Multilevel converters have become the direct accepted solution for high power converter applications. They are used in wide variety of power electronic applications like power transmission and distribution, electric motor drives, battery management and renewable energy management to name a few. For medium and high voltage motor drives, especially induction motor drives, the use of multilevel voltage source inverters have become indispensible. A high voltage multilevel inverter could be realized using low voltage switching devices which are easily available and are of low cost. A multilevel inverter generates voltage waveforms of very low harmonic distortion by switching between voltage levels of reasonably small amplitude differences. Thus the dv/dt of the output voltage waveform is small and hence the electromagnetic interference generated is less. Because of better quality output generation, the switching frequency of the multilevel inverters could be reduced to control the losses. Thus, a multilevel converter stands definitely a class apart in terms of performance from a conventional two-level inverter. Many multilevel inverter topologies for induction motor drives are available in the literature. The basic multilevel topologies are the neutral point clamped (NPC) inverter, flying capacitor (FC) inverter and the cascaded H-bridge (CHB) inverter. Various other hybrid multilevel topologies have been proposed by using the basic multilevel inverter topologies. It is also possible to obtain multilevel output by using conventional two-level inverters feeding an open-end winding induction motor from both sides. All the conventional multilevel voltage source inverters generate hexagonal (6 sided polygons) voltage space vector structures. When an inverter with hexagonal space vector structure is operated in the over modulation range, significant low order harmonics are generated in the phase voltage output. Over modulation operation is required for the full utilization of the available DC-link voltage and hence maximum power generation. Among the harmonics generated, the fifth and seventh harmonics are of significant magnitudes. These harmonics generate torque ripple in the motor output and are undesirable in high performance motor drive applications. The presence of these harmonics further creates problems in the closed loop current control of a motor, affecting the dynamic performance. Again, the harmonic currents generate losses in the stator windings. Therefore, in short, the presence of harmonic voltages in the inverter output is undesirable. Many methods have been proposed to eliminate or mitigate the effect of the harmonics. One solution is to operate the inverter at high switching frequency and thereby push the harmonics generated to high frequencies. The stator leakage inductance offers high impedance to the high frequency harmonics and thus the harmonic currents generated are negligible. But, high switching frequency brings switching losses and high electromagnetic interference generation in the drive system. And also, high switching frequency operation is effective only in the linear modulation range. Another solution is to use passive harmonic filters at the inverter output. For low order harmonics, the filter components would be bulky and costly. The loss created by the filters degrades the efficiency of the drive system as well. The presence of a filter also affects the dynamic performance of the drive system during closed loop operation. Special pulse width modulation (PWM) techniques like selective harmonic elimination (SHE) PWM can prevent the generation of a particular harmonic from the phase voltage output. The disadvantages of such schemes are limited modulation index, poor dynamic performance and extensive offline computations. An elegant harmonic elimination method is to generate a voltage space vector structure having more number of sides like a dodecagon (12 sided polygons) or an octadecagon (18 sided polygons) rather than a hexagon. Inverter topologies generating dodecagonal voltage space vector structure eliminate fifth and seventh order harmonics, represented as 6n 1; n = odd harmonics, from the phase voltages and hence from the motor phase currents, throughout the entire modulation range. The first harmonics appearing the phase voltage are the 11th and 13th harmonics. Another advantage is the increased linear modulation range of operation for a given DC-link voltage, because geometrically dodecagon is closer to circle than a hexagon. An octadecagonal structure eliminates the 11th and 13th harmonics as well from the phase voltage output. The harmonics present in the phase voltage are of the order 18n 1; n = 1; 2; 3; :::. Thus the total harmonics distortion (THD) of the phase voltage is further improved. The linear modulation range also gets enhanced compared to hexagonal