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This paper proposes a control strategy for a single-stage, three-phase, photovoltaic (PV) system that is connected to a distribution network. The control is based on an inner current-control loop and an outer dc-link voltage regulator. The current-control mechanism decouples the PV system dynamics from those of the network and the loads. The dc-link voltage-control scheme enables control and maximization of the real power output. Proper feedforward actions are proposed for the current-control loop to make its dynamics independent of those of the rest of the system. Further, a feedforward compensation mechanism is proposed for the dc-link voltage-control loop, to make the PV system dynamics immune to the PV array nonlinear characteristic. This, in turn, permits the design and optimization of the PV system controllers for a wide range of operating conditions. A modal/sensitivity analysis is also conducted on a linearized model of the overall system, to characterize dynamic properties of the system, to evaluate robustness of the controllers, and to identify the nature of interactions between the PV system and the network/loads. The results of the modal analysis confirm that under the proposed control strategy, dynamics of the PV system are decoupled from those of the distribution network and, therefore, the PV system does not destabilize the distribution network. It is also shown that the PV system dynamics are not influenced by those of the network (i.e., the PV system maintains its stability and dynamic properties despite major variations in the line length, line X/R ratio, load type, and load distance from the PV system). Index Terms--Control, eigenvalue analysis, feedforward, modal analysis, participation factor, photovoltaic (PV), power electronics, voltage-source converter (VSC).

This paper deals with modeling and stability analysis of a doubly-fed induction generator (DFIG)-based wind-power unit that is interfaced with the grid via a series-compensated transmission line. A detailed mathematical model is developed in this paper that takes into account dynamics of the flux observer, phase-locked loop (PLL), controllers of the power-electronic converter, and wind turbine. Using the model and based on eigenvalue/participation-factor analysis, the system and controller parameters that substantially influence the system stability have been identified. The developed model is validated through a comprehensive set of simulation studies in the Matlab/Simulink and PSCAD/EMTDC software environments. Index Terms--Control, doubly-fed induction generator (DFIG), eigenvalue analysis, participation factor, power electronics, series compensation, small-signal analysis, wind power.

This paper presents an active islanding detection method for a distributed resource (DR) unit which is coupled to a utility grid through a three-phase voltage-sourced converter (VSC). The method is based on injecting a negative-sequence current through the VSC controller and detecting and quantifying the corresponding negative-sequence voltage at the point of common coupling of the VSC by means of a unified three-phase signal processor (UTSP). UTSP is an enhanced phase-locked loop system which provides high degree of immunity to noise, and thus enable islanding detection based on injecting a small (< 3%) negative-sequence current. The negative-sequence current is injected by a negative-sequence controller which is adopted as the complementary of the conventional VSC current controller. Based on simulation studies in the PSCAD/EMTDC environment, performance of the islanding detection method under UL1741 anti-islanding test is evaluated, and its sensitivity to noise, grid short-circuit ratio, grid voltage imbalance, and deviations in the UL1741 test parameters are presented. The studies show that based on negative-sequence current injection of about 2% to 3%, islanding can be detected within 60 ms even for the worst case scenario. Index Terms--Distributed resource (DR), islanding detection, UL1741 test.

This paper introduces and develops i) a novel unified dynamic model, and ii) new control schemes for a grid-connected static energy conversion system that is based on the voltage-sourced converter (VSC) configuration. The model and the controls are applicable to both the conventional two-level and the three-level neutral point diode clamped (NPC) VSCs. The model is unified since it represents the dynamic behavior of the VSC with respect to both positive- and negative-sequence components, due to unbalanced grid conditions and disturbances. To develop the unified model, the concepts of 'generalized sinusoidal pulse-width modulation (SPWM)' and 'VSC sequence subsystems' are presented. Based on the negative-sequence subsystem control, a new control strategy, to either balance the ac line currents or to mitigate dc-bus voltage ripples, is introduced. Application of the developed model and the controls to a variable-speed wind-power unit is presented in the paper. Index Terms--Instantaneous symmetrical components, neutral point diode clamped (NPC) converter, sequence subsystems, unbalanced operation, voltage-sourced converter (VSC), wind power.

