Highly reliable and efficient voltage optimizer based on direct PWM AC-AC buck converter

A voltage optimizer (VO) to regulate overrated electricity supply voltages to optimized values with high reliability and efficiency has been developed based on a direct PWM ac-ac buck converter. A comprehensive control strategy is proposed for the ac-ac converter to address existing commutation problems and also to offer a fault handling capability against short circuits. A number of switching states are defined depending on the input voltage, whereby continuous and reliable current paths are maintained at all times especially around voltage zero-crossing points, and safe transitions between them are achieved as well. Based on a detailed power loss analysis, a hybrid scheme with silicon carbide (SiC) MOSFETs and silicon IGBTs is proposed which significantly improves the efficiency while minimizes the cost, and simultaneously enables the use of a high switching frequency, with the potential to reduce the volume of a VO. Results from simulation and experiment confirm that the proposed strategies give reliable operation of the VO with an efficiency higher than 99%

Control techniques with system efficiency comparison for micro-wind turbines

This paper presents the implementation of a sensorless speed controller and active rectification in a micro-wind turbine intended for battery charging. The controller was tested in a wind turbine emulator test rig using real wind data available from British bases in Antarctica. The control algorithm was successfully tested up to 14 m/s wind speed. Beyond this point the electrical unbalance in the turbine generator compromised the stability and performance of the system. Also, a system efficiency comparison of different control algorithms is given to demonstrate the advantages of using active rectification instead of passive diode rectifiers in microwind turbines. This comparison was done between the sensorless control plus active rectifier, a DC-DC converter regulator and the direct connection between the turbine and battery by means of a diode rectifier. The turbine with an active rectifier and sensorless control achieved the highest power coefficient over the range of wind speeds showing that this technique is an attractive and relatively low cost solution for maintaining good performance of micro-wind turbines at low and moderate wind speeds

The Density of Lyman-alpha Emitters at Very High Redshift

We describe narrowband and spectroscopic searches for emission-line star forming galaxies in the redshift range 3 to 6 with the 10 m Keck II Telescope. These searches yield a substantial population of objects with only a single strong (equivalent width >> 100 Angstrom) emission line, lying in the 4000 - 10,000 Angstrom range. Spectra of the objects found in narrowband-selected samples at lambda ~5390 Angstroms and ~6741 Angstroms show that these very high equivalent width emission lines are generally redshifted Lyman alpha 1216 Angstrom at z~3.4 and 4.5. The density of these emitters above the 5 sigma detection limit of 1.5 e-17 ergs/cm^2/s is roughly 15,000 per square degree per unit redshift interval at both z~3.4 and 4.5. A complementary deeper (1 sigma \~1.0 e-18 ergs/cm^2/s) slit spectroscopic search covering a wide redshift range but a more limited spatial area (200 square arcminutes) shows such objects can be found over the redshift range 3 to 6, with the currently highest redshift detected being at z=5.64. The Lyman alpha flux distribution can be used to estimate a minimum star formation rate in the absence of reddening of roughly 0.01 solar masses/Mpc^3/year (H_0 = 65 km/s/Mpc and q_0 = 0.5). Corrections for reddening are likely to be no larger than a factor of two, since observed equivalent widths are close to the maximum values obtainable from ionization by a massive star population. Within the still significant uncertainties, the star formation rate from the Lyman alpha-selected sample is comparable to that of the color-break-selected samples at z~3, but may represent an increasing fraction of the total rates at higher redshifts. This higher-z population can be readily studied with large ground-based telescopes.Comment: 7 pages, 5 encapsulated figures; aastex, emulateapj, psfig and lscape style files. Separate gif files for 2 gray-scale images also available at http://www.ifa.hawaii.edu/faculty/hu/emitters.html . Added discussion of foreground contaminants. Updated discussion of comparison with external surveys (Sec. 5 and Fig. 5). Note: continuum break strength limits (Fig. 3 caption) are correct here -- published ApJL text has a sign erro

Brushless doubly fed machine magnetic field distribution characteristics and their impact on the analysis and design

