Stand-alone Mode Research Articles

System Modeling and Reliability Assessment of Microgrids: A Review

The world today is plagued with problems of increased transmission and distribution (T&D) losses leading to poor reliability due to power outages and an increase in the expenditure on electrical infrastructure. To address these concerns, technology has evolved to enable the integration of renewable energy sources (RESs) like solar, wind, diesel and biomass energy into small scale self-governing power system zones which are known as micro-grids (MGs). A de-centralised approach for modern power grid systems has led to an increased focus on distributed energy resources and demand response. MGs act as complete power system units albeit on a small scale. However, this does not prevent them from large operational sophistication allowing their independent functioning in both grid-connected and stand-alone modes. MGs provide greater reliability as compared to the entire system owing to the large amount of information secured from the bulk system. They comprise numerous sources like solar, wind, diesel along with storage devices and converters. Several modeling schemes have been devised to reduce the handling burden of large scale systems. This paper gives a detailed review of MGs and their architecture, state space representation of wind energy conversion systems & solar photovoltaic (PV) systems, operating modes and power management in a MG and its impact on a distribution network.

Optimal scheduling of the combined power and desalination system

In this paper, the scheduling model is proposed that addresses the operational optimization of the combined power and desalination (CPD)system, which is described as a mixed integer non-linear programming (MINLP) problem with the maximizing total economic benefits and the time-dependent electricity price is considered. The typical day of each season is selected as one scheduling cycle. The results showed: The multi-stage flash (MSF) system keeps running in the total scheduling cycle and the water production rate varies between 2,000 t/h and 4,000 t/h. The reverse osmosis (RO) system only keeps running in the Spring. In the other seasons, a part group’s module of RO system would be shutdown following the increasing demand of electricity. The higher electricity price, the more electricity is supplied to the users and the less water is produced. Vice versa, less electricity and more water are produced. On stand-alone operation mode, the largest fluctuation range of electricity varies from 360 to 510 MW in Summer. The least range varies from 270 to 330 MW in Spring. On the scheduling mode, the difference of the largest and least value varies between 10 to 20 MW. The operation stability and efficiency will be improved by the optimal scheduling of the CPD system.

Implementation of a Multi-Zone Numerical Blow-by Model and Its Integration with CFD Simulations for Estimating Collateral Mass and Heat Fluxes in Optical Engines

Nowadays reducing green-house gas emissions and pushing the fossil fuel savings in the field of light-duty vehicles is compulsory to slow down climate change. To this aim, the use of new combustion modes and dilution strategies to increase the stability of operations rich in diluent is an effective technique to reduce combustion temperatures and heat losses in throttled operations. Since the combustion behavior in those solutions highly differs from that of typical market systems, fundamental analyses in optical engines are mandatory in order to gain a deep understanding of those and to tune new models for improving the mutual support between experiments and simulations. However, it is known that optical accessible engines suffer from significant blow-by collateral flow due to the installation of the optical measure line. Thus, a reliable custom blow-by model capable of being integrated with both mono-dimensional and three-dimensional simulations was developed and validated against experimental data. The model can work for two different configurations: (a) stand-alone, aiming at providing macroscopic data on the ignitable mixture mass loss/recover through the piston rings; (b) combined, in which it is integrated in CFD engine simulations for the local analysis of likely collateral heat release induced by blow-by. Furthermore, once the model was validated, the effect of the engine speed and charge dilution on the blow-by phenomenon in the optical engine were simulated and discussed in the stand-alone mode.

