Large Power Variations Research Articles

Improved wind forecasting with wavelets

An important consequence of a high wind penetration and the difficulty to predict the power generated is the need to entrust the coverage of a large power variations to power spinning reserve. A forecasting system of generation would resolve, at least in part, this problem. The wind variability has a significant negative influence on the forecast models. To avoid the influence of variability, we applied a wavelet filter that can work with softer signals. The results improve significantly when wavelet filtering is applied.

Large diameter electrically tunable lens for ophthalmic distance accommodation.

Electrically tunable liquid crystal lens with 30 mm diameter is presented based on the refractive Fresnel concept. Relatively large optical power variation range (from - 0.74 to +0.71 Diopters) is demonstrated along with very low root mean square aberrations (≤0.15 µm). Optical characterizations, including with Snellen chart, show that good vision recovery may be obtained with fast response time (under 500 msec) and relatively low haze. The proposed design is very simple and may be fabricated by using single step lithography. Perspectives on its applications are discussed.

Explantation of a light-adjustable intraocular lens: case report with laboratorial analyses

Introduction: A case in which a light-adjustable lens (LAL) underwent explantation and extensive laboratory analyses is reported. Patient and Clinical Findings: A 61-year-old man underwent uneventful cataract surgery with in-the-bag implantation of a first-generation LAL in the right eye. Examination on the second postoperative day revealed limbus-to-limbus corneal edema, moderate anterior chamber cells, and a mid-dilated, minimally responsive pupil, suggesting toxic anterior segment syndrome (TASS). Diagnosis, Intervention, and Outcomes: Endothelial keratoplasty was performed 2 months after LAL implantation. The patient reported noncompliance with required UV-protective eyewear, and an attempted light adjustment confirmed macromer reserve depletion, likely secondary to ambient UV contamination. Gross and microscopic examinations of the explanted LAL showed a decentered round elevation on the optic. The light transmittance (%T) curve was normal, with 95.9%T on average in the visible spectrum. Optical coherence tomography revealed irregular curvatures of the anterior and posterior optic surfaces. Wavefront analysis displayed aberrations and large power variations, which corresponded to decreased focus and contrast through the optic. Conclusions: To the authors' knowledge, this is the first reported case of LAL implantation complicated by TASS, leading to a prolonged clinical course and nonadherence to proprietary UV-protective spectacles. Extensive laboratory analyses of the explanted LAL demonstrated findings that can be expected with uncontrolled polymerization of the LAL.

Enhancing dynamic stability performance of hybrid ac/dc power grids using decentralised model predictive control

High voltage direct current (HVDC) networks are transforming ac power grids to hybrid ac/dc power grids; hence hybrid ac/dc power grids are facing new stability challenges. In this paper, a decentralised control scheme based on model predictive control (MPC) is proposed to enhance the stability of hybrid ac/dc power grids under disturbances. The proposed scheme controls the dc voltage, frequency, active and reactive power simultaneously with minimum deviations from steady-state values. A communication network is used to share the sampled grid data with the local MPC of each voltage source converter (VSC). Hence, the central controller can be avoided. Moreover, the active power-sharing between converters can be pre-designed. Thus, fast and accurate active power control can be realised. Finally, the proposed control scheme is validated using a five-terminal VSCHVDC test system with MATLAB. The VSCHVDC network with the proposed control scheme shows a smaller deviation and a faster response compared with the conventional droop control scheme and centralised MPC scheme under small disturbances. Under large power variations, the proposed control scheme assists to reach the steady-state rapidly with a minimum impact on the entire power grid. The proposed control scheme has also proved its robustness under communication delays and outages.

