Rodriguez, P.; Luna, A.; Teodorescu, R.; Blaabjerg, F. Energy 2030 Conference, 2008. ENERGY 2008. IEEE p. 517-524 DOI: 10.1109/ENERGY.2008.4781065 Data de presentació: 2018-11-17 Presentació treball a congrés
This work employs the double synchronous reference frame PLL (DSRF-PLL) as an effective method for grid synchronization of WT's power converters in the presence of transient faults in the grid. The DSRF-PLL exploits a dual synchronous reference frame voltage characterization, adding a decoupling network to a standard SRF-PLL in order to effectively separate the positive- and negative-sequence voltage components in a fast and accurate way. Experimental evaluation of the proposed grid synchronization method and simulations regarding its application to ride through transient faults verify and validate the excellent behavior of the DSRF-PLL in the grid synchronization of WT's power converters under unfavorable grid conditions
Remon, D.; Zhang, W.; Luna, A.; Candela, J.; Rodriguez, P. IEEE International Conference on Renewable Energy Research and Applications p. 169-174 DOI: 10.1109/ICRERA.2017.8191261 Data de presentació: 2017-11-05 Presentació treball a congrés
The increasing penetration of renewable energy sources in power systems has increased the amount of generating units employing power electronics as their grid interface, instead of synchronous generators. Their different dynamics have brought new challenges for power system operators; in particular, the reduction of the total inertia of power systems, which leads to larger frequency disturbances. Different methods have been proposed to address this issue, with virtual synchronous machines as a promising solution; however, the start-up process of these controllers has not received much attention. This paper proposes a start-up strategy for the synchronous power controller, explaining its working principle in detail, and assessing its performance through time-domain simulation.
Verdugo, C.; Candela, J.; Luna, A.; Rodriguez, P. IEEE International Conference on Renewable Energy Research and Applications p. 768-773 DOI: 10.1109/ICRERA.2017.8191163 Data de presentació: 2017-11-05 Presentació treball a congrés
Most of the large scale photovoltaic power plants (LS-PVPP) count on power converters with a central configuration. Advantages such as robustness, low maintenance and installation cost makes this configuration the preferred specially suitable in large scale systems. However, important drawbacks like the low efficiency level make necessary to develop new solutions for future power plants. In this paper a power station for large scale PV systems is proposed, which consists of power inverters synchronized with an interleaving modulation and connected to a multi-winding transformer. The main principles that support this proposal, as well as, simulation results are presented to validate the effectiveness of the proposed configuration.
Grid-connected power converters, which are frequently used to link renewable generation plants with the grid, are required to provide a better functionality for large scale integration of renewables. They are expected to be gridfriendly, or even grid-supportive, instead of simply grid-feeding or grid-demanding. This paper designs a synchronous power controller for grid-connected converters in detail, emulating the electromechanical characteristics of synchronous machines and improving even its actual performance, as it is based on a virtual approach. Based on this design, the grid-interfacing units are capable of showing inertia, damping, and droop characteristics as synchronous machines and presenting thus a grid-supporting behavior. The detailed control design and experimental validation on a 10 kW laboratory setup acts as the main contribution of this paper, compared with the existing studies on generator emulation controls.
The integration of AC-DC power converters to manage the connection of generation to the grid has increased exponentially over the last years. PV or wind generation plants are one of the main applications showing this trend. High power converters are increasingly installed for integrating the renewables in a larger scale. The control design for these converters becomes more challenging due to the reduced control bandwidth and increased complexity in the grid connection filter. A generalized and optimized control tuning approach for converters becomes more favored. This paper proposes an algorithm for estimating the dynamic performance of the stationary frame current controllers, and based on it a generalized and optimized tuning approach is developed. The experience-based specifications of the tuning inputs are not necessary through the tuning approach. Simulation and experimental results in different scenarios are shown to evaluate the proposal.
El concepto de inercia virtual se está convirtiendo en una parte imprescindible de los sistemas de energía modernos y en la actualidad hay diferentes líneas de investigación orientadas a estudiar diferentes métodos de emulación de inercia virtual en diferentes partes del sistema.Esta tesis doctoral se centra en el modelado, análisis y aplicación del concepto de inercia virtual en el control de frecuencia y en el control automático de generación (AGC), los cuales son elementos clave en el control de alto nivel de los sistemas eléctricos interconectados AC / DC.Dada la evolución tanto en las topologías, como en las prestaciones de control de los convertidores electrónicos de potencia, los enlaces HVDC y el control a alto nivel de los dispositivos implicados para emular inercia constituirán el foco principal de esta tesis doctoral.El AGC es un mecanismo muy útil en un sistema de potencia multi-zona durante, sobre todo cuando hay variaciones en la carga o en la generación, ya que puede facilitar diversas tareas como: la restauración de frecuencia, control de potencia de líneas de enlace entre las áreas y el despacho económico de los sistemas de generación. En este contexto en AGC, dada su condición de control a alto nivel, generará las consignas para todos los elementos locales, tales como generadores o estaciones de conversión de energía, que están bajo el control de sus controladores locales.En esta tesis se proponen dos métodos diferentes para emular inercia virtual, los cuales se introducirán, se modelarán y se aplicarán al control de sistemas de potencia AC / DC interconectados. El primer método, que es uno de los métodos más comunes para la emulación de inercia en otras aplicaciones, es la técnica del control derivativo. En esta tesis, esta técnica se utiliza para llevar a cabo la emulación de inercia a alto nivel. Éste método de emulación de inercia se ha desarrollado para un sistema AGC de dos área que está interconectado mediante líneas de transmisión de CA / CC en paralelo. Sobre la base de la técnica propuesta, el efecto dinámico de la inercia emulada sobre el control de frecuencia y potencia activa será formulado y evaluado. Los efectos del retardo mediciones de frecuencia y los efectos del bucle de sincronización (PLL) también se consideran en el análisis y posterior ensayo. Las simulaciones llevadas a cabo demostrarán cómo la emulación de inercia virtual puede mejorar efectivamente el rendimiento del sistema de potencia.Dado que el primer método se basa en la derivación de la frecuencia de red, la medición de la frecuencia es muy importante y la aplicación de los métodos de medida de la misma, como las PLL, conlleva algunas limitaciones en la aplicación y respuesta del mismo. Por lo tanto, como una solución definitiva, se introduce el segundo método para la emulación de inercia virtual en esta tesis. El segundo método se basa en el concepto de energía síncrona virtual (VSP). El concepto de VSP, el cual permite simular los efectos dinámicos de las emulaciones de inercia por enlaces HVDC, se presentará y aplicará en sistemas de control de alto nivel, aplicándolo finalmente en el modelo AGC multi-zona. Tal y como se demostrará, mediante el uso de esta combinación propuesta en el modelo de AGC, el comportamiento dinámico de los sistemas muestra una mejora significativa. El control de bucle de potencia activa en el enlace HVDC basado en el VSP tiene una característica de segundo orden que genera una respuesta instantánea y permite controlar la amortiguación y la inercia en el sistema. La eficacia de concepto propuesto en las mejoras dinámicas se probará en plataformas de simulación que representarán un sistema multi-zona. Por último, se demostrará que la inercia virtual añade un grado de libertad adicional a la dinámica del sistema, lo cual ayuda a controlar el sobre pico así como el amortiguamiento en los transitorios en los enlaces HVDC.