Recent and ongoing developments in wind turbine technology indicate a trend towards utilization of high capacity (e.g., up to 5 MW) wind power units in large wind farms. Higher capacity of the wind turbine necessitates operation of the corresponding electric machine and the static converter system at higher voltages. This paper presents a neutral point diode clamped (NPC) converter system that inherently accommodates higher voltage and power ratings of a high capacity wind power unit. The overall control strategy of an NPC-based wind power unit and the details of the ac side and the dc side controls of the NPC converter system are also described. The generator-side NPC converter provides torque-speed control of the turbine-generator unit. The network-side NPC converter controls real and reactive power flow to the network and thus regulates the dc bus voltage and the ac side power-factor (or voltage) respectively. The paper also presents a new control approach to balance the dc capacitor voltages. The NPC converter system is augmented with a dc chopper that controls the synchronous generator field current. The NPC-based converter system is used to interface a 3 MW, direct-drive (gearless), synchronous machine based wind power unit to the utility grid. Performance of the overall NPC-based wind power unit, under the proposed controls, is evaluated based on time domain simulations in the power systems computer aided design (PSCAD) electromagnetic transient for DC (EMTDC) environment. Index Terms--Current control, dc-side voltage balancer, electromagnetic transients, neutral-point diode clamped (NPC) converter, variable-speed wind-power system.

This paper presents a comprehensive model of the Back-to-Back (BtB) HVDC system based on the three-level Neutral-Point Diode Clamped (NPC) converter. Based on the developed model, a systematic design procedure for i) the ac-side controllers, ii) the voltage balancer of the dc-side capacitors, and iii) the net dc-bus voltage controller, are presented. The model is developed based on the generalized state-space averaging method and the principle of power balance. The developed model precisely describes the system dynamics if the ac grids are strongly or moderately stiff, and offers acceptable precision otherwise. The averaged nature of the model inherently renders itself for analysis in the SIMULINK/MATLAB environment, and thus provides a computationally efficient tool for the design and the performance evaluation of the control. The accuracy of the developed model and the controls are validated by comparing the results from MATLAB/SIMULINK with those obtained from the exact switching model of the system, based on digital time-domain simulation studies, using the PSCAD/EMTDC software package. Index Terms--Back-to-Back HVDC system, current control, generalized state-space averaging, multilevel converter, Neutral Point Diode Clamped (NPC) converter.

This paper presents an in depth analysis to show that the conventional power balance model for the dc-side control of the Neutral Point Diode Clamped (NPC) converter is not adequately accurate for all operating conditions. The inaccuracy is due to the fact that the instantaneous power of the ac-side inductors is assumed to be negligible in the power balance equation. This paper shows that this assumption obscures (i) the impact of operating point which is a direct consequence of the inherent nonlinear nature of the problem and (ii) the nonminimum-phase property of the plant in the rectifying mode of operation. Thus, the conventional dc-side control model may lead to unsatisfactory performance or even instability. To overcome the shortcoming, this paper also develops accurate nonlinear and linearized models for the dc-side controller design, including the impact of the ac-side interface reactors. The analytical results and the conclusions are validated by digital time-domain simulation of a study system in the PSCAD/EMTDC environment. The mathematical developments and conclusions of the paper are equally applicable to the conventional two-level VSC. Index Terms--Generalized state-space averaging, multilevel converter, neutral point diode clamped (NPC) converter, nonminimum-phase plant, power balance.

A comprehensive dynamic model of the three-level Neutral Point Diode Clamped (NPC) converter, based on the generalized state-space averaging method, is presented. The developed model mathematically describes (i) the reason for and (ii) the impacts of the system parameter tolerances on the drift/imbalance of the DC-side capacitor voltages. Then, based on the developed model (i) a novel controller to prevent DC capacitor voltage drift/imbalance and (ii) a decoupled current controller in the dq-frame are designed. The paper also presents a feed-forward control to eliminate the coupling between the voltage balancer and the current controller. The accuracy of developed NPC converter model and the effectiveness of the proposed controls are verified by time-domain simulations of a study system in the PSCAD/EMTDC environment. Index Terms--Generalized averaging method, multi-level converter, neutral point voltage control, unipolar PWM.

