137 research outputs found
A TWELVE-PULSE LOAD COMMUTATED CONVERTER DRIVE SYSTEM WITH VSI FOR STARTING UP AND ACTIVE POWER FILTERING IN AN LNG APPLICATION
Variable Frequency Drives (VFDs) are an integral component of the industry in today’s age. VFDs provide a great range of control for electrical machines, and can be integrated in a variety of applications to meet the desired objectives of operation with improved reliability and efficiency.
This thesis presents the Load-Commutated Converter (LCC) drive, which belongs to the Current Source Converter (CSC) based drive system family. Such drives are widely used in high power applications, due to power handling capabilities and the maturity of the drive system. The application under study is that of a helper/starter motor for a turbine compressor in a Liquefied Natural Gas (LNG) plant. Primarily, the thesis presents real-life scenarios of drive system operation such as constant/variable speed operation at constant/varying torque. The respective controllers for the LCC drive are presented alongside their results.
In addition to simulating the drive system in this LNG application, current harmonic mitigation measures are presented in this study. The typical converter topology presented in this thesis is the 12-pulse type, however comparisons with different topologies (6, 18, and 24-pulse) have also been presented.
Finally, a dual-purpose external Voltage Source Inverter (VSI) is used both as a starter and an Active Power Filter (APF), therefore addressing the issues of drive/load induced harmonics and LCC starting. As a conclusion, a controlled LCC drive model is simulated in SIMULINK to emulate the drive operation in actual plant conditions. The controlled drive is further studied for the presence of harmonics and their subsequent mitigation, by using passive as well as active power filters. The results obtained present the adequacy of the control system as well as the efficacy of the filters used for harmonics mitigation.
Future work revolves around improving the efficiency of the APF, and the drive control system to be more robust and reliable. The system can further be investigated for enhancements as per operational requirements
Robust Generator System Using PM Assisted Synchronous Reluctance Generator with Current-fed Drive
The growth of embedded generation and portable electrical installations has led
to an increased demand for low cost, flexible and reliable generator systems for military
and commercial applications. An interior permanent magnet (IPM) machine has high
power density due to its reluctance torque and magnetic torque components so it can
produce a large constant power-speed range. However, an IPM machine needs
demagnetizing current at high-speed during the flux-weakening region and thus develops
an inverter shutdown problem in an uncontrolled generator mode operation. In order to
overcome the disadvantages of the IPM machine, the permanent magnet assisted
synchronous reluctance generator (PMa-SynRG) can be a good solution for low cost,
high efficiency reliable generator systems. A PMa-SynRG can produce a high efficiency
drive by utilizing the proper amount of magnet and reluctance torque. This work
proposes a PMa-SynRG with two flux barriers and permanent magnets embedded in the
second layer of the rotor. A neodymium magnet (NdFeB) was used as permanent magnets in the rotor to prevent demagnetization. Finding the minimum amount of
magnet is one of the goals of the optimization process.
The objectives of this work are to build an optimal design for the 3kW generator
and an advanced power electronics converter for the PMa-SynRG drive system. In order
to find the optimized 3kW machine, a Lumped Parameter Model (LPM) was used to
achieve fast computation, and Differential Evolution Strategy (DES) was used to embed
the LPM in an efficient numerical optimization routine to identify optimum designs.
Finite Element Analysis (FEA) was used for test performance of optimum designs. On
the basis of differences between LPM and FEA, model predictions were used to fine
tune the LPM model. For new optimum design converges, numerical optimizations and
iterations were performed to produce LPM and FEA predictions.
For the drive system, the thyristor based, current-fed drive is much simpler and
has lower power losses compared to the pulse width modulation (PWM) drive.
Eliminating the requirement for self-controlled switches is a distinct advantage for lower
cost. Another feature of the developed current-fed drive is its inherent capability to
provide generating action by making the PMa-SynRG operates as a generator, rectifying
the phase voltages by means of the three-phase rectifier and feeding the power into the
load. These features make the current-fed drive a good candidate for driving any type of
synchronous generators including the proposed PMa-SynRG
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System optimization studies related to stator design and AC/AC converter selection for brushless doubly-fed machines
The advent of superior power semiconductor devices and converter topologies
has renewed interest in ac drive systems. Although considerable research efforts have
gone into improving power electronic converter devices and topologies, very little has
been reported on the overall performance optimization of induction machine drive
systems. The report of the research work presented in this thesis is an endeavor in that
direction where enhancement in the system performance is achieved through optimization
of the overall system.
