Rouzbehi, K.; Candela, J.; Gharehpetian, G.B.; Harnefors, L.; Luna, A.; Rodriguez, P. Renewable and sustainable energy reviews p. 1-10 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.
Abdollahi, M.; Candela, J.; Rocabert, J.; Muñoz-Aguilar, R. S.; Hermoso, J.R. IEEE International Conference on Renewable Energy Research and Applications Data de presentació: 2016-11-20 Presentació treball a congrés
Abdollahi, M.; Candela, J.; Rocabert, J.; Muñoz-Aguilar, R. S.; Hermoso, J.R. IEEE International Conference on Renewable Energy Research and Applications Data de presentació: 2016-11-20 Presentació treball a congrés
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. 10, num. 13, p. 1-9 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.; 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.
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.
Rouzbehi, K.; Candela, J.; Luna, A.; Gharehpetian, G.B.; López Rodríguez, Pedro IEEE Journal of emerging and selected topics in power electronics Vol. 4, num. 3 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. PP, num. 99, p. 1-11 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 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.
Remon, D.; Mir, A.; Zhang, W.; Candela, J.; Rodriguez, P. IEEE Power and Energy Society General Meeting p. 1-5 DOI: 10.1109/PESGM.2016.7741525 Data de presentació: 2016-07-17 Presentació treball a congrés
Distributed renewable energy sources progressively gain importance in power systems, with which new challenges, but also new opportunities, arise. Therefore, it is necessary to analyze the interaction between this type of generators and the grid, and to study adequate control methods that allow small generating units to operate harmoniously within power systems and contribute to their control, stability, and reliability. In this paper, the stability of a distribution system with several renewable energy generators is studied, focusing on its ability to maintain power supply during a transmission blackout. The response of the distribution system after a sudden disconnection from the main network is simulated and analyzed, comparing the results obtained using a conventional control for the renewable generators with those of controllers that enable a synchronous interaction between these generators and the power system.
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.
One of the major issues associated with the implementation of direct-current distribution systems is the design of a proper protection scheme. The fault current characteristics in direct-current distribution systems are quite different than those in conventional alternating-current grids. Thus, the performance of conventional protection schemes can adversely be affected, and it is necessary to modify the con-ventional protection schemes or design new protection methods for direct-current networks. This paper proposes a multi-zone differential protection scheme for direct-current distribution systems embedding distributed generators. The proposed method provides a selective and fast protection through the use of a communication link between two sides of a protected feeder. Moreover, the method provides a differential-based backup for the adjacent relays, which can enhance the protection system reliability. In addition, the method proposed in this paper also utilizes directional over-current elements to provide backup protection if the communication network fails. The effectiveness of the proposed protection scheme is evaluated through comprehensive hardware-in-the-loop simulation studies to obtain more realistic results and to investigate the impact of the communication delay. The results show that the proposed method can provide a selective and fast protection and effectively protect components of direct-current distribution systems against different types of faults.
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
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.
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.
In this paper, a communication-assisted protection scheme is proposed for medium voltage dc (MVDC) distribution networks. MVDC grids are applicable for connection between microgrids, upgrading the transmission capacity of ac lines by the conversion to dc, and integration of renewable energy systems to the distribution grids. However, protection issues are one of the main challenges in the development of the VSC-based dc networks. Due to the special behavior of the dc fault currents, it is almost impossible to coordinate the overcurrent relays based on the time inverse grading. Hence, in the proposed scheme, to provide a fast and selective protection, each proposed relay communicates with two other relays to operate as the main protection for a dc feeder and the backup protection of the adjacent feeder. Hardware in the loop simulation approach by use of the OPAL-RT real time simulator is used to verify the performance of the proposed method.
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. 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.
Remon, D.; Cantarellas, A.M.; Rakhshani, E.; Candela, J.; Rodriguez, P. IEEE International Conference on Renewable Energy Research and Applications p. 424-429 DOI: 10.1109/ICRERA.2014.7016421 Data de presentació: 2014-10-22 Presentació treball a congrés
Renewable energy sources are increasing their penetration in power systems, making necessary new control systems that offer services usually provided only by conventional generators. In this paper, an active power controller able to achieve synchronization with the grid and to control the DC link voltage is proposed. This controller allows identifying the converter with a virtual synchronous generator whose inertia can be modified online, considering that its virtual kinetic energy is stored in the DC link. Additionally, the resulting active power loop is a second order system whose damping factor can be defined freely.
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. 268-271 DOI: 10.1109/ICRERA.2014.7016568 Data de presentació: 2014-10-19 Presentació treball a congrés
In this paper, DC-link voltage control in DC microgrids with photovoltaic (PV) generation and battery, is addressed based on an intelligent approach. The proposed strategy is based on the modeling of the power interface, i.e. power electronic converter, located between the PV array, battery and DC bus, by use of measurement data. For this purpose, a local model network (LMN) is developed to model the converter and then a local linear control (LLC) strategy is designed based on the LMN. Simulation results on a DC microgrid demonstrates efficacy of the proposed approach.