Role of optimal sized dual energy storage based microgrid for ancillary service support to utility grid

Electrical energy storage system (ESS) has been used worldwide as ancillary support to the grid, aiding in frequency regulation and grid stability. Intermittent renewable energy sources (RESs) can be effectively integrated into the grid by maintaining power balance using different types of energy storage. However, the power requirement during small frequent, and large persistent power variations in the load demand can be more effectively controllable with the combination of the different energy storage according to their operational characteristics. This paper evaluates the suitability of a dual energy storage system (DESS) integrated in a grid connected microgrid (MG) system for providing ancillary services to the utility grid. The DESS is a combination of battery bank storage (BBS) and pump hydro storage (PHS). The BBS is very fast responding and it can counter the immediate variations in the power whereas, the PHS system can counter the large variations of the power as needed in the MG system. Four different operating cases have been studied to obtain the optimal size of the DESS for optimizing not only the operational cost but also for the best combination size of the storage systems to counter the variability in power generations and the load demand in the power system. Day-ahead electricity market scheduling has been performed on the MG to control the net power demand variations on the distribution feeder of the utility grid. The simulation results show that the higher capacity of the DESS does not always provide the optimal operational cost of the MG system during the grid-connected mode. Therefore, optimal sizing of the storage system also plays an important role in providing the optimal solution. It has been found that the optimal size of the individual energy storage and the DESS is different for microgrids supporting or not supporting the grid. The study enables the system operator (SO) to choose the best combination of storage systems for the supportive and non-supportive operation of microgrid system.

Implications of Multiple Corona Bursts in Lightning Processes for Radio Frequency Interferometer Observations

AbstractRecent observations from LOFAR indicate that multiple, spatially distributed corona bursts can occur in lightning processes with a timescale of 10 microseconds. The close proximity of the corona bursts in space and time poses a great observation challenge for short‐baseline (typically ≤100 m) radio interferometers. This paper reports simulations to show the interferometry results that would be obtained with such an interferometer. In particular, spatially‐separated corona bursts at fixed locations may be seen as a fast (>107 m/s) propagating source with large power variation if the resolution of the instrument is greater than the spatial separation of the bursts. The implications and suggestions for lightning interferometry studies are discussed in the paper.

Inline Microring Resonator Based Microwave Photonic Phase Shifter With Self-Mitigation of RF Power Variations

We present a new technique to self-mitigate the RF power variations in microwave photonic phase shifters (MPPS) based on microring resonators by exploiting the root problem of varying optical intensities at different optical phase shifts to harness a varying and opposing optical equalization gain. Whilst optical gain mediums are commonly present in microwave photonic systems to overcome optical loss, choosing an appropriate equalization gain point to strike a balance between the optical gain transformation of weak and preservation of strong signals allows the proposed system to effectively achieve minimal radio frequency (RF) power variation across all signals without interfering with the RF phase shifting operation. Simulation results show an inline MPPS based on the phase and amplitude of conventional resonant structures with a capability to successfully minimize the RF power variations by 80% for up to 25 dB of phase-dependent power variations throughout the full 360° phase tuning range. Experimental results verify the MPPS operation based on a single microring resonator by significantly reducing the large RF power variations from 9 dB to less than ±1.25 dB, while maintaining the same RF phase shifting operation. We then verify the use of the proposed inline MPPS to adaptively minimize the self-interference by optimizing the phase compensation in a self-interference cancellation system. We demonstrate a cancellation bandwidth of 100 MHz with peak in-band cancellation of up to 40 dB while exhibiting the capability to recover weak signals of interest at operating center frequencies ranging from 5 – 20 GHz.

Grid Synchronization of WEC-PV-BES Based Distributed Generation System Using Robust Control Strategy

In this article, a renewable (solar and wind) energy sources based distributed generation system (DGS) is controlled to operate in the grid-connected (GC) mode and an islanded mode using the robust and fast IAPV (Improved Affine Projection Versoria) and proportional resonant based control strategy. Due to the use of IAPV-based control in the GC mode, the grid current becomes immune to the dc offset and the other nonfundamental harmonics during an unbalanced load, which significantly improves the grid current quality. It also presents the flexible operation of DGS in the GC mode using the variable and constant power injection into the grid, which ensures the grid stability during large and fast wind and solar power variations. Moreover, a modified second-order sequence frequency locked loop is used to improve the synchronization and seamless mode switching (islanded to GC and vice versa) performance by quickly and accurately estimating the phase angle and frequency at distorted and unbalanced grid voltages. Simulation and test results validate the microgrid operation and robustness of the microgrid control.

