Showing papers by "Josep M. Guerrero published in 2015"
TL;DR: In this paper, a distributed controller for secondary frequency and voltage control in islanded microgrids is proposed, which uses localized information and nearest-neighbor communication to collectively perform secondary control actions.
Abstract: In this paper, we present new distributed controllers for secondary frequency and voltage control in islanded microgrids. Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions. The frequency controller rapidly regulates the microgrid frequency to its nominal value while maintaining active power sharing among the distributed generators. Tuning of the voltage controller provides a simple and intuitive tradeoff between the conflicting goals of voltage regulation and reactive power sharing. Our designs require no knowledge of the microgrid topology, impedances, or loads. The distributed architecture allows for flexibility and redundancy, eliminating the need for a central microgrid controller. We provide a voltage stability analysis and present extensive experimental results validating our designs, verifying robust performance under communication failure and during plug-and-play operation.
600 citations
TL;DR: A novel distributed coordinated controller combined with a multiagent-based consensus algorithm is applied to distributed generators in the Energy Internet, which keeps voltage angles and amplitudes consensus, while providing accurate power-sharing and minimizing circulating currents.
Abstract: With the bidirectional power flow provided by the Energy Internet, various methods are promoted to improve and increase the energy utilization between Energy Internet and main grid (MG). This paper proposes a novel distributed coordinated controller combined with a multiagent-based consensus algorithm, which is applied to distributed generators in the Energy Internet. Then, the decomposed tasks, models, and information flow of the proposed method are analyzed. The proposed coordinated controller installed between the Energy Internet and MG keeps voltage angles and amplitudes consensus, while providing accurate power-sharing and minimizing circulating currents. Finally, the Energy Internet can be integrated into the MG seamlessly if necessary. Hence, the Energy Internet can be operated as a spinning reserve system. Simulation results are provided to show the effectiveness of the proposed controller in an Energy Internet.
329 citations
TL;DR: In this paper, technical literature about optimization techniques applied to microgrid planning has been reviewed and the guidelines for innovative planning methodologies focused on economic feasibility can be defined, some trending techniques and new micro-grid planning approaches are pointed out.
Abstract: Microgrids are expected to become part of the next electric power system evolution, not only in rural and remote areas but also in urban communities. Since microgrids are expected to coexist with traditional power grids (such as district heating does with traditional heating systems), their planning process must be addressed to economic feasibility, as a long-term stability guarantee. Planning a microgrid is a complex process due to existing alternatives, goals, constraints and uncertainties. Usually planning goals conflict each other and, as a consequence, different optimization problems appear along the planning process. In this context, technical literature about optimization techniques applied to microgrid planning have been reviewed and the guidelines for innovative planning methodologies focused on economic feasibility can be defined. Finally, some trending techniques and new microgrid planning approaches are pointed out.
312 citations
TL;DR: In this article, the authors give a description of state of the art for distributed power generation systems (DPGS) based on renewable energy and explore the power converters connected in parallel to the grid which are distinguished by their contribution to the formation of the grid voltage and frequency and are accordingly classified in three classes.
Abstract: The introduction of microgrids in distribution networks based on power electronics facilitates the use of renewable energy resources, distributed generation (DG) and storage systems while improving the quality of electric power and reducing losses thus increasing the performance and reliability of the electrical system. The hierarchical control structure, which consists of primary, secondary, and tertiary levels for microgrids that mimic the behavior of the mains grid, is reviewed. The main objective of this article is to give a description of state of the art for distributed power generation systems (DPGS) based on renewable energy and explore the power converters connected in parallel to the grid which are distinguished by their contribution to the formation of the grid voltage and frequency and are accordingly classified in three classes. This analysis is extended focusing mainly on the three classes of configurations: grid-forming, grid-feeding, and grid-supporting. The article ends up with an overview and a discussion of the control structures and strategies to control distribution power generation system (DPGS) units connected to the network.
305 citations
TL;DR: A double-quadrant state-of-charge (SoC)-based droop control method for distributed energy storage system is proposed to reach the proper power distribution in autonomous dc microgrids and the simulation results are shown to verify the proposed approach.
