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Motivated by the global need for greener energy, the exploration of renewable energies is on the rise. Due to their intermittent availability, inverters are increasingly used as interfaces to provide power conditioning. In contrast to the conventional power systems with fewer generators, numerous distributed inverter-interfaced renewable generation units must be connected to meet load demands. This fact requires a paradigmatic change in power system operation. One solution to facilitate this change are autonomous microgrids, which allow renewable powered systems to operate independent of the fossil-fuelled power grids. To achieve this, inverters must have grid-forming capabilities, operate in a synchronised manner and share power optimally. However, the absence of the stabilising inertia makes the control and stabilising of autonomous microgrids a complex problem. In this thesis, we consider the main control objectives associated with operating inverter-based microgrids in autonomous mode, which are frequency and voltage regulation, and power sharing. More precisely, we design grid-forming control policies which satisfy conditions for stability and achieve the aforementioned control objectives, and appropriate stability assessments are performed. This thesis is structured as follows. Considering the amount of literature on autonomous inverter-based microgrids, we start by reviewing models of different accuracy. In particular, an investigation of the accuracy of reduced models used to describe autonomous microgrids is performed relative to that of a detailed average model, and it is shown that various simplifications upon which the reduced models rely can impact their quality. Based on the findings, recommendations are given on the models to be used for microgrid stability assessment. The main results of the thesis are then presented and these are summarised below: (i) Passivity techniques are applied to the problem of stabilising an inverterbased microgrid. Representing the inverter and network models in the common reference frames, it is shown that microgrids can be described as the negative feedback interconnection of the two subsystems. Using the passivity framework, local conditions are imposed for stability at each inverter, the passivity properties of various existing control strategies for microgrids are assessed numerically, and improvement on these is discussed. Simulations on advanced inverter models verify the improved performance when the passivity property is satisfied. (ii) A new grid-forming control strategy is proposed, which satisfies a decentralised passivity condition that guarantees stability in the local reference frame. This proposed frequency scheme circumvents the nonlinearity in the feedback policy associated with active power-based frequency droop control, provides active power sharing without a communication link and strict passivity of the inverter model in the local reference frame. Associated conditions for power sharing, strict passivity and closed-loop asymptotic stability conditions are derived. DC and AC-side voltage control schemes which provide voltage regulation and allow the passivity properties of the inverter to be maintained are proposed. The benefit of the passivity-based design is demonstrated via the simulations of advanced inverter models. (iii) A new control architecture for frequency and voltage control with good scalability properties is proposed. At slower timescales, it allows to incorporate a distributed secondary control policy for which is provided an analytical stability result with line conductances taken into account. At faster timescales, it satisfies a passivity property in the common reference frame for a wide range of parameters. The voltage control scheme has a double loop structure that uses the DC voltage in the feedback control policy to improve performance through power imbalance elimination, and provides current limiting capability. The frequency control policy employs the inverter output current and angle to provide an improved angle droop policy. This scheme provides inverters with sufficient stabilising inertia and damping similar to the dynamical behaviour of synchronous generators. The performance of the control policy is illustrated via advanced simulations. (iv) The problem of synchronisation of droop-controlled inverters interconnected in lossy microgrids is considered by investigating conditions for which an equilibrium point exists for synchronisation. A reduced model of the interconnected inverters is derived via singular perturbation analysis and the behaviour of the boundary layer model is investigated. Using the reduced model sufficient conditions for the existence of an equilibrium point are derived. In particular, computable 2-norm and ∞-norm sufficient conditions are derived given that the bus angle differences are sufficiently small. These conditions take into account line conductances in contrast to those in the literature. Also, results on the worst synchronisation rate and the expanded Laplacian of the lossy network are presented. The conservativeness of the proposed 2-norm condition is compared to that of the ∞-norm condition numerically using IEEE test cases.

This thesis contributes to improve the photovoltaic Distributed Generation (DG) systems by proposing three novel methods to the system. On DC conversion side, a new integrated magnetic structure for interleaved converter and a new method to calculate the eddy current and hysteresis losses in the magnetic core were proposed. On inversion side, A new synchronisation method for grid tie inverters was suggested. The technique is using the Recursive Discrete Fourier Transform (RDFT) to find fundamental in grid waveform. On the DC converter side, the benefits of the new structure is to produce magnetic flux that alternate in the core across both directions of the BH curve. The advantages of alternating magnetic flux are, to increase the Root Mean Square (RMS) value of produced current with respect to core volume that lead to reduce the core size and reducing losses by using high permeability material. Furthermore, the proposed structure led to reduce the number of magnetic components which helped to improve the efficiency. The converter was tested and evaluated were the results show that the topology is able to produce high gain and it shows that the new interleaved structure is efficient. A new method to calculate the eddy current loss was proposed, where the flux waveform in the core was analysed to its original frequency component. Each of the components were utilized individually to find the loss. The effect of changing the duty cycle of the converter was taken into consideration on the total eddy current loss, as it will effect on the total harmonics content in the flux waveform. On the inverter side, due to recent developments combined with the increasing power demand by single phase non-linear loads where voltage spikes, harmonics and DC component were impacted the electric grid quality. These effects can likewise make the synchronisation process a challenge, where filters or Digital Signal processing (DSP) analysers are required to acquire the fundamental component as a consequence to the waveform deformation. A new linear approximation with RDFT is presented in this thesis for grid tie inverters. The new method provides a computation reduction as well as high accuracy in tracking the fundamental frequency in a distorted grid during synchronisation. The method accuracy was proved mathematically and simulated with different input signals. Error in magnitude and frequency measurement were measured, presented and compared with other research in order to verify the proposed method.

