Advanced control architectures for grid connected and standalone converter systems

This dissertation proposes new control algorithms dedicated towards improving the reliability, computational burden and stability in grid-connected and stand-alone based power electronic converter systems applicable for ac microgrids. Two voltage sensorless control architectures, one for stand-alone applications and the other for grid-connected application are established in this thesis. The output voltage of a standalone single-phase inverter is controlled directly by controlling the output filter capacitor current without using a dedicated output voltage sensor. A method to estimate the output filter capacitance is also presented. For the grid connected converter, a novel closed loop estimation is presented to estimate the grid voltage. In addition to the estimation of the grid voltage, the proposed method also generates the unit vectors and frequency information similar to a conventional phase-locked loop structure. The voltage sensorless algorithm is then extended to LCL filter based grid connected converters thereby proposing a new indirect method of controlling the grid current. Furthermore, addressing the stability issues in current-controlled grid tied converters, this dissertation also analyzes the power angle synchronization control of grid-tied bidirectional converters for low voltage grids. The power flow equations for the low voltage grid are analyzed and compensators are designed to ensure the decoupled control of active and reactive power. It is demonstrated that the proposed compensators are immune to grid fluctuations and ensure stable operation controlling the desired power flow to and from the grid. Detailed plant modeling, controller design, simulation and experimental results are presented for all of the proposed schemes --Abstract, page iv

Contributions to impedance shaping control techniques for power electronic converters

El conformado de la impedancia o admitancia mediante control para convertidores electrónicos de potencia permite alcanzar entre otros objetivos: mejora de la robustez de los controles diseñados, amortiguación de la dinámica de la tensión en caso de cambios de carga, y optimización del filtro de red y del controlador en un solo paso (co-diseño). La conformación de la impedancia debe ir siempre acompañada de un buen seguimiento de referencias. Por tanto, la idea principal es diseñar controladores con una estructura sencilla que equilibren la consecución de los objetivos marcados en cada caso. Este diseño se realiza mediante técnicas modernas, cuya resolución (síntesis del controlador) requiere de herramientas de optimización. La principal ventaja de estas técnicas sobre las clásicas, es decir, las basadas en soluciones algebraicas, es su capacidad para tratar problemas de control complejos (plantas de alto orden y/o varios objetivos) de una forma considerablemente sistemática. El primer problema de control por conformación de la impedancia consiste en reducir el sobreimpulso de tensión ante cambios de carga y minimizar el tamaño de los componentes del filtro pasivo en los convertidores DC-DC. Posteriormente, se diseñan controladores de corriente y tensión para un inversor DC-AC trifásico que logren una estabilidad robusta del sistema para una amplia variedad de filtros. La condición de estabilidad robusta menos conservadora, siendo la impedancia de la red la principal fuente de incertidumbre, es el índice de pasividad. En el caso de los controladores de corriente, el impacto de los lazos superiores en la estabilidad basada en la impedancia también se analiza mediante un índice adicional: máximo valor singular. Cada uno de los índices se aplica a un rango de frecuencias determinado. Finalmente, estas condiciones se incluyen en el diseño en un solo paso del controlador de un convertidor back-to-back utilizado para operar generadores de inducción doblemente alimentados (aerogeneradores tipo 3) presentes en algunos parques eólicos. Esta solución evita los problemas de oscilación subsíncrona, derivados de las líneas de transmisión con condensadores de compensación en serie, a los que se enfrentan estos parques eólicos. Los resultados de simulación y experimentales demuestran la eficacia y versatilidad de la propuesta.Impedance or admittance shaping by control for power electronic converters allows to achieve among other objectives: robustness enhancement of the designed controls, damped voltage dynamics in case of load changes, and grid filter and controller optimization in a single step (co-design). Impedance shaping must always be accompanied by a correct reference tracking performance. Therefore, the main idea is to design controllers with a simple structure that balance the achievement of the objectives set in each case. This design is carried out using modern techniques, whose resolution (controller synthesis) requires optimization tools. The main advantage of these techniques over the classical ones, i.e. those based on algebraic solutions, is their ability to deal with complex control problems (high order plants and/or several objectives) in a considerably systematic way. The first impedance shaping control problem is to reduce voltage overshoot under load changes and minimize the size of passive filter components in DC-DC converters. Subsequently, current and voltage controllers for a three-phase DC-AC inverter are designed to achieve robust system stability for a wide variety of filters. The least conservative robust stability condition, with grid impedance being the main source of uncertainty, is the passivity index. In the case of current controllers, the impact of higher loops on impedance-based stability is also analyzed by an additional index: maximum singular value. Each of the indices is applied to a given frequency range. Finally, these conditions are included in the one-step design of the controller of a back-to-back converter used to operate doubly fed induction generators (type-3 wind turbines) present in some wind farms. This solution avoids the sub-synchronous oscillation problems, derived from transmission lines with series compensation capacitors, faced by these wind farms. Simulation and experimental results demonstrate the effectiveness and versatility of the proposa

High-performance motor drives

This article reviews the present state and trends in the development of key parts of controlled induction motor drive systems: converter topologies, modulation methods, as well as control and estimation techniques. Two- and multilevel voltage-source converters, current-source converters, and direct converters are described. The main part of all the produced electric energy is used to feed electric motors, and the conversion of electrical power into mechanical power involves motors ranges from less than 1 W up to several dozen megawatts

Hysteretic control of grid-side current for a single-phase LCL grid-connected voltage source converter