Abstract: In this paper, a double-quadrant state-of-charge (SoC)-based droop control method for distributed energy storage system is proposed to reach the proper power distribution in autonomous dc microgrids. In order to prolong the lifetime of the energy storage units (ESUs) and avoid the overuse of a certain unit, the SoC of each unit should be balanced and the injected/output power should be gradually equalized. Droop control as a decentralized approach is used as the basis of the power sharing method for distributed energy storage units. In the charging process, the droop coefficient is set to be proportional to the nth order of SoC, while in the discharging process, the droop coefficient is set to be inversely proportional to the nth order of SoC. Since the injected/output power is inversely proportional to the droop coefficient, it is obtained that in the charging process the ESU with higher SoC absorbs less power, while the one with lower SoC absorbs more power. Meanwhile, in the discharging process, the ESU with higher SoC delivers more power and the one with lower SoC delivers less power. Hence, SoC balancing and injected/output power equalization can be gradually realized. The exponent n of SoC is employed in the control diagram to regulate the speed of SoC balancing. It is found that with larger exponent n, the balancing speed is higher. MATLAB/simulink model comprised of three ESUs is implemented and the simulation results are shown to verify the proposed approach.
271 citations
TL;DR: A comprehensive small-signal model is derived by analyzing the interface converters in each stage of a converter-based dc microgrid, and virtual-impedance-based stabilizers are proposed to enhance the damping of dc microgrids with CPLs and guarantee the stable operation.
Abstract: In this paper, a converter-based dc microgrid is studied. By considering the impact of each component in dc microgrids on system stability, a multistage configuration is employed, which includes the source stage, interface converter stage between buses, and common load stage. In order to study the overall stability of the above dc microgrid with constant power loads (CPLs), a comprehensive small-signal model is derived by analyzing the interface converters in each stage. The instability issue induced by the CPLs is revealed by using the criteria of impedance matching. Meanwhile, virtual-impedance-based stabilizers are proposed in order to enhance the damping of dc microgrids with CPLs and guarantee the stable operation. Since droop control is commonly used to reach proper load power sharing in dc microgrids, its impact is taken into account when testing the proposed stabilizers. By using the proposed stabilizers, virtual impedances are employed in the output filters of the interface converters in the second stage of the multistage configuration. In particular, one of the virtual impedances is connected in series with the filter capacitor, and the other one is connected at the output path of the converter. It can be seen that by using the proposed stabilizers, the unstable poles induced by the CPLs are forced to move into the stable region. The proposed method is verified by the MATLAB/Simulink model of multistage dc microgrids with three distributed power generation units.
247 citations
TL;DR: In this article, an improved droop control method was proposed to improve the reactive power sharing accuracy, which mainly includes two important operations: error reduction operation and voltage recovery operation, which is activated by the low-bandwidth synchronization signals.
Abstract: For microgrid in islanded operation, due to the effects of mismatched line impedance, the reactive power could not be shared accurately with the conventional droop method. To improve the reactive power sharing accuracy, this paper proposes an improved droop control method. The proposed method mainly includes two important operations: error reduction operation and voltage recovery operation. The sharing accuracy is improved by the sharing error reduction operation, which is activated by the low-bandwidth synchronization signals. However, the error reduction operation will result in a decrease in output voltage amplitude. Therefore, the voltage recovery operation is proposed to compensate the decrease. The needed communication in this method is very simple, and the plug-and-play is reserved. Simulations and experimental results show that the improved droop controller can share load active and reactive power, enhance the power quality of the microgrid, and also have good dynamic performance.
245 citations
TL;DR: New distributed controllers for secondary frequency and voltage control in islanded microgrids Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions.
Abstract: In this work we present new distributed controllers for secondary frequency and voltage control in islanded microgrids. Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions. The frequency controller rapidly regulates the microgrid frequency to its nominal value while maintaining active power sharing among the distributed generators. Tuning of the voltage controller provides a simple and intuitive trade-off between the conflicting goals of voltage regulation and reactive power sharing. Our designs require no knowledge of the microgrid topology, impedances or loads. The distributed architecture allows for flexibility and redundancy, and eliminates the need for a central microgrid controller. We provide a voltage stability analysis and present extensive experimental results validating our designs, verifying robust performance under communication failure and during plug-and-play operation.
