Improvements in RF Monitoring System on Generators

This paper presents new developments for an improved RF monitoring system on generators. A capacitive coupling has been shown to be much more sensitive than the currently used magnetic coupling through a current transformer (CT). The increase in sensitivity resulted in an improved coupling between the generator´s neutral grounding lead and the monitor. An R-C high-pass filter was shown to be an effective coupler. This is due to its inherent nature that it will not attenuate the RF noise or create saturation problems as in the case of magnetic coupling through the CT. By selecting the R-C filter with higher value of capacitance than the stray capacitances created by the typical generator grounding transformer, the coupler can detect arcing signals in any generator, regardless of the type of neutral grounding transformer used. Even though there is an ohmic contact to the neutral lead, the protections to the added RF instrumentation, assure that the protective relaying scheme will not be degraded. Low frequency interference (typical in a power plant) can be easily picked up by the magnetic coupler (CT). There is no proper shielding that can be provided for this problem. This interference may have magnetizing effects in the CT core material. Saturation produces non-linear transforming action resulting in a decrease of the CT output level and modulation of the RF output by the power line frequency. The proposed electrostatic coupling to the generator neutral lead filters out such a low frequency pick-up.