Cascaded H-Bridge Multilevel Inverter Based Statcom with a Novel DC-Voltage Detection Technique

Abstract

This thesis is dedicated to a comprehensive study of state synchronous compensator (STATCOM) utilizing cascaded H-bridge (CHB) multilevel inverter. Two challenging problems exist in the CHB-based STATCOM. Firstly, unbalanced voltages across de capacitors of H-bridge units would occur because a large number of capacitors are utilized in the CHB inverter for high-voltage operation. The de capacitor-voltage unbalance may result in uneven voltage stress on swithces and the distortion of inverter output voltage due to the degradation of total harmonic distortion (THD). Secondly, all de-voltages should be measuresed and controlled separately to perform de capacitor-voltage balance control. In doing so, a large number of voltage sensors are requuired, which increases the cost and complexity of the SATACOM system. Therefore, the main objectives of this thesis are to develop new techniques for the balance of de capacitor voltages and reduction of the number of voltage senors used in the CHB-based STATCOM system. Therefore, the main objectives of this thesis are to develop new techniques for the balance of de capacitor voltages and reduction of the number of voltage sensors used in the CHB-based STATCOM system. A novel de-voltage detection technique, referred to as single multiple-voltage (SMV) detector, is developed to obtain de voltages of all H-bridges. The proposed SMV algorithm can substantially reduce the number of voltage sensors. Only three voltage sensors are needed to obtain all de capacitor voltages from the measured inverter output voltage. As a result, the CHB-based STATCOM with SMV detector reduces the cost and complexity of the system. The system reliability is enhanced as well. Furthermore, this technique can be extended to the STATCOM with high-level CHB inverter and suitable to high-voltage high-power applications. A new de-voltage balance control method is proposed to assure balanced voltages accross all capacitors in the CHB inverter. The method combines the phase shifting technique and sinusoidal pulse modulation strategy for the de-voltage control. PI refulators in all control loops can be identical due to the modular structure of CHB topology and phase-shifted PWM strategy. This feature makes the de capacitor-voltage balance easy to be implemented. To verify the performance of the CHB-based STATCOM and the effectiveness of the proposed STATCOM shows superior performance in steady-state operation and can rapidly respond to the reactive power demand as well. All individual de-voltages can be detected accurately from the SMV detector without direct de-voltage measurement. With the detected de-voltages, all capacitor de-voltages can be well balanced based on the proposed de-voltage balance control method.