This paper contributes to the characterisation of the brushless doubly fed induction generator (BDFIG), which is attractive as a variable speed generator in applications (offshore wind turbine) with minimum maintenance requirements. The BDFIG has two three-phase stator windings of different pole numbers housed within the same stator slots and a shortcircuited rotor winding capable of coupling fields of different pole numbers. The stator windings and rotor winding create a magnetic field distribution with a range of characteristics different to those of conventional induction generators. This paper presents an analysis to identify the field characteristics and discusses their impact on the analysis and design of the BDFIG. The characteristics are determined from an analysis of the sum of two rotating sinusoidal field waveforms and confirmed by comparison with time-stepping finite element results and measured magnetic flux density data

Modelling, control and sensorless speed estimation of micro-wind turbines for deployment in Antarctica

This paper presents the modelling, control and sensorless speed estimation of two micro-wind turbines deployed by the British Antarctic Survey (BAS) in Antarctica. Mathematical models for the generators attached to an Ampair 100 and Rutland 913 wind turbines and their experimental validation are given. Also, a model for the wind turbines, particularly taking into account the power coefficient Cp versus tip speed ratio λ relationship was proposed and successfully evaluated on a wind turbine emulator test rig. This paper describes an analogue speed estimator board and a Kalman filter for estimating the shaft speed. These estimators use only DC side measurements to match the characteristics of the current version of the turbine control board. The wind turbine control and speed estimators were tested on the emulator test rig using real wind data from BAS research bases in Antarctica. Using only DC side measurements leads to low computation requirements to execute the algorithms in comparison to commonly used schemes that rely on AC measurements. In addition, the estimation algorithms are based on the model of a PM generator connected to a diode rectifier, as they can be used in a wider range of applications, including DC to DC converters with MPPT algorithms based on speed measurements

Practical considerations in the deployment of ground source heat pumps in older properties - a case study

A ground-sourced heat pump (GSHP) was installed in a former Vicarage in Cambridgeshire, with a mix of solid wall structure built in the late 1800s and cavity wall section built in the 2000s, previously heated by oil. This type of building is usually considered unsuitable for heat pumps, unless substantial insulation work and extensive replacement of radiators are undertaken. Although the building had undergone a degree of retrofit to increase insulation, the GSHP was installed with the existing radiators. A detailed thermal model for the house was built in ESP-r and validated against experimental measurements taken from sensors in every room. The expected heating demands were computed from the model based on weather data and the GSHP system was designed accordingly. A compromise was made between minimizing the size of the heat pump and the achievable energy savings, which could have important implications for the way incentives for low-emissions heating systems are set up. Using the initial SAP assessment would have led to a substantial oversizing of the heat pump. The data collected so far show that an SPF of 2.9 has been achieved whilst maintaining comfortable (18 °C) internal temperatures and emissions of CO2 have been reduced by 70%

Impact of transmission topology for protective operations in multi-terminal HVDC networks

This paper presents an outcome of a comprehensive study which evaluates the transient behaviour of point-to-point and multi-terminal high voltage direct current (MT-HVDC) networks. The behaviour of the HVDC system during a permanent pole-to-pole and pole-to-ground fault is assessed considering a range of fault resistances, fault positions along the line, and operational conditions. The emphasis of this investigation is on DC fault characteristics which would facilitate a reliable method of faulty line discrimination in a multi-terminal direct current (MTDC) system using local measurements only (i.e. assuming that no communication media is used). All the simulated waveforms (and subsequent analysis) utilise the sampling frequency of 96 kHz in compliance with IEC-61869 and IEC-61850:9-2 for DC-side voltages and currents

Development of a 50 kW wireless power transfer system

A high-power modular wireless power transfer system has been developed intended for use in larger vehicles. This paper presents the design methodology and evolution of the system, including test results. The system utilises SiC switching devices and an 85 kHz operating frequency. 50 kW of wireless power transfer has been achieved over a 200 mm gap at 89% efficiency, showing good promise for the system