This article presents a dynamic model and a current-mode control for a dc–dc modular multilevel converter with the ultimate aim of expanding the converter to a three-leg configuration to reduce the filtering requirements of the dc ports. A complete model of the converter including a systematic approach for the control of the power throughput is developed. It is then shown that the expansion from a single-leg to three-leg configuration requires no additional modeling, and that the three-leg converter significantly alleviates the filtering requirements on both the high-voltage and low-voltage sides. The proposed model and control technique are verified using simulation results based on the MATLAB / Simulink software environment for both the single and three-leg configuration. Further verification is carried out through a single-leg 1-kW experimental prototype. [ABSTRACT FROM AUTHOR]

The dynamic performance of a microgrid is governed by the decentralized primary control strategy that is embedded in each of its hosted Distributed Energy Resource (DER) units. The primary control computes the voltage synthesized by the interfacing voltage-sourced converter such that the DER unit contributes active and reactive powers in support of the microgrid voltage amplitude and frequency. These operational requirements must be satisfied with respect to the converter safety and physical limitations, such as the limited magnitude of the converter output current and terminal voltage. In this paper, all the control requirements mentioned above are taken into account in a single constrained optimization problem using the framework of Model-based Predictive Control (MPC). Solving the primary control problem in this way allows the microgrid to respond to major disturbances such short-circuit faults and transitions between modes of operation, while the electronically-interfaced DER units operate within operational and safety limits. [ABSTRACT FROM AUTHOR]

In the future power systems, a large number of offshore wind farms will be connected to the AC grids through high voltage DC (HVDC) and multi-terminal DC (MTDC) grids. As wind power penetration level increases, complex grid codes and regulations will be imposed on wind turbines for frequency support. To follow any grid code and requirement for frequency support, two important features should be included in the wind turbine frequency support: i) It should be able to adjust the maximum additional power that the wind turbine temporarily provides for frequency support; ii) It should be capable of adjusting the time interval in which the wind turbine provides additional temporary power. The first feature is mainly important for reducing rate of change of frequency (RoCoF) and improving the frequency nadir while the second one is mainly important for fast frequency recovery from its nadir and improving the second frequency drop. This paper indicates that the conventional method cannot offer both of the two aforementioned features. To address this issue, two approaches are proposed for frequency support by wind turbines. The first one uses P-ωr and P-ƒWF droops in each wind turbine controller, where P, ωr, and ƒWF represent the wind turbine power, rotor speed, and wind farm frequency. The second method employs P-ωr and P-ƒWF droops in each wind turbine controller. Performance and effectiveness of the proposed methods are evaluated by time-domain simulation studies on an MTDC grid in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

This paper proposes a state feedback primary control strategy for microgrids with multiple distributed energy resource units, improving their transient behavior in both islanded and grid-connected modes of operation. To that end, the interaction of each distributed energy resource unit within the microgrid is modeled as a lumped dynamic system, which results to be nonlinear and multivariable. For the closed-loop control of such multivariable systems, the full state feedback formulation is preferred which requires a suitable state observer. For the design of the observer and feedback gains, the solution of the linear-quadratic estimation and regulation problems is considered. For simplicity, an approximate linear model at a representative operating point is derived. The linear quadratic Gaussian/loop-transfer recovery is adopted as the design procedure to optimize the trajectory of the state variables subject to a desirable actuation effort, in this case of the voltage amplitude and frequency, yielding a solution that is robust to model uncertainties. The effectiveness of the strategy is assessed through time-domain simulation on the CIGRE benchmark medium-voltage distribution network with three distributed energy resource units. These results are compared to those obtained with conventional static droop gains and with a state-of-the-art technique from the literature. [ABSTRACT FROM AUTHOR]

