Influence of spacer aperture size on PD-induced electromagnetic wave measured with UHF external sensor in 66 kV GIS model

Partial discharge detection in ultra high frequency band (300 MHz to several GHz) is one of the effective methods for diagnosing the insulation condition of a gas insulated switchgear. This method which detect electromagnetic wave with high S/N ratio has been rapidly developed in recent years. We need to install an internal sensor or an external sensor on GIS tank in order to detect partial discharge (PD) signals. External sensor is used to detect leaked EM wave signals from insulation spacer of GIS tank. In this paper, we conduct experiment in 66 kV GIS model with metal flange type (MT) spacer and leaked EM wave will be investigated using spiral type antenna. External sensor without any filter and amplifier will be used for our experimental work. We arrange the size of metallic aperture for external sensor from small aperture size of 25 × 15 mm to the maximum 65 × 15 mm. We took data using an oscilloscope and a spectrum analyzer. The experimental result suggested that EM wave magnitude emitted to the aperture window sensor increased from the aperture size of 30 × 15 mm to the maximum (means without the aperture). It is concluded from the frequency response analysis that the dominant frequency of small aperture tends to be higher than the one of larger aperture. The highest dominant frequency is achieved with the smallest aperture 25 × 15 mm.