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Generalized integrator-based orthogonal signal generator phase-locked loops (GI-OSG PLLs) are very popular in single-phase grid synchronization. Generally speaking, the OSG of this PLL is realized from a second-order integrator perspective. As a result, the OSG implementation impacts the PLL dynamics. Improving the PLL performance by employing a reduced-order OSG implementation is the main motivation for this paper. Therefore, a reduced order-OSG PLL is proposed. In the proposed PLL, a single state and two output equations are employed to obtain a pair of orthogonal signals. This results in the loss of orthogonality between the OSG outputs and the single-phase supply voltage. Also, harmonic filtering capability provided by SOGI-OSG becomes non-existent. These issues are resolved by incorporating steady-state phase angle-offset error compensation at the PLL output and a moving average filter (MAF) within the PLL loop. To counteract the influence of MAF on the PLL’s stability and dynamics performance, a phase-lead compensator is designed. Through experimental studies, it is shown that the proposed reduced order-OSG PLL offers excellent dynamic performance. Also, the proposed PLL achieves minimal fluctuation of PV power when employed for grid synchronization of a PV system. [ABSTRACT FROM AUTHOR]

Fast dc fault detection in high-voltage dc (HVdc) grids is paramount due to the possibility of attaining a severely high fault current within few milliseconds. This article proposes a new traveling-wave (TW) based protection scheme to detect and locate faults in HVdc grids. In contrast to the existing TW-based protection and fault location approaches, the proposed scheme, first, utilizes only the first locally measured TW after the inception of a fault, and, second, focuses on the frequency content and polarity of the TW, rather than its arrival time. The direction of the TW is identified based on the pattern of the current TW waveform. A frequency spectral ratio of the current TW is proposed to identify the fault zone and fault location with high accuracy. The proposed method ensures a detection speed of 50  $\mu$ s on overhead lines and 500  $\mu$ s on underground cables. A remote backup scheme is coordinated with the primary protection and operates within 1.05 ms, which meets the speed requirement of HVdc grids. The performance of the proposed scheme has been, first, assessed using the CIGRE B4 HVdc test grid under various conditions, and, second, compared to existing single- and two-terminal TW methods. [ABSTRACT FROM AUTHOR]

In AC microgrids, voltage source inverters (VSIs) have become widely employed to interface DC energy sources to the AC network. Apart from power generation, VSI-based energy sources can also perform ancillary services such as demand side management using conservation voltage reduction (CVR). In conventional distribution systems, CVR involves lowering the voltage levels of the system within permissible limits to reduce the amount of power consumed by certain loads. However, in autonomous microgrids, the application of CVR for demand side management has not been fully explored yet. Thus, in this paper, a CVR strategy for a droop-controlled VSI-based distributed generation (DG) operating in an autonomous microgrid is proposed. This strategy employs the deviation in DG output voltage magnitude and frequency, caused by an increase in power demand during peak and overloading conditions, to determine the amount of voltage compensation required to reduce the microgrid bus voltages. In addition, a controller to coordinate between the proposed CVR strategy and load shedding is also developed to operate the DGs during overloading conditions. The proposed strategy is verified with a microgrid test system, and its performance is validated on MATLAB/Simulink software environment. [ABSTRACT FROM AUTHOR]

This paper provides a critical assessment of oscillatory modes for a droop-controlled multi-microgrid (MMG) consisting of inverter-based and synchronous-based distributed generation (DG). Local and inter-microgrid oscillatory modes similar to conventional interconnected power systems are observed. However, an additional new and more “dominant” oscillatory mode caused by interactions between close-by coherent synchronous-based and inverter-based DGs in the MMG is identified. Sensitivity analysis is employed to assess the impact of various MMG configurations and parameters such as: interconnecting line length, droop gain, exciter gain, and governor gain on MMG oscillatory modes. Parametric studies are conducted to highlight the impact of oscillatory modes on MMG transient and dynamic performances. Consequently, power system stabilizers (PSSs) are developed for both inverter-based and synchronous-based DGs to improve the transient and dynamic performances of the MMG. Residues are calculated and therefore utilized to determine the effect of PSS placement on the MMG oscillatory modes. Finally, small-signal stability analysis results and the performances of the PSSs are validated through time domain simulations on MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

