Sensorless Predictive Direct Power Control PDPC_SVM For PWM Converter Under Different Input Voltage Conditions

In this paper, a new virtual flux (VF) based predictive direct power control (VF_PDPC) applied for three-phase pulse width modulation (PWM) rectifier is proposed. The virtual flux estimation is performed using a pure integrator in series with a new adaptive algorithm in order to cancel dc offset and harmonic distortions in the estimated VF. The introduced structure is able to produce two virtual flux positive sequence components orthogonal output signals under unbalanced and distorted voltage conditions. The main features of the proposed virtual flux estimator are, it's simple structure, accuracy, and fast VF estimation over the excited integrators. Therefore,  the estimated VF is then used for robust sensorless VF-PDPC with a constant switching frequency using space vector modulation (SVM) and tested through numerical simulations. The instantaneous active and reactive powers provided by orthogonal (VF) positive sequence components are directly controlled. More importantly, this configuration gives quasi-sinusoidal and balanced current under different input voltage conditions without using the power compensation methods. The results of the simulation confirmed the validity of the proposed virtual flux algorithm and demonstrated excellent performance under different input voltage conditions, complete rejection of disturbances

Grid Voltages Estimation for Three-Phase PWM Rectifiers Control Without AC Voltage Sensors

This paper proposes a new ac voltage sensorless control scheme for the three-phase pulse-width modulation rectifier. A new startup process to ensure a smooth starting of the system is also proposed. The sensorless control scheme uses an adaptive neural (AN) estimator inserted in voltage-oriented control to eliminate the grid voltage sensors. The developed AN estimator combines an AN network in series with an AN filter. The AN estimator structure leads to simple, accurate, and fast grid voltages estimation, and makes it ideal for low-cost digital signal processor implementation. Lyapunov-based stability and parameters tuning of the AN estimator are performed. Simulation and experimental tests are carried out to verify the feasibility and effectiveness of the AN estimator. Obtained results show that the proposed AN estimator presented faster convergence and better accuracy than the second-order generalized integrator-based estimator; the new startup procedure avoided the overcurrent and reduced the settling time; and the AN estimator presented high performances even under distorted and unbalanced grid voltages

Sliding mode approach for control and observation of a three phase AC-DC pulse-width modulation rectifier

Introduction. For AC-DC conversion systems, the electrical systems typically use thyristor or diode bridge rectifiers, which have relatively poor performance. Nowadays, three-phase pulse-width modulation rectifiers are widely applied in various applications for their well-known intrinsic benefits, such as adjustable DC link voltage, unity power factor, bidirectional power flow and very low total harmonic distortion. Purpose. The objective of this work is to achieve better stability and dynamic performance using sliding mode strategy for control and observation. Methods. For that purpose, first a sliding mode controller is introduced on the DC-link side to ensure a fast and accurate response of the output load voltage. Then, the sliding mode approach is employed to control the quadrature and direct components of power to maintain the input power factor at unity. Finally, this approach is used to design two observers for grid voltage estimation and online variation of load resistance. To overcome the problem associated with the use of the classical low-pass filter, an adaptive compensation algorithm is used to compensate the attenuation of the amplitude and phase delay of the observed grid voltages. This algorithm is based on the use of the two low-pass filters in cascade and ensures the minimization of chattering. Results. Comparative studies have been carried out between sliding mode control method for controlling the three-phase AC-DC pulse-width modulation rectifier and other conventional techniques. The validation by simulation and the tests carried out gave very satisfactory results and proved the effectiveness and feasibility of the sliding mode for both control and observation of three phase pulse-width modulation rectifier.Вступ. Для AC-DC систем перетворення електричні системи зазвичай використовують тиристорні або діодні мостові випрямлячі, які мають відносно погані характеристики. В даний час трифазні випрямлячі з широтно-імпульсною модуляцією широко застосовуються з різними цілями завдяки їх добре відомим внутрішнім перевагам, таким як регульована напруга у ланці постійного струму, одиничний коефіцієнт потужності, двонаправлений потік потужності та дуже низькі загальні гармонічні спотворення. Метою даної роботи є досягнення кращої стабільності та динамічних характеристик з використанням стратегії ковзного режиму для контролю та спостереження. Методи. З цією метою спочатку на стороні ланки постійного струму вводиться регулятор режиму ковзання, щоб забезпечити швидку і точну реакцію на вихідну напругу навантаження. Потім використовується метод ковзного режиму для управління квадратурною та прямою складовими потужності, щоб підтримувати вхідний коефіцієнт потужності рівним одиниці. Нарешті цей підхід використовується для розробки двох спостерігачів для оцінки напруги мережі та зміни опору навантаження в режимі онлайн. Для подолання проблеми, пов'язаної з використанням класичного низькочастотного фільтру, використовується алгоритм адаптивної компенсації, що компенсує загасання амплітуди і фазової затримки напруг мережі, що спостерігаються. Цей алгоритм заснований на використанні двох низькочастотних фільтрів у каскаді та забезпечує мінімізацію брязкоту. Результати. Були проведені порівняльні дослідження між методом керування ковзним режимом для керування трифазним випрямлячем AC-DC з широтно-імпульсною модуляцією та іншими традиційними методами. Перевірка за допомогою моделювання та проведені випробування дали дуже задовільні результати та довели ефективність та здійсненність ковзного режиму як для управління, так і для спостереження за трифазним випрямлячем з широтно-імпульсною модуляцією

