Materials and emerging test techniques
By Simon Sutton, Chair, & Gordon Wilson, Secretary
Scope
The activities of Study Committee D1 (SC D1) include the assessment of the performance of new and improved materials, and the development of diagnostic and test methods. These activities may be summarised as:
- New and existing materials for electrotechnology
- Diagnostic techniques and related knowledge rules
- Emerging test techniques which may be expected to have a significant impact on power systems in the medium to long-term.
Since the publication of the CIGRE 2030 Strategic Plan the scope of SC D1 has been broadened to include aspects of the energy transition, such as, energy storage, hydrogen, solar PV and wind, sustainability and climate, and electric vehicles. In all cases the scope is focused on the medium to long-term.
Organisation and Membership
The current membership of the SC consists of 28 regular members, 3 additional regular members and 9 observer members. Plus, from 2024, there are additional positions on the Study Committee for Women in Energy (WiE) and the Next Generation Network (NGN).
The activities of the SC are grouped into four focus areas:
- Liquids and liquid impregnated insulation systems
- High voltage and high current testing and diagnosis
- Solid materials
- Insulating gases
Each of these areas is overseen by an Advisory Group comprised of global experts who collectively identify topics for future study and develop the Terms of Reference for new Working Groups. There are two additional Advisory Groups, namely, (1) Tutorial Advisory Group and (2) Strategic and Customer Advisory Group. Figure 1 shows the current structure of the Study Committee.
More information about current activities can be found on the Study Committee website.
Figure 1 - High level structure of Study Committee D1
Activities
SC D1 deals with the characteristics and performance of materials for electrotechnology including conducting and insulating materials, superconductors and nanocomposites. The scope of SC D1 also covers emerging test techniques and the development of knowledge rules.
Testing encompasses the application of electrical, thermal, mechanical, and electro-chemical stresses, as well as ageing performance under various stresses. Such testing can be applied for the purpose of design, type, sample, or routine tests. The development of test techniques is driven by various factors, such as the introduction of new materials, new requirements on accuracy or an increase in stress levels. The need for new or modified test methods can also be driven by failures in service. The increasing use of HVDC technologies has driven the need for new or improved international standards, and SC D1 is supporting the work of the IEC.
As one of two horizontal CIGRE Study Committees, SC D1 strives to support the other CIGRE Study Committees in their activities and consequently has many joint Working Groups. In 2025, SC D1 had 18 WGs, including four JWGs led by D1 and additionally SC D1 is involved in a further eight JWGs led by other SCs. During 2025 a further three WG or JWGs have been established (Figure 2) which involve the members of D1:
- D1.83 Optical spectral sensing technologies for partial discharge (PD) and arc detection in gas-insulated equipment
- D1/A2.84 Silver corrosion in power transformers
- A2/C4/D1.77 Design of transformers for very fast transient overvoltages
The following two CIGRE Technical Brochures were published since the last annual report:
Liquids and liquid impregnated insulation systems, organised by AG D1.01 (Q. Liu)
This Advisory Group focuses on basic phenomena and mechanisms of dielectric performance, ageing process, and thermal characteristics of liquids and liquid impregnated insulation systems; covering traditional mineral oil-based insulating liquids as well as alternative liquids (e.g. natural and synthetic esters, silicone). The work aims to discover and understand the basic physical and chemical mechanisms associated with ageing, thus forming the basis for diagnostics and asset management of products like liquid-filled transformers and liquid-insulated cable accessories. The most recent meeting was during A2/D1 Colloquium in Seoul (Figure 3).
WG D1.65 was dealing with mechanical properties of insulating materials and conductors for liquid-immersed transformers. The final technical brochure was published this year, TB 967 “Mechanical properties of insulating materials and insulated conductors for oil insulated power transformers”. The follow-up JWG D1/A2.79 on the dynamic behaviour of winding insulation materials in liquid immersed transformers is actively progressing. Five sub-groups were formed to review the loading cases, static behaviour of insulation materials, dynamic behaviour of insulation materials, processing and modelling aspects.
