENTS

• MANUFACTURING EQUIPMENT & TECHNOLOGIES

• LABORATORY TESTING

• POLLUTION & DESIGN

• LIGHTNING PERFORMANCE, SYSTEM PROTECTION & INSULATION COORDINATION

• APPLICATION & SERVICE EXPERIENCE

• CONDITION MONITORING & MAINTENANCE

INMR is confident that with Florent’s valued participation, the 2027 INMR WORLD CONGRESS will once again rank among the world’s best technical conferences and skills building events for T&D engineers and professionals. Thank you.

Marvin Zimmerman
Congress Chair
mzimmerman@inmr.com

Florent Giraudet
Conference Director
fgiraudet@inmr.com

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This edited contribution to INMR by Armando Pastore and Laura De Fina at GE Vernova in Italy described research and testing to verify dry RIP bushing reliability as well as to assess various design criteria to assure long service life.

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Internal Insulation Design & Tests

Dry type condenser bushings are produced by wrapping paper or synthetic material on a central tube or conductor as thin metal foil electrodes of certain lengths are inserted to grade radial and axial voltage stress not to exceed design limits.

To assess the reliability of RIP insulation materials subjected to service voltage stress, accelerated ageing tests were performed on specimens under multiple stresses, including thermal and electrical, to represent actual service conditions. One approach to accelerate the ageing process is by increasing the frequency of the applied voltage. Similarly, ageing of RIP bushings for DC applications was also evaluated through a DC voltage endurance test to assess reliability.

Accelerated ageing tests were conducted on RIP insulation materials to verify reliability by applying increased stresses for sufficient time duration to represent the stress experienced by a bushing over its typical service life. Samples were exposed to these stresses up to discharge or alternatively for a time duration comparable to required equivalent life. To evaluate behavior of these samples, the following conditions were applied:

• oven with controlled temperature at 90°C
• three different voltage/electrical gradient levels (kV) increased compared with design criteria
a) |E_{1_test}|=140 % |E_radial|_max design
b) |E_{2_test}|=175 % |E_radial|_max design
c) |E_{3_test}|=230 % |E_radial|_max design

The temperature that the insulation could experience in service (as a maximum average) was selected as 90°C. To reduce test time and accelerate the ageing process, frequency of the applied voltage was increased from 50 Hz to 5 kHz. The following relationship was assumed between life at power frequency f, i.e. L_f and the life L_h, at the test frequency, f_h:

For the RIP specimens being tested, given a service frequency 50 Hz and test frequency 5000 Hz, time to failure was estimated at 100 times the test time. Aged samples that had not failed were then tested at lightning voltages (1.2/50 s) to determine if there was any decrease in performance following ageing.

In terms of test methodology, the RIP specimens had been manufactured from the same material used in standard production but machined in a shape to have a central uniform stress area. RIP thickness in this uniform stress area was the average thickness between two conductive layers in the bushing’s condenser core. Several finite element analyses were carried out to fine tune both electrode and sample profiles (see Fig. 1).

The external surfaces of the uniform stress area were metallized with graphite paint to allow good electrical contact with the HV and ground electrodes with minimum risk of surface discharges. To avoid any other partial discharge activity inside the oven, the HV electrodes were rounded on the air side facing the test specimens (see Figs. 2 & 3).

Before performing these tests, the RIP samples were pre-conditioned in an oven under vacuum for 48 hours and then placed inside PET cells and filled with de-gassed silicon oil to prevent external flashover in the surrounding medium.

A distribution line was built to supply test voltage for up to 5 specimens in parallel inside the oven through a PET ‘bushing’ with air shields on both sides – inside the oven and outside (see Fig. 5). A set of wires had been put in place to connect the bushing terminals to the specimens’ air side electrodes. Oven temperature was set at 90°C during the entire test while voltage and frequency on the test specimens was constantly monitored. Moreover, a system for measuring partial discharge was used to record, in advance, any signs of insulation ageing. The measuring system was connected to the ground electrodes of the specimens and continuously measured and recorded discharges while, at same time, a circuit board recorded voltage measurements.

The tests were stopped for all voltage levels at 5000 hours, which corresponded to roughly 57 years of service, according to the above assumptions. No partial discharge activities were measured on specimens without defects.

In regard to the three-voltage levels and samples:

• At |E_{1_test}|=140 % |E_radial|_max, two out of the 5 samples failed due to metallization process issues at the initial stage of the test and were therefore not considered.
• At |E_{2_test}|=175 % |E_radial|_max, one out of the 5 failed just after energization due to a material defect.
• At |E_{3_test}|=230 % |E_radial|_max one out of the 5 failed due to material ageing after an equivalent of more than 36 years’ service.

No failures were observed on samples without defects, even with voltage stress up to 175% of normal service stress over an equivalent service life of 57 years. This was much greater than the average required for bushing life expectancy (i.e. 30 to 40 years) and served to confirm the conservativeness of the design criteria.

Lightning breakdown tests were also performed on aged specimens after the accelerated testing. A series of sets of three waves of equal peak voltages were applied, starting from 110 kVpk and increasing by steps of 5 kVpk. The breakdown voltages (BD) show that there was no significant difference among the specimens aged at different stress values (see Fig. 7).

DC Voltage Endurance Testing on RIP Samples

To obtain proper material characterization for DC applications, DC voltage endurance tests were performed on epoxy impregnated paper material at 90°C to represent both electrical and thermal stresses. As used in the AC ageing tests, 90°C was selected as the maximum average temperature that insulation will experience in service. Also, as in the earlier tests described above, test specimens were machined from RIP material in such a shape to have a uniform stress area and to minimize possibility of surface discharges. Sample surfaces in contact with electrodes were metallized to allow good electrical contact between electrodes and sample surfaces.

