Search

This paper proposes a generalized and efficient power-flow algorithm for islanded hybrid ac/dc microgrids. The algorithm considers the microgrid operational aspects, i.e., absence of a slack bus, unbalanced ac subgrid, droop-controlled ac and dc voltages and ac frequency, and coupling between the ac frequency and dc voltage through interlinking converters. To attain high computational efficiency, the algorithm adopts three features. First, it models the ac subgrid elements in sequence components, thereby dividing the subgrid's set of equations into three smaller sets for faster parallel solution. This approach also accurately represents the different types of ac distributed generators. Second, the algorithm sequentially solves for the power-flow variables of the ac and dc subgrids, thus reducing the number of equations to be solved simultaneously, once again for further computational cost alleviation. Third, the algorithm implements the quadratically convergent Newton–Raphson technique to solve the decoupled sets of equations. The proposed algorithm is validated through comparisons with time-domain simulations, in MATLAB/Simulink, for test hybrid ac/dc microgrids of different configurations. Moreover, three case studies are introduced to examine the proposed algorithm's effectiveness in solving large-scale microgrids, to investigate its limits-enforcement capabilities, and to evaluate its performance as compared to conventional methods. [ABSTRACT FROM AUTHOR]

This paper proposes a model for planning isolated microgrids. The goal of the proposed model is to minimize the investment, operational, and total costs of an isolated microgrid through the full identification of the system configuration. The model designates the optimal type of microgrid, i.e., ac, dc, or hybrid ac/dc, and the optimal sizing of both the distributed energy resources (DERs) and the electronic power converters, if needed. To render this planning approach generic, the model accommodates a wide variety of DERs, including ac and dc generators, capacitors, and energy storage systems. The detailed operational criteria of each power apparatus are taken into account in order to provide reliable operational scenarios. As a means of guaranteeing active and reactive power adequacy in isolated microgrids, the stochastic nature of generation and demand are also considered. The proposed model was developed analytically as a mixed integer nonlinear problem so that obtaining solution optima is thus possible with the use of a deterministic branch-and-bound nonlinear solver. The validity and effectiveness of the new formulation have been demonstrated through several case studies involving varied load topologies. [ABSTRACT FROM AUTHOR]

This paper proposes a novel branch-based load flow approach for isolated hybrid microgrids. This evolving network configuration involves the application of a distinctive operational philosophy that poses significant challenges with respect to conventional load flow techniques. Hybrid microgrids are characterized by small-rating, droop-based distributed generators (DGs) and by variable but coupled frequency and dc voltage levels. In particular, the absence of a slack bus that results from the small DG rating impedes the application of traditional techniques such as branch-based methods. To overcome these limitations, a modified branch-based approach provided the basis for the development of the proposed algorithm. The new algorithm solves the load flow sequentially by dividing the problem into two coupled ac and dc subproblems. The coupling criterion is established by modeling and updating the power exchange between the subgrids, hence enabling an accurate and efficient formulation of the subproblems. For each subproblem, a modified directed forward-backward sweep has been developed to perform the load flow analysis based on consideration of the individual characteristics of each subgrid. Case study results demonstrate that the algorithm is applicable and effective for the steady-state analysis of several operational factors associated with an isolated hybrid microgrid system: 1) load changing; 2) converter outages; and 3) the probabilistic nature of renewable generation and loads. [ABSTRACT FROM AUTHOR]

This paper proposes a generic steady-state modeling and power-flow analysis approach for droop- and isochronously controlled microgrids. The proposed framework adopts symmetrical sequence component models, rather than phase-coordinate models, of microgrid elements. Such approach immensely reduces the power-flow execution time, as it breaks down the system model into independent equation sets with considerably reduced sizes. To render the proposed approach practical and generic, it integrates different types and control schemes of distributed generation (DG), including synchronous generator-based DG (SGDG) and electronically interfaced DG units. Furthermore, it incorporates unbalanced loads and feeders, transformer connections, different load characteristics, and configurations, as well as microgrid droop features. A novel power-flow algorithm based on a modified Newton–Raphson method is proposed to solve for the microgrid steady-state voltage magnitudes, angles, and frequency. The accuracy of the models and algorithm is verified through comparison with detailed time-domain simulations in MATALB/Simulink. Additionally, the proposed approach is shown to outperform the reported Newton-Trust Region approach in generality, accuracy, and performance. Two case studies, incorporating IEEE 123-node test microgrid, are further performed to examine the effectiveness of the proposed approach in solving complicated droop-controlled microgrids, and to examine the behavior of droop-controlled DGs in isochronous microgrids. [ABSTRACT FROM AUTHOR]

This paper provides a novel power flow algorithm as a viable tool to accurately analyze the steady-state behavior of islanded hybrid ac/dc microgrids. The operational philosophy of these networks is sophisticated since both the ac frequency and dc voltage are variable, coupled, and sensitive to the control schemes of the distributed generators (DGs) and interlinking converters (ICs). In order to render the analysis generic, mathematical models are developed for control schemes of the DGs and ICs under unbalanced conditions. The presented models are then incorporated into an optimized power flow formulation and the solution is found using the globally convergent Newton-Trust Region (NTR) method. The algorithm accuracy is verified using detailed time-domain simulations in PSCAD/EMTDC environment. The effectiveness of the introduced power flow algorithm is illustrated via two case studies. The first case investigates parallel operation of droop-controlled ICs and highlights the effects of IC settings on load sharing among subgrids and power-transfer sharing through installed ICs. The second case reveals the significance of analyzing the loading condition for each phase of the DGs rather than for the total three-phase power generation, which is commonly followed in steady-state analysis in the reported literature. [ABSTRACT FROM AUTHOR]

This paper proposes multiagent supervisory control for precise power management in isolated dc microgrids. Two power management aspects are considered: 1) equal power sharing, which is realized via a proposed distributed equal power sharing algorithm; and 2) optimal power dispatch, which is achieved through a proposed distributed equal incremental cost (DEIC) algorithm. Both algorithms offer the additional advantage of the ability to restore the average system voltage to its nominal value. The proposed algorithms are based on the application of the average consensus theory along with voltage sensitivity analysis. Each distributed generation (DG) unit exchanges information with its neighbors, thus locally updating its no-load voltage setting to achieve the supervisory control objectives. The incorporation of DG droop-based control renders the proposed algorithms fully distributed with a reduced number of agents. The stability of the proposed algorithms is addressed, as well as the convergence of the proposed DEIC algorithm. Real-time OPAL-RT simulations demonstrate the effectiveness of the proposed algorithms in a hardware-in-the-loop application. [ABSTRACT FROM PUBLISHER]