The main focus of this PhD thesis is fundamental investigations into control techniques of inverter-based microgrids. It aims to develop new and improved control techniques to enhance performance and reliability. It focuses on the modelling, stability analysis and control design of parallel inverters in a microgrid. In inverter-based microgrids, the paralleled inverters need to work in both grid-connected mode and stand-alone mode and should be able to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In stand-alone mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using droop control. Inverters can have different power set-points during grid-connected mode but in stand-alone mode they all need their power set-points to be adjusted according to their power ratings. However, during sudden unintentional islanding (due to loss of mains), transient power can flow from inverters with high power set-points to inverters with low power set-points, which can raise the DC link voltage of the inverters causing them to shut down. This thesis investigates the transient circulating power between paralleled inverters during unintentional islanding and proposes a controller to limit it. The controller monitors the DC link voltage and adjusts the power set-point in proportion to the rise in the voltage. A small signal model of an island microgrid has been developed and used to design the controller. The model and the controller design have been validated by simulation and practical experimentation. The results confirmed the performance of the proposed controller for limiting the DC link voltage and supporting a seamless mode transfer. The limitation of the droop controller, that is utilized to achieve load sharing between parallel-operated inverters in island mode, has also been addressed. Unequal output impedances among the distribution generation (DG) units lead to the droop control being inaccurate, particularly in terms of reactive power sharing. Many methods reported in the literature adopt low speed communications to achieve efficient sharing. However, the loss of this communication could lead to inaccuracy or even instability. An improved reactive power-sharing controller is proposed in this thesis. It uses the voltage at the point of common coupling (PCC) to estimate the inductance value of the output impedance including the impedance of the interconnecting power cables and to readjust the voltage droop controller gain accordingly. In an island microgrid consisting of parallel-connected inverters, the interaction between an inverter’s output impedance (dominated by the inverter’s filter and voltage controller) and the impedance of the distribution network (dominated by the other paralleled inverters’ output impedances and the interconnecting power cables) might lead to instability. This thesis studies this phenomenon using root locus analysis. A controller based on the second derivative of the output capacitor voltage is proposed to enhance the stability of the system. Matlab simulation results are presented to confirm the validity of the theoretical analysis and the robustness of the proposed controller. A laboratory-scale microgrid consisting of two inverters and local load has been built for the experimental phase of the research work. A controller for a voltage source inverter is designed and implemented. A dSPACE unit has been used to realize the controller and monitor the system in real time with the aid of a host computer. Experimental results of the two voltage source inverters outputs are presented.

Conventional 'hard switching' converters suffer from significant switching loss due to the simultaneous imposition of high values of current and voltage on the devices during commutation. Resonant converters offer a solution to this problem. A review of resonant circuit topologies is presented, which includes a summary of the interference problems which may occur when using power converters in the rail traction environment. Particular attention is given to the Resonant DC Link Inverter (RDCLI) which shows a great deal of pronuse using currently available devices. The frequency domain simulation of RDCLIs is discussed as a means of rapidly evaluating circuit behaviour, especially in relation to modulation strategies. A novel modulation strategy is proposed for Resonant DC Link Inverters, based on a procedure known as Simulated Annealing which allows complex harmonic manipulations such as han-nonic minimisation, to be performed. This is despite the fact that RDCLIs are constrained to use Discrete Pulse Modulation whereby switch commutations are restricted to specific moments in time. The modulation algorithms were verified by use of a low-power test rig and the results obtained are compared against theoretical values. Details of the hardware implementation are also included. A single-phase pulse-converter input stage is described which may be incorporated into the Resonant DC Link Inverter topology. This input stage also benefits from soft-sVVItching and allows four-quadrant operation at any desired power factor. A modulation scheme based on SiMulated Annealing is proposed for the pulse-converter, to achieve hannomc control whilst also synchronising with the supply wavefon-n. Practical results are presented and compared with those obtained by simulation and calculation. Finally the design of Resonant DC Link Converters is discussed and reconunendations made for the choice of resonant components based on the minimisation of overall losses. Comparisons are made between hard-switching and soft-switching converters in terms of loss and harmonic performance, in an attempt to quantify the benefits which may be obtained by the application of soft-switching.

