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Multi-pulse converters have been widely used for a multi-megawatt scale power generating system to comply with harmonic regulations. Among all types of multi-pulse converters, a 12-pulse converter is the most widely used due to the simple structure, which consists of a delta-delta and a delta-wye phase-shift transformer pair and it effectively mitigates undesirable harmonics from the nonlinear load. In the early 2000s, a shunt type passive front-end for a shipboard power system was proposed. By shunting the two gensets with 30° phase angle difference, a single phase-shift transformer effectively eliminates 5th and 7th harmonics. It achieves a significant size and weight reduction compared to a 12-pulse converter while keep the comparable harmonic cancellation performance. Recently, a hybrid type front-end was proposed. On top of the passive front-end, 3 phase active power filter was added and an additional harmonic cancellation was achieved to further eliminate 11th and 13th harmonics. However, the performance of both the passive and hybrid type front-end are highly dependent on the size of the line reactor in ac mains. A back to back active power filter is proposed in this dissertation to replace the phase-shift transformer in the multi-generator power architecture. The proposed front-end does not include phase-shift transformer and the size and the weight of the overall front-end can be significantly reduced. Due to the active harmonic compensation, the back to back front-end achieves better power quality and the line reactor dependency is improved. The number of required dc-link capacitors is reduced by half by introducing a back to back configuration and the capacitor size is reduced by adjusting the phase angle difference of genset to cancel out the most significant voltage harmonics in the shared dc-link bus. The overview of the existing shunt type front-end is provided and the concept of back to back active power filter is validated by simulation and prototype hardware. The comparison between existing front-end and the proposed front-end is provided to highlight the superior performance of back to back active front-end. The dc-link bus current and voltage ripple analysis is provided to explain the dc-link ripple reduction mechanism.

This thesis provides a summary on renewable energy sources integration into the grid, using an inverter, along with a comprehensive literature research on variety of available control methods. A new generalized method for grid side inverter control under unbalanced operating conditions is also proposed. The presented control method provides complete harmonic elimination in line currents and DC link voltage with adjustable power factor. The method is general, and can be used for all levels of imbalance in grid voltages and line impedances. The control algorithm proposed in this work has been implemented by using MATLAB Simulink and dSPACE RT1104 control system. Simulation and experimental results presented in this thesis are in excellent agreement.

This thesis presents experimental verification of a generalized control method for constant switching frequency Three-phase PWM Boost Type Rectifier under extremely unbalanced operating conditions in the power system. In this model, a hysteresis current controller has been implemented using a high-speed micro-controller to generate the gate signals. Even though a constant switching frequency was achieved in the previous model, while in simulation using MATLAB/Simulink, it wasn’t achieved experimentally due to the low processing speed of the DSpace processor ds1104. This problem has been resolved by using the “C2000 Delfino MCUs F28377S Launchpad ” from Texas Instruments to implement Hysteresis Current Controller that generate the gate signal of the three-phase PWM boost rectifier. The model has been verified to work under the conditions of extremely unbalanced input voltages and unbalanced input impedances with constant switching frequency in closed-loop and open-loop control.

This thesis illustrates the principle and the behavior for doubly fed electric machines used in variable speed wind power systems under the balanced and unbalanced operating conditions. A generalized control method for complete stator pulsating power elimination and harmonic elimination in grid currents with adjustable power factor of a doubly fed induction generator under the unbalanced operating conditions is proposed. The theoretical analysis of this proposed control method is presented and then demonstrated by the simulation in Powersim®. The grid ride-through-fault ability by using the proposed method is also tested.