Gavriluta, C.; Candela, J.; Rocabert, J.; Etxeberria, I.; Rodriguez, P. IEEE Energy Conversion Congress and Exposition p. 5323-5330 DOI: 10.1109/ECCE.2014.6954131 Data de presentació: 2014-09-18 Presentació treball a congrés
Large penetration of renewable energy is currently attenuated by concerns regarding their impact on the controllability and reliability of the electrical system. As the inclusion of energy storage is to a great extent the solution to these issues, this paper proposes a methodology for approaching the calculation of the size of the energy storage to be connected to a PV power plant for providing inertia emulation, primary control, and the reduction of power fluctuation. A complete control strategy, developed for the inclusion of these services in the operation of a dc-ac grid connected converter has been implemented and validated through simulation results. The obtained results are validated by experimental tests performed on a scaled 10 kW prototype.
Remon, D.; Cantarellas, A.M.; Rakhshani, E.; Candela, J.; Rodriguez, P. IEEE Power & Energy Society General Meeting DOI: 10.1109/PESGM.2014.6939250 Data de presentació: 2014-07-31 Presentació treball a congrés
The increasing penetration of renewable energy sources in power systems requires new control systems that provide services similar to those of conventional generators. This paper proposes an active power controller that is able to automatically achieve synchronization with the grid and to control the DC link voltage. Through this controller, the converter can be identified with a virtual synchronous generator whose kinetic energy is stored in the DC link. Moreover, the active power loop becomes a second order system whose damping can be programmed freely.
In this paper, application of Posicast control method to generator excitation system is presented. The method is one of the simplest possible control design methods that can be applied to damp the oscillations caused by changing the excitation reference signal. Stability and robustness of the designed controller are shown using extensive time domain simulations. All the detailed simulations are carried out in MATLAB/Simulink environment.
Ghorbani, H.; Candela, J.; Luna, A.; Rodriguez, P. IEEE International Symposium on Industrial Electronics p. 58-63 DOI: 10.1109/ISIE.2014.6864586 Data de presentació: 2014-06-01 Presentació treball a congrés
Application of Posicast control method to generator excitation system is presented in this paper. Presented control method is one of the simplest control design methods which can be installed in generators excitation to damp the oscillations caused by changing the excitation reference signal. Stability of the designed controller is shown using extensive time domain simulations. Performance of Posicast controller in IEEE power system standard models with presence of PV power plant is evaluated in MATLAB/Simulink environment.
Hasan, K.N.Md; Kalle, R.; Luna, A.; Candela, J.; Rodriguez, P. IEEE transactions on industry applications Vol. 50, num. 3, p. 2050-2060 DOI: 10.1109/TIA.2013.2286216 Data de publicació: 2014-05-16 Article en revista
The harmful effects of harmonics are an important issue in wind power plants (WPPs), especially in offshore applications. In offshore WPPs, the wind turbines are linked to the network through high-power converters that produce harmonics at relative low frequencies. Moreover, in the network of a WPP, the propagation of noncharacteristic harmonics and the effect of resonance contribute as well in boosting the harmonic distortion. In this paper, a solution for compensating the harmonics in a WPP by means of using hybrid filters is proposed. In this paper, not only the filtering solution is tested, but also a method for finding the best place where to connect the filter, based on a modal analysis or a harmonic modal resonance analysis, is implemented. The proposed solution has been tested by considering the model of a real 400-MW offshore WPP as the study case. As it will be shown, the hybrid filter is capable of damping the resonances in the plant, while the analysis conducted permits us to optimize its location in the plant
Muñoz-Aguilar, R. S.; Rodriguez, P.; Doria-Cerezo, A.; Candela, J.; Luna, A. International journal of electrical power and energy systems Vol. 60, p. 275-282 DOI: 10.1016/j.ijepes.2014.03.003 Data de publicació: 2014-04-01 Article en revista
This paper presents a sliding mode control for a wound rotor synchronous machine acting as an isolated generator connected to an unbalanced load. In order to simplify the control methodology, the standard dq-model of the machine is connected to a balanced resistive load. A switching function is defined in order to fulfill the control objective. From the desired surface, the standard sliding methodology is applied to obtain a robust and very simple controller. Then, the actual measured voltage of the machines is acquired and treated trough a frequency locked loop algorithm in order to extract the positive sequence and control it to guarantee a good definition of the park transformation. A phase locked loop algorithm is also used to avoid speed sensors. Numerical simulations and experimental results validate the control law and show good performance and a fast response to load and reference changes
Rouzbehi, K.; Miranian, A.; Candela, J.; Luna, A.; Rodriguez, P. IEEE/PES Transmission and Distribution Conference and Exposition: Latin America p. 6863334-1-6863334-5 Data de presentació: 2014-04 Presentació treball a congrés
This paper proposes an intelligent control scheme for DC voltage regulationin a DC micro-grid integrating photovoltaic (PV) generation, energy storage and electric loads. The maximum power generation of the PV panel is followed using the incremental conductance (IC) maximum power point tracking (MPPT) algorithm while a high-performance local linear controller (LLC)is developed for the DC voltage control in the micro-grid.The LLC, as a data-driven control strategy, controls the bidirectional converter located between the battery and the DC micro-grid to regulate the micro-grid's voltage at the specified reference value.In order to validate the controller different simulations including variations in the load and in the solar irradiance are performed. Finally a comparison with PI based controller demonstrates the advantages of the proposed control technique for regulating theDC micro-grid's voltage.