Virtual inertia is known as an inevitable part of the modern power systems. Recent trend of research is oriented in different methods of emulating virtual inertia in different part of the systems. This dissertation is focused on modelling, analysing and application of virtual inertia concept in frequency control and Automatic Generation Control (AGC) issue in high level control AC/DC interconnected power systems. Since the virtual inertia is provided by advanced control concepts of power electronic based components, the HVDC links are the main focus of this dissertation for emulating inertia.
AGC in a multi-area power system during load and resource variation is known as a very important mechanism that could facilitate various tasks like: frequency restoration, tie-line power control between authority areas and economic dispatch of generation units. The AGC concept is known as higher level control at the transmission level. This higher level control will generate the set-points for all the local components, like generators or power converter stations, which are under control by their local controllers.
In this thesis two different methods for emulating virtual inertia are proposed and introduced in AGC modelling and control of AC/DC interconnected power systems.
The first method which is one of the common methods for emulating inertia in various filed of applications, is derivative control technique. In this thesis, derivative control technique is used for higher level application of inertia emulation. This
method of inertia emulation is developed for two-area AGC system which is connected by parallel AC/DC transmission lines. Based on the proposed technique, the dynamic effect of inertia emulated for frequency and active power control of interconnected systems are evaluated. The effects of frequency measurements delay and Phase Locked Loop (PLL) effects are also considered by introducing a second-order function. Simulations performed by Matlab software demonstrate how virtual inertia emulation can effectively improve the performance of the power system. A detailed eigenvalue and sensitivity analyses have been also performed to support the positive effects of the proposed method.
Since the first method is based on derivation for grid frequency, the measurement of frequency is very important and application of different method for frequency measurements like PLL will bring some limitations for this method. Therefore, as an ultimate solution, the second method for virtual inertia emulation is introduced in this thesis. The second method is based on Virtual Synchronous Power (VSP) concept. The concept of VSP to simulate the dynamic effects of inertia emulations by HVDC links for higher level control applications is introduced and reflected in the multi-area AGC model. By using this proposed combination in AGC model, the dynamic performance of the systems shows a significant improvement. The active power loop control on VSP based HVDC link has second-order characteristic which make a simultaneous enabling of damping and inertia emulations into the system. Trajectory sensitivities and eigenvalue analyses are used to analyse the effects of VSP on the system stability. The effectiveness of proposed concept on dynamic improvements is tested through Matlab simulation of multi-area test system.
Finally, it became clear that virtual inertia will add additional degree of freedom to the system dynamics which makes a considerable improvement in first overshoot in addition to damping characteristics of HVDC links. Comparing the results of these two different methods of inertia emulation shows that VSP technique has better performance with several advantages for emulating the inertia. In the VSP technique, PLL and frequency estimation are not required. Also considering the fact that simultaneous damping and inertia could be emulated, a powerful method based on VSP for improving the system dynamics during the contingencies is proposed.
Rouzbehi, K.; Candela, J.; Gharehpetian, G.B.; Harnefors, L.; Luna, A.; Rodriguez, P. Renewable and sustainable energy reviews Vol. 70, p. 886-895 DOI: 10.1016/j.rser.2016.11.270 Data de publicació: 2016-12-06 Article en revista
Nowadays, some Multi-terminal DC (MTDC) systems are in operation around the world. Soon, MTDC grids will be built and overlay the present AC grids. The main driver for the construction of such a grid is to facilitate large-scale integration of remote renewable energy sources to existing AC grids and to develop the energy market. This paper presents a comprehensive analogy between the control and operation aspects of the emerging MTDC grids to those of the traditional AC power grids. Similarities and difference between the two technologies are presented and highlighted. Based on the performed detailed overview, even though a three-layered control system, i.e., primary, secondary, and tertiary control layers is state-of-the-art in large-scale AC power systems, a two-layered control system will satisfy MTDC grids control and operation requirements. This paper also addresses some control and operational issues and limitations of MTDC grids.
Garcia, J.Ignacio; Candela, J.; Luna, A.; Catalan, P. Annual Conference of the IEEE Industrial Electronics Society p. 2313-2318 DOI: 10.1109/IECON.2016.7792973 Data de presentació: 2016-10-23 Presentació treball a congrés
This paper presents a grid synchronization structure for three-phase electric power systems based on the use of a filtered quadrature signal generator (FQSG) and a phase-locked loop (PLL) structure, named Adaptive Vector Grid Synchronization system (AVGS). This system estimates the magnitude, frequency and phase of a signal, specially three-phase voltages and currents, and allows fast and accurate detection of the symmetrical components meet with the transient operating requirements imposed by grid codes. The adaptive vector based PLL (VB-PLL) permits offering a proper performance under generic grid conditions, especially under faulty scenarios. For the particular case of grid voltage synchronization of wind turbine converters, the AVGS is adjusted to provide a good response considering the most critical situation, which is the low voltage ride through (LVRT). This paper includes a detailed study of the proposed grid synchronization system and simulations with registers of real grid disturbances provided by Ingeteam Power Technology S.A.
Rouzbehi, K.; Zhang, W.; Candela, J.; Luna, A.; Rodriguez, P. IET generation, transmission and distribution Vol. 11, num. 3, p. 750-758 DOI: 10.1049/iet-gtd.2016.0665 Data de publicació: 2016-10-11 Article en revista
Multi-terminal dc (MTDC) grids are expected to be built and experience rapid expansion in the near future as they have emerged as a competitive solution for transmitting offshore wind generation and overlaying their ac counterpart. The concept of inertia sharing for the control and operation of MTDC grids, which can be achieved by the proposed unified reference controller. The control objectives of the MTDC grids voltage source converter (VSC) stations are no longer limited to the stabilisation of MTDC grid, instead, the requirements of ac side are also met. The interaction dynamics between the ac and dc grid is analysed to illustrate the proposed concept. In addition, the voltage source converter stations can work in different operation modes based on the proposed unified control structure, and can switch among the operation modes smoothly following the secondary control commands. Simulation results exhibit the merits and satisfactory performance of the proposed control strategy for stable MTDC grid operation.
Zhang, W.; Cantarellas, A.M.; Rocabert, J.; Luna, A.; Rodriguez, P. IEEE Transactions on Sustainable Energy Vol. 7, num. 4, p. 1572-1582 DOI: 10.1109/TSTE.2016.2565059 Data de publicació: 2016-10-01 Article en revista
The increasing amount of renewable power generation systems is a challenging issue for the control and operation of the electrical networks. One of the main issues is their lack of inertia, which is becoming a greater problem as much as the share of the power plants based on traditional synchronous generators gets reduced. In this regard, the new grid codes ask these plants to provide new functionalities such as the frequency support and inertia emulation. In this paper, a synchronous power controller for grid-connected converters is proposed as a good solution for the renewable generation systems with energy storage. It provides inertia, damping, and flexible droop characteristics. Different from the faithful replication of the swing equation of synchronous machines, an alternative control structure is proposed, by which the damping and inherent droop slope can be configured independently to meet the requirements in both dynamics and frequency regulations. Analysis and experimental results are both shown to validate the proposed controller.
