As an increasing number of distributed power generation systems (DPGS) are being connected to the utility grid, there is a growing requirement for the DPGS to be able to ride through short grid disturbances. This requires improvements to be made to the grid-side control scheme of the DPGS. An important part of the grid-side control scheme is the grid synchronization method, which is responsible for tracking the phase angle of the grid voltage vector. The state-of-the-art grid synchronization methods being used today are phase-locked loops. This thesis presents a modified phase-locked loop which is more robust towards grid disturbances. It consists of a multi-block adaptive notch filter (ANF) integrated into a conventional three-phase synchronous reference frame phase-locked loop (SRF-PLL). The addition of the multi-block ANF to the system allows it to become frequency adaptive. Also, since the multi-block ANF consists of multiple ANF blocks in parallel with one another, the system is able to remove multiple input signal distortions. Thus, the proposed system is able to eliminate the double frequency ripple that is caused in the conventional three-phase SRF-PLL by input unbalance, as well as harmonic errors, despite the presence of frequency variations in the input signal. Simulation results found using Matlab/Simulink, and experimental results found using the dSPACE DS1103 DSP board, demonstrate the feasibility of the modified SRF-PLL. Also, the modified SRF-PLL is compared to a conventional three-phase SRF-PLL, as well as to a conventional three-phase SRF-PLL with a simple notch filter, and the advantages of the modified SRF-PLL are discussed. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2008-12-17 12:38:02.589

Environmentally friendly renewable energy technologies such as wind and solar energy systems are among the fleet of new generating technologies driving the demand for distributed generation of electricity. Power Electronics has initiated the next tech¬nological revolution and enables the connection of distributed generation (DG) systems to the grid. The challenge is to achieve system functionality without extensive custom engineering, yet still have high system reliability and generation placement flexibility. Nowadays, it is a general trend to increase the electricity production using DG systems. If these systems are not properly controlled, their connection to the utility network can generate problems on the grid side. Therefore, considerations about power generation, safe running and grid synchronization must be done before connecting these systems to the utility network. This thesis introduces a new grid-synchronization, or more visibly a new “power signal processor” adaptive notch filtering (ANF) tool that can potentially stimulate much interest in the field and provide improvement solutions for grid-connected operation of DG systems. The processor is simple and offers high degree of immunity and insensitivity to power system disturbances, harmonics and other types of pollutions that exist in the grid signal. The processor is capable of decomposing three-phase quantities into symmetrical components, extracting harmonics, tracking the frequency variations, and providing means for voltage regulation and reactive power control. In addition, this simple and powerful synchronization tool will simplify the control issues currently challenging the integration of distributed energy technologies onto the electricity grid. All converter-interfaced equipments like FACTS (flexible ac transmission systems) and Custom Power Controllers will benefit from this technique. The theoretical analysis is presented, and simulation and experimental results confirm the validity of the analytical work. / Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2009-01-27 11:37:07.279

Renewable power sources with a relatively uneven or constant DC power production require synchronization methods to work with the current utility power grid. The solution to this synchronization problem has been solved with semiconductor based converters and advanced switching algorithms. To enable switching algorithms that work well with the grids amplitude, phase-shift and frequency, the current waveform has to be measured and estimated.    There are many sources of noise that will add distortion of the current waveform, making its appearance less similar to the grids. The distorted measurement affects the accuracy of the converters negatively. Therefore, using a filter algorithm to attenuate the grid noise is required.  This project uses a Kalman filter with the aim to decrease the noise and estimate the current phase shift for a three phase power-grid.  To achieve reliable and fast calculation, implementing the Kalman filter within a FPGA were done.The project contains results from both simulated MATLAB data and the FPGAs real time data. The method was able to estimate the grid within a few Hz frequency deviation and enable some noise reduction. For larger degree of harmonic distortion during steady state operation, the Kalman filter could remove more of the harmonic distortion. Limits and differences with MATLAB are discussed for the FPGA implemented Kalman filter.