High performance of sensorless sliding mode control of doubly fed induction motor associated with two multilevel inverters fed by VFDPC_SVM rectifier

A robust sensorless control based on the sliding-mode observer applied to a doubly fed induction motor associated with two three-level NPC-type voltage inverters fed by PWM rectifier with constant switching frequency and without line voltage sensors, is presented in this paper.  Also, we present an improved direct power control with virtual flux (VFDPC_SVM) for the control of three phase rectifier. Simulation results of this proposed system were analyzed using MATLAB environment

New Control Algorithms for the Distributed Generation Interface in Grid-Connected and Micro-grid Systems

Driven by economic, technical, and environmental reasons, the energy sector is moving into an era where large portions of increases in electrical energy demand will be met through widespread installation of distributed resources or what's known as distributed generation (DG). DG units can operate in parallel to the main grid or in a micro-grid mode. The later is formed by a cluster of DG units connected to a distribution network to maintain the reliability of critical loads, mainly when the grid supply is not available. Distributed resources include variable frequency sources, high frequency sources, and direct energy conversion sources producing dc voltages or currents. The majority of distributed resources are interfaced to the utility grid or to the customer load via dc-ac pulse-width-modulated (PWM) voltage source inverter (VSI) systems. However, these interfaces introduce new issues, such as the absence of the physical inertia, wide-band of dynamics, limited overload capability, susceptibility to parameters variation, and switching harmonics generation. In addition, the uncertain and dynamic nature of the distribution network challenges the stability and control effectiveness of a grid-connected inverter-based DG interface. Generally, difficulties appear in the form of grid impedance and interfacing parameter variations, fast and slow grid-voltage disturbances, grid distortion and unbalance, and interactions between the inverter ac-side filter and the grid. On the other hand, a micro-grid system will be dominated by inverter-based DG units. Unlike conventional power system generators, inverter-based DG units have no physical inertia. This fact makes the micro-grid system potentially susceptible to oscillations resulting from system disturbances. Severe and random disturbances might be initiated in a micro-grid system, due to load changes, the power sharing mechanism of the inverters and other generators, and interactions between the DG interface and the network. Motivated by the aforementioned difficulties, this thesis presents new control algorithms for the DG interface that guarantee stable and high power quality injection under the occurrence of network disturbances and uncertainties, in both the grid-connected and micro-grid systems. The control architecture of the proposed DG interface relies on the following subsystems. First, a newly designed deadbeat current regulation scheme is proposed. The proposed design guarantees high power quality current injection under the presence of different disturbing parameters such as grid voltage distortion, interfacing parameter variation, and inverter system delays. Further, it utilizes the maximum dynamic performance of the inverter in a way that provides a high bandwidth and decoupled control performance for the outer control loops. Different topologies of the ac-side filter are considered for the current control design. Second, a novel adaptive discrete-time grid-voltage sensorless interfacing scheme for DG inverters is proposed. The adaptive interface relies on a new interface-monitoring unit that is developed to facilitate accurate and fast estimation of the interfacing impedance parameters and the grid voltage vector (magnitude and position) at the point of common coupling. The estimated grid voltage is utilized to realize a grid-voltage sensorless interfacing scheme, whereas the interfacing parameters are utilized for the self-tuning control and interface-parameter monitoring. Further, a simple and robust synchronization algorithm and a voltage-sensorless average power control loop are proposed to realize an adaptive voltage-sensorless DG interface. The voltage-sensorless interface positively contributes to the elimination of the residual negative sequence and voltage feed-forward compensation errors, and to the robustness of the power sharing mechanism in paralleled inverter systems, where the power-sharing mechanism is generally based on open-loop controllers. Third, a new voltage control scheme for the DG interface featuring fast load voltage regulation and effective mitigation of fast voltage disturbances is proposed. The proposed voltage control scheme targets the problem of fast and large-signal-based voltage disturbances, which is common in typical distribution feeders. A hybrid voltage controller combining a linear with a variable-structure-control element is proposed for the DG interface. Positive and dual-sequence versions of the proposed voltage controller are developed to address the issue of unbalanced voltage disturbances. The proposed voltage controller successfully embeds a wide band of frequency modes through an equivalent internal model. Subsequently, wide range of balanced and unbalanced voltage perturbations, including capacitor-switching disturbances, can be effectively mitigated. Fourth, to constrain the drift of the low frequency modes in a conventional droop-controlled micro-grid, a new transient-based droop controller with adaptive transient-gains is proposed. The proposed power-sharing controller offers an active damping feature that is designed to preserve the dynamic performance and stability of each inverter unit at different loading conditions. Unlike conventional droop controllers, the proposed droop controller yields two-degree of freedom tunable controller. Subsequently, the dynamic performance of the power-sharing mechanism can be adjusted, without affecting the static droop gain, to damp the oscillatory modes of the power-sharing controller. The overall robust DG interface facilitates a robust micro-grid operation and safe plug-and-play integration of DG units on existing distribution systems; hence increasing the system penetration of DG. The direct result of this development is huge financial saving for utilities by capturing the salient features of deploying DG into existing utility networks. Further, these developments are significant to the industry as they provide the blue print for reliable control algorithms in future DG units, which are expected to operate under challenging system conditions

