Exploitable results
Existing line protection shows deficiences in safety when operating in the presence of explosive methane and air mixtures and coal dust. Two alternative selective monitoring systems have been tested which monitor directly the three conductors of a power cable and which detect a cable fault in about 100 us. In existing monitoring methods the fault information is determined from the 50 Hz values of voltage and current. Both methods tested superimpose an additional system to the 50 Hz system, in one case a pulsed system with a monitor which operates in the time domain and in the other case a sinusoidal system with a higher frequency to reduce the time needed to detect faults. The operatability of the pulsed system has been proven by laboratory-examinations. All cable faults with a significant change of the characteristic impedance are detected. Because the system is independent of operation voltage and frequency, the disconnected line can also be monitored and, in principle, the operation of a frequency converter is possible. But in this case the prototype shows need for improvement, as the harmonic waves generated by the frequency converter cause fault detections. With the medium frequency method, basically both the increase of the fault current and the change of the phase angle between fault current and medium-frequency voltage can be analysed in the auxiliary circuit. Because of the high capacitive load of the cables, the maximum fault resistance is limited to some 10 Ohms if the fault current value is analysed and to some 100 Ohms if the phase angle is analysed. No limitation of the maximum fault resistance occurs if the fault voltage rather than the fault current is analysed and the medium-frequency auxiliary source shows the characteristic of a current source. Using this method some more components must be considered to decouple the fault voltage. The fault voltage value depends on the connected load. Both methods are basically suitable to monitor lines.
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