Three-Phase Voltage Source Converters (VSCs) are finding widespread applications in grid integrated power conversion systems. The control systems of such VSCs are in an increasing number of these applications required to operate during voltage disturbances and unbalanced conditions. Control systems designed for grid side voltagesensor- less operation are at the same time becoming attractive due to the continuous drive for cost reduction and increased reliability of VSCs, but are not commonly applied for operation during unbalanced conditions. Methods for voltage-sensor-less grid synchronization and control of VSCs under unbalanced grid voltage conditions will therefore be the main focus of this Thesis. Estimation methods based on the concept of Virtual Flux, considering the integral of the converter voltage in analogy to the flux of an electric machine, are among the simplest and most well known techniques for achieving voltage-sensor-less grid synchronization. Most of the established techniques for Virtual Flux estimation are, however, either sensitive to grid frequency variations or they are not easily adaptable for operation under unbalanced grid voltage conditions. This Thesis addresses both these issues by proposing a simple approach for Virtual Flux estimation by utilizing a frequency-adaptive filter based on a Second Order Generalized Integrator (SOGI). The proposed approach can be used to achieve on-line frequency-adaptive varieties of conventional strategies for Virtual Flux estimation. The main advantage is, however, that the SOGI-based Virtual Flux estimation can be arranged in a structure that achieves inherent symmetrical component sequence separation under unbalanced conditions. The proposed method for Virtual Flux estimation can be used as a general basis for voltage-sensor-less grid synchronization and control during unbalanced conditions. In this Thesis, the estimated Virtual Flux signals are used to develop a flexible strategy for control of active and reactive power flow, formulated as generalized equations for current reference calculation. A simple, but general, implementation is therefore achieved, where the control objective and the power flow characteristics can be selected according to the requirements of any particular application. Thus, the same control structure can be used to achieve for instance balanced sinusoidal currents or elimination of double frequency active power oscillations during unbalanced conditions. In case of voltage sags, current references corresponding to a specified active or reactive power flow might exceed the current capability of the converter. The limits for active and reactive power transfer during unbalanced conditions have therefore been analyzed, and generalized strategies for current reference calculation when operating under current limitations have been derived. The specified objectives for active and reactive power flow characteristics can therefore be maintained during unbalanced grid conditions, while the average active and reactive power flow is limited to keep the current references within safe values. All concepts and techniques proposed in this Thesis have been verified by simulations and laboratory experiments. The SOGI-based method for Virtual Flux estimation and the strategies for active and reactive power control with current limitation can also be easily adapted for a wide range of applications and can be combined with various types of inner loop control structures. Therefore, the proposed approach can potentially be used as a general basis for Virtual Flux-based voltage-sensor-less operation of VSCs under unbalanced grid voltage conditions.

This Master of Applied Science thesis presents a single phase grid connected DC/AC inverter with reactive power (VAR) control for residential photovoltaic (PV) applications. The inverter, utilizing the voltage sourced inverter (VSI) configuration, allows the local residential PV generation to actively supply reactive power to the utility grid. A low complexity grid synchronization method was introduced to generate the parallel and orthogonal components of the grid voltage in a highly computationally efficient manner in order to create a synchronized current reference to the current control loop. In addition, the inverter is able to use a small long life film type capacitor on the DC-link by utilizing a notch filter on the voltage control loop. Simulations were performed on PSCAD/EMTDC platform and a prototype was also developed in the lab to prove the effectiveness of the controllers and the grid synchronization method.

This Master of Applied Science thesis presents a single phase grid connected DC/AC inverter with reactive power (VAR) control for residential photovoltaic (PV) applications. The inverter, utilizing the voltage sourced inverter (VSI) configuration, allows the local residential PV generation to actively supply reactive power to the utility grid. A low complexity grid synchronization method was introduced to generate the parallel and orthogonal components of the grid voltage in a highly computationally efficient manner in order to create a synchronized current reference to the current control loop. In addition, the inverter is able to use a small long life film type capacitor on the DC-link by utilizing a notch filter on the voltage control loop. Simulations were performed on PSCAD/EMTDC platform and a prototype was also developed in the lab to prove the effectiveness of the controllers and the grid synchronization method.