Test specimens and electrodes were of Bruce profile type, to minimize field increase at their borders. The Bruce profile electrode is composed of three sections: a linear section of radius R0 followed by a sinusoidal section extending over a radial distance of A and terminated by a circular section. This allows for a negligible border effect and nearly constant uniform field region. Effective area of uniform fields can be greater with a Bruce profile compared to other uniform field profiles.

These tests consisted of two parts. In the first, DC breakdown tests were conducted at three different uniform temperatures: 90°C, 70°C and 50°C. As a second step, a set of ageing tests were performed at 90°C and at voltage levels defined as percentages of DC breakdown voltage (as per the first test). During all tests, specimens were kept at constant temperature in a dedicated test vessel, using heating resistors placed on the outer surface that had been calibrated to achieve the desired temperature inside.

Standard IEC 60243-2 was followed for the first test to determine DC breakdown voltage of the samples, using the 60 s step-by-step method. This states that the test shall be applied starting from 40% of probable breakdown voltage and increased by steps, that, for the voltage of the test, have a value of 5 kV. If the test specimen withstands this voltage for 60 s without failure, the voltage shall be increased in incremental steps. Each increased voltage shall be kept for 60 s until failure occurs. The output of the first test is average DC breakdown voltage at 90°C, 70°C and 50°C.

The second part consisted of ageing tests performed at 90°C. Specimens were kept at constant temperature and constant voltage level until breakdown. Voltage levels were defined as 90%, 75% and 65% of the short time DC BD average voltage. The output of these tests was a DC voltage-time (V-t) curve at 90°C (shown in Fig. 11).

With regards to set-up, dedicated mock-ups for solid material ageing were used to simultaneously test 5 samples in a gas-insulated environment that allows having pressurized gas (SF6, dry air, etc.) with humidity control and constant temperature. The vessel system allowed disconnecting any punctured specimen and continuing testing the remaining specimens.

Concerning methodology, the following procedure was established for these ageing tests:
1. Conditioning the samples at 75°C for 2 days using N₂ at 4 bar abs to reduce humidity content within the bulk material and ‘de-sorb’ any moisture;
2. Installing samples in the insulated test vessel;
3. Applying ageing voltage (90% of the short time DC breakdown voltage) up to breakdown;
4. After each breakdown, recording the time and re-applying the voltage to the remaining specimens.

Leakage current on each sample was also measured over the test’s duration to monitor evolution in material conductivity with time and to obtain additional information on possible deterioration.

The short time DC breakdown voltage tests were completed and, as expected, voltages at breakdown were lower at higher temperatures. Considering average breakdown stress at 50°C as reference, Fig. 13 shows a plot of average BD fields per unit. Increased temperature almost halved breakdown field.

Comparing these results with average design criteria, a safety margin was estimated for each temperature.

With regards to ageing tests, Fig. 15 plots time to breakdown of each sample versus the ratio between testing voltage and average breakdown voltage. When comparing the V-t endurance curve at 90°C (based on preliminary results of these tests) with design voltage, it is evident that present criteria guarantee highly reliable service life.

Partial Discharge Performance of RIP Bushings

To advance knowledge on RIP insulation, continuous effort has been devoted to collecting and analysing data and experience with faults by partial discharge measurements done with the conventional method (as per IEC 60270). These records on PD patterns permitted identifying different types of faults that can occur. The literature describes much work devoted to recognizing partial discharge activity but this is mostly general in nature and not really applicable to specific equipment such as a bushing.

Phase resolved partial discharge (PRPD) patterns, which visualize occurrence of PD activity in reference to the phase of AC voltage allows classifying defects. Based on this classification, it becomes easier to recognize them. In a PRPD pattern, important parameters include discharge location in reference to phase angles and also variation in discharge magnitude with voltage and time.

Delamination is a typical defect that can appear in solid insulation. A void, for example, can generate partial discharge activity at the interface between the foils and the RIP materials. Delamination PD patterns are symmetrical with discharge pulses on the positive half-cycle larger than on the negative half-cycle. Other PD patterns based on experience are listed in Table 1 and refer to contamination on insulator surfaces and discharges in air from a sharp point at ground potential. Such continuous research has offered useful support to analyze bushing defects and has also provided valuable feedback about process reliability and recognizing systematic process deviation.

Special Tests on Dry Type Bushings

Beyond the above tests on materials, extended DC special tests were also performed on dry type bushings to assess service reliability, considering the polarization effect. The example shown in Fig. 16 involved a DC RIP bushing, rated Um at 530 kV and foreseen to be applied for a service voltage of 400 kV. It was successfully subjected to a 12-hour test at 690 kV, which corresponded to 30% more than rated voltage. The bushing was placed inside a turret that houses a barriers system to represent its actual service condition in an HVDC converter transformer. The bushing was tested with continuous PD measurements to detect any abnormal behaviour. After completing the DC extended test, success was confirmed based on the type tests as well as with AC partial discharge tests performed both before and after the long duration test and which showed no change in main parameters.

Mechanical Design & Testing

The main aim of mechanical design for a bushing is to ensure the best service conditions for materials by predicting stress and strain during its life cycle. Proper design must therefore evaluate stress under both test and service conditions. In this regard, the main loads generally come from the external connections that are subject to bending, gravity (due to mounting angle), earthquakes and short circuit forces. As a first step, advanced numerical analysis is useful to schematize the physical model using different failure criteria and mechanical properties as input for the different materials. The main components of a bushing can then be verified for different loads, acting simultaneously, to simulate test and service conditions. This helps designers to ensure the correct criteria are adopted. Fig. 17, for example, shows the ratio between predicted stress of a 550 kV RIP design subjected to seismic test on a shake table and maximum allowable stress according to design rules and material tests.

