Performance analysis of a sliding mode control for distributed generations

This paper presents the performance of a sliding mode based hybrid controller for three phase voltage source inverter. The main objective of this analysis is to observe the effectiveness of the controller for fault ride through (FRT) capability improvement of the DC based distributed generations (DG). The performance of the conventional PI based cascaded controller is also presented for comparison purpose

A Voltage Unbalance Mitigation Technique for Low-voltage Applications with Large Single-phase Loads

In this paper a voltage unbalance mitigation technique for low-voltage microgrids or feeders in presence of large single-phase loads is introduced. In order to take maximum advantage of the existing hardware, the proposed solution consists of a sequence-based decentralized voltage control to be embedded in three-phase VSC connecting distributed generation to the considered system. Furthermore, a centralized controller is proposed to define optimal negative and zero sequence voltage reference. Control effectiveness is numerically verified considering a low-voltage feeder case study

Estrategias de control para sistemas de generación distribuida durante huecos de tensión

The objective of this thesis is the development of control strategies for distributed generation systems during voltage sags. The proposed control strategies present different options to improve the behavior of inverters during voltage unbalances and grid faults. Therefore, it could be possible to contribute to ride-through the perturbation and to avoid the disconnection of power suppliers during these contingencies. The scope of the proposed strategies covers a wide range of possibilities, from static voltage unbalance to dynamic voltage sags. The control algorithms are versatile and their usefulness is discussed in detail. The utility of these algorithms is focused on distributed generation in power networks with high penetration of renewables energy sources, mainly wind farms, located in remote zones and connected in weak grids. The proposed control strategies are: (i). Voltage support control of the maximum and minimum phase voltages within the limits for continuous operation. (ii). Control of positive and negative active and reactive powers to flexibilize the low-voltage ridethrough services, by injecting the maximum current of the inverter. (iii). On the use of effective power factor for reducing the voltage unbalance in static grid voltage perturbations.El objetivo de esta tesis es el desarrollo de estrategias de control para sistemas de generación distribuida durante huecos de tensión. Las estrategias de control propuestas presentan diferentes alternativas para mejorar el comportamiento de los inversores durante desbalanceos y fallos de red. De esta forma, es posible contribuir a la superación de la perturbación y evitar la desconexión de los sistemas de potencia ante este tipo de contingencias. El ámbito de aplicación de las estrategias propuestas cubre un gran abanico de posibilidades, desde desbalanceos de tensión estáticos hasta huecos de tensión dinámicos. Los algoritmos de control son versátiles y su aplicabilidad es discutida en detalle. La utilidad de estos algoritmos está enfocada hacia sistemas de generación distribuida en redes eléctricas con gran integración de renovables, principalmente eólica, ubicados en zonas remotas y conectados a una red blanda. Las estrategias de control presentadas son (i). Control para el soporte de tensión máxima y mínima dentro de los límites para la operación continua. (ii). Control de potencias activa y reactiva de secuencia positiva y negativa para flexibilizar los servicios durante huecos de tensión, inyectando el máximo de corriente permitida por el inversor. (iii). Alternativas para la utilización del factor de potencia efectivo para reducir el desbalanceo en perturbaciones estáticas de tensión

Uncertainty and disturbance estimator design to shape and reduce the output impedance of inverter

Power inverters are becoming more and more common in the modern grid. Due to their switching nature, a passive filter is installed at the inverter output. This generates high output impedance which limits the inverter ability to maintain high power quality at the inverter output. This thesis deals with an impedance shaping approach to the design of power inverter control. The Uncertainty and Disturbance Estimator (UDE) is proposed as a candidate for direct formation of the inverter output impedance. The selection of UDE is motivated by the desire for the disturbance rejection control and the tracking controller to be decoupled. It is demonstrated in the thesis that due to this fact the UDE filter design directly influences the inverter output impedance and the reference model determines the inverter internal electromotive force. It was recently shown in the literature and further emphasized in this thesis that the classic low pass frequency design of the UDE cannot estimate periodical disturbances under the constraint of finite control bandwidth. Since for a power inverter both the reference signal and the disturbance signal are of periodical nature, the classic UDE lowpass filter design does not give optimal results. A new design approach is therefore needed. The thesis develops four novel designs of the UDE filter to significantly reduce the inverter output impedance and maintain low Total Harmonic Distortion (THD) of the inverter output voltage. The first design is the based on a frequency selective filter. This filter design shows superiority in both observing and rejecting periodical disturbances over the classic low pass filter design. The second design uses a multi-band stop design to reject periodical disturbances with some uncertainty in the frequency. The third solution uses a classic low pass filter design combined with a time delay to match zero phase estimation of the disturbance at the relevant spectrum. Furthermore, this solution is combined with a resonant tracking controller to reduce the tracking steady-state error in the output voltage. The fourth solution utilizes a low-pass filter combined with multiple delays to increase the frequency robustness. This method shows superior performance over the multi-band-stop and the time delayed filter in steady-state. All the proposed methods are validated through extensive simulation and experimental results

Grid-adaptive control and active-filter functionality of grid-connected PWM converters in renewable energy generation

