Dissertations / Theses on the topic 'Single-phase grid system'

This thesis focuses on the single-phase voltage-source inverter for use in photovoltaic (PV) electricity generating systems in both stand-alone and grid-tied applications. In many cases, developments in single-phase PV systems have followed developments in three-phase systems. Time-variant systems are more difficult to control than time-invariant systems. Nevertheless, by using suitable transformation techniques, time-variant systems can often be modelled as time-invariant systems. After the transformation, the control signals that are usually time-variant (often varying sinusoidally in time) become time-invariant at the fundamental frequency, and are hence much easier to deal with. With this approach, synchronous rotating frame control techniques have been previously proposed for high performance three-phase inverter applications. The transformation theory cannot be applied directly in single-phase systems without modification, and the d-q components would not be time-invariant in situations where harmonics, resonances or unbalance is present. Single-phase inverter controller designs based on the use of a synchronous rotating reference frame have been proposed, but such designs do not always perform as well as expected. This thesis aims to improve single-phase voltage-source inverters. The main objective is to address, in terms of cost, efficiency, power management and power quality, the problems found with single-phase designs based on a synchronous rotating frame single-phase inverter controller. Consequently, this thesis focuses on a novel controller approach in order to obtain a more reliable and flexible single-phase inverter. As the first step, this thesis investigates the single-phase inverter switching gate-drive algorithms and develops a form of space-vector pulse-width-modulation (SVPWM) in order to reduce total harmonic distortion. The results of the new SVPWM algorithm demonstrate its superior performance when compared with sinusoidal pulse-width-modulation (SPWM) which is often used with single-phase inverters. The second step, which is further reviewed and presented in this thesis, is the modelling of the single-phase inverter control based on the synchronous rotating frame. A mathematical analysis is conducted to determine the mechanism of the coupling that exists between the voltage phase and amplitude terms, and a new transformation strategy is proposed based on using the voltage phase as a reference at the Park transformation stages, and the current phase as a reference for the current at the transformation stages. The line-frequency components of the feedback signals are transformed to time-invariant components, thus eliminating the ripple and reducing the computational burden associated with the controller stage. Consequently, the inverter feedback controller stage is designed so that the coupling terms are decoupled within the controller itself. The effectiveness of the techniques proposed in this thesis are demonstrated by simulation using the MATLAB/SIMULINK environment. The proposed technique was also investigated through a practical implementation of the control system using a Digital Signal Processor (DSP) and a single-phase inverter. This practical system was tested up to 1 kW only (limited by the available inverter hardware). Nevertheless, the correlation between the simulation and the practical results is high and this gives confidence that the developed mechanism will allow the 2.5kW goal to be achieved. Practical test cases illustrate the effectiveness of the models. In addition, the comparisons between experimental and simulation results permit the system's behaviour and performance to be accurately evaluated. With the development of the new controller, small-scale single-phase renewable energy systems will become more useful in the field of power quality management through their ability to separately control the phase and amplitude of the output voltage. Consequently, incorporation of this type of generator within the national electrical distribution network, as distributed generators (DG) at low-voltage level, can assist with power quality management at the consumer side of the grid. In addition, such a generator can also operate in stand-alone mode if the grid becomes unavailable. The third step in this thesis investigates small-scale single-phase renewable energy systems operating as decentralized distributed generators within a local network. This operation is achieved by controlling the inverter side using the quantities measured at the common coupling point between the grid and the inverter, without requiring other extensive communications. Thus, the small-scale single-phase renewable energy distributed generator systems will contain only a local controller at each installation.

Multilevel inverters offer many well-known advantages for use in high-voltage and high-power applications, but they are also well suited for low-power applications. A single phase inverter is developed in this paper to deliver power from a residential-scale system of Photovoltaic panels to the utility grid. The single-stage inverter implements a novel control technique for the reversing voltage topology to produce a stepped output waveform. This approach increases the granularity of control over the PV systems, modularizing key components of the inverter and allowing the inverter to extract the maximum power from the systems. The adaptive controller minimizes harmonic distortion in its output and controls the level of reactive power injected to the grid. A computer model of the controller is designed and tested in the MATLAB program Simulink to assess the performance of the controller. To validate the results, the performance of the proposed inverter is compared to that of a comparable voltage-sourced inverter.

The number of distributed power generation systems (DPGSs), mostly based on photovoltaic (PV) energy sources is increasing exponentially. These systems must conform to grid codes to ensure appropriate power quality and to contribute to grid stability. A robust and reliable synchronization to the grid is an important consideration in such systems. This is due to the fact that, fast and accurate detection of the grid voltage parameters is essential in order to implement stable control strategies under a broad range of grid conditions. The second-order generalized integrator (SOGI) based phase-locked loop (PLL) is widely used for grid synchronization of single-phase power converters. This is because it offers a simple, robust and flexible solution for grid synchronization. However, the SOGI-PLL is affected by the presence of a dc offset in the measured grid voltage. This dc voltage offset is typically introduced by the measurements and data conversion process, and causes fundamental-frequency ripple in the estimated parameters of the grid voltage (i.e. voltage amplitude, phase angle and frequency). In addition to this ripple, the unit amplitude sine and cosine signals of the estimated phase angle (i.e. unit vectors), that are used to generate reference signals in the closed-loop control of grid-connected PV converters will contain dc offset. This is highly undesirable since it can cause dc current injection to the grid, and as a consequence, the quality of the power provided by the DPGSs can be degraded. To overcome this drawback, a modified SOGI-PLL with dc offset rejection capability is proposed. The steady-state, transient and harmonic attenuation performance of the proposed PLL scheme are validated through simulation and experimental tests. The overall performance demonstrates the capability of the proposed PLL to fully reject such dc current injection as well as to provide a superior harmonic attenuation when compared with the SOGIPLL and two other existing offset rejection approaches. It is shown that, the proposed PLL scheme can enhance the overall total harmonic distortion (THD%) of the injected power by 15% when compared to the conventional SOGI-PLL. In addition to the synchronization, grid-connected PV systems require a current control scheme to regulate the output current. Due to the simple implementation, proportional-integral (PI) controllers in the stationary reference frame are commonly used for current controlled inverters. However, these PI-controllers exhibit a major drawback of failure to track a sinusoidal reference Abstract ii without steady-state error, which may result in low-order harmonics. This drawback can be overcome if the PI-controllers are implemented in direct-quadrature (dq) rotating reference frame. In single-phase systems, the common approach is to create a synthesized phase signal orthogonal to the fundamental of the real single-phase system so as to obtain dc quantities by means of a stationary-to-rotating reference frame. The orthogonal synthesized signal in conventional approaches is obtained by phase shifting the real signal by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. This thesis proposes an alternative way of implementing such PI-controllers in the dq reference frame without the need of creating such orthogonal signals. The proposed approach, effectively improves the poor dynamic of the conventional approaches while not adding excessive complexity to the controller structure. The results show that, in addition to its ability to regulate the current and achieve zero steady-state error, the proposed approach shows superior dynamic response when compared with that of conventional delay-based approach.