Recent advances in semiconductor switches, new multilevel converter topologies and advanced converter modulation techniques have contributed to the expansion of voltage source converters (VSCs) to higher voltage and power ratings for utility-scale and motor drive applications. Multilevel VSC topologies extend the advantages of the fully controlled, four-quadrant, two-level converter by improving the quality of the output waveforms and minimizing filtering requirements. Additionally, the stresses across the switching devices are reduced, converter losses and electromagnetic interference are decreased and the overall efficiency is improved. The challenges associated with the multilevel converters are well documented and include: capacitor voltage deviation and voltage balancing issues, increased complexity in the circuit configuration, control and regulation of voltages and currents. The main research efforts focus on further minimizing the losses, increasing the efficiency of the converter topologies and providing tight regulation of the voltages and currents while delivering a cost-effective solution for commercial applications. The thesis deals with selective harmonic elimination pulse width modulation (SHE-PWM) for three-phase, two-level and multilevel converter topologies. SHE-PWM for the three-phase two-level converter is initially treated. Different formulations of SHE-PWM based on relaxing the symmetry requirements, previously imposed, are investigated. New solution sets are calculated by imposing half-wave symmetry or completely eliminating the symmetry requirements. Based on multiple harmonic performance factors, an evaluation identifies the solution sets that exhibit superior harmonic performance. The theoretical analysis and simulations are verified through experimental work on a laboratory prototype. Multilevel SHE-PWM (MSHE-PWM) techniques are analyzed for various number of levels in the output waveforms and harmonics eliminated from the output voltage harmonic spectrum. The prime challenge of MSHE-PWM is the acquisition of the different solutions while ensuring their continuity over the modulation index range. The solutions are evaluated for their harmonic performance. The operation of hybrid multilevel converters, namely the five-level hybrid H-bridge based converter and the seven-level hybrid cascaded ANPC converter under MSHE-PWM is also analyzed. In both topologies, the regulation of the voltages in the H-bridge cells of the converter together with the application and performance of MSHE-PWM are investigated. Extended simulation and experimental results from both topologies are provided. MSHE-PWM is further extended to include the voltage levels of the PWM waveform as variables. The proposed approach offers advantages in terms of the acquired solutions. The switchings assume constant values over the modulation index range, the voltage levels vary linearly with the increase of the modulation index and the formulation of the problem allows for elimination of additional harmonics from the output voltage harmonic spectrum. The advantages of the proposed formulations come at the cost of increased complexity in the regulation of the DC voltages, particularly in closed loop implementations. Simulation and experimental results based on laboratory setups for five and seven-level waveforms are provided. The thesis also deals with the modulation of the modular multilevel converter (MMC) and its operation in utility-scale applications. The MMC is the state-of-the-art multilevel converter based on the cascaded connection of half-bridge sub-modules (SMs) and can be extended to provide large number of levels in the output voltage waveform. The circuit configuration and modular concept make the MMC a particularly attractive multilevel topology for medium and high-voltage, medium and high-power utility-scale applications. Two different modulation methods for the MMC are proposed and analyzed. The two methods, under the same switching frequency for the switches, provide different operating characteristics. MSHE-PWM techniques for the MMC are also proposed. The techniques, requiring calculation of switchings for large number of levels, are combined with a method to balance the SM capacitor voltages through sorting of the SM voltage values. The operation of the MMC under both modulation methods, based on the MSHE-PWM is verified through simulation and experimental results. Finally, the thesis discusses the back-to-back configuration of MMC topologies. The converters are operated with sinusoidal PWM, under both the modulation techniques and the voltage balancing method presented earlier in the thesis. Based on controllers in the synchronous rotating reference frame, the system is investigated for its operation under steady state and during transients in the active and reactive power references of the converters. Extensive simulation results are provided to illustrate the system behavior.

In this thesis, the implementation of a generalized method of harmonic elimination for the PWM boost type rectifier under severe fault conditions in the power system is presented. This method has been implemented by using DSPACE RT1104 digital control system. Control loop has been designed for real-time DC link voltage regulation. The simulation and experimental results verify the feasibility of the control scheme for input-output harmonic elimination. With the proposed method, unity power factor can be achieved as well as low-order harmonic elimination. High quality input and output waveforms of the three-phase PWM boost type rectifier are obtained under extremely unbalanced operating conditions.

A novel cascaded multilevel converter is proposed in this thesis. The thesis proposes to produce more voltage levels from fewer H-bridges in a cascaded multilevel converter. The converter uses fewer H-bridges and the proposed switching scheme renders more voltage level in the staircase waveform with equal steps. Since the resulting voltage levels are equal, the angles are determined for the complete elimination of more unwanted harmonics. The implementation of the switching scheme, in single and three-phase configurations were simulated with Ansoft Simplorer© and the frequency spectrum of the resulting waveform and its total harmonic distortion are shown to verify the results. The number of switches employed in the converter is halved. The impact of voltage magnitude variations on harmonic elimination is analyzed. Source and switch utilization is also evaluated.

The switching mode power supplies used in personal computers are major sources of harmonic currents. Measured and calculated waveforms and harmonic levels of input currents of various types of personal computers are presented. Harmonic currents of a group of personal computers reinforce each other. Neutral currents resulting from the addition of triplen harmonics are analyzed and recommendations are made to safeguard against potential problems. The impact of personal computer harmonic currents on distribution transformer losses are explored. Harmonic currents result in the overheating of transformer due to excessive eddy current losses. In addition, the influence of three-phase transformer connection on the distribution system neutral current is examined. In the past ten years several methods of power factor improvement through harmonic elimination have been developed that use high frequency switches to actively shape the input current. A new single- phase harmonic reduction circuit based on the voltage- doubler circuit is presented that uses a switch operated on line-frequency. Significant reduction in the THD of the supply current is achieved with the proposed method. Theoretical, simulation and laboratory waveforms of the input current and its harmonics are presented. The advantages and disadvantages of the proposed circuit are outlined.