The dc–dc modular multilevel converter (MMC), derived from the ac–dc MMC, is an attractive converter topology for interconnection of medium-/high-voltage dc grids. The dc–dc MMC generates an ac voltage component in each arm to drive an ac circulating current for maintaining the energy balance of its submodule capacitors. The power transfer capability of the dc–dc MMC and the amplitude of the ac circulating current are closely coupled with the available arm ac voltage headroom. As the voltage conversion ratio of the dc–dc MMC deviates from 0.5, the power transfer capability of the converter diminishes. This paper proposes an enhanced closed-loop control strategy, which increases the power transfer capability and at the same time reduces the ac circulating current of the dc–dc MMC. Performance and effectiveness of the proposed control strategy are demonstrated by simulation studies in the MATLAB/Simulink as well as experiments conducted on a laboratory prototype. [ABSTRACT FROM AUTHOR]

This paper proposes a current-controlled voltage-mode control scheme for dispatchable electronically coupled distributed energy resource (DER) units based on sliding-mode control (SMC) strategy. The proposed control strategy provides fast and stable control on the terminal voltage and frequency of DER unit and ensures protection of the power-electronic interface to external faults. In addition it maintains the quality of the output voltage of the host DER unit in spite of unbalanced and/or distorted load currents. Performance of the proposed SMC strategy is demonstrated through time-domain simulation of a single islanded DER unit, starting from black-start and subjected to various operating scenarios, and it is compared to the performance of a proportional-integral (PI) based control strategy. Also a current-mode control strategy is proposed for DER units based on SMC. The performance of both voltage- and current-mode control strategies are demonstrated in a sample master-slave organized three-unit microgrid. [ABSTRACT FROM AUTHOR]

This paper proposes a power mismatch elimination strategy for a medium-voltage modular multilevel converter (MMC) -based photovoltaic (PV) system. In the MMC-based PV system, each submodule of the MMC is energized by multiple PV generators, each interfaced with the dc port of the submodule by a corresponding isolated dual active bridge dc–dc converter. This configuration allows for a transformerless connection to the host grid and independent maximum power point tracking for the PV generators. The paper then proposes a power mismatch elimination strategy that ensures that the current delivered to the host grid is balanced in spite of unbalanced PV generator outputs. The proposed power mismatch elimination strategy employs a dc differential current to equalize the leg powers, and an ac differential current to stabilize the dc voltages of the submodules. The effectiveness of the proposed power mismatch elimination strategy is demonstrated by time-domain simulations conducted on a model of the PV system in PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

An analytical steady-state model of the modular multilevel converter (MMC) can be used for component sizing and assessment of the impact of different parameters on the MMC performance. Thus, this paper proposes an enhanced steady-state model for the MMC, which enables one to calculate the amplitudes of the harmonic components of MMC arm currents, submodule capacitor voltages, and arm voltages. In addition, the proposed model allows for the calculation of the amplitude and phase angle of the modulating signal, and it also presents a criterion for resonance of the arm currents. This is achieved by means of a polynomial function of the modulation index, and through checking the stability criteria established by the proposed model. The capability of formulating the modulating signal enables a more accurate calculation of the steady-state amplitudes of the variables, as compared to the previously published models. This paper also presents an accurate method of submodule capacitor sizing, based on the proposed steady-state model. The proposed method gives the exact value of the submodule capacitance (i.e., the capacitance that results in dc voltage fluctuations within a desired range) for a prespecified submodule capacitor voltage variation and a given circulating current amplitude. The stability criterion given by the proposed steady-state model, used for sizing the arm inductance, and the submodule capacitor voltage variation given by the proposed capacitor sizing method, used for sizing the submodule capacitor, allows for the determining of the operating limits of the MMC in terms of its parameters. Extensive simulation results are presented to demonstrate the efficacy of the proposed steady-state model and the capacitor sizing method. Further, the results obtained from the proposed capacitor sizing method are compared with experimental results from the literature. [ABSTRACT FROM AUTHOR]

