Search

The increasing penetration of renewable energy resources (RERs) is challenging power grid planning and operation in maintaining grid reliability. Potential power grid reliability and stability issues may arise when large-scale RERs are connected to a weak power grid. These issues may become more significant due to the mutual interactions among RERs that are interconnected electrically close through a power network. Weakness analysis based on grid strength assessment is useful for identifying the potential weak grid issues. However, when taking into account the impact of the interactions among RERs, the weakness analysis becomes computationally challenging. Different combinations of points of interconnections (POIs) of RERs may have different impacts on grid strength at each POI. Due to the combination nature, such weakness analysis may be time-consuming when identifying the weakest combination of POIs from a large number of potential candidates in realistic power grids. To address this concern, the impact of the power network structure on grid strength is analyzed in this paper. It is found that the impact mainly depends on the impedance ratios at POIs. The findings can be summarized as two criteria to develop a recursive method for fast identification of the weakest combination in this paper. The efficacy of the proposed method is demonstrated through case studies on the IEEE 39-bus system and a realistic power grid.

The increasing penetration of renewable energy sources (RESs) can challenge both power system planners and operators to maintain system reliability. Potential power system stability issues may arise when a large amount of RESs are connected to a weak power system. The short-circuit ratio (SCR) with some modifications has been used to analyze power system strength. However, the existing SCR-based methods for system strength assessment neglect real electrical network connections among multiple RESs, which may not reflect the effect of the interactions among multiple RESs at different sites on system strength. To account for the interactions among electrically interconnected RESs, the so-called site-dependent SCR (SDSCR) is proposed in this paper by analyzing the relationship between system strength and voltage stability. The efficacy of the proposed SDSCR is demonstrated through numerical simulation case studies on the IEEE 39-bus system.

Practical power curve estimation is necessary for evaluating the actual power output of a wind farm; since a power curve provided by the wind turbine manufacture will be different with the actual power curve following several years of operation. It can be estimated using the collected power output data including wind power generation and wind speed. This data is commonly ill-distributed due to a noticeable number of outliers, which impose a serious bias to the estimation models obtained from this data. It introduces an interesting challenge in estimation of a power curve. In this paper, an intelligent algorithm is proposed for estimating a power curve using the measured data while modeling and bias errors, imposed to the estimation model by the outliners, are minimized. More specifically, this algorithm is designed based on the Statistical Analysis Software (SAS) programming software package in order to facilitate analyzing and managing big datasets of wind speed and wind power generation. The effectiveness and practical application of the proposed algorithm is demonstrated using a real-world dataset.

In this study, the authors present an approach to validate the voltage schedule for operation of generation fleet within a balancing area managed by Independent System Operators/Regional Transmission Organizations or a balancing authority. An appropriate voltage schedule is an important requirement for the generator operators to maintain adequate VAR reserves and ensure the voltage stability. Based on current industry requirements, an empirical approach is developed to validate the voltage schedule following significant variations in generation pattern and load pattern, as possible results of integration of power generation from variable renewable resources or responsive demand. The approach can validate voltage schedule of generator fleet and transfer buses connected to the generator buses while monitoring the ranges of set-point voltages and VAR levels of the generators. Moreover, they use the approach to investigate the impact of variations in the generation and load patterns on the voltage schedule in a real power grid. The findings are useful for an improved management of voltage stability, voltage schedule validation, and estimation of reactive power adequacy for operation support of future power grid with high penetration of renewables and responsive demands.

The number and locations of transmission lines that are likely to get simultaneously congested are useful information for analysis of power market. In this paper, we present a study that demonstrates the number of simultaneously congested transmission lines is very limited, as well as an algorithm for identifying a small set of transmission lines that may become simultaneously congested based on the current states of the system and market. To this end, we first investigate the maximum number of simultaneously congested lines. Then, a novel algorithm is developed to identify a smaller set using two scientific criteria – one is concerned with the absolute impact of generation dispatch decisions, and the other, the relative sensitivities of line flows with respect to the changes of generation outputs. The effectiveness of the proposed algorithm is demonstrated through two simulation studies, one based on IEEE 39-bus for illustration of concepts, and the other based on a large-scale, real-world power system for showing the practical application and significance of results.

In this paper, we analyze the impacts of major electrical properties, including node constraints, line limits, and flow direction, on vulnerability assessment of power grid using several types of electrical betweenness measures. Specifically, we first propose a set of new electrical betweenness measures, which takes into account flow direction in power grids. Then, the impacts of major electrical properties on vulnerability assessment of power grid are analyzed by comparing the identification results of critical components based on the proposed electrical betweenness measures with those based on the other two types of electrical betweenness measures reported in the literature, which take into consideration node constraints and line limits, respectively. Analysis results show the important impact of flow direction on the identification of critical components. The results lead us to introduce a set of combined electrical betweenness measures that take into account node constraints, line limits, and flow direction together. Simulation results on the IEEE 300-bus system and the Italian power grid show that the combined electrical betweenness measures are superior in identifying critical components and more useful in assessing power grid vulnerability.

This paper presents a transient energy based screening approach for quickly identifying potential critical attacks that might have significant impacts on power system transient stability. Specifically, the proposed approach focuses on the total transient energy injected into power systems as the result of assumptive cyber attacks. The computational improvements of the proposed method are significant as the time-domain simulations can be avoided. The efficacy of the proposed approach is demonstrated using a practical power system with various cyber attack scenarios. The identification results of the proposed method can be used to guide more detailed impact analysis and to develop more effective countermeasures against cyber attacks.

In this work, the Ag-SiO2 nanoparticles were successfully synthesized and their antibacterial property was confirmed using the plate colony counting method. The SiO2 and Ag-SiO2 nanoparticles were used to prepare the PVDF/SiO2 and PVDF/Ag-SiO2 nanocomposite membranes, respectively. Pure water flux, contact angle and mechanical strength measurement analyses were conducted to characterize and compare the performance of the neat and nanocomposite membranes. Moreover, in order to investigate the structure of the prepared membranes scanning electron microscope (SEM) was used to obtain surface and cross-section images. A long-term filtration test was carried out in a bench scale submerged membrane bioreactor (MBR) system, fed by real pharmaceutical wastewater, to evaluate the antibiofouling performance of the prepared neat and nanocomposite membranes. In comparison to the neat PVDF, the pure water flux of the nanocomposite PVDF membrane (PVDF/Ag-SiO2; 0.6 wt%) increased about 60% and the water contact angle decreased from about 99° to 89°. The obtained results showed that the nanocomposite PVDF/Ag-SiO2 membrane exhibits considerable antibiofouling properties such that the accumulated dried biofilm as well as the extra cellular polymeric substances (EPSs) collected from the cake layer decreased considerably for the nanocomposite membrane. Moreover, the flux recovery ratio increased from 58% for the neat PVDF membrane to 76% for the nanocomposite PVDF/Ag-SiO2 membrane. The excitation and emission matrix (EEM) fluorescence spectroscopy analysis revealed that the accumulated protein on the surface of nanocomposite membranes decreased considerably in a way that the peak corresponding to tryptophan protein-like substances diminished completely, indicating the high antibiofouling potential of nanocomposite membranes. The chemical oxygen demand (COD) and ammonium removal efficiencies of the neat and nanocomposite membranes were higher than 90% and 95%, respectively, indicating negligible impact of the membrane modification on the effluents’ quality.

One of the important features of low-carbon electric power system is the massive deployment of renewable energy resources in the advent of a new carbon-strained economy. Wind generation is a major technology of generating electric power with zero carbon dioxide emission. In a power system with the high penetration of wind generation, the displacement of conventional synchronous generators with variable speed wind turbines reduces system inertia. This leads to larger system frequency deviation following a loss of large generation. In this paper, the impact of the reduction of system inertia on system frequency is analyzed as the result of the integration of a significant amount of wind generation into power systems. Furthermore, we present a preliminary study of the impact of the distribution of the inertia contributions from those online conventional synchronous generators on the rate of change of frequency (ROCOF) based on the total energy injected into the system due to the fault. The total fault energy is represented using Hamiltonian formulism. With the IEEE 39-bus system, it is shown that for a fault with the given injected total energy, clearing time, and location, the distribution of inertia contributions can significantly affect the magnitude of ROCOF. Moreover, for such a fault with different locations, the average of the magnitudes of ROCOF caused by the fault at different locations is larger when the distribution of the inertia contributions is more dispersed.

Graph energy is the form of energy often used in a simple system of physics, typically expressed with the potential energy in springs and the kinetic energy of interlinked moving particles. In this paper, we present a study to show a new piece of evidence of the significance of graph energy interlinks in topology of the power grid, which is formed under the specific reliability principles of power system engineering. Particularly, based on the comparisons of the graphs, generated with a modified graph energy based layout method, to the physical layouts of three real power grids, it is found that the generated graphs are highly consistent with the actual layouts. The result is an important piece of evidence, showing that the graph energy interlinks play an important role in the topology formation of the power grid.

This paper presents a study of the impact of severe generation imbalance on reactive power margins. In particular, the focus of the study is the analysis of characteristics of the reactive power margins under two common strategies A) generation redispatch and B) load shedding when a significant loss of generation occurs in a power system. Simulation results show that, at some buses, Strategy A may yield higher reactive power margins than Strategy B. To understand this counterintuitive incident, an index is proposed to estimate the reactive power support capabilities that load buses can obtain from generation units based on the reactive power capacities of generation units and the electrical distances between load buses and generation units. It is found that the counterintuitive incident occurs when the buses of interest are close to the generation units with sufficient reactive power capacities. The analysis results are demonstrated in the IEEE 39-bus system.