Multi-level converters for large capacity motor drive

The two-level PWM inverter has expanded in industrial applications because it was able to achieve maintainability and high performance to surpass the DC drive by the development of the vector control and the IGBT (Insulated Gate Bipolar Transistor). However, as the main motor drive of the metal plant rolling mill, the two-level inverter output voltage was too low and capacity was too small, so the DC drive was used for a long time as the main drive. The 3-level NPC circuit (Neutral Point Clamped) was introduced as a new circuit which can output two times higher voltage and the two times cleaner waveform than the 2-level circuit. The improved power devices such as GCT (Gate Commutated Turn-off) Thyristor and IEGT (Injection Enhanced Gate bipolar Transistor) were developed and applied to 3-level inverter after 2000. The switching performance was improved and the efficiency of the inverter was improved to about 99%. Now, most of the new main drives of the metal plant rolling mill are using the 3-level inverters. There are other large motor drive applications, such as fans, pumps, and compressors. Usually, these motors are directly connected to the commercial power line and flow is changed by the mechanical means, such as dampers and bypass valves, which are inefficient. Recently, our society is becoming more focused on saving energy, and MV inverters, directly operating (without mechanical means) the electric motors in these applications at adjustable speed, are used to save energy. Recently, LNG (Liquefied Natural Gas) is considered as an important energy source that can be transported from the gas fields to the consumers that need it. The LNG compressors are often very large capacity of 75MW or greater. Initially it was driven by the gas turbine system. Now, it is changing to an all electric motor drive system because of higher efficiency and maintainability. To meet the very large capacity required, a 5-level inverter was developed, with the rating of 7.2kV and 30MVA. It can be applied in parallel up to 4 sets, for a maximum capacity of 120MVA. The 5-level inverter has an excellent performance of high voltage and clean output voltage waveform.