Khoshooei, A.; Moghani, J.S.; Milimonfared, J.; Luna, A.; Candela, J.; Rodriguez, P. IEEE Energy Conversion Congress and Exposition p. 1-8 DOI: 10.1109/ECCE.2016.7854965 Data de presentació: 2016-09-18 Presentació treball a congrés
The safe operation of grid connected power converters during abnormal condition is a key issue in order to guarantee its operation and to avoid undesired trips. In this paper different control methods for the operation of a D-STATCOM are evaluated, where the reference currents are determined in such a way that none of the phase currents goes over the limits, as well as the DC voltage fluctuations remain in safe operation limit. Therefore, the contribution of this paper lays on the combination of the DC voltage oscillations and the current limit control. As it is shown in the following, three different control strategies are evaluated. The amplitude of the oscillations which are superimposed on the DC voltage as well as peak amplitude of the phase currents are calculated for each, considering a generic imbalance in the network. The effectiveness of the presented control strategies are verified by simulating a D-STATCOM tied to an industrial distribution network. Moreover a scaled scenario has been reproduced experimentally which shows that the results cope well with the analytical equations and the simulation results.
Zhang, W.; Rouzbehi, K.; Candela, J.; Luna, A.; Rodriguez, P. IEEE Energy Conversion Congress and Exposition p. 1-8 DOI: 10.1109/ECCE.2016.7854969 Data de presentació: 2016-09-18 Presentació treball a congrés
High voltage dc (HVDC) systems act as the prevailed solution for transmitting offshore wind energy to onshore main grids. Control of the voltage source converters (VSC) in HVDC systems is decisive for the performance. This paper proposes the control of VSC-HVDC with electromechanical characteristics and unified primary strategy, as a reaction to the updated requirements of the ac grid transmission system operators. As two important aspects of VSC-HVDC control, converter control and primary control are both designed in detail. Electromechanical characteristics make the VSC capable of providing inertia to the ac networks as well as simplicity in island operation. Besides, unified primary control is given as a universal primary strategy for VSC stations, and especially takes into account frequency support and control mode transition. The proposed converter control is validated in scaled-down 10 kW laboratory setups, while the proposed primary control is endorsed by the simulation tests on a CIGRE multi-terminal HVDC model.
Shahparasti, M.; Catalán, P.; Candela, J.; Luna, A.; Rodriguez, P. IEEE Energy Conversion Congress and Exposition p. 1-7 DOI: 10.1109/ECCE.2016.7854829 Data de presentació: 2016-09-18 Presentació treball a congrés
This paper addresses the stability problems of a high power converter connected to a weak grid. The wide range values that grid impedance can take, challenges the stability and the performance of the controllers, which are responsible of regulating the current injection in such converters. In this work, a control strategy based on stationary reference frame controllers is selected and implemented using a proportional resonant (PR) controller, with capacitor voltage feedforward and a phase shifter. As it will be demonstrated in this paper, although the feedforward contributes to enhance the transient response of the converter, it may cause also deep unstable dynamics near to the medium frequency and decreases the phase margin in low frequency ranges. Therefore, it can be used to damp the unstable dynamics near to resonance frequency range and the LCL-filter can be adopted for the high frequency one. In order to improve the controller performance, a new phase shifter is added to the control scheme to enhance the phase margin at low frequency ranges. Simulation and experimental results considering weak grid conditions are shown to validate the proposed method.
Roslan, N.; Suul, J.; Luna, A.; Rocabert, J.; Candela, J.; Rodriguez, P. IEEE Energy Conversion Congress and Exposition p. 1-6 DOI: 10.1109/ECCE.2016.7854660 Data de presentació: 2016-09-18 Presentació treball a congrés
Grid connected Voltage Source Converters (VSCs) with LCL filters usually have voltage measurements at the filter capacitors, while it can be important to control the active or reactive power injection at the grid-side of the LCL filter, for instance at a Point of Common Coupling (PCC). Synchronization to the PCC voltage can be obtained by Virtual Flux (VF) estimation, which can also allow for voltage sensor-less operation of VSCs. This paper is presenting a comparative evaluation of methods for estimating the VF at the PCC, considering a VSC connected to the grid through an LCL filter with a Proportional Resonant (PR) controller as the inner current control loop. The VF estimation is achieved by using frequency adaptive dual SOGI-QSGs (DSOGI-VF). The Frequency Locked Loop (FLL) is used in order to keep the positive and negative sequence (PNS) VF estimation inherently frequency adaptive. Three different methods are considered for obtaining the capacitor current needed for estimating the VF at the grid side of the LCL filter which are based on fully estimation by using the voltage sensor-less method, by estimating the capacitor current from the measured voltage or by using additional capacitor current sensors. The results have been compared and validated by simulation studies.
Zhang, W.; Remon, D.; Rocabert, J.; Luna, A.; Candela, J.; Rodriguez, P. IEEE Energy Conversion Congress and Exposition p. 1-8 DOI: 10.1109/ECCE.2016.7855127 Data de presentació: 2016-09-18 Presentació treball a congrés
Grid-connected converters with primary frequency control and inertia emulation have emerged and are promising for future renewable generation plants because of the contribution in power system stabilization. This paper gives a synchronous active power control solution for grid-connected converters. As design considerations, the virtual angle stability and transient response are both analyzed, and the detailed implementation structure is also given without entailing any difficulty in practice. The analysis and validation of frequency support characteristics are particularly addressed. The 10 kW simulation and experimental frequency sweep tests on a regenerative source test bed present good performance of the proposed control in terms of showing inertia and droop characteristics, and the controllable transient response is also demonstrated.
Rouzbehi, K.; Candela, J.; Luna, A.; Gharehpetian, G.B.; Rodriguez, P. IEEE Journal of emerging and selected topics in power electronics Vol. 4, num. 3, p. 1135-1144 DOI: 10.1109/JESTPE.2016.2574458 Data de publicació: 2016-09-01 Article en revista
This paper proposes an efficient control framework that utilizes dc-dc converters to achieve flexible power flow control in multiterminal dc (MTDC) grids. The dc-dc converter employed in this paper is connected in cascade with the dc transmission line, and is therefore named cascaded power flow controller (CPFC). In this paper, a two-layer control strategy is developed for the operation and control of voltage source converter stations and CPFC station in MTDC grids. At the primary control layer, a novel differential voltage droop control is developed, while at the secondary control layer, a modified dc power flow algorithm-employing the new CPFC framework-is implemented. The overall control strategy enables the CPFC to regulate the power flow in the dc transmission line. The primary control guarantees the transient stability of the CPFC, and the secondary control system ensures the desired steady-state operation. The proposed voltage droop control framework helps the MTDC grid to remain stable in the event of a communication failure between the primary and secondary control layers. Static analysis and dynamic simulations are performed on the CIGRE B4 dc grid test system, in order to confirm the effectiveness of the proposed control framework for power flow regulation in MTDC grids.
Monadi, M.; Gavriluta, C.; Luna, A.; Candela, J.; Rodriguez, P. IEEE transactions on power delivery Vol. 32, num. 1, p. 430-440 DOI: 10.1109/TPWRD.2016.2600278 Data de publicació: 2016-08-15 Article en revista
This paper presents a centralized protection strategy for medium voltage dc (MVDC) microgrids. The proposed strategy consists of a communication-assisted fault detection method with a centralized protection coordinator and a fault isolation technique that provides an economic, fast, and selective protection by using the minimum number of dc circuit breakers (DCCBs). The proposed method is also supported by a backup protection which is activated if communication fails. The paper also introduces a centralized self-healing strategy that guarantees successful operation of zones that are separated from the main grid after the operation of the protection devices. Furthermore, to provide a more reliable protection, thresholds of the protection devices are adapted according to the operational modes of the microgrid and the status of distributed generators (DGs). The effectiveness of the proposed protection strategy is validated through real-time simulation studies based on the hardware in the loop (HIL) approach.
