Torsional Mode Damping for Electrically Driven Gas Compression Trains in Extended Variable Speed Operation

The oil and gas industry has a growing demand for electrically driven trains operated at variable speeds. Variable frequency electrical drives enable increased operational flexibility and energy efficiency. This is of great importance in applications requiring high power, such as gas compression. Load commutated inverters (LCIs) are one of the most widespread technologies for driving large gas compression trains because of excellent reliability records. One drawback of power electronics driven systems is the generation of nonfundamental air-gap torque ripple components due to electrical harmonics. The air-gap torque ripple can interact with the mechanical system at torsional natural frequencies of the drive train. Torsional vibration is an oscillatory angular motion that causes alternating twisting in shaft sections and machinery couplings. A consequence of uncontrolled excited torsional vibration may be a protective trip of the motor, to prevent mechanical damage, such as a failed coupling or a broken shaft. This paper discusses illustrative design details of applying a torsional mode damping control system to LCI driven multi-Megawatt centrifugal gas compressors. The coincidence of electrical drive harmonics and torsional natural frequencies of the mechanical system is sometimes unavoidable due to the large variable speed range of the compressor such as for process requirements. For these types of applications, a power electronic damping system technology can be applied to new units or as a retrofit solution to existing variable speed trains. The so-called integrated torsional mode damping (ITMD) unit is based on a torsional vibration measurement in the mechanical system and an interface to the existing inverter control of the drive system. The dc-link inductor of the LCI is partially used as an integrated energy storage unit and is combined with a smart damping controller, which reacts to a torsional vibration by modulating a small amount of the stored energy and sending it to the motor without impacting the normal operation of the system. As a result, the active power modulation at a torsional natural frequency of the mechanical system has a strong damping effect for torsional vibrations. Intensive simulations and several tests were performed on large LCIs (up to 50 MW) over the last three years. Selected experimental results will be presented and discussed to validate the suggested