Research On The Control Strategy Of Multilevel Active Neutral-Point-Clamped Converters

In order to achieve the switching device’s loss balance and reliable clamping and output five or more voltage level, two active neutral-point-clamped(ANPC) topologies are studied in this dissertation:three-level ANPC converter and five-level ANPC converter. The main contents are as followes:Three-level ANPC converter has the advantages of balanced switching device’s loss and realiable clamping compared to the traditional three-level diode neutral-point-clamped(NPC).Four switching states are selected for normal operating after the detailed analysis of it switching pattern and the current path would not be influenced. Three pulse width modulation(PWM) strategies are studied and compared for realizing the switching device’s loss balance. To compare the loss distribution of switching devices in three-level NPC converter and three-level ANPC converter, loss calculation under a special condition is carried out. The results proved the superiority of three-level ANPC converter in the aspect of switching device’s loss balance.Five-level ANPC converter has excellent prospect in the medium-high votage high power applications. Some PWM techniques are studied and carried out, including the phase-disposition(PD) PWM, phased-shifted(PS) PWM and selected harmonic elimination(SHE) PWM. The output voltage level would be abnormal during the dead time when use PD PWM while wouldn’t be when use PS PWM. In order to improve the charge efficiency of floating capcacitors, a modified pre-charge strategy is proposed based on the existing strategy which can charge the capacitors through the dc-bus directly.The voltages of dc-bus neutral-point and floating capcacitors in five-level ANPC converter must be maintained balance to ensure the normal operation of the system and improve the quality of the output voltage.The floating capcacitors voltages balancing strategies based on the selecting or distribution of the redundant switch states are studied when using PD PWM, PS PWM and SHEPWM respectively. The dc-bus neutral-point voltage balance control is based on zero-sequence voltage injection. To choose the appropriate zero-sequence voltage, The relationship between the neutral-point current and zero-sequence voltage is discussed and the range of zero-sequence voltage is analyzed. Two neutral-point voltage balancing algorithms are presented under using PS PWM:search optimization method and the optimal zero-sequence voltage injection method. A neutral-point voltage balancing algorithm based on zero-sequence voltage injection is presented.A five-level ANPC converter for experiment is designed and set up. The experimental results show the validity and feasibility of various control methods which are put forward in this article.