Search

This paper deals with the control and performance improvement of parallel-operated voltage-source inverters (VSIs) controlled as virtual synchronous generators (VSGs). In publications regarding the parallel-operated VSGs, transmission lines are considered to be mainly inductive. However, less analytical works have been done regarding the control of paralleled VSGs in low-voltage grids with dominant resistive impedances. Once VSIs are controlled as VSGs in a microgrid with more resistive transmission lines, swing equation and system representation for the power-angle synchronization will change leading to a new control structure. Therefore, this paper deals with the control of parallel-operated converter-based VSGs in low-voltage grids with dominant resistive line impedances. In this way, the VSG representation, comprising the swing equation and V-P droop characteristic, for applications in highly resistive microgrids is presented, in which the swing equation and VSG frequency are related to reactive power. Then, the V-P droop characteristic is modified and an enhanced P-V droop characteristic for proper sharing of active power between the VSGs in highly resistive microgrids is proposed. Next, the VSG control is modified so that the R/X ratio at the VSG output increases and thus the decoupled control of active/reactive powers in relatively inductive cases is realized as well. [ABSTRACT FROM AUTHOR]

Blade passing effects (BPEs) including tower shadow and wind shear cause 3P oscillations on mechanical and electrical active power profiles in a grid-connected doubly fed induction generator (DFIG) wind turbine. The basic aim of this paper was to present an effective method to mitigate the destructive problems caused by BPEs and to smooth the active power profile in DFIG, in addition to mitigate 3P oscillations on the voltage profile of the weak connected grid. The proposed approach was based on design and application of a band pass filter (BPF) to the external loop of DFIG active power control in the rotor side converter to remove 3P oscillations of the rotor reference current at mean wind speeds of 9-16 m/s. At high wind speeds, the proposed BPF is simultaneously used in the pitch angle control loop of DFIG to reduce 3p pulsations in the pitch angle profile. The simulation process is carried out at multiple wind speeds considering the new model of BPEs, which was derived to model these phenomena with a simple equation. The proposed model could be effective to analyze the DFIG performance, suitably via considering the adverse effects of tower shadow and wind shear, and to study the grid power quality problems. Simulation results are obtained for DFIG active power and grid RMS voltage profiles. These results validated the usefulness of the proposed method to improve the DFIG performance by smoothing the produced active power profile. Moreover, the periodic pulsations of the grid voltage profile are mitigated to enhance the grid power quality. [ABSTRACT FROM AUTHOR]

Integration of wind power generation into power distribution grid may result in stability and power quality challenges. In this way, this paper first reviews the grid integration issues of wind power generation. Then, as the main contribution, the paper develops in-depth theoretical analyses for examination the grid integration issues such as voltage rise, output power fluctuations, and static and dynamic voltage variations created due to grid integration of wind turbines and corresponding aerodynamic power fluctuations. Besides, the paper addresses these issues by proposing elaborate control approaches and by using the static compensator (STATCOM). The paper first addresses the problem of voltage rise by active power curtailment through pitch control. However, this method may result in a large waste of wind energy that may not be cost effective. Next, the STATCOM is implemented and efficient control approaches are proposed for addressing the problem of voltage rise, improvement of voltage profile and suppression of voltage and power fluctuations. At the end, simulation studies for the study distribution grid are given. The study system is an actual distribution grid in Iran with approximately 7.9 MW wind power generation and 21.9 MW load. [ABSTRACT FROM AUTHOR]

A risk-based approach for short term security assessment with voltage stability constraint is presented in this paper. This approach includes voltage-stability considerations in reactive power adequacy assessment of a composite power system. The proposed method is based on probabilistic criteria because deterministic risk criteria do not define consistently the true risk in the system. For risk calculation, in this paper, load and parametric uncertainties and system contingencies are considered. Then the effects of uncertainties and contingencies on bus voltage violation are calculated. The required reactive power is determined based on the calculated voltage violation in order to maintain the bus voltage in an acceptable range. At the end, to make a trade-off between reliability and economics, the local reactive power adequacy is calculated using the reliability cost/worth concept. The effect of voltage violations on reliability indices and required reactive power is illustrated in a reliability test system (6-bus RBTS). [ABSTRACT FROM AUTHOR]