This paper proposes a modular multilevel converter (MMC) topology for high-voltage dc (HVdc) system applications. The proposed MMC employs two half-bridge converters, a cascade of networks that consist of electronic switches, and multiple capacitors. Thus, charged capacitors are inserted in series with the arm current path, for a desired level of the output ac voltage. The switches that must be turned on are judiciously selected for the most efficient current path. The proposed MMC also offers dc-side fault handling capability. The paper compares the proposed MMC with the half-bridge submodule-based MMC, full-bridge submodule-based MMC, and clamp-double submodule-based MMC technologies. It is shown that the proposed MMC is more efficient than the three aforementioned MMC technologies, while it offers the same fault handling capability as that of an equivalent full-bridge-based MMC. Time-domain simulation studies confirm the effectiveness of the proposed MMC under various normal and faulted operating scenarios, using detailed as well as reduced models. [ABSTRACT FROM AUTHOR]

In some mature power systems located in densely populated areas, an increase in demand and supply is resulting in very high short-circuit levels that are either too close to or over the safe breaker operating limits. Fault current limiters (FCLs) built with advanced materials and methods are a potent solution in this situation wherein an in-line FCL offers a very low impedance and power loss under normal operating conditions, but a high impedance and hence a lower short-circuit current during faults. From a planning perspective for complex meshed networks where faults are concurrently fed from several sources, it is important to optimally locate and size FCLs, which is a difficult mixed integer nonlinear optimization challenge as commonly used location sensitivity index methods and heuristic search techniques seem inadequate. In this paper, an iterative mixed integer nonlinear optimization method is proposed to optimally locate and size FCLs in a power system by searching the entire solution space such that costs are the least and fault currents are curtailed to levels within breakers' limits, without prior knowledge of the best locations irrespective of the size of the system. The IEEE 9-bus, IEEE 30-bus, and a real North American 395-bus transmission system were chosen to test and demonstrate the proposed method. [ABSTRACT FROM PUBLISHER]

This paper proposes a voltage control strategy for islanded operation of dispatchable electronically coupled distributed energy resource (DER) systems, for remote (off grid) electrification systems. The proposed control strategy is based on fractional-order sliding-mode control, offers black-start functionality, maintains the quality of the output voltage of the host DER system in spite of unbalanced and/or distorted load currents, and ensures protection of the power-electronic interface to external faults. Moreover, the fractional derivative order of the controller can be used as an additional tuning parameter. Performance of the proposed control strategy is demonstrated through time-domain simulation of single- and multi-DER islanded networks, starting from black-start and subjected to various operating scenario, and it is also compared to the performance of a proportional-integral (PI)-based control strategy. [ABSTRACT FROM PUBLISHER]

The increasing capacity of power systems and the continuing growth in interconnections within transmission networks to improve the reliability may cause the short-circuit fault current level of the equipment in the system, including the existing circuit breakers, to exceed their rated capacities. Therefore, the equipment must be either upgraded or replaced, which is costly and requires time-intensive procedures. Fault current-limiting techniques offer benefits to the system in such cases. Using passive elements, such as current-limiting reactors, is a well-known practice in power systems: however, they impact the power flow under normal operation, cause voltage drop, and might reduce the transient stability. Alternatively, resonant fault current limiters (RFCL) offer a dynamic solution based on proven technologies of current-limiting reactors and series capacitors. This paper presents a comprehensive framework to design RFCLs in bulk power systems. The presented approach uses a combination of mathematical analyses and numerical time-domain simulations to design the RFCL elements, and its effectiveness is assessed in test power systems. [ABSTRACT FROM PUBLISHER]

Stability analysis of average value models (AVMs) of vector-controlled modular multilevel converters (MMCs) is the subject matter of this paper. Stability analysis of fundamental frequency phasor-based AVMs of MMCs can be conducted in a traditional linear time-invariant framework through eigenvalue computation. This class of models does not consider circulating current control loop and hence fails to capture system instability that occurs in a certain range of gains of the circulating current controller. We propose stability analysis in a linear time-periodic (LTP) framework to solve this issue. To that end, a nonlinear AVM is presented that considers the submodule capacitor insertion dynamics and takes into account the output and the circulating current control schemes in the vector control approach. Upon linearization, an LTP model is derived from this averaged model. It is shown that the Poincar \acutee multipliers are indicative of system instability corresponding to a certain range of gains of the circulating current controller. [ABSTRACT FROM AUTHOR]