and dodecagonal structures. Multilevel dodecagonal and octadecagonal space vector structures combines the advantages of both multilevel structure and dodecagonal and octadecagonal structure and hence are very attractive solutions for high performance induction motor drive schemes. Chapter 1 of this thesis introduces the multilevel in-verter topologies generating hexagonal, dodecagonal and octadecagonal voltage space vector structures. Inverter topologies generating multilevel dodecagonal and octadecago-nal voltage space vector structures have been proposed before but using multiple DC sources delivering active power. The presence of more than one DC source in the inverter topology makes the back to back operation (four-quadrant operation) of the drive system difficult. And also the drive system becomes more costly and bulky. This thesis proposes induction motor drive schemes generating multilevel dodecagonal and octadecagonal volt-age space vector structures using a single DC source. In Chapter 2, an induction motor drive scheme generating a six-concentric multilevel dodecagonal voltage space vector structure using a single DC source is proposed for an open-end winding induction motor. In the topology, two three-level inverters drive an open-end winding IM, one inverter from each side. DC-link of primary inverter is from a DC source (Vdc) which delivers the entire active power, whereas the secondary inverter DC-link is maintained by a capacitor at a voltage of 0:289Vdc, which is self-balanced during the inverter operation. The PWM scheme implemented ensures low switching frequency for primary inverter. Secondary inverter operates at a small DC-link voltage. Hence, switching losses are small for both primary and secondary inverters. An open-loop V/f scheme was used to test the topology and modulation scheme. In the work proposed in Chapter 3, the topology and modulation scheme used in the first work is modified for a star connected induction motor. Again, the scheme uses only a single DC source and generates a six-concentric multilevel space vector struc-ture. The power circuit topology is realized using a three-level flying capacitor (FC) inverter cascaded with an H-bridge (CHB). The capacitors in the CHB inverter are maintained at a voltage level of 0:1445Vdc. The FC inverter switches between volt-age levels of [Vdc; 0:5Vdc; 0] and the CHB inverter switches between voltage levels of [+01445Vdc; 0; 0:1445Vdc]. The PWM scheme generates a quasi-square waveform output from the FC inverter. This results in very few switchings of the FC inverter in a funda-mental cycle and hence the switching losses are controlled. The CHB inverter switches Ch. 0: at high frequency compared to the FC inverter and cancels the low order harmonics (6n 1; n = odd) generated by the FC inverter. Even though the CHB operates at higher switching frequency, the switchings are at low voltage thereby controlling the losses. The linear modulation range of operation is extended to 48:8Hz for a base frequency of 50Hz. An open-loop V/f scheme was used to test the topology and modulation scheme. In Chapter 4, a nine-concentric multilevel octadecagonal space vector structure is proposed for the first time, again using a single DC source. The circuit topology remains same as the work in Chapter 3, except that the CHB capacitor voltage is maintained at 0:1895Vdc. The 5th; 7th; 11th and 13th harmonics are eliminated from the phase voltage output. The linear modulation range is enhanced to 49:5Hz for a base speed of 50Hz. An open-loop V/f scheme and rotor field oriented control scheme were used to test the proposed drive system. All the proposed drive schemes have been extensively simulated and tested in hard-ware. Simulation was performed in MATLAB-SIMULINK environment. For implement-ing the inverter topology, SKM75GB12T4 IGBT modules were used. The control al-gorithms were implemented using a DSP (TI’s TMS320F28334) and an FPGA (Xilinx Spartan XC3S200). A 1kW , 415V , 4-pole induction motor was used for the experiment purpose. The above mentioned induction motor drive schemes generate phase voltage outputs in which the low order harmonics are absent. The linear modulation range is extended near to the base frequency of operation compared to hexagonal space vector structure. In the inverter topologies, the secondary inverters or the CHB inverters functions as harmonic filters and delivers zero active power. The primary inverter in the topologies switches at low frequency, reducing the power loss. Single DC source requirement brings down the cost of the system as well as permitting easy four-quadrant operation. This is also advantageous in battery operated systems like EV applications. With these features and advantages, the proposed drive schemes are suitable for high performance, medium voltage induction motor drive applications.