231 citations
TL;DR: In this article, a direct cell-to-cell battery equalizer based on quasi-resonant LC converter (QRLCC) and boost dc-dc converter (BDDC) is proposed.
Abstract: In conventional equalizers, the facts of bulky size and high cost are widespread. Particularly, the zero-switching loss and zero-voltage gap (ZVG) between cells are difficult to implement due to the high-frequency hard switching and the voltage drop across power devices. To overcome these difficulties, a direct cell-to-cell battery equalizer based on quasi-resonant LC converter (QRLCC) and boost dc-dc converter (BDDC) is proposed. The QRLCC is employed to gain zero-current switching, leading to a reduction of power losses. The BDDC is employed to enhance the equalization voltage gap for large balancing current and ZVG between cells. Moreover, through controlling the duty cycle of the BDDC, the topology can online adaptively regulate the equalization current according to the voltage difference, which not only effectively prevents overequalization but also abridges the overall balancing time. Instead of a dedicated equalizer for each cell, only one balancing converter is employed and shared by all cells, reducing the size and implementation cost. Simulation and experimental results show the proposed scheme exhibits outstanding balancing performance, and the energy conversion efficiency is higher than 98%. The validity of the proposed equalizer is further verified by a quantitative and systematic comparison with the existing active balancing methods.
227 citations
TL;DR: A detailed analysis and design of dqCDSC-PLL (PLL with in-loop dq-frame CDSC operator) is provided and it is shown that how using the proportional-integral-derivative controller as the loop filter can improve the response time of dQC DSC- PLL.
Abstract: To improve the performance of phase-locked loops (PLLs) under adverse grid conditions, incorporating different filtering techniques into their structures have been proposed in the literature. These filtering techniques can be broadly classified into in-loop and preloop filtering techniques depending on their position in the PLL structure. Inspired from the concept of delayed signal cancellation (DSC), the idea of cascaded DSC (CDSC) has recently been introduced as an effective solution to improve the performance of the PLL under adverse grid conditions. However, the focus has been on the application of CDSC operator as the prefiltering stage of PLL, and little work has been conducted on its application as the in-loop filtering stage of PLL. This paper provides a detailed analysis and design of dqCDSC-PLL (PLL with in-loop dq-frame CDSC operator). The study is started with an overview of this PLL. A systematic design method to fine tune its control parameters is then proposed. The performance of the dqCDSC-PLL under different grid scenarios is then evaluated in detail. It is then shown that how using the proportional-integral-derivative controller as the loop filter can improve the response time of dqCDSC-PLL. A detailed comparison between the dqCDSC-PLL and moving average filter (MAF) based PLL (MAF-PLL) is then carried out. Through a detailed mathematical analysis, it is also shown that these PLLs are equivalent under certain conditions. The suggested guidelines in this paper make designing the dqCDSC-PLL a simple and straightforward procedure. Besides, the analysis performed in this paper provides a useful insight for designers about the advantages/disadvantages of dqCDSC-PLL for their specific applications.
206 citations
TL;DR: Based on the proposed SSDC approach, flexible power control of each ESS/RES unit can be obtained with seamless modes changes and decentralized power management can be achieved by executing frequency bus-signaling.
Abstract: Coordinated operation of microgrids requires that energy management system takes into account both the available power in renewable energy sources (RES) and storage capacity of energy storage systems (ESS). In this paper, a coordinated architecture of islanded ac microgrids with smooth switching droop control (SSDC) is derived. Based on the proposed SSDC approach, flexible power control of each ESS/RES unit can be obtained with seamless modes changes. Furthermore, decentralized power management can be achieved by executing frequency bus-signaling. The power management principle based on different operational modes is explained in detail and small-signal analysis is carried out for SSDC. Real-time hardware-in-the-loop results of an islanded microgrid are provided under several scenarios to validate the proposed coordinated control strategy.
TL;DR: A novel bus-signaling method (BSM) is proposed to achieve autonomous coordinated performance of system according to different state of charge conditions and real-time simulation results show the feasibility of the proposed approach by presenting the operation of an islanded dc microgrid in different testing scenarios.
