Generation of Higher Number of Voltage Levels by Stacking Inverters of Lower Multilevel Structures with Low Voltage Devices for Drives

ABSTRACT

This paper proposes a new method of generating higher number of levels in the voltage waveform by stacking multilevel converters with lower voltage space vector structures. An important feature of this stacked structure is the use of low voltage devices while attaining higher number of levels. This will find extensive applications in electric vehicles since direct battery drive is possible. The voltages of all the capacitors in the structure can be controlled within a switching cycle using the switching state redundancies (pole voltage redundancies). This helps in reducing the capacitor size. Also, the capacitor voltages can be balanced irrespective of modulation index and load power factor. To verify the concept experimentally, a 9-level inverter is developed by stacking two 5-level inverters and an induction motor is run using V/f control scheme. Both steady state and transient results are presented.

KEYWORDS

  1. Induction motor drive
  2. PWM
  3. Multilevel inverter
  4. Topology
  5. CHB
  6. Flying capacitor
  7. Low voltage devices

SOFTWARE: MATLAB/SIMULINK

 CONCLUSION

In this paper, a new method of generating higher number of voltage levels by stacking multilevel converters having lower space vector structures is presented. Here each of the stacked inverter is having only one DC supply. The proposed stacked multilevel inverter has a modular structure which is realized by stacking the FC and cascading it with series connected capacitor fed H-bridges. Since the voltage across the H-bridge switches are low, the switching loss can be further reduced. Also the H-bridges can be bypassed if it fails. Thus using this system has a improved reliable operation. Also when one of the FC fails, inverter can still be operated with reduced voltage and power levels. The concept of stacking can be generalized to obtain higher voltage levels. As the number of levels increases, blocking voltages of switches reduces and the proposed structure can be fed from low voltage battery cells. Also, higher number of voltage levels imply lower switching frequency and therefore higher efficiency, which makes it suitable for application in electric vehicles. Hysteresis based  capacitor voltage balancing algorithm is used to maintain the capacitor voltages irrespective of modulation index and load power factor. Detailed experimental results, using a stacked 9- level inverter, showing the steady state operation at different frequencies and the transient results, ensure that the proposed structure will be a viable scheme for high power applications with improved reliability.

REFERENCES

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