Cascaded H-bridge multilevel inverter based STATCOM with a novel dc-voltage detection technique.

摘要

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