To achieve final product qualification, mechanical type tests are necessary to evaluate bushing performance. The tests recommended in most relevant standards are seismic (for bushings installed in areas where earthquakes occur) and cantilever (to simulate stress on the connection and on a bushing’s structure during service). Since utilities often require type test certificates not older than 5 to 7 years, continuous upgrade in product qualification is necessary to sustain any manufacturer’s product portfolio.

Focusing on the seismic aspects of dry bushing technology, laboratory tests were carried out using a shake table to validate RIP bushing classes 245 kV, 362 kV and 550 kV equipped with hollow core composite insulators. In general, seismic stress has a specific spectrum of frequencies from 1 to 30 Hz. Maximum peak in frequencies is from 2 to 10 Hz while for other frequencies stress is lower. The critical point in seismic bushing design is the resonance frequencies that can occur within this spectrum, usually in the lower range for HV bushings and at the higher range for LV bushings. To mitigate seismic impact during the design stage, numerical analyses are useful to evaluate any reinforcement that might be needed to a bushing’s structure or any change in material thickness that can modify its natural frequencies. While it is relatively easy to modify the vibration mode for a low voltage bushing, this can be complex and costly for a high voltage bushing. A test using a shake table is therefore useful to assess the response of different materials subjected at the same time to mechanical stress relative to their design limits.

Insofar as test set-up, bushings were positioned on a structure that simulates a transformer turret. Using a specially designed plate, the bushings being tested were installed with a mounting angle of 45°, which is generally the maximum required in service for Um ≥ 245kV. Referring to the reference system according to the IEEE standard, the spectrum in Fig 31 was applied for 245 kV and 362 kV class bushings and the spectrum in Fig. 32 was applied for the high voltage class, i.e. 550 kV. The spectrums were applied along X and Y axes with a damping factor of 2% while along the vertical Z axis the spectrum was applied at 80%.

An additional mass of either 7 kg or 11 kg was positioned on the top terminal, depending on bushing size in terms of rated voltage. According to IEEE’s standard, 7 kg is required to be added for bushing in the range 145 kV ≤ Um ≤ 500 kV and 11 kg is added for Um > 500 kV. By contrast, the IEC standard requires 7 kg for Um < 420 kV and 11 kg for Um ≥ 420 kV. With the aim of testing under the more restrictive conditions, the IEC standard was considered as the reference when it came to additional mass. Before performing this test, each bushing was subjected to a routine test in a UHV test hall. The test objects were also equipped with strain gauges placed near the bottom of their hollow core composite insulators, where highest stress and strain are normally expected. To apply the transducers, four 20 x 30 mm openings were made by cutting the silicone sheds all the way to the surface of the fiberglass inner tube (see Fig. 20).

In addition, four accelerometers were placed as follows:
• A0 on the base, at ground level – reference position placed on shake table;
• A1 on the terminal at the top of the bushing;
• A2 near the center of gravity;
• A3 on the shield at the bottom of the bushing;
• A4 on the main flange, on top of the rigid adaptor near the mounting location.

While the bushings were mounted on the shake table according to their class, (as shown in Table 2), pre-seismic static pull tests were carried out by applying a 600N or 1200 N force to the top terminal. This procedure was necessary to calibrate the strain gauges for the test limit based on results of a bending type test previously performed on the hollow composite insulator.

To measure vibration modes, a pre-test was then conducted by imposing an input sine sweep at 0.07g in the main three directions (X, Y, Z) with frequency scanning for resonance research. Such values are useful to validate the input used during any preliminary finite element analysis.

Before starting the final test at 2.00 g for 245 kV and 362 kV bushings, several steps were completed. Triaxial time history shake table tests for calibration and instrumentation checks were performed at 0.08g, 0.12g, 0.18g, 0.24g, 0.36g, 0.50g, 0.70g, 1.00g and 1.42g ZPA. Deformations at each step, measured by the strain gauges, were monitored and these values did not exceed defined limits. Finally, the 2×1.00g =2.00g ZPA time history test was performed according to the demanding requirements of IEEE 693-2018.

The same procedure for calibration and instrumentation checks was performed at 0.08g, 0.12g, 0.18g, 0.24g, 0.36g, 0.50g and 0.70g for the 550 kV class bushing. After the final test, post-seismic inspection was carried out consisting of the following:

1. Visual inspection for any signs of damage;
2. Comparison of bolt torque values before and after the test;
3. Evaluation of the test response spectra of the shake table motion about its capability to envelop the required response spectra from the technical specification.

Moreover, a post-test resonance search was performed for all bushings by imposing an input sine sweep at 0.07g in the three main directions (X, Y, Z). According to the IEEE standard, a change of more than 20% in the resonant frequencies can be used as the parameter to determine whether there are structural changes because of damage to materials. Resonance tests were successfully carried out for each test object, i.e. with no change in the values before and after testing. After this mechanical test, the bushings passed routine electrical checks with no change in partial discharge behavior. As a point of interest, these tests were witnessed by an independent external group to validate the product with an official qualification report, as mandatory for access to certain end user markets.

Conclusions

Special electrical and mechanical tests were carried out at GE Grid Solutions – RPV Unit in Italy to verify reliability of RIP dry bushings up to 550 kV. Since the market is moving rapidly toward a dry solution to replace traditional OIP technology, their reliability aspects in service are key points that must be included in each product development program from the start. Moreover, dry bushing technology is also growing because of the large demand for HVDC and AC oil-free and SF6-free bushings.