Multilevel inverters play a major role in the modern day medium and high power energy conversion processes. The classic two level voltage source inverter generates PWM pole voltage output having two levels with strong fundamental component and harmonics centered around the switching frequency and its multiples. With higher switching frequency, its components can be easily filtered and results in better Total harmonic distortion (THD) output voltage and current. But with higher switching frequency, switching loss of power devices increases and electromagnetic interferences also increases. Also in two level inverter, pole voltage switches between zero and DC bus volt-age Vdc. This switching results in high dv=dt and causes EMI and increased stress on the motor winding insulation. The attractive features of multilevel inverters compared to a two level inverter are reduced switching frequency, reduced switching loss, improved volt-age and current THD, reduced dv=dt, etc. Because of these reasons, multilevel invertersultilevelinvertersplayamajorroleinthemoderndaymediumandhighpower find application in electric motor drives, transmission and distribution of power, transportation, traction, distributed generation, renewable energy systems like photo voltaic, hydel power, energy management, power quality, electric vehicle applications, etc. AC motor driven applications are consuming the significant part of the generated electrical energy (more than 60%) around the world. The multilevel inverters are ideal for such applications, since the switching frequency of the devices can be kept low with lower out-put voltage dv=dt. Also by using multilevel inverters, the common mode voltage (CMV) switching can be made zero and associated motor bearing failure can be mitigated. For multilevel inverter topologies, as the number of level increases, the power circuit becomes more complex by the increase in the number of DC power supplies, capacitors, switching devices and associated control circuitry. The main focus of development in multilevel inverter for medium and high power applications is to obtain an optimized number of voltage levels with reduced number of switching devices, capacitors and DC power sources. In this thesis, a new hybrid seven level inverter topology with a single DC supply is proposed with reduced switch count. The inverter is realized by cascading two three level flying capacitor inverters with a half bridge module. Compared to the conventional seven level inverter topologies, the proposed inverter topology uses lesser number of semiconductor devices, capacitors and DC power supplies for its operation. For this topology, capacitor voltage balancing is possible for entire modulation range irrespective of the load power factor. Also capacitor voltage can be controlled over a switching cycle and this result in lowering the capacitor sizing for the proposed topology. A simple hysteresis band based capacitor voltage balancing scheme is implemented for the inverter topology. For a voltage source inverter fed induction motor drive system, the inverter pole voltage is the sum of motor phase voltage and common mode voltage. In induction motors, there exists a parasitic capacitance between stator winding and stator iron, and between stator winding and rotor iron. Common mode voltage with significant magnitude and high frequency switching causes leakage current through these parasitic capacitances and motor bearings. This leakage current can cause ash over of bearing lubricant and corrosion of ball bearings, resulting in an early mechanical failure of the drive system. In this thesis, analysis of extending the linear modulation range of a general n-level inverter by allowing reduced magnitude of common mode voltage (CMV) switching (only Vdc/18) is presented. A new hybrid seven level inverter topology, with a single DC supply and with reduced common mode voltage (CMV) switching is presented in this thesis for the first time. Inverter is operated with zero CMV for modulation index less than 86% and is operated with a CMV magnitude of Vdc/18 to extend the linear modulation range up to 96%. Experimental results are presented for zero CMV operation and for reduced common voltage operation to extend the linear modulation range. A capacitor voltage balancing algorithm is designed utilizing the pole voltage redundancies of the inverter, which works for every sampling instant to correct the capacitor voltage irrespective of load power factor and modulation index. The capacitor voltage balancing algorithm is tested for different modulation indices and for various transient conditions, to validate the proposed topology. In recent years, model predictive control (MPC) using the system model has proved to be a good choice for the control of power converter and motor drive applications. MPC predicts system behavior using a system model and current system state. For cascaded multilevel inverter topologies with a single DC supply, closed loop capacitor voltage control is necessary for proper operation. This thesis presents zero and reduced common mode voltage (CMV) operation of a hybrid cascaded multilevel inverter with predictive capacitor voltage control. For the presented inverter topology, there are redundant switching states for each inverter voltage levels. By using these switching state redundancies, for every sampling instant, a cost function is evaluated based on the predicted capacitor voltages for each phase. The switching state which minimizes cost function is treated as the best and is switched for that sampling instant. The inverter operates with zero CMV for a modulation index upto 86%. For modulation indices from 86% to 96% the inverter can operate with reduced CMV magnitude ( Vdc/18) and reduced CMV switching frequency using the new space-vector PWM (SVPWM) presented herein. As a result, the linear modulation range is increased