The Brushless Doubly-Fed Machine (BDFM) can reduce the drive system cost
and also retain the robustness of a cage rotor induction machine. Proof-of-concept
prototypes have been used in the laboratory for investigation of BDFM operating modes.
These prototypes, though providing insight into the operation of the BDFM were far
from optimum. Thus, design procedures for optimizing the machine design needed to
be developed. The optimized machine can then be integrated into an optimized system
by using a realistic, application-dependent converter selection scheme.
As mentioned earlier, recent developments in power semiconductor and converter
technologies have led to a proliferation of circuit topologies and their modifications with
sometimes contradictory performance claims. Consequently, for the non-specialist
application engineer designing the BDFM system, this can often lead to uncertainty
which potentially can result in non-optimum converter selection. An extensive and
comprehensive literature review of presently available converter topologies is presented
with a detailed comparative evaluation. Converter selection criteria, as applied to the
BDFM, are also discussed in detail. These provide sufficient guidelines for selecting
an optimum technology for a given application.
Stator design optimization, as discussed in this thesis uses a design parameter
search algorithm and the BDFM steady state d-q model. Projected performance of an
optimized stator BDFM design is compared with simulation results of present and past
laboratory machines. An optimized design with good overall performance is presented.
It is also shown that this optimization scheme lowers the power rating requirement of
the converter.
Thus, for a given rotor structure and its parameters, the converter comparison and
selection scheme for BDFM applications along with the optimization scheme for the
stator design can lead to an overall optimized system with stator losses, reduced power
converter rating and thus lower the initial investment and operational costs
SCR-Based Wind Energy Conversion Circuitry and Controls for DC Distributed Wind Farms
The current state of art for electrical power generated by wind generators are in alternating current (AC). Wind farms distribute this power as 3-phase AC. There are inherent stability issues with AC power distribution. The grid power transfer capacity is limited by the distance and characteristic impedance of the lines. Furthermore, wind generators have to implement complicated, costly, and inefficient back-to-back converters to generate AC. AC distribution does not offer an easy integration of energy storage. To mitigate drawbacks with AC generation and distribution, direct current (DC) generation and high voltage direct current (HVDC) distribution for the wind farms is proposed. DC power distribution is inherently stable. The generators convert AC power to DC without the use of a back-to-back converter. DC grid offers an easy integration of energy storage.
The proposed configuration for the generator is connected to a HVDC bus using a 12 pulse thyristor network, which can apply Maximum Power Point Tracking (MPPT). To properly control the system, several estimators are designed and applied. This includes a firing angle, generator output voltage, and DC current estimators to reduce noise effects. A DSP-based controller is designed and implemented to control the system and provide gate pulses. Performance of the proposed system under faults and drive train torque pulsation are analyzed as well. Additionally, converter paralleling when turbines operate at different electrical power levels are also studied. The proposed new Wind Energy Conversion System (WECS) is described in detail and verified using MATLAB®/ Simulink® simulation and experimental test setup. The proposed solution offers higher reliability, lower conversion power loss, and lower cost. The following is proposed as future work:
1) Study different control methods for controlling the SCR\u27s.
2) Investigate reducing torque pulsations of the PMSG and using the proposed power conversion method for DFIG turbines.
3) Explore options for communication/control between PMSG, circuit protection and grid-tied inverters.
4) Investigate the best possible configuration for DC storage/connection to the HVDC/MVDC bus.
5) Study the filtering needed to improve the DC bus voltage at the generator
Advanced wind energy convertors using electronic power conversion.