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.

Within recent years, interest in the installation of solar-based, wind-based, and various other renewable Distributed Energy Resources (DERs) and Energy Storage (ES) systems has risen; in part due to rising energy costs, demand for cleaner power generation, increased power quality demands, and the need for additional protection against brownouts and blackouts. A viable solution for these requirements consists of installation of small-scale DER and ES systems at the single-phase (1Φ) distribution level to provide ancillary services such as peak load shaving, Static-VAr Compensation (STATCOM), ES, and Uninterruptable Power Supply (UPS) capabilities through the creation of microgrid systems. To interconnect DER and ES systems, power electronic converters are needed with not only control systems that operate in multiple modes of operation, but with islanding detection and resynchronization capabilities for isolation from and reclosure to the grid. The proposed system includes control architecture capable of operating in multiple modes, and with the ability to smoothly transfer between modes. Phase-Locked Loops (PLLs), islanding detection schemes, and resynchronization protocols are developed to support the control functionality proposed. Stationary frame PLL developments proposed in this work improve upon existing methods by eliminating steady-state noise/ripple without using Low-Pass Filters (LPFs), increasing frequency/phase tracking speeds for a wide range of disturbances, and retaining robustness for weakly interconnected systems. An islanding detection scheme for the stationary frame control is achieved through the stability of the PLL system interaction with the converter control. The proposed detection method relies upon the conditional stability of the PLL controller which is sensitive to grid-disconnections. This method is advantageous over other methods of active islanding detection mainly due to the need for those methods to perturb the output to test for islanding conditions. The PLL stability method does not inject signal perturbations into the output of the converter, but instead is designed to be stable while grid-connected, but inherently unstable for grid-disconnections. Resynchronization and reclosure to the grid is an important control aspect for microgrid systems that have the ability to operate in stand-alone, backup modes while disconnected from the grid. The resynchronization method proposed utilizes a dual PLL tracking system which minimizes voltage transients during the resynchronization process; while a logic-based reclosure algorithm ensures minimal magnitude, frequency, and phase mismatches between the grid and an isolated microgrid system to prevent inrush currents between the grid and stand-alone microgrid system.<br>Ph. D.