Electricity generation from renewable energy sources has significantly increased in recent years, and they are projected to become the primary power sources in the near future. Power electronic converters are necessary for converting and integrating this power into the grid and are equipped with advanced controlling techniques for faster responses. The absence of control and visibility over these systems by operators leads to issues in power distribution, particularly during grid disturbances. Researchers are focusing on understanding the impacts of grid disturbances on distributed photovoltaic (DPV) inverters to ensure a reliable network. Although technical standards provide guidance on DPV inverters for expected performance and behaviour, some guidelines are unclear about different grid disturbances. This thesis aims to provide extensive experimental evidence on the behaviour of residential DPV inverters under different grid disturbance conditions. The undesirable behaviour of DPV inverters is classified, and the reasons behind these behaviours are identified. This thesis identifies and classifies the undesirable behaviours presented by DPV inverters in response to grid disturbances, such as fast voltage sag and voltage phase angle jump (VPAJ). These behaviours are undesirable because various inverters respond differently to the same grid disturbance. An extensive experimental study was conducted on 31 commercially available off-the-shelf DPV inverters. The results provide insights into the undesirable behaviours exhibited by inverters and their potential impacts on the power system. This study classified the behaviours of DPV inverters as: 1) ride-through, 2) power curtailment, and 3) disconnection. Statistical analysis showed that a large proportion of installed inverters could exhibit undesirable behaviours that could negatively impact the operation and stability of the power system. The outcomes from this contribution have been far-reaching, including a necessary update to the Australian standard for grid-connected inverters. Further, the causes behind these undesirable behaviours of DPV inverters are classified into disconnections and power curtailments to a similar VPAJ grid disturbance. The inverters’ misinterpretation of VPAJ disturbance caused these undesirable behaviours. The outcomes from this research provided a necessary update to the Australian standard for grid-connected inverters by including emergency voltage disturbance ride-through tests enforced in South Australia and provided essential behaviours and data for a load-photovoltaic composite model for the Australian Energy Market Operator. Finally, simulation and experimental investigations are conducted to identify the impacts of grid disturbances occurring at a specific point-on-wave. The study aimed to identify the points on the voltage waveform that are most susceptible to VPAJ disturbance. A sensitivity analysis is conducted to identify the vulnerability of inverters for VPAJ that occur at different points-on-wave. The results of this study help identify the points on the voltage waveform that are most prone to VPAJ disturbances and understand the appropriate response that inverters should exhibit during such disturbance and at-risk generation scale. The research aims to demonstrate the importance of understanding and providing information about inverters’ responses to different grid faults because of the constantly evolving grid under high penetration from distributed energy resources. Such types of behaviour potentially threaten the operation of highly-penetrated systems because a large portfolio of generation can suddenly disconnect, leaving a large contingency to be satisfied.

In the United States, smart PV inverters integrated with residential distribution systems are becoming a more common occurrence. With integration of smart PV inverters, power utilities are experiencing an increase of number of operations with regards to switched capacitor banks, voltage regulators and on load tap changers. These increases can lead to excess wear and tear on the devices causing power utilities to perform unwanted replacement and maintenance. However, smart PV inverters when controlled under specific functions can enable these inverters to provide reactive power and voltage control which in turn lowers the number of operations for switched capacitor banks, voltage regulators and on load tap changers. Furthermore, the standard basis is that when implementing Unbalanced Residential Distribution Systems into the grid, centralized control is a well-known choice, however, decentralized control provides a strong case for usage when using smart PV inverters in residential distribution systems. The objective of this thesis is to provide a better understanding of Unbalanced Residential Distribution Systems tied into the distribution side of the power grid when using control functions. Furthermore, better understand and prove the theory of using decentralize control for smart PV inverters in a residential distribution system. The future work will be analyzing the role of restoration practices and islanded mode with control algorithms that are used in grid connected mode. The specific areas below will be discussed in this thesis

Integration of inverter-based resources (IBRs) such as solar photovoltaic, wind, and battery storage, is both a boon and a bane for electric power systems. On one hand, IBRs have helped in making electrical energy a clean (carbon-free) source of energy. On other hand, the dynamics of IBRs have changed the way power system studies have been carried out. With the advantages IBRs offer over conventional resources, it is assumed that a small distribution power system (microgrid) will have a 100% IBRs penetration in the future. Such a microgrid can operate in grid-connected mode or in an islanded mode. Both modes of operation bring challenges to system stability due to faster control dynamics of IBRs and can pose problems such as blackouts if the dynamics are not studied and mitigated properly. IBRs in a microgrid can operate in a grid-following (GFL) mode or grid-forming mode (GFM). We studied the effect of GFM inverters, GFL inverters, and elements of microgrid (such as lines, and transformers) on microgrid stability. We studied how solar radiation affects the bus voltage and how the inverter gains and line impedance can make a system unstable. Based on the study stable limits for a single inverter connected to a strong point of common coupling (PCC) were established. Since the study of a microgrid is still an evolving topic it has been observed that some elements such as a transformer are not modeled in detail for the stability study of the system. It was shown how detailed modeling of different transformer models will help in understanding the cause of high frequencies in the system following a disturbance. Droop type GFM inverters mimic the droop characteristics of the synchronous generator to control the frequency and voltage. Droop control is best suited for a power system where lines are highly inductive which is not the case for lines of a microgrid that are highly resistive. This results in a weak microgrid system. A modified primary control technique was developed for improving reactive power sharing between the inverters and thus making the microgrid system strong. Virtual Synchronous Generator (VSG) type of control of GFM inverters is another widely used control technique for GFM type of inverters. In an islanded microgrid GFM and GFL inverters work in parallel and interaction between these different types of inverter controls plays a huge role in system stability. Therefore, it is necessary to study the interactions of dynamics between GFM and GFL inverters to understand and define the stability limits of a microgrid system. We studied the inter-inverter dynamics in an islanded microgrid and demonstrated how in a deregulated market one inverter can aid another inverter to make the overall system more stable and reliable. A nonlinear study of inverters for large-signal stability was also performed for islanded microgrid system.