In this paper, a modified backward/forward sweep (BFS) load flow algorithm is proposed to solve power flow for islanded droop-controlled ac microgrids. The proposed method has been formulated such that the system frequency is one of the system’s unknown variables which is typical for an islanded network justifying further the absence of a slack bus. As a result, new equations have been systematically developed and incorporated with the conventional BFS method. The modified BFS method has been applied to the 33-bus test system and the results obtained are compared with simulation results from the direct BFS method as well as PSCAD/EMTDC. In comparison to the conventional methods, the proposed method is accurate, simple, and easy to implement thus serving as a useful tool for microgrid analysis and planning. [ABSTRACT FROM AUTHOR]

Representing a major technology toward the proliferation of sustainable energy resources, droop controlled microgrids have attracted numerous modeling approaches with a view to improving their operation and applicability. Acknowledging the low inertia of distributed generators and the network characteristics at the distribution level, this paper proposes a reduced order model for inter-inverter oscillations in autonomous droop controlled microgrids. At first, a simplified equivalent network on the basis of Kron reduction is suggested as an effective topology for the study of inter-inverter oscillating currents. The focus shifts toward low-frequency modes following the detailed small signal model and its simplification according to the singular perturbation theory with multiple time scales. Assuming accurate and instantaneous tracking of the voltage by the inner controllers on their local reference frame, the quasi steady state study results in a simple closed-loop model. This model is successfully benchmarked against the detailed model and other germane models over the full range of operational damping ratios. [ABSTRACT FROM AUTHOR]

Increasing the penetration level of distributed generation (DG) may lead to improper protection coordination. Previous work introduced a protection coordination index (PCI) that can quantify the impact of installing a DG at individual locations. This paper extends on previous work and proposes a generalized approach for calculating PCI considering multiple DG locations simultaneously. Furthermore, this paper proposes a solution for enhancing the PCI using fault current limiters (FCL). The problem is formulated as a mixed-integer nonlinear programming problem to determine the optimal FCL size and location as well as enhanced PCI. [ABSTRACT FROM AUTHOR]

This paper proposes a new topological configuration for a unified power quality conditioner (UPQC). Generally, the power structure of the three-phase three-wire UPQC consists of two back-to-back connected six-switch inverters. For this configuration, out of 12 switches, six of the series inverter switches will be underutilized most of the time. To improve the semiconductor utilization and consequently to reduce the total switch count, this paper proposes a new reduced switch topology for the UPQC. The proposed topology is realized using only ten switches and retains all the performance merits of the 12-switch UPQC while minimizing its underutilization without increasing the switch VA rating. The paper provides a detailed analytical study and evaluation by comparing the proposed topology with the twelve- and nine-switch-based UPQC system configurations. The feasibility of the proposed topology is validated through experimental investigation. [ABSTRACT FROM AUTHOR]

The study of power flow analysis for microgrids has gained importance where several methods have been proposed to solve these problems. However, these schemes are complicated and not easy to implement due to the absence of a slack bus as well as the dependence of the power on frequency as a result of the droop characteristics. This paper proposes simple and effective modifications to the conventional method (Newton Raphson) to compute the power flow for microgrids. The presented method provides a simple, easy to implement, and accurate approach to solve the power flow equations for microgrids. The proposed method is applied to two test systems: a 6-bus system and a 38-bus system. The results are compared against simulation results from PSCAD/EMTDC which validate the effectiveness of the developed method. The proposed technique can be easily integrated in current commercially available power system software and can be applied for power system studies. [ABSTRACT FROM PUBLISHER]

It is known that the choice of gain ( K ) in the Sandia frequency shift (SFS) scheme has direct impacts on the stability of a system with grid-connected distributed generations (DGs). In this paper, a scheduled perturbation technique is proposed to reduce the stability impact of K . In the proposed technique, chopping fraction (cf) is used to compensate for reduction in the value of K , where higher cf values are used to achieve zero nondetection zone (NDZ) under low gain SFS. It is shown by analysis that theoretical reduction of NDZ can be always achieved for a nonzero value of cf. Simulations for single- and multi-DGs systems are performed to verify the analytical analysis. It is shown that an appropriate design of scheduled signal duty cycle ( d ) is of critical importance to realize the proposed reduction in NDZ. While close synchronization of perturbation signals for multi-DG system is required, a delay of 0.33 s is shown to be tolerated for a two-DG system. Synchronization can be achieved either through locally synchronized timers or by limited communication among DGs. The proposed technique provides an attractive option for systems with high DG penetration by reducing the negative impact of K on stability. [ABSTRACT FROM PUBLISHER]

