用于分布式在线UPS中的并联逆变器的一种无线控制器
A Wireless Controller for Parallel Inverters in Distributed Online UPS Systems
Josep M. Guerrero', Luis Garcia de Vicufia", Jose Matas'*, Jaume Miret", and Miguel Castilla"
. Departament #Enginyeria de Sistemes, Automatica i Informhtica Industrial. Universitat Polithica de Catalunya
C. Comte d'Urgell, 187.08036 -Barcelona. Spain. Email: .. Departament #Enginyeria Electrbnica. Universitat Polit6cnica de Catalunya
AV. Victor BaLguer s/n. 08800I - Vilanova i la Geltrh. Spain
Absiract - In this paper, a novel controller for parallelconnected
online-UPS inverters without control wire
interconnections is presented. The wireless control technique is
based on the well-known droop method, which consists in
introducing P-oand Q-V schemes into the inverters, in order to
share properly the power drawn to the loads. The droop method
has been widely used in applications of load sharing between
different parallel-connected inverters. However, this method
has several drawbacks that limited its application, such as a
trade-off between output-voltage regulation and power sharing
accuracy, slow transient response, and frequency and phase
deviation. This last disadvantage makes impracticable the
method in online-UPS systems, since in this case every module
must be in phase with the utility ac mains. To overcome these
limitations, we propose a novel control scheme, endowing to the
paralleled-UPS system a proper transient response, strictly
frequency and phase synchronization with the ac mains, and
excellent power sharing. Simulation and experimental results
are reported confirming the validity of the proposed approach.
1. INTRODUCTION
The parallel operation of distributed Uninterruptible Power
Supplies (UPS) is presented as a suitable solution to supply
critical and sensitive loads, when high reliability and power
availability are required. In the last years, many control
schemes for parallel-connected inverters has been raised,
which are derived from parallel-schemes of dc-dc converters
[I], such as the master-slave control [2], or the democratic
control [3]. In contrast, novel control schemes have been
appeared recently, such as the chain-structure control [4], or
the distributed control [ 5 ] . However, all these schemes need
control interconnections between modules and, hence, the
reliability of the system is reduced since they can be a source
of noise and failures. Moreover, these communication wires
limited the physical situation ofthe modules [6].
In this sense, several control techniques has been proposed
without control interconnections, such as the droop method.
In this method, the control loop achieves good power sharing
making tight adjustments over the output voltage frequency
and amplitude of the inverter, with the objective to
compensate the active and reactive power unbalances [7].
This concept is derived from the power system theory, in
which the frequency of a generator drops when the power
drawn to the utility line increases [8].
0-7803-7906-3/03/$17.00 02003 IEEE. 1637
However, this control approach has an inherent trade-off
between voltage regulation and power sharing. In addition,
this method exhibits slow dynamic-response, since it requires
low-pass filters to calculate the average value of the active
and reactive power. Hence, the stability and the dynamics of
the whole system are hardly influenced by the characteristics
of these filters and by the value of the droop coefficients,
which are bounded by the maximum allowed deviations of
the output voltage amplitude and frequency.
Besides, when active power increases, the droop
characteristic causes a frequency deviation from the nominal
value and, consequently, it results in a variable phase
difference between the mains and the inverter output voltage.
This fact can be a problem when the bypass switch must
connect the utility line directly to the critical bus in stead of
its phase difference. In [9], two possibilities are presented in
order to achieve phase synchronization for parallel lineinteractive
UPS systems. The first one is to locate a particular
module near the bypass switch, which must to synchronize
the output voltage to the mains while supporting overload
condition before switch on. The second possibility is to wait
for the instant when phase matching is produced to connect
the bypass.
However, the mentioned two folds cannot be applied to a
parallel online-UPS system, since maximum transfer time
ought to be less than a % of line period, and all the modules
must be always synchronized with the mains when it is
present. Hence, the modules should be prepared to transfer
directly the energy from the mains to the critical bus in case
of overload or failure [lo].
In our previous works [11][12], we proposed different
control schemes to overcome several limitations of the
conventional droop method. However, these controllers by
themselves are inappropriate to apply to a parallel online-
UPS system. In this paper, a novel wireless control scheme is
proposed to parallel different online UPS modules with high
performance and restricted requirements. The controller
provides: 1) proper transient response; 2) power sharing
accuracy; 3) stable frequency operation; and 4) good phase
matching between the output-voltage and the utility line.
