Partial Discharge Monitoring on Power Cables
The main task of the partial discharge monitoring system type ICMmonitor CA is to detect internal partial discharge in power cables caused by cracks in joints and terminations or degradation in the insulation system at an early stage. PD signals are picked up by a current transformer and preprocessed by an FCU3. A monitoring control rack (PDMCR) with ICMmonitor CA software installed receives the measured data from the monitoring units and processes it.
Due to the dielectric properties of cable materials and their attenuation, partial discharge activity in power cables is typically measured in a frequency band of several tenth of MHz. The frequency converter unit (FCU3) is a signal conditioning unit with a demodulating logarithmic transfer function. It picks up the HF signal from a current transformer (CT). The output of the FCU3 is the envelope of the HF signal down-converted into the frequency range of the instrument (< 1 MHz). The FCU3 is mainly used with monitoring applications and for gating purposes. This unit is remote powered by a DC phantom voltage provided by the ICMmonitor CA. A malfunction of the FCU3 will be detected automatically by the monitoring unit and will be indicated in the main panel of the software. An external synchronization signal is usually derived from a VT, which is installed in short distance to the PD measuring device. If such a short distance installation is not possible, due to space restrictions e. g., Power Diagnostix offers a special synchronization unit FSYNC1, which allows transmission of synchronization signals over long distances via fiber optic cables. The number of current transformers to be monitored depends on the type of cable and the individual specification of the customer. The signal of each CT is converted into lower frequencies by an FCU3. Up to six signal cables from current transformers are brought together into one ICMmonitor CA instrument.
All monitoring instruments are interconnected via a fiber optic LAN ring, providing n+1 communication redundancy. A monitoring control rack (PDMCR) receives the measured data and processes it. With this system architecture a nearly infinite number of current transformers can be continuously monitored and observed.
A special software reads the measured data of all stand-alone ICMmonitor CAs, each scanning up to six signal sources.