Mohammed E. Nassar

Adaptive Self-Adequate Microgrids Using Dynamic Boundaries

Intensive research is being directed at microgrids because of their numerous benefits, such as their ability to enhance the reliability of a power system and reduce its environmental impact. Past research has focused on microgrids that have predefined boundaries. However, a recently suggested methodology enables the determination of fictitious boundaries that divide existing bulky grids into smaller microgrids, thereby facilitating the use of a smart grid paradigm in large-scale systems. These boundaries are fixed and do not change with the power system operating conditions. In this paper, we propose a new microgrid concept that incorporates flexible fictitious boundaries: “dynamic microgrids.” The proposed method is based on the allocation and coordination of agents in order to achieve boundary mobility. The stochastic behavior of loads and renewable-based generators are considered, and a novel model that represents wind, solar, and load power based on historical data has been developed. The PG&E 69-bus system has been used for testing and validating the proposed concept. Compared with the fixed boundary microgrids, our results show the superior effectiveness of the dynamic microgrid concept for addressing the self-adequacy of microgrids in the presence of stochastically varying loads and generation.

A review of Volt/Var control techniques in passive and active power distribution networks

Volt/Var control problem of distribution systems has been extensively investigated in literature. Many control models and algorithms have been proposed to achieve better system quality, security, reliability, efficiency, loadability, and cost effectiveness. Early distribution systems are built based on centralized power generation which is named passive distribution system (PDS), where power flow is unidirectional. Nowadays, the topology of the distribution system allows for bidirectional power flow which is named active distribution system (ADS) due to the presence of active resources, such as distributed generations (DGs). The complexity of controlling each system depends on the topology and size of the network, as well as the control devices used. However, in general there are mainly two main control strategies used to control power networks: centralized and decentralized. This paper provides a review for both control strategies in the distribution system based on Volt/Var control techniques. It introduces the most commonly used techniques and algorithms in the literature for passive and active distribution systems. Moreover, it provides the reader with a comprehensive review on the common optimization techniques and the different objective functions used in terms of loss minimization, voltage deviation, and minimum control variable operation.

A novel probabilistic load model and probabilistic power flow

A precise planning and operation of the power system requires an accurate load model that account for the load stochastic nature. In addition, the probabilistic power flow is a vital tool for power system engineers. Hence, this paper presents a novel probabilistic load model that models the load with an average load curve in addition to a probabilistic error around this average value. The goodness of fit tests are used to find the best fit PDF to model that error. A probabilistic power flow algorithm is presented, this algorithm employs the proposed load model and solves the power flow using the forward/backward sweep technique.

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.

Statistical evaluation study for different wind speed distribution functions using goodness of fit tests

Modeling wind generation for use in many power system applications requires a massive database of historical wind speeds so that the stochastic nature of the wind at a particular site can be accurately analyzed. The alternative is to use reliable estimates of a probability distribution function (PDF) that can preserve the variable characteristics of wind speed and generate the desired synthetic data. This paper presents a statistical evaluation study for different collections of PDFs in order to find the best model to precisely reflect the variable characteristics of the wind at a particular site. The most commonly used PDFs, along with some advanced PDFs, have been verified against the observed wind data based on consideration of two well-known goodness of fit statistical tests. A further case study is conducted in order to evaluate the impact of sample size on the selection of the best-fit PDFs. From a variety of candidate PDFs, the results indicate that the Generalized Logistic and Dagum distributions are the PDFs that best maintain the main characteristics of the observed wind data.

Optimal sizing of wind/solar mix for supply security of active power distribution systems

Planning of renewable resources is an essential study in active power distribution systems. In this paper, an optimal sizing of wind/solar mix is presented with the consideration of the security of the generation resources. The proposed methodology employs the negative correlation between solar and wind output power to ensure a certain level of supply security and load fulfillment. The effect of formulating the design optimization problem on the optimal wind/solar mix and power system operation is addressed. The IEEE 38-bus system with selected candidate locations for renewable resources demonstrates the proposed design methodology. The forward/backward sweep power flow technique is used to evaluate the obtained design results. Minimizing the squared difference between generated power and load demand, as the objective function led to a significant performance in meeting load demand without degrading the power system operational constraints.

A generic algorithm for peak fault current calculation in HVDC links

This paper studies the effect of different DC-links parameters on the magnitude of peak fault current in the DC link. Formulae are presented to calculate the peak fault current. These formulae are presented for different controllers response time considering/neglecting source inductance (overlap). This paper proposes two algorithms to calculate the peak fault current, based on the results obtained, an approximate formula is derived to give an estimate for the peak fault current for different pre-fault current values. Also, a generic algorithm is proposed to calculate the peak fault current accurately for a delayed or an instantaneous controller action considering or neglecting source reactance.

Distributed Energy Storage Based Series Compensator to Mitigate Power Quality Problems

Power quality is gaining more interest especially in the deregulated markets with competitive electrical suppliers. Moreover, some critical loads require power supply contracts with premium power quality. Usually, to improve power quality utilities install power quality conditioners. However, new approaches are presented and implemented to improve the power quality through controlling the interfacing converters used with distributed generators and energy storage units. This paper presents a voltage control based power quality mitigation technique. The proposed technique is applied to the voltage source inverter used to interface energy storage to mitigate some power quality problems such as harmonic distortion, voltage sag and voltage swell occurs after fault clearing actions. The system is modeled and simulated using MATLAB/SIMULINK to validate the proposed technique. Results show successful action of the proposed technique in improving the power quality by significant reduction in the harmonic contents in the presence of nonlinear loads. Moreover, the compensator mitigates the voltage sag problem by keeping the voltage at load terminals constant during disturbances with 2–3 % change in voltage. In addition, the proposed technique allows the uninterruptable power supply feature by supplying the load during supply interruption for duration depends on the stored energy.