대용량 추진전동기 구동시스템을 위한 H-브리지 멀티레벨 인버터 설계 및 해석

Abstract

Recently, the industrial field has begun to demand higher power equipment, and the use of motors with several megawatt output level is increasing. Despite rapid development of power electronics technology, it is difficult to connect a single power switch device to medium voltage grids such as 3.3 or 6.6kV. For this reason, a series of multilevel inverters have been studied and developed for higher voltage and higher power applications since the 1970s.

In multilevel inverters, each power device divides total DC bus voltage, so an increase of level permits an increase of DC bus voltage with a limited voltage stress on power semiconductors. Furthermore, multilevel inverters reduce dv/dt which could cause insulation failure of the motors electric winding. They also reduce common mode voltage which causes bearing current which could make motor mechanical failure.

This thesis analyzes some properties and modulation strategies of multilevel inverters, and investigates the recent trends of research. It is focused on the cascaded H-bridge inverter which has been known to have many advantages compared to others. In general, H-bridge inverters utilize symmetric cell voltage structures like 7-level from 3-cells, or 9-level from 4-cells. They can achieve more output levels from asymmetric structures in the same output voltage RMS. The asymmetric H-bridge inverter gives us several advantages such as better harmonic properties, lower dv/dt, and common mode voltage without increasing important power electronic parts, weight, or equipment size.

The characteristics of an asymmetric 11-level H-bridge inverter with 3-cell structures are investigated based on high-power propulsion motor system power converters. The system structure, output characteristics, and the load balance of each cell are analyzed. The computer simulation and experiment are implemented using a rate reduced inverter.

Furthermore, a new space vector modulation strategy is proposed, which is very easy to understand and is fast to implement in modulation software. The proposed method does not need identification of the nearest three vectors and duty cycle calculation for each vector, which is essential to the conventional method.

The proposed space vector modulation strategy is implemented with the CAN serial communication protocol. The stability and high communication speed of CAN made it possible to transfer the important data to the phase controller from the master controller at every 2kHz modulation cycle from a distance. This thesis describes that the proposed modulation strategy using CAN serial communication systems can be successfully applied to the space vector modulation purpose of a multi-level inverter.

In conclusion, this thesis presents that the proposed asymmetric 11-level H-bridge inverter can be successfully applied to a propulsion motor system with good performance for high-power propulsion motor systems. Furthermore, it presents that the proposed modulation strategy can be successfully applied to the space vector modulation of the multilevel inverters using the CAN communication protocol. Several disadvantages on the proposed inverter topology were also identified such as the complex of input rectifier circuits and the unbalance of the frequency of the inverter cells, which need further studies for improvement.