Nominal Grid Research Articles

A Hybrid Filtering Technique-Based PLL Targeting Fast and Robust Tracking Performance under Distorted Grid Conditions

In most grid-connected power converter applications, the phase-locked loop (PLL) is probably the most widespread grid synchronization technique, owing to its simple implementation. However, its phase-tracking performance tends to worsen when the grid voltage is under unbalanced and distorted conditions. Many filtering techniques are utilized to solve this problem, however, at the cost of slowing down the transient response. It is a major challenge for PLL to achieve a satisfactory dynamic performance without degrading its filtering capability. To tackle this challenge, a hybrid filtering technique is proposed in this paper. Our idea is to eliminate the fundamental frequency negative sequence (FFNS) and other harmonic sequences at the prefiltering stage and inner loop of PLL, respectively. Second-order generalized integrators (SOGIs) are used to remove FFNS before the Park transformation. This makes moving average filters (MAFs) eliminate other harmonics with a narrowed window length, which means the time delay that is caused by MAFs is reduced. The entire hybrid filtering technique is included in a quasi-type-1 PLL structure (QT1-PLL), which can provide a rapid dynamic behavior. The small-signal model of the proposed PLL is established. Based on this model, the parameter design guidelines targeting the fast transient response are given. Comprehensive experiments are carried out to confirm the effectiveness of our method. The results show that the settling time of the proposed PLL is less than one grid cycle, which is shorter than most of the widespread PLLs. The harmonic rejection capability is also better than other methods, under both nominal and adverse grid conditions.

Traceable X,Y self-calibration at single nm level of an optical microscope used for coherence scanning interferometry

Coherence scanning interferometry used in optical profilers are typically good for Z-calibration at nm-levels, but the X,Y accuracy is often left without further notice than typical resolution limits of the optics, i.e. of the order of ~1 µm. For the calibration of metrology tools we rely on traceable artefacts, e.g. gauge blocks for traditional coordinate measurement machines, and lithographically mask made artefacts for microscope calibrations. In situations where the repeatability and accuracy of the measurement tool is much better than the uncertainty of the traceable artefact, we are bound to specify the uncertainty based on the calibration artefact rather than on the measurement tool. This is a big drawback as the specified uncertainty of a calibrated measurement may shrink the available manufacturing tolerance. To improve the uncertainty in X,Y we can use self-calibration. Then, we do not need to know anything more than that the artefact contains a pattern with some nominal grid. This also gives the opportunity to manufacture the artefact in-house, rather than buying a calibrated and expensive artefact. The self-calibration approach we present here is based on an iteration algorithm, rather than the traditional mathematical inversion, and it leads to much more relaxed constrains on the input measurements.In this paper we show how the X,Y errors, primarily optical distortions, within the field of view (FOV) of an optical coherence scanning interferometry microscope, can be reduced with a large factor. By self-calibration we achieve an X,Y consistency in the 175 × 175 µm2 FOV of ~2.3 nm (1σ) using the 50× objective. Besides the calibrated coordinate X,Y system of the microscope we also receive, as a bonus, the absolute positions of the pattern in the artefact with a combined uncertainty of 6 nm (1σ) by relying on a traceable 1D linear measurement of a twin artefact at NIST.

Stability Augmentation of a Grid-Connected Wind Farm by Fuzzy-Logic-Controlled DFIG-Based Wind Turbines

