A converter topology for high speed motor drive applications

In the area of natural gas industry, there is a large interest in converters for driving high speed, medium voltage induction motors. High-speed drive systems are capable of directly driving turbo-compressors at speeds of up to 20,000 rpm / 8 MW. In high speed motor drive applications, the motor fundamental frequency can vary from 100 Hz to 300 Hz, which requires a high switching frequency. However, switching losses increase with switching frequency and these losses represent a critical point for high voltage semiconductors. Soft switching topology could represent a relevant solution to work at high switching frequency. The drawback of such a converter is the number of added components and the complexity of the control. Multilevel inverters provide output signals with low harmonic distortion. When the number of level increases, the output voltage THD decreases but the size and the cost of the system increases as well. The switching voltage is reduced when the number of level increases, depending on the topologies, or when a series connection of components is made. This paper reports a 3-level converter structure for high speed motor drive applications that allows decreasing switching losses in comparison with the well known 3-level NPC (neutral point clamped) inverter. This feature leads to the possibility to increase the switching frequency or the load current level thanks to series connection of IGBTs. As the neutral point is actively piloted, this structure is named NPP (neutral point piloted).