Protection schemes for a battery energy storage system based microgrid

This paper evaluates directional and adaptive overcurrent protection schemes in microgrids. A microgrid supported by a centralised Battery Energy Storage System (BESS) is chosen for the study. The stringent PQ controller of BESS will not allow it to dissipate into a fault, during its charging mode, causing the conventional directional schemes to mal-operate. A direction estimation scheme using magnitude and angle of superimposed positive sequence impedance is proposed to address this issue in BESS. Further, two fault detection techniques are proposed for BESS integrated feeders. The Main Protection Unit (MPU) detects an internal fault when there is a mismatch in the direction of relays at either end of a feeder. The second scheme uses adaptive overcurrent relay settings. Since the fault current is limited with PQ control, the pickup current is calculated dynamically with this control. Whereas a fixed pickup current is chosen, when the BESS inverter is voltage controlled. Simulations have been carried out using MATLAB/SIMULINK software. The proposed schemes work well for bidirectional power flows and in grid connected and standalone modes of microgrid operation.

An Enhanced Control Strategy for Mitigation of State-Transition Oscillation Phenomena in Grid-Forming Self-Synchronized Converter System with Islanded Power System

This paper proposes an enhanced control strategy for mitigating state-transition oscillations in active and reactive power responses of self-synchronized converter system to secure the islanded power system stability. The self-synchronized converter is well known for “grid-forming” that is able to operate to stand-alone mode (SAM) providing grid voltage and frequency without phase synchronization units. Although the grid-forming (GFM) is self-synchronized, the inherent synchronization principle causes system degradation in which should maintain a point of common coupling (PCC) voltage for critical loads as well as transitions from grid-connected mode (GCM) to SAM and vice versa. Therefore, this paper focuses on resolving the inherent oscillatory issues in GFM self-synchronized converter system (especially adopted ‘synchronverter’ principle), and proposes a control strategy for controllability improvement based on stability analysis for smooth state-transition under islanded power system. The efficacy of the proposed control method is verified through a high-fidelity electromagnetic transient (EMT) simulation with case studies on 30kW synchronverter system and further experimental hardware-in-loop system (HILS) test with Opal-RT (OP-5707) platform.

Performance of a capstone gas turbine based power plant working on high Butane LPG

This paper presents the operational performance results of a 30 kW microturbine generator (MTGs) fed with high butane content liquefied petroleum gas, while subjected to a test procedure involving steady and transient load conditions. To evaluate the performance, measurements of turbine and generator parameters were gathered from its original unit controller, as load changes were applied. For the stand-alone mode detailed graphs of the test results are presented, showing the transparency and robustness of the turbine-generator set to the used fuel, judging by the quality of the output electric parameters. The results from this performance testing provide good insight into the use of high-butane content liquefied petroleum gas as fuel for the tested microturbine. The continuous use of a fuel would need more tests to establish that the life of the critical components of the microturbine are not hampered from what they are on the baseline fuel.
 The course of selected performance parameters in the microturbine generator is described in this document. The analysis of test results of the microturbine under steady-state and transient operation have been made. Both in steady state and transient conditions, values of output power, speed fluctuation, emissions, noise levels, and exhaust gas temperatures remained under acceptable levels.

Hardware Acceleration of High-Performance Computational Flow Dynamics Using High-Bandwidth Memory-Enabled Field-Programmable Gate Arrays

Scientific computing is at the core of many High-Performance Computing applications, including computational flow dynamics. Because of the utmost importance to simulate increasingly larger computational models, hardware acceleration is receiving increased attention due to its potential to maximize the performance of scientific computing. Field-Programmable Gate Arrays could accelerate scientific computing because of the possibility to fully customize the memory hierarchy important in irregular applications such as iterative linear solvers. In this article, we study the potential of using Field-Programmable Gate Arrays in High-Performance Computing because of the rapid advances in reconfigurable hardware, such as the increase in on-chip memory size, increasing number of logic cells, and the integration of High-Bandwidth Memories on board. To perform this study, we propose a novel Sparse Matrix-Vector multiplication unit and an ILU0 preconditioner tightly integrated with a BiCGStab solver kernel. We integrate the developed preconditioned iterative solver in Flow from the Open Porous Media project, a state-of-the-art open source reservoir simulator. Finally, we perform a thorough evaluation of the FPGA solver kernel in both stand-alone mode and integrated in the reservoir simulator, using the NORNE field, a real-world case reservoir model using a grid with more than 10 5 cells and using three unknowns per cell.