Wind farm (WF) grid codes require wind generators to have low voltage ride through (LVRT) capability, which means that normal power production should be resumed quickly once the nominal grid voltage has been recovered. However, WFs with fixed-speed wind turbines with squirrel cage induction generators (FSWT-SCIGs) have failed to fulfill the LVRT requirement, which has a significant impact on power system stability. On the other hand, variable-speed wind turbines with doubly fed induction generators (VSWT-DFIGs) have sufficient LVRT augmentation capability and can control the active and reactive power delivered to the grid. However, the DFIG is more expensive than the SCIG due to its AC/DC/AC converter. Therefore, the combined use of SCIGs and DFIGs in a WF could be an effective solution. The design of the rotor-side converter (RSC) controller is crucial because the RSC controller contributes to the system stability. The cascaded control strategy based on four conventional PI controllers is widely used to control the RSC of the DFIG, which can inject only a small amount of reactive power during fault conditions. Therefore, the conventional strategy can stabilize the lower rating of the SCIG. In the present paper, a new control strategy based on fuzzy logic is proposed in the RSC controller of the DFIG in order to enhance the LVRT capability of the SCIG in a WF. The proposed fuzzy logic controller (FLC) is used to control the reactive power delivered to the grid during fault conditions. Moreover, reactive power injection can be increased in the proposed control strategy. Extensive simulations executed in the PSCAD/EMTDC environment for both the proposed and conventional PI controllers of the RSC of the DFIG reveal that the proposed control strategy can stabilize the higher rating of the SCIG.

Modified <italic>pq</italic>-Theory-Based Control of Solar-PV-Integrated UPQC-S

This paper proposes a modified pq-theory-based control of a solar photovoltaic (PV)-array-integrated unified power quality conditioner (PV-UPQC-S). The system incorporates clean energy generation along with power quality improvement, thus increasing functionality of the system. The fundamental-frequency positive-sequence components of voltages at the point of common coupling (PCC) are extracted using the generalized cascaded delay signal cancellation technique, which are then used in pq-theory-based control to estimate reference signals for the PV-UPQC-S. This modification in pq theory enables its application for PV-UPQC-S control under conditions of distorted PCC voltages. The series voltage-source converter (VSC) of the PV-UPQC-S operates such that it shares a part of the reactive power of the load even under nominal grid conditions. This increases the utilization of the series VSC while reducing the rating of shunt VSC. The PV array is integrated at the dc bus of the UPQC and provides a part of active load power, thus reducing demand on the supply system. The dynamic performance of the modified pq-theory-based PV-UPQC-S is verified by simulating the system in MATLAB-Simulink with a combination of linear and nonlinear loads. The steady-state and dynamic performances of the system are then experimentally validated through extensive testing on a scaled-down laboratory prototype.

Harmonic Profile Injection-Based Hybrid Active Islanding Detection Technique for PV-VSC-Based Microgrid System

In this paper, a new investigation on islanding detection method is presented for PV-based VSC system. The proposed PV-VSC is represented as one of the distributed energy resource (DER) for a microgrid environment. A new multivariable dynamic model is presented in terms of two stage conversion with a dc–dc converter for the proposed PV-VSC system. The proposed VSC system is controlled primarily by a positive sequence P–Q loop, at the PCC. A new backstepping fast sliding mode control (BFSMC) is proposed for primary PWM control of VSC system. During sudden grid disconnection an islanding protection loop based on rate of change in voltage harmonic profile injection is proposed. A binary tree (BT) threshold filter is designed for the proposed hybrid islanding protection loop, to avoid false detection during other grid operational issues. UL 1741 (section 46) standard is followed for the PV-VSC system to consider the worst-case scenario. Performance of the proposed hybrid islanding protection scheme is investigated for IEEE 9 bus system with multiple DERs, implemented in MATLAB/Simulink environment. The effectiveness of the proposed scheme is determined by result analysis, where no significant effect on power quality is recorded during nominal grid operation.

Indirect measurement of high grid strip densities over Nyquist sampling rate based on the moiré pattern analysis for quality assurance in grid manufacturing

Abstract We have recently developed very precise carbon-interspaced antiscatter grids having a wide range of grid strip densities ( i.e. , 85–234 lines/in.) by adopting the sawing process, depending on the demands of specific imaging modalities. However, because grid strips of the recently developed grids are mostly invisible through X-ray nondestructive testing due to their high strip densities, quality assurance in the manufacturing has not yet satisfactorily made. In this work, we proposed a pragmatic method to measure actual grid strip densities over the Nyquist sampling rate based on the moire pattern analysis. We prepared several samples of the carbon-interspaced grids having nominal grid strip densities of 85, 103, 130, 150, 178, 203, 215, and 234 lines/in. and measured their strip densities by using the proposed method to demonstrate its viability assessment. The differences between the nominal and the measured grid strip densities were less than about 1.8%, indicating the proposed method is useful for the quality assurance in grid manufacturing.

