The Design Of A Medium Voltage Frequency Control System Based On The Five-Level NPC/H-Bridge

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). The 3-level NPC circuit (Neutral Point Clamped) 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 were developed and applied to 3-level inverter. The switching performance was improved and the efficiency of the inverter was improved to about 99%.Now, to meet the very large capacity required, a 5-level NPC/H bridge 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.This paper is presented in three parts to discuss the 5-level NPC/H bridge inverter.Part One, Multi-pulse Diode Rectifiers, covers 12-, 18- and 24-pulses rectifier topologies commonly used in the MV drive for the reduction of line current distortion. The configuration of phase-shifting (zigzag) transformers is discussed. Part Two, Multilevel Voltage Source Inverters, presents detailed analysis on various multilevel voltage source inverter (VSI) topologies, including neutral point clamped and cascaded H-bridge neutral point clamped inverters. Space-vector modulation schemes for the 5 level NPC/H bridge inverters are elaborated: A novel flexible 3-segment switching sequence for the space vector modulation (SVM) of high-power multilevel converters. The switching sequence is optimized for the improvement of harmonic spectrum and the minimization of device switching frequency. Compared to other commonly used switching sequences, the output voltage spectrum of the proposed scheme shows higher inverter equivalent switching frequencies and lower device switching frequencies. The proposed switching sequence is simulated on a 5-level neutral point clamped (NPC) H-bridge based inverter.Part Three, The field-oriented control (FOC) schemes for high power PMSM drives are presented. There exist a variety of field oriented control schemes with many different variations and approaches. To make the subject matter easy to understand, this part focuses on the rotor flux oriented control scheme. The PMSM control algorithms for the rotor flux orientation are elaborated.