Hany E. Farag

A Novel and Generalized Three-Phase Power Flow Algorithm for Islanded Microgrids Using a Newton Trust Region Method

A new formulation is required to provide a proper power flow analysis in islanded microgrids taking into consideration their special philosophy of operation. In this paper, a novel and generic three-phase power flow algorithm is formulated for islanded microgrids. The algorithm is novel since it adapts the real characteristics of the islanded microgrid operation; i.e., 1) some of the distributed generation (DG) units are controlled using the droop control methods and their generated active and reactive power are dependent on the power flow variables; 2) the steady-state system frequency is considered as one of the power flow variables. The proposed algorithm is generic, where the features of distribution systems, i.e., three-phase feeder models, unbalanced loads and load models have been taken in consideration. Further, all possible operation modes of DG units (droop, PV, or PQ) have been considered. The problem has been formulated as a set of nonlinear equations. A globally convergent Newton-trust region method has been proposed to solve this set of nonlinear equations. The proposed algorithm is a helpful tool to perform accurate steady state studies of the islanded microgrid. Different case studies have been carried out to test the effectiveness and the robustness of the proposed algorithm.

A Two Ways Communication-Based Distributed Control for Voltage Regulation in Smart Distribution Feeders

Smart grid initiative is based on several pillars among which integrating a wide variety of distributed generation (DG) is of particular importance. The connection of a large number of DG units among loads may result in a severe voltage regulation problem and the utility-side voltage regulators might no longer be able to use conventional control techniques. In addition, smart grid should provide new digital technologies such as monitoring, automatic control, and two way communication facilities to improve the overall performance of the network. These technologies have been applied in this paper to construct a distributed control that has the capability to provide proper voltage regulation in smart distribution feeders. The functions of each controller have been defined according to the concept of intelligent agents and the characteristics of the individual DG unit as well as utility regulators. To verify the effectiveness and robustness of the proposed control structure, a real time simulation model has been proposed. The simulation results show that distributed control structure has the capability to mitigate the interference between DG facilities and utility voltage regulators.

A Novel Cooperative Protocol for Distributed Voltage Control in Active Distribution Systems

In this paper, a novel cooperative protocol has been proposed to provide a proper voltage control for multiple feeders having a transformer tap-changer (LTC), unbalanced load diversity (station with different feeder loads) and multiple distributed generation (DG) units in each feeder. The proposed cooperative protocol has been defined according to the distributed control technology, where LTC and DG units are considered as control agents. Two conflicting objectives have been defined for each control agent. The first objective aims to achieve the system requirements by minimizing the voltage deviation and the second objective aims to achieve the device requirements by reducing the tap operation and maximizing the energy capture for LTC and DG units, respectively. The interior structures of the control agents and the communication acts between them have been designed to achieve the best compromise between the two objectives of each control agent. The effectiveness of the proposed cooperative scheme has been verified via different case studies.

Voltage and Reactive Power Impacts on Successful Operation of Islanded Microgrids

This paper proposes a probabilistic technique to evaluate the success of islanded microgrids taking into consideration the impacts of voltage and reactive power constraints and the special features and operational characteristics of both dispatchable and wind distributed generators in islanded microgrids. New adequacy and reliability indices are proposed to account for the effect of voltage and reactive power constraints. To facilitate these studies, the proposed technique employs a microgrid model that reflects the special characteristics of microgrid operation. Simulation studies have been carried out to validate the proposed technique. The simulation results show that voltage and reactive power constraints have considerable effects on the microgrids successful operation.

Voltage regulation in distribution feeders with high DG penetration: From traditional to smart

This paper addresses the negative impacts of distributed generations (DGs) on the utility voltage regulators and hence on the voltage profile of distribution feeders. These negative impacts have been verified through carrying out different simulations on an unbalanced distribution feeder. The results show that a significant regulation problem might arise due to the increased levels of DG penetration correlated with traditional utility voltage regulators control practices. Consequently, DGs inevitably need to cooperate and communicate with distribution network voltage control devices. Smart grid generally aims to combine the existing utility grids with new digital technology to substantially improve the overall efficiency of the network. In this paper, a communication based multi-agent cooperative control structure has been proposed as a solution to mitigate the voltage regulation issues in distribution feeders with high DG penetration.

A multilayer control framework for distribution systems with high DG penetration

In this paper, a multilayer distributed multi-agent framework under the smart grid umbrella is proposed as a control structure to tackle technical challenges for distribution systems with high penetration of distributed generation. The proposed multi-agent control structure has been divided into three main layers. In the first layer, the functions of the local agents is defined according to the concept of intelligent agents and the characteristic of the individual DG and utility devices such as load tap changer, shunt capacitors and protection devices. Based on the concepts of microgrids, cells and virtual power plants, regional coordination agents are defined and chosen as a second layer. To achieve global objective(s), the distribution management system DMS is selected as the highest layer supervisor agent.

Optimum Reconfiguration of Droop-Controlled Islanded Microgrids

This paper proposes a new formulation for the optimum reconfiguration of islanded microgrid (IMG) systems. The reconfiguration problem is casted as a multi-objective optimization problem, in order to: 1) minimize the IMG fuel consumption in the operational planning horizon for which islanded operation is planned; 2) ensure the IMG capability to feed the maximum possible demand by enhancing its voltage instability proximity index taken over all the states at which the islanded system may reside; and 3) minimize the relevant switching operation costs. The proposed problem formulation takes into consideration the systems operational constraints in all operating conditions based on the consideration of the uncertainty associated with renewable resources output power and load variability. Moreover, the proposed formulation accounts for droop controlled IMG special operational characteristics as well as the availability/unavailability of a supervisory microgrid central controller (MGCC). The formulated problem is solved using non-dominated sorting genetic algorithm II (NSGA-II). MATLAB environment has been used to test and validate the proposed problem formulation. The results show that the implementation of appropriate IMG reconfiguration problem formulations will enhance the performance of IMG systems and facilitate a successful integration of the microgrid concept in distribution networks.

Network reconfiguration in balanced and unbalanced distribution systems with high DG penetration

This paper presents a simple and efficient reconfiguration approach for balanced and unbalanced distribution networks with DG. The proposed approach that is based on load flow, begins with meshed networks by closing all tie switches. Branch currents were used as switching index to determine the open/closed states of the tie and sectionalizing switches with an objective of system losses reduction. The radial configuration was restored by opening the switch with smallest switching index in each loop. The proposed method has been tested on one balanced and two unbalanced systems with and without distributed generation, and compared with those reported in the literature.

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.

Voltage Regulation in Islanded Microgrids Using Distributed Constraint Satisfaction

Droop control is a key control method for operating islanded microgrids (IMGs). The settings of the droop parameters for distributed generation (DG) units can considerably affect the ability of an IMG to satisfy the required voltage tolerance boundary prescribed in steady-state voltage regulation standards. This paper analyzes the complexity of voltage regulations in droop-controlled IMGs. A new algorithm is proposed to satisfy the voltage regulation requirements of IMGs. The proposed algorithm obviates the need for a centralized secondary controller, where each DG unit updates its own voltage droop parameters, autonomously, via interaction with other DG units, using a low-bandwidth, peer-to-peer communication network. To that end, a distributed constraint satisfaction approach is adopted to formulate the problem of voltage regulation in a multi-agent environment. An asynchronous weak commitment technique is proposed to solve the formulated problem. Several case studies are simulated to evaluate the performance of the proposed algorithm. The results show that the proposed algorithm can effectively mitigate the challenges of voltage regulation in IMG systems.