Online grid measurement and ENS detection for PV inverter running on highly inductive grid

Photovoltaic (PV) and other sources of renewable energy are being used increasingly in grid-connected systems, for which stronger power quality requirements are being issued. Continuous grid monitoring should be considered so as to provide safe connections and disconnections from the grid. This letter gives an overview of the methods used for online grid measurement with PV inverters. Emphasis is placed on a method based on the injection of a noncharacteristic harmonic in the grid. Since this injection is regarded as a disturbance for the grid, different issues, i.e., the influence on total harmonic distortion (THD), the accuracy of line impedance measurement and the ENS (German abbreviation of Main Monitoring units with allocated Switching Devices) detection are studied. Laboratory results conducted on an existing PV inverter are presented to demonstrate the behavior of the PV inverter under different grid conditions. Some of the injection parameters are tuned in order to get an accurate measurement of line impedance.     

Generating hydrogen-rich fuel-cell feeds from dimethyl ether (DME) using physical mixtures of a commercial Cu/Zn/Al2O3 catalyst and several solid–acid catalysts

Homogeneous physical mixtures containing a commercial Cu/ZnO/Al₂O₃ catalyst and a solid–acid catalyst were used to examine the acidity effects on dimethyl ether hydrolysis and their subsequent effects on dimethyl ether steam reforming (DME-SR). The acid catalysts used were zeolites Y [Si/Al = 2.5 and 15: denoted Y(Si/Al)], ZSM-5 [Si/Al = 15, 25, 40, and 140: denoted Z(Si/Al)] and other conventional catalyst supports (ZrO₂, and γ-Al₂O₃). The homogeneous physical mixtures contained equal amounts, by volume, of the solid–acid catalyst and the commercial Cu/ZnO/Al₂O₃ catalyst (BASF K3-110, denoted as K3). The steam reforming of dimethyl ether was carried out in an isothermal packed-bed reactor at ambient pressure.

The most promising physical mixtures for the low-temperature production of hydrogen from DME contained ZSM-5 as the solid–acid catalyst, with hydrogen yields exceeding 90% (T = 275 °C, S/C = 1.5, τ = 1.0 s and P = 0.78 atm) and hydrogen selectivities exceeding 94%, comparable to those observed for methanol steam reforming (MeOH-SR) over BASF K3-110, with values equaling 95% and 99%, respectively (T = 225 °C, S/C = 1.0, τ = 1.0 s and P = 0.78 atm). Large production rates of hydrogen were directly related to the type of acid catalyst used. The hydrogen production activity trend as a function of physical mixture was

     

Compensation of cable voltage drops and automatic identification of cable parameters in 400 Hz ground power units

In this paper a new cable voltage drop compensation scheme for ground power units (GPU) is presented. The scheme is able to predict and compensate the voltage drop in an output cable by measuring the current quantities at the source. The prediction is based on an advanced cable model that includes self and mutual impedance parameters. The model predicts the voltage drop at both symmetrical and unbalanced loads. In order to determine the cable model parameters an automatic identification concept is derived. The concept is tested in full scale on a 90-kVA 400-Hz GPU with two different cables. It is concluded that the performance is significantly improved both with symmetrical and unsymmetrical cables and with balanced and unbalanced loads.     

Where the wind blows

The power systems in Denmark, Spain, Ireland, and New Zealand have some of the highest wind penetrations in the world (see Table 1). The management of the different power systems to date, with increasing amounts of wind energy, has been successful. There have been no incidents in which the wind has directly or indirectly been a major factor causing operational problems for the system. However, there are a number of parameters that are being monitored that indicate the need for active management in the near future (and in some cases already today). In this article, we briefly describe the situations in these four countries, giving special emphasis to the market integration of wind power, the use of wind forecasting, and curtailment experience. The final section provides an overview of the main wind forecasting methodologies and challenges.     

Simulation of acoustic wave propagation in dispersive media with relaxation losses by using FDTD method with PML absorbing boundary condition

We present a method to incorporate the relaxation dominated attenuation into the finite-difference time-domain (FDTD) simulation of acoustic wave propagation in complex media. A dispersive perfectly matched layer (DPML) boundary condition, which is suitable for boundary matching to such a dispersive media whole space, is also proposed to truncate the FDTD simulation domain. The numerical simulation of a Ricker wavelet propagating in a dispersive medium, described by second-order Debye model, shows that the Ricker wavelet is attenuated in amplitude and expanded in time in its course of propagation, as required by Kramers-Kronig relations. The numerical results also are compared to exact solution showing that the dispersive FDTD method is accurate and that the DPML boundary condition effectively dampens reflective waves. The method presented here is applicable to the simulation of ultrasonic instrumentation for medical imaging and other nondestructive testing problems with frequency dependent, attenuating media.     

