–1 MV DC Filter and High-Voltage DC Measurement System for ITER Neutral Beam Injector System

This article proposes the one-package structure of -1 MV direct current (dc) filter circuit and dc measurement system composed of dc voltage dividers and dc current transducers (DCCTs). These apparatuses are becoming more important for the expansion of high-voltage dc (HVDC) power grids. However, at present, they are separately installed keeping a huge distance for air insulation, and an increase in equipment size and footprint has become a major issue. We succeeded in reducing the footprint to 1/100 by storing the aforementioned HVDC vital parts in one package of a pressure vessel. Our developed configuration uses sulfur hexafluoride (SF6) gas not only for insulation purpose but also as a refrigerant to forcibly ventilate vertically in order to allow the dc filter circuit, which is heat generation source, and the dc measuring instruments, which need to minimize the effect of temperature drift, to coexist. We also addressed another issue caused by the limited space in the one-package structure, where the stray capacitance of the dc voltage divider tends to become large and then lowers the measurement accuracy for transient voltage variation. As a countermeasure, the dc voltage divider is composed of much larger capacitance than the stray capacitance and additional damping resistance to avoid LC resonance with wiring inductance. The actual-product tests demonstrated the followings; the dc measurement system has satisfied high measurement accuracy of 0.5% and fast responsivity of 3.3 μs retaining decay rate within -3 dB; the temperature rise of the dc voltage dividers and the DCCTs is reasonably suppressed; the dc measurement system and the dc filter circuit withstood DC -1.2 MV.