Due to interconnection requirements and network structure, HV substations are sometimes being sited in areas of high seismic activity or with heavy pollution. It has therefore become necessary to offer a fully proven technical solution for such environments and applications. Bushings are an essential component of transformers and their failure for any reason can result in system failure with serious financial consequences for utilities. In this regard, most relevant standards require higher test levels for bushings than for transformers, aiming to obtain reliable products with long service life. In addition to the requirements prescribed by standards, certain special tests performed by bushing OEMs can confirm the service behavior of optimized solutions focused on performance as well as cost reduction.

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Hydrophobicity is an especially critical factor and must be guaranteed over the entire life cycle of the insulator, even when exposed to severe temperatures, high UV, pollution, biogenic contamination and combinations thereof. If hydrophobicity is lost due to ageing, leakage currents can flow through the moistened pollution layer.

At high voltages, electrically sufficient hydrophobicity is assumed given a receding contact angle of approximately 40° or a classification of HC2. But at lower dynamic contact receding angles, significant current can flow through the pollution layer and occasional initial discharges (so-called dry band discharges) can ignite. If such discharges occur over a prolonged period, thermal and chemical damage can occur until the insulator fails.

This edited past contribution to INMR by Professor Stefan Kornhuber at the University of Applied Science Zittau/Görlitz in Germany offered an overview of hydrophobicity test methods and findings.

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While the pollution flashover exponent for non-hydrophobic insulators is 0.25, in the case of hydrophobic insulators it falls between 0.01 and 0.1. This means less reduction in pollution flashover voltage with increasing conductivity of the pollution layer. Due to different stress and multi-stress situations, the hydrophobicity property of a material can diminish over time, which leads to lower flashover performance from the hydrophobic curve to the non-hydrophobic curve and significantly reduced flashover performance.

Silicone materials also exhibit a certain ability to recover hydrophobicity and insulators in service can revert back to their original hydrophobic pollution flashover performance. Given this, creepage distance reduction factors have been introduced for so-called hydrophobicity transfer materials (HTM). Requirements to be termed HTM include:

• retention of hydrophobicity against certain stresses such as partial discharges under wet conditions (water droplet corona);

• recovery of hydrophobicity after a rest period; and

• transfer of hydrophobicity into polluted surfaces.

Understanding hydrophobic behaviour and the processes that lead to its reduction and recovery is therefore crucial in order to select the proper insulator material and design. Hydrophobicity transfer is already included in the new version of IEC TR 62039 and results of multi-stress testing to investigate hydrophobicity retention and recovery (e.g. the Dynamic Drop Test) have been summarized by CIGRE WG D1.58.

Test Methods

1. Hydrophobicity Transfer Test

In the case of silicone-housed materials in particular, the polymeric structure can be covered by a foreign layer representing pollution. It is known that short and medium-length polymeric chains, i.e. the so-called low molecular weight (LMW) and medium molecular weight (MMW) components, have the ability to penetrate this layer and form a new hydrophobic layer (so-called ‘hydrophobization’). This is particularly necessary to prevent wetting of the layer and thus occurrence of dry band discharges, as described earlier (see Fig. 3).

Testing for this hydrophobicity transfer mechanism has been specified in the new revision of IEC 62039, published in 2021. In principle two different test methods are possible:

• Method A, with quartz powder according to CIGRE;

• Method B. with Kieselguhr according to DL/T 810.

Method A

Test specimens are covered with adhesive foil to obtain a window of 30 mm × 30 mm (L x W). Thickness of these foils defines thickness of the pollution layer and 0.36 mm thickness is to used. Inside the area marked by the foil window, the specimens are coated by applying a slurry made of 7.5 g untreated silica powder (i.e., not silanized, medium grain size of approximately 3 µm) as well as approximately 3.5 ml of a mixture of water and isopropanol (65% water and 35% isopropanol by volume) that has been homogenized by stirring. A clean medical blade is used to wipe off any excess slurry. Since isopropanol tends to evaporate, the slurry is to be used shortly after its preparation. The result is a smooth and even surface.

After application of the slurry, samples are stored for 96h in desiccators under controlled 53% ± 10% relative humidity and temperature of 23°C ± 2°C (e.g. using a saturated solution with magnesium nitrate or a climate chamber).

Method B

The area of the test specimens should be around 30 cm² to 50 cm² and their thickness between 3 mm and 6 mm. Polishing test specimens is not permitted before the test. The pollutants include Kieselguhr, in accordance with IEC 60507:2013 Table 2, and NaCl. The Kieselguhr is weighed and put on the surface of each specimen and the NaCl solution is dropped on the Kieselguhr using a pipette or syringe. The Kieselguhr and NaCl solution are mixed and then evenly applied on the specimen using a small paintbrush.

Desired amounts of Kieselguhr and NaCl are 0.5 mg/cm² and 0.1 mg/cm² respectively. Mass of Kieselguhr is calculated according to the area of the test samples. The 5 polluted specimens are then placed in a dust-proof container for 96h under standard laboratory ambient conditions (i.e. 40% to 70% relative humidity and 20°C to 25°C).

Evaluation Criteria

Both methods provide results which lead to the same acceptance criteria in regard to the static contact angle from 5 measurements:

• Average value: ≥90°;

• Minimum value: ≥80°.

The criterion is related to static contact angle, which is measurable anywhere across the globe. In addition, several publications as well as IEC 62073 show that the dynamic receding contact angle provides better information in regard to related leakage current behaviour. Dynamic receding contact angle is used in the evaluation and the static contact angle should be measured for conformity with the standard.

Comparison of Test Results

Several measurements were done at different laboratories to compare test methods during the course of development of the standard and related discussions. Fig. 3 for example compares results of Methods A and B, whereby the values shown are the average mean value of measurements according to the specification.