Adaptive Nonsingular Fast Terminal Sliding Mode Control for Maximum Power Point Tracking of a WECS-PMSG

This paper investigates maximum power extraction from a wind-energy-conversion system (WECS) with a permanent magnet synchronous generator (PMSG) operating in standalone mode. This was achieved by designing a robust adaptive nonsingular fast terminal sliding mode control (ANFTSMC) for the WECS-PMSG. The proposed scheme guaranteed optimal power generation and suppressed the system uncertainties with a rapid convergence rate. Moreover, it is independent of the upper bounds of the system uncertainties as an online adjustment algorithm was utilized to estimate and compensate them. Finally, four case studies were carried out, which manifested the remarkable performance of ANFTSMC in comparison to previous methods reported in the literature.

An efficient hybrid reconfigurable wind gas turbine power management system using MPPT algorithm

To improve power management scheme in standalone mode using hybrid wind-gas turbine system. To reduce electrical fluctuations due to permanent magnet synchronous generator (PMSG) in wind turbine system. For power generation, the wind turbine system is a main source. When there is reduction in wind turbine power generation, then gas turbine gets activated immediately and produces the required electricity in effective manner. This reconfigurable power generation system is controlled by perturb and observe maximum power point tracking (P&O_MPPT) algorithm. The proposed wind-gas power management system algorithm and device performance was analysed and simulated using MATLAB R2021a for various wind turbine experiment parameters. The simulation result shows that the proposed model and algorithm effectively meets the load demand when the wind turbine speed falls below the minimum required value.

Stand-Alone Operation of Distributed Generation Systems With Improved Harmonic Elimination Scheme

Improvement in the efficiency of distributed generation (DG) inverters is a concern and challenge for researchers across the globe. To address the concern, an inverter control technique is developed. Inverters have issues with voltage regulation and harmonics when operating in the grid-connected (GC) and stand-alone (SA) mode. This article proposes a control strategy capable of operating DGs in both SA and GC environments. The GC operation of the inverter is achieved by the current control mode, and the SA control features a voltage control loop capable of overcoming the drawbacks due to load shedding or load switching. Besides, the harmonics due to various conditions of transition and load switching are eliminated by adapting a harmonic elimination pulsewidth modulation (PWM) scheme. To enhance the performance of the inverter and eliminate the problem of flexibility associated with conventional and offline switching angle calculations in the PWM technique, a bioinspired intelligent algorithm is adapted. The developed system is verified for various reduced total harmonic distortions (THDs) by performing simulations and experiments. The results depict that the output voltage is regulated for varying load conditions, and the THD is observed to be 2.4% under varying load conditions.

Advanced single-loop discrete-time control for T-type voltage source inverter with minimum capacitor voltage ripple modulation

This research focuses on improving the performance of the voltage source inverter (VSI), which has been widely used in practical applications such as uninterruptible power supply (UPS), photovoltaic (PV) systems in standalone mode. To reduce the total harmonic distortion (THD) of the output voltage without increasing the switching frequency, the T-type three levels inverter with enhanced modulation strategy, which minimizes the voltage ripple of the two input capacitors, is employed. In addition, a new single-loop fully digital control strategy in which various advanced control techniques such as proportional-integral observer, one-step ahead minimum prediction error, and model-based current command generator is also proposed to fulfill the strict requirements of the output voltage. The proposed control strategy is capable of achieving excellent output voltage regulation and high robustness against the parameters variation with low computational cost. Real-time simulations based on hardware in the loop (HIL) platform are conducted to confirm the validity of the method.