This paper proposes an energy management strategy (EMS) for a dc distribution system for power system integration of plug-in electric vehicles (PEVs). The dc distribution system is intended to integrate electric vehicles in a parking lot with an ac grid through bidirectional dc–dc converters (battery chargers) and a central voltage-sourced converter. Using an online constrained optimization algorithm, the proposed EMS manages the power flow within the dc system. Thus, the PEV owners can charge or discharge their batteries based on the state-of-charge of the batteries and their upcoming trip plans. The EMS offers two energy exchange options to the PEV owners: 1) the fast energy exchange option for the owners wishing to minimize the energy exchange time; and 2) the optimal energy exchange option for the owners intend to either minimize their costs of charging or maximize their revenues through selling their stored energy. The proposed EMS seamlessly handles all charging/discharging requests from the PEV owners with different options and, at the same time, it takes into account the power demand and power generation limits of the ac grid to preclude under-voltage, over-voltage, and reverse power flow issues. Various energy exchange scenarios are simulated to demonstrate the effectiveness of the proposed EMS. [ABSTRACT FROM PUBLISHER]

The small-signal stability of an interconnected source and load can be analyzed using the frequency-response methods of stability analysis. Existing frequency-response methods of stability analysis introduce artificial conservativeness and do not explicitly address stability robustness against load perturbations, that is, by how much the load can change such that the system will remain stable in the small-signal sense. This paper presents a frequency-response method of stability analysis which provides a less conservative stability condition compared to existing methods, in the sense of having less restriction on the control system; further, the proposed method establishes a robust stability margin in terms of perturbations in load parameters. The proposed method is based on μ analysis, and models the source-load dynamic interaction via a closed-loop system of impedances and admittances. The proposed method is used to examine small-signal stability of an islanded subnetwork extracted from a university campus microgrid, composed of a distributed energy resource (DER) unit feeding a load, providing a study with realistic parameter values. Two case studies are presented to show the features of the proposed method, namely, reduced conservativeness and the establishment of a robust stability margin. Another example is provided to show the application of the proposed stability analysis method in a multi-DER-unit network. The findings of the frequency-domain analysis are illustrated through time-domain simulations around the operating point on the system under study. [ABSTRACT FROM AUTHOR]

Widespread penetration of distributed generators (DGs) into the power system has aggravated the problem of temporary overvoltages (TOVs), which can inflict substantial damages on the DGs, equipment, and other customers. Through slight modifications to the power-electronic circuit and its control, this paper proposes a strategy for limiting the TOVs caused by single-stage grid-connected photovoltaic (PV) energy systems. This paper also proposes a reduced TOV mitigation strategy for applications where a somewhat less effective, but also less expensive, solution is acceptable. The effectiveness of the proposed TOV mitigation strategy is demonstrated through time-domain simulation studies conducted on a detailed switched model of a study PV system in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

This paper proposes a systematic method for developing a model of a dc distribution system, based on the configuration of the system. The dc distribution system is assumed to host electric vehicles and photovoltaic modules, using dc–dc converters, and to integrate them with an ac power grid. The developed model is of the matrix form and, therefore, can be readily expanded to represent a dc distribution system of any desired number of dc–dc converters. The model captures both the steady-state and dynamic characteristics of the system and includes port capacitors of the converters, as well as the interconnection cables. Thus, it can be used for identifying the condition for existence of a steady state, as well as for stability analysis. This paper further proposes an alternative set of characteristic equations that are less computationally intensive than the original matrix representation, for example, for online stability assessment tasks. The adequacy of the proposed model has been demonstrated through a number of case studies conducted and compared in the PSCAD/EMTDC and MATLAB software environments. [ABSTRACT FROM PUBLISHER]

