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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]

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]

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]

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]

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 introduces a novel coordinated voltage-control (CVC) scheme for distributed generations (DGs) that relies on adaptively changing the roles (master or slave) of the devices [inverter-based DG, diesel generator, and online tap changer (OLTC)] within the smart grid, depending on system conditions. In addition, the proposed scheme imposes different control response and bandwidth on the devices to coordinate the reactive power among distributed generations (DGs) and OLTC steps. The main objective of the proposed method is twofold: 1) to maximize the reactive power reserve of DGs and, hence, facilitate reaction during contingency situations and 2) to provide voltage regulation during normal operating conditions. The simulated distribution system includes inverter-based DGs (photovoltaic and wind turbine), diesel generator, and OLTC and the potential of the CVC scheme is evaluated and analyzed in view of improving voltage profile, maximizing the reactive power reserve, enhancing fault ride through and improving the transient stability margin. The control algorithm is examined under steady state, load excursion, and three-phase-to-ground fault conditions. The results demonstrate the ability of the proposed CVC scheme to satisfy the targeted objectives with significant improvement in the maximum critical clearing time. The proposed scheme is independent of real-time measurements and is widely adaptive to the dynamics of power systems, thus making it quite suitable for utility implementation. [ABSTRACT FROM AUTHOR]

Planning distribution systems without considering the operation status of multiple distributed generation (DG) units could result in constraining the network, lowering the utilization of its assets and minimizing the total DG capacity that can be accommodated. In this paper, the impact of multiple DG configurations on the potential of active network management (ANM) schemes is firstly investigated. Secondly, the paper proposes a multi-configuration multi-period optimal power flow (OPF)-based technique (MMOPF) for assessing the maximum DG capacity under ANM schemes considering 1) variability of demand and generation profiles (multi-period scenarios), and 2) different operational status of DG units (multi-configurations). The results show that the availability of DGs at certain locations could critically impact the amount of DG capacity at other locations. If DGs are properly allocated and sized at certain locations up to the optimal limits, even with a “fit-and-forget” approach, the total connected DG capacity can be maximized, with minimum utilization of ANM schemes. However, exceeding these optimal limits may lead to minimizing the total DG penetration in the long term, impacting the system reliability due to the operational status of multiple DG units, and consequently, imposing more investments on ANM schemes to increase the amount of connected DG capacity. [ABSTRACT FROM AUTHOR]

This paper presents an optimization procedure that enables the optimal dispatching of distributed generators and storage systems in a medium-voltage islanded microgrid. The network is assumed to be supplied by programmable (dispatchable) and nonprogrammable generators (i.e. nondispatchable, such as renewable energy sources—based units). The optimization goal is to minimize the overall microgrid operating cost and the pollutants emission of the programmable generators, assuming that all of the power made available by the renewable generators (photovoltaic and wind systems) is either directly injected into the network or stored in order to be subsequently delivered according to the proposed storage units' management strategy. The optimization is carried out by a niching evolutionary algorithm (NEA) that is able to find multiple optima and the variation of the objective function in their neighborhood. NEAs allow overcoming the performance of standard algorithms used for optimal power-flow calculations in power systems by avoiding falling into local optima. The optimization procedure is performed on a test microgrid and verified by computer simulations. The numerical results show that the solutions can always improve the microgrid performances irrespective of the network operating conditions in all of the considered cases. [ABSTRACT FROM AUTHOR]

In this paper, a new passive islanding detection method for grid-connected inverter-based distributed-generation (DG) systems is proposed. A statistical signal-processing algorithm known as estimation of signal parameters via rotational invariance techniques is used to extract new features from measurements of the voltage and frequency at the point of common coupling as islanding indicators. The new features are defined based on a damped-sinusoid model for power system voltage and frequency waveforms, and include modal initial amplitudes, oscillation frequencies, damping factors, and initial phases. A set of training cases generated on the IEEE 34-bus system was used to train a naïve-Bayes classifier that discriminates islanding and nonislanding events. Cross-validation was used to evaluate the performance of the proposed islanding detection method. The results show that by using the new features extracted from ESPRIT, the classifier is capable of discriminating islanding and nonislanding events with an accuracy close to 100%. [ABSTRACT FROM AUTHOR]

Islanding detection is an essential protection requirement for distribution generation (DG). Antiislanding techniques for inverter-based DG are typically designed and tested on constant RLC loads and, hence, do not take into account frequency-dependent loads. In this paper, a new antiislanding technique, based on simultaneous P-f and Q-V droops, is designed and tested under different islanding conditions considering different load types. The behavior of frequency-dependent static loads during islanding operation is discussed. The developed antiislanding technique is designed to fully address the critical islanding cases with frequency-dependent static loads and RLC-based loads. The performance of the proposed method is tested to accommodate load switching disturbances and grid voltage distortions. Theoretical investigation coupled with intensive simulation results using the Matlab/Simulink software is presented to validate the performance of the proposed antiislanding technique. Unlike previously proposed methods, the results show that the proposed islanding detection method is robust to frequency-dependent loads and system disturbances. [ABSTRACT FROM AUTHOR]

Sandia frequency shift (SFS) falls under the active islanding detection methods that rely on frequency drift to detect an islanding condition for inverter-based distributed generation. Active islanding detection methods are commonly tested on constant RLC loads where the load's active power is directly proportional to the square of voltage and is independent on the system frequency. Since the SFS method relies primarily on frequency to detect islanding, the load's active power frequency dependence could have an impact on its performance and the nondetection zone (NDZ). In this paper, the impact of the load's active power frequency dependence on the performance of the SFS method, during an islanding condition, is analyzed. A NDZ model that takes into account the load's frequency dependence parameter is derived mathematically and validated through digital simulation. The results show that the load's frequency dependence has a significant impact on the NDZ of the SFS method and thus is an important factor to consider when designing and testing this method. [ABSTRACT FROM AUTHOR]

An active islanding detection technique for an inverter-based distributed generator (DG) is presented. Two control strategies are studied: one in which the DG only supplies active power and one in which the DG supplies both active and reactive power to improve the power factor. The nondetection zone (NDZ) of the over/under voltage and frequency protection (OVP/UVP and OFP/UFP) method for each control strategy is determined. For the presented interface control, it is observed that the NDZ of the OVP/UVP and OFP/UFP is larger when the DG interface is designed to improve the power factor. In order to decrease the NDZ, an islanding detection method is designed that is based on creating a mismatch between the load and DG reactive power, thus forcing the frequency to deviate outside the OFP/UFP limits. It is shown that the proposed islanding detection technique is robust and has a negligible NDZ. [ABSTRACT FROM AUTHOR]