An Unbiased CO Survey toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. I. Overview: CO Cloud Distributions

Abstract We have analyzed the data from a large-scale CO survey toward the northern region of the Small Magellanic Cloud (SMC) obtained with the Atacama Compact Array (ACA) stand-alone mode of ALMA. The primary aim of this study is to comprehensively understand the behavior of CO as an H2 tracer in a low-metallicity environment (Z ∼ 0.2 Z ⊙). The total number of mosaic fields is ∼8000, which results in a field coverage of 0.26 deg2 (∼2.9 ×105 pc2), corresponding to ∼10% of the area of the galaxy. The sensitive ∼2 pc resolution observations reveal the detailed structure of the molecular clouds previously detected in the single-dish NANTEN survey. We have detected a number of compact CO clouds within lower H2 column density (∼1020 cm−2) regions whose angular scale is similar to the ACA beam size. Most of the clouds in this survey also show peak brightness temperature as low as <1 K, which for optically thick CO emission implies an emission size much smaller than the beam size, leading to beam dilution. The comparison between an available estimation of the total molecular material traced by thermal dust emission and the present CO survey demonstrates that more than ∼90% of H2 gas cannot be traced by the low-J CO emission. Our processed data cubes and 2D images are publicly available.

Assessment of the levelized cost of electricity for a standalone power plant in Morocco: A case study

This paper presents a detailed analysis of the levelized cost of electricity (LCOE) for a real photovoltaic(PV)-wind-battery system installed in a remote area in Morocco. Besides the evaluation of the existing autonomous system, different scenarios are discussed. A techno-economic evaluation of a PV-battery system is carried out. Additionally, in order to enrich the discussion, the case of possible connection to the national grid, either in a medium- or low-voltage level is considered. Costs were analyzed using on-site collected data of energy production and initial (realistic) costs of the different components of the power plant with various discount and degradation rates. In contrast to the grid-connected mode where all the available energy could be exploited, the detailed analysis shows that the LCOE is affected by the energy management strategy (droop control) in the standalone mode where a part of the energy cannot be used. Furthermore, the degradation rate, the discount rate, and the battery lifetime affect the LCOE. The results show that the LCOE for the PV-wind-battery system is optimal compared to the PV-battery system under different system designs, especially for the low voltage grid-connection where the grid parity could be achieved.

Optimal energy management system for microgrids considering energy storage, demand response and renewable power generation

To ensure the autonomous power supply in microgrids (MGs) in stand-alone mode while also maintaining stability, energy storage systems (ESSs) and demand-side flexibility can be utilized together. Motivated by this fact, in this study, a scenario-based energy management system (EMS) modelled as a mixed-integer linear programming (MILP) problem is presented by taking the stochastic nature of wind and photovoltaic (PV) sources into account in order to analyze the operational behaviour of MGs and thereby to reduce the network energy losses. Direct load control (DLC) based demand response (DR) program is implemented to the system with the objective of exploiting the remarkable potential of thermostatically controllable appliances (TCAs) for energy reduction while satisfying comfort and operational constraints. Furthermore, a common ESS with a bi-directional power flow facility is incorporated in the proposed structure and electric vehicles (EVs) are employed as an additional flexible load in grid-to-vehicle (G2V) mode. To testify the effectiveness of the proposed optimization algorithm, different case studies are conducted considering diverse scenarios. Moreover, the performance is compared with a deterministic method from the perspective of achieving loss reduction and capturing the uncertainties.

Did you see it? A Python tool for psychophysical assessment of the human blind spot.

The blind spot is a region in the temporal monocular visual field in humans, which corresponds to a physiological scotoma within the nasal hemi-retina. This region has no photoreceptors, so is insensitive to visual stimulation. There is no corresponding perceptual scotoma because the visual stimulation is "filled-in" by the visual system. Investigations of visual perception in and around the blind spot allow us to investigate this filling-in process. However, because the location and size of the blind spot are individually variable, experimenters must first map the blind spot in every observer. We present an open-source tool, which runs in Psychopy software, to estimate the location and size of the blind spot psychophysically. The tool will ideally be used with an Eyelink eye-tracker (SR Research), but it can also run in standalone mode. Here, we explain the rationale for the tool and demonstrate its validity in normally-sighted observers. We develop a detailed map of the blind spot in one observer. Then, in a group of 12 observers, we propose a more efficient, pragmatic method to define a "safe zone" within the blind spot, for which the experimenter can be fully confident that visual stimuli will not be seen. Links are provided to this open-source tool and a user manual.