DC power distribution has gained popularity in sustainable buildings, renewable energy utilization, transportation electrification and high-efficiency data centers. This dissertation focuses on two aspects of facilitating the application of dc systems: (a) system-level control to improve load sharing, voltage regulation and efficiency; (b) design of a high-efficiency interface converter to connect dc microgrids with the existing low-voltage ac distributions, with a special focus on common-mode (CM) voltage attenuation. Droop control has been used in dc microgrids to share loads among multiple sources. However, line resistance and sensor discrepancy deteriorate the performance. The quantitative relation between the droop voltage range and the load sharing accuracy is derived to help create droop design guidelines. DC system designers can use the guidelines to choose the minimum droop voltage range and guarantee that the sharing error is within a defined range even under the worst cases. A nonlinear droop method is proposed to improve the performance of droop control. The droop resistance is a function of the output current and increases when the output current increases. Experiments demonstrate that the nonlinear droop achieves better load sharing under heavy load and tighter bus voltage regulation. The control needs only local information, so the advantages of droop control are preserved. The output impedances of the droop-controlled power converters are also modeled and measured for the system stability analysis. Communication-based control is developed to further improve the performance of dc microgrids. A generic dc microgrid is modeled and the static power flow is solved. A secondary control system is presented to achieve the benefits of restored bus voltage, enhanced load sharing and high system efficiency. The considered method only needs the information from its adjacent node; hence system expendability is guaranteed. A high-efficiency two-stage single-phase ac-dc converter is designed to connect a 380 V bipolar dc microgrid with a 240 V split-phase single-phase ac system. The converter efficiencies using different two-level and three-level topologies with state-of-the-art semiconductor devices are compared, based on which a two-level interleaved topology using silicon carbide (SiC) MOSFETs is chosen. The volt-second applied on each inductive component is analyzed and the interleaving angles are optimized. A 10 kW converter prototype is built and achieves an efficiency higher than 97% for the first time. An active CM duty cycle injection method is proposed to control the dc and low-frequency CM voltage for grounded systems interconnected with power converters. Experiments with resistive and constant power loads in rectification and regeneration modes validate the performance and stability of the control method. The dc bus voltages are rendered symmetric with respect to ground, and the leakage current is reduced. The control method is generalized to three-phase ac-dc converters for larger power systems.<br>Ph. D.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior<br>Este trabalho apresenta o estudo, desenvolvimento e aplicação de novas técnicas de controle aos sistemas de geração de energia fotovoltaica, conectados à rede elétrica monofásica, visando a melhoria da eficiência destes sistemas em relação aos métodos de controle tipicamente utilizados. O trabalho é dividido em duas partes principais, as quais estão relacionadas ao rastreamento do ponto de máxima potência (Maximum Power Point Tracking – MPPT) e, depois, ao controlador interno dos conversores boost-quadrático – lado dos painéis fotovoltaicos – assim como aos controladores do inversor de tensão, no lado da rede elétrica monofásica. A primeira parte do trabalho consiste no desenvolvimento de um algoritmo de meta heurística para o MPPT, o qual é baseado no método do recozimento simulado (Simulated Annealing - SA) e tem como objetivo a determinação do ponto de máximo global, buscando soluções fora de uma vizinhança próxima, de modo a evitar o atracamento em máximos locais da curva de potência de saída do arranjo de painéis fotovoltaicos, melhorando, assim, o aproveitamento da energia em situações de sombreamento parcial. A segunda parte do trabalho apresenta o projeto de controladores baseados no modelo interno (Internal Model Control – IMC) com 1 grau de liberdade (1 Degree of Freedom – 1 DOF) aplicados, primeiramente, ao conversor CC-CC Boost Quadrático, utilizado para o MPPT, e, posteriormente, a um inversor de tensão com filtro LCL, conectado à rede elétrica monofásica, operando no modo de injeção de corrente. É apresentada a modelagem matemática de ambos os conversores e analisada a compensação da realimentação interna ao sistema do inversor de tensão, correspondente ao desacoplamento da tensão da rede, através da estratégia de alimentação à frente (Feedforward). Visando-se uma base de comparação, para a análise de desempenho do conversor IMC – 1DOF, também é aplicado ao sistema de injeção de corrente, o controlador Proporcional-Ressonante (P+Res). São apresentados resultados de simulação computacional e experimentais de ambos os 8 controladores, os quais permitem verificar o desempenho do sistema em situação de rede fraca e carga local não-linear.<br>This paper presents the study, development and application of new control techniques for photovoltaic power generation systems, connected to single-phase power grid in order to improve the efficiency of these systems in relation to control methods typically used. The work is divided into two main parts, which are related to tracking the maximum power point (Maximum Power Point Tracking - MPPT) and then the internal controller of boost quadratic converters - side of the photovoltaic panels - as well as the controllers the voltage inverter, the side of the single-phase mains. The first part of the work is the development of a heuristic goal algorithm for MPPT, which is based on the method of simulated annealing (Simulated Annealing - SA) and aims at determining the overall maximum point, seeking out solutions in a close vicinity, so as to prevent the docking local maxima in the curve of power output of the photovoltaic array, thereby improving the utilization of energy in partial shading situations. The second part presents the design of controllers based on internal model (Internal Model Control - IMC) with 1 degree of freedom (1 Degree of Freedom - 1 DOF) applied, first, the DC-DC Boost Quadratic converter, used for MPPT, and thereafter, a voltage inverter with LCL filter connected to the single phase power supply operating in current injection mode. A mathematical modeling of both converters and analyzed the compensation of internal feedback to the voltage inverter system corresponding to the disconnection of the mains voltage through the power strategy a head (feedforward). A basis of comparison for the performance analysis of IMC-1DOF converter is aiming at, is also applied to the current injection system, Proportional-Resonant Controller (P + Res). They are presented computer simulation and experimental results of both controllers, which allow you to check the system performance in low and non-linear local load network situation.<br>Tese (Doutorado)

A single phase grid connected with a photovoltaic (PV) power system that will provide high voltage gain with state model analysis for the control of the system has been presented. First the photovoltaic system is designed and simulated using MATLAB SIMULINK software. The output voltage of a PV array is comparatively low thus high voltage gain is necessary for grid-connection and synchronization. The PV system has been provided with a boost converter which will boost the low voltage of the PV array to high dc-voltage. A steady state model is obtained and is verified with the help of simulation. A full bridge inverterwith bidirectional power flow is used as the second power processing stage, which stabilizes the dc voltage and the output current. Further, a maximum-power-point-tracking method is employed in the PV system to obtain a high performance.

碩士<br>明志科技大學<br>電機工程研究所<br>100<br>Two technology improvements in the grid-connected PV generation system were proposed in the thesis. One is a variable step-size maximum power point tracking method with noise interference reduction for the PV array. The other is a digital phase locked loop improvement for single-phase grid-connected inverters. Eventually, a DSP-based test system was built and tested to validate the performance of the methods. In the thesis, a PV model was built and simulated. From the results of simulations, some useful conclusions were found. The experiments in the PV system were also performed. Based on the conclusions of the simulations and experiments, a variable step size maximum power point tracking method using |dPpv/dVpv| and |dPpv/dIpv| to determine step size for noise interference reduction was proposed. This method can increase the speed of maximum power point tracking, and the output power and utility of PV modules. Using synchronous frame to obtain essential parameters from the utility power such as frequency, phase and amplitude in a single phase grid-connected systems, how to convert a single phase signal into orthogonal two phase signals by using memory table method and SOGI-QSG method are discussed. In addition, an improved synchronizing signal-detecting circuit was also proposed. Finally, a digital phase-lock-loop (DPLL) was built and tested to validate the performance of the system. The experimental results show that a high performance DPLL with harmonic and noise rejection and wide frequency variation stability have been realized and validated.