This study proposes a single‐objective optimal sizing approach for an islanded microgrid (IMG). The approach determines the optimal component sizes for the IMG, such that the life‐cycle cost is minimised while a low loss of power supply probability (LPSP) is ensured. As wind speed and solar irradiation exhibit both diurnal and seasonal variations, the proposed algorithm takes advantages of the typical meteorological year‐based chronological simulation and enumeration‐based iterative techniques. The mathematical models presented in this study for the IMG components consider the non‐linear characteristics as well as the reactive power. The LPSP is also formulated based on the supply‐demand balances of both real and reactive powers, and an economic evaluation model is presented. The proposed sizing approach identifies the global minimum, and simultaneously provides the optimal component sizes as well as the power management strategies. This study also presents a number of sensitivity analyses as well as comparisons with a commercial software package. [ABSTRACT FROM AUTHOR]

This paper proposes a transformerless dc–dc converter, which features a large voltage ratio, for medium-voltage dc (MVDC) applications. The proposed converter combines a boost converter with a series-parallel resonant converter which enables soft switching of the power switches and diodes. Further, the power switches and diodes of the proposed converter experience lower voltage stresses compared to the other topologies. The proposed converter is modular and, therefore, permits series and/or parallel connection of multiple units for the desired voltage and/or power ratings. This paper discusses the principles of operation, analysis, and design of the proposed converter. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed converter. [ABSTRACT FROM AUTHOR]

The ever-increasing penetration of wind power into the electric power systems indicates the need that wind energy conversion systems (WECSs) should be able to regulate their output real power, since their participation in power-flow control is expected to be essential for power systems stability. On the other hand, the tendency to build larger WECSs at competitive costs encourages lighter and, consequently, more flexible drive-trains. However, a softer drive-train may give rise to undamped drive-train oscillations when the WECS exercises output power regulation. This paper proposes an enhanced control strategy that enables a direct-drive WECS based on the permanent magnet synchronous generator (PMSG) to damp its drive-train torsional mode, even if the output real power of the WECS is regulated. A procedure is presented for tuning of the proposed control to ensure the internal stability of the WECS, considering the impacts of the proposed control on the dynamic performance of the host WECS. [ABSTRACT FROM PUBLISHER]

This paper proposes a method for enhancing the stability of a dc distribution system that integrates plug-in hybrid electric vehicles (PHEVs) with an ac power grid. The dc distribution system is interfaced with the host ac grid via a voltage-sourced converter and can also embed photovoltaic (PV) modules. Thus, bidirectional dc–dc electronic power converters act as battery chargers and interface the PHEVs with the dc distribution system, while PV modules are interfaced with the dc distribution system via unidirectional dc–dc converters. The dc distribution system is expected to be more efficient and economical than a system of ac–dc battery chargers directly interfaced with an ac grid, but it is prone to instabilities due to the constant-power property of the dc–dc converters. Using a nonlinear control strategy, the proposed stability enhancement method mitigates the issue of instability by altering the power setpoints of the battery chargers, bidirectional dc–dc converters, without a need for changing system parameters or hardware. The paper presents mathematical models for the original and modified systems and demonstrates that the proposed technique expands the stable operating region of the dc distribution system. Simulation results demonstrate the effectiveness of the proposed method for a study system in the PSCAD/EMTDC software environment. [ABSTRACT FROM PUBLISHER]

This paper proposes a control strategy to enhance transient performance and stability of a droop-controlled active distribution network. The dependency of dynamics on the droop gains, steady-state power flow, and network/load is studied in a droop-controlled distributed energy resource (DER) unit. These dependencies result in poor transient performance or even instability of the network in the event of a disturbance in the system. To eliminate these dependencies, a gain-scheduled decoupling control strategy is proposed which reshapes the characteristics of conventional droop by means of supplementary control signals; these control signals are based on local power measurements and supplement the d- and q-axis voltage reference of each DER unit. The impact of the proposed control on the DER and network dynamics is studied by calculating the eigenvalues of a test active distribution system assuming proposed control. The proposed control is shown to stabilize the system for a range of operating conditions. The effectiveness of the proposed control is further demonstrated through simulations carried out in the PSCAD/EMTDC software environment, on the active distribution system under study. [ABSTRACT FROM AUTHOR]