The USArray Transportable Array as a Platform for Weather Observation and Research

Abstract The USArray Transportable Array (TA), a component of the National Science Foundation’s EarthScope Initiative, has proven to be a successful model for large-scale real-time monitoring networks. Approximately 400 stations are deployed simultaneously in the continental United States on a nominal Cartesian grid across an area of approximately 2,000,000 km2. Each station was originally designed to operate autonomously as a seismic observing platform capable of recording and transmitting data at 1 and 40 samples per second in real time. The expansion of onboard instrumentation to include surface atmospheric pressure sensors improved the USArray’s real-time capability in monitoring the atmosphere and weather phenomena at the same sample rates. Though not a traditional weather monitoring station, the combination of these seismic and pressure sensors at each TA station can contribute to the observation of surface weather phenomena and has facilitated broader-scale observational applications. Twenty-five TA stations have a Vaisala WXT520 weather station installed in order to create an observational array of “full meteorological” (full-met) stations. Meteorological phenomena have been recorded in high detail, including a data quality comparison between a pair of TA and National Weather Service (NWS) stations, a gust-front passage, rainfall measurements following a squall-line passage, and a near pass of a tornado by one station. Additional products have been constructed from the TA data for visualization and research purposes. The large spatial extent and station configuration of the TA network increases the probability for making high-resolution recordings of data from rare encounters with atmospheric events.

Frequency adaptability of harmonics controllers for grid-interfaced converters

A wider spread adoption of power electronic converters interfaced renewable energy systems has brought more attention to harmonic issues to the electrical grid, and means are taken to improve it in the control. More advanced closed-loop harmonic controllers are thus demanded to enhance the renewable energy integration in order to be grid-friendly. However, usually being treated as a constant factor in the design of harmonic controllers, the grid frequency varies with the generation-load imbalance, and thus may lead to deterioration of the power quality. This paper explores the frequency sensitivity of the most popular harmonic controllers for grid-interfaced converters. The frequency adaptability of these harmonic controllers is evaluated in the presence of a variable grid frequency within a specified reasonable range, e.g., ± 1% of the nominal grid frequency (50 Hz). Solutions to the improvement of the frequency variation immunity of the discussed harmonic controllers are also emphasised. Case studies of a single-phase grid-connected photovoltaic system are provided to verify the analysis.

Simulation sensitivities of the major weather regimes of the Southeast Asia region

This study evaluates the Weather Research and Forecasting model’s ability to simulate major weather phenomena [dry conditions, tropical cyclones (TCs) and monsoonal flow] over East and Southeast Asia. Sensitivity tests comprising different cumulus (Kain–Fritsch and Betts–Miller–Janjic) and microphysics (Purdue Lin, WSM3, WSM6 and Thompson) are used together with different placement of lateral boundaries to understand and identify suitable model configuration for weather and climate simulations over the Asia region. All simulations are driven with reanalysis data and use a nominal grid spacing of 36 km, with 51 levels in the vertical. The dry season showed little sensitivity to any configuration choices, while the TC case shows high sensitivity to the cumulus scheme and low sensitivity to the microphysical scheme. Monsoon simulations displayed significant sensitivity to the placement of the lateral boundaries.