2. Dynamic Drop Test

Test Method 

The Dynamic Drop test (DDT) permits accelerated evaluation of hydrophobicity retention when the surface of a material is subjected to electrical micro-discharges caused by water droplets. This is achieved by supplying an electrolyte with defined volume conductivity and flow rate under simultaneous electric field stress.

Samples are arranged at an inclination angle to the horizontal axis (see Fig. 4). The evaluation criterion is time to loss of hydrophobicity. This stage is achieved when a conductive electrolytic path between high voltage and ground electrodes is formed, which can be detected by measurement of leakage current.

At the beginning of the test, the electrolyte forms discrete droplets that roll down the specimen surface with more or less constant frequency. Presence of the droplet can result in field intensification and hence in ignition of electrical micro-discharges which could lead to localized reduction in hydrophobicity. This process continues until the electrolytic path becomes completely hydrophilic and a wetted path bridges the electrode distance. The failure criterion is reached with increase in leakage current to the mA range.

The upper-grounded electrode is supplied with an electrolyte that continuously provides droplets onto its lower side. As such, droplets flow on the surface of the insulation material to the energized electrode at the bottom. As a result of continuous electrolytic and electrical field stress on the drop-off sliding track, its hydrophobicity property is lost.

At the start of the test, individual electrolyte residues are formed and increase progressively in number and size. At the end, the number of residues is so high that, in conjunction with a draining drop, a continuous electrolyte connection of both electrodes occurs, which can carry a resistive current. The time taken to reach this state is defined as the retention time (tA) and can be detected by a current measurement system. Table 1 summarizes test parameters during this investigation.

Reference Measurement

For reference measurement and validation of the test set-up, a special material was found, i.e. the medical silicone, Replisil 22 NO. Not used in high voltage engineering, it is two component curing material at room temperature. Figs. 5 and 6 show initial results with this medical silicone material.

In late 2021 again investigations were done for evaluating the test setups with the same procedure and the same medical silicone, which is shown in Fig. 7.

In summary, the medical silicone Replisil 22 NO seems to show suitable properties for evaluating the test set-up and for comparing measurements. However, once produced, this type of silicone exhibits changing behaviour over time and the same timing must therefore be satisfied.

Summary & Conclusions

The following key properties are necessary for reliable, long-term functionality of composite insulators manufactured with hydrophobicity transfer materials:

• retention of hydrophobicity against stresses such as partial discharges under wet conditions (water droplet corona);

• recovery of hydrophobicity after a rest period; and

• transfer of hydrophobicity into polluted surfaces.

The new version of IEC 62039 includes testing for this hydrophobicity transfer property using two different test methods (A and B), which nonetheless show similar results. However, having two tests for only one property seems not an ideal solution, Moreover, the static contact angle by itself is still not fully representative of behaviour for electrical purposes.

The Working Group behind the new document will now get the first response and experience and, based on this, adjust the next version.

For the Dynamic Drop Test (DDT), CIGRE WG D1.58 has been defining test set-up and the possible evaluation criteria since 2014. After several Round Robin tests, test description and procedure can be recognized and minimum retention time to validate a material have been reached. In terms of subsequent steps, a final brochure will be written and all remaining open questions will need to be solved in a new WG. Moreover, IEC WG 5 decided at the TC 112 meeting in 2019 that this test will soon be proposed as a new work item.

References
[1]    Bär, Christiane; Schmuck, Frank; Kornhuber, Stefan; Bärsch, Roland; Brade, Volker: Influence of the Material Composition on the Dynamic Hydrophobicity of Silicone Elastomers for high-voltage Outdoor Application. In: CIGRE Session 2018. Paris, 2018
[2]    IEC, TR 62039:2021 Selection guide for polymeric materials for outdoor use under HV stress. 2021.
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[4]    C. Bär, R. Bärsch, A. Hergert, and J. Kindersberger, “Evaluation of the retention and recovery of hydrophobicity of insulating materials in high voltage outdoor applications under AC and DC stresses with the Dynamic Drop Test,” IEEE Trans. Dielect. Electr. Insul., vol. 23, no. 1, pp. 294–303, Sep. 2016.
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[6]    F. Schmuck, “Zur zeitraffenden Alterungsprüfung von Silikongummi-Oberflächen unter Fremdschichtbelastung und simultaner 50-Hz-Spannungsbeanspruchung,” Zittau, Techn. Hochsch., Diss., 1992., 1992.
[7]    IEC, TS 60815-4:2016 Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 4: Insulators for d.c. systems, Oct. 2016.
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[14]  Alexander Hergert: Test methods for evaluating the dynamic properties of hydrophobicity of polymeric insulating materials. Dissertation, Technische Universität München, 2017
[15]  Hergert, Josef Kindersberger, Christiane Bär, Roland Bärsch: Transfer of hydrophobicity of polymeric insulating materials for high voltage outdoor application. IEEE Transactions on Dielectrics and Electrical Insulation 24 (2017), 1057–1067
[16]  Heike Herzig, Stefan Kornhuber: Defined silicone rubber surface structures in a long-term test. IEEE 2nd International Conference on Dielectrics, Budapest, 2018
[17]  Cervinka, Rüdiger, Stefan Kornhuber, und Christiane Bär. „Investigation for a reference material for the dynamic drop test (DDT) with a reproduceable and repeatable retention time“. In 2019 IEEE 4th International Conference on Condition Assessment Techniques in Electrical Systems (CATCON), 2019.

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Important trends have been driving development of transmission and distribution networks, including:

1. Increasing demand for electrical energy by developing countries;
2. Shift from fossil fuels to electricity for key end use energy sectors worldwide; and
3. Change in the power generation mix toward more de-carbonization along with a rapid increase of renewable energy sources that are often highly fluctuating and relatively difficult to dispatch.

