Mohamed Moustafa

Utility-connected microgrids (MGs) provide ancillary services using distributed energy resources (DERs). However, existing DER control strategies are complex and may not yield reliable results under diverse MG configurations. To address this challenge, this paper presents a unified secondary control strategy for effective real and reactive power regulation of dispatchable energy sources. These configurations are simulated based on the practical setup of the industry-grade MG at the Center for Microgrid Research (CMR). Real and reactive power regulation strategies based on a unified proportional-integral-derivative (PID) scheme across DERs are developed and evaluated for providing secondary control at the point of common coupling (PCC). Proportional power sharing algorithms for the DERs are implemented within the PCC control, incorporating the constraints of variable renewable energy such as solar photovoltaic (PV) arrays. The proposed strategies are assessed using time-domain simulations for multiple source configurations in the real-time simulator.

Grid-forming (GFM) inverters typically employ droop-based primary control, which necessitates the implementation of robust secondary control to maintain nominal frequency and voltage, especially under dynamic conditions. While Microgrid secondary control research has primarily focused on simulation studies, practical implementation on industry-grade controllers remains a critical gap. This paper bridges the gap between theoretical and practical implementation by proposing an automation-based approach for enhanced secondary voltage and frequency regulation in GFM inverters. In this approach, secondary control architecture is simulated based on an industry-employed real-time automation controller (RTAC) architecture. RTAC parameters are optimized using a grid search optimization technique. It employs iterative random gain sampling to minimize the step response parameters. The developed controller is validated in real-time using RTAC-in-the-loop. The simulation and hardware experimental results are validated in the Center of Microgrid Research (CMR). The results confirm that the proposed method can be effectively used to replicate the industry-grade controller performance.

Modern power systems are shifting toward decarbonization and incorporation of distributed energy resources (DERs) to replace fossil fuel generators. Although promising, DERs introduce uncertainty because of their intermittent nature. This study provides a comprehensive survey of current approaches for modeling system uncertainties and methods of analysis, particularly in the context of static voltage stability studies within modern power systems. Emphasis is placed on evaluating various models applied to different system random variables (RVs), focusing on their suitability for those particular RVs. Additionally, the study examines the characteristics and frameworks of prominent probabilistic methods (PM), evaluates their efficacy, and discusses static voltage stability analysis approaches, emphasizing solution structures and appropriate applications. It concludes by thoroughly reviewing both numerical and analytical PM methods and offering insights into their strengths and limitations. The provided comprehensive survey reveals that, considering system uncertainties, voltage stability studies have gained the most share, followed by small-signal stability studies, whereas the frequency stability studies have gained the least share.

The integration of distributed energy resources (DERs) and unpredictable loads has increased uncertainty in power systems. Traditional methods struggle to assess performance under these uncertainties, and existing probabilistic methods face challenges with complexity and accuracy. This paper introduces a new combined analytical-numerical probabilistic method to assess the impact of DERs on voltage stability. Using Bayesian Parameter Estimation (BPE), the method derives the analytical properties of random variables (RVs) associated with DERs and loads, obtaining posterior distributions. The Metropolis–Hastings sampling technique then estimates these posteriors numerically, enabling accurate predictions of DERs and load outputs. Voltage stability analysis was performed using the continuation power flow method and validated on the IEEE 59-bus test system in MATLAB/Simulink. The results show that integrating DERs significantly improves voltage stability. The proposed method outperforms the Monte Carlo simulation (MCS)-based method in accuracy and computational speed, increasing DERs penetration and voltage stability limits by 3%. It closely matches MCS voltage estimates but requires fewer iterations (500 per loading increment) compared to MCS’s 1000, leading to faster computation times (a few hours to one day versus up to three days for MCS). This method provides an efficient solution for managing uncertainties in power systems.

Addition of distributed energy resources (DERs) in power systems (PS) coupled with uncertain loading has increased system uncertainties. The usual deterministic stability solution is no longer sufficient. While probabilistic methods (PM) have been explored before, their focus has mainly been on system events like faults or the realization of microgrids composed of DERs. Voltage stability (VS) analysis of a PS mixed with DERs has not received sufficient attention. In this work, a Bayesian parameter estimation (BPE) method is proposed. BPE works efficiently with efficient sampling techniques such as the Markov Chain Monte Carlo (MCMC) to accurately estimate uncertain parameters with good computation speed while using smaller data sample sizes. DERs and loads are represented by their respective statistical models. The models are then transformed into the Bayesian inferential framework. Using the BPE algorithm, uncertain parameters are estimated and their corresponding power outputs are obtained. The estimated powers are injected in the continuous power flow (CPU) to determine the VS of the PS. The proposed BPE has been tested on the 14 generator, 59 bus Australian IEEE benchmark. Test results show that with a 4.33% generation increase from DERs, leads to 11% enhancement in voltage stability margin of the PS.