One of the greatest technological challenges of the world today is reducing the size and weight of the existing products to make them portable. Specifically, in electric vehicles such as electric cars, UAVs and aero planes, the size of battery chargers and inverters needs to be reduced so as to make space for more parts in these vehicles. Electromagnetic Interference (EMI) filters take up a more than 80 % of these power converters, the size of these filters can be reduced by pushing the switching frequency higher. High frequency operation (> 300 kHz) leads to a size in reduction of EMI filters though it also leads to an increase in switching losses thus compromising on efficiency. Thus, soft switching becomes necessary to reduce the losses, adding more electrical components to the converter to achieve soft switching is a common method. However, it increases the physical complexity of the system. Hence, advanced control methods are adopted for today's power converters that enable soft switching for devices specifically ZVS turn-on as the turn-off losses of next generation WBG devices are negligible. Thus, the goal of this research is to discover novel switching algorithms for soft turn-on. The state-of the-art control methods namely CRM and TCM achieve soft turn-on by enabling bi-directional current such that the anti-parallel body diode starts conducting before the device is turned on. CRM and TCM result in variable switching frequency which leads to asynchronous operation in multi-phase and multi-converter systems. Hence, TCM is modified in this dissertation to achieve constant switching frequency, as the goal of this research is to be able to achieve ZVS turn-on for a three-phase converter. Further, Triangular Current Mode (TCM) to achieve soft switching and phase synchronization for three-phase two-level converters is proposed. It is shown how soft switching and sinusoidal currents can be achieved by operating the phases in a combination of discontinuous conduction mode (DCM), TCM and clamped mode. The proposed scheme can achieve soft switching ZVS turn-on for all the three phases. The algorithm is tested and validated on a GaN converter, 99% efficiency is achieved at 0.7 kW with a density of 110 W/in3. The discussion of TCM in current literature is limited to unity power factor assumption, however this limits the algorithm's adoption in real world applications. It is shown how proposed TCM algorithm can be extended to accommodate phase shift with all the three phases operating in a combination of DCM+TCM+Clamped modes of operation. The algorithm is tested and validated on a GaN converter, 99% efficiency is achieved at 0.7 kVA with a density of 110 W/in3. TCM operation results in 33 % higher rms current which leads to higher conduction losses, as WBG devices have lower on-resistance, these devices are the ideal candidates for TCM operation, hence to accurately obtain the device parameters, a detailed device characterization is performed. Further, proposed TCM+DCM+Clamped control algorithm is extended to three-level topologies, the control is modified to extract the advantage of reduced Common Mode Voltage (CMV) switching states of the three-level topology, the switching frequency can thus be pushed to 3 times higher as compared to state-of-the-art SVPWM control while maintaining close to 99 % efficiency. Two switching schemes are presented and both of them have a very small switching frequency variation (6%) as compared to state-of-the-art methods with >200% switching frequency variation.

The finite control set model predictive control (FCS-MPC) for motor drives has been vigorously investigated during the past decade for control of current, torque, stator flux linkage, and other performances in a motor drive system. Model predictive torque control (MPTC) and flux control (MPFC) are two popular categories of FCS-MPC for motor drives. In FCS-MPTC, a finite number of possible voltage vectors are evaluated by a cost function in an iterative prediction loop. The cost function includes several control objectives, such as minimization of torque and flux errors, the neutral-point voltage of multi-level inverters and inverter switching frequency. The control algorithm determines the optimal voltage vector that minimizes the pre-defined cost function. Each variable included in the cost function has a weighting factor according to different magnitudes and units. Weighting factor tuning is a non-trivial and complicated task, particularly when the control algorithm has two control-objectives which are prime variables i.e., torque and flux. In such a case, the control algorithm may choose a global optimal solution for torque which will be a sub-optimal solution for flux. In general, a conventional FCS-MPTC algorithm has a large computational burden due to a large number of available voltage vectors as in multi-level inverters or discrete SVM MPC technique and B) the presence of weighting factors in the cost function with several control objectives. This thesis developed a two-stage optimization-based FCS-MPC algorithm for an induction motor drive that uses reduced voltage control sets (RVCS) to evaluate the pre-defined cost function in the prediction loop. The proposed algorithm lowers the computational burden of the controller in two cascaded stages for a three-level neutral-point clamped voltage source inverter (3L-NPC VSI) fed IM drive. The voltage vector selection in the first stage of the proposed two-stage algorithm is executed using two different approaches: six long voltage vectors in the first stage and three long voltage vectors in the first stage. The first approach presented in Chapter 3 evaluates all six long voltage vectors in the first stage to obtain a long optimal voltage vector. In the second stage, the nearest 11 voltage vectors of the optimal long voltage vector are evaluated to reach the final optimal voltage vector. The cost function values from both stages are compared to each other to select the final optimal voltage vector. In chapter 4, the second approach evaluates three long voltage vectors instead of 6 in the first stage. It uses the sign of stator flux deviation and the position of stator flux to select the voltage vectors in the first stage. The second stage of this flux-error based approach is the same as Chapter 3. The proposed FCS-MPC two-stage optimization algorithms of Chapter 3-5 for an IM drive reduces the computational burden of the conventional algorithm which evaluates all available 27 voltage vectors. In Chapter 3, the proposed algorithm’s cost function combined torque and flux as prime control variables along with some other constraints i.e. neutral point voltage balance, switching frequency reduction, over current protection. This proposed algorithm reduced the computational burden of the conventional algorithm from 27 to 17 but suffered from sub-optimal decisions. The sub-optimality occurs when the reduced control set of the voltage vectors does not include the optimal vector and therefore the algorithm does not select the same voltage vector as the conventional all voltage vector-based MPC would have selected. The issue from Chapters 3 and 4 led to the development of a low-complexity finite control set model predictive flux control (FCS-MPFC) algorithm presented in Chapter 5 which eliminates the flux weighting factor in the cost function. A reference stator flux vector calculator (RSFVC) with an inner proportional-integral torque regulator is incorporated into the proposed two-stage optimization-based algorithm to convert the torque and flux amplitude references into an equivalent stator flux reference vector. The stator flux reference vector is then included in the cost function for flux deviation calculation. This allowed the elimination of flux weighting factor and elimination of flux-error based look-up table used in the first stage (the second approach) and solved the sub-optimality issue of the two-stage optimization-based FCS-MPTC algorithms. The developed FCS-MPFC algorithm experimentally demonstrated that this algorithm can be used for an induction motor drive to accurately control the electromagnetic torque, stator flux, and NPV with zero sub-optimal decision and reduced computation burden. Hence the original performance of the conventional FCS-MPFC is preserved with a lower computational burden. Operation with conventional FCS-MPC results in high torque and flux ripples in a 2-level inverter drive. Recently, MPC algorithms with integrated discrete space vector modulation (DSVM) for two-level inverter fed drives have been reported. DSVM technique creates a number of virtual voltage vectors using the real available voltage vectors. This allows for better optimization and minimization of the torque and flux errors/ripple due to the virtually created sub-divisions in the space vector region. However, all conventional DSVM integrated with FCS-MPTC yields a very high and complex calculation load. Therefore, these techniques are not practical for a cost-effective drive system. This thesis also proposes a simplified DSVM based FCS-MPFC algorithm for the two-level inverter (2L-VSI) fed IM drive system. This technique applies optimal voltage vectors that yield the desired zero sub-optimality and therefore the performance of the original DSVM-algorithm is retained. Lastly, a switching frequency reduction technique for the developed DSVM based FCS-MPFC is proposed which generates a minimum number of switching transitions by an effective synthesis of the virtual voltage vectors. This reduced switching is effective in any operating region for the 2L-VSI fed IM drive. This achieves low switching frequency of the inverter while preserving the same performances of the drive as the conventional algorithm. All the proposed methods which achieved zero sub-optimality have been experimentally tested in parallel with their conventional forms to detect the sub-optimality and compared with recently published algorithms to demonstrate their superior performances in a wide operating range.