Optimal configuration of solar and wind-based hybrid renewable energy system with and without energy storage including environmental and social criteria: A case study

The Hybrid renewable energy system (HRES) has the potential to better match the demand load profile with power by using the complementary nature of the variable renewables. The sizing of the HRES should be done carefully to better match with demand load without oversizing and under-sizing. This study presents the use of the Generalized Reduced Gradient Method to optimize the size of components of HRES. The case study is performed to demonstrate the HRES for a remote rural region. Cases of both the standalone and grid-connected modes are explored. The standalone system with reliability from 100% up to 70% is investigated. A grid-connected system is studied with and without payment of power supplied to the grid to explore the feasibility of HRES for different price regimes. The methodology used is validated using HOMER software by comparing its results with HOMER results for the cases considered. Higher renewable share in the standalone systems leads to more need for employments at every stage of the lifecycle of components. In addition to that, the negative environmental impact of the HRES system in comparison to the conventional system is significantly less. It can be concluded that the standalone system proves to be better in terms of job creation and carbon emission. In contrast, grid-connected comes out to be better in terms of reliability and economics.

Techno-Commercial Study of a Solar Hybrid Renewable Energy Generator With an Initial Sizing Strategy

Abstract Solar energy-based hybrid renewable energy generators are techno-economically designed for various standalone uses such as far-distance area electrification, commerical building electrification, polygeneration, and industrial application. This article attempts to address the problem of high cost of energy of such generators, which needs to be optimized. The research gap is that there is hardly any work on techno-economic optimization of such generators using an initial sizing technique, thereby resulting in a design with higher cost of energy. In this work, a hybrid renewable energy generator comprising solar photovoltaic-biogas-battery storage is techno-economically optimized with the aim of minimizing its cost of energy for electrification of a radio-broadcasting station in India by using simulation software called hybrid optimization model for electric renewables. An initial sizing strategy has been proposed in the modeling of the hybrid system. Optimal designs are obtained under different scenarios like in the standalone mode with and without capacity shortage fraction and in grid-connected mode with scheduled and random outages. The results show that during initial sizing in the standalone mode, consideration of larger size (25 kW) of the component with lower unit capacity cost (biogas unit) and higher load sharing by it (102,625 kWh/year) in the energy management of the hybrid design with a small capacity shortage (5%) reduces the levelized cost of energy to 0.0956 $/kWh compared to some of the literature designs. Of all the optimal designs, the grid-connected system has the lowest levelized cost of energy of 0.0873 $/kWh with a renewable fraction of 0.821. The present work contributes to the component size planning of hybrid renewable energy generator to generate lower levelized cost of energy. The novelty of the present work is that in the component size planning in the standalone mode, following the initial sizing strategy, if the component with lower unit capacity cost shares majority of the load, then the levelized cost of energy with or without grid-connected mode can be further reduced compared to some existing designs, which can be verified from the simulation results. Therefore, the present results will be useful for component planning and sizing of hybrid renewable generators so as to preset the size of its components as per their unit capacity cost to result in a lower cost of energy.