Highlights: [•] The paper improves the LVRT capability of DFIG under unbalanced voltage dips. [•] LVRT method is realized through joint use of RSC control and an asymmetrical SDR. [•] Asymmetrical SDR resistors are activated only in phase(s) experiencing low voltage. [•] DFIG is controlled such that no unbalance voltage appears on the stator voltage. [•] The method removes fluctuation of transient response appeared during the fault. [Copyright &y& Elsevier]

This paper deals with the coordinated control of rotor- and grid-side converters in wind turbines with doubly fed induction generators (DFIGs) to improve the low-voltage ride-through capability. The rotor-side converter control and additional equipment, called stator damping resistor, are used to limit the rotor inrush current and to reduce the oscillations and settling time of DFIG transient response during the voltage dip. Also, the grid-side converter is controlled to limit the dc-link overvoltage during the voltage drop. It is found that the dynamics of the grid-side converter and dc-link voltage exhibit nonminimum phase behavior, and thus there is an inherent limitation on the achievable dynamic response during the fault. Since the dc-link dynamics is nonlinear, the linear control scheme cannot properly limit the dc-link voltage under large voltage dips. Thus, a nonlinear control scheme applied to the grid-side converter is proposed, which stabilizes the internal dynamics and limits the dc-link voltage fluctuations during the fault. The proposed ride-through approaches limit the peak values of rotor current and dc-link voltage at the instants of occurring and clearing the fault. They also limit the oscillations of electromagnetic torque, and consequently, improve the DFIG voltage dip behavior. [ABSTRACT FROM PUBLISHER]

Abstract: This paper analytically investigates the effects of system and controller parameters and operating conditions on the dynamic and transient behavior of wind turbines (WTs) with doubly-fed induction generators (DFIGs) under voltage dips and wind speed fluctuations. Also, it deals with the design considerations regarding rotor and speed controllers. The poorly damped electrical and mechanical modes of the system are identified, and the effects of system parameters, and speed/rotor controllers on these modes are investigated by modal and sensitivity analyses. The results of theoretical studies are verified by time domain simulations. It is found that the dynamic behavior of the DFIG-based WT under voltage dips is strongly affected by the stator dynamics. Further, it is shown that the closed loop bandwidth of the rotor current control, rotor current damping, DFIG power factor and the rotor back-emf voltages have high impact on the stator modes and consequently on the DFIG dynamic behavior. Moreover, it is shown that the dynamic behavior of DFIG-based WT under wind speed fluctuation is significantly dependent on the bandwidth and damping of speed control loop. [Copyright &y& Elsevier]

Summary: This paper deals with the improvement of stability margin, dynamic behavior, and steady state performance of a direct current (DC) microgrid system consisting of wind energy sources, battery energy storage, and back up generation source. The DC microgrid control system should set the DC bus voltage at the allowable range and provide proper load sharing among generation sources. Accurate load sharing, proper voltage regulation, and dynamic stability of the system at presence of constant power loads (CPLs) are the main problems that may be raised in operation of DC microgrid systems. In order to address these issues, this paper proposes a generalized droop‐based compensator that not only stabilizes the DC microgrid dynamics at high penetration of CPLs, but also addresses the issues such as inaccurate load sharing and improper voltage regulation. The proposed generalized droop‐based compensator comprises three parts: the first part enhances the DC microgrid stability even at high penetration of CPL, the second part compensates the transmission lines voltage drops and improves the function of voltage regulation, and the third part performs proper load sharing among different generation sources. At the end, capability of the proposed compensator is examined by frequency response analysis and time domain simulations. [ABSTRACT FROM AUTHOR]