Die hohe Durchdringung öffentlicher Energienetze mit zumeist leistungselektronisch angebundenen regenerativen Energieerzeugungsanlagen (kurz: EZAs) führt zu einem Wandel der Energieerzeugung. Die Netzimpedanz stellt dabei einen Schlüsselparameter bei der Erzeugung und Verteilung elektrischer Energie dar. In dieser Arbeit wird ein Gesamtkonzept für die Erweiterung der Regelung eines Netzpulsstromrichters einer dezentralen EZA um die Adaption variierender ohmsch-induktiver Netzimpedanzen (kurz: netzadaptive Regelung) und einer zusätzlichen Aktiv-Filter Funktionalität (kurz: AFF) entwickelt, untersucht und unter Laborbedingungen validiert. Für die Detektion der äquivalenten Netzparameter werden zwei Verfahren berücksichtigt: (1) Die Messung der Netzimpedanz durch die Einspeisung eines interharmonischen Oberschwingungsstromes und (2) die Schätzung der Netzimpedanz mittels eines Erweiterten Kalman-Filters. Die Analysen dieser Detektionsverfahren zeigen, dass sich die Messung der Netzimpedanz durch eine hohe Detektionsgenauigkeit und die Schätzung der Netzimpedanz durch eine hohe Detektionsgeschwindigkeit auszeichnet. Diese Erkenntnis führt zu der Entwicklung eines kombinierten Identifikationskonzeptes, welches die Detektion der Netzparameter in Echtzeit und mit minimaler Systemanregung ermöglicht. Das kombinierte Identifikationskonzept dient als Grundlage für die Entwicklung eines netzadaptiven Regelungskonzeptes eines Netzpulsstromrichters mit netzseitigem LCL-Filter. Auf der Grundlage einer detaillierten Modellierung der Regelstrecke in einem rotierenden (dq) Koordinatensystem wird ein zeitdiskreter Stromregler mit idealerweise vollständiger Entkopplung der nieder- und hochfrequenten Verkopplungsdynamiken von LCL-Filtern entworfen. Es wird ein auf einer Sollwertfilterung basierendes aktives Dämpfungsnetzwerk entwickelt und um eine Adaption variierender Netzparameter erweitert. Die experimentellen Untersuchungen zeigen, dass die entwickelte netzadaptive Regelung insbesondere bei sprunghaften Änderungen der Netzimpedanz zu einer Erhöhung des Stabilitätsbereiches führt ohne die Dynamik der gesamten Stromregelung reduzieren zu müssen. Zudem wird die netzadaptive Regelung um eine AFF erweitert. Die zusätzliche AFF führt zu einer Reduzierung bzw. vollständigen Kompensation von Spannungsunsymmetrien und -oberschwingungen am Anschlusspunkt des Netzpulsstromrichters einer dezentralen EZA. Die theorischen und praktischen Analysen zu der AFF offenbaren, dass eine lastadaptive Betriebsführung für die Ausnutzung der maximalen Betriebsgrenzen eines Netzpulsstromrichters zielführend ist. Hierfür wird ein lastadaptiver Begrenzungsregler entwickelt. Das neu entwickelte lastadaptive Regelungskonzept zur Umsetzung der AFF führt zu einer deutlichen Verbesserung der Spannungsqualität am Anschlusspunkt einer regenerativen und denzentralen EZA.The high penetration of public energy distribution networks with distributed energy production units mainly connected through power electronic converters leads to a high utilization of the existing grid structures. This trend is mainly driven by the efforts to push electrical power generation toward green and sustainable energy production. In this context the grid impedance is a key parameter for the generation and distribution of electrical energy. In this work an overall control concept for the grid impedance adaptive control and an additional active-filter functionality of grid-connected PWM converters is proposed, developed and validated with experimental results. The detection of equivalent grid parameters is examined using two different methods: (1) the measurement of the grid impedance by means of inter-harmonic current injection and (2) the estimation of the grid impedance using an Extended Kalman-Filter. The analysis of these two methods reveals that measurement of the grid impedance leads to a high detection precision whereas the estimation of the grid impedance leads to high detection dynamics. This insight motivates the development of a combined identification method that is able to detect the grid impedance conditions in a real-time manner with minimal system excitation. The combined identification method is the basis for the development of a grid impedance adaptive control concept for grid connected PWM converters with LCL-filters. The control is based on a detailed model of the control plant in the rotating (dq) reference frame whereas a discrete current controller with improved decoupling dynamics of both low and high frequency coupling dynamics of a LCL-filter is proposed. An active damping of the LCL-filter resonance is achieved using a grid impedance adaptive reference value filter. The measurement analysis shows that the proposed grid impedance adaptive control is able to guarantee stable converter operation with high control performance in the presence of step-wise grid impedance changes without the need of decreasing the current control dynamics. In addition to high performance current control and grid impedance detection capability, the grid impedance adaptive control is superimposed with an active-filter functionality. The active-filter functionality leads to a mitigation or compensation of voltage-unbalances and lower-order voltage-harmonics at the grid-connection point of the renewable energy system. The analysis of the active-filter functionality motivated the development of the load adaptive operation point controller to utilize the maximum capability limits of the grid-connected converter. A measurement analysis validates the proposed control concept and demonstrates that a considerable voltage quality improvement is achieved by the additional active-filter functionality