Control strategy of grid connected power converter based on virtual flux approach

A la portada consta el nom del programa interuniversitari: Joint Doctoral Programme in Electric Energy Systems [by the] Universidad de Málaga, Universidad de Sevilla, Universidad del País Vasco/Euskal Erriko Unibertsitatea i Universitat Politècnica de CatalunyaDistributed Generation (DG) provides an alternative to the Centralized Generation (CG) by means of generating electricity near to the end user of power with the employment of small-scale technologies to produce electricity, mainly using Renewable Energy Sources (RES). The prospects of renewable energy integration during the next years are still very optimistic. This PhD dissertation is made to provide an alternative control framework for the grid connected power converter by adopting the virtual flux concept in the control layer. This dissertation can be divided into three main topics. The 1st topic presents the voltage sensorless control system for the grid-connected power converter. The control system presented is done without depending on AC-voltage measurement where the grid synchronization is based on the Virtual Flux (VF) estimation. In this regard, the Frequency Locked Loop (FLL) is used in conjunction with the estimation scheme to make the system fully adaptive to the frequency changes. This voltage sensorless application is useful for reducing cost and complexity of the control hardware. It is also can be utilized in case of limited reliability or availability of voltage measurements at the intended point of synchronization to the grid. Considering that most previous studies are based on the VF estimation for the case of power converter connected to the grid through the L-filter or LC-filter, this dissertation is focused on the power converter connected to the grid through the LCL filter. The Proportional Resonant (PR) current controller is adopted in the inner loop control of the power electronics-based converter to test the performance of such system. Another control method based on VF synchronization that permits to control the active and reactive power delivery in a remote point of the grid is also presented in this dissertation. This is due to the fact that the VF is implemented that the voltage in a remote point of the line can be estimated. As it will be shown in simulations and experiments, the proposed control scheme provides a good tracking and dynamic performance under step changes in the reference power. The fast synchronization and the smooth reference tracking achieved in transient conditions have demonstrated the effectiveness of the Dual Second Order Generalized Integrator controlled as Quadrature Signal Generator (DSOGI-QSG) and also the current controller used in the proposed system. In addition to the power control itself, this study could also benefit the frequency and the voltage regulation methods in distributed generation applications as for instance in microgrid. Considering the fact that the grid connected power converter can be controlled as a virtual synchronous generator where the flux is a variable to be used for controlling its operation, this dissertation also presents a Virtual Synchronous Flux Controller (VSFC) as a new control framework of the grid connected power converter. In this regard, a new control strategy in the inner loop control of the power converter will be proposed. The main components of the outer loop control of VSFC are based on the active and reactive power control. The results presented show that the VSFC works well to control the active and reactive power without considering any synchronization system. The inner loop control is able to work as it is required, and the measurement flux is able to track the reference flux without any significant delays. All the work presented in this dissertation are supported by mathematical and simulation analysis. In order to endorse the conclusions achieved, a complete experimental validations have been conducted before wrapping this dissertation with a conclusion and recommendation for future enhancement of the control strategies that have been presented.Postprint (published version