CHAPTER 3

POWER TRANSMISSION AND DISTRIBUTION CABLES & TRANSFORMERS

Robert S. Sokolowski

INTRODUCTION

The motivation for applying superconducting materials to a power transmission and distribution system is the promise of power delivery and conversion without the electric losses that result from I²R or Joule heating. The period of 25 years from 1961 to 1986 saw considerable activity in development of power transmission cables using metallic or low temperature superconductors (LTS). Had it not been for the energy crisis of the early 1970s and the subsequent decline in energy demand, today there might be superconducting power transmission cables in use throughout the world. Target power ratings per circuit for superconducting cable systems dropped from 5,000-10,000 MW in the 1970s to 1,000 MW by the early 1980s (Engelhardt, Von Dollen, and Samm 1992). Although economic considerations continue to dominate the criteria for deciding whether a superconducting solution to electric power problems is appropriate, other factors are becoming increasingly important in the minds of decisionmakers. These include growing public concern over environmental issues and safety and the uncertain effects of deregulation on the generation and distribution of electric power. The responses to many of these issues will be known only after lengthy debate and no doubt countless pages of legislation. While the actual need for superconducting cables and transformers will be determined by local market conditions, aided perhaps by varying legislative requirements, the technology of superconducting systems is being developed globally, and competitors in the United States, Europe, and Japan who are looking for a stake in an anticipated multibillion dollar business are making excellent progress. When leaders in the field of superconductivity convened in Japan in May 1996 for the Fifth International Superconductivity Industrial Summit, they agreed that the world market for electric power devices based just on superconductivity will exceed $10 billion by the year 2010.

In spite of worldwide efforts to develop superconducting cables and transformers using LTS materials, the expense of cryogenic cooling systems for liquid He operation at 4.2 K with the strict operational reliability demanded by electric utilities, and the difficulty of developing a suitable low loss ac superconductor, presented seemingly insurmountable barriers to their introduction into the network. The discovery since 1986 of high temperature superconducting (HTS) materials in oxide-based systems with increasingly high transition temperatures has rekindled an interest in superconductivity in everyone in the power delivery chain, from generator to consumer. The operating temperature of HTS materials of up to 77 K (liquid nitrogen temperature) is considerably higher than the 4.2 K (liquid helium temperature) on which design of the LTS power systems of the 1970s and early 1980s was based. With higher temperatures come not only reduced refrigeration costs but also enhanced reliability.