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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]

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]

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]

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 presents modeling guidelines and a benchmark system for power system simulation studies of grid-connected, three-phase, single-stage Photovoltaic (PV) systems that employ a voltage-sourced converter (VSC) as the power processor. The objective of this work is to introduce the main components, operation/protection modes, and control layers/schemes of medium- and high-power PV systems, to assist power engineers in developing circuit-based simulation models for impact assessment studies, analysis, and identification of potential issues with respect to the grid integration of PV systems. Parameter selection, control tuning, and design guidelines are also briefly discussed. The usefulness of the benchmark system is demonstrated through a fairly comprehensive set of test cases, conducted in the PSCAD/EMTDC software environment. However, the models and techniques presented in this paper are independent of any specific circuit simulation software package. Also, they may not fully conform to the methods exercised by all manufacturers, due to the proprietary nature of the industry. [ABSTRACT FROM AUTHOR]

This paper proposes a single-stage three-phase photovoltaic (PV) system that features enhanced maximum power point tracking capability, and an improved energy yield under partial shading conditions. Further, the proposed PV system can effectively double the maximum permissible dc voltage of a grounded conventional single-stage PV system, with no need for insulators, fuses, disconnects, and switchgear of a higher voltage class, with respect to safety/insulation standards or common system integration practices exercised for conventional grounded single-stage PV systems. The proposed PV system is realized through the parallel connection of an auxiliary half-bridge converter to the dc link of a conventional single-stage PV system and, therefore, is also an option for retrofit applications. This paper presents the mathematical model, principles of operation, and the control loops of the proposed single-stage PV system. The performance of the proposed single-stage PV system is demonstrated by time-domain simulation studies conducted on a detailed switched model in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

This paper proposes a multimode control strategy for a doubly fed induction generator-based wind-power unit, which enables operation in the islanded (autonomous) mode as well as the grid-connected mode. The configuration can be an option for electricity generation in remote communities with favorable wind conditions, but limited or unreliable connection to the grid. In the grid-connected mode, the proposed control enables the wind power unit to operate with or without battery energy storage, as desired. In case that the wind-power unit is augmented with battery energy storage, in the islanded mode, the proposed control strategy enables voltage and frequency regulation for the network, and parallel operation with constant-speed wind-power units, induction motor loads, and passive loads. The proposed control strategy employs a unified controller for all of the foregoing modes of operation and, therefore, relieves the need for switching between different controllers or reconfiguration of the hardware. The effectiveness and robustness of the proposed control strategy are demonstrated for faulted as well as normal operating conditions through simulation studies carried out on a detailed switched model of the system in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper proposes a control strategy for important transients of a single-stage, three-phase, PV system that is connected to a distribution network. The proposed control strategy adopts an inner current-control loop and an outer DC-link voltage control loop. The current-control mechanism renders the PV system protected against external faults, enables control of the DC-link voltage and, therefore, controls/maximizes the PV system power output. The paper also proposes a feed-forward compensation strategy for the DC-link voltage control loop to mitigate the impact of the nonlinear characteristic of the PV array on the closed-loop stability, and to permit design and optimization of the DC-link voltage controller for a wide range of operating conditions. A mathematical model and a control design methodology are presented for the PV system, and it is shown that under the proposed control, the PV system fulfills the operational requirements of a grid-connected PV system. The effectiveness of the proposed control strategy and the most important transients of the PV system are evaluated through simulation studies conducted on a detailed switched model of the PV system in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

• A power regulation scheme is proposed for grid-connected DER units based on ILC. • The proposed control compensates the harmonic distortion of the grid current • The paper presents the mathematical model , as well as the control design methodology. • A MIMO ILC with perfect power tracking and harmonic rejection performance is proposed. This paper proposes a power regulation scheme for dispatchable grid-connected distributed energy resource (DER) units based on the iterative learning control (ILC) strategy. The proposed control provides fast and stable control of the real and reactive powers that the DER unit delivers to the grid. In addition, it compensates the harmonic distortion of the grid current caused by the local nonlinear loads and delivers a high quality power to the grid. To this end, the paper presents the mathematical model on which the proposed control is based, as well as the control design methodology. A new MIMO iterative learning control with perfect power tracking and harmonic rejection performance as well as minor measurement requirements is proposed. The performance of the proposed ILC strategy is demonstrated through time-domain simulation of a grid-connected DER unit under various operating scenarios, and it is compared with the performance of the conventional control based on the instantaneous power theory (IPT). [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes an integrated wind-photovoltaic-battery hybrid system that features a simple power management strategy, requires a lower number of power-electronic converters, and eliminates the need for dump loads. Thus, it is expected to offer a lower cost and higher efficiency, and to enable easier integration with distribution networks, as compared with a set of three stand-alone system. The power management strategy of the proposed hybrid system enables (1) rapid control of the wind and photovoltaic (PV) power outputs for tightly regulating the battery current, (2) off-grid operation with black-start capability, (3) grid-connected operation, and (4) safe transition from the grid-connected mode to the off-grid mode, and vice versa, without a need for communications with the host grid. Further, the proposed hybrid system is expected to have plug-and-play and power sharing capabilities and, therefore, suited for multi-generator remote electrification systems. The effectiveness of the proposed hybrid system is demonstrated through time-domain simulation studies in the PSCAD/EMTDC software environment. [Copyright &y& Elsevier]