This dissertation covers several issues related to the ac-dc bus-interface bi-directional converters in renewable energy systems. The dissertation explores a dc-electronic distribution system for residential and commercial applications with a focus on the design of an ac-dc bi-directional converter for such application. This converter is named as the "Energy Control Center" due to its unique role in the system. First, the impact of the unbalanced power from the ac grid, especially the single-phase grid, on the dc system operation is analyzed. Then, a simple ac-dc two-stage topology and an advanced digital control system is proposed with a detailed design procedure. The proposed converter system significantly reduces the dc-link capacitor volume and achieves a dynamics-decoupling operation between the interfaced systems. The total volume of the two-stage topology can be reduced by upto three times compared with the typical design of a full-bridge converter. In addition, film capacitors can be used instead of electrolytic capacitors in the system, and thus the whole system reliability is improved. A set of ac passive plus active filter solutions is proposed for the ac-dc bus-interface converter which significantly reduces the total power filter volume but still eliminate the total leakage current and the common-mode conducted EMI noises by more than 90%. The dc-side low-frequency CM voltage ripple generated by the unbalanced ac voltages can be eliminated as well. The proposed solution features a high reliability and fits three types of the prevalent low-voltage ac distribution systems. Grid synchronization, a critical interface control in ac-dc bus-interface converters, is discussed in detail. First, a novel single-phase grid synchronization solution is proposed to achieve the rejection of multiple noises as well as the capability to track the ac voltage amplitude. Then, a comprehensive modeling methodology of the grid synchronization for three-phase system is proposed to explain the output frequency behaviors of grid-interface power converters at the weak grid, at the islanded condition, and at the multi-converter condition. The proposed models provide a strong tool to predict the grid synchronization instabilities raised from industries under many operating conditions, which is critical in future more-distributed-generation power systems. Islanding detection issues in ac-dc bus-interface converters are discussed in detail. More than five frequency-based islanding detection algorithms are proposed. These solutions achieve different performances and are suitable for different applications, which are advantageous over existing solutions. More importantly, the detailed modeling, trade-off analysis, and design procedures are given to help completely understand the principles. In the end, the effectiveness of the proposed solutions in a multiple-converter system are analyzed. The results drawn from the discussion can help engineers to evaluate other existing solutions as well. / Ph. D.

Ce travail s’intéresse à l’étude d’un système de pompage turbinage à vitesse variable avec une machine asynchrone doublement alimentée face aux perturbations de réseau électrique. L’objectif est d’assurer la continuité de service de cet élément stabilisateur de réseau électrique de sorte qu’il reste connecté au réseau même en cas de perturbations. Le contrôle du système dans les différentes phases de fonctionnement en mode sain a été tout d’abord développé ainsi qu'une étude de stabilité de système utilisant l’analyse modale. Les différents outils nécessaires pour cette analyse ont été tout d’abord mis en oeuvre. Ensuite, les limites de stabilité du système ont été étudiées tenant compte de la variation de longueur de ligne. Le comportement du système en présence des défauts a été par la suite étudié. Les défauts de réseau considérés sont les défauts symétriques et les défauts asymétriques. Une solution basée sur la modification de la stratégie de contrôle a été adoptée pour le cas des défauts symétriques et une solution basée sur l’ajout d’éléments au circuit de puissance a été adoptée pour les cas des défauts asymétriques. Un dispositif expérimental de 4kW a été mis en oeuvre pour la validation des développements menés dans le cadre de cette thèse. / This work focuses on the study of a variable speed pumped storage system based on a doubly fed induction machine in case of grid disturbances. Thus the main objective is to improve the fault ride through capabilities of this grid stabilizer and to guarantee its connection with the grid even under disturbances. The system control in the different operating phases in healthy conditions is developed and then a system stability study is conducted using the eigenvalue analysis. The system stability limits have been investigated taking into account the variation of grid line length so the grid impedance variation. Then the system behavior under disturbances is analyzed. Theconsidered grid faults are symmetric and asymmetric faults. The investigated fault ride through capabilities of the pumped storage system consist of two solutions, one based on the modification of the control strategy was adopted for the symmetric faults and one based on hardware modification has been adopted for the asymmetric ones. A 4kW laboratory set-up has been developed for experimental validation.