to 96% as compared to 86% for zero CMV operation. Simulation and experimental results are presented for the inverter topology for various steady state and transient operating conditions by running an induction motor drive with open loop V/f control scheme. The operation of a two level inverter in the over-modulation region (maximum peak phase fundamental output of inverter is greater than 0:577Vdc) results in lower order harmonics in the inverter output voltage. This lower order harmonics (mainly 5th, 7th, 11th, and 13th) causes electromagnetic torque ripple in motor drive applications. Also these harmonics causes extra losses and adversely affects the efficiency of the drive system. Also inverter control becomes non linear and special control algorithms are required for inverter operation in the over modulation region. In conventional schemes, maximum fundamental output voltage possible is 0:637Vdc. In that case inverter is operated in a square wave mode, also called six-step mode. This operation results in high dv=dt for the inverter output voltage. With multilevel inverters also, the inverter operation with peak phase fundamental output voltage above 0:577Vdc results in lower order harmonics in the inverter output voltage and results in electromagnetic torque pulsation. In this thesis, a new space vector PWM (SVPWM) method to extend the linear modulation range of a cascaded five level inverter topology with a single DC supply is presented. Using this method, the inverter can be controlled linearly and the peak phase fundamental output voltage of the inverter can be increased from 0:577Vdc to 0:637Vdc without increasing the DC bus voltage and without exceeding the induction motor voltage rating. This new technique makes use of cascaded inverter pole voltage redundancy and property of the space vector structure for its operation. Using this, the induction motor drive can be operated till the full speed range (0 Hz to 50 Hz) with the elimination of lower order harmonics in the phase voltage and phase current. The ve level topology presented in this thesis is realized by cascading a two level inverter and two full bridge modules with floating capacitors. The inverter topology and its operation for extending the modulation range is analyzed extensively. Simulation and experimental results for both steady state and dynamic operating conditions are presented. Zero common mode voltage (CMV) operation of multilevel inverters results in reduced DC bus utilization and reduced linear modulation range. In this thesis two reduced CMV SVPWM schemes are presented to extend the linear modulation range by allowing reduced CMV switching. But using these SVPWM schemes the peak phase fundamental output voltage possible is only 0:55Vdc in the linear region. In this thesis, a method to extend the linear modulation range of a CMV eliminated hybrid cascaded multilevel inverter with a single DC supply is presented. Using this method peak fundamental voltage can be increased from 0 to 0:637Vdc with zero CMV switching inside the linear modulation range. Also inverter can be controlled linearly for the entire modulation range. Also, various PWM switching sequences are analyzed in this thesis and the PWM sequence which gives minimum current ripple is used for the zero CMV operation of the inverter. The inverter topology with single DC supply is realized by cascading a two level inverter with two floating capacitor fed full bridge modules. Simulation and experimental results for steady state and dynamic operating conditions are presented to validate the proposed method. A three phase, 400 V, 3.7 kW, 50 Hz, two-pole induction motor drive with the open-loop V/f control scheme is implemented in the hardware for testing proposed inverter topology and proposed SVPWM algorithms experimentally. The semiconductor switches that were used to realize the power circuit for the experiment were 75 A, 1200 V IGBT half-bridge modules (SKM-75GB-12T4). Optoisolated gate drivers with de-saturation protection (M57962L) were used to drive the IGBTs. For the speed control and PWM timing computation, TMS320F28335 DSP is used as the main controller and Xilinx SPARTAN-3 XC3S200 FPGA as the PWM signal generator with dead time of 2.5 s. Level shifted carrier-based PWM algorithm is implemented for the normal inverter operation and zero CMV operation. From the PWM algorithm, information about the pole voltage levels to be switched can be obtained for each phase. In the sampling period, for capacitor voltage balancing of each phase, the DSP selects a switching state using the capacitor voltage information, current direction and pole voltage data for each phase. This switching state information along with the PWM timing data is sent to an FPGA module. The FPGA module generates the gating signals with a dead time of 2.5 s for the gate driver module for all the three phases by processing the switching state information and PWM signals for the given sampling period. For fundamental frequencies above 10Hz, synchronous PWM technique was used for testing the inverter topology. For modulation frequencies 10Hz and below, a constant switching frequency of 900 Hz was used. Various steady state and transient operation results are provided to validate the proposed inverter topology and the zero and reduced CMV operation schemes and extending the linear modulation scheme presented in this thesis. With the advantages like reduced switch count, single DC supply requirement, zero and reduced CMV operation, extension of linear modulation range, linear control of induction motor over the entire modulation range with zero CMV, lesser dv=dt stresses on devices and motor phase windings, lower switching frequency, inherent capacitor balancing, the proposed inverter power circuit topologies, and the SVPWM methods can be considered as good choice for medium voltage, high power motor drive applications.

