Separate space vector control with P-Q compensation for cascaded multilevel inverter applied for field oriented induction motor

This paper introduces a new control technique for the cascaded multilevel inverter which is constructed by connecting two three-level diode clamped multilevel inverters to both sides of splitted neutral ac motors. One of them is called the bulk inverter and the other one is called the conditioning inverter which is installed without DC power supply to reduce the inverter cost and complexity. This type of multilevel inverter which is also called hybrid multilevel inverter reduces the number of switches and dc sources for the same output levels. The proposed control technique is based on calculation of the conditioning inverter space vector instead of passing the real and reactive powers (P, Q) of the bulk inverter through low pass filters to generate the controlled space vector reference. In this proposal P, Q compensation is used only to regulate the conditioning inverter capacitor voltage. The proposed control technique avoids sudden jumps in the output voltage waveform. The total harmonic distortion is improved over a wide range of modulation indices and no extra switching is required in the bulk inverter to regulate the conditioning inverter capacitor voltage. A simple algorithm is used to identify the space vector reference location (nearest three states) for space vector modulation. The proposed algorithm could be extended easily to higher number of levels. The proposed control technique is verified by simulation for an (R-L) load and for a 200 hp field oriented controlled induction motor.