Zhang, W.; Rouzbehi, K.; Candela, J.; Luna, A.; Gharehpetian, G.B.; Rodriguez, P. IEEE Power and Energy Society General Meeting p. 1-5 DOI: 10.1109/PESGM.2016.7741768 Data de presentació: 2016-07-17 Presentació treball a congrés
Multi-terminal high-voltage dc (HVDC) grids are expected to experience a continuous expansion in the near future, and have appeared as competitive strategies in transmitting offshore wind and interconnecting multiple ac areas. This paper proposes the inertia sharing concept for the operation of multi-terminal HVDC (MTDC) grids, which can be achieved by the proposed Unified Reference Controller. The control objectives of the VSC stations are no longer limited to the stabilization of dc grid, instead, the requirements in ac side are also met. The interaction dynamics between the ac and dc grid is analyzed for illustrating the proposed concept. In addition, the VSC stations can work in different operation modes based on the unified control architecture, and further smoothly switch the operation modes for flexible maneuver. Simulation results exhibit good performance of the proposed strategy.
Monadi, M.; Koch-Ciobotaru, C.; Luna, A.; Candela, J.; Rodriguez, P. IET generation, transmission and distribution Vol. 10, num. 14, p. 3517-3528 DOI: 10.1049/iet-gtd.2016.0183 Data de publicació: 2016-06-28 Article en revista
Voltage source converters (VSCs) are highly vulnerable to DC fault current; thus, protection is one of the most important concerns associated with the implementation of multi-terminal VSC-based DC networks. This paper proposes a protection strategy for medium voltage DC (MVDC) distribution systems. The strategy consists of a communication-assisted fault location method and a fault isolation scheme that provides an economic, fast and selective protection by means of using the minimum number of DC circuit breakers (DCCBs). This paper also introduces a backup protection which is activated if communication network fails. The effectiveness of the proposed protection strategy is analyzed through real-time simulation studies by use of the hardware in the loop (HIL) approach. Furthermore, the effects of fault isolation process on the connected loads are also investigated. The results show that the proposed strategy can effectively protect multi-terminal DC distribution networks and VSC stations against different types of faults.
The high-voltage multi-terminal dc (MTDC) systems are foreseen to experience an important development in the next years. Currently, they have appeared to be a prevailing technical and economical solution for harvesting offshore wind energy. In this study, inertia mimicry capability is added to a voltage-source converter-HVDC grid-side station in an MTDC grid connected to a weak ac grid, which can have low inertia or even operate as an islanded grid. The presented inertia mimicry control is integrated in the generalised voltage droop strategy implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid to provide higher flexibility, and thus controllability to the network. Besides, complete control framework from the operational point of view is developed to integrate the low-level control of the converter stations in the supervisory control centre of the MTDC grid. A scaled laboratory test results considering the international council on large electric systems (CIGRE) B4 MTDC grid demonstrate the good performance of the converter station when it is connected to a weak islanded ac grid.
En los últimos años se han dedicado considerables esfuerzos en el diseño y desarrollo de tecnologías para el transporte de electricidad a largas distancias, especialmente en el caso de sistemas de generación renovables "off-shore", los cuales han de contribuir a satisfacer la siempre creciente demanda de energía eléctrica y limitar a su vez el uso de combustibles fósiles.En este sentido, los parques off-shore se han convertido en una de las apuestas más destacadas, sobre todo en el Mar del Norte, debido al enorme potencial de la energía eólica en esa región. La instalación de grandes parques eólicos marinos en el Mar del Norte plantea desafíos en la integración a red de los mismos, tales como la necesidad de la transmisión de energía a larga distancia y el control y la gestión de la energía eólica producida. Estos desafíos pueden abordarse adecuadamente mediante haciendo uso de redes de continua multiterminales en alta tensión (MTDC).El trabajo realizado en el marco de esta tesis doctoral tiene como objetivo el realizar aportaciones que contribuyan a facilitar el control y la operación de las futuras redes MTDC. Los enfoques propuestos se basan en utilización de una arquitectura de control jerárquico,inspirada en la estrategia de control de generación automática (AGC),la cual se viene aplicando en redes de corriente alterna desde hace varios años. En la propuesta realizada el control primario de la red MTDC está descentralizado mediante la propuesta de estrategia de control generalizado de tensión Droop (GVD). La utilización del GVD propone una solución de control más genérico y flexible que tiene en cuenta los modos de funcionamiento de las estaciones de conversión y los estados de la futura red híbrido ac-dc. Tal y como se describe en la tesis, la estrategia de control GVD puede trabajar en tres modos de control diferentes, incluyendo el control de caída de tensión convencional, control de potencia activa fija,y de control de tensión continua fijo, mediante el ajuste de las características GVD de los convertidores encargados dela regulación. Este ajuste se lleva a cabo por la capa secundaria de la estructura jerárquica de control propuesta. La estrategia propuesta mejora las capacidades de control y distribución de la energía en comparación con otras anteriores,y mejora su capacidad de maniobra.Esta tesis también se ocupa de la sintonización de los controladores en las diferentes estaciones de la red MTDC, proporcionando una metodología que permite alcanzar el ajuste óptimo de los parámetros que influyen en su comportamiento. Como los convertidores electrónicos son plantas no lineales, se ha propuesto una metodología específica para su ajuste,dado que los enfoques clásicos para el ajuste del sistema de control, que se basan por lo general en el modelo lineal aproximada de las plantas, no conducen a resultados óptimos.En este caso algoritmos basados en Particle Swarm Optimization (PSO) se han utilizado para la sintonización de los sistemas conectados en los lados ac y dc. En la última parte de la estructura de control jerárquica propuesta, la capa de control secundario se ha diseñado siguiendo una estructura centralizada, la cual se encarga de regular el punto de funcionamiento de la red de manera que se consigue el flujo de potencia óptimo (OPF). En el enfoque propuesto, un algoritmo OPF se ejecuta en el nivel de control secundario de la red MTDC para encontrar los valores de referencia óptimos para las tensiones en continua y la potencia activa de los convertidores. De este modo, en el nivel de control primario, las características GVD de los convertidores están sintonizados en base a los resultados OPF. A través de esta estructura de control, los controladores GVD óptimamente sintonizados conducen a la operación óptima de la red MTDC. En caso de variación en la carga o en nivel de generación en la red, un nuevo punto de operación estable se logra a través de las características GVD de las estaciones de conversión
In recent years, there have been considerable efforts in the design and development of technologies and techniques for more efficient harvesting of renewable energy resources to meet the ever increasing electric power demand and to limit the use of fossil fuels. In this regard, offshore wind farms have emerged as a promising solution, particularly in the North Sea, due to the vast potential of offshore wind energy. Large-scale offshore wind farms in the North Sea pose grid integration challenges such as the need for long distance submarine power transmission and managing the harvested wind energy. These challenges can be properly addressed by developing of Multi terminal dc (MTDC) systems. Future MTDC grids are expected to be built overlaying the present ac grids as well as harvesting offshore wind to build so-called "Supergrid"
The work presented in this dissertation is oriented to facilitate the control and operation of future MTDC grids. The proposed approaches rely on hierarchical control architecture, inspired by the well-established automatic generation control (AGC) strategy have applied to ac grids. In the inspired hierarchical control architecture, the primary control of the MTDC grid is totally decentralized and implemented using the proposed Generalized Voltage Droop (GVD) control strategy.
