High Voltage Insulation Diagnostics Utilizing HTS-SQUID Based Amplifiers
Doctoral thesis, 2001
In the design and development of high voltage insulation systems, different diagnostic methods are often used for system characterization. It is therefore crucial to continuously improve existing methods and to develop new ones. An interesting possibility is to use one of the most sensitive sensors available, the Superconducting Quantum Interference Device, SQUID. The scope of this thesis is the design and evaluation of SQUID based amplifiers in a number of insulation diagnostic systems. Two amplifier concepts, a voltage and a current amplifier, are presented. The amplifiers are implemented into three different applications, as a voltage amplifier for Rogowski type sensors and as a current amplifier for partial discharge and dielectric response measurements. Since the discharge phenomenon is such a rapid process, we find that the slew-rate of the voltage amplifier is insufficient for transient applications, while it shows very good performance for harmonic signals. The current amplifier, on the other hand, show good applicability to rapid signals if the correct input circuitry is selected. Two main input solutions are evaluated; one is optimized by maintaining a flat frequency response and the other is optimized for sensitivity. Performance data for the different systems are presented and compared to conventional technology. The current amplifier is a crucial part in dielectric response measurements. The SQUID based amplifier offers a number of features, which can be advantageous in such measurements. A three input coil solution is suggested for maximum noise immunity. Measurements show that the SQUID system competes well with conventional technology even though the current noise should be decreased further in order to fully benefit from the advantages that it offers. As a general conclusion, the SQUID amplifiers compete well with conventional technology, especially when considering the limited time they have been used. In some areas the SQUID system out performs conventional technology, but semi-conducting amplifiers are still the preferred choice for most applications.