With the increasing attention to the concept of microgrids, it has become important to identify the requirements that enable an electric distribution network to operate as a microgrid. This paper studies a realistic distribution network to accomplish the following: 1) determine the conditions under which the network can operate as a microgrid and 2) present the control scenarios that allow the islanded operation of the network. In particular, the employment of electronically coupled distributed energy resources with the fault-ride-through and synchronization capabilities is investigated. Another objective of this paper is to propose practical protection strategies that are effective in both operational modes of a microgrid (i.e., the grid-connected mode and the islanded mode), considering the system constraints; the protection strategies provide the microgrid with acceptable reliability against different fault scenarios. The effectiveness of the control and protection strategies is demonstrated and discussed through a comprehensive set of simulation studies conducted in the PSCAD/EMTDC software environment. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel single-switch quasi-active power factor correction (PFC) converter based on two flyback dc/dc modules. To achieve a high efficiency, part of the input power is directly transferred to the load by the first dc/dc module. The rest of the input power is stored in the dc-bus capacitor and reprocessed by the second dc/dc module. Together, the dc-bus capacitor and the second dc/dc module also serve as a regenerative snubber. Furthermore, a quasi-active PFC circuit is provided to improve the power factor and to ensure the continuous current mode (CCM) operation of the input inductor without the need for sophisticated feedforward sensing and control mechanism. The quasi-active PFC circuit is driven by a magnetic switch, through the tertiary winding of the transformer of the first flyback module. The input current harmonics of the proposed converter meet the IEC 61000-3-2 (Classes A, C, and D) requirements. The dc-bus capacitor voltage is always less than the input voltage regardless of the load condition and is fairly invariant to large variations of the input voltages. High power factor with CCM operation and fast output voltage regulation have been achieved through a single-loop controller. Operating principles, analysis, and experimental results are presented to verify the effectiveness of the proposed converter. [ABSTRACT FROM PUBLISHER]

As the penetration of wind energy into the power system continues to grow, wind energy conversion systems (WECSs) are increasingly expected to be able to control their output real power, while retaining their maximum power-point tracking (MPPT) capability. This paper proposes a simple real power control strategy, which augments the MPPT feature of modern WECSs, and is based on rapid torque control as opposed to the traditional pitch-angle control. This paper presents the implementation of the proposed control strategy for a direct-drive WECS that employs the permanent-magnet synchronous generator, even though the proposed method can also be extended to other classes of electronically interfaced WECSs. The paper also presents a parameter-tuning procedure for the proposed control strategy. The effectiveness of the proposed control strategy is demonstrated through mathematical analysis and time-domain simulation studies. [ABSTRACT FROM PUBLISHER]

This paper proposes a communication-assisted protection strategy implementable by commercially available microprocessor-based relays for the protection of medium-voltage microgrids. Even though the developed protection strategy benefits from communications, it offers a backup protection strategy to manage communication network failures. The paper also introduces the structure of a relay that enables the proposed protection strategy. Comprehensive simulation studies are carried out to verify the effectiveness of the proposed protection strategy under different fault scenarios, in the PSCAD/EMTDC software environment. [ABSTRACT FROM PUBLISHER]

This paper proposes an adaptive feedforward compensation that alters the dynamic coupling between a distributed-resource unit and the host microgrid, so that the robustness of the system stability to droop coefficients and network dynamic uncertainties is enhanced. The proposed feedforward strategy preserves the steady-state effect that the conventional droop mechanism exhibits and, therefore, does not compromise the steady-state power sharing regime of the microgrid or the voltage/frequency regulation. The feedforward compensation is adaptive as it is modified periodically according to the system steady-state operating point which, in turn, is estimated through an online recursive least-square estimation technique. This paper presents a discrete-time mathematical model and analytical framework for the proposed feedforward compensation. The effectiveness of the proposed control is demonstrated through time-domain simulation studies, in the PSCAD/EMTDC software environment, conducted on a detailed switched model of a sample two-unit microgrid. [ABSTRACT FROM AUTHOR]