The power generation capacities based on solar and wind resources have grown significantly over the last decade because of the successful implementation of national policies to reduce greenhouse gas emissions and stimulate the integration of renewable energy sources. These distributed generation units are typically connected to the medium and low voltage level of the distribution network using power electronic interfaces (also known as inverters) and can be grouped in clusters operating as an Inverter based Microgrid (IMG). Multiple benefits might result from the IMG implementation in the current power system; however, it implies the need to deal with new implementation and operation problems resulting from the integration of inverter-based distributed generators. Although considerable research has been reported on the IMG area, multiple challenges require further research to allow a feasible integration of the IMG concept. The work presented in this thesis is directed towards addressing this gap in the literature focusing on the current IMG modelling and control challenges. In the modelling area, the cyber-physical characteristics of IMGs are considered and using a cyber-physical modelling methodology multiple IMG models are developed for different power-sharing controllers including a novel primary control approach proposed in this work. Next, the accurate proportional power-sharing problem is tackled as one of the essential control requirements for the IMG islanded operation, and a novel power sharing control scheme is proposed based on a Direct Current Droop Primary Controller (DCD) and a Consensus-based Secondary Controller (CBQC). This novel scheme achieves accurate proportional active and reactive power sharing in practical scenarios with impedance feeder mismatches, unknown network parameters and with low inter-unit communication requirements. Finally, the economic operation of the IMG is addressed where the control objective is to minimize the total generation IMG cost guarantying the generation and demand power balance. Addressing this challenge three novel energy management systems (EMSs) are proposed. These novel EMSs achieve optimal operation in real-time using simple algorithms and low-bandwidth centralized or distributed communications, and under strategic scenarios with information sharing restrictions. Apart from these major contributions, additional results on modelling and control are also proposed for the grid-connected operation.

This thesis compares the performance of a nine-level transformerless cascaded H-bridge (CHB) inverter with integrated battery energy storage system (BESS) using SiC power MOSFETs and Si IGBTs. Two crucial performance drivers for inverter applications are power loss and efficiency. Both of these are investigated in this thesis. Power devices with similar voltage and current ratings are used in the same inverter topology, and the performance of each device is analyzed with respect to switching frequency and operating temperature. The loss measurements and characteristics within the inverter are discussed. The Saber® simulation software was used for the comparisons. The power MOSFET and IGBT modeling tools in Saber® were extensively utilized to create the models of the power devices used in the simulations. The inverter system is also analyzed using Saber-Simulink cosimulation method to feed control signals from Simulink into Saber. The results in this investigation show better performances using a SiC MOSFET-based grid-connected BESS inverter with a better return of investment.