碩士<br>明志科技大學<br>電機工程系碩士班<br>102<br>A single-phase grid-connected PV system is developed in this project. In the maximum power point tracking (MPPT), a variable step-size MPPT method is used to maximize the output power of PV with faster response, smaller perturbance, and reducing noise interference. The synchronizing signals are detected by a second-order generalized integrator quadrature-signal generator (SOGI-QSG), a synchronous frame transformation, and digital phase-lock loop (DPLL) with faster response and smaller noise interference. Using the synchronous frame transformation power control, active power and reactive power are controlled independently and respectively. In the right side of the maximum power point(MPP), the derivative of PV output current with respect to the duty cycle of push-pull converter is larger , so the interference of measurement noise is relative smaller and the is getting smaller as the output power approaches to maximum power point, finally to zero. Conversely, the output voltage of PV has the same characteristic in the left side of MPP. Therefore, the value of multiplied by gain is used as a variable step-size during the MPPT procession in the right side of the MPP and the value of multiplied by gain is used as a variable step-size in the left side of the MPP. The single-phase voltage and current are transferred to two phase orthogonal signals, , , ,and , by SOGI-QSG. Then, the frequency, phase angle, and amplitude of the grid voltage are obtained by synchronous frame conversion and DPLL. Moreover, the active power and reactive power can be calculated using the signals, , , ,and . PI controllers are used to regulate the output of active power and reactive power from the system. The outputs of PI controllers are taken inverse park transfer and voltage drop compensation to get the need of the inverter output voltage. A DSP TMS320F28335 is used to realize the control system by writing program for sampling data, performing the calculation and control algorithms, and generating the trigger signals of power devices. Finally, the performance of the system is validated by simulations and experiments.

碩士<br>明志科技大學<br>電機工程研究所<br>100<br>In this work it implements a power control for a single-phase grid-connected PV System. The power control consists of a digital phase-lock-loop (DPLL) and synchronous frame current regulator. Based on the DPLL, the frequency, phase angle, and amplitude of the grid voltage are obtained by the second-order generalized integrator quadrature-signal generator (SOGI-QSG), synchronous frame transfer, and phase angle control loop. The DPLL can reduce the interference of the harmonics. The DPLL has provided the orthogonal voltage Signals, and in the stationary frame, phase angle , , and the rotating d-axis voltage, . Involving in the orthogonal voltage Signals, and , the active power and reactive power can be calculated easily. The inverter output current setting is obtained through the power control loop and inverse Park transform. The inverter output current controller is implemented by a proportional-resonant control strategy (PR). The PR controller can achieve zero steady-state errors in an AC current control system. Single-phase grid-connected inverter circuit had twice frequency power pulsation inherently. It is affected dc bus voltage which causes the output current distortion. It also produces PV module voltage and current fluctuation and interfere Maximum Power Point Tracking, hence reducing the utility of PV module. In this work the power pulsation is discussed and a method for compensating this problem is proposed.

The Photo Voltaic (PV) energy system, used in this project, is a very new concept in use, which is gaining immense popularity due to increasing importance to research on alternative sources of energy over depletion of the conventional fossil fuels all around the world. The systems which are being developed extract energy from the sun in the most efficient manner and suit them to the available loads without affecting their performance. In this project, The design and control issues associated with the development of a 1.8 kW prototype single-phase grid-connected photovoltaic system a multilevel cascaded inverter are discussed in this project. For the current controller a ramp time zero average current error control algorithm combined with an optimized cyclic switching sequence is suggested. Simulation results have been presented to demonstrate the suitability of the control method. Simulation results exhibits improved performance under the presence of harmonics and the studied system is modeled and simulated in the MATLAB/Simulink.

Power quality which involves power factor and the current wave form mainly affected by the power electronic loads which are connected to the grid. In this work reactive power compensation theory is applied to the inverter which feeds the power to the grid from the solar grid. Solar cell works on the principle of photo voltaic effect, which has nonlinear voltage and current characteristics. These characteristics are improved with the help of maximum power point tracking (MPPT) controller. MPPT controller helps to feed the inverter with maximum power from the solar grid. The Mat lab/Simulink model for the photo voltaic cell are implemented, MPPT controller has been modeled for driving the boost converter. MATLAB/simulation results are verified for the single phase grid involves the current waveform and total harmonic distortion level.

碩士<br>國立臺灣科技大學<br>電機工程系<br>101<br>This thesis aims to develop a single-phase grid-connected battery energy storage system for bi-directional power regulation. Overall system consists of four-arm type battery module buck-boost dc–dc power converter and single-phase full-bridge dc-ac power converter. Four-arm type buck-boost dc power converter performs boost and buck functions, respectively, for battery charging and discharging, resulting in wide adjustment for the input and output voltages. Continuous conduction mode for each arm is maintained through dc power converter and its input inductors. Reduction in current ripple is achieved with interleaved pulse-width modulation and equal-current control. In addition, feed forward compensation and power compensation control strategy are used for inverter to improve the transient response speed, reduce the steady state error, and complete the grid-connected system with the closed-loop control of dc link voltage and ac inductor current. In this thesis, a high-performance digital signal processor, TMS320F28035, is used as the control core. The system is administered by software control strategy in order to achieve the digital control to increase the stability and reliability. This system is integrated into two modes: the battery discharges output power of 1kW to the grid, the overall system efficiency is 85%, the current total harmonic distortion of inverter is 5.18%; in battery charging mode, input power is 300W from grid, the overall system efficiency is 81%, and the corresponding total current harmonic distortion of inverter is 7.61%. Experimental results validate the feasibility of theoretical analysis and control strategy.

This thesis presents adaptive control techniques to improve the stability and performance of grid-tied single-phase inverter systems. There are three main control systems involved in the conversion of DC to AC power: a synchronizer, a current-controller, and a DC-bus controller. The focus of this thesis is to create a control system which responds to the changing operating conditions. In order to adapt to changing operating conditions, estimations of the variables which influence stability and performance are necessary. A synchronization system is proposed, which delivers very accurate estimation of the grid frequency and voltage amplitude across all operating conditions. The synchronization system also provides an accurate and normalized set of orthogonal vectors for the AC output current reference phase. An adaptive parameter estimation technique is proposed to passively estimate the LCL-filter parameters and DC input current. Using the parameter estimations of the output filter, an adaptive state-feedback system is proposed. Next, a frequency-adaptive current controller is proposed, which dynamically tunes to the frequency provided by the synchronizer. Finally, the DC-bus controller is enhanced with the grid-voltage estimate and DC current estimator. The proposed techniques are verified for performance and stability though simulation and experimental results.<br>Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2014-05-05 15:25:47.342

