A 300-kV Compressed Gas Standard Capacitor with Negligible Voltage Dependence

A compressed gas capacitor now in use at the National Bureau of Standards (NBS) changes capacitance by less than 2 parts per million from 0-300 kV. This extremely small variation results primarily from rugged construction, and thus requires only reasonably careful centering of its three cylindrical electrodes. It has a hollow cantilevered high-voltage electrode suspended between an outer 100-pF low-voltage electrode and a segmented inner low-voltage electrode having values of 1, 2, 4, 8, and 10 pF, or any parallel combination thereof. The capacitor's small voltage dependence was inferred from the following independent measurements: 1) comparison with another high-voltage capacitor having a small experimentally determined coefficient; 2) comparison of the inner and outer sections with each other; and 3) measurement of capacitance change resulting from application of known forces on the cantilevered electrodes, and correlating this with calculated high-voltage electrostatic forces. The capacitor is used in determining the voltage coefficients of NBS-owned commercial standard capacitors; in the calibration of customers' standard capacitors, voltage transformers, and dividers to 200 kV and above; and in research on coupling capacitor voltage transformers and other high-voltage devices and materials.