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.
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