The demand to improve grid infrastructure is of utmost importance especially with the increased expansion and addition of distributed generation units to cope with today's era of transforming to smart grids. The health status of distribution transformers in an electrical grid plays a vital role in maintaining network service continuity, safety from catastrophic events, and reliability of utility operation. Monitoring transformer's health is essential for minimizing outages and extending its life in service. Various diagnostic tests and fault detection techniques have developed to effectively support utility operators in predicting failure, estimating life expectancy and condition monitor its status. For these reasons, traditional and advanced methods are required to reduce maintenance time, cost, and help in finding fault causes and location easily. This paper gives a thorough overview about widely used and latest condition monitoring techniques while highlighting strengths and limitations of each.

This paper presents a novel robust current droop controller (RCDC) using a single droop loop. This scheme is unified supporting dual mode of operation for micro-grids (MGs), including grid connected mode (GCM) and islanded mode (ISM) while ensuring seamless transition between the two modes with proportional power sharing maintained. The proposed controller is further incorporated with an improved maximum power point tracking (MPPT) technique presented for the parallel operation of single-stage inverters fed by multi-string PV array topology. In addition, an improved phase-locked-loop-less (PLL-less) method is presented supporting self-synchronization strategy of the parallel operation of PV-inverters with the main grid while maintaining the full capabilities of the unified control architecture. This obviates the usage of conventional PLLs, which are widely used with active synchronization techniques. The performance of the proposed control scheme is validated using real time simulations (RTS) developed by dSPACE MicroLabBox.

The overcurrent protection relays are a main component of the power system protection. The design of the protection scheme has to ensure the capability of relays to detect the faulty condition and trip the suitable circuit breaker to disconnect only the faulty part of the network without any effect on the healthy loads in the network. The coordination between OCRs may be affected by the penetration of the distributed generation units to the conventional network. This is due to the variable topology nature of networks with multiple feeding sources, which leads to variable network impedances, variable power flow and variable fault current levels. In this paper, a design of an optimized adaptive overcurrent protection scheme is done for an industrial microgrid with distributed generation. The setting of the relays are selected according to a study of the network operation topologies using ETAP software, and the protection setting of relays is optimized by genetic algorithm using MATLAB optimization toolbox to obtain the optimum coordination scheme.

Micro-grid (MG) operation using voltage control methods (VCMs) has been widely recommended for parallel operation of three phase voltage source inverters, especially during islanded mode to maintain the voltage level in the MG. In contrast, this paper presents a new unified control strategy for MG parallel operation using a current control method (CCM). A hybrid control scheme which combines a universal robust droop controller (URDC) and quasi-proportional resonant (QPR) regulator is introduced. It ensures equal power sharing among parallel operated inverters during all MG's operating modes. Moreover, an improved adaptive estimator for the reference current magnitude is integrated to meet dynamic load variations. Furthermore, the scheme is enhanced with a self-synchronized capability using a simple synchronous reference frame phase locked loop (SRF-PLL). This obviates the necessity of communication links and external synchronous clocks required for synchronization process. In addition, a decentralized control action is locally managed to restore the inverter's frequency upon any reactive load change. Modeling and simulation results using MATLAB/SIMULINK software are presented to show the effectiveness of the proposed strategy. (C) 2019 The Authors. Published by Elsevier B.V. on behalf of Faculty of Engineering, Alexandria University.

This paper introduces a new self-synchronization mechanism for three phase current controlled voltage source inverter (CCVSI). The grid connected operation of the inverters is smoothly managed with the use of a hybrid quasi-proportional resonant (QPR) regulator embedded with a universal droop controller. Prominently, the commonly used phase-locked-loop (PLL) that is required to for synchronization with the main grid is obviated. This way, isolation and reconnection with the main grid is achieved seamlessly without the need of a dedicated synchronization unit. Modeling and simulation results are presented using MATLAB/SIMULINK software to show the effectiveness of the proposed strategy.