Abstract: A low-voltage islanded dc microgrid contains a number of renewable energy sources, local loads, and energy storage systems (ESS). To avoid the over-charging and over-discharging situations of ESS, a coordinated control strategy should be used in islanded dc microgrids. In this paper, a novel bus-signaling method (BSM) is proposed to achieve autonomous coordinated performance of system according to different state of charge conditions. Additionally, a secondary coordinated control is introduced to restore the voltage deviation produced by primary control level without decaying coordinated performance. The proposed control algorithm and controller implementation based on BSM are also presented. Finally, real-time simulation results show the feasibility of the proposed approach by presenting the operation of an islanded dc microgrid in different testing scenarios.
TL;DR: This paper shows that the conventional SRF-PLL is a first-order adaptive complex bandpass filter and the accuracy of this transfer function is confirmed through numerical results.
Abstract: Despite the wide acceptance and use of the conventional synchronous reference frame phase-locked loop (SRF-PLL), no transfer function describing its actual input–output relationship has been developed so far. Arguably, the absence of such transfer function has hampered the application of SRF-PLL as a filter or controller inside the closed-loop control systems. In this paper, the transfer function describing the actual input–output relationship of the conventional SRF-PLL is presented. Using this transfer function, it is shown that the conventional SRF-PLL is a first-order adaptive complex bandpass filter. The accuracy of this transfer function is confirmed through numerical results.
TL;DR: A mathematical model for islanded microgrids with linear loads and inverters under frequency and voltage droop control is proposed and shows that the currents' dynamics influence the stability of the microgrid, particularly for high values of the frequency Droop control parameters.
Abstract: Three-phase inverters subject to droop control are widely used in islanded microgrids to interface distributed energy resources to a network and to properly share loads among different units. In this paper, a mathematical model for islanded microgrids with linear loads and inverters under frequency and voltage droop control is proposed. The model is constructed by introducing a suitable state-space transformation that allows to write the closed-loop model in an explicit state-space form. Then, the singular perturbations technique is used to obtain reduced order models that reproduce the stability properties of the original closed-loop model. The analysis shows that the currents' dynamics influence the stability of the microgrid, particularly for high values of the frequency droop control parameters. It is also shown that a further reduction of the model order leads to a typical oscillator model that is not able to predict the possible instability of the droop-controlled system. Numerical and experimental results demonstrate the validity of the proposed models.
TL;DR: The novel reactive power sharing algorithm is developed, which takes into account the converters parameters as apparent power limit and maximum active power and is verified in simulation and compared with other known reactive power control methods.
Abstract: A microgrid (MG) is a local energy system consisting of a number of energy sources (e.g., wind turbine or solar panels among others), energy storage units, and loads that operate connected to the main electrical grid or autonomously. MGs provide flexibility, reduce the main electricity grid dependence, and contribute to changing large centralized production paradigm to local and distributed generation. However, such energy systems require complex management, advanced control, and optimization. Moreover, the power electronics converters have to be used to correct energy conversion and be interconnected through common control structure is necessary. Classical droop control system is often implemented in MG. It allows correct operation of parallel voltage source converters in grid connection, as well as islanded mode of operation. However, it requires complex power management algorithms, especially in islanded MGs, which balance the system and improves reliability. The novel reactive power sharing algorithm is developed, which takes into account the converters parameters as apparent power limit and maximum active power. The developed solution is verified in simulation and compared with other known reactive power control methods.
TL;DR: In this paper, a power sharing unit (PSU) composed of three single-phase back-to-back converters is proposed to realize the power exchange and coordinated control of load power sharing among phases, as well as to allow full utilization of the energy generated by DGs.
Abstract: With the fast proliferation of single-phase distributed generation (DG) units and loads integrated into residential microgrids, independent power sharing per phase and full use of the energy generated by DGs have become crucial. To address these issues, this paper proposes a hybrid microgrid architecture and its power management strategy. In this microgrid structure, a power sharing unit (PSU), composed of three single-phase back-to-back (SPBTB) converters, is proposed to be installed at the point of common coupling. The aim of the PSU is mainly to realize the power exchange and coordinated control of load power sharing among phases, as well as to allow full utilization of the energy generated by DGs. Meanwhile, the method combining the modified adaptive backstepping-sliding mode control approach and droop control is also proposed to design the SPBTB system controllers. With the application of the proposed PSU and its power management strategy, the loads among different phases can be properly supplied and the energy can be fully utilized, as well as obtaining better load sharing. Simulation and experimental results are provided to demonstrate the validity of the proposed hybrid microgrid structure and control.
