1,000 research outputs found
Investigation into power distribution _grid interfacing for subsea tidal generation - Rectification to a common DC-bus
The research objective was to investigate the generation and collection of power generated in a distributed tidal generation site, and realise the most economic grid connection methodology for a practical installation. Current tidal steam prototype designs utilise variable speed gear boxes, power frequency converters and variable pitch blades in the nacelle, thus requiring complicated, heavy and expensive equipment in the nacelle. Moreover the variable blade pitch mechanism has a large power demand, and has to operate day after day submerged in water at depths between 30m – 80m. The approach taken in this research is to replace major cost components with much cheaper ones, as well as locating as many components as possible onshore where costs are less.
This thesis investigates a novel tidal stream power generation system referred to as “passive rectification to a common DC-bus” where an array of 3-phase synchronous generators operating at diverse speeds are connected to a common DC bus via passive diode rectifiers. The proposed tidal stream topology shows that variable pitched blades, variable speed gearboxes and frequency converters in the nacelle can be replaced by a much simpler system that uses fixed pitch blades, a fixed ratio gearbox, generator excitation control system, and a diode bridge rectifier. These components are generally cheaper, lighter, have a lower power demand and are more reliable than the components they replace. Also some of the voltage and frequency conversion is carried out ashore where costs are less, in addition to the overall reduction in the number of components required as some of the components are shared by multiple devices. As a result system efficiency is maximised, in addition to facilitating a reduction in capital, installation and operational costs.
Power conversion from the tidal generation power system to shore is achieved via passive rectification to a common DC-bus, thus system power regulation is achieved by individual generator field current control, to attain optimum system operation including, maximum power extraction, power limitation and stall control, in turn improving system reliability and controllability. Furthermore passive stall control via generator field current regulation eliminates the need for mechanical brakes which reduces cost and improves reliability.
The feasibility of the proposed power generation system was carried out via computer simulations and later validated via small scale laboratory hardware simulations
Glottal-synchronous speech processing
Glottal-synchronous speech processing is a field of speech science where the pseudoperiodicity
of voiced speech is exploited. Traditionally, speech processing involves segmenting
and processing short speech frames of predefined length; this may fail to exploit the inherent
periodic structure of voiced speech which glottal-synchronous speech frames have
the potential to harness. Glottal-synchronous frames are often derived from the glottal
closure instants (GCIs) and glottal opening instants (GOIs).
The SIGMA algorithm was developed for the detection of GCIs and GOIs from
the Electroglottograph signal with a measured accuracy of up to 99.59%. For GCI and
GOI detection from speech signals, the YAGA algorithm provides a measured accuracy
of up to 99.84%. Multichannel speech-based approaches are shown to be more robust to
reverberation than single-channel algorithms.
The GCIs are applied to real-world applications including speech dereverberation,
where SNR is improved by up to 5 dB, and to prosodic manipulation where the importance
of voicing detection in glottal-synchronous algorithms is demonstrated by subjective
testing. The GCIs are further exploited in a new area of data-driven speech modelling,
providing new insights into speech production and a set of tools to aid deployment into
real-world applications. The technique is shown to be applicable in areas of speech coding,
identification and artificial bandwidth extension of telephone speec
Development of a scaled doubly-fed induction generator for assessment of wind power integration issues
Years of experience have been dedicated to the advancement of thermal power plant technology, and in the last decade the investigation has focused on the wind energy conversion system (WECS). Wind energy will play an important role in the future of the energy market, due to the changing climate and the fossil fuel crisis. Initially, wind energy was intended to cover a small portion of the energy market, but in the long term it should compete with conventional fossil fuel power generation. The movement of the power system towards this new phenomena has to be investigated before the wind energy share increases in the network. Therefore, the wind energy integration issues serve as an interesting topic for authors to improve the perception of integration, distribution, variability and power flow issues. Several simulation models have been introduced in order to resolve this issue, however, the variety in types of wind turbines and the network policies result in these models having limited accuracy or being developed for specific issues. The micro-machine is introduced in order to overcome the challenges of simulation models and the costs involved in field tests. In the past, the grid integration issue of large turbo-alternators was solved by the micro-machines. A variety of tests are possible with the micro-machines and they also increase the flexibility of the system. The increased accuracy as well as the ability to carry out real-time analysis and compare actual field test data are strengths worth utilizing. This project involves the designing and the prototyping of a scaled doubly-fed induction generator (micro-DFIG). The machine is also analysed and tested. The scaling of the micro-machine is achieved by means of a dimensional analysis, which is a mathematical method that allows machines and systems to be downscaled by establishing laws of similitude between the reference model and its scaled model. MATLAB/SIMULINK, Maxwell and Solid Work are employed to achieve the objectives of this project
The low bit-rate coding of speech signals
Imperial Users onl
Time and frequency domain algorithms for speech coding
The promise of digital hardware economies (due to recent advances in
VLSI technology), has focussed much attention on more complex and sophisticated
speech coding algorithms which offer improved quality at relatively
low bit rates.
