Safety Considerations in the Design, Fabrication, Testing, and Operation of the DC Bias Coil of a Saturated Iron-Core Superconducting Fault Current Limiter

The function of the superconducting bias coil in a saturated iron-core fault current limiter is to magnetize the iron cores. For practical utility devices of HV power grids, the rated capacity is usually 100 MVA or higher. The size and weight of the iron cores of a saturated iron-core fault current limiter with such capacity is quite large, so the bias coil needs to have high magnetization capacity and consequently a large diameter. The stability and safety of the bias coil are of critical importance for the reliability required by power utilities. Experience taught us that great care must be taken in the design, fabrication, testing, and grid operation to avoid potentially fatal risks. In this paper, we will discuss and analyze the possible safety hazards for a large size superconducting bias coil, which may result in mechanical failure and electrical impact, causing significant damage to the coil. Possible causes of the hazards are improper configuration, mismatch of component materials during thermal cycling, manufacturing failures in fabrication, lack of protection in testing and operation, etc. We will also report measures we took to eliminate or reduce these safety hazards.

I. Introduction

With size of power grids growing constantly, the problem of ever-increasing fault current is becoming more critical to large size power system [1]–[4]. Former solutions, such as improving switch device rated current, using high-impedance transformers or series reactors, closed path operation, etc., cannot solve the problem successfully because of insurmountable negative effects to the power grid, including high transmission loss, harmonic problems, lowered grid reliability, etc. Saturated iron-core superconducting fault current limiter (SISFCL), which has a development future, has many superior attributes, such as low steady-state loss, immediate current limiting, fast recovery, and having no adverse effect on power quality and reliability [5]–[8]. It has gained increasing attention.