TL;DR: Simulation results are given that show the seamless transitions from islanded to grid-connected and vice versa for a single phase microgrid made up from voltage controlled voltage source inverters and current controlled voltage sources working together in both modes of operation.
Abstract: Microgrids are an effective way to increase the penetration of distributed generation into the grid. They are capable of operating either in grid-connected or in islanded mode, thereby increasing the supply reliability for the end user. This paper focuses on achieving seamless transitions from islanded to grid-connected and vice versa for a single phase microgrid made up from voltage controlled voltage source inverters (VC-VSIs) and current controlled voltage source inverters (CC-VSIs) working together in both modes of operation. The primary control structures for the VC-VSIs and CC-VSIs is considered together with the secondary control loops that are used to synchronize the microgrid as a single unit to the grid. Simulation results are given that show the seamless transitions between the two modes without any disconnection times for the CC-VSIs and VC-VSIs connected to the microgrid.
TL;DR: In this article, a new control strategy is proposed to achieve the coordinate control of power and current quality without the need for a phase-locked loop (PLL) or voltage/current positive/negative sequence extraction calculation.
Abstract: Power oscillation and current quality are the important performance targets for the grid-connected inverter under unbalanced grid faults. First, the inherent reason for the current harmonic and power oscillation of the inverter is discussed with a quantitative analysis. Second, a new control strategy is proposed to achieve the coordinate control of power and current quality without the need for a phase-locked loop (PLL) or voltage/current positive/negative sequence extraction calculation. Finally, the experimental tests are conducted under unbalanced grid faults, and the results verify the effectiveness of the propose method.
TL;DR: The proposed hybrid filter is operated as variable harmonic conductance according to the voltage total harmonic distortion; therefore, harmonic distortion can be reduced to an acceptable level in response to load change or parameter variation of the power system.
Abstract: Unintentional series and/or parallel resonances, due to the tuned passive filter and the line inductance, may result in severe harmonic distortion in the industrial power system. This paper presents a hybrid active filter to suppress harmonic resonance and to reduce harmonic distortion. The proposed hybrid filter is operated as variable harmonic conductance according to the voltage total harmonic distortion; therefore, harmonic distortion can be reduced to an acceptable level in response to load change or parameter variation of the power system. Since the hybrid filter is composed of a seventh-tuned passive filter and an active filter in series connection, both dc voltage and kVA rating of the active filter are dramatically decreased compared with the pure shunt active filter. In real application, this feature is very attractive since the active power filter with fully power electronics is very expensive. A reasonable tradeoff between filtering performances and cost is to use the hybrid active filter. Design consideration are presented, and experimental results are provided to validate effectiveness of the proposed method. Furthermore, this paper discusses filtering performances on line impedance, line resistance, voltage unbalance, and capacitive filters.
TL;DR: The results show that the system efficiency is improved by using tertiary optimization control and the desired transient response is ensured with system damping secondary control.
Abstract: Droop control by means of virtual resistance (VR) control loops can be applied to paralleled dc–dc converters for achieving autonomous equal power sharing. However, equal power sharing does not guarantee an efficient operation of the whole system. In order to achieve higher efficiency and lower energy losses, this paper proposes a tertiary control level including an optimization method for achieving efficient operation. As the efficiency of each converter changes with the output power, VR values are set as decision variables for modifying the power sharing ratio among converters. A genetic algorithm is used in searching for a global efficiency optimum. In addition, a secondary control level is added to regulate the output voltage drooped by the VRs. However, system dynamics is affected when shifting up/down the VR references. Therefore, a secondary control for system damping is proposed and applied for maintaining system stability. Hardware-in-the-loop simulations are conducted to validate the effectiveness of this method. The results show that the system efficiency is improved by using tertiary optimization control and the desired transient response is ensured with system damping secondary control.
TL;DR: This paper shows that the problem of a slow transient response for the PLL can be alleviated by adding a phase-lead compensator in the MAF-PLL control loop.
Abstract: A basic approach to improve the filtering capability of a standard phase-locked loop (PLL) is to incorporate a moving average filter (MAF) into its control loop. This improvement, however, is at the cost of a slow transient response for the PLL, which is undesirable in most applications. It is shown in this paper that this problem can be alleviated by adding a phase-lead compensator in the MAF-PLL control loop. The effectiveness of the suggested approach is confirmed through numerical results.
