Test Bench and Frequency Response of a Magnetic Antenna used in GIS PD Measurements

Gas insulated substations (GIS) have increased in the electric power system in recent years and is expected to continue increasing in the following years. The GIS has remarkable advantages over air insulated substation in offshore windfarms where the installation space is limited. The outlying location of the GIS demands a robust monitoring system. It is well known from literature that partial discharge (PD) measurements are an accepted method for insulation diagnosis and in many cases a required part of the acceptance protocol for many high voltage (HV) assets. In a recent investigation, a new concept of PD measurements in GIS using a Very High Frequency (VHF) Sensor (so-called Magnetic Antenna) has been demonstrated. Unlike Ultra High Frequency (UHF) technology, the presented PD sensor has a sensitivity better than 5 pC and yield an estimation of the charge magnitude, thus providing information about the severity of the insulation degradation. In this paper, a test bench for the characterization of the magnetic antenna is proposed. The magnetic antenna and a traditional UHF antenna are tested in the frequency domain up to 1 GHz, and in the time domain using different wave-shapes. Additionally, using the test bench, it is demonstrated the homogeneity of the PD current propagating in the GIS in the transverse electromagnetic (TEM) mode. The charge is estimated using the double integral voltage method, showing low errors. The results from the research gives a preface for further investigation in the design of the magnetic antenna.