Not long ago the European Union announced plans to generate fully one-fifth of its electricity from renewable energies while some scenarios foresaw an even more rapid increase. One study in this regard was titled ‘100% Renewable Electricity: A Road Map to 2050 for Europe and North Africa’. According to the participants, a substantial and rapid de-carbonization of electricity generation will have to take place to limit climate change.

To move towards this goal, one scenario envisaged that fully 100% of electricity generation by 2050 will come from renewables, e.g. by integrating wind and hydraulic resources of Europe along with the wind and solar resources of North Africa and possibility using the hydraulic basins of Northern Europe for energy storage.

For example, the energy produced at night by wind farms in Denmark or England could be stored as hydraulic energy in Norwegian fjords and used the following day. Such a scenario would require a ‘power network revolution’. Super transmission grids would need reinforcement of current HVAC interconnections between countries and integrated HVDC super grid long distance connections. Along with such transmission systems, new grids have been foreseen for distribution and connection to de-centralized renewable generation sites

Of course, this is but one possible scenario. The European power system remains split into separate synchronous grids. Moreover, this transmission system is mostly old, sometimes inefficient and typically congested. In addition, the North African grid is only partially linked with that of Europe. A similar if not greater transmission (and distribution) upgrade will be needed in the United States as well. That grid remains basically a ‘relic’ given the minor investments made over the past 50 years.

But things have been changing. A past initiative in Europe by ‘Friends of the Super-Grid’ proposed a Phase 1 project to connect England, Scotland, Germany and Norway at a cost of some €34 billion – an amount close to that foreseen by the North Sea Countries Offshore Grid Initiative. The estimated investment for a more systematic intervention over the next two decades is on the order of hundreds of billions of Euros in Europe and an order of magnitude higher worldwide (i.e. USD 1.8 trillion based on past IEA estimates).

Such costs and resources make it urgent to make use of the smartest technologies to optimize system exploitation. In other words, these future grids would have to be smart, reliable and cheap, while also integrating new networks with old. While ‘smart grid’ means different things to different people and its definition varies by country and discipline, most agree that it should be more efficient, resilient, strong, reliable, predictable and cost efficient.

The massive capital expenditures expected on smart grids over coming decades will create unparalleled opportunities for manufacturers of advanced materials and insulators. Meeting the many and varied expectations for these new grids will probably also require development of new designs, materials and diagnostics, including:

Smarter Insulator Materials

‘Smarter’, higher-performance insulator materials would contribute to assure the reliability of smart grids, while also reducing costs. Ceramic insulators have already undergone tremendous improvement in quality and consistency, with better manufacturing and development of new designs specifically for contaminated environments. While the history of non-ceramic insulators is far shorter, their development process has been extremely rapid. Present designs, materials, and production methods are ‘generations’ ahead of the original attempts, to the point that today they are considered essentially a mature product and no longer seen as a high-risk alternative to conventional insulators. Superior materials (i.e. super-hydrophobic and environmentally friendly as a result of nanotechnology) may lead to a whole new generation of such insulators, designed and optimized for each service climate and pollution situation.

Smarter Insulator Designs & Manufacturing

Availability of ever more sophisticated software will permit more customized insulator designs, both from the mechanical and electrical points of view. For example, the mechanical characteristics of composite housings will be able to be adjusted, case-by-case, to meet seismic and pressure requirements as well as any other mechanical stresses (taking into account the statistical nature of such events). Insulator profiles will be optimized, along with their bulk material, for each application and environment, e.g. selecting profiles to avoid pollution accumulation and improve dielectric performance. In particular, smarter insulators will be developed for DC applications, being that present insulator technology sets a limit on the evolution of UHVDC. Such customized insulators will imply new investments for more flexible and economic manufacturing processes.

Smarter & Less Costly Insulator Diagnostics

On-line condition monitoring of critical assets is one way the electrical insulation industry can contribute to smart grids by avoiding system outages due to insulator failure. Together with new designs of insulators, those units already in service will have to be better ‘controlled’ to improve smart grid reliability. In this regard, improved prediction methods together with less costly off-line as well as on-line monitoring tools will permit operators to know the overall insulation condition of their systems and streamline maintenance costs in the process. Development of better insulator diagnostics will also permit more live line work with guaranteed safety and eliminate the costly shutting down of lines for maintenance.

Smarter Insulation Coordination

Broad use of smart and less costly surge arresters could lead to systematically limiting overvoltages, thereby leading to ever more compact lines and further contributing to optimization of smart grids.

These are some of the ways insulators have the potential to become ‘smarter’.

Hivolt Power System

China

PFISTERER

Germany

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This lecture discusses measures for improving lightning performance of transmission lines, based on experience acquired facing the challenge of improving performance of numerous transmission lines in South America, installed in a variety of distinct service environments.

The post Techniques to Improve Transmission Lines Performance: Conventional & Non-Conventional Approaches (Video) appeared first on .

One of the strategic goals at the Danish TSO, Energinet, has been to become an Energy Hub by establishing additional interconnections with other countries. This way, when there are periods of calm winds, energy can be imported from other areas and, when there is large wind production, excess power can be exported. But having such large interconnectors places great new requirements on transmission grids given the huge amounts of power that will need to be transferred. To ensure enough capacity within the grid, new overhead lines (OHL) are needed and now being planned.

This edited past contribution to INMR by Karl Emil Steenholt-Eliasson of Energinet discussed how new tower designs have been researched and tested to help overcome public opposition to new lines while at the same time meeting the requirement of reduced cost.