Detecting False Data Injection Attacks on Power Grid by Sparse Optimization

State estimation in electric power grid is vulnerable to false data injection attacks, and diagnosing such kind of malicious attacks has significant impacts on ensuring reliable operations for power systems. In this paper, the false data detection problem is viewed as a matrix separation problem. By noticing the intrinsic low dimensionality of temporal measurements of power grid states as well as the sparse nature of false data injection attacks, a novel false data detection mechanism is proposed based on the separation of nominal power grid states and anomalies. Two methods, the nuclear norm minimization and low rank matrix factorization, are presented to solve this problem. It is shown that proposed methods are able to identify proper power system operation states as well as detect the malicious attacks, even under the situation that collected measurement data is incomplete. Numerical simulation results both on the synthetic and real data validate the effectiveness of the proposed mechanism.

Grounding-line migration in plan-view marine ice-sheet models: results of the ice2sea MISMIP3d intercomparison

AbstractPredictions of marine ice-sheet behaviour require models able to simulate grounding-line migration. We present results of an intercomparison experiment for plan-view marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no buttressing effects from lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameters permitted the evolution of curved grounding lines, generating buttressing effects. The experiments showed regions of compression and extensional flow across the grounding line, thereby invalidating the boundary layer theory. Steady-state grounding-line positions were found to be dependent on the level of physical model approximation. Resolving grounding lines requires inclusion of membrane stresses, a sufficiently small grid size (<500 m), or subgrid interpolation of the grounding line. The latter still requires nominal grid sizes of <5 km. For larger grid spacings, appropriate parameterizations for ice flux may be imposed at the grounding line, but the short-time transient behaviour is then incorrect and different from models that do not incorporate grounding-line parameterizations. The numerical error associated with predicting grounding-line motion can be reduced significantly below the errors associated with parameter ignorance and uncertainties in future scenarios.

Comparison of intelligent charging algorithms for electric vehicles to reduce peak load and demand variability in a distribution grid

A potential breakthrough of the electrification of the vehicle fleet will incur a steep rise in the load on the electrical power grid. To avoid huge grid investments, coordinated charging of those vehicles is a must. In this paper, we assess algorithms to schedule charging of plug-in (hybrid) electric vehicles as to minimize the additional peak load they might cause. We first introduce two approaches, one based on a classical optimization approach using quadratic programming, and a second one, market based coordination, which is a multi-agent system that uses bidding on a virtual market to reach an equilibrium price that matches demand and supply. We benchmark these two methods against each other, as well as to a baseline scenario of uncontrolled charging. Our simulation results covering a residential area with 63 households show that controlled charging reduces peak load, load variability, and deviations from the nominal grid voltage.

Augmentation of Wind Farms Ride Through by DFIG-based Variable Speed Wind Generators

Wind farm grid codes require wind generators to ride through voltage sags, which means that normal power production should be re-initiated once the nominal grid voltage has been recovered. Doubly Fed Induction Generator (DFIG) based wind farm is gaining popularity these days because of its inherent advantages like variable speed operation and independent controllability of active and reactive power over conventional Induction Generator (IG). This paper proposes a new control strategy using DFIGs for stabilizing a wind farm composed of DFIGs and IGs. Simulation analysis by using PSCAD/EMTDC shows that the DFIGs can effectively stabilize the IGs and hence the entire wind farm through the proposed control scheme by providing sufficient reactive power to the system.

Wind Farm Stabilization by using DFIG with Current Controlled Voltage Source Converters Taking Grid Codes into Consideration

Recent wind farm grid codes require wind generators to ride through voltage sags, which means that normal power production should be re-initiated once the nominal grid voltage is recovered. However, fixed speed wind turbine generator system using induction generator (IG) has the stability problem similar to the step-out phenomenon of a synchronous generator. On the other hand, doubly fed induction generator (DFIG) can control its real and reactive powers independently while being operated in variable speed mode. This paper proposes a new control strategy using DFIGs for stabilizing a wind farm composed of DFIGs and IGs, without incorporating additional FACTS devices. A new current controlled voltage source converter (CC-VSC) scheme is proposed to control the converters of DFIG and the performance is verified by comparing the results with those of voltage controlled voltage source converter (VC-VSC) scheme. Another salient feature of this study is to reduce the number of proportionate integral (PI) controllers used in the rotor side converter without degrading dynamic and transient performances. Moreover, DC-link protection scheme during grid fault can be omitted in the proposed scheme which reduces overall cost of the system. Extensive simulation analyses by using PSCAD/EMTDC are carried out to clarify the effectiveness of the proposed CC-VSC based control scheme of DFIGs.

