Thesis (M.E.)--University of Canterbury, 2006. / Typescript (photocopy). "March 2006." Includes bibliographical references (p. 171-179). Also available via the World Wide Web.

Driven by economic, technical and environmental factors, the energy sector is currently undergoing a profound paradigm shift towards a smarter grid setup. Increased intake of Distributed and Renewable Generation (DG) units is one of the Smart Grid (SG) pillars that will lead to numerous advantages among which lower electricity losses, increased reliability and reduced greenhouse gas emissions are the most salient. The increase of DG units’ penetration will cause changes to the characteristics of distribution networks from being passive with unidirectional power flow towards Active Distribution Networks (ADNs) with multi-direction power flow. However, such changes in the current distribution systems structure and design will halt the seamless DG integration due to various technical issues that may arise. Voltage and reactive power control is one of the most significant issues that limit increasing DG penetration into distribution systems. On the other hand, the term microgrid has been created to be the building block of ADNs. A microgrid should be able to operate in two modes of operation, grid-connected or islanded. The successful implementation of the microgrid concept demands a proper definition of the regulations governing its integration in distribution systems. In order to define such regulations, an accurate evaluation of the benefits that microgrids will bring to customers and utilities is needed. Therefore, there is a need for careful consideration of microgrids in the assessment, operation, planning and design aspects of ADNs. Moreover, SG offers new digital technologies to be combined with the existing utility grids to substantially improve the overall efficiency and reliability of the network. Advanced network monitoring, two ways communication acts and intelligent control methods represent the main features of SG. Thus it is required to properly apply these features to facilitate a seamless integration of DG units in ADNs considering microgrids. Motivated by voltage and reactive power control issues in ADNs, the concept of microgrids, and SG technologies, three consequent stages are presented in this thesis. In the first stage, the issues of voltage and reactive power control in traditional distribution systems are addressed and assessed in order to shed the light on the potential conflicts that are expected with high DG penetration. A simple, yet efficient and generic three phase power flow algorithm is developed to facilitate the assessment. The results show that utility voltage and reactive power control devices can no longer use conventional control techniques and there is a necessity for the evolution of voltage and reactive power control from traditional to smart control schemes. Furthermore, a probabilistic approach for assessing the impacts of voltage and reactive power constraints on the probability of successful operation of islanded microgrids and its impacts on the anticipated improvement in the system and customer reliability indices is developed. The assessment approach takes into account: 1) the stochastic nature of DG units and loads variability, 2) the special philosophy of operation for islanded microgrids, 3) the different configurations of microgrids in ADNs, and 4) the microgrids dynamic stability. The results show that voltage and reactive power aspects cannot be excluded from the assessment of islanded microgrids successful operation. The assessment studies described in the first stage should be followed by new voltage and reactive power planning approaches that take into account the characteristics of ADNs and the successful operation of islanded microgrids. Feeders shunt capacitors are the main reactive power sources in distribution networks that are typically planned to be located or reallocated in order to provide voltage support and reduce the energy losses. Thus, in the second stage, the problem of capacitor planning in distribution network has been reformulated to consider microgrids in islanded mode. The genetic algorithm technique (GA) is utilized to solve the new formulation. The simulation results show that the new formulation for the problem of capacitor planning will facilitate a successful implementation of ADNs considering islanded microgrids. In the third stage, the SG technologies are applied to construct a two ways communication-based distributed control that has the capability to provide proper voltage and reactive power control in ADNs. The proposed control scheme is defined according to the concept of multiagent technology, where each voltage and reactive power control device or DG unit is considered as a control agent. An intelligent Belief-Desire-Intention (BDI) model is proposed for the interior structure of each control agent. The Foundation for Intelligent Physical Agents (FIPA) performatives are used as communication acts between the control agents. First, the distributed control scheme is applied for voltage regulation in distribution feeders at which load tap changer (LTC) or step voltage regulators are installed at the begging of the feeder. In this case, the proposed control aims to modify the local estimation of the line drop compensation circuit via communication. Second, the control scheme is modified to take into consideration the case of multiple feeders having a substation LTC and unbalanced load diversity. To verify the effectiveness and robustness of the proposed control structure, a multiagent simulation model is proposed. The simulation results show that distributed control structure has the capability to mitigate the interference between DG units and utility voltage and reactive power control devices.