Multilevel converters are finding increased attention in industry and academia as the preferred choice of electronic power conversion for high power applications. They have a wide application area in a variety of industries involving transportation and energy management, a significant portion of which comprises of multilevel inverter fed induction motor drives. Multilevel inverters are ideally suitable for high power drives, since the switching frequency of the devices is limited for high power applications. In low power drives, the switching frequency is often in the range of tens of kHz, so that switching frequency harmonics are pushed higher in the frequency spectrum thereby the size and cost of the filter are reduced. But higher switching frequency has its own drawbacks, in particular for high voltage, high power applications. They cause large dv/dt stress on the motor and the devices, increased EMI problems and higher switching losses. An engineering trade-o is thus needed to select the minimum switching frequency without compromising on the output voltage quality. The present work is an alternate approach in this direction. Here, new inverter topologies and PWM strategies are developed that can eliminate a set of harmonics in the phase voltage using 12-sided polygonal space vector diagrams, also called dodecagonal space vector diagrams. A dodecagonal space vector diagram has many advantages over a hexagonal one. Switching space vectors on a dodecagon will not produce any harmonics of the order 6n 1, (n=odd) in the phase voltage. The next set of harmonics thus reside at 12n 1, (n=integer). By increasing the number of samples in a sector, it is also possible to suppress the lower order harmonics and a nearly sinusoidal voltage can be obtained. This is possible to achieve at a low switching frequency of the inverters. At the same time, a dodecagon is closer to a circle than a hexagon; so the linear modulation range is extended by about 6.6% compared to the hexagonal case. For a 50 Hz rated frequency operation, under constant V/f ratio, the linear modulation can be achieved upto a frequency of 48.3 Hz. Also, the harmonics of the order 6n 1, (n=odd) are absent in the over-modulation region. Maximum fundamental voltage is obtained from this inverter at the end of over-modulation region, where the phase voltage becomes a 12-step waveform. The present work is developed on dodecagonal space vector diagrams. The entire work can be summarized and explained through Fig. 1. This figure shows the development of hexagonal and dodecagonal space vector diagrams. It is known that, 3-level and 5-level space vector diagrams have been developed as an improvement over 2-level ones. They Figure 1: Development of hexagonal and dodecagonal space vector diagrams have better harmonic performance, reduced dv/dt stress on the motor and devices, better electromagnetic compatibility and improvement of efficiency over 2-level space vector diagrams. This happens because the instantaneous error between the reference vector and the switching vectors reduces, as the space vector density increases in the diagram. This is shown at the top of the figure. In the bottom part, the development of the dodecagonal space vector diagram is shown, which is the contribution of this thesis work. This is explained in brief in the following lines. Initially, a space vector diagram is proposed which switches on hexagonal space vectors in lower-modulation region and dodecagonal space vectors in the higher modulation region. As the reference vector length increases, voltage vectors at the vertices of the outer dodecagon and the vertices from the outer most hexagon is used for PWM control. This results in highly suppressed 5th and 7th order harmonics thereby improving the harmonic profile of the motor current. This leads to the 12-step operation at rated voltage where all the 5th and 7th order harmonics are completely eliminated. At the same time, the linear range of modulation extends upto 96.6% of base speed. Because of this, and the high degree of suppression of lower order harmonics, smooth acceleration of the motor upto rated speed is possible. The presence of multilevel space vector structure also limits the switching frequency of the inverters. In the next work, the single dodecagonal space vector diagram is improved upon to form two concentric dodecagons spanning the space vector plane (Fig. 1). The radius of the outer dodecagon is double the inner one. It reduces the device rating and the dv/dt stress on the devices to half compared to existing 12-sided schemes. The entire space vector diagram is divided into smaller sized isosceles triangles. PWM switching on these smaller triangles reduces the inverter switching frequency without compromising on the output voltage quality. The space vector diagram is further refined to accommodate six concentric dodecagons in the space vector plane (Fig. 1). Here the space vector diagram is characterized by alternately placed dodecagons which become closer to each other at higher radii. As such the harmonics in the phase voltage are reduced, in particular at higher modulation indices. At the same time, because of the dodecagonal space vector structure, all the 6n ± 1, (n=odd) harmonics are eliminated from the phase voltage. A nearly sinusoidal phase voltage can be generated without resorting to high frequency switching of the inverters. The above space vector diagrams are developed using different inverter circuits. The first work is developed from cascaded combination of three 2-level inverters, while the second and third works use 3-level NPC inverters feeding an open end induction motor drive. The circuit topologies are explained in detail in the respective chapters. Apart from this, PWM switching schemes and detailed analysis on duty cycle calculations using the concept of volt-second balance are also presented. They show that with proper switching schemes, the proposed configurations can substantially reduce the overall loss of the inverter. Other operational issues like capacitor voltage balancing of 3-level NPC inverters and improvement of input current drawn from the grid are also covered. All the above propositions are first simulated by MATLAB and subsequently verified by an experimental laboratory prototype. Motor current waveforms both at steady state and transient conditions during motor acceleration show that the induction motor can be fed from nearly sinusoidal voltage at all operating conditions. Simplified comparative studies are also made with the proposed converters and higher level inverters in terms of output voltage quality and losses. These are some of the constituents for chapters 2, 3 and 4 in this thesis. Additionally, the first chapter also covers a brief survey on some of the recent progresses made in the field of multilevel inverter. The thesis concludes with some interesting ideas for further thought and exploration.