GVD proposes an alternative to the conventional voltage droop characteristics of voltage-regulating VSC stations, providing more generic and flexible control solution that takes into account the states of the converter stations and the ac+dc grid. The GVD control strategy can perform three different control modes, including conventional voltage droop control, fixed active power control, and fixed dc voltage control, by adjusting the GVD characteristics of the voltage-regulating converters. Such adjustment is driven by the secondary layer of the proposed hierarchical control structure. The proposed strategy improves the control and power-sharing capabilities of the conventional voltage drop, and enhances its maneuverability.
This dissertation also addresses the tuning of the controllers of VSC-HVDC stations, by providing a methodology for optimized tuning of the parameters that influence their behavior. Since the VSC stations are nonlinear plants in nature, the classical approaches for tuning of the control system, which are usually based on the approximate linear model of the plants, do not lead to optimal results. Refereeing to the successful application of particle swarm optimization (PSO) algorithm in the tuning of ac grids parameters, this algorithm again is used to find optimal control parameters of VSC-HVDC stations in MTDC grids.
As the last part of the proposed hierarchical control structure, the secondary control is centralized and it regulates the operating point of the grid so that optimal power flow (OPF) is achieved. In the proposed approach, an OPF algorithm is executed at the secondary control level of the MTDC grid to find the optimal reference values for the dc voltages and active power of the voltage-regulating converters. Then, at the primary control level, the GVD characteristics of the voltage-regulating converters are tuned based upon the OPF results. Via this control structure, the optimally-tuned GVD controllers lead to the optimal operation of the MTDC grid. In case of variation in load or generation of the grid, a new stable operating point is achieved based on the GVD characteristics of converter stations. Then by executing a new OPF, the GVD characteristics are re-tuned for optimal operation of the MTDC grid.
En los últimos años se han dedicado considerables esfuerzos en el diseño y desarrollo de tecnologías para el transporte de electricidad a largas distancias, especialmente en el caso de sistemas de generación renovables ¿off-shore¿, los cuales han de contribuir a satisfacer la siempre creciente demanda de energía eléctrica y limitar a su vez el uso de combustibles fósiles.En este sentido, los parques off-shore se han convertido en una de las apuestas más destacadas, sobre todo en el Mar del Norte, debido al enorme potencial de la energía eólica en esa región. La instalación de grandes parques eólicos marinos en el Mar del Norte plantea desafíos en la integración a red de los mismos, tales como la necesidad de la transmisión de energía a larga distancia y el control y la gestión de la energía eólica producida. Estos desafíos pueden abordarse adecuadamente mediante haciendo uso de redes de continua multiterminales en alta tensión (MTDC).El trabajo realizado en el marco de esta tesis doctoral tiene como objetivo el realizar aportaciones que contribuyan a facilitar el control y la operación de las futuras redes MTDC. Los enfoques propuestos se basan en utilización de una arquitectura de control jerárquico,inspirada en la estrategia de control de generación automática (AGC),la cual se viene aplicando en redes de corriente alterna desde hace varios años. En la propuesta realizada el control primario de la red MTDC está descentralizado mediante la propuesta de estrategia de control generalizado de tensión Droop (GVD). La utilización del GVD propone una solución de control más genérico y flexible que tiene en cuenta los modos de funcionamiento de las estaciones de conversión y los estados de la futura red híbrido ac-dc. Tal y como se describe en la tesis, la estrategia de control GVD puede trabajar en tres modos de control diferentes, incluyendo el control de caída de tensión convencional, control de potencia activa fija,y de control de tensión continua fijo, mediante el ajuste de las características GVD de los convertidores encargados dela regulación. Este ajuste se lleva a cabo por la capa secundaria de la estructura jerárquica de control propuesta. La estrategia propuesta mejora las capacidades de control y distribución de la energía en comparación con otras anteriores,y mejora su capacidad de maniobra.Esta tesis también se ocupa de la sintonización de los controladores en las diferentes estaciones de la red MTDC, proporcionando una metodología que permite alcanzar el ajuste óptimo de los parámetros que influyen en su comportamiento. Como los convertidores electrónicos son plantas no lineales, se ha propuesto una metodología específica para su ajuste,dado que los enfoques clásicos para el ajuste del sistema de control, que se basan por lo general en el modelo lineal aproximada de las plantas, no conducen a resultados óptimos.En este caso algoritmos basados en Particle Swarm Optimization (PSO) se han utilizado para la sintonización de los sistemas conectados en los lados ac y dc.En la última parte de la estructura de control jerárquica propuesta, la capa de control secundario se ha diseñado siguiendo una estructura centralizada, la cual se encarga de regular el punto de funcionamiento de la red de manera que se consigue el flujo de potencia óptimo (OPF). En el enfoque propuesto, un algoritmo OPF se ejecuta en el nivel de control secundario de la red MTDC para encontrar los valores de referencia óptimos para las tensiones en continua y la potencia activa de los convertidores. De este modo, en el nivel de control primario, las características GVD de los convertidores están sintonizados en base a los resultados OPF. A través de esta estructura de control, los controladores GVD óptimamente sintonizados conducen a la operación óptima de la red MTDC. En caso de variación en la carga o en nivel de generación en la red, un nuevo punto de operación estable se logra a través de las características GVD de las estaciones de conversión
Rouzbehi, K.; Zhang, W.; Candela, J.; Luna, A.; Rodriguez, P. IEEE International Conference on Renewable Energy Research and Applications p. 1568-1574 DOI: 10.1109/ICRERA.2015.7418670 Data de presentació: 2015-11-24 Presentació treball a congrés
Vector current control based on Phase-locked loop, regardless of its popularity in control of grid-connected converters, performs under the limitation of the short-circuit capacity of the connected ac system.
Vector current control based on Phase-locked loop,
regardless of its popularity in control of grid-connected
converters, performs under the limitation of the short-circuit
capacity of the connected ac system.
A previously proposed method, power-synchronization control
(PSC), by L. Zhang et al. has been demonstrated good
performance in HVDC links especially in case of weak ac grid
interconnection. This method utilizes the internal synchronization
mechanism, analogous to the operation of a synchronous
machine in ac grids. By using this technique, the voltage source
converter (VSC) avoids the instability caused by a standard
PLL in a weak ac-system connection. Moreover, a VSC
terminal can give the weak ac system strong voltage support,
just like a common synchronous machine does.