Driven by economic, technical and environmental factors, the energy sector is currently undergoing a profound paradigm shift towards a smarter grid setup. Increased intake of Distributed and Renewable Generation (DG) units is one of the Smart Grid (SG) pillars that will lead to numerous advantages among which lower electricity losses, increased reliability and reduced greenhouse gas emissions are the most salient. The increase of DG units’ penetration will cause changes to the characteristics of distribution networks from being passive with unidirectional power flow towards Active Distribution Networks (ADNs) with multi-direction power flow. However, such changes in the current distribution systems structure and design will halt the seamless DG integration due to various technical issues that may arise. Voltage and reactive power control is one of the most significant issues that limit increasing DG penetration into distribution systems. On the other hand, the term microgrid has been created to be the building block of ADNs. A microgrid should be able to operate in two modes of operation, grid-connected or islanded. The successful implementation of the microgrid concept demands a proper definition of the regulations governing its integration in distribution systems. In order to define such regulations, an accurate evaluation of the benefits that microgrids will bring to customers and utilities is needed. Therefore, there is a need for careful consideration of microgrids in the assessment, operation, planning and design aspects of ADNs. Moreover, SG offers new digital technologies to be combined with the existing utility grids to substantially improve the overall efficiency and reliability of the network. Advanced network monitoring, two ways communication acts and intelligent control methods represent the main features of SG. Thus it is required to properly apply these features to facilitate a seamless integration of DG units in ADNs considering microgrids. Motivated by voltage and reactive power control issues in ADNs, the concept of microgrids, and SG technologies, three consequent stages are presented in this thesis. In the first stage, the issues of voltage and reactive power control in traditional distribution systems are addressed and assessed in order to shed the light on the potential conflicts that are expected with high DG penetration. A simple, yet efficient and generic three phase power flow algorithm is developed to facilitate the assessment. The results show that utility voltage and reactive power control devices can no longer use conventional control techniques and there is a necessity for the evolution of voltage and reactive power control from traditional to smart control schemes. Furthermore, a probabilistic approach for assessing the impacts of voltage and reactive power constraints on the probability of successful operation of islanded microgrids and its impacts on the anticipated improvement in the system and customer reliability indices is developed. The assessment approach takes into account: 1) the stochastic nature of DG units and loads variability, 2) the special philosophy of operation for islanded microgrids, 3) the different configurations of microgrids in ADNs, and 4) the microgrids dynamic stability. The results show that voltage and reactive power aspects cannot be excluded from the assessment of islanded microgrids successful operation. The assessment studies described in the first stage should be followed by new voltage and reactive power planning approaches that take into account the characteristics of ADNs and the successful operation of islanded microgrids. Feeders shunt capacitors are the main reactive power sources in distribution networks that are typically planned to be located or reallocated in order to provide voltage support and reduce the energy losses. Thus, in the second stage, the problem of capacitor planning in distribution network has been reformulated to consider microgrids in islanded mode. The genetic algorithm technique (GA) is utilized to solve the new formulation. The simulation results show that the new formulation for the problem of capacitor planning will facilitate a successful implementation of ADNs considering islanded microgrids. In the third stage, the SG technologies are applied to construct a two ways communication-based distributed control that has the capability to provide proper voltage and reactive power control in ADNs. The proposed control scheme is defined according to the concept of multiagent technology, where each voltage and reactive power control device or DG unit is considered as a control agent. An intelligent Belief-Desire-Intention (BDI) model is proposed for the interior structure of each control agent. The Foundation for Intelligent Physical Agents (FIPA) performatives are used as communication acts between the control agents. First, the distributed control scheme is applied for voltage regulation in distribution feeders at which load tap changer (LTC) or step voltage regulators are installed at the begging of the feeder. In this case, the proposed control aims to modify the local estimation of the line drop compensation circuit via communication. Second, the control scheme is modified to take into consideration the case of multiple feeders having a substation LTC and unbalanced load diversity. To verify the effectiveness and robustness of the proposed control structure, a multiagent simulation model is proposed. The simulation results show that distributed control structure has the capability to mitigate the interference between DG units and utility voltage and reactive power control devices.