© 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/This paper proposes a new approach to control the grid-side current of LCL-grid connected voltage source converters using hysteretic relay feedback controllers. The closed loop system is stabilized by designing a local feedback around the relay element. The compensator allows the use of relay feedback controllers by making the controlled plant almost strictly positive real. The article proposes the use of the locus of the perturbed relay system as analysis and design tool and studies orbital stability for several plant and controller conditions. The approach is validated by means of simulation testing.Postprint (author's final draft

AC Current Controller with Error-Free Feedback Acquisition System

In this paper, we introduce an improved ac current controller with robustness against the noise in the feedback path. This three-phase current controller is suited for inverter supplied ac machines and grid-connected power converters. Conventional solutions make use of symmetric pulse-width modulation (PWM) techniques with feedback sampling in the middle of the voltage pulses. Single-sample-based feedback acquisition gets affected by the noise and parasitic phenomena. We perform a thorough analytical and experimental study on feedback errors with conventional sampling, and we consider the impact of the lockout time, motor cables, winding capacitance, and anti-aliasing filters. In order to suppress a significant spectral content in the area of low-order harmonics, we apply an improved acquisition technique, which uses a period-average and removes any PWM noise from the feedback signals. Both anti-aliasing filter and proposed period-average filter introduce delays, which impair the control-loop performance. The conventional current controller is then extended to suppress the effects of the delays. Parameter setting procedure is devised to achieve both the bandwidth and the robustness against the parameter changes. Analytical and experimental studies prove that the proposed feedback acquisition technique improves the robustness in the presence of variable delays and switching transients. Experimental tests show that the extended current controller with period-average feedback acquisition reaches the same bandwidth and robustness as the state-of-the-art controller

Synchronverter-based control for wind power

More and more attention has been paid to the energy crisis due to the increasing energy demand from industrial and commercial applications. The utilisation of wind power, which is considered as one of the most promising renewable energy sources, has grown rapidly in the last three decades. In recent years, many power converter techniques have been developed to integrate wind power with the electrical grid. The use of power electronic converters allows for variable speed operation of wind turbines, and enhanced power extraction. This work, which is supported by EPSRC and Nheolis under the DHPA scheme, focuses on the design and analysis of control systems for wind power. In this work, two of the most popular AC-DC-AC topologies with permanent magnet synchronous generators (PMSG) have been developed. One consists of an uncontrollable rectifier, a boost converter and an inverter and a current control scheme is proposed to achieve the maximum power point tracking (MPPT). In the control strategy, the output current of the uncontrollable rectifier is controlled by a boost converter according to the current reference, which is determined by a climbing algorithm, to achieve MPPT. The synchronverter technology has been applied to control the inverter for the grid-connection. An experimental setup based on DSP has been designed to implement all the above mentioned experiments. In addition, a synchronverter-based parallel control strategy, which consists of a frequency droop loop and a voltage droop loop to achieve accurate sharing of real power and reactive power respectively, has been further studied. Moreover, a control strategy based on the synchronverter has been presented to force the inverter to have capacitive output impedance, so that the quality of the output voltage is improved. Abstract The other topology consists of a full-scale back-to-back converter, of which the rectifier is controllable. Two control strategies have been proposed to operate a three-phase rectifier to mimic a synchronous motor, following the idea of synchronverters to operate inverters to mimic synchronous generators. In the proposed schemes, the real power extracted from the source and the output voltage are the control variables, respectively, hence they can be employed in different applications. Furthermore, improved control strategies are proposed to self-synchronise with the grid. This does not only improve the performance of the system but also considerably reduces the complexity of the overall controller. All experiments have been implemented on a test rig based on dSPACE to demonstrate the excellent performance of the proposed control strategies with unity power factor, sinusoidal currents and good dynamics. Finally, an original control strategy based on the synchronverter technology has been proposed for back-to-back converters in wind power applications to make the whole system behave as a generator-motor-generator system

Application of Optimal Switching Using Adaptive Dynamic Programming in Power Electronics

In this dissertation, optimal switching in switched systems using adaptive dynamic programming (ADP) is presented. Two applications in power electronics, namely single-phase inverter control and permanent magnet synchronous motor (PMSM) control are studied using ADP. In both applications, the objective of the control problem is to design an optimal switching controller, which is also relatively robust to parameter uncertainties and disturbances in the system. An inverter is used to convert the direct current (DC) voltage to an alternating current (AC) voltage. The control scheme of the single-phase inverter uses a single function approximator, called critic, to evaluate the optimal cost and determine the optimal switching. After offline training of the critic, which is a function of system states and elapsed time, the resulting optimal weights are used in online control, to get a smooth output AC voltage in a feedback form. Simulations show the desirable performance of this controller with linear and nonlinear load and its relative robustness to parameter uncertainty and disturbances. Furthermore, the proposed controller is upgraded so that the inverter is suitable for single-phase variable frequency drives. Finally, as one of the few studies in the field of adaptive dynamic programming (ADP), the proposed controllers are implemented on a physical prototype to show the performance in practice. The torque control of PMSMs has become an interesting topic recently. A new approach based on ADP is proposed to control the torque, and consequently the speed of a PMSM when an unknown load torque is applied on it. The proposed controller achieves a fast transient response, low ripples and small steady-state error. The control algorithm uses two neural networks, called critic and actor. The former is utilized to evaluate the cost and the latter is used to generate control signals. The training is done once offline and the calculated optimal weights of actor network are used in online control to achieve fast and accurate torque control of PMSMs. This algorithm is compared with field-oriented control (FOC) and direct torque control based on space vector modulation (DTC-SVM). Simulations and experimental results show that the proposed algorithm provides desirable results under both accurate and uncertain modeled dynamics