The nature of adolescent learner interaction in a virtual high school setting

This study used survey data to measure the effect of learners' reported interactions with content, peers, and instructors on several course outcomes in two virtual high school courses that emphasized interactive learning. Surveys found that the large majority of students viewed all investigated types of interaction as educational and motivational. Students perceived learner–instructor and learner–content interactions to have significantly higher educational value (α < 0.01) than learner–learner interactions, and viewed learner–instructor interaction to be significantly more motivational (α < 0.01) than learner–content interaction. Furthermore, nine significant correlations were found involving the time students reported spending on human interaction and course outcomes. Seven of the significant correlations were related to the time students reported spending in human interaction and the more affective outcomes, such as course satisfaction and disposition towards the subject area. Outcomes also indicate that learner–learner interaction had higher correlations with course outcomes than learners' interactions with the content or their instructor. Students' perceived learning was not significantly correlated with any type of interaction, and only students' total reported time spent on learner–learner interaction and students' social learner–learner interaction were significantly correlated with their grade.     

Formulation and validation of Berenger's PML absorbing boundary forthe FDTD simulation of acoustic scattering

Berenger's perfectly matched layer (PML) absorbing boundary condition for electromagnetic (EM) waves is derived to absorb 2-D and 3-D acoustic waves in finite difference time domain (FDTD) simulation of acoustic wave propagation and scattering. A PML medium suitable for acoustic waves is constructed. Plane wave propagation in the PML medium is solved for both 2-D and 3-D cases and explicit FDTD boundary conditions are derived. The equations show that a matched PML medium is a perfect simulation of free space in that a plane wave does not change its direction of propagation or its speed when it propagates from free space into a matched PML medium. FDTD simulation of a pulsed point source propagating in two dimensions is carried out to test the performance of the PML boundary for acoustic waves. Results show that an eight layer PML boundary condition reduces the reflected error 40 dB over Mur's second order boundary condition     

High-temperature thermoelectric properties of Ca0.9Y0.1Mn1−x Fe x O3 (0 ≤ x ≤ 0.25)

Polycrystalline compounds of Ca_0.9Y_0.1Mn_{1? x} Fe _x O₃ for 0 ≤ x ≤ 0.25 were prepared by solid-state reaction, followed by spark plasma sintering process, and their thermoelectric properties from 300 to 1200 K were systematically investigated in terms of Y and Fe co-doping at the Ca- and Mn-sites, respectively. Crystal structure refinement revealed that all the investigated samples have the O′-type orthorhombic structure, and the lattice parameters slightly increased with increasing Fe concentration, causing a crystal distortion. It was found that with increasing the content of Fe doping, the Seebeck coefficient of Ca_0.9Y_0.1Mn_{1? x} Fe _x O₃ tended to increase, while the tendency toward the electrical conductivity was more complicated. The highest power factor was found to be 2.1 × 10^?4 W/mK² at 1150 K for the sample with x = 0.05 after annealing at 1523 K for 24 h in air. Thermal conductivity of the Fe-doped samples showed a lower value than that of the x = 0 sample, and the highest dimensionless figure of merit, ZT was found to be improved about 20 % for the sample with x = 0.05 as compared to that of the x = 0 sample at 1150 K.     

Sensitivity of water quality indicators in a large tropical reservoir to selected climate and land‐use changes

This study compared the sensitivity of water quality in tropical Aguamilpa Reservoir, as represented by normalized algae mass and dissolved oxygen, to selected projected changes from global climate change and development. The sensitivity of reservoir stratification as an indicator of reservoir sensitivity also was analysed. Model simulations indicated the reservoir was more sensitive to changes during the warm‐dry season than at other times. Both indexes (normalized algal mass and dissolved oxygen mass) were more sensitive to changes in air temperature (climate change) and nitrogen loading (development) than to changes in flow. The sensitivity to air temperature was similar to, but generally less than, the sensitivity to nutrient inflow. At the bounding values for change (3 °C for temperature; 50% increase in nitrogen loading), the algae mass sensitivities were 0.15 mg L^?1 per 3 °C and 0.2 mg L^?1 per 50% nitrogen load increase, and the dissolved oxygen mass sensitivities were 0.7 mg L^?1 per 3 °C and 2.0 mg L^?1 per 50% load increase. Changes in air temperature and nitrogen loadings affect the reservoir in different ways, air temperature mostly changing the timing of the algal growth with little change in peak values, while nutrient loadings change the peak values with little change in the timing. While the sensitivities are similar, the total algal mass change is significantly larger for nitrogen loading, compared to air temperature changes. These results imply global climate change effects can be partially mitigated by implementing management measures in the surrounding watersheds to minimize nutrient inflows, especially nitrogen in the case of Aguamilpa Reservoir.