This thesis describes the results (obtained from computer simulations)
of research into various efficient (time and frequency domain) speech
encoders operating at a transmission bit rate of 16 Kbps.
In the time domain, Adaptive Differential Pulse Code Modulation (ADPCM)
systems employing both forward and backward adaptive prediction were
examined. A number of algorithms were proposed and evaluated, including
several variants of the Stochastic Approximation Predictor (SAP). A
Backward Block Adaptive (BBA) predictor was also developed and found to
outperform the conventional stochastic methods, even though its complexity
in terms of signal processing requirements is lower. A simplified
Adaptive Predictive Coder (APC) employing a single tap pitch predictor
considered next provided a slight improvement in performance over ADPCM,
but with rather greater complexity.
The ultimate test of any speech coding system is the perceptual performance
of the received speech. Recent research has indicated that this
may be enhanced by suitable control of the noise spectrum according to
the theory of auditory masking. Various noise shaping ADPCM
configurations were examined, and it was demonstrated that a proposed
pre-/post-filtering arrangement which exploits advantageously the
predictor-quantizer interaction, leads to the best subjective
performance in both forward and backward prediction systems.
Adaptive quantization is instrumental to the performance of ADPCM systems.
Both the forward adaptive quantizer (AQF) and the backward oneword
memory adaptation (AQJ) were examined. In addition, a novel method
of decreasing quantization noise in ADPCM-AQJ coders, which involves the
application of correction to the decoded speech samples, provided
reduced output noise across the spectrum, with considerable high frequency
noise suppression.
More powerful (and inevitably more complex) frequency domain speech
coders such as the Adaptive Transform Coder (ATC) and the Sub-band Coder
(SBC) offer good quality speech at 16 Kbps. To reduce complexity and
coding delay, whilst retaining the advantage of sub-band coding, a novel
transform based split-band coder (TSBC) was developed and found to compare
closely in performance with the SBC.
To prevent the heavy side information requirement associated with a
large number of bands in split-band coding schemes from impairing coding
accuracy, without forgoing the efficiency provided by adaptive bit
allocation, a method employing AQJs to code the sub-band signals together
with vector quantization of the bit allocation patterns was also
proposed.
Finally, 'pipeline' methods of bit allocation and step size estimation
(using the Fast Fourier Transform (FFT) on the input signal) were examined.
Such methods, although less accurate, are nevertheless useful in
limiting coding delay associated with SRC schemes employing Quadrature
Mirror Filters (QMF)
Component and system design of a mild hybrid 48 V powertrain for a light vehicle
This thesis presents contributions in three areas relevant for the development of 48 V mild hybrid electric powertrains for cars. The first part comprises methodologies and extensive testing of lithium-ion battery cells in order to establish the electric and thermal performance using equivalent circuit models.\ua0 Empirical, lumped-parameter models are used to ensure fast simulation execution using only linear circuit elements. Both electrochemical impedance spectroscopy and high-current pulse discharge testing is used to extract model parameters. Plenty of parameter results are published for various cells, temperatures and SOC levels. Further on, the model accuracy in voltage response is also evaluated. It is found that an R+2RC equivalent circuit offers the lowest error, 11 mV RMSE in a 1.5 h drive cycle, which is among the lowest numbers found in the literature for similar models. In the second part, electric machines with tooth-coil windings are explored as a viable candidate for mild hybrids. First, a method of analytically calculating the high-level electro-magnetic properties for all possible combinations of three-phase, dual layer tooth-coil winding machines is established and presented in a graphically appealing manner.\ua0 Then, a pair of pseudo-6-phase 50 kW PMSMs are designed, constructed and validated in a custom designed calorimetric dynamo test stand. These machines feature in-stator and in-slot forced oil cooling, enabling very high current densities of 25\ua0A/mm\ub2 continuous and 35\ua0A/mm\ub2 peak. A high net power density (19 kW/l) and a large area of high peak efficiency (95%) is shown numerically and validated by calorimetric measurements. Finally, low-level design, construction and evaluation of 48 V inverter hardware is explored. By using high-performance, extra-low-voltage silicon-based MOSFETs with custom designed metal substrate printed circuit boards, custom made gate drivers, and water cooling, 3x220 A RMS is reached experimentally on a 154 cm\ub2 area and an efficiency of 95.6%
Brushless asynchronous induction machines with leading VAR capability
Imperial Users onl
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