Electrical Insulation Systems at Cryogenic Temperatures

Abstract

High voltage superconducting power apparatus and systems have been developed in the world, e.g. a 275 kV high temperature superconducting (HTS) cable in Japan or a 220 kV HTS fault current limiter in Russia. CIGRE SC D1 has so far established WGs D1.15 and D1.38 for the state-of-the-art R&D on superconducting power apparatus and systems and published TBs No. 418 (June 2010) and No. 644 (December 2015), respectively. Especially in WG D1.38, HTS materials, electrical insulation, and cryogenics were focused as the common and critical factors for successful R&D and practical development of superconducting power apparatus and systems. 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
industrial applications. This TB of WG D1.64 discusses the fundamentals and applications of the electrical insulation systems at cryogenic temperatures. Not only the dielectric properties and physical mechanisms of insulation materials at cryogenic temperatures, but also thermomechanical properties are discussed in Chapters 2, 3, and 4. The state-of-the-art R&D projects on superconducting power apparatus and their design, test, and failure experiences are introduced in Chapters 5 and 6. The main results in each chapter can be summarized as follows:
Chapter 2: 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, which move in the liquid influenced by buoyancy or electric field during the device operation. 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 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: Discharge characteristics and mechanisms The chapter discusses the discharge processes of the insulation medium, which have been identified
and influence the breakdown and partial discharge characteristics and their dependence on electric field non-uniformity, which is defined with the utilization factor η: uniform (η > 0.9), quasi-uniform (0.9 ≥ η > 0.6), weakly non-uniform (0.6 ≥ η > 0.3) and strongly non-uniform (η ≤ 0.3). The chapter starts by 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. Ageing mechanisms usually for about 30 years under electrical stress and cryogenic conditions are addressed with the statistical and lifetime calculations.
Chapter 4: Mechanical properties and fatigue under thermal stress Mechanisms of a 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 limiter (SFCL) technology, adjusting thermal expansion of
epoxy resin for redundant superconducting coils and thermal cycling stability, and the effect of polyimide insulation on the improvement of heat transfer.
Chapter 5: 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 of 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 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 continuous thermal stress could be too conservative. 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 state-of-the-art worldwide projects.
Chapter 6: 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 conditions and in the dynamic regime, i.e. during current limitation. Dielectric test experience in steady-state conditions under AC and DC SFCL is described. Validating the dielectric withstand in the dynamic regime requires power tests under 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 is 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 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.