Rapid advances in the field of integrated circuit design has been advantageous from the point of view of cost and miniaturization. Although technology scaling is advantageous to digital circuits in terms of increased speed and lower power, analog circuits strongly suffer from this trend. This is becoming a crucial bottle neck in the realization of a system on chip in scaled technology merging high-density digital parts, with high performance analog interfaces. This is because scaled technologies reduce the output impedance (gain) and supply voltage which limits the dynamic range (output swing). One way to mitigate the power supply restrictions is to move to current mode circuit circuit design rather than voltage mode designs. This thesis focuses on designing Process Voltage and Temperature (PVT) tolerant base band circuits at lower supply voltages and in lower technologies. Inverter amplifiers are known to have better transconductance efficiency, better noise and linearity performance. But inverters are prone to PVT variations and has poor CMRR and PSRR. To circumvent the problem, we have proposed various biasing schemes for inverter like semi constant current biasing, constant current biasing and constant gm biasing. Each biasing technique has its own advantages, like semi constant current biasing allows to select different PMOS and NMOS current. This feature allows for higher inherent inverter linearity. Similarly constant current and constant gm biasing allows for reduced PVT sensitivity. The inverter based OTA achieves a measured THD of -90.6 dB, SNR of 78.7 dB, CMRR 97dB, PSRR 61 dB wile operating from a nominal power of 0.9V and at output swing of 0.9V{pp,diff} in TSMC 40nm general purpose process. Further the measured third harmonic distortion varies approximately by 11.5dB with 120C variation in temperature and 9dB with a 18% variation in supply voltage. The linearity can be increased by increasing the loop gain and bandwidth in a negative feedback circuit or by increasing the over drive voltage in open loop architectures. However both these techniques increases the noise contribution of the circuit. There exist a trade off between noise and linearity in analog circuits. To circumvent this problem, we have introduced nonlinear cancellation techniques and noise filtering techniques. An analog-to-digital converter (ADC) driver which is capable of amplifying the continuous time signal with a gain of 8 and sample onto the input capacitor(1pF) of 1 10 bit successive approximation register (SAR) ADC is designed in TSMC 65nm general purpose process. This exploits the non linearity cancellation in current mirror and also allows for higher bandwidth operation by decoupling closed loop gain from the negative feedback loop. The noise from the out of band is filtered before sampling leading to low noise operation. The measured design operates at 100MS/s and has an OIP_3 of 40dBm at the nyquist rate, noise power spectral density of 17nV/sqrt{Hz} and inter modulation distortion of 65dB. The intermodulation distortion variation across 10 chips is 6dB and 4dB across a temperature variation of 120C. Non linearity cancellation is exploited in designing two filters, an anti alias filter and a continuously tunable channel select filter. Traditional active RC filters are based on cascade of integrators. These create multiple low impedance nodes in the circuit which results in a higher noise. We propose a real low pass filter based filter architecture rather than traditional integrator based approach. Further the entire filtering operation takes place in current domain to circumvent the power supply limitations. This also facilitates the use of tunable non linear metal oxide semiconductor capacitor (MOSCAP) as filter capacitors. We introduce techniques of self compensation to use the filter resistor and capacitor as compensation capacitor for lower power. The anti alias filter designed for 50MHz bandwidth is fabricated in IBM 65nm process achieves an IIP3 of 33dBm, while consuming 1.56mW from 1.2 V supply. The channel select filter is tunable from 34MHz to 314MHz and is fabricated in TSMC 65nm general purpose process. This filter achieves an OIP3 of 25.24 dBm at the maximum frequency while drawing 4.2mA from 1.1V supply. The measured intermodulation distortion varies by 5dB across 120C variation in temperature and 6.5dB across a 200mV variation in power supply. Further this filter presents a high impedance node at the input and a low impedance node at the output easing system integration. SAR ADCs are becoming popular at lower technologies as they are based on device switching rather than amplifying circuits. But recent SAR ADCs that have good energy efficiency have had relatively large input capacitance increasing the driver power. We present a 2X time interleaved (TI) SAR ADC which has the lowest input capacitance of 133fF in literature. The sampling capacitor is separated from the capacitive digital to analog converter (DAC) array by performing the input and DAC reference subtraction in the current domain rather than as done traditionally in charge domain. The proposed ADC is fabricated in TSMC's 65nm general purpose process and occupies an area of 0.0338 mm^2. The measured ADC spurious free dynamic range (SFDR) is 57dB and the measured effective number of bits (ENOB) at nyquist rate is 7.55 bit while using 1.55mW power from 1 V supply. A sub 1V reference circuit is proposed, that exploits the complementary to absolute temperature (CTAT) and proportional to absolute temperature (PTAT) voltages in the beta multiplier circuit to attain a stable voltage with temperature and power supply. A one-time calibration is integrated in the architecture to get a good performance over process. Chopper stabilization is employed to reduce the flicker noise of the reference circuit. The prototype was simulated in TSMC 65nm process and we obtain the nominal output of 236mW, while consuming 0.7mW from power supply. Simulations show a temperature coefficient of 18 ppmC from -40 to 100C and with a power supply ranging from 0.8 to 2V.

