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High-power LCI grinding mill drive under faulty conditions

Mining plants present harsh operating conditions because locations at high altitude in the mountains, with large temperature variations and dusty environment, that can affect the integrity of the electronics producing loss of reliability with undesirable effects on safety and production. This paper presents a problem and corresponding solution related with a mining issue, where a 6.6 kV fed load-commutated-inverter (LCI) drive is employed for driving a 6500 Hp-grinding mill. In the last time, the drive has shown a variety of faulty conditions causing several downtimes and the concern of the staff. The drive has a control system with hybrid technology comprising analogue and digital components, which was installed in year 1991 with a history of few operational problems. Recent failures have shown a different and complex nature. Because the employed technology has not an intelligent diagnostics system integrated to the distributed control system (DCS) of the plant, it is a challenging task to understand the origin of the problems. A study was carried out in order to find the causes of the failures. A procedure based on a theoretical and experimental approach was developed in order to establish hypothesis for finding the roots of the problem. Among other factors, the study indicated that a lost of firing pulses of the machine converter was the most likely reason for a commutation failure. Simulated results and measurements confirmed that hypothesis. Therefore, overcurrents caused by failure in the pulses pattern of machine side converter were detected and a focalised search in specific printed boards allowed the finding of defects and remedial actions could be taken.