WG D1.76 has been dealing with Degree of Polymerisation (DP) measurements and the relationship between functional mechanical properties of cellulose products and measured DP‑values. A recommendation on revising the DP measurement standard IEC60450 was proposed to IEC TC112. JWG D1/A2.77 has been evaluating liquid insulation tests in terms of data accuracy, data reporting format and data interpretation. A data template to collect transformer basic information and oil test results was proposed. Both WGs are preparing the final technical brochures, which are expected to be completed before Paris session in 2026.
JWG D1/A2.80 focuses on the functional properties of solid insulation materials used in liquid filled transformers and reactors and their compatibility with insulating liquids. Two task forces are formed this year, one focusing on liquid impregnable materials and another on non‑impregnable materials. The group is co-ordinating the work with IEC TR60076-26 JMT on “Functional requirements of insulating liquids for use in power transformers”.
There are three A2 led joint WGs with close collaboration with D1.01:
- JWG A2/D1.71 focuses on the qualification of modern insulating liquids for key transformer components and accessories.
- JWG A2/D1.72 addresses retrofilling mineral oil-filled transformers with alternative liquids.
- JWG A2/D1.74 considers online moisture monitoring of transformers for ageing assessment.
In addition, a new JWG D1/A2.84 on silver corrosion in power transformers was approved in Nov 2025; this follows a Task Force run by SC A2 which developed the Terms of Reference.
Figure 3 - Huge attendance for the Advisory Group meeting on Liquids and Liquid Impregnated Insulation Systems during the Korean Colloquium
High voltage and high current testing and diagnosis, organised by AG D1.02 (U. Riechert)
One of the most important foundations for this Advisory Group is the co-operation with IEC TC 42 “High-voltage and high-current test techniques”. WG D1.60 was established to coordinate the development of suitable hardware and software for traceable measurement techniques for very fast transient overvoltages (VFTO), to widen the basis for reference measurement capabilities meeting the relevant calibration requirements. In contrast, WG D1.61 accepted the challenge to develop objective methods and indicators which can be used for optical corona measurements on overhead lines and equipment. The group performed comparative measurements at different sites and laboratories using different types of cameras. Both groups have largely completed their work and will publish their brochures shortly.
In recent years, this Advisory Group has seen increasing interest in topics related to the properties and applications of materials and equipment under DC stress. This includes discussing and defining how to test gas insulated HVDC systems, the investigation of partial discharge activities and surface arcing under DC, methods to measure the AC and DC resistance of conductors of cables and overhead lines, and the evaluation of dynamic hydrophobicity of polymeric insulating materials under AC and DC voltage stress.
WG D1.72 is defining a material resistance test against surface arcing under DC and WG D1.74 is investigating PD measurements on insulation systems stressed from HV power electronics. To support the development of existing IEC standards, WG D1.63 is studying methods of partial discharge detection under DC stress and the analysis of partial discharge activity with respect to pulse patterns and the sequence of pulses under various factors of influence, e.g., slope of voltage change, voltage magnitude, and material properties of the test object. The response of measuring instruments and possibilities of noise suppression will also be addressed. JWG B1/B3/D1.79 was formed to harmonise the different requirements for different materials and products and focused on recommendations for dielectric testing of HVDC gas insulated cable sealing ends. The recommendations from TB 957 for dielectric testing of HVDC gas-insulated cable connection assemblies was published in 2025 and the topic was presented in several tutorials. The work is already being incorporated directly into IEC standardisation. The harmonisation of voltage designations and definitions across different HVDC component technologies is another topical issue in this area. SC D1 is actively involved in a joint task force along with various other SCs under the leadership of SC B4.
Life cycle assessment and data handling are also becoming increasingly important. SC D1 is also active in this area and started WG D1.81, “Methods and Common Data File Format for Time-Domain Reflectometry” and participates in the JWG B3/A2/A3/C3/D1.66, “Guidelines for Life Cycle Assessment in Substations Considering the Carbon Footprint Evaluation”.
Supporting the practical work of service engineers will be provided by JWG A2/D1.67 dealing with guidelines for online dissolved gas analysis monitoring and reflects the close cooperation between SC D1 and A2.
