excitation system HIEE300900R1

These findings resulted in a protection solution that has been implemented in the drives to avoid the effect observed in the failure of ball mill 2. At the same time, a study was begun on the basic causes of the failure of the over-voltage arrester modules.

Specifically in a balanced three-phase system, the sum of the currents is at all times zero. Monitoring of the cycloconverter output currents was implemented in the PSR program. If the difference between the actual motor phase currents is higher than 5 % for longer than 60 ms, then this function trips the drive. Later, there was a detection of this stator current unbalance protection due to the failure of the over-voltage arrester modules.

It can be said that the protection fulfilled its function, as it eliminated the effects observed in the first failure of ball mill 2. In a time of 3 hours taken to replace the over-voltage arrester modules, the equipment was fit to operate. Later factory analysis of the over-voltage arrester modules thyristors showed that the cause of their failure was due to the variations in voltage. This resulted in checking the characteristics of the thyristors in the modules.

Thyristors that are more robust from a voltage point of view were specified, and up until now they have been successful as there have been no further module faults.