Ground fault detection in NSTX coil power system

Abstract

The National Spherical Torus Experiment (NSTX) has been designed and installed in the existing facilities at Princeton Plasma Physic Laboratory (PPPL). Most of the hardware, plant facilities, auxiliary sub-systems, and power systems originally used for the Tokamak Fusion Test Reactor (TFTR) have been used with suitable modifications to reflect NSTX needs. The NSTX power system has eleven (11) circuits. Each circuit is a floating DC system. Ground detection and protection device is provided for each circuit. The TFTR ground detection scheme had to be modified for the new application. The coaxial helicity injection system (CHI) requires special scheme to detect grounds in the main power loop since the circuit can be back fed from during neutral beam injection. For CHI, the vacuum vessel is divided into a) inner vessel and b) outer vessel, insulated from one another. Special ground protection devices are being designed for the CHI. An additional system to detect grounds and generate alarms while performing maintenance on the vacuum vessel has also been installed. This was necessary to maintain the insulation integrity of the vessel. Also techniques used to trouble shoot vessel grounds are outlined. This paper describes the changes and addition to the electrical power circuit ground detection and monitoring for the NSTX magnet systems and CHI electrodes.