Cavity resonance effect in large HV laboratories equipped with electromagnetic shield

The electromagnetic screening of High Voltage laboratories against external interference is required in order to facilitate measurements inside the laboratory of partial discharges and radio interference voltages during dielectric testing. Such laboratories have been found to behave as high quality cavity resonators when excited from inside by the rapid discharges of impulse generators. Due to the large physical dimensions of high voltage halls, some resonant frequencies may be as low as a few megahertz. The oscillatory electromagnetic fields produced by the currents which circulate in the cavity induce stray signals in the impulse voltage measuring circuits. Such circuits usually have pass-band widths sufficiently large to reproduce these oscillations. This interference is then superimposed on the impulse oscillograms and, in certain circumstances may hamper the interpretation of the test voltage oscillograms. The interference can be suppressed by coating the inner side of the shield with a special resistive paint, which reduces the Q factor of the resonant cavity. Although such paint is not yet commercially available the method has been experimentally verified on a reduced scale model.