Superconducting Fault Current Limiter

The following sections are included:

• Foreword
• Preface
• Contents

In this chapter, we will address the structure of the electrical grid. An emphasis will be stressed on the effects of failure on the stability and security of the grid. The recourse to superconducting fault current limiter (SFCL) could be interesting to help the management of the network in case of failure. We will also address new trends in the electrical industry that would be an opportunity to use SFCL.

This chapter aims to give the basic properties of superconducting materials used for fault current limiter (FCL): electric field versus current, critical current, basic model of the superconductor, normal zone propagation velocity, etc. The AC losses are addressed in Chapter 3.

In spite of having zero electrical resistance, superconductors exhibit dissipation in AC transport. We show that the physical origin of this phenomenon is intimately linked with the ability to carry large electrical currents, and critically depends on the way in which the magnetic field interacts with the superconducting material. Therefore, the AC loss is not a property of material itself, but can be significantly influenced by the configuration of conductors in a superconducting device. From the variety of arrangements in which the AC loss has been investigated, those that are most relevant for the design of superconducting fault current limiters (SFCLS) are discussed: AC transport by a single wire and by a bifilarly wound coil. Basic engineering formulas are presented allowing the reader not familiar with the issue to obtain the first guess for the expected AC loss level.

This chapter aims to present the superconducting materials used for fault current limiter (FCL) with their implementation. First the Bismuth compounds are discussed and then the coated conductors.

Many different types of conventional fault current limiters (FCLs) and superconducting fault current limiters (SFCLs) have been investigated in the past and several prototypes and demonstrators have been built and successfully tested, see Refs. 1 and 2. This chapter concentrates on the most basic types of SFCLs only and explains the general setup, the work principle and the equivalent electrical circuit of these basic SFCL types.

This chapter aims to give a preliminary design of the SC element of a resistive FCL: how to calculate its approximate section and length taking into account the specifications and the inescapable inhomogeneity of the SC conductor. The design will also be discussed in terms of economical considerations. The implementation of the SC element will be introduced. At least some electric circuits of the resistive FCL will also be presented.

This chapter gives an overview of the requirements and approaches for modeling resistive superconducting fault current limiters in power system transient simulators. Such simulators are routinely used by power system engineers to predict the network behavior under various types of disturbances and find ways to keep the network stable and reliable in all circumstances. However, these simulations are useful only if the device models are sufficiently fast and accurate, which is not yet the case for rSFCL models. The chapter starts with general considerations about power system device models, then the discussion rapidly focuses on rSFCLs. An original classification of the rSFCL models is proposed and then used to point out the differences between the few existing models found in the literature. The physical relevance of each model is discussed all along the chapter. Detailed equations are derived for each model, which provides interested readers with a good basis to start implementing their own models. Finally, after a brief overview of the numerical methods that can be used to discretize and solve the various models, future perspectives to implement fast and accurate rSFCL models are discussed.

Electrical insulation in cryogenic environment is one of the most critical issues for the practical and rational development of superconducting fault current limiter (SFCL). Electrical breakdown of SFCL in liquid nitrogen loses the safe and reliable operation of electric power transmission system with SFCL. This chapter reviews the fundamental and peculiar characteristics on electrical insulation of SFCL.

This chapter aims to demystify cryogenics for electrical engineers. Cryogenics appears exotic for electrical engineers, but it is a fully mastered industrial and reliable technique. Its implementation in space is now conventional with cryocooler lifetime of 10 years, for example. Different aspects from cryostat to cryocoolers, including safety concerns, are presented here.

Superconducting fault current limiters (SFCLs) are now commercially available systems at least at medium voltages and even high voltages (115 kV). They must be considered by utility engineers as a component, a tool to design existing and future grids. This chapter aims to give a few possible SFCL locations with their advantages and disadvantages including some economical aspects, and discusses how the FCLs alter the way electrical grids are designed and planned, though not very comprehensively.

The following sections are included:

• Introduction
• General Requirements for Current Limiter Components
• MCP BSCCO 2212 Components
• 2G Components
• HV Insulation
• Assembling and Current Leads
• Cooling Concepts and Cryogenic Management
• Current Limiter Projects
• Conclusions
• References

Superconducting fault current limiter (SFCL) devices have been an important issue within the Ricerca sul Sistema Energetico (RSE) S.p.A. portfolio. Since 2009, in fact, RSE has been involved in the design of resistive-type SFCL for MV applications to be installed in the A2A Reti Elettriche (A2A) S.p.A distribution grid in the Milano area. The project, funded in the framework of a R&D national project, started focusing the attention on a preliminary single-phase device, which was tested in the RSE and CESI labs, respectively, against dielectric discharges and short-circuit current. The first success paved the way for the finalization of the missing two phases and the result of this research activity is a three-phase resistive-type 9 kV/3.4 MVA SFCL device, based on first generation (1G) BSCCO tapes and installed in the S. Dionigi substation, belonging to A2A, inside the Milan MV distribution grid. The in-filed activity lasted for more than two years, demonstrating the SFCL capability to cope with the everyday operation conditions of the grid, moreover, at the end of the experimentation, the device was able to perform a true limitation during a three-phase fault, thereby officially becoming one of the first SFCL devices in the world (the first in Italy) installed in a real grid and to have limited a real short-circuit current. Further to this first success, the design of an upgraded device to be installed in the same substation was already launched: the new device will be devoted to the protection of four different feeders, thereby implying an enhancement in rated power up to 15.6 MVA.

The following sections are included:

• Introduction
• SFCL Design Principle
• Realizations
• Conclusions and Perspectives
• References

In Korea, a 22.9 kV SFCL was developed and demonstrated in Icheon Substation. The operation had been very stable. In parallel, a 154 kV single-phase SFCL was developed and has been under field tests.

Superconducting fault current limiter (SFCL) for Moscow 220 kV grid is designed as the highest voltage rating superconducting fault current limiter in the world. Production of such device generates massive challenges and requires extensive research to overcome them. This chapter covers aspects of high-voltage insulation, superconductor qualification and processing, low-temperature cooling that impact the overall design of the fault current limiter.

The slow normal zone propagation velocity (NZPV) in commercial second generation high temperature superconductor coated conductors (2G HTS CCs) upon partial quenching is problematic for resistive superconducting fault current limiters (rSFCLs) since it reduces the performance of rSFCLs and may lead to the development of destructive hot spots. A promising approach to increase the NZPV is the current flow diverter (CFD) concept. The CFD concept can be implemented by slightly modifying the architecture of 2G HTS CCs, either by inserting a highly resistive layer at the HTS–silver interface, or by putting most of the stabilizer on the substrate side. Recent experiments demonstrated that the CFD concept can increase the NZPV of 2G HTS CCs by at least one order of magnitude at 77 K.

We review briefly some of the properties of the superconducting fault current limiter (SFCL) elements prepared on EFG sapphire ribbons. It turns out that high quality YBCO layers can be grown without polishing these ribbons, with a single and thin YSZ buffer layer. SFCL elements comprising a thin Au shunt about 100 nm thick have been tested. The voltage restored upon switching approaches 1 kV/m. The maximum power that can be dissipated without burnout at the time of switching is of the order of 1000 W/cm², 1–2 orders of magnitude larger than that for conventional coated conductors. This allows a large reduction in the amount of conductor necessary for a given device, thus reducing its footprint and eventually its cost.

The following sections are included:

• Conclusion
• Index