Thus, this new approach is especially suitable for paralleled-
UPS systems with true redundancy, high reliability and
power availability. Simulation and experimental results are
reported, confirming the validity of this control scheme.
Fig. 1. Equivalenl cimuif ofan invener connecled 10 a bus
t"
Fig. 2. P-odraop function.
11. REVlEW OF THE CONVENTIONAL DROOP METHOD
Fig. 1 shows the equivalent circuit of an inverter connected
to a common bus through coupled impedance. When this
impedance is inductive, the active and reactive powers drawn
to the load can be expressed as
EVcosQ - V2 Q=
where Xis the output reactance of an inverter; Q is the phase
angle between the output voltage of the inverter and the
voltage of the common bus; E and V are the amplitude of the
output voltage of the inverter and the bus voltage,
respectively.
From the above equations it can be derived that the active
power P is predominately dependent on the power angle Q,
while the reactive power Q mostly depends on the outputvoltage
amplitude. Consequently, most of wireless-control of
paralleled-inverters uses the conventional droop method,
which introduces the following droops in the amplitude E
and the frequency U of the inverter output voltage
u = w -mP (3)
E = E ' - n Q , (4)
being W* and E' the output voltage frequency and amplitude
at no load, respectively; m and n are the droop coefficients
for the frequency and amplitude, respectively.
Furthermore, a coupled inductance is needed between the
inverter output and the critical bus that fixes the output
impedance, in order to ensure a proper power flow. However,
it is bulky and increase:; the size and the cost of the UPS
modules. In addition, tho output voltage is highly distorted
when supplying nonlinezr loads since the output impedance
is a pure inductance.
It is well known that if droop coefficients are increased,
then good power sharing is achieved at the expense of
degrading the voltage regulation (see Fig. 2).
The inherent trade-off of this scheme restricts the
mentioned coefficients, which can be a serious limitation in
terms of transient response, power sharing accuracy, and
system stability.
On the other hand, lo carry out the droop functions,
expressed by (3) and (4), it is necessary to calculate the
average value over one line-cycle of the output active and
reactive instantaneous power. This can be implemented by
means of low pass filters with a smaller bandwidth than that
of the closed-loop inverter. Consequently, the power
calculation filters and droop coefficients determine, to a large
extent, the dynamics and the stability of the paralleledinverter
system [ 131.
In conclusion, the droop method has several intrinsic
problems to be applied 1.0 a wireless paralleled-system of
online UPS, which can he summed-up as follows:
Static trade-off between the output-voltage regulation
(frequency and amplitude) and the power-sharing
accuracy (active an4d reactive).
2) Limited transient response. The system dynamics
depends on the power-calculation filter characteristics,
the droop coefficients, and the output impedances.
Lost of ac mains synchronization. The frequency and
phase deviations, due to the frequency droop, make
impracticable this method to a parallel-connected
online UPS system, in which every UPS should be
continuously synchronized to the public ac supply.
1)
3)
111. PROPOSED CONTROL FOR PARALLEL ONLINE UPS
INVERTERS
In this work, we will try to overcome the above limitations
and to synthesize a novel control strategy without
communication wires that could be appropriate to highperformance
paralleled industrial UPS. The objective is to
connect online UPS inverters in parallel without using
control interconnections. This kind of systems, also named
inverter-preferred, should be continuously synchronized to
the utility line. When an overload or an inverter failure
occurs, a static bypass switch may connect the input line to
the load, bypassing the inve:rter [14][15].
Fig. 3 shows the general diagram of a distributed online
UPS system. This system consists of two buses: the utility
bus, which is connected lo the public ac mains; and the
secure bus, connected to the distributed critical loads. The
interface between these buses is based on a number of online
UPS modules connected in parallel, which provides
continuously power to the: loads [16]. The UPS modules
include a rectifier, a set of batteries, an inverter, and a static
bypass switch.
1
1638
Q ac mains
utility bus
I I I
j distributed loads !
Fig. 3. Online distributed UPS system.
syposr /
I 4
(4
Fig. 4. Operation modes of an online UPS.
