Condition Monitoring of Subsea Connectors - Partial Discharge Detection Using a High Frequency Sensor

This thesis discusses possible causes of failure and potential condition indicators that may be used to monitor the condition of the insulation system of a subsea connector. The main focus is to describe how high frequency capacitive couplers can be used to detect the presence of partial discharges in subsea connectors. Fac- tors relevant to the sensitivity and implementation of high frequency capacitive couplers are discussed and an approximate model of the coupler is created. The experimental work were conducted at the high voltage laboratory at NTNU/Sintef Energy Research where capacitive couplers were constructed using copper foil before being mounted on the outer semi-conductive screen of a 12 kV medium voltage cable. The functionality and sensitivity of the couplers were investigated with both pulses from a calibrator and real partial discharges. The signal received from the capacitive couplers were analyzed using a spectrum ana- lyzer and the signal power were compared to the measured apparent charge from a conventional partial discharge measuring setup. The results show that by connecting a simple high frequency capacitive coupler, mounted on the outside of the outer semi-conductive layer to a spectrum analyzer, partial discharges can be detected. The experiments suggest that calibration sig- nals down to a magnitude equivalent of an apparent discharge of 2 pC can be detected, depending on the location of the discharge. The frequency spectrum of the partial discharge signal is shown to be dependent on the type of discharge and the measured signal magnitude increases with increasing partial discharge activity. Based on the results obtained it was concluded that high frequency capacitive couplers is a promising way of detecting and monitoring partial dis- charges for the purpose of on-line condition monitoring of high voltage subsea connectors