Hivolt Power System

China

DALEKOVOD OSO

Croatia

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Establishing any new overhead line these days is seldom without issues since there is often strong public opposition. For example, the most recent new overhead transmission line in Denmark, running between Kassø and Tjele, had to employ special design structures, called Eagle Towers (see Fig. 1). Construction of the line was deemed a success and use of these design towers received a high level of public appreciation.

The Eagle Tower design, however, came with the downside of increased cost compared to the lattice towers traditionally used in Denmark but now no longer accepted. At the time of construction of this line, the only real alternative to these design towers was putting cables in the ground, which came with a host of other issues. Now, as the need arises for new transmission interconnections, a project started to investigate possibilities for a new design tower where the focus was still on aesthetics but also on reducing cost.

Optimizing Eagle Tower Design

The reason for looking to optimize the Eagle Tower design was that a new OHL is in development and planned to use these due to their success on the earlier project. Tower optimization would aim not to compromise design but rather to maintain the same basic visual expression as the existing design. To ensure that cost and aesthetics were both taken into account during this process, an architect and an engineering firm were hired – the former to ensure the design was kept the same or improved and the latter to calculate the impact of each idea to reduce cost. The first task was to establish a baseline budget using experience and information from the earlier Kassø-Tjele Project. The budget included all relevant items that would directly or indirectly be affected by tower design. For the sake of simplicity, it was decided to limit investigations in order to ensure that the all the different ideas could be compared with regard to total cost. The baseline was then calculated according to the following:

• Cost to be evaluated in price/km;
• Cost to be based only on the tallest suspension tower and not include tension towers, etc.;
• Span to be used as baseline would be 330 m (instead of the original 360 m) since this was the actual average between towers on the last project;
• All new prices would be converted back to their 2013 equivalents to ensure comparability to data from the Kassø-Tjele line.

Based on these prerequisites, the baseline budget was made and grouped into the following cost categories:

• 42 m Eagle Tower
* Cost of towers including delivery to site but without installation;
• Foundation
* Including monopile, cast-in section, ramming of monopile and installation of cast-in section;
• Insulator Arrangements
* Including insulators, fittings, etc.;
• Installation of Tower
* Including tower installation and drive plates
• Compensation to Landowners
• Conductors
* Including earth and phase conductors as well as their installation.

To evaluate visual impact of various cost reduction proposals, architects came up with a scoring scheme whereby the visual impact of any changes could be compared with the existing Eagle design. Scores went from 1 to 5, where 5 was considered as having a negative visual impact and 1 was considered to represent a significant improvement. A score of 3 was considered neutral, with no clear aesthetic difference to the original design.

Identifying Cost Saving Possibilities

With the evaluation scheme in place, work began identifying various options to reduce costs. To start, the advisors were asked to review existing design and specifications and return with ideas to optimize the tower. At the same time, experience within Energinet in regard to design, production and installation was gathered and investigated to establish whether any obvious steps could be taken into consideration. This was done by contacting all individuals who had worked in development or installation of the original Eagle towers – both internally and externally. During this phase, the basic approach set out to all participants was that all aspects of the existing design could be challenged since all previous decisions could affect cost, one way or the other. Based on this investigation, a list items were identified for further study of which the following were eventually selected as most appropriate:

• Using high strength steel (i.e. S460 instead of S355);
• Optimizing conductor geometry;
• Materials and surface treatment;
• Using slip-joints instead of internal flanges;
• Reducing conductor diameter;
• Increasing tower diameter, including investigation of tapered tower sections;
• Simplifying cross-arm design;
• Establishing monopile foundation without cast-in section.

Together, all these items were estimated to allow cost of the tower to be reduced by about 180,000 Euro per km, yet with only minor changes to visual impact (see Fig. 2).

Testing Tower

Due to the scale of proposed changes in design, it was decided to test the optimized tower design according to EN 50341. This allowed the opportunity to determine whether actual cost savings were in line with the expectations prior to purchasing optimized towers for the complete line and also if these new towers could be erected as expected. The test was therefore considered to consist of the following:

• Production of Tower
* To investigate if use of S460 would yield the calculated reduction in cost;              
* To test constructability of the tower.
• Establishment of Test Tower (Complete Construction)
* To test if the installation of monopile could be done as expected (without cast-in section);
* To evaluate the consequences of ramming directly on the flange of the monopile;
* To observe rotation of the monopile during ramming                                   
* To evaluate the shim-plate solution, designed to correct for any deviation during ramming;
* To evaluate any complications from establishing the tower with oval bolt holes (to correct for any rotation in the monopile).
• Static Test
* To ensure that the tower could withstand the static requirements set out in EN 50341.

In each part of this test, another requirement was that all reflections, observations and ideas for further optimization had to be captured and noted.

Conclusions From Eagle Tower Optimization

Even though some of the updates made to the Eagle Tower were considered optimistic and potentially risky compared to former practices, the test ensured confidence in the optimized design. Moreover, it also gave valuable insights into the consequences of decisions made during the optimization process.

Based on the tower test, some items previously identified as offering potential for cost savings proved to be not as expected. For example, during procurement of the test tower, it became clear from the tenders that working with S460 came with downsides not considered during the evaluation. It was therefore decided to revert back to using S355 prior to final procurement of optimized towers for the new OHL. Other items, however, were deemed a success even though some had been considered as risky to constructability of the tower. One example here was the choice to remove the cast-in section to connect monopile foundations with towers. This was because ramming the monopile proved to be within acceptable deviations and yielded confidence in this solution.

Another important decision came from investigating use of the slip joint. Even though this had been deemed as not possible given the size of this tower, it still gave insight into the potential for cost savings of removing a single flange connection. A great deal of effort was therefore put into trying to optimize tower section lengths to determine if the number of sections could be reduced. This process concluded by eliminating one section on the largest towers and contributed greatly toward cost reduction.

