Partial discharge (PD) is a small electrical avalanche caused by locally disrupted electric fields in dielectric materials, and is known to be one of the major factors which accelerate the degradation of electrical insulation. This thesis deals with a relatively new and challenging application of conducting on-line high frequency PD measurements for the monitoring of falling trees on covered-conductor (CC) overhead distribution lines.
A measuring test set-up was arranged in the high voltage laboratory for real-time analysis. A pine tree was leaned against a 20 kV energized conductor and PDs were measured at different locations on the CC line using a Rogowski coil. The time domain reflectometry (TDR) measurement technique is presented to extract the frequency-dependent wave propagation characteristics (attenuation, phase constant, and propagation velocity) of CC overhead distribution lines. The theoretical modeling of the CC line based on its geometry is presented using two-wire transmission line theory and its frequency-dependent line characteristics are derived. The theoretical model is verified experimentally using TDR measurements taken on a certain length of the line.
The entire single-phase on-line PD monitoring system including CC line and Rogowski coil is simulated in the electromagnetic transient program-alternative transient program (EMTP-ATP) simulation environment for detecting falling trees on CC overhead distribution lines. The model is confirmed by the measurement results taken in the laboratory. The model can be used to estimate the length of the CC line at which the PDs due to falling trees can be detected, thus deciding the number and positioning of the sensors over a particular length of the CC line. Moreover, the challenges in on-line condition monitoring of falling trees on CC lines using wireless sensors are also discussed. The wavelet transform technique is applied as a powerful tool to de-noise on-line PD signals, which are completely buried by electromagnetic interference.
Automatic detection of falling trees will reduce visual inspection work after storms and it will improve the reliability and safety of the distribution system. The system can be planned to be integrated into the distribution automation system to reduce the overall costs of CC lines
