Development of an exponential tapered impedance transformer for UHF-PD sensor

Among the methods used in partial discharge (PD) detection in GIS, ultra-high-frequency (UHF) method has gained broad acceptance and applications as an effective and efficient way to monitor PD, the key part of which is the sensor. Now the ultra wide band antenna has been usually used as the sensor for the broadband detection ranging from 300MHz to 3GHz. However, the impedance of the coaxial-cable, widely used in practical application, relatively differs from the broadband antennas, and that can result a bad impedance matching. To match the balance structure and the wideband character of the antenna, a kind of microstrip impedance transformer which is exponentially gradual type was designed in this paper, the parameters of which were defined through simulation. Optimization of the designed transformer has been done to make an easy package of the antenna and the transformer. The simulation results show that impedance matching in the ultra broadband range was achieved by the designed impedance transformer. The return loss was reduced and the sensitivity of the sensor increased simultaneously, thus the requirements of the on-line monitoring for the PD in GIS were better met.