Overall, the entire optimization effort was deemed a success since the cost reduction target proved close to expectations, after correction for inflation. In fact, the new optimized tower is now being built on a new OHL that runs in south Jutland from Kassø to the border with Germany.

Development of New Design Towers

A final element of this same Project was to develop a totally new structure design to replace the Eagle Tower on future overhead line projects. This work began after cost optimization of the Eagle Tower concluded and knowledge gained during the process was kept in mind when searching for a new design tower. The architects and engineers continued to work together in this part of the project since it was felt that engineers could challenge the design and propose more cost-efficient solutions while architects could ensure that any final design kept in line with the desired design philosophy. The basic requirements for development of a new tower included:

• More cost-efficient than the optimized Eagle Tower;
• Lower in height than the Eagle Tower;
• Minimum one lattice solution;
• Same technical requirements as the optimized Eagle Tower.

With these in mind, the team of architects and engineers came up with sketches of possible design solutions, each of which was then evaluated in regard to expected cost (see Fig. 4 for some of these proposals).

Based on estimated cost and aesthetic evaluation of each solution, a decision was made to allow the team to proceed with two solutions: one a tubular solution (in two versions); and the other a lattice solution (shown in Fig. 5). Both solutions are referred to as Thor Towers, differentiated by adding Lattice or Tubular, depending on type of tower.

The tubular solution was then selected due to positive recent experience with the Eagle Tower, which had received mostly favorable comments from the public. Moreover, it was different than traditional lattice towers and a tubular solution was already known to Energinet and therefore came with only minor uncertainties in regard to constructability. This tubular solution is designed in two versions, with the insulator arrangement being the main difference between them. Using a V-insulator configuration, total width of the tower can be reduced but at higher cost. With an I-insulator configuration, total cost of this tower solution was approximately 33,500 Euro less per km than the optimized Eagle Tower. This tower is about 10m lower than the optimized Eagle Tower and 10m wider.

The lattice tower solution is designed using round steel members instead of the angle steel members used on traditional lattice towers. The properties of round steel members make it possible to reduce the amount of members throughout the tower, making it far simpler and ‘lighter’ compared to traditional lattice towers. This tower solution is expected to be some 115.000 Euro less per km than the optimized Eagle Tower. Compared to the traditional Donau lattice towers formerly used in Denmark, the Thor lattice version is expected to cost about 30,000 Euro more but with significantly greater aesthetic expression.

After adoption of both solutions as potential towers for use on future overhead line projects in Denmark, one of the unknowns was: how would a lattice tower design be received by the public?

Public Hearings

When the need arose for a new line on the Westcoast of Jutland, the lattice design was taken into consideration during project planning and development. Due to the cost reduction expected, it was decided to use the new tower and, when public hearings commenced, the lattice version was shown alongside the Eagle Tower. Both were presented as 3-D models together with illustrations of how they would appear in the local environment, to offer an idea of how they compared in appearance. In general, there was strong opposition to a new OHL but this was not focused so much on tower design as on the fact that the line would be “in their backyard”. During hearings, a lot of people stated they would prefer the lattice tower over the Eagle Tower since it was deemed less visible in the landscape. This result gave Energinet comfort in using this tower design for the new line. In fact, the Lattice Thor Tower is now in the detailed engineering phase given that it has been chosen for the full length of the new 170 km line that will run along the West coast of Jutland.

Procurement of Test Tower

As with the Eagle Tower, the process of procuring the Thor Lattice Tower, has started but not quite with the results expected during the design phase. Even considering that some increased cost should be expected on any prototype tower, as work progresses there is an effort to better understand which elements of the design are responsible for increasing costs. It is still believed that the cost target will be achievable but since use of tubular steel of this scale is comparatively new, more work will be needed on this tower.

Conclusions

Returning to traditional lattice tower designs is not an option for Energinet and the last few years have made it clear that the public appreciates every effort made to reduce visual impact of new overhead lines. Moreover, it has also become clear that adjusting how “lattice structures” are viewed can create elegant solutions while also helping control costs. The processes involved when optimizing the cost of the original Eagle Tower prior to the development of new towers gave engineers an indication of which ideas would work best. Subsequently, as new towers are being produced and tested, additional ideas for optimization may become clear and these should be implemented before final procurement of towers for any new OHL.

Requiring that engineers and architects work together during all aspects of tower development and optimization ensures that solutions will be a compromise between design and cost efficiency. Architects often think ‘outside the box’ and this challenges standard decisions while engineers ensure that solutions proposed are realistic, possible to manufacture and cost effective. Though the process has so far proven mostly positive, the new towers developed are not yet perfect in regard to design. It is therefore important that these methods be maintained to ensure that solutions to reinforce the transmission grid in Denmark, and elsewhere, are aesthetic as well as cost efficient.

CATU Test Laboratory

FRANCE

Powertech Labs Inc.

Canada

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To qualify a bushing for seismic application, there are guidelines to support manufacturers such as IEEE 693:2018 & IEC TS 61463:2016 that instruct how to proceed for product type tests. But in terms of design, engineering criteria at the product development stage are defined by manufacturers and they are applied based on past testing and service experience from the field.

The post Seismic Solutions & Testing for Oil-to-Air & Air-to-Air Bushings (Video) appeared first on .

A comprehensive assessment of two monitoring devices deployed in Itaipu Binacional’s power system was carried out as part of the utility’s predictive maintenance strategy. Laboratory evaluations were conducted to compare total and resistive leakage current measurements against an offline reference under controlled conditions.

The post Assessment of Resistive Leakage Current in Surge Arresters: Monitoring Versus Offline Measurements (Video) appeared first on .