Electrical insulation systems at cryogenic temperatures

Introduction

From the above technical background, a new WG D1.64 was established in 2016 to focus on the electrical insulation systems at cryogenic temperatures as one of the common and critical issues described in the above TB No. 644. Electrical insulation systems at cryogenic temperatures have been recognized as one of the key technologies for the efficient, reliable and practical development of superconducting power apparatus. Besides the superconducting power apparatus, the electrical insulation systems at cryogenic temperatures will also be helpful for fusion reactors, accelerators and industry applications.

Scope/Methodology

The scope of CIGRE WG D1.64 was to study the fundamentals and applications on electrical insulation techniques for superconducting power apparatus and other applications to be operated at cryogenic temperatures, i.e.

1. Summary of discharges in solids, liquids, gases, vacuum and their composite insulation systems at cryogenic temperatures,
2. Principles and mechanisms of electrical insulation phenomena at cryogenic temperatures,
3. Major design and test issues for electrical insulation systems of superconducting power apparatus, superconducting magnets and their components to be operated at cryogenic temperatures.

Not only the dielectric properties and physical mechanisms of insulation materials at cryogenic temperatures, but also thermomechanical properties were discussed. The state-of-the-art R&D projects on superconducting power apparatus and their design, test and failure experiences were also introduced.

Description of the TB

The TB is divided into 2 categories; fundamentals (Chapters 2, 3 and 4) and applications (Chapters 5 and 6).

Chapter 2 deals with dielectric properties of insulation materials at cryogenic temperatures. Dielectric properties and data of insulation materials (gases, liquids, solids and new insulation materials) at cryogenic temperatures are summarized. Cryogenic gases still follow Paschen’s law, if it is related to the respective density of the gas, instead of pressure. A new approach for universal Paschen diagrams for typical cryogenic gases is presented. For certain applications, mixtures of gases are more suitable than their pure counterparts. Liquid nitrogen is the most studied and used liquid insulation material for HTS power applications. The dielectric strength of liquid nitrogen is greatly reduced by bubbles during operation, which move in the liquid influenced by buoyancy or electric field. The incompatibility of room temperature solid insulation materials and techniques for cryogenic temperatures is primarily due to the additional mechanical stresses and microstructural changes present at cryogenic temperatures. Most of new solid materials are polymeric nanocomposites. Significant progress has been made in preparing and improving the electrical properties of conventional polymeric materials with nanometre size particle fillers for cryogenic applications. Other composites can also improve dielectric properties, such as the combination of Tyvek with polyethylene and functionally graded materials.

Chapter 3 focuses on discharge characteristics and mechanisms. Discharge processes of the insulation medium, which have been identified and influence the breakdown and partial discharge characteristics, are discussed in dependence on the non-uniformity of the electric field, which is defined with the utilization factor : uniform ( > 0.9), quasi uniform (0.9 ≥  > 0.6), weakly non-uniform (0.6 ≥  > 0.3) and strongly non-uniform ( ≤ 0.3). The chapter starts with describing how the gas breakdown behaviour changes from room temperature to cryogenic temperature and how the cryogenic gases are influenced. Although the discharge theory for liquids is less well established, the state-of-the-art discharge mechanisms of liquid nitrogen are presented in uniform, quasi uniform and non-uniform electric fields, respectively. The discharge processes in solid insulation materials are discussed in terms of the effects of structure, temperature, applied voltage, magnetic field, irradiation and interfaces of different materials. Aging mechanisms usually for about 30 years under electrical stress and cryogenic conditions are addressed with the statistical and lifetime calculations.

Chapter 4 addresses with mechanical properties and fatigue under thermal stress. Mechanisms of thermally-induced mechanical breakdown are described in this chapter. Exact control of the thermomechanical behaviour during cool-down/warm-up cycles is essential for a normal operation of superconducting systems. Quite often, the design of the insulation system for superconducting equipment is a compromise between appropriate dielectric and thermomechanical performance determined by thermal conductivity, thermal expansion and mechanical properties at cryogenic temperature. Thermal and mechanical properties of cryogenic materials are summarized with the fatigue under thermal stress. Practical examples are also introduced, e.g. huge heat conductivity of sapphire in fault current limiters (SFCL) technology, adjusting thermal expansion of epoxy resin for redundant superconducting coils and for thermal cycling stability, and the effect of polyimide insulation on improvement of heat transfer.

Chapter 5 discusses experience in design of insulation systems and devices. From the application viewpoint, this chapter starts with the insulation of superconducting wire and tapes, and continues with superconducting devices and systems. Polyimide is used in a wide variety of cryogenic applications and in particular for insulating HTS wires and tapes. High performance polyimide may offer superior electrical and mechanical performance, easy application, and flexibility for different voltage withstand needs. Enamel made out with polyvinyl acetate, polyetherimide or polyurethane varnishes is used for coil windings of electrical motors, transformers, MRI/NMR magnets, particle colliders or high energy physics equipment.

Design experience of superconducting devices and systems has been accumulated primarily for cables and SFCL. For superconducting cables with electrical insulation layers composed of liquid nitrogen and polypropylene laminated paper (PPLP) there are lots of design experiences for both AC and DC. Recently, gas cooled superconducting cables are investigated, where a 4 mol% GH2 and 96 mol% GHe mixture possessed about 80 % higher AC and DC breakdown strength compared to pure GHe at various pressure levels at 77 K. Shrinkage at long cable length is also described in this chapter.

Among several types of SFCLs, resistive SFCLs are the more demanding in terms of insulation design. When a fault current flows in a resistive SFCL, a quench of HTS materials may occur and generate bubbles in liquid nitrogen, hence so-called dynamic breakdowns can be induced by the bubbles under the operating voltage of SFCL. The assumption of a continuous thermal stress could be too conservative. In order to rationalize the dielectric insulation design for SFCLs, the study of dynamic breakdown is therefore important.

Design experience of other superconducting devices such as transformers, rotating machines and magnets is summarized with the state-of-the-art worldwide projects.

Chapter 6 describes dielectric testing of cryogenic insulation systems. Testing methods of superconducting devices and systems are introduced mainly for cables and SFCLs. For superconducting cables, shipping tests and performance tests on site are described. The dielectric withstand of resistive SFCL has to be verified both in steady-state condition and in the dynamic regime, i.e. during current limitation. Dielectric test experience in steady-state condition under AC and DC SFCL is described. Validating the dielectric withstand in the dynamic regime require the power tests in short-circuit conditions. During these power tests, the internal insulation will be verified at the same time as the current limiting performance and the general withstand of the device, which has not yet been authorized in the testing methods of SFCLs. Test experience of transformers, DC reactors, fusion magnets are also described.

In many cases, not limited to low-temperature insulation technology, reports on design development, experimental research, or operation of a device or equipment indicate successful results achieved according to the initial design or schedule. In contrast, few reports describe unexpected trouble. When trouble occurs, it is often difficult to investigate and to identify the cause. However, for subsequent R&D, it is important to examine failure experiences. Investigations into failure experiences associated with cryogenic insulation are collected in this chapter.

Conclusions

The data, knowledge and experiences on electrical insulation at cryogenic temperatures for applied superconductivity are increasing with the number of researchers and papers in the recent international conferences and academic journals. The continuous and further effort in this community is highly expected toward the future sustainable society with environment-friendliness, efficiency and reliability of the electric power networks.