Damping torsional interharmonic effects of large drives

Current source thyristor converters are the most widespread technology for large drives and today still a suitable choice for supplying high power variable speed drives for pumps and compressors because of excellent reliability records. The integer and non-integer harmonics generated by line commutated converters cause pulsating torque harmonics on the motor and on the grid side of the converter. Intersections of harmonic excitation frequencies with torsional natural frequencies of mechanical drive trains (motor driven load or generator train) cannot always be avoided in the operating- speed range of the motor. Continuously generated harmonic torque excitations could have a critical impact on the torsional behavior of the entire train. It is therefore mandatory to perform torsional analyses during the design stage of large drives, as specified in API 617. In multi-unit plants, more sophisticated analyses have to be performed, considering the fully coupled electrical and mechanical system. The risk for operational problems increases with increasing percentage of converter loads (converter power relative to the short circuit level of the grid). Some oil and gas production sites, e.g. offshore platforms, are based on island or island-like power systems. To mitigate the risk of torsional issues with increasing percentage of converter loads in weak power systems, new devices for damping torsional resonance modes have been developed and successfully tested. The effect of torsional mode damping will be explained by coupled electro-mechanical simulations and by measurement results from applying an Integrated Torsional Mode Damper (ITMD) to a 30 MW gas compression train. Torsional Mode Damping (TMD) does not require changes to the mechanical or electrical system design and can be designed as a retrofit system extension for variable speed drives from different manufacturers.