Research Doctorate - Doctor of Philosophy (PhD)<br>The characteristics of single- and binary-phase turbulence in an oscillating grid system were investigated experimentally by using time-resolved, non-intrusive particle image velocimetry (PIV) technique. Experiments were conducted in three different fields of view (FoV) namely: 10 mm × 10 mm, 30 mm × 30 mm and 60 mm × 60 mm in the center of the tank for grid oscillation frequency 0 to 5 Hz. The grid Reynolds number (Re_g) and Taylor Reynolds number (Re_λ) ranges were 1080–10800 and 12–60, respectively. Specifically, the focus was on: (i) characterisation of single-phase homogeneous isotropic flow based on specific energy dissipation rate; (ii) modulation of turbulence due to particle-fluid and bubble-fluid interaction; and (iii) quantification of scaling properties of pressure spectrum for single- and binary-phase flow. In order to understand the homogeneous and isotropic turbulence inside the oscillating grid system turbulent length scales, isotropy ratio (IR = v_rms/u_rms), specific energy dissipation rate, and energy spectra were determined from the measured experimental fluctuating velocity field for different grid Reynolds numbers. Fluctuating velocity was found to increase linearly with increasing Re_g which was in agreement with the previous reported results. It was found that the turbulence length scale decreased with increase in grid oscillation frequency. The isotropy ratio ranged from 0.77-0.85 which indicated the presence of isotropic homogeneous turbulence in the system. The energy dissipation rate of single-phase flow was determined using the following methodologies: (i) dimensional analysis; (ii) velocity gradient; (iii) structure function; and (iv) energy spectrum. In general, the specific energy dissipation rate increased with increase in grid oscillation frequency. It was found that the specific energy dissipation rates were different for each of the four methodologies. Whilst the analysis identified uncertainties in all four approaches, it was concluded that the energy spectrum methodology was likely to be most reliable since it was able to satisfy the energy balance of the system—this was not possible for other three methods. Moreover, the energy spectra exhibited a slope close to Kolmogorov’s -5/3 in the inertial subrange. White noise was observed in the dissipation range, which was able to be removed by an exponential filter. The modulation of homogeneous and isotropic turbulence was experimentally investigated in the presence of a single stationary particle. The particle diameter varied in the range of 1 to 8 mm (~ 10 to 77 times larger than the flow Kolmogorov length scale). It was found that the fluid-only fluctuating velocity increased by up to 2-25 percent depending upon the particle diameter. The isotropy ratio of the fluid-only phase also increased with the size of the particle; but was much less influenced by the Reynolds number of the grid, Re_g. The energy dissipation rate of the fluid-only phase increased with increase in particle size; and followed a power law trend with grid Reynolds number. Longitudinal and transverse integral length scales were determined using the autocorrelation function for both fluid-only and particle-fluid case. The fluid-only phase integral length scales followed a power law dependency with Re_g, and decreased when a particle was present. Both longitudinal and transverse energy spectrums in the inertial subrange exhibited a slope less steep than the -5/3 predicted by Kolmogorov when a particle was present. It is thought that the particle presence resulted in the production of turbulence in the inertial region, leading to an energy enhancement in that part of the spectrum. Finally, turbulence intensity was determined as a function of the particle-diameter-to-integral-length-scale ratio, d_p/L_x; and it was found that below d_p/L_x = 0.41 the turbulence intensity was attenuated, and above this d_p/L_x value the turbulence intensity was enhanced. As per the particle-fluid experiments, modulation of homogeneous and isotropic turbulence was also studied due to bubble-fluid interaction. The bubble equivalent spherical diameter was varied in the range 2.7-3.52 mm, that corresponded to approximately 26-34 times larger than the Kolmogorov length scale. In the presence of a bubble, the single-phase fluctuating velocity along the transverse direction was found to be significantly enhanced when compared to that in the longitudinal direction. The presence of the bubble also influenced the isotropy of the fluid flow field; whereby at low grid Reynolds number the isotropy ratio increased with increase in the bubble equivalent spherical diameter, whilst at high Re_g the isotropy ratio showed no significant bubble equivalent spherical diameter dependence. The specific energy dissipation rate was found to be influenced by the shape of the bubble, and followed a (positive exponent) power law dependence with the bubble equivalent spherical diameter. Conversely, the integral length scale of the single-phase decreased with increase in the bubble equivalent spherical diameter. It also followed that the spectral slope was less steep than -5/3 in the inertial subrange—corresponding to an enhancement of energy—for both longitudinal and transverse energy spectra. Any effect of bubble size on the energy spectrum in the dissipative region could not be conclusively demonstrated due to the presence of white noise. Finally, the pressure spectrum for both single- and binary-phase flows was obtained by taking the fast Fourier transformation (FFT) of the instantaneous pressure field which was computed from the measured, instantaneous 2D velocity field. It was found that in the inertial subrange the pressure spectra exhibited a -7/3 slope for single-phase flow, whilst that for the binary-phase flow exhibited a less steep slope. The pressure-based integral length scale as well as the Taylor microscale were estimated from autocorrelation function and pressure spectrum, respectively. For single-phase flow, at low grid Reynolds number, the pressure-integral-length-scale-to-velocity-integral-length-scale ratio was found to be constant at around 0.67; whilst the pressure Taylor microscale was approximately 79 percent of the velocity Taylor microscale. Both of these values were consistent with theoretical predictions and published direct numerical simulation results. Finally, a methodology has been proposed whereby the specific energy dissipation rate can be computed from the pressure spectrum. It was found that the values obtained from this approach were approximately 25 percent higher than those calculated directly from velocity spectrum.