Online Monitoring of the Power Transfer in a DC Test Grid

One solution for increasing the use of renewables is to find new strategies to promote the connection of distributed energy resources (DERs) within the existing power system. A solution that allows a flexible integration of dispersed generation (DG) into energy networks is the use of dc at the user layer. This seems a reasonable choice since several renewable sources and the presently available storage systems deliver electricity in dc form (e.g., fuel cells and solar cells). Furthermore, other generators (wind turbines and microhydro or microgas turbines) deliver electricity in ac form but mostly at variable or nonstandard frequencies. Consequently, the output of these generators must be rectified, converted again into ac, and conditioned to meet the nominal grid parameters. Anticipating the extensive use of low-power DC-based intelligent devices, one can avoid losses in energy transfer by using a dc layer within the distribution networks, at least where the energy is produced in dc form. DC grids in buildings are expected to be one of the main applications and challenges in the future. The development of a dc test network, including measurement and communication, is presented in this paper. The development of promising applications showing the effective integration of DG into intelligent networks with low-power loads is illustrated and discussed. Specific indicators are formulated to characterize power quality issues of dc systems.

Nondestructive measurement of the grid ratio using a single image

The antiscatter grid is an essential part of modern radiographic systems. Since the introduction of the antiscatter grid, however, there have been few methods proposed for acceptance testing and verification of manufacturer-supplied grid specifications. The grid ratio (r) is an important parameter describing the antiscatter grid because it affects many other grid quality metrics, such as the contrast improvement ratio (K), primary transmission (Tp), and scatter transmission (Ts). Also, the grid ratio in large part determines the primary clinical use of the grid. To this end, the authors present a technique for the nondestructive measurement of the grid ratio of antiscatter grids. They derived an equation that can be used to calculate the grid ratio from a single off-focus flat field image by exploiting the relationship between grid cutoff and off-focus distance. The calculation can be performed by hand or with included analysis software. They calculated the grid ratios of several different grids throughout the institution, and afterward they destructively measured the grid ratio of a nominal r8 grid previously evaluated with the method. They also studied the sensitivity of the method to technical factors and choice of parameters. With one exception, the results for the grids found in the institution were in agreement with the manufacturer's specifications and international standards. The nondestructive evaluation of the r8 grid indicated a ratio of 7.3, while the destructive measurement indicated a ratio of 7.53 +/- 0.28. Repeated evaluations of the same grid yielded consistent results. The technique provides the medical physicist with a new tool for quantitative evaluation of the grid ratio, an important grid performance criterion. The method is robust and repeatable when appropriate choices of technical factors and other parameters are made.

Evaluation of a CCSM3 Simulation with a Finite Volume Dynamical Core for the Atmosphere at 1° Latitude × 1.25° Longitude Resolution

Abstract A simulation of the present-day climate by the Community Climate System Model version 3 (CCSM3) that uses a Finite Volume (FV) numerical method for solving the equations governing the atmospheric dynamics is presented. The simulation is compared to observations and to the well-documented simulation by the standard CCSM3, which uses the Eulerian spectral method for the atmospheric dynamics. The atmospheric component in the simulation herein uses a 1° latitude × 1.25° longitude grid, which is a slightly finer resolution than the T85-grid used in the spectral transform. As in the T85 simulation, the ocean and ice models use a nominal 1-degree grid. Although the physical parameterizations are the same and the resolution is comparable to the standard model, substantial testing and slight retuning were required to obtain an acceptable control simulation. There are significant improvements in the simulation of the surface wind stress and sea surface temperature. Improvements are also seen in the simulations of the total variance in the tropical Pacific, the spatial pattern of ice thickness distribution in the Arctic, and the vertically integrated ocean circulation in the Antarctic Circumpolar Current. The results herein demonstrate that the FV version of the CCSM coupled model is a state-of-the-art climate model whose simulation capabilities are in the class of those used for Intergovernmental Panel on Climate Change (IPCC) assessments. The simulated climate is very similar to that of the T85 version in terms of its biases, and more like the T85 model than the other IPCC models.

