On-line PD monitoring system for rotating machines using narrow band detection of EM wave in GHz range

On-line PD monitoring system with higher sensitivity and low noise features was developed for rotating machines such as turbine generator and high voltage motor. By using narrow band detection in GHz region, the features were achieved. The system is composed of an antenna, a specifically designed signal processing circuit and data acquisition equipment. The antenna is so-called patch antenna and its dimension is approximately 120x120mm and is designed for detecting a specific narrow band frequency in GHz range. This type of antenna is very easy to install because the thin plate structure does not interfere other high voltage parts inside the rotating machine if installed on the inner wall of the machine housing. The antenna is connected to the amplifier adjusted for the frequency band of the antenna resulting in superior noise suppression. The sensitivity of the system was measured and confirmed to have a good linear correlation with conventional off-tine Qmax value from several hundreds pico Coulomb. This sensitivity is enough for on-line insulation monitoring for rotating machines because the low level PD activity during operation does not infer abnormality of the machines.