碩士<br>健行科技大學<br>電機工程系碩士班<br>103<br>This thesis focuses on design and implementation of a single-phase transformerless grid-connected photovoltaic generation system. A 1.14kW photovoltaic generation system model was built by Powersim (PSIM) software. The simulation and analysis results were used to design the control algorithms and programs for system testing. First,we observed the output characteristics of solar cell, the perturb-and-observe method was adopted to control the boost DC chopper for maximum power point tracking. Finally, the current control method was adopted to control a new single-phase full-bridge inverter for grid connected control to convert DC/AC power and connect to the grid. A digital signal processor (DSP, TMS320F28335) was applied to implement 1kW prototype single-phase transformerless grid-connected photovoltaic generation system. Experiment was then conducted to verify the theory.

碩士<br>國立雲林科技大學<br>電機工程系碩士班<br>96<br>This thesis presents the design and implementation of converters for the grid-connected single-phase photovoltaic energy conversion system. In order to provide the functions of the energy management and the load distribution, the system is combined with the photovoltaic cells and utility network. Due to that the output voltage and current of the photovoltaic cells are easily changed by light and load fluctuations, it will cause the maximum power points drift. Therefore, the maximum power point tracking and power electronic technique are required to draw the available maximum power from the photovoltaic cells. The proposed system makes use of the perturbation-and-observation method to achieve maximum power point tracking. The output frequency, phase and amplitude of the single-phase inverter are the same with utility network by using the zero-crossing control method. Therefore, it can match the condition of the grid-connected system. A digital signal processor is applied for the system control. Finally, a 500W generation system is constructed and tested to verify the system performance.

碩士<br>國立臺北科技大學<br>車輛工程系<br>106<br>The objective of this dissertation is to propose a new control strategy for the active filter. It controls the output current of the distributed energy devices for simplifying the control loop design of the active filter, and therefore the output current of the filter can be adjustable indirectly. The proposed control strategy can be used to reduce the instantaneous power phenomenon with double line frequency of the single-phase DC/AC conversion system. Thus, the low output ripple and long-life cycle characteristics of distributed energy devices can be achieved. This dissertation covers the root causes of the double line-frequency ripple and how to reduce that with the proposed active filter. Besides, the circuit simulation, steady-state analysis, small-signal model derivation, control loop design, and hardware implementation are made. Finally, the proposed active filter is built for single-phase grid-connected system with input voltage of 36V and output AC voltage of 110Vrms / output rated power of 500W. According to the simulation results and experimental verifications for the proposed active filter, the low ripple characteristics of the DC side for the single-phase grid-connected system can be achieved.

With the reduction of available fossil fuels, the research in the area of renewable energy system has been exponentially increased in order to find out an effective solution. Considering the case of a PV panel, it is observed that it produces a DC voltage which has to be converted to AC for grid connected applications. So a Voltage Source Inverter (VSI) with a PWM control of switching action is used for this purpose. This project presents the comparative study between unipolar and bi-polar switching scheme for single–phase grid-connected inverter system with filter design on the grid side. For that, LCL filter is considered and its performance for current ripple reduction on grid-side is compared with normal L and LC filter. It is seen that the response for LCL filter show the best results with lower current ripple as compared to L and LC Filter. Further, the obtained results show that the obtained LCL-filter can provide sufficient attenuation of current harmonics and mean while injects a sinusoidal current into the grid for maintaining power quality standard. In addition to that, the method to design the inductor is also studied. Further, the optimal design of LCL filter for grid connected inverter system is studied. For that, initially normal design is considered. Then the conduction and switching losses that are caused by the filter are calculated and are optimized considering the level of reduction of harmonics. Hence the main aim of the study is to attenuate higher order harmonics along with the reduction in switching losses to ensure sinusoidal current injection into the grid. Further, the different switching schemes for single phase unipolar full bridge inverter are studied and compared to get the switching scheme which gives lesser switching losses. The LCL filter is designed accordingly and optimal inductance and capacitance values are obtained. The complete model of the study is simulated in MATLAB-Simulink environment for feasibility of the study.

With the reduction of available fossil fuels, the research in the area of renewable energy system has been exponentially increased in order to find out an effective solution. Considering the case of a PV panel, it is observed that it produces a DC voltage which has to be converted to AC for grid connected applications. So a Voltage Source Inverter (VSI) with a PWM control of switching action is used for this purpose. This project presents the comparative study between unipolar and bi-polar switching scheme for single–phase grid-connected inverter system with filter design on the grid side. For that, LCL filter is considered and its performance for current ripple reduction on grid-side is compared with normal L and LC filter. It is seen that the response for LCL filter show the best results with lower current ripple as compared to L and LC Filter. Further, the obtained results show that the obtained LCL-filter can provide sufficient attenuation of current harmonics and mean while injects a sinusoidal current into the grid for maintaining power quality standard. In addition to that, the method to design the inductor is also studied. Further, the optimal design of LCL filter for grid connected inverter system is studied. For that, initially normal design is considered. Then the conduction and switching losses that are caused by the filter are calculated and are optimized considering the level of reduction of harmonics. Hence the main aim of the study is to attenuate higher order harmonics along with the reduction in switching losses to ensure sinusoidal current injection into the grid. Further, the different switching schemes for single phase unipolar full bridge inverter are studied and compared to get the switching scheme which gives lesser switching losses. The LCL filter is designed accordingly and optimal inductance and capacitance values are obtained. The complete model of the study is simulated in MATLAB-Simulink environment for feasibility of the study.