In this paper, generalized voltage droop (GVD) strategy is
developed based on PSC to provide more flexibility in control
paradigm of MTDC grids and smoother transition among
different operation modes of converter stations. GVD is
implemented at the primary layer of a two-layer hierarchical
control structure of MTDC grid, and brings about some new
features such as fixed power control and fixed dc voltage
control, additional to the conventional voltage droop
characteristics. The mode transition can be achieved according
to the set point given by the secondary layer of the control
framework. Analytical justifications are given to demonstrate
the effectiveness in each control mode and the capability in soft
Rouzbehi, K.; Zhu, J.; Zhang, W.; Gharehpetian, G.B.; Luna, A.; Rodriguez, P. IEEE International Conference on Renewable Energy Research and Applications p. 1556-1561 DOI: 10.1109/ICRERA.2015.7418668 Data de presentació: 2015-11-22 Presentació treball a congrés
The level of inertia in modern power systems is reducing drastically due to the increasing penetration of renewables to the power grid. In the near future the grid will face high penetration of renewable resources because of countries policies. However, renewables provides no inertia. On the other hand, voltage source converter (VSC) based multi terminal HVDC system is a promising solution for connection of offshore wind farms to transmit huge amount of harvested energy to ac grids. This paper proposes a complete VSC-HVDC control structure which enables candidate converter station to mimic the inertia of a synchronous machine considering generalized voltage droop strategy. The emulated inertia comes from the capacitor of the dc link. The proposed strategy enables a VSC station with a fixed dc link capacitor to mimic inertia constant in a considerable range, by configuring a range of dc-link dc voltage variation. Sudden load changes in weak ac grid will be compensated by the emulated inertia that is provided by the candidate grid side VSC station.
Roslan, N.; Suul, J.; Luna, A.; Candela, J.; Rodriguez, P. Annual Conference of the IEEE Industrial Electronics Society p. 1934-1941 DOI: 10.1109/IECON.2015.7392383 Data de presentació: 2015-11-12 Presentació treball a congrés
This paper discusses the implementation of proportional resonant (PR) current controllers for a Voltage Source Converter (VSC) with LCL filter which is synchronized to the grid by virtual flux (VF) estimation with inherent sequence separation. Even though there is an extensive amount of literature and studies on the PR current controller for tracking the current reference of a VSC in the stationary reference frame, there is no discussion taking into account voltage sensor-less operation based on virtual flux estimation with an LCL-filter. Separate estimation of the positive and negative sequence virtual flux components at the grid-side of the LCL-filter, as well as current sequence separation, using the Second Order Generalized Integrator-Frequency Locked Loop (SOGI-FLL) is presented as part of a proposed method. The LCL filter is characterized in order to reduce the parameter deviation that might affect the virtual flux estimation. The stability of the proposed method is analyzed in the frequency domain while the operation and performance of the proposed system is verified by simulation studies.
The integration of distributed generation (DG) units into distribution networks has challenged the operating principles of traditional AC distribution systems, and also motivated the development of emerging DC systems. Of particular concern are the challenges associated with the Operation of conventional protection schemes and/or devices. This paper first analyses the fault current characteristics in AC and DC distribution systems; it then presents a comprehensive review of the latest protection methods proposed for distribution systems embedding DGs. In addition, the advantages and disadvantages of each method are outlined and compared. The differences between the protection algorithms employed in/proposed for AC and DC systems are also discussed. Finally, this study identifies the future trends and provides recommendation for researches in the field of protections of DC distribution networks. (C) 2015 Elsevier Ltd. All rights reserved.
Rouzbehi, K.; Davarifar, M.; Martino, M.; Citro, C.; Luna, A.; Daneshifar, Z.; Rodriguez, P. International Congress on Technology, Communication and Knowledge p. 536-541 DOI: 10.1109/ICTCK.2015.7582725 Data de presentació: 2015-11 Presentació treball a congrés
Global warming is already having significant and costly effects on our communities, our health, and our
climate. Nowadays, In developing countries (G20) around the world, decisions are being considered and a shifting of
our dependence from fossil fuels to renewable energy sources (RES). Among RES solar energy, specially photovoltaic
system (PV) play a key role in this challenge, thus, this paper provides a comparative study on the performances of
three transformer-less inverters for photovoltaic applications (H-Bridge, H5, HERIC and NPC inverters) in terms of
power losses, considering the use of different kinds of switches. Silicon (Si) and Silicon Carbide (SiC) diodes have
been combined with two topologies of IGBTs in the performed tests. Simulation results have been obtained for each
of the systems in different operating conditions by using PLECS software. Power losses and European Efficiency (EE)
have been calculated for all the topologies, taking into account the real characteristics of the switches by using the
thermal models provided by the software library.
The electric networks of the future will make an extensive use of DC grids. Therefore, the control of Multi-terminal DC (MTDC) grids is a key issue, which is gathering the attention of the industry and the research community. In this regard, this paper proposes a grid control strategy for voltage-source converter (VSC)-based MTDC networks, based on the use of the particle swarm optimization (PSO) technique. In the proposed approach, the controllers of the power converters belonging to the MTDC grid are acting based on the concept of vector control, in which the AC currents and voltages are transformed into a rotating direct-quadrature (dq) reference frame for controlling of the active and reactive powers as well as the DC and AC voltages. Since the VSCs are nonlinear plants in nature, the classical approaches for tuning of the control system, which are usually based on the approximate linear model of the plants, do not lead to optimal results. As an alternative, in this paper an efficient PSO algorithm is used for tuning optimally the parameters of the controllers in the MTDC grid. In addition, the voltage droop control scheme is utilized to ensure the active power balance within the MTDC grid. The simulation results, obtained through a detailed model of a four-terminal DC grid, demonstrate the effi-ciency of the proposed approach. Finally a comparison with PI controllers which have been conventionally tuned also confirmed the favorable performance of the proposed PSO-tuned controllers.
Monadi, M.; Koch-Ciobotaru, C.; Luna, A.; Candela, J.; Rodriguez, P. IEEE Energy Conversion Congress and Exposition p. 3380-3385 DOI: 10.1109/ECCE.2015.7310137 Data de presentació: 2015-09-24 Presentació treball a congrés
Development of Medium Voltage DC (MVDC) distribution systems require using proper protection schemes whereas due to the behavior of dc fault current, the conventional fault detection methods for ac distribution systems cannot provide a selective protection for these networks. In this paper, a differential based protection method using Ethernet communication has been implemented for a radial MVDC distribution system with integrated PV and WT. This method must be fast enough to detect the faults before involving the main converters and separate only the faulty feeder in order to increase the reliability of the power system. The experimental validation is performed by using Hardware-in-the-Loop (HIL) approach and communication based on the IEC61850 protocol. A real time simulator (OPAL-RT) and a development board (DK60) are used to evaluate the method and to calculate the time delay of this fault detection algorithm.
Rodriguez, P.; Citro, C.; Candela, J.; Rocabert, J.; Luna, A. IEEE Energy Conversion Congress and Exposition p. 450-459 DOI: 10.1109/ECCE.2015.7309723 Data de presentació: 2015-09-20 Presentació treball a congrés
Photovoltaic power plants (PV) are equipped with anti-islanding algorithms, embedded in the converters controller, to avoid the island operation. However, the current trends in the development of the future electrical networks evidence that it is technically feasible and economically advantageous to keep feeding islanded systems under these situations, without cutting the power supply to the loads connected to the network. Nevertheless, commercial PV power converters are programmed as grid-feeding converters, and they are unable to work in island mode if there is not an agent forming the grid. In order to overcome this problem the Synchronous Power Controller (SPC) is presented in this paper as a suitable alternative for controlling PV inverters. As it will be further discussed this controller permits PV plants to operate seamless in grid connected and island mode, with no need of changing the control structure in any case. Moreover, the participation of SPC based power converters integrating energy storage enables other grid-feeding systems to contribute to the grid operation in island conditions. The results achieved with the SPC will be shown in simulations and also with experiments considering a real PV power plant combining SPC and comercial converters.