TL;DR: A novel distributed active synchronization strategy is proposed, which takes into account not only the fundamental component, but also positive and negative sequences of the harmonic components, so that a seamless reconnection to the main grid can be performed.
Abstract: Microgrids can operate in both grid-connected and islanded modes. In order to seamlessly transfer from islanded to grid-connected modes, it is necessary to synchronize microgrid voltage and frequency, and phase to the main grid. However, since the microgrid is often based on power electronic converters, the synchronization process is quite different compared with the quasi-synchronism control in conventional power systems. First, in order to address this concern, the microgrid synchronization criteria are derived. Based on these criteria, a novel distributed active synchronization strategy is proposed, which takes into account not only the fundamental component, but also positive and negative sequences of the harmonic components. This way, a seamless reconnection to the main grid can be performed. The proposed method is implemented in the secondary control level of a hierarchical control structure. Real-time hardware-in-the-loop results show the feasibility of the proposed technique.
01 Sep 2015
TL;DR: This paper presents an overview of the LVDC distribution systems used in residential applications, and different power architectures and topologies are discussed.
Abstract: The concept of a microgrid has drawn the interest of research community in recent years. The most interesting aspects are the integration of renewable energy sources and energy storage systems at the consumption level, aiming to increase power quality, reliability and efficiency. On top of this, the increasing of DC-based loads has re-open the discussion of DC vs AC distribution systems. As a consequence a lot of research has been done on DC distribution systems and its potential for residential applications. This paper presents an overview of the LVDC distribution systems used in residential applications. Several publications that study the potential energy savings and overall advantages of the LVDC distribution systems are analysed. Different power architectures and topologies are discussed. The existing demonstration facilities where LVDC distribution systems have been implemented are also shown.
TL;DR: The increased power handling capacity and improved voltage profile of the ac distribution feeder using the proposed system is shown through time domain simulations on a test system.
Abstract: The development of distributed generation system and electric vehicles is bound to strain the distribution network. A typical radial distribution feeder suffers from voltage fluctuation and feeder overload in the presence of a large amount of variable renewable generation. This paper presents a concept of enhancing the power handling capacity of distribution networks using dc grid interconnections. Control of both the active and reactive power exchange between the ac feeder and the interconnecting power converter has been proposed for the voltage regulation at the ac feeder terminal. Besides, the dc grid interconnection also allows the introduction of a common storage system that can be shared by the connected ac feeders and the dc grid connection to other renewable energy resources. The increased power handling capacity and improved voltage profile of the ac distribution feeder using the proposed system is shown through time domain simulations on a test system.
TL;DR: This paper reveals that the current ripple can be significantly reduced by adjusting the gate patterns of space vector modulation between the rectifier and the inverter in a back-to-back converter.
Abstract: Back-to-back converters have been typically used to interconnect microgrids. For a back-to-back current-source converter, the dc-link current ripple is one of the important parameters. A large ripple will cause electromagnetic interference, undesirable high-frequency losses, and system instability. Conventionally, with a given switching frequency and rated voltage, the current ripple can be reduced by increasing the dc-link inductor, but it leads to bulky size, high cost, and slow dynamic response. To solve this problem, this paper reveals that the current ripple can be significantly reduced by adjusting the gate patterns of space vector modulation between the rectifier and the inverter in a back-to-back converter. The experimental results verify the effectiveness of the proposed method.
29 Oct 2015
TL;DR: In this paper, a quasi-proportional-resonant (Quasi-PR) current controller is designed for the capacitive-coupling grid-connected inverter.
Abstract: The capacitive-coupling grid-connected inverter (CGCI) is able to achieve reactive power compensation and active power transfer simultaneously with a low operational voltage. The CGCI is coupled to the point of common coupling (PCC) via a second-order LC circuit, which makes its modeling and current control characteristics differs from the conventional inductive-coupling grid-connected inverter. The direct current tracking with hysteresis pulse width modulation (PWM) was used in previous studies. However, this method suffers from widely varying switching frequency and large current ripples. A Quasi-proportional-resonant (Quasi-PR) current controller is designed for the CGCI in this paper. Its modeling and parameter selection are studied in detail. In contrast with proportional-integration (PI) current controller, the Quasi-PR controller reduces steady-state error. It also generates a voltage reference for applying the carrier-based PWM to improve output waveform quality. Simulation results are provided to verify the Quasi-PR controller and comparison with the PI controller is also done. A lab-scale prototype is built. Experimental results are given to show the validity of the proposed control method and its design.