The distribution network planning under active network management (ANM) schemes is becoming of interest due to substantial benefits in facilitating the increasing integration of renewable energy sources. This paper presents various potential ANM schemes based on the photovoltaic inverter control (PVIC) considering enhanced utilization of the inverter reactive power capability. Depending on the active power generation of PV arrays, inverter size and desired reactive power settings, several PVIC schemes are proposed. The PVIC schemes are incorporated in the optimal power flow (OPF) and formulated as a nonlinear programming (NLP) problem. In this study, the PVIC schemes are applied to maximize the total wind-distributed generation (DG) penetration on a typical U.K. distribution system. Various case studies are presented and compared to evaluate the performance. The results show that the proposed schemes can significantly increase the wind penetration levels by 45.4% and up to 92.3%. [ABSTRACT FROM PUBLISHER]

In this paper, a new formulation for distribution system reconfiguration (DSR) is proposed for minimizing the annual energy losses considering the variability in active and reactive power demand and distributed generation (DG) profiles. A fuzzy C-means clustering algorithm is used to obtain representative centroids from annual DG and power demand profiles. The DSR study is formulated as a mixed-integer nonlinear programming optimization problem and tested on a 33-bus and an 84-bus system. The minimization is subject to the power balance, bus voltage, and distribution system radiality constraints. Using this formulation, a single optimum configuration that minimizes annual energy losses is found and shown to be different from the optimum configuration, found in previous literature, which focuses on minimizing power losses at peak or average loads. In addition, the prospective advantages of grid automation on DSR are demonstrated to provide further energy loss reduction by including the possibility of interchanging a predefined number of configurations that minimize annual energy losses. [ABSTRACT FROM AUTHOR]

Dual-rotor permanent-magnet synchronous motors (PMSMs) offer the merits of higher efficiency, higher torque density, and inherent electric differential (ED). This paper proposes a novel four-axis vector-controlled dual-rotor PMSM drive for driving plug-in electric vehicles (PEVs). A four-axis $d$– $q$ model of the dual-rotor PMSM is developed with the proposed three-phase to four-phase transformation. This transformation separately resolves the stator variables to the $d$– $q$-axes of the inner and outer rotor. Instead of applying back electromotive forces and rotor power angles, the $d$-axis fluxes are used for computing the displacement between the two $d$– $q$-axes, which is essential for the transformation. The proposed four-axis vector control of dual-rotor PMSM allows for the independent control of stator quantities corresponding to the inner and outer rotors. This facilitates ED and better control under parametric differences, such as rotor inertia and permanent magnet flux for the inner and outer rotors. The comparison of the dual-rotor PMSM-based PEV operation with the proposed control and two earlier reported schemes reveals that the proposed scheme offers effective ED without the loss of vehicle control. Furthermore, the proposed configuration is validated with different drive cycles, including the loss modeling, sensitivity analysis, and efficiency mapping. [ABSTRACT FROM PUBLISHER]

With the integration of distributed generation (DG) to meshed distribution systems, the operating time of the protective system becomes a major concern in order to avoid nuisance DG tripping. This paper proposes a new time–current–voltage tripping characteristic for directional overcurrent relays (DOCRs) that can achieve a higher possible reduction of overall relays operating time in meshed distribution networks. The proposed tripping characteristic is described in detail. Moreover, the protection coordination problem is formulated as a constrained nonlinear programming problem to determine the optimal relay settings. The proposed characteristic is tested on the power distribution system of the IEEE 14 bus and IEEE 30 bus with inverter-based and synchronous-based DG units. The outcome of this study reveals that the new tripping characteristic for DOCRs achieves notable reduction in total relays' operating time over the conventional characteristic. [ABSTRACT FROM AUTHOR]