Zhang, W.; Remon, D.; Cantarellas, A.M.; Luna, A.; Rocabert, J.; Candela, J.; Rodriguez, P. IEEE Energy Conversion Congress and Exposition p. 3780-3787 DOI: 10.1109/ECCE.2015.7310194 Data de presentació: 2015-09-20 Presentació treball a congrés
The impact caused by the large scale penetration of renewable energy sources into electrical grid has been given an increasing concern in the past decade. Multiple challenges will occur in the future when the share of the traditional synchronous generators is reduced. One of the main issues is the lack of rotational inertia in the grid, which may cause stability issues. Therefore, the renewable energy generation plants have been asked for new control objectives and services.
In this paper, three types of active power synchronizing controllers are analyzed and compared with each other. All the three types of controllers are able to provide inertia, damping and droop characteristics, which are the favored features for the future renewable energy generation plants. The comparisons are hence conducted in these three aspects. Theoretical and experimental comparisons are both given, with the comments and conclusions.
Ghorbani, H.; Monadi, M.; Luna, A.; Candela, J.; Rodriguez, P. European Conference on Power Electronics and Applications p. 1-8 DOI: 10.1109/EPE.2015.7309365 Data de presentació: 2015-09-10 Presentació treball a congrés
Transient stability enhancement through using a Static Var Compensator (SVC) and Rotor Speed Deviation Signal (RSDS) of synchronous machine are thoroughly investigated in this paper. Speed deviation signal of generator as a remote signal is sent to SVC in order to achieve more transient stability. Studies are carried out on Kundur's four machine two-area test system which SVC is implemented on midway of transmission system. The simulation results verify the effectiveness of the proposed method to improve rotor angle stability versus different kinds of disturbances scenarios. The standard benchmark model is used for the analysis in the MATLAB/Simulink environment.
Monadi, M.; Koch-Ciobotaru, C.; Luna, A.; Candela, J.; Rodriguez, P. European Conference on Power Electronics and Applications p. 1-9 DOI: 10.1109/EPE.2015.7309213 Data de presentació: 2015-09-10 Presentació treball a congrés
This paper presents a communication-assisted protection technique for dc microgrids. The technique, consist of a centralized fault detection, by use of the differential method; and a fault isolation strategy by coordinated operation of dc breakers and isolator switches. Hardware in the loop simulation is used to evaluate the proposed protection.
Ghorbani, H.; Moghadam, D.; Luna, A.; Candela, J.; Rodriguez, P. European Conference on Power Electronics and Applications DOI: 10.1109/EPE.2015.7309389 Data de presentació: 2015-09-10 Presentació treball a congrés
This paper proposes a new auxiliary Subsynchronous Damping Controller (SSDC) for the Static Var Compensator (SVC) to damp out subsynchronous oscillations in power system containing series compensated transmission lines. The arrival of Wide Area Measurement (WAM) technology has made it possible to measure the states of a large power system interconnected with synchronized Phasor Measurement Units (PMU). This paper presents the idea of using remote signals obtained from PMU to damp SSR. An auxiliary subsynchronous damping controller (SSDC) has been proposed for a SVC, using the generator rotor speed deviation signal as the stabilizing signal to damp subsynchronous oscillations. Sturdiness of the controller has been examined by applying the disturbances in the system that causes significant changes in generator's operating point. The IEEE Second Benchmark (SBM) model is used for the analysis and the SVC is simulated using the Power System Block set (PSB) in the MATLAB/Simulink environment.
Luna, A.; Rocabert, J.; Candela, J.; Hermoso, J.R.; Teodorescu, R.; Blaabjerg, F.; Rodriguez, P. IEEE transactions on industry applications Vol. 51, num. 4, p. 3414-3425 DOI: 10.1109/TIA.2015.2391436 Data de publicació: 2015-07-01 Article en revista
The actual grid code requirements for the grid connection of distributed generation systems, mainly wind and photovoltaic (PV) systems, are becoming very demanding. The transmission system operators (TSOs) are especially concerned about the low-voltage-ride-through requirements. Solutions based on the installation of STATCOMs and dynamic voltage regulators (DVRs), as well as on advanced control functionalities for the existing power converters of distributed generation plants, have contributed to enhance their response under faulty and distorted scenarios and, hence, to fulfill these requirements. In order to achieve satisfactory results with such systems, it is necessary to count on accurate and fast grid voltage synchronization algorithms, which are able to work under unbalanced and distorted conditions. This paper analyzes the synchronization capability of three advanced synchronization systems: the decoupled double synchronous reference frame phase-locked loop (PLL), the dual second order generalized integrator PLL, and the three-phase enhanced PLL, designed to work under such conditions. Although other systems based on frequency-locked loops have also been developed, PLLs have been chosen due to their link with dq0 controllers. In the following, the different algorithms will be presented and discretized, and their performance will be tested in an experimental setup controlled in order to evaluate their accuracy and implementation features.
Zhang, W.; Luna, A.; Candela, J.; Rocabert, J.; Rodriguez, P. IEEE International Symposium on Power Electronics for Distributed Generation Systems DOI: 10.1109/PEDG.2015.7223055 Data de presentació: 2015-06-22 Presentació treball a congrés
Improving the dynamics of the widely applied gridconnected power converters has been drawing a lot of interests in recent years. Since the currently installed grid-connected converters doesn’t shown inertia effect in power processing, the lack of inertia in the electrical networks will become increasingly significant and less admissible when more and more renewable energy sources penetrate into the grid through grid-connected converters.
An active power synchronizing controller for grid-connected power converters with configurable natural droop characteristics is proposed in this paper. The proposed controller is able to incorporate inertia, damping and configurable natural droop effect in the dynamics of the controlled converter. And in addition to the inertia characteristics, the droop and damping behaviors are discussed in detail. By a modified form of the swing equation of the synchronous generators, the droop behavior can be properly configured without undermining the damping characteristics. Experimental results are shown to validate the proposed controller
Gavriluta, C.; Candela, J.; Luna, A.; Gomez-Exposito, A.; Rodriguez, P. IEEE Transactions on Smart Grid Vol. 6, num. 3, p. 1502-1510 DOI: 10.1109/TSG.2014.2365854 Data de publicació: 2015-05-01 Article en revista
This paper proposes a hierarchical control architecture designed for an arbitrary high voltage multiterminal dc (MTDC) network. In the proposed architecture, the primary control of the MTDC system is decentralized and implemented using a generalized droop strategy. Design criteria for dimensioning the primary control parameters, including voltage limits, are offered by analyzing the transients appearing in the system. The proposed secondary control is centralized and regulates the operating point (OP) of the network so that optimal power flow (OPF) is achieved. Compared to previous works, this paper further elaborates, both analytically and through simulations, on the coordination between the primary and secondary control layers. This includes how local primary controllers have to be driven by the centralized controller in order to ensure a smooth transition to the optimal OP.