Development of a robust, accurate and efficient coupling between PLEIADES/ALCYONE [formula omitted] fuel performance code and the OpenCalphad thermo-chemical solver

In this paper, a robust, accurate and efficient coupling between the PLEIADES/ALCYONE 2.1 fuel performance code and the OpenCalphad thermo-chemical solver is presented. A challenging 3.5D simulation of a power ramp, consisting of the 3D simulations of all the fuel pellets within a complete rodlet, serves as a base case to illustrate the difficulty inherent to extensive thermochemical equilibrium calculations (more than 1.5 million in this example) within fuel performance simulations, related both to the wide temperature and pressure range encountered in irradiated nuclear fuels. In the first part of this paper, a detailed analysis of the main quantities of interest (i.e. temperature, hydrostatic pressure, fission products concentration) is provided to illustrate the couplings at hand during a power ramp and, above all, to demonstrate the good agreement of the results with those already published in the literature and with available measurements of xenon, cæsium and iodine releases. In the core of this paper, several numerical strategies are proposed to reduce the overall time spent in the thermochemical solver by defining at each node and for each time step a good initial estimate of the set of stable phases that are likely to form. The first one, referred to as the spatial strategy, relies on an intelligent reorganization of the mesh nodes as a function of temperature or hydrostatic pressure with an appropriate initialization of the thermochemical equilibrium calculation at a node by the phase composition calculated at the node with the closest thermo-mechanical conditions. The second strategy, referred to as the hybrid strategy, combines the spatial strategy during the power transient (where large power variations take place at each time step) with an initialization of the nodal equilibria from the solution obtained at the previous time step in case of small power variations. The 3.5D simulation has been run several times in order to assess and compare the spatial and hybrid strategies. The simulations show that the spatial strategy is the most efficient. It leads to an overall increase of the calculation time related to the incorporation of thermochemistry in the 3.5D fuel performance simulation of less than 2.25%, showing the feasibility of large thermal-chemical–mechanical simulations within the PLEIADES/ALCYONE 2.1 fuel performance code. Finally, as a first step towards more complicated simulations (i.e. including a greater number of phases and/or performed with a finer spatial discretization), a primitive parallelization algorithm based on a multiprocess approach is proposed. Combined with the most efficient spatial strategy, it leads to a substantial reduction of the extra (real) calculation time related to thermochemistry which become less than 0.65% of the total computation time when the thermochemical equilibrium calculations are distributed over 4 cores.

Large thermoelectric power variations in epitaxial thin films of layered perovskite GdBaCo2O5.5±δ with a different preferred orientation and strain

Oxygen stoichiometry in epitaxial GdBaCo2O5.5±δ films accommodates the strain, which substantially affects thermoelectric properties, bringing the material from p-type (tensile strain c⊥-oriented on STO) to n-type thermopower (compressive b⊥ on LAO).

Integrated microwave photonic phase shifter with full tunable phase shifting range (> 360°) and RF power equalization.

We report a novel microwave photonic phase and amplitude control structure based on a single microring resonator with a tunable Mach Zehnder interferometer reflective loop, which enables the realization of a continuously tunable microwave photonic phase shifter with enhanced phase tuning range while simultaneously compensating for the RF power variations. The complimentary tuning of the phase and amplitude presents a simplistic approach to resolve the inherent trade-off between maintaining a full RF phase shift while eliminating large RF power variations. Detailed simulations have been carried out to analyze the performance of the new structure as a microwave photonic phase shifter, where the reflective nature of the proposed configuration shows an effective doubling of the phase range while the amplitude compensation module provides a parallel control to potentially reduce the RF amplitude variations to virtually zero. The phase range enhancement, which is first verified experimentally with a passive only chip, demonstrates the capability to achieve a continuously tunable RF phase shift of 0-510° with an RF amplitude variation of 9 dB. Meanwhile, the amplitude compensation scheme is incorporated onto an active chip with a continuously tunable RF phase shift of 0-150°, where the RF power variations is shown to be reduced by 5 dB while maintaining a constant RF phase shift.