Islanding detection is important to ensure the reliability and safety of distributed generation (DG). In this paper, a new active islanding detection method (IDM) is proposed, and it depends on individually estimating an overall transient stiffness measure for any multi-DG system to establish a clear separation between prior- and post-islanding stiffness. For the multi-DG system to avoid spectrum overlapping, each of its DGs is required to perturb at distinct frequencies. By using this concept of perturbation separation, the proposed technique can be applied to multi-DG systems without requiring any communication among the DGs. Simulation results show that the proposed technique is scalable and robust against different loading conditions and variations of grid stiffness levels as well as with respect to the number of connected DGs and different types of DG controllers. It is also shown that the proposed technique can successfully distinguish islanding conditions from other disturbances that may occur in power system networks. [ABSTRACT FROM AUTHOR]

This paper presents a novel dc-link scheme for enhancing the fault ride-through (FRT) capability of doubly fed induction generator-based wind turbine (DFIG-WT). The proposed system consists of parallel capacitors with a dedicated control strategy designed to provide means for power evacuation during grid fault conditions. This technically simple and cost-effective scheme was developed considering transmission line autoreclosing which may cause multiple fault inceptions. Simulation studies were carried out to compare the performance of the introduced solution with a DFIG-WT, equipped with the dc chopper and crowbar. The simulation results demonstrate the enhanced performance of the proposed approach in maintaining the dc-link voltage, transient rotor voltages, and currents within the permissible operating range during a bolted three-phase-to-ground fault. The proposed schemes were also tested in response to asymmetrical grid faults, and the enhancement in transient response has been verified. An experimental setup was developed to emulate the behavior of the dc-link circuit during fault conditions. These experimental results demonstrate the effectiveness of the switching parallel capacitors in preventing dc-link overvoltage during imbalance power operation. The discharging capacitor circuit highlighted the capability of tackling the multiple fault inception problems while adhering to grid code requirements. [ABSTRACT FROM PUBLISHER]

In this paper, a new islanding detection method (IDM) is proposed to dynamically estimate islanding occurrence. The proposed dynamic estimators estimate amplitudes and phase angles of the current injected by the grid at the point of common coupling with the distributed generation (DG) in addition to the DG's bus voltage. A distributed two-level algorithm is proposed to detect an islanding condition for single and multi-DG configurations. Analytical design and transient analysis are carried out for the islanding detection problem to determine the nondetection zone (NDZ) of the proposed islanding detection algorithm. A local low-frequency meshed communication network is sufficient to achieve distributed islanding detection capability for a general multi-DG network with negligible NDZ. It is shown through simulations that the proposed IDM can successfully distinguish an islanding condition from other disturbances that may occur in power system networks. [ABSTRACT FROM PUBLISHER]

In the presence of distributed generation (DG), it is important to assure a fast and reliable protection system for the distribution network to avoid unintentional DG disconnection during fault conditions. In this paper, dual setting directional over-current relays are proposed for protecting meshed distribution systems with DG. Dual setting relays are equipped with two inverse time-current characteristics whose settings will depend on the fault direction. The protection coordination problem for the dual setting directional relay is formulated as a nonlinear programming problem where the objective is to minimize the overall time of operation of relays during primary and backup operation. The proposed protection coordination scheme using dual setting relays is compared against the conventional approach, which relies on the conventional one setting directional relay. The proposed scheme is applied to the power distribution network of the IEEE 30-bus system equipped with synchronous and inverter-based DG. The results show that the proposed protection coordination scheme with dual setting relay can significantly reduce the overall relay operating time, making it an attractive option for distribution systems with DG. [ABSTRACT FROM PUBLISHER]

This paper presents a computational method that employs Natural Language Processing (NLP) and text mining techniques to support requirements engineers in extracting and modeling goals from textual documents. We developed a NLP-based goal elicitation approach within the context of KAOS goal-oriented requirements engineering method. The hierarchical relationships among goals are inferred by automatically building taxonomies from extracted goals. We use smart metering system as a case study to investigate the proposed approach. Smart metering system is an important subsystem of the next generation of power systems (smart grids). Goals are extracted by semantically parsing the grammar of goal-related phrases in abstracts of research publications. The results of this case study show that the developed approach is an effective way to model goals for complex systems, and in particular, for the research-intensive complex systems. [ABSTRACT FROM PUBLISHER]