A comparison of cage and nested-loop BDFM rotors

The Brushless Double-Fed Machine (BDFM) is a type of variable speed generator or drive. Using theoretical analysis of simple BDFM rotors this paper establishes trends in how rotor structures determine the rotor's equivalent circuit resistance, leakage inductance and turns ratio. The variation in measured parameters of five prototype rotors is then analysed in light of the trends found. Both the theory and experimental results suggest a significant performance advantage in using cage+loops type rotors as opposed to the simple nested loop type more usually employed.

Trailing edge noise theory for rotating blades in uniform flow

This paper presents a new formulation for trailing edge noise radiation from rotating blades based on an analytical solution of the convective wave equation. It accounts for distributed loading and the effect of mean flow and spanwise wavenumber. A commonly used theory due to Schlinker and Amiet (1981) predicts trailing edge noise radiation from rotating blades. However, different versions of the theory exist; it is not known which version is the correct one and what the range of validity of the theory is. This paper addresses both questions by deriving Schlinker and Amiet's theory in a simple way and by comparing it to the new formulation, using model blade elements representative of a wind turbine, a cooling fan and an aircraft propeller. The correct form of Schlinker and Amiet's theory (1981) is identified. It is valid at high enough frequency, i.e. for a Helmholtz number relative to chord greater than one and a rotational frequency much smaller than the angular frequency of the noise sources.

Experimental LVRT performance of a 250 kW brushless DFIG

Nowadays, all new wind turbine generators have to meet strict grid codes, especially riding through certain grid faults, such as a low voltage caused by grid short circuits. The Low-Voltage Ride Through (LVRT) capability has become a key issue in assessing the performance of wind turbine generators. The mediumspeed Brushless DFIG in combination with a simplified two-stage gearbox shows commercial promise as a replacement for conventional DFIGs due to its lower cost and higher reliability. Furthermore, the Brushless DFIG has significantly improved LVRT performance when compared with the DFIG due to its inherent design characteristics. In this paper, the authors propose a control strategy for the Brushless DFIG to improve its LVRT performance. The controller has been implemented on a prototype 250 kW Brushless DFIG and test results show that LVRT is possible without a need for any external protective hardware such as a crowbar.

Design and performance analysis of a 6 MW medium-speed Brushless DFIG

The paper presents the design and performance analysis of a 6 MW medium-speed Brushless Doubly-Fed Induction Generation (Brushless DFIG) for a wind turbine drivetrain. Two machines with different frame sizes have been designed to show the flexibility of the design procedure. The mediumspeed Brushless DFIG in combination with a two stage gearbox offers a low-cost, low-maintenance and reliable drivetrain for wind turbine applications.

The BDFM as a generator in wind turbines

The Brushless Doubly-Fed Machine (BDFM) is attractive for use in wind turbines, especially offshore, as it offers high reliability by virtue of the absence of brushgear. Critical issues in the use of the BDFM in this role at a system level include the appropriate mode of operation, the sizing of associated converter and the control of the machine. At a machine level, the design of the machine and the determination of its ratings are important. Both system and machine issues are reviewed in the light of recent advances in the study of the BDFM, and preliminary comparisons are made with the well-established doubly fed wound rotor induction generator. © 2006 IEEE.

The single-phase brushless doubly-fed machine as a generator for wind turbines

This paper proposes a single-phase variant of the Brushless Doubly-Fed Machine, the Single-Phase BDFM, SPB. Like the BDFM it is a variable speed generator which requires a converter rated at only a fraction of the machine rating, using structure with no brushes. Unlike the BDFM, most of its power is delivered directly into a single-phase grid connection. As such it is a potential replacement for PM generators in small-ish wind turbines, potentially with a much lower cost. In this paper we give details of its operation, with reference to results from the first prototype SPB. We also suggest how it may be controlled, again with experimental results. © 2011 IEEE.

A 4 GHz non-resonant clock driver with inductor-assisted energy return to power grid

Power consumption of a multi-GHz local clock driver is reduced by returning energy stored in the clock-tree load capacitance back to the on-chip power-distribution grid. We call this type of return energy recycling. To achieve a nearly square clock waveform, the energy is transferred in a non-resonant way using an on-chip inductor in a configuration resembling a full-bridge DC-DC converter. A zero-voltage switching technique is implemented in the clock driver to reduce dynamic power loss associated with the high switching frequencies. A prototype implemented in 90 nm CMOS shows a power savings of 35% at 4 GHz. The area needed for the inductor in this new clock driver is about 6% of a local clock region. © 2006 IEEE.

Spray visualisation and acoustic analysis in a gas-turbine type burner

This paper presents the characterisation of self-excited oscillations in a kerosene burner. The combustion instability exhibits two different modes and frequencies depending on the air flow rate. Experimental results reveal the influence of the spray to shift between these two modes. Pressure and heat release fluctuations have been measured simultaneously and the flame transfer function has been calculated from these measurements. The Mie scattering technique has been used to record spray fluctuations in reacting conditions with a high speed camera. Innovative image processing has enabled us to obtain fluctuations of the Mie scattered light from the spray as a temporal signal acquired simultaneously with pressure fluctuations. This has been used to determine a transfer function relating the image intensity and hence the spray fluctuations to changes in air velocity. This function has identified the different role the spray plays in the two modes of instability. At low air flow rates, the spray responds to an unsteady air flow rate and the time varying spray characteristics lead to unsteady combustion. At higher air flow rates, effective evaporation means that the spray dynamics are less important, leading to a different flame transfer function and frequency of self-excited oscillation. In conclusion, the combustion instabilities observed are closely related with the fluctuations of the spray motion and evaporation.

Towards topology-free optimization - An application to turbine internal cooling geometries

The application of automated design optimization to real-world, complex geometry problems is a significant challenge - especially if the topology is not known a priori like in turbine internal cooling. The long term goal of our work is to focus on an end-to-end integration of the whole CFD Process, from solid model through meshing, solving and post-processing to enable this type of design optimization to become viable & practical. In recent papers we have reported the integration of a Level Set based geometry kernel with an octree-based cut- Cartesian mesh generator, RANS flow solver, post-processing & geometry editing all within a single piece of software - and all implemented in parallel with commodity PC clusters as the target. The cut-cells which characterize the approach are eliminated by exporting a body-conformal mesh guided by the underpinning Level Set. This paper extends this work still further with a simple scoping study showing how the basic functionality can be scripted & automated and then used as the basis for automated optimization of a generic gas turbine cooling geometry. Copyright © 2008 by W.N.Dawes.

Dynamic analysis of the brushless doubly-fed induction generator during symmetrical three-phase voltage dips

The Brushless Doubly-Fed Induction Generator (BDFIG) shows commercial promise as replacement for doublyfed slip-ring generators for wind power applications by offering reduced capital and operational costs due to its brushless operation. In order to facilitate its commercial deployment, the capabilities of the BDFIG system to comply with grid code requirements have to be assessed. This paper, for the first time, studies the performance of the BDFIG under grid fault ride-through and presents the dynamic behaviour of the machine during three-phase symmetrical voltage dips. Both full and partial voltage dips are studied using a vector model. Simulation and experimental results are provided for a 180 frame BDFIG.