CVaR-Constrained Optimal Bidding of Electric Vehicle Aggregators in Day-Ahead and Real-Time Markets

An electric vehicle aggregator (EVA) that manages geographically dispersed electric vehicles offers an opportunity for the demand side to participate in electricity markets. This paper proposes an optimization model to determine the day-ahead inflexible bidding and real-time flexible bidding under market uncertainties. Based on the relationship between market price and bid price, the proposed optimal bidding model of EVA aims to minimize the conditional expectation of electricity purchase cost in two markets considering price volatility. Moreover, the penalty cost of the deviation between the bidding quantities is included to avoid large power variation and arbitrage. The conditional expectation optimization model is formulated as an expectation minimization problem with the conditional value-at-risk constraints. Based on the price data in the PJM market, simulation results verify that our model is a decision-making tool in electricity markets, which can help market players comprehend the variants of bid price, expected cost and probability of successful bidding.

Optical modelling of a supplementary tunable air-spaced goggle lens for rodent eye imaging.

Aberration variations severely degrade retinal imaging in small animal eyes. Previously, the approach of a goggle lens with a matching corneal index was proposed to overcome the on-axis resolution limit of static imaging systems, which allows the use of the full eye pupil. But this technique didn’t address the problem of the large power variation, and the ensuing aberration on and off-axis, when dealing with small animal eyes. In this study, we present the concept of a tunable goggle lens, designed to compensate individual ocular aberration for different rodent eye powers. Ray tracing evidences that lens-fitted goggles permit, not only to adjust individual eyes power, but also to surpass conventional adaptive correction technique over large viewing angle, provided a minimum use of two spaced liquids. We believe that the overlooked advantage of the 3D lens function is a seminal finding for further technological advancements in widefield retinal imaging.

Effect of Supercapacitors Supply on the High Torque Flywheel for Satellite Power System

During the operation of the satellite, the rapid adjustment of the satellite attitude is needed to monitor the hot spots. The rapid adjustment of the satellite attitude requires the rapid change of the speed of the flywheel, which demands a large instantaneous power from satellite power system. The traditional satellite power supply system is unable to meet the requirements of High Torque Flywheel due to the limited satellite space and the limited battery capacity. The character of the supercapacitor in the satellite power system is helpful to meet the demand of the large load power variation. In this paper, the traditional satellite power system model is established by Matlab/Simulink and integrated the large torque flywheel motors and supercapacitors to study the whether the traditional satellite power system can meet the demand of the acceleration of High Torque Flywheel and the bidirectional flow of braking energy. According to the simulation results, the energy distributions of lithium battery and supercapacitors are determined.

Dynamic Optimal AC Line Current Regulation Method for Three-Phase Active Power Conditioners

The main feature of the three-phase back-to-back active power conditioner (APC) is to improve the power quality of the microgrid. During the large power variation transient, the severe voltage surge/sag on the dc-link of the APC may cause its malfunction. Therefore, the dynamic optimal ac line current regulation (DOCR) with flexible dc-link voltage slope (FDVS) is proposed in this paper. The proposed DOCR can determine the accurate input current reference value to regulate the dc-link voltage under the large power variation condition. On the other hand, the proposed FDVS can further mitigate the dc-link voltage variation during the large power variation transient. The mathematical equations of the proposed control strategy are derived thoroughly. Furthermore, procedures to determine the dc-link capacitance with the proposed DOCR are developed. Finally, the experimental results obtained from a 5-kVA back-to-back APC verify the feasibility and the performance of the proposed line current regulation strategies.

A Reconfigurable Diode Topology for Wireless Power Transfer With a Wide Power Range

This letter presents a reconfigurable diode topology which obtains the independently tuned ultra-low threshold voltage and large breakdown voltage simultaneously. Measured I-V curves show that the proposed topology obtains both ultra-low threshold voltage (250 mV) and tunable breakdown voltage up to −51 V. By applying the topology in rectifier design, high efficiency of above 50% can be maintained within a wide power range from 0.1 W to above 10 W in an inductive wireless power transfer (WPT) test environment. It provides a practical diode topology design for rectifiers to maintain high efficiency for large power variation scenarios such as coil misalignment.