Ice-sheet modelling at different spatial resolutions: focus on the grounding zone

AbstractA high-resolution time-dependent two-dimensional flowline model was developed, which takes into account all relevant stresses (i.e. both shear and longitudinal stresses), and which is solved on a fixed finite-difference grid. Special attention was paid to the computational efficiency. A comparison is made with a similar model that solves the velocity field according to the shallow-ice approximation. Both models were applied to an East Antarctic flowline for which the data on surface and bedrock elevation were sampled at different spatial resolutions. The numerical analysis deals with the importance of longitudinal stresses at the ice-sheet/ice-shelf interface at different grid sizes" and the importance of basal drag vs driving stress in basal sliding laws. At a grid resolution of 5–10 km the force balance indicates that all stress components are of equal importance in an area that is larger than the nominal grid size. When sliding becomes dominant, this transition zone considerably widens. Furthermore, the longitudinal stress deviator is found to vary considerably with depth due to the thermomechanical coupling.

Pure lead and the tin effect in deep-cycling lead/acid battery applications

The use of leadcalcium or pure lead grids in valve-regulated lead/acid (VRLA) batteries has been generally satisfactory, but one drawback of these materials is the unpredictable build-up of a passivation layer on the surface of the positive grid. In deep-cycling applications, this passivation layer can result in a rapid loss of discharge capacity that is reversible with proper treatment. This phenomenon is reminiscent of the ‘memory effect’ in nickel/cadmium cells and although it is reversible its unpredictability renders products not entirely suitable for deep-cycling applications. The use of small amounts of tin, alloyed into the positive grid or as a surface coating, has been promoted as a cure for this shortcoming, and to a large extent is effective. This paper presents a careful review of the ‘tin effect’ and provides information on how best to take advantage of minimal tin levels so that optimal cycling performance is realized. For example, it is noted that the nominal grid tin content may be less important than the way in which the metal is distributed. In VRAL products designed for deep-cycling applications, incorporation of tin into or on the surface of the positive grid in the proper way is of substantial benefit, if not crucial.

Using geology to control geostatistics in the Hellyer deposit

The Hellyer orebody, a polymetallic massive sulfide deposit, was discovered in western Tasmania by Aberfoyle in 1983. Delineation diamond drilling was carried out in 1984 on a nominal 50-m square grid pattern to outline the resource. Resource estimation methods were influenced by the requirement to develop a regular block model for conceptual mine planning studies. Detailed geological interpretation indicates that the interpolation technique must take into account several important features to retain geological credibility. The deposit has sharp limits defined by visual geological contacts with virtually barren enclosing rocks. Lateral terminations are rapid with no interfingering internal waste. The dip and strike are variable and a major fault with a measurable displacement cuts acutely through the center of the deposit. Ore grades are reasonably correlateable within specific layers from hole to hole indicating a significant across-dip anisotropy. A hanging wall enriched zone is well-defined throughout the deposit. To overcome the variable geometry problems, a stratigraphic coordinate system was defined arbitrarily to replace the normal z coordinate. This allowed variography in stratigraphic layers. Blocks to be estimated were constrained by hand-drawn and subsequently digitized hanging wall and footwall contours. Each block was ascribed a stratigraphic coordinate by calculating its spatial position in relation to nearby stratigraphic unit boundaries within the massive sulfide body. Estimates were generated for each element by ordinary linear kriging. Despite the relatively sparse data in a large massive deposit, the customized technique developed for Hellyer has provided a reliable model of spatial grade distribution by combining conventional geostatistical methods with careful geological observation and interpretation. Some geometry problems remain which are the subject of ongoing studies.