Resonances and overvoltages in a medium voltage fan motor drive with long cables in an underground mine

Underground mining is characterized by space constraints forcing the use of long cables to feed large machines, where the application of variable speed drives can generate overvoltages at the machine terminals. This paper presents the analysis and mitigation of harmful overvoltages; detected in inverter-fed 1400 HP 13.8 KV motors used for air ventilation in an underground mine. The possible causes of the over-voltages, namely wave reflection and resonance are described theoretically and simulated. On-site direct voltage measurements at the 13.8 kV motor terminals confirmed that resonance was the dominant phenomenon. Based on this study a suitable low-pass filter was designed, constructed and installed. Finally, the adequate operation of such filter is demonstrated by further on-site measurements.