(a) Normal operation. (b) Bypass operation. (c) Mains failure
The main operation modes of a distributed online UPS
1) Normal operation: The power flows to the load, from
the utility through the distributed UPS units.
2) Mains failure: When the public ac mains fails, the
UPS inverters supply the power to the loads, from the
batteries, without disruption.
Bypass operation: When an overload situation occurs,
the bypass switch must connect the critical bus
directly to the ac mains, in order to guarantee the
continuous supply of the loads, avoiding the damage
of the UPS modules.
For this reason, the output-voltage waveform should be
synchronized to the mains, when this last is present.
system are listed below (see Fig. 5):
3)
Nevertheless, as we state before, the conventional droop
method can not satisfy the need for synchronization with the
utility, due to the frequency variation of the inverters, which
provokes a phase deviation.
To obtain the required performance, we present a transient
P-w droop without frequency-deviation in steady-state,
proposed previously by OUT in [ 111
w=o -mP (5)
where is the active power signal without the dccomponent,
which is done by
. -
I t -1s
P= p ,
( s + t - ' ) ( s + o , )
being zthe time constant of the transient droop action.
The transient droop function ensures a stable frequency
regulation under steady-state conditions, and 'at the same
time, achieves active power balance by adjusting the
frequency of the modules during a load transient. Besides, to
adjust the phase of the modules we propose an additional
synchronizing loop, yielding
o=w'-m%k,A$, (7)
where A$ is the phase difference between the inverter and the
mains; and k, is the proportional constant of the frequency
adjust. The steady-state frequency reference w* can be
obtained by measuring the utility line frequency.
The second term of the previous equality trends to zero in
steady state, leading to
w = w' - k4($ -@'), (8)
being $and $* the phase angles of the output voltage inverter
and the utility mains, respectively.
Taking into account that w = d $ / d t , we can obtain the
next differential equation, which is stable fork, positive
d$ *
dt dt
- + km$ = - + k,$' . (9)
Thus, when phase difference increases, frequency will
decrease slightly and, hence, all :he UPS modules will be
synchronized with the utility, while sharing the power drawn
to the loads.
IV. CONTROLLIEMRP LEMENTATION
Fig. 5 depicts the block diagram of the proposed
controller. The average active power P , without the dc
component, can be obtained by means of multiplying the
output voltage by the output current, and filtering the product
........................................................................................
io
",.
L
Sj'nchronirorion loop
.......................................................................................
Fig. 5. Block diagram of the proposed controller.
using a band-pass filter. In a similar way, the average
reactive power is obtained, hut in this case the output-voltage
must be delayed 90 degrees, and using a low-pass filter.
In order to adjust the output voltage frequency, equation
(7) is implemented, which corresponds to the frequency
mains drooped by two transient-terms: the transient active
power signal term; and the phase difference term, which
is added in order to synchronize the output voltage with the
ac mains, in a phase-locked loop (PLL) fashion. The outputvoltage
amplitude is regulated by using the conventional
droop method (4).
Finally, the physical coupled inductance can be avoided by
using a virtual inductor [17]. This concept consists in
emulated an inductance behavior, by drooping the output
voltage proportionally to the time derivative of the output
current. However, when supplying nonlinear loads, the highorder
current-harmonics can increase too much the outputvoltage
THD. This can be easily solved by using a high-pass
filter instead of a pure-derivative term of the output current,
which is useful to share linear and nonlinear loads [I 1][12].
Furthermore, the proper design of this output inductance can
reduce, to a large extent, the unbalance line-impedance
impact over the power sharing accuracy.
v. SIMULATION AND EXPERIMENTARELS ULTS
The proposed control scheme, (4) and (7), was simulated
with the parameters listed in Table 1 and the scheme shown
in Fig. 6, for a two paralleled inverters system. The
coefficients m, n, T, and kv were chosen to ensure stability,
proper transient response and good phase matching. Fig. 7
shows the waveforms of the frequency, circulating currents,
phase difference between the modules and the utility line,
and the evolution of the active and reactive powers. Note the
excellent synchronization between the modules and the
ACmiiinr 4 j. ...L...I.P...S...1... ..........................B...u...n...r.r..r..e..s... ................................... i
Fig. 6. Parallel operation oftwa online UPS modules,
mains, and, at the same time, the good power sharing
obtained. This characteristik let us to apply the controller to
the online UPS paralleled systems.