Comparative Analysis of Voltage Control in Battery Power Converters for Inverter-Based AC Microgrids

A microgrid is a micro-power system composed of local distributed generators, energy storage systems, loads, and other components in a local power network. Because renewable energy sources show relatively large output power variation, the integration of distributed generators in a microgrid often requires the installation of a large-scale energy storage system. The energy storage system is connected to a local AC bus via the DC/AC converter with an output inductor-capacitor (LC) filter. The energy storage system power converters generally form the local AC bus voltage. This grid-forming operation requires fast and robust voltage control to properly maintain a stable energy flow and high power quality in the local AC bus. In this paper, two major voltage control schemes—double-loop control and direct voltage control—are analytically compared, and their effects on the power quality of the microgrid are illustrated. The dynamic performance is compared through simulations and experimental results.

The Hydrogen Grand Challenge

More than 90% of the world’s growing energy demand is satisfied by fossil fuels (BP Statistical Review …, 2015)1. One consequence of the unrestrained use of this technology is the continuous increase of the CO2 level of the atmosphere2. There are also the challenges associated with the limitations of the corresponding resources (Hubbert, 1956; BP Statistical Review …, 2015). Climate change as a consequence of the growing CO2 level (see text footnote 2, ESRL Global Monitoring Division, 2015) has been identified as one of the most critical challenges facing mankind and requires immediate action: “The Paris Agreement aims to strengthen the global response to the threat of climate change, ( … ) by low greenhouse gas emissions development, in a manner that does not threaten food production” (United Nations Framework …, 2015). How to reach the corresponding significant reduction of CO2 emission by 2050 is not defined in this document, but it implies that mankind must transform its energy technology from a fossil to a renewable basis. Numerous studies and publications have indicated that the sun’s energy and its derivatives (wind, water) are by far sufficient to supply world’s energy demand (see, e.g., Smalley, 2005; Zuttel et al., 2010); but the large daily and seasonal power variation of renewable energy is an additional complication for a wide spread replacement of fossil energy by renewable energy.

Offshore wind farms energy injection in the electrical grid - Lithium battery to mitigate power fluctuations

The wind farm energy injection in the electrical grid generates the economic and technical challenges which must to be solved for successful large scale wind energy integration in electrical grids. Due to wind speed variations, the wind farm output power presents the fluctuating components which are not compatible to the electrical grid requirements. Today, this problem is a major challenge with the wind farm projects increasing in the world. Large power variations cause the deviations of rms voltage and the frequency compared to nominal values in point of the coupling. These deviations cause generally the protective equipment activation which causes a disconnection of wind farms from the grid. To avoid these problems, the electrical grid managers use the grid code requirements to regulate the wind energy injection process in the grid. Using energy storage devices such as a lithium battery or ultracapacitors to compensate the power fluctuations from wind farm is an interesting solution. This paper deals this issue to improve the injected energy in electrical grid by the wind farm using lithium battery. Proposed approach allows compensating the wind energy fluctuations, so that the energy destined to electrical grid becomes compatible to electrical grid code requirements. Studied system configuration includes a wind farm with rated power of 300MW, a lithium battery module with a maximum power of 30MW and electrical grid. Lithium battery is connected to grid through an inverter and the power fluctuations from wind farm are assigned to battery. To show the performances of the control strategy, some simulation results are presented and analyzed using Matlab/SimPowerSystem software.

A Coordination Control Strategy of Voltage-Source-Converter-Based MTDC for Offshore Wind Farms

Similar to other major electrical apparatuses, the reliability and stability of the dc network is becoming the most important issue when using the voltage-source-converter-based multiterminal dc (VSC-MTDC) system for offshore wind power integration. A coordinated control strategy of VSC-MTDC named master–auxiliary is proposed by combining the advantages of the voltage margin and voltage droop control. This strategy has three advantages. First, the master converter station with the constant dc voltage control can provide reference to the system dc voltage and is helpful for the stabilization of dc voltage. Second, the integrated control of the dc voltage in both master and auxiliary converter stations are helpful for providing adequate active power control (APC) and restraining large power variation. Third, the APC converter station can serve as a backup for the dc voltage control in abnormal conditions. In order to guarantee the reliability and stability of the system under various operating conditions, this paper introduces the priority of dc voltage control to the coordination control strategy. Moreover, a parameter optimizing method of controllers for this strategy is also proposed. Finally, the effectiveness of the master–auxiliary control is verified by simulations under normal and abnormal conditions.