This Master of Applied Science thesis presents an inverter control system design and implementation with active damping of LCL filter resonance for a single phase grid-connected Distributed Power Generation (DPGS). The focus of the thesis is to actively damp the LCL filter resonance while keeping inverter control variables well regulated. The mathematical model of the LCL filter is analyzed and the filter is designed. Then, a PLL, and a PI compensator in the synchronous reference frame, and a PR compensator in stationary reference frame along a notch filter in cascade are designed and implemented. System level simulation and implementation are conducted. The idea of systematic applying the low loss power conversion topology, effective grid condition detection, grid synchronization, and advanced signal processing theory provides some advantages for single phase grid-connected inverter control design to meet the standard specifications of the interaction between the DPGS and utility grid.<br>Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2014-06-26 17:06:03.693

This thesis focuses on the boost converter and single phase VSI used with photovoltaic electricity generating systems in grid tied applications.A simple power control method is proposed.The control of time variant systems is more complicated compared to time invariant systems. However, by using suitable transformation techniques, time-variant systems (often vary sinusoidally with time) become time-invariant at the fundamental frequency, and are hence much easier to deal with. The direct application of transformation theory in single phase power systems is not possible without modification,and the d-q components would not be time-invariant in situations where harmonics, resonances or unbalance of system is present. The main objective is to address,the problems found with single-phase designs based on hysteresis controller in terms of cost, efficiency,power management and power quality. So a reliable single-phase inverter controller design based on voltage oriented control, with the implementation of direct quadrature reference frame has been proposed.As the first step the photovoltaic module is analyzed using SIMULINK software. By using MPPT extraction of maximum possible power from the PV can be done and feed this power to the inverter via boost converter, in which required magnitude of voltage can be acquired in steps. The second step, is the modelling of the single-phase inverter control using synchronous rotating frame. A mathematical analysis is conducted to determine the mechanism of the coupling that exists between the voltage phase and amplitude terms. The line frequency components of the feedback signals are transformed to time-invariant components, thus eliminating the ripple and reducing the computational burden associated with the controller stage.The inverter is control by bipolar sinusoidal pulse width modulation (SPWM) technique. Grid and VSI synchronization by SOGI-PLL is presented.By using SOGI generation of fictitious phase is presented. By park transform two-axis stationary (αβ) is converted into rotary (dq) frame. Finally, PI-controller based active and reactive power controller is implemented and the results areobtained using MATLAB/ SIMULINK.

碩士<br>國立高雄應用科技大學<br>電機工程系博碩士班<br>101<br>Based on single-phase Quasi Z-source inverter, a grid-connected PV system was built; besides, a Digital Signal Processor (DSP) is used to implement the power control and improve the voltage quality problem caused by voltage dip. The zero-point detector circuit by PLL control is used to facilitate the parallel connection of the inverter to grid; in the meantime, the DC bus and the output voltage can be effectively regulated by shoot-through and non-shoot-through modes. In addition, the power angle and the voltage amplitude are used to regulate active and reactive power injected to the utility. To verify the feasibility of the system, the system model was built by MATLAB/Simulink, and the operation of the system was simulated. Finally, a prototype of Quasi Z-source inverter was tested, and the results illustrate that the parallel connection of inverter to grid and the power control of the system can be satisfied.

碩士<br>國立臺灣科技大學<br>電機工程系<br>97<br>This thesis is concerned with the analysis and implementation of boost-type dc-dc power converter and single-phase dc-ac power converter system. The output power of overall system is adjusted by detecting the input voltage and current as well as the dc-link voltage of boost converter. The designed system can be operated in grid-connected fashion. In single-phase dc-ac power converter, the utility angle is used to calculate the arc-tangent for frame transformation. The controllers, which use the d-q current control modes, are performed under stationary reference frame , thereby yielding current and power stably to utility. Finally, the introduction of the dc-link fixed-voltage control and power compensating control in overall power converter system have improved system performance and reduce the input current harmonics. In this thesis, the mathematical models of dc-dc and dc-ac power converters are built and simulated by Matlab/Simulink. Then, a high-performance digital signal processor, TMS320F28335, is used to control the system with the voltage and current feedbacks. The control of overall system is conducted by software to reduce circuit component and improve system reliability. A prototype of 520W power conversion system is developed under grid-connection. The input voltage of boost converter is 140V. The dc-link voltage is 200V. Under single-phase grid-connected operation, the output voltage is 110V, 60Hz. Besides, the experimental data show that the efficiency of the whole system reaches 91.2% with current harmonic distortion of 6.71% and power factor of 0.996.

The design and control issues associated with the development of single phase grid-connected photovoltaic system incorporating a multi-level cascaded inverter are discussed in this paper. The advantages of transformer less inverter over a full-bridge inverter in combination with a line frequency transformer which is a common topology has been described in this report. Attractive features of multi-level inverters have been studied and descriptive details of photovoltaic system along with control and grid synchronization has been given this paper. Simulation results are presented to demonstrate the suitability of the control method.

The environmental awareness has led to many political decisions and initiated laws that regulate the market towards responsible energy usage. The demand of sustainable power has led to an increasing integration of renewable energy sources to the electric grid. Solar power is the 3rd largest renewable power source after wind and bio-power. One of the main reasons to this fast expansion is the German renewable energy act that has motivated households to install PV systems in their houses. This has led to a large amount of producers on the low-voltage network. The small scale producers receive compensation for electricity generated from the PV systems, both when it is used directly in the producer’s home and when it is sold to the grid due to low usage. The systems can be more profitable by storing the energy instead of selling it on the grid. In this way the amount of bought electricity can be reduced. There are concerns regarding the connection of renewable sources to the grid. This project aims to examine the impact from single-phase PV systems on the low-voltage grid. The focus of this bachelor thesis is understanding problems such as harmonic distortion and grid asymmetry. Simulations have been carried out using the software MATLAB in order to study harmonic distortion in the output of a single-phase PV system. Grid asymmetry is examined through calculations and simulations of a worst case scenario in the software NEPLAN. This scenario contains a low voltage grid with a star-star connected transformer, where all PV-systems are connected to the same phase. The simulations in combination with a literature review have provided the conclusion that harmonic distortion caused by the inverter becomes higher when the voltage supply is too low. Integration of battery energy storage systems together with PV systems does not cause additional harmonic distortion. The results also show how single-phase systems contribute to the asymmetry in the grid. When the production from the PV systems is high, and all systems are connected to a certain phase, the current and voltage will also have an impact on the other phases in the worst case scenario.