Two I-kVA UPS modules were built and tested in order to
show the validity of the proposed approach. Each UPS
inverter consisted of a single-phase IGBT full-bridge with a
switching frequency of 20 kHz and an LC output filter, with
the following parameters: 1. = 1 mH, C = 20 WF, Vi" = 400V,
v, = 220 V, I50 Hz. The controllers of these inverters were
based on three loops: an inner current-loop, an outer PI
controller that ensures voltage regulation, and the loadsharing
controller, based on (4) and (7). The last controller
was implemented by means of a TMS320LF2407A, fixedpoint
40 MHz digital sigrial processor (DSP) from Texas
Instruments (see Fig. 8), using the parameters listed in Table
I. The DSP-controller also includes a PLL block in order to
synchronize the inverter with the common bus. When this
occurs, the static bypass switch is tumed on, and the droopbased
control is initiated.
1640
big 7 Wa\cfc)rms for twu.invencr, ;mnectcd in parallel. rpchrontred io Ihc ac mdnl.
(a) Frequencics ufhoth UPS (b) Clrculattng currcni among modulcs. (CJ Phmc d!Nercn;: betucen ihc UPS a#>dth e ai mum
(d) Ikiril uf the phze diNmncc (e) md (0 Activc and rcactlw pouerr "I ooih UPS
Note that the iimc-acs arc deliheratcly JiNercni due in thc disiinct timuion*uni) ofthe \ inrblrr
1641
TABLEI.
PARAMETEROSF THE PARALLELESDYS TEM.
Filter Order I I
Filter Cut-off Frequency I 0, I 10 I rags
Fig. 8 shows the output-current transient response of the
UPS inverters. First, the two UPS are operating in parallel
without load. Notice that a small reactive current is circling
between the modules, due to the measurement mismatches.
Then, a nonlinear load, with a crest factor of 3, is connected
suddenly. This result shows the good dynamics and loadsharing
of the paralleled system when sharing a nonlinear
load.
Fig. 8. Output current for the two paralleled UPS, during the connection of B
common nonlinear load with a crest factor of 3. (Axis-x: 20 mddiv. Axis-y:
5 Mdiv.).
VI. CONCLUSIONS
In this paper, a novel load-sharing controller for parallelconnected
online UPS systems, was proposed. The controller
is based on the droop method, which avoids the use of
control interconnections. In a sharp contrast with the
conventional droop method, the controller presented is able
to keep the output-voltage frequency and phase strictly
synchronized with the utility ac mains, while maintaining
good load sharing for linear and nonlinear loads. This fact let
us to extend the droop method to paralleled online UPS.
On the other hand, the proposed controller emulates a
special kind of impedance, avoiding the use of a physical
coupled inductance. Th.e results reported here show the
effectiveness of the proposed approach.
电气工程：
1
Electrical Engineering
My decision to pursue graduate study in the United States is underscored by my desire to be a part of the graduate program at your institution. Purdue University offers the flexibility needed for such a vast and rapidly changing field. The research facilities and the faculty at the university are par excellent.
Communications is an industry that has changed our lives. In a very short period it has changed the way we have looked at things since centuries. It is one industry that is going to shape our future for centuries to come. Hence my desire to do masters in electrical engineering with communications as my major.
My interest in electronics blossomed during my high school years. It was the time when technology had begun to make an impact on the lives of people in India. Hence engineering with electronics as my major was the first choice for my undergraduate studies. Right since the beginning of my undergraduate study electronics is a subject that has fascinated me with its power of applications. The subjects that I have studied include Linear Electronics, Digital Electronics. These laid the foundation for my courses in Electronic Communication & Communication Systems at a later stage. My undergraduate studies already focus on the communications aspect of electronics. A masters degree in electrical engineering with communications as major field is the next logical step.
For the past four months I have been working as a project trainee at the Indian Institute for Advanced Electronics. I am working on the design and development of a "PC Controlled Digital Serial Data Generator". This short stint has given me invaluable practical experience. It has given me the confidence to pursue a masters degree and also kindled a desire to do research.
During the course of my work at IIAE, I have come across several scientists. Most of them work in different areas of communications. Interactions with them have made me realize the vastness and the scope of communications. My discussions with them convinced me that specializing in communications will suit me very