Solid materials, organised by AG D1.03 (J. Castellon)
Currently this Advisory Group is leading four WGs D1.62, D1.73, D1/B1.75 and D1.82 at varying stages of completion.
WG D1.62 has made slow progress in recent years after the retirement of its original convenor and the disruption of Covid. However, Fabian Lehretz, has stepped up to the role and reinvigorated the work.
WG D1.73 deals with the multi-functionality of nanostructured dielectrics and continues previous work (TBs 451 and 661) in this important field of improving material properties with the addition of small amounts of nano-fillers. Nanocomposite samples were made by University of Southampton and sent to the labs involved in material characterisations. However, too few laboratories have carried out the measurements to highlight the multifunctionality of the materials produced. This group has also suffered the retirement of its convenor (replaced by Jérôme Castellon) and Covid. Moreover, it seems that many of the original members are now focused on other topics.
WG D1.82, “Additive Manufacturing/3D Printing in Service of the Electrical Power Industry” was launched in July 2024. WG members were recruited until February 2025: there are 15 members from 10 different countries, including 5 members from the NGN. The WG had 2 meetings so far with a third meeting scheduled in December 2025. The table of contents has been defined and work allocated to task force groups and individual WG members. This WG aims to investigate the potential for 3D printing for the electrical power industry and informs on the opportunities and limitations of the technology.
The JWG D1/B1.75, “Strategies and tools for corrosion prevention for cable systems” has completed and the Technical Brochure has been reviewed and approved by the Study Committee and should be published soon. JWG B1/D1.75 also completed its work and published TB 968, “Interaction between cable and accessory materials in HVAC and HVDC applications”.
Insulating gases, organised by AG D1.04 (K. Juhre)
The main topic related to this Advisory Group is still the change from sulphur hexafluoride (SF6) towards alternative gases and gas mixtures with significantly lower global warming potential (GWP), reaching technological maturity at all significant voltage levels. This transition is motivated by efforts to enhance sustainability, mitigate global warming, and comply with regulatory frameworks such as the European F-gas regulation enacted in 2024. Ongoing research and development into these alternatives continue both within academia and industry. The work of AG D1.04 encompasses both SF6 and its potential substitutes.
Partial discharge monitoring (PDM) systems are essential for assessing SF6 equipment and gas-insulated systems (GIS). WG D1.66 defined GIS PDM requirements to ensure reliable detection and interpretation of PD signals. Its Technical Brochure (TB 933) was published in 2024 and provides guidance in selecting a suitable PDM system based on their needs and expertise. Recently, the new WG D1.83 “Optical Spectral Sensing Technologies for Partial Discharge (PD) and Arc Detection in Gas-Insulated Equipment” was launched. This group aims to provide a comprehensive overview of the application potential of optical spectral sensing technologies for detecting partial discharge (PD) and/or arc faults in gas-insulated equipment, including comparison to conventional technologies.
Natural-origin gases like carbon dioxide or synthetic air, as well as their mixtures with fluorinated compounds, are emerging technologies whose properties still require further evaluation. WG D1.78 “Partial discharge properties of non-SF6 insulating gases and gas mixtures” started in 2023 and focuses on reviewing and summarizing respective partial discharge (PD) fundamentals, measurement techniques, and existing data on PD properties of all major alternative gas mixtures, in comparison to SF6. This part will be published in a Technical Brochure in 2026. Furthermore, the group is currently preparing round robin tests, and respective pre-tests were already conducted. WG D1.78 intends to assess the applicability of current PD measurement techniques and defect detection in the new gases.
JWG B3/D1.63 is developing guidelines for the determination and classification of used SF6 gas toxicity, considering both onsite and lab methods with practical applications in mind.
New activities concerning the gas decomposition and gas tightness of SF6 alternatives are presently under review.