SIGLEAvailable from British Library Document Supply Centre-DSC:DXN013000 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Optimised design of isolated industrial power systems and system harmonics
This work has focused on understanding the nature and impact of non-linear loads on isolated industrial power systems. The work was carried out over a period of 8 years on various industrial power systems: off-shore oil and gas facilities including an FPSO, a wellhead platform, gas production platforms, a mineral processing plant and an LNG plant. The observations regarding non-linear loads and electrical engineering work carried out on these facilities were incorporated into the report.A significant literature describing non-linear loads and system harmonics on industrial power systems was collected and reviewed. The literature was classified into five categories: industrial plants and system harmonics, non-linear loads as the source of current harmonics, practical issues with system harmonics, harmonic mitigation strategies and harmonic measurements.Off-shore oil and gas production facilities consist of a small compact power system. The power system incorporates either its own power generation or is supplied via subsea cable from a remote node. Voltage selection analysis and voltage drop calculation using commercially available power system analysis software are appropriate tools to analyse these systems. Non-linear loads comprise DC rectifiers, variable speed drives, UPS systems and thyristor controlled process heaters. All nonlinear loads produce characteristic and non-characteristic harmonics, while thyristor controlled process heaters generate inter-harmonics. Due to remote location, harmonic survey is not a common design practice. Harmonic current measurements during factory acceptance tests do not provide reliable information for accurate power system analysis.A typical mineral processing plant, located in a remote area includes its own power station. The power generation capacity of those systems is an order of magnitude higher than the power generation of a typical off-shore production facility. Those systems comprise large non-linear loads generating current and voltage interharmonics. Harmonic measurements and harmonic survey will provide a full picture of system harmonics on mineral processing plants which is the only practical way to determine system harmonics. Harmonic measurements on gearless mill drive at the factory are not possible as the GMD is assembled for the first time on site.LNG plants comprise large non-linear loads driving gas compressor, however those loads produce integer harmonics. Design by analysis process is an alternative to the current design process based on load lists. Harmonic measurements and harmonic survey provide a reliable method for determining power system harmonics in an industrial power system
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Gate-Turn-Off thyristor commutation of DC machines : The development of a rotating DC machine with static commutation of armature coil current using Gate-Turn-Off thyristor devices.
The thesis Is concerned with the development of a separately
excited DC machine In which gate turn-off thyristor devices with
their associated firing and protective circuits are used to provide
the static commutation of armature coil current. The developed
machine has Its armature winding with 24 tapping points located on
the stator and Interconnected In "Lop" configuration. The
Initiation of the conduction periods of armature switching devices
Is defined by a digital control logic circuit. In conjunction with
an Incremental rotary encoder which provides the necessary feedback
Information relating to shaft speed and shaft angular position.
This Is arranged such that, under normal running conditions of the
machine, the axis of the radial field of the armature winding
maintains the normal space-quadrature relationship with that of the
main field winding, giving the optimal torque angle of 000.
Provision Is made, however, within the digital control circuit for
controlled departure of the armature switch tapping points from the
quadrature axis positions, and the effect of this, In Improving
commutation Is Investigated. The effect of Interpoles Is also
explored. On the basis of the analysis carried out, a proposal Is
made for the future development of the machine employing a reduced
number of armature switching devices without the need for Interpole
windings.Electricity Directorate, Ministry of Works, Power and Water, Manama, Bahrain
Assessment of novel power electronic converters for drives applications
Phd ThesisIn the last twenty years, industrial and academic research has produced over one hundred new
converter topologies for drives applications. Regrettably, most of the published work has been
directed towards a single topology, giving an overall impression of a large number of
unconnected, competing techniques. To provide insight into this wide ranging subject area, an
overview of converter topologies is presented. Each topology is classified according to its
mode of operation and a family tree is derived encompassing all converter types. Selected
converters in each class are analysed, simulated and key operational characteristics identified.
Issues associated with the practical implementation of analysed topologies are discussed in
detail.
Of all AC-AC conversion techniques, it is concluded that softswitching converter topologies
offer the most attractive alternative to the standard hard switched converter in the power range
up to 100kW because of their high performance to cost ratio. Of the softswitching converters,
resonant dc-link topologies are shown to produce the poorest output performance although
they offer the cheapest solution. Auxiliary pole commutated inverters, on the other hand, can
achieve levels of performance approaching those of the hard switched topology while retaining
the benefits of softswitching. It is concluded that the auxiliary commutated resonant pole
inverter (ACPI) topology offers the greatest potential for exploitation in spite of its relatively
high capital cost.
Experimental results are presented for a 20kW hard switched inverter and an equivalent 20kW
ACPI. In each case the converter controller is implanted using a digital signal processor. For
the ACPI, a new control scheme, which eliminates the need for switch current and voltage
sensors, is implemented. Results show that the ACPI produces lower overall losses when
compared to its hardswitching counterpart. In addition, device voltage stress, output dv/dt and
levels of high frequency output harmonics are all reduced. Finally, it is concluded that
modularisation of the active devices, optimisation of semiconductor design and a reduction in
the number of additional sensors through the use of novel control methods, such as those
presented, will all play a part in the realisation of an economically viable system.Research Committee of the University of
Newcastle upon Tyn
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