Medium voltage three-level inverter for high speed applications

Key component of modern gas compressor systems in the megawatt range is the high speed induction motor fed by a medium voltage inverter. The high speed motor can be directly coupled with the compressor without needing a gearbox. This makes the compressor system more reliable and efficient. For the design of this drive system it is mandatory to follow the requirements of the high speed motor regarding harmonic distortions. The paper outlines the impact of harmonic distortions on the high speed motor. Different inverter topologies are discussed. The three-level NPC inverter with a reasonable output filter can be optimized to meet the motor requirements while also considering the constraints of the inverter. Different filter topologies combined with optimized pulse patterns are described and the harmonic distortions of the motor and the losses of the inverter are determined. The optimization shows that in many cases just an output choke in series to the motor is sufficient as a filter to meet the requirements, if the pulse pattern is optimized accordingly. Applying press pack IGBTs the three-level inverter is well fitting for the higher power range of gas compressor systems.