TL;DR: A systematic procedure for accurate dynamics assessment and tuning of synchronous-frame proportional-integral current controllers, which is based on linear control for multiple-input-multiple-output (MIMO) systems, and is suitable for wind turbine applications.
Abstract: Current controller performance is key in grid-connected power converters for renewable energy applications. In this context, a challenging scenario is arising in multi-megawatt wind turbines, where sampling and switching frequencies tend to be lower and lower as power ratings increase. This strongly affects achievable control time constant. With this perspective, this paper presents a systematic procedure for accurate dynamics assessment and tuning of synchronous-frame proportional–integral current controllers, which is based on linear control for multiple-input–multiple-output (MIMO) systems. The dominant eigenvalues of the system are calculated with explicit consideration of time-delay and cross-coupling terms, two factors which clearly impair the system dynamics when considering a low sampling frequency. The proposed methodology is summarized as follows. First, the plant and controller matrices are modeled in state space. Subsequently, the characteristic polynomial of the closed-loop system is obtained and a computer-aided parametric analysis is performed to calculate the MIMO root locus as a function of the control gain. By its inspection, it is possible to identify the gain, which minimizes the current closed-loop time constant. This tuning is suitable for wind turbine applications, taking into consideration cascaded-control structures and grid-code requirements. The validity and accuracy of the analysis is fully supported by experimental verification.
15 Mar 2015
TL;DR: In this article, the authors present the development of a microgrid central controller in an inverter-based intelligent microgrid (iMG) lab in Aalborg University, Denmark.
Abstract: This paper presents the development of a microgrid central controller in an inverter-based intelligent microgrid (iMG) lab in Aalborg University, Denmark. The iMG lab aims to provide a flexible experimental platform for comprehensive studies of microgrids. The complete control system applied in this lab is based on the hierarchical control scheme for microgrids and includes primary, secondary and tertiary control. The structure of the lab, including the lab facilities, configurations and communication network, is first introduced. Primary control loops are developed in MATLAB/Simulink and compiled to dSPACEs for local control purposes. In order to realize system supervision and proper secondary and tertiary management, a LabVIEW-based microgrid central controller is also developed. The software and hardware schemes are described. An example case is introduced and tested in the iMG lab for voltage/frequency restoration and voltage unbalance compensation. Experimental results are presented to show the performance of the whole system.
TL;DR: In this paper, a hierarchical controller for a hybrid PV-battery-hydropower microgrid is proposed in order to achieve the parallel operation of the hydropower and PVbattery system with different rates and to guarantee power sharing performance among PV voltage-controlled inverters.
Abstract: Hybrid photovoltaic (PV)-battery-hydropower microgrids (MGs) can be considered to enhance electricity accessibility and availability in remote areas. However, the coexistence of different renewable-energy sources with different inertias and control strategies may affect system stability. In this paper, a hierarchical controller for a hybrid PV-battery-hydropower MG is proposed in order to achieve the parallel operation of the hydropower and PV-battery system with different rates and to guarantee power sharing performance among PV voltage-controlled inverters, while the required power to the hydropower-based local grid is supplied. In this case, the PV-battery system will operate as a $PQ$ bus to inject the desired active and reactive powers to the local grid, while the hydropower station will act as a slack bus which maintains its voltage amplitude and frequency. An integrated small-signal state-space model is derived to analyze the system stability of the hybrid MG. The simulation results show system frequency and voltage stability for a hybrid MG demonstration which includes the 2-MWp PV installations, a 15.2-MWh battery system, and a 12.8-MVA hydropower plant. The experimental results on a small-scale laboratory prototype verify the validity of the theoretical analysis and proposed control strategy.
TL;DR: In this article, an improved control strategy for grid-connected inverters within microgrids is presented, which is based on the classical P − ω and Q − V droop method.