Multi-terminal dc networks based on voltage source converters (VSC) are the latest trend in dc-systems; the interest in the area is being fueled by the increased feasibility of these systems for the large scale integration of remote offshore wind resources. Despite the active research effort in the field, at the moment, issues related to the operation and control of these networks, as well as sizing, are still uncertain. This paper intends to make a contribution in this field by analyzing the sizing of droop control for VSC together with the output capacitors. Analytical formulas are developed for estimating the voltage peaks during transients, and then it is shown how these values can be used to dimension the dc-bus capacitor of each VSC. Further on, an improved droop control strategy that attenuates the voltage oscillations during transients is proposed. The proposed methods are validated on the dc-grid benchmark proposed by the CIGRE B4 working group. Starting from the structure of the network and the power rating of the converters at each terminal, the output capacitors and the primary control layer are designed together in order to ensure acceptable voltage transients.(C) 2014 Elsevier B.V. All rights reserved.
Multiterminal dc (MTDC) systems are drawing a lot of interest lately in applications related to distributed generation, especially in those that integrate wind or photovoltaic (PV) generation with energy storage (ES). Several approaches for controlling the operation of such systems have been proposed in the literature; however, the existing structures are mainly application specific and, thus, can be still improved in order to provide a more generic approach. This paper proposes an improved primary control layer for an MTDC system. The concept is based on the combination of a droop control method and dc bus signaling in order to provide a more generic and flexible solution. In this paper, different droop characteristics are proposed for the various elements connected to the dc bus. All of them are specifically tailored around five operation bands, which depend on the dc bus voltage level. Special attention is paid to the integration of ES: the state of charge (SoC) is considered at the primary control level, yielding a surface characteristic that depends on the SoC and the dc bus voltage. The scaling of the system has been analyzed together with the proposed control strategy and the overall operation has been validated through simulations by considering a 100 kW PV system with energy storage. Experimental results were obtained on a scaled laboratory prototype rated at 10 kW.
Rouzbehi, K.; Miranian, A.; Candela, J.; Luna, A.; Rodriguez, P. IEEE transactions on industry applications Vol. 51, num. 1, p. 607-618 DOI: 10.1109/TIA.2014.2332814 Data de publicació: 2015-02 Article en revista
This paper proposes a generalized voltage droop (GVD) control strategy for dc voltage control and power sharing in voltage source converter (VSC)-based multiterminal dc (MTDC) grids. The proposed GVD control is implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid, and constitutes an alternative to the conventional voltage droop characteristics of voltage-regulating VSC stations, providing higher flexibility and, thus, controllability to these networks. As a difference with other methods, the proposed GVD control strategy can be operated in three different control modes, including conventional voltage droop control, fixed active power control, and fixed dc voltage control, by adjusting the GVD characteristics of the voltage-regulating converters. Such adjustment is carried out in the secondary layer of the hierarchical control structure. The proposed strategy improves the control and power-sharing capabilities of the conventional voltage droop, and enhances its maneuverability. The simulation results, obtained by employing a CIGRE B4 dc grid test system, demonstrate the efficiency of the proposed approach and its flexibility in active power sharing and power control as well as voltage control. In these analysis, it will be also shown how the transitions between the operating modes of the GVD control does not give rise to active power oscillations in the MTDC grids.
Gavriluta, C.; Candela, J.; Citro, C.; Rocabert, J.; Luna, A.; Rodriguez, P. IEEE transactions on industry applications Vol. 50, num. 6, p. 4122-4131 DOI: 10.1109/TIA.2014.2315715 Data de publicació: 2014-11-01 Article en revista
Multiterminal dc networks are drawing a lot of interest lately in applications related to distributed generation, particularly in those that also integrate energy storage (ES). A few approaches for controlling the operation of such systems have been proposed in the literature; however, the existing structures can be significantly enhanced. This paper proposes an improved primary control layer, based on custom droop characteristics obtained by combining concepts of droop and dc-bus signaling control. This approach is designed to be generic and takes into account the various operating states of the network. Five operating bands, similar to the operating states of the ac grids, as well as various droop characteristics for different elements connected to the dc network, are defined. For the ES, the state of charge is taken into account at the primary control level and included in the droop characteristic, creating a two-variable droop surface. The proposed control strategy is validated through simulation and experimental results obtained from a case study that involves amicro dc network composed of a photovoltaic generator, a lead-acid battery, and a connection point to the ac grid.
Rouzbehi, K.; Miranian, A.; Candela, J.; Luna, A.; Rodriguez, P. IEEE International Conference on Renewable Energy Research and Applications p. 180-184 DOI: 10.1109/ICRERA.2014.7016553 Data de presentació: 2014-10-22 Presentació treball a congrés
The current route of achieving the ultimate plan for flawless operation and control of the multi-terminal DC (MTDC) grids can be significantly accelerated by learning from the vast and valuable experiences gained from the operation of the AC power grids for more than a century. This paper introduces concept of flexible DC transmission system (FDCTS), inspired by the successful operation of flexible AC transmission systems (FACTS), to provide voltage regulation, power control and load flow control within MTDC grids. Considering the current advancements in the field of power electronics, this paper recognizes DC-DC converters as the first element of the FDTCS for providing voltage and power control in MTDC grids. By use of DC-DC converters, this paper developes two elements of the FDCTS, namely the cascaded power flow controller (CPFC) and hybrid power flow controller (HPFC). In this paper, to demonstrate the eligibility of the CPFC and HPFC to play the role of an FDCTS, they are included in the DC power flow formulation for DC voltage regulation and power flow control purposes.
Rouzbehi, K.; Miranian, A.; Candela, J.; Luna, A.; Rodriguez, P. IEEE International Conference on Renewable Energy Research and Applications p. 550-555 DOI: 10.1109/ICRERA.2014.7016445 Data de presentació: 2014-10-22 Presentació treball a congrés
This paper develops a hybrid power flow controller (HPFC) to take one step forward for building future flexible DC transmission system (FDCTS). The idea for proposing HPFC is inspired by the successful operation of flexible AC transmission systems (FACTS) devices, the FDCTS includes static power electronics-based elements to provide voltage regulation, power control and load flow control in the multi-terminal DC (MTDC) grids. The HPFC, as one of the devices of the FDCTS, has a hybrid connection scheme (i.e. series-parallel) to the grid and allows power flow control through DC transmission lines by imposing a series voltage to the controlled DC line. In this work, in order to demonstrate the eligibilities of the HPFC, the performance of this device is included in the DC power flow formulation for the power flow control purposes. Finally, it will be shown through dynamic simulations that the proposed HPFC is able to control the power flow through a particular DC transmission line.
Monadi, M.; Koch-Ciobotaru, C.; Luna, A.; Candela, J.; Rodriguez, P. IEEE International Conference on Renewable Energy Research and Applications p. 496-501 DOI: 10.1109/ICRERA.2014.7016434 Data de presentació: 2014-10-22 Presentació treball a congrés
A fault location method and a fault clearance strategy are presented in this paper for medium voltage dc (MVDC) distribution system. MVDC systems are applicable for connection between microgrids (MGs) and integration of renewable energy systems (RESs) to distribution systems. Due to the specifications of fault current in dc systems, it is difficult to coordinate the over current (O/C) relays based on the time inverse grading. Hence, in this paper, a communication link between O/C relays is used to diagnose the fault location. On the other hand, the fault clearance is done by the operation of dc circuit breakers (DCCB) and isolator switches. In this protection strategy, O/C relays detect the faulty part using communication links and after the fault extinguishing by DCCBs, the dc switches isolate the faulty part. Finally, the sound parts of the system re-energize when DCCB are re-closed. Moreover, data transmission by communication links is based on the standard messages of IEC61850 protocol.