Small-signal stability is of great concern for distributed power systems with a large number of regulated power converters. These converters are constant-power loads (CPLs) exhibit a negative incremental input resistance within the output voltage regulation bandwidth. In the case of dc systems, design requirements for impedances that guarantee stability have been previously developed and are used in the design and specification of these systems. In terms of three-phase ac systems, a mathematical framework based on the generalized Nyquist stability criterion (GNC), reference frame theory, and multivariable control is set forth for stability assessment. However, this approach relies on the actual measurement of these impedances, which up to now has severely hindered its applicability. Addressing this shortcoming, this research investigates the small-signal stability of three-phase ac systems using measured d-q frame impedances. Prior to this research, negative incremental resistance is only found in CPLs as a results of output voltage regulation. In this research, negative incremental resistance is discovered in grid-tied inverters as a consequence of grid synchronization and current injection, where the bandwidth of the phase-locked loop determines the frequency range of the negative incremental resistance behavior, and the power rating of inverter determines the magnitude of the resistance. Prior to this research, grid synchronization stability issue and sub-synchronous oscillations between grid-tied inverter and its nearby rectifier under weak grid condition are reported and analyzed using characteristic equation of the system. This research proposes a more design oriented analysis approach based on the negative incremental resistance concept of grid-tied inverters. Grid synchronization stability issues are well explained under the framework of GNC. Although stability and its margin of ac system can be addressed using source and load impedances in d-q frame, method to specify the shape of load impedances to assure system stability is not reported. This research finds out that under unity power factor condition, three-phase ac system is decoupled. It can be simplified to two dc systems. Load impedances can be then specified to guarantee system stability and less conservative design.

Electrical power is the bedrock of modern civilization, and the large-scale hierarchical structure of bulk generation, transmission, and distribution has served us well for more than one hundred years. Currently however, the landscape of energy production is shifting, as economic, environmental, and technological factors are pushing power generation towards a future dominated by distributed generation from renewable energy sources. While grid-wide control strategies and architectures in bulk power systems were designed for slow time-scales and large synchronous generators, renewable energy interfaced through power electronic inverters allows for rapid response and greater flexibility in both local controller design and grid-wide control architectures.This thesis focuses on exploring the limitations of both local controller design and control architectures for inverter-based power grids. Our contributions can be broadly divided into two categories. First, we study the classic primary droop controllers proposed for inverter-based power grids, and provide the first nonlinear analysis of the closed-loop frequency and voltage dynamics resulting from the controllers. We present tight analytic conditions for the existence and uniqueness of stable equilibrium points, thereby quantifying the fundamental limits of these controllers. In the second portion of the thesis, we propose and analyze a distributed control architecture which takes the place of the classical centralized secondary control layer. The distributed controllers combine classic droop ideas from power systems with agreement algorithms studied in cooperative control and multi-agent systems, leading to a scalable control architecture which achieves centralized performance. The algorithms require only sparse communication between nearby inverters in the power grid, and require no a priori knowledge of the grid topology or load demands. We present extensive analysis results, along with experimental results validating our designs, and outline directions for future research.

All power electronics consist of solid state devices that generate heat.  Managing the temperature of these devices is critical to their performance and reliability.  Traditional methods involving liquid-cooling systems are expensive and require additional equipment for operation.  Air-cooling systems are less expensive but are typically less effective at cooling the electronic devices.  The cooling system that is used depends on the specific application. Until recently, silicon based devices have been used for the solid-state devices in power electronics.  Newly developed silicon-carbide based wide band gap devices operate at maximum temperatures higher than traditional silicon devices.  Due to the permissible increase in operating temperatures, it has been proposed to develop an air-cooling system for an inverter consisting of silicon carbide devices.  This thesis presents recent research efforts to develop the proposed air-cooling system.  The thermal performance of the each design iteration was determined by numerical simulations via the finite element method in both steady state and transient mode using COMSOL Multi-physics software version 3.5a.  For all simulations presented in this thesis, the heat dissipated in the MOSFETS and diodes are specified as functions of current, voltage, switching frequency, and junction temperature.  For both the steady state and transient simulations, the junction temperature was determined iteratively.  Additionally in the transient simulations, the current distribution is a function of time and was deduced from the EPA US06 drive cycle.  After several design iterations, a thermally feasible design has been reached.  This design is presented in detail in this thesis.  Under transient simulations of the final design, the maximum junction temperatures were determined to be below 146 ºC for air flow rates of 30 and 60 CFM, which is substantially lower than the 250 ºC maximum allowable junction temperature of Si-C devices.  However for steady state simulations, junction temperatures were found to be much higher than the transient simulations.  It was determined that a minimum flow rate of 50 CFM is required to meet the temperature requirements of the Si-C devices under steady state operating conditions.  The power density of this air-cooled final design is 11.75 kW/L, and it is competitive with liquid-cooled systems.

Optimal force-commutated current-source ac/dc, voltage-source dc/ac, and chopper-type ac/ac converters with minimum input-current or output-voltage harmonics are proposed. The minimization of rectifier input-current harmonics or inverter output- voltage harmonics is achieved through numerical solution of optimal firing and extinction angles for the switches. The method is superior to the conventional harmonic elimination techniques, in terms of its harmonic distortion, operating zone, and flexibility. Conventional harmonic-elimination methods are compared with the proposed technique for ac/dc, dc/ac, and ac/ac single-phase converters. The distortion- minimization technique eliminates certain chosen harmonics while minimizing harmonic distortion in rectifier input current or inverter output voltage. Choosing fewer harmonics to be eliminated results in lower harmonic distortion; the lowest possible distortion results from allowing all harmonics to have non-zero values. A striking characteristic of the optimal solutions for the ac/dc, and ac/ac chopper converter is the absence of quarter- wave symmetry, resulting in either leading or lagging power factors of the converters. Other applications of the proposed method are: the ac/dc control of static VAR compensators, and active filters with minimum harmonic distortion. The SVC compensates reactive power demand and minimizes the harmonic distortion of the other phase-controlled converters as well as its own distortion. The active filter injects harmonics into the ac bus out of phase with existing ones, resulting in harmonic cancellation. The optimized injection has an advantage over the conventional harmonic elimination technique in terms of the lower total distortion and converter rating. In addition the optimization method can be applied to the cascaded, phase-controlled HVDC converters. An optimal firing strategy for N cascaded, 6- or 12-pulse converters of a HVDC link is proposed, in either unipolar or bipolar operation. Simultaneous control of the firing angles of the N converters (for both the inverters and rectifiers) can be utilized for either harmonics minimization, or reactive power demand reduction. With relatively low values of N (4 12-pulse, or 8 6-pulse), the resultant ac-current harmonics, or reactive power demand are substantially lower than for comparable 12- or 6-pulse converters. Also, the dc-voltage harmonics are greatly reduced over phase-control operation.