Voltage stability is the ability of a power system to maintain acceptable voltages at all buses under normal and abnormal operating conditions. The scarcity of reactive power or reactive power imbalance is the main reason of voltage instabilities. The energy supply of Germany will be dominated by renewable energy sources (RES) within energy transition actions leading to a decrease in conventional power plants especially in the transmission grid. Since the conventional power plants are still main reactive power sources, key challenge in the future will be the provision of system services such as control of reactive power to maintain voltage stability in the transmission networks. A wide range of technology that is able to provide the required dynamic reactive power compensation is already available. However, in order to find the most effective implementation, it is necessary to investigate and compare these different technologies for a voltage stable grid operation. The main challenge regarding comparative studies that incorporate reactive power compensation devices is the development and the implementation of reliable comparison strategies. This thesis analyses the impact of reactive power on the voltage stability phenomena both in long-term and short-term time frames under various grid situations. Voltage stability margins in the long-term time frame are quantified by gradually increasing the reactive power injection into certain buses until the voltage collapses. Voltage stability in the short-term time frame is assessed by applying grid faults and simulating the time-domain grid response. A transient voltage severity ratio (TVSR) is developed in this thesis as the main comparison metric to evaluate and compare the performance of investigated compensation devices. Additionally, TVSR and other developed comparison metrics in this thesis are used to determine the optimal settings of the controller parameters of compensation devices. As a result, the submitted thesis shows a practical and reliable approach to analyze the performance of different compensation devices under different grid situations.

Le réglage de la tension et la gestion de la puissance réactive est d’une importance capitale pour le bon fonctionnement du système électrique. Les réseaux de distribution connaissent des modifications profondes qui sont tant techniques avec l’insertion de la production décentralisée ou l’enfouissement des lignes aériennes, que réglementaires avec l’entrée en vigueur des codes de réseaux Européens. Aussi, des alternatives aux réglages traditionnels de tension et la mise en place de contrôle des échanges de puissance réactive à l’interface réseau de transport/distribution peuvent être développées. Dans le cadre de ces travaux de doctorat, une solution basée sur l’amélioration du réglage temps réel des réseaux de distribution ainsi que la mise en place d’une gestion conjointe de la puissance réactive entre les gestionnaires du réseau de transport et distribution a été proposée. Ce réglage temps réel est basé sur une méthode de commande prédictive, et s’appuie notamment sur le régleur en charge ou les productions décentralisées pour réaliser un réglage de la tension au sein d’un réseau de distribution et contrôler les échanges de puissance réactive. Les références de puissance réactive à atteindre à l’interface entre réseau de distribution et de transport sont déterminées par le gestionnaire du réseau de transport pour ses propres besoins et en connaissance des réserves de puissance réactive disponible depuis les réseaux de distribution. Par rapport à la littérature, notre démarche a pour originalité de prendre en compte les problèmes à l’interface des réseaux de distribution et de transport et démontre l’intérêt de mener des études conjointes entre gestionnaires / Voltage and reactive power control are of paramount importance to ensure safe and reliable operation of the power system. Distribution grids are undergoing major changes, namely the insertion of distributed generation and the replacement of overhead lines. Along with these physical evolutions, new distribution networks should comply with the requirements of the European Grid Codes on the reactive power exchange at the HV/MV interface. To handle these new operational concerns, alternative solutions to the traditional voltage and reactive power control can be found. In our work, a scheme based on the evolution of the real-time Volt Var Control (VVC) of distribution networks and a joint coordination for the reactive power management of a HV system has been proposed. The real-time VVC of MV grids is based on a predictive control method. This control uses in a coordinated manner the on load tap changer, the distributed generation and the capacitor banks to enforce a suitable MV voltage profile and an appropriate HV/MV reactive power exchange. The reactive power targets at the HV/MV interface are determined by the Transmission System Operator for its own requirements but considering the true MV reactive power reserve. Compared to the literature, the novelty of our approach consists in considering the concerns at the HV/MV system interface. Next our works have shown the relevance of performing joint transmission and distribution system operators analysis.