碩士<br>東南科技大學<br>電機工程研究所<br>99<br>This thesis studies a single-phase grid-connected photovoltaic system based on a Z-source inverter. The system directly converts the DC power generated by the photovoltaic modules into AC power and connects to the utility power system. Moreover, Z-source inverter is a single-stage topology, so its efficiency could be higher than other multi-stage systems. When the sun moves during a day or cloud cover changes, the system makes use of the buck-boost feature of the Z-source and applies the maximum-power-point-tracking scheme to output the maximum power of the photovoltaic modules. In addition, the system is based on a 32-bit digital signal processor TMS320F2808 to realize a full digital photovoltaic power system. Because the system only uses the PWM generator of the DSP TMS320F2808 to output all the Z-source trigger signals, it does not need the extra digital circuit hardware and the cost of system is decreased. Finally, the experimental results show that the system could work on a wide voltage range from 130-300 volts. The maximum output power of the system is up to 1kWatt and the efficiency of the system can reach to 97%. It is very suitable to apply to small power of solar power system.

In general, the reliability of power supply to load can be increased by grid connected inverter systems. But harmonics produced in the inverter, may lead to the harmonic injection in to the grid and this can cause non linearity in grid current which will produce undesirable effects like high Total harmonic distortion (THD) and low grid power factor. These unwanted effects can be reduced by current control techniques where the controller controls the inverter switching that indirectly controls the grid current. The current control technique used in this work is based on the error between the actual grid current and a reference grid current trajectory. The error value is confined in a hysteresis band which is bounded between two limits. Hence, it is called Hysteresis current control. This current controller makes use of relay that switches only when the input given to it reaches the extreme value. In this hysteresis current control, the input given to relay is error value which is the difference between the actual and a reference grid current. The relay output triggers the inverter switches. Here, the switching frequency depends on the relay output which depend son hysteresis band. Thus, a proper selection of band is required to minimise switching loss. In this method, the grid voltage is used to generate the reference sinusoidal signal which after multiplication with a proper factor generates reference current signal. The reference current is compared to actual grid current which in turn forces the grid current to be in phase with grid voltage. In other words, the controller forces power consumption from the grid to be in unity power factor condition. In one way, the reactive power of the system is controlled. In this project, another type of current controller is used. It is called adaptive hysteresis controller that can work in a similar manner. In this method, the factor used for reference current generation depends on load. In this method, a new switching technique is used to ensure the prevention of short circuit between inverter DC power supply terminals. The comparative analysis is made between conventional and adaptive hysteresis current controllers through its implementation in MATLAB SIMULINK and simulation results are compared.

Single-phase distribution is a preferred approach for setting up microgrids or supplying power to rural and remote locations due to its lower infrastructure costs when compared to a three-phase grid. On the other hand, three-phase induction machines exhibit smooth torque-speed characteristics, offer better starting torque, have higher power density and are of lower cost compared to their single-phase counterparts. Due to the modernization of the technologies used in agriculture and mechanization of production processes, the demand for electrical energy on single-phase distribution grid has increased considerably in the past decade. However, the sole availability of single-phase power limits the usage of three-phase induction motors in certain applications such as auxiliary services in rail- ways, pumps or mills in the agricultural sector and in small urban and rural industries. Conventionally, power electronics based double-conversion drive systems are available that power three-phase machines with single-phase supply. However, inexpensive and low-efficiency power conversion solutions are often adopted to achieve cost benefits at the expense of performance and power quality. In this context, there is a need for power electronic phase converters that facilitate the use of three-phase induction machines on the single-phase grid while offering high performance at low cost. This work investigates reduced switch-count active phase converters (APC) that serve three-phase loads from a single-phase grid. Specifically, the focus is on topologies that process only a fraction of the load power while still maintaining the quality of power at the grid and load ends. An auxiliary capacitor based active phase converter (AC- APC) topology is shown to have reduced current stress in the switching devices and the dc-link capacitors. At the rated load of 3.3 kVA/0.8 pf, the total VA processed by the semiconductor devices of the conventional topology is 180% of the load VA. Whereas the semiconductor VA of the APC and AC-APC in the experimental prototype are 88% and 40% respectively. Analytical power loss evaluation shows that the device power loss at rated conditions for the APC and AC-APC are 46% and 18% respectively, of that of a conventional double conversion con figuration. Moreover, 50% reduction in the double frequency dc-link ripple is observed in the experimental prototype of AC-APC when compared to that of a standard APC at rated load. The proposed control structure allows asymmetric control of the three legs of the APC. It also facilitates independent selection of optimized components for higher order filters, to meet the independent design requirements at the grid and the load. The control to generate modulation signals for the APC with a shared leg is implemented as two decoupled single-phase converter controllers. A method for soft-starting an induction motor is incorporated in the APC. Moreover, the bidirectional power ow capability of the APC also facilitates injection of power to a single-phase grid. A common-mode filter and its design method are presented for the asymmetric APC, which significantly reduces the effects of common-mode voltage in the system. All the methods proposed in this work are validated on an experimental 5 kVA laboratory converter prototype.

碩士<br>國立勤益科技大學<br>電機工程系<br>107<br>The main purpose of this thesis is to propose a three phase Multi-input Single-stage Grid-tied inverter (MSG-inverter) which can be used in distributed photovoltaic power generation systems (PVPGS). The proposed inverter is composed of micro dc-dc buck-boost converters (BBCs) and a dc-ac unfolder. Each micro BBC is connected to a single photovoltaic module (PV) in a PVPGS. The micro BBCs are connected to the central dc-ac unfolder through a dc bus. The micro BBCs are operating in discontinuous conduction mode (DCM). The pulsating dc current reduces stress and losses in the dc bus. Each micro BBC is equipped with a simple maximum power point tracker (MPPT) designed specifically to achieve maximum energy transfer. Each micro BBC is equipped with an interleaving power control scheme which is controlled using a signal from a phase lock loop (PLL). The architecture and control scheme of the proposed three phase MSG-inverter achieves a distributed MPPT (DMPPT) control and distributed power control of the PV array in distributed PVPGS. Finally some computer simulations results are presented to verify the performance of the proposed MSG-inverter.