Investigations on Stacked Multilevel Inverter Topologies Using Flying Capacitor and HBridge Cells for Induction Motor Drives
Abstract
Conventional 2level inverters have been quite popular in industry for drives applications. It used pulse width modulation techniques to generate a voltage waveform with high quality. For achieving this, it had to switch at high frequencies and also the switching is between 0 and Vdc. Also additional LC filters are required before feeding to a motor. 3phase IM is the work horse of the industry. Several speed control techniques have been established namely the V/f control technique and for high performance, vector control is adopted.
An electric drive system comprises of a rectifier, inverter, a motor and a load. each module is a topic by itself. This thesis work discusses the novel inverter topologies to overcome the demerits of a conventional 2level inverter or even the basic multilevel topologies, for an electric drive. The word ‘multilevel’ itself signifies that inverter can generate more than two levels. The idea was first originated by Nabae, Takahashi and Akagi to bring an additional voltage level so that the waveform becomes a quasi square wave. This additional voltage level brought additional benefits in terms of reduced dv/dt and requirement of low switching frequency. But this was not without any cost. The inverter structure is slightly more complicated than a 2level and also required more devices. But the advantage it gave was superior enough to such an extent that the above topology (popularly known as NPC) has become quite popular in industry. This topology was later modified to equalize the semiconductor losses among switches by replacing the clamping diodes with controllable switches and such topologies are popularly known as Active NPCs (ANPCs) because of the replacement of diodes with active switches.
3level flying capacitors were then introduced where the additional voltage level is provided using charged capacitors. But this capacitor voltage has to be maintained at its nominal value during the inverter operation. An additional floating capacitor, which is an electrolytic capacitor is needed for this. Increasing the number of electrolytic capacitors reduces the reliability of the inverter drive since they are the weakest link in any inverters and its count has to be kept to the minimum.
By using a Hbridge cell in each of the three phases, three voltage levels can be easily obtained.This is commonly known as Cascaded Hbridge (CHB) multilevel inverter.
The above three topologies have been discussed with respect to generation of three pole voltage levels and these topologies are quite suited also. A higher number of voltage levels will reduce the switching frequency even lesser and also the dv/dt. On increasing the number of levels further and further, finally the inverter need not do any PWM switching and just generating the levels is sufficient enough for a good quality waveform and also low dv/dt. But when the above topologies are scaled for more than three voltage levels, all of them suffer serious drawbacks which is briefly discussed below.
The diode clamped inverter (known as NPC if it is 3level), when extended to more than three levels suﬀers from the neutral point balancing issue and also the count of clamping diodes increase drastically. FC inverters, when extended beyond 3level, the number of electrolytic capacitors increases and also balancing of these capacitors to their nominal voltages becomes complicated. In the case of multilevel CHB, when extended beyond 3level, the requirement of isolated DC sources also increases. To generate isolated supplies, phase shifting transformer and 8, 12 or 24 pulse diode rectifier is needed which increases the weight , size and cost of the drive. Therefore its application is limited.
In this thesis, the aim is to develop a novel method to develop a multilevel inverter without the drawbacks faced by the basic multilevel topologies when scaled for higher number of voltage levels. This is done through stacking the basic or hybrid combination of these basic multilevel topologies through selector switches. This method is experimentally verified by stacking two 5level inverters through a 2level selector switch (whose switching losses can be minimized through soft cycle commutation). This will generate nine levels.Generating 9levels through scaling the basic topologies is disadvantageous, the comparison table is provided in the thesis. This is true for any higher voltage level generation. Each of the above 5level inverter is developed through cascading an FC with a capacitor fed Hbridge. The device count can be reduced by making the FCCHB module common to the selector switches by shifting the selector switches between the DC link and the common FCCHB module. Doing so, reduces the modular feature of the drive but the device count can be reduced. The FFT plot at different frequencies of operation and the switching losses of the different modulesFC, CHB and the selector switches are also plotted for different frequencies of operation.
The next step is to check whether this method can be extended to any number of stackings for generation of more voltage levels. For this, a 49level inverter is developed in laboratory by stacking three 17level inverters. Each of the 17level inverter is developed by cascading an FC with three CHBs. When there are 49 levels in the pole voltage waveform, there is no need to do any regular PWM since the output waveform will be very close to a sine wave even without any PWM switching. The technique used is commonly known in literature as Nearest Level Control (NLC). This method of stacking and cascading has the advantage that the FC and the CHB modules now are of very low voltages and the switching losses can be reduced. The switching losses of the diﬀerent modules are calculated and plotted for diﬀerent operating frequencies in the thesis. To reduce the voltages of the modules further, a 6phase machine has been reconfigured as a 3phase machine, the advantage being that now the DC link voltage requirement is half of that needed earlier for the same power. This further reduces voltages of the modules by half and this allows the switches to be replaced with MOSFETs, improving the eﬃciency of the drive. This topology is also experimentally verified for both steady state and transient conditions.
So far the research focussed on a 3phase IM fed through a stacked MLI. It can be observed that a stacked MLI needs as many DC sources as the number of stackings. A 6phase machine apart from reduced DC link voltage requirement, has other advantages of better fault tolerant capability and better space harmonics. They are serious contenders for applications like ship propulsion, locomotive traction, electric vehicles, more electric aircraft and other high power industrial applications. Using the unique property of a 6phase machine that its opposite windings always draw equal and opposite current, the neutral point (NP) (formed as a result of stacking two MLIs) voltage can be balanced. It was observed that the net mid point current drawn from the mid point can be made zero in a switching interval. It was later observed that with minimal changes, the mid point current drawn from the NP can be made instantaneously zero and the NP voltage deviation is completely arrested and the topology needs only very low capacity series connected capacitors energized from a single DC link. This topology is also experimentally
verified using the stacked 9level inverter topology discussed above but now for 6phase
application and experimental results are provided in the thesis. Single DC link enables direct back to back conversion and power can be fed back to the mains at any desired power factor.
All the experimental verification is done on a DSP (TMS320F28335) and FPGA
(Spartan 3 XCS3200) platform. An IM is run using V/f control scheme and the above
inverter topologies are used to drive the motor. The IGBTs used are SKM75GB123D
for the stacked 9level inverter in the 3phase and 6phase experiments. For the 49level inverter experiment, MOSFETsIRF260N were used. Both steady state and transient
results ensure that the proposed inverter topologies are suitable for high power applications.
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