Several space vector modulation schemes have been analyzed for three-leg and four-leg voltage source inverters. The analysis is performed with respect to a) switching losses, b) total harmonic distortion, c) peak-to-peak ripple in the line current and d) the ease of digital implementation. The analysis is performed over the entire range of modulation index and for varying load power factors (leading and lagging) under both balanced and unbalanced load conditions. The analysis shows that the performance of four-leg inverters is similar to three-leg inverters for various space vector modulation schemes. The analysis also verifies the fact that a modulation scheme with good harmonic performance usually has high switching losses and vice-versa. The analysis is verified using simulation and experiments. A novel algorithm for the calculation of total harmonic distortion of PWM signals has been proposed.

With the advent of power electronics, inverter-driven induction motor are finding increased use in industries because of applications that demand variable speed operations and because of the potential savings in energy usage. However, these drives sometimes produce unacceptably high levels in vibration and acoustic noise. A literature survey has revealed that while there has been intensive research on the design of inverters to minimize acoustic noise radiation from these drives, the vibro-acoustic behaviour of an induction motor structure has received relatively little attention. The primary objective of this research project, therefore, is to develop a general strategy/algorithm for estimating the acoustic noise radiated from inverter-driven induction motors. By using a three-phase, 2.2 kW induction motor, the vibration modes due to various structural components (such as the rotor, the stator/casing, the endshields and the base plate) of the motor structure were analysed by experimental modal testing. Results indicate that the vibration modes due to the rotor are only important at low frequencies. It has been found that the power injection method gives more accurate measurement of the damping of a motor structure than the modal testing and the time decay methods. If a point force excitation is used, then it is more accurate to measure the sound radiation efficiency than the power conversion efficiency for motor structures. The effect of three different inverter designs (an ideal ‘almost sinusoidal’ controller and two commercially available PWM inverters) on the radiated acoustic power were assessed for both no-load and load conditions using sound intensity measurements conducted in an anechoic room. The results indicate that although the sound power level due to aerodynamic and mechanical noise increases at a rate of 12 dB per doubling of the motor speed, the electromagnetic noise dominates at low motor speeds and is still a significant noise source even at high motor speeds. For inverters with low switching frequencies, the radiated sound power level is almost 15 dB higher than the ideal case at low speeds and is relatively insensitive to the motor speed. For inverters that implement the random modulation technique, the change in the total sound power level with the level of the random modulation is very small but the tonal nature of the noise is greatly reduced. The vibration behaviour of a motor structure was modeled using the finite element method (FEM) and validated using the experimental modal testing results. It has been found that it is essential to model the laminated stator as an orthotropic structure. While the details of other structural components (such as the endshields, the teeth in the stator and the windings) are not so important, it is essential that they are incorporated into the structural model as simplified structures to account for their mass, stiffness and boundary conditions imposed on the motor structure. Based on this structural model, the radiated acoustic power for various operating conditions has been predicated using the boundary element (BEM) and the electromagnetic force calculated from an electromagnetic finite element model. The predicted results agree reasonably well with experimental measurements. Despite the success of the FEM/BEM approaches, they can be prohibitively expensive (in terms of computer resources required) to apply to large motors and high frequencies. Thus the feasibility of using a statistical method, namely, the statistical energy analysis (SEA), to estimate the radiated acoustic sound power from an inverter-driven induction motor has been examined. In order to carry out this analysis, analytical expressions for calculating the natural frequencies and radiation efficiency of finite length circular cylindrical shells (which are simplified models of the stator and casing of a motor structure) were firstly derived. The internal loss factors and coupling loss factors of the motor structure were determined experimentally using the power injection method. Then by introducing an equivalent surface mobility of circular cylindrical shells for the electromagnetic force, the vibration response and the acoustic noise radiated from each part of the motor structure were estimated. Results indicate that SEA method is potentially an efficient and effective tool in estimating the noise radiated from inverter-driven induction motors.

This thesis reviews in general the topic of electromagnetic compatibility, and electromagnetic interference and their origin and effects in modern electronically controlled motor drives. The measurement techniques for EMI noise are reviewed. The sources of noise of a switching power circuit are described. This thesis investigates the establishment of a procedure for measuring conducted emission produced by motor drives, using a virtual instrument. This procedure is based on the traditional methodology of EMI measurement and the use of simulation techniques. A test bench is designed. The thesis covers the detailed design of a virtual instrument for measuring conducted current produced by motor drives. A line impedance stabilisation network LISN is designed and built. A series of measurements were carried out using the developed instrument. The results show very little difference between the conducted emission produced by induction, permanent magnet and reluctance motor drives. Comparing one of the experimental results with published results from a major test laboratory assesses the validity of the designed instrument. The experimental results refer to drive systems under no load conditions. Useful conclusions are drawn and future research studies recommended.