CIGRE active Working Groups / Call for experts
Tutorials, organised by TAG (I. Atanasova-Hoehlein)
Tutorials from two recent WGs have been presented several times in the last year. The convenor of TB 933, “Requirements and Application of UHF PD Monitoring Systems for gas insulated Systems” (W. Koltunowicz) has been active disseminating the important findings of WG D1.66 at the following international events:
- VDE Hochspannungstechnik, 11-13 November 2024, Berlin
- Kabel 2025 Conference, 18-21 March 2025, Tutorial Poland
- AWTr Transformer Conference, 13-15 October 2025), Baiona, Spain
- IEEE PES 2025 GTD Conference and Exposition, 26-29 November 2025, Bangkok, Thailand
Similarly, the work from WG D1.70, which delivered two Technical Brochures (TB 856 and TB 927), has been summarised and presented by Lars Lundgaard at two events under the title, “Dielectric performance of insulating liquids for transformers – a CIGRE study”:
- Technical Presentation for Electric Energy Society of Australia (EESA), 4th June 2025
- DEIS Webinar on 10th April 2025
Joint A2/D1 Colloquium in Seoul
At the end of October SC D1 joined forces with A2 (Power Transformers and Reactors) for a colloquium organised by the Korean National Committee (KNC). The week started with WG meetings and Advisory Group meetings (Figure 3); I used the Chairman’s privilege to sit in some of the meetings and listen to the great work which is taking place. Many of the meetings allowed guests from Korea to join and get a better understanding of what happens in a WG or AG: hopefully they will be inspired to join future WGs.
During the middle of the week, the Study Committee meetings took place. Each SC held its own meeting (Figure 3) to cover the topics specific to their remit before coming together for a half day joint meeting. Members from SC D1 community travel from as far away as Brazil, Canada, New Zealand and USA, as well as many representatives from Asia and Europe (Figure 4). This joint A2/D1 meeting (Figure 5) gave plenty of opportunity to discuss the topics of mutual interest such as ageing of oils and solid insulation in transformers. We were also privileged to be joined by Rannveig Loken, chair of CIGRE’s International Technical Council who gave a speech at the start of the joint SC session.
Figure 4 - Study Committee D1 meeting attendees
Figure 5 - Joint Study Committee with A2 and D1 meeting attendees
The afternoon after the joint SC meeting was filled with four excellent one-hour tutorials (two from each SC). Each deserved longer time to be able to spend the time needed to dive into the topics and allow plenty of time for questions. The tutorial topics were:
- Effect of DC Bias on Power Transformers
- Impulse Testing of Power Transformers
- Condition of Cellulose Insulation in Oil-Immersed Transformers after Factory Acceptance Test
- New Laboratory Methodologies for Investigating of Insulating Liquids (Figure 6)
That evening a welcome reception for the colloquium was hosted by the KNC, which gave everyone the chance to relax over some delicious Korean food (Figure 6).
Figure 6 - Ivanka Atanasova-Hoehlein (DE) presents a tutorial (left) and Jin-Ho Nam the Korean Regular Member for Korea gives a speech at the welcome reception (right)
After all the formal business of the SC meetings was concluded, the colloquium attracted 350 participants from 39 countries, which everyone agreed was a great success. A total of 88 high quality presentations were made by speakers from 21 countries. All the speakers in the oral sessions received many questions and the poster session was characterised by lively discussions at all the papers.
Following the conference there was a Technical Tour to KEPCO’s Research and Development Centre in Gochang. The site is huge and was originally built to support the development of 765 kV overhead transmission in Korea (Figure 7). The site now boasts facilities for cables, switchgear, superconductivity, distribution automation, as well as research to support the energy transition such as DC distribution, photovoltaics and battery technology. In the short time we were there, we only had time to see a fraction of the laboratories.
Figure 7 - Technical Tour to KEPCO’s Gochang Research and Development Centre
Looking Forward
The next year will be busy in the lead up to the Paris Session in August. Reviewing the synopses has already been completed; the quality of the received synopses was much higher than in 2024. SC D1 is looking forward to receiving and reviewing the full papers early next year.
The SC has several WGs coming to the end of their activities, so I’m expecting more Technical Brochures to be published in 2026 than 2025. There are also plans for new WGs and the Terms of Reference are in the early stages of development. Anyone with suggestions for work within the remit of D1 should get in contact with the Study Committee.