Ultra-portable, smartphone-based spectrometer for heavy metal concentration measurement in drinking water samples

Heavy metals are very toxic and hazardous for human health. Onsite screening of heavy metal contaminated samples along with location-based automation data collection is a tedious job. Traditionally high-end equipment’s such as gas chromatography mass spectrometer (GC–MS) and atomic absorption spectrometers have been used to measure the concentration of different heavy metals in water samples but most of them are costly, bulky, and time consuming, and requires expert human intervention. This manuscript reports an ultra-portable, rapid, cost-effective, and easy-to-use solution for onsite heavy metal concentration measurement in drinking water samples. Presented solution combines off-the-shelf available chemical kits for heavy metal detection and developed spectrometer-based readout for concentration prediction, quality judgment, and automatic data collection. Two chemical kits for copper and iron detection have been imported form Merck and have been used for overall training and testing. The developed spectrometer has capability to work with smartphone-based android app and also can work in standalone mode. The developed spectrometer uses white light-emitting diode as a source and commercially imported spectral sensor (AS7262) for visible radiation reception. A low-power sub-GHZ-based wireless embedded platform has been developed and interfaced with source and detector. A power management module also has been designed to monitor the battery status and also to generate low battery indication. Overall modules has been packaged in custom designed enclosure to avoid external light interference. The developed system has been trained using standard buffer samples with known heavy metal concentrations and further tested for water samples collected from institute colony and nearby villages. The obtained results have been validated with commercially imported system from HANNA instruments, and it has been observed that developed system has shown excellent accuracy to predict heavy metal concentration (tested for Fe and Cu) in water samples.

Rapid and reliable analysis of underivatized amino acids in urine using tandem mass spectrometry

Nowadays there is a great interest in the development of fast and reliable methods for the determination of amino acids (AAs) in biological samples due to their biological importance. In the present work, a method based on the use of a guard column (gC) prior to tandem mass spectrometry (gC-MS/MS) is proposed for the determination of proteinogenic AAs in urine. Heptafluorobutyric acid (HFBA) is used as ion pairing reagent. Comparison of the gC-MS/MS method versus tandem mass spectrometry (MS/MS) in stand-alone mode showed improved sensitivity and peak shape, and solved some problems related to interfering compounds, with a total analysis time of 2.8 min. All the proteinogenic AAs were adequately determined using the gC-MS/MS method, except glutamic acid (Glu).To confirm quantitative results obtained with gC-MS/MS for individual AAs, an ion-pair liquid chromatography tandem mass spectrometry method (LC-MS/MS) has also been developed. Both methods (gC-MS/MS and LC-MS/MS) were validated using synthetic urine. For the gC-MS/MS method, LODs and LOQs values were found to be between 0.004 and 0.425 mg/L and 0.01 and 1.40 mg/L, respectively. Aspartic acid (Asp) showed the highest LOD and LOQ values (3 mg/L and 9 mg/L, respectively). A one-point standard addition method and internal standard normalization were used for the quantification because matrix effects were observed. L-alanine-1-13C (Ala-13C) and L-leucine-1-13C (Leu-13C) were used as isotopically labeled internal standards.To demonstrate the applicability of the gC-MS/MS method in the reliable determination of AAs in real samples, urine from eighteen healthy volunteers were analyzed using both gC-MS/MS and LC-MS/MS methods. Similar quantitative results were obtained for individual AAs with both of them. In addition, possible differences in AAs concentrations related to sex were checked, but the results did not show significant differences for the evaluated compounds.

Frequency Estimation for Grid-Tied Inverters Using Resonant Frequency Estimator

This paper introduces a new strategy of frequency estimation for grid-tied and stand-alone mode AC/DC power converters. Frequency stabilization is required, especially when operating in stand-alone mode with a Droop Control strategy, or in hybrid mode with a Diesel generator. It is also required to reduce or increase power output along with grid frequency changes. The novel strategy utilizes a resonant filter to estimate the frequency of the voltage grid and is referenced as a resonant-frequency-estimator (RFE). A mathematical background is presented for the proposed estimator and its performance is evaluated. It is compared with three common frequency estimation algorithms: SRF-FLL, DDSRF-FLL, and DSOGI-FLL. Results are presented for three cases: frequency swing, harmonics injection, and type B short-circuit. Results are analyzed and the conclusion shows that the proposed novel strategy has comparable parameters to commonly used frequency estimation algorithms while having a loopless structure.