Amr A. Hamad

Multiagent Supervisory Control for Power Management in DC Microgrids

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.

A Novel Multiagent Control Scheme for Voltage Regulation in DC Distribution Systems

This paper proposes a novel multiagent control scheme to mitigate the voltage regulation challenges of dc distribution systems (DCDSs) with high penetration of distributed and renewable generation (DG). The proposed control scheme consists of two sequential stages. In the first stage, a distributed state estimation algorithm is implemented to estimate the voltage profile in a DCDS, thus enhancing the ac/dc converter operation to keep the system voltages within specified limits. The second stage is activated only when the ac/dc fails to regulate the system voltages. Two distributed power management control strategies are proposed in the second stage. The first is based on a distributed equal curtailment, at which all DG units responsible for the voltage violation are equally curtailed. The second strategy aims to optimize the output power in order to maximize the revenue of DG units. The formulated problem in the second strategy is classified as a convex optimization problem under global constraints. A distributed Lagrangian primal-dual subgradient (DLPDS) algorithm is proposed in order to obtain the global optimal solution of the formulated problem. Various case studies are performed to prove the effectiveness, robustness, and convergence characteristics of the proposed control schemes.

A Sequential Power Flow Algorithm for Islanded Hybrid AC/DC Microgrids

This paper proposes a sequential power flow algorithm for hybrid ac/dc microgrids operating in the islanded mode. Unlike in grid-connected systems, variable, rather than fixed, frequency and voltage are utilized for power coordination between the ac and dc microgrids, respectively. The main challenge is to solve the power flow problem in hybrid microgrids while considering the absence of a slack bus and the coupling between the frequency and dc voltage. In the proposed algorithm, the ac power flow is solved using the Newton-Raphson (NR) method, thereby updating the ac variables and accordingly utilizing these variables in a proposed interlinking converter model for the dc problem. This sequential algorithm is iterated until convergence. The proposed algorithm is generic and can include different operational modes not only for the distributed generation units (DGs), but also for the interlinking converters. Detailed time-domain simulations using PSCAD/EMTDC have validated the algorithms accuracy. Its robustness and computational cost are contrasted to those of conventional algorithms.

A Novel Load Flow Algorithm for Islanded AC/DC Hybrid Microgrids

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.

Generalized steady-state modeling for hybrid AC/DC microgrids

In this paper, a generalized models are presented for different hybrid ac/dc microgrids components in the islanded mode of operation. In this scenario, power sharing schemes are based on local information to manage the system power with the absence of the main substation. Instead, the DG units operate based on droop characteristics, thereby forming a multi-slack distribution system, and accordingly stabilize the system frequency and voltage. The introduced models for the DG units and interlinking converters consider the variation of the ac frequency and dc voltage results from the multi-slack nature and different power sharing strategies between the subgrids. The solution of the nonlinear models, represent the system steady-state behavior, are contrasted with detailed time-domain simulations performed in PSCAD/EMTDC environment to evaluate the modeling accuracy.

Steady-state analysis for hybrid AC/DC microgrids

This paper demonstrates the generic models of different hybrid ac/dc microgrids components. The operational philosophy is oriented towards decentralized schemes while considering the absence of a single slack bus in the entire system. Instead, the DG units operate based on droop characteristics, thereby forming a multi-slack distribution system, and accordingly stabilize the system frequency and voltage. Unlike in grid-connected systems, the system frequency and voltage are variable, rather than fixed. These new operational characteristics are highlighted in the models proposed for the DG units, loads, and interlinking converters as well. The solution of the nonlinear equations, represent the system steady-state behavior, are contrasted with detailed time-domain simulations performed in PSCAD/EMTDC environment to evaluate the modeling accuracy.

Realization of market clearance alternatives in AC/DC hybrid microgrids

The main pillar of a smart grid setup is the evolution from a vertically integrated, partially-automated and producer-controlled electric power network to a decentralized one that enables interactions among customers, network operators, and power producers. Accordingly, the existing and emerging stakeholders will change their roles. Furthermore, the smart networks are characterized by paradigm shift from exclusively ac to dc and hybrid ac/dc networks. In this work, the economic model among the producers/consumers (prosumers) in the hybrid networks is developed based on the mathematical formulation of their interactions along with the network technical aspects. The results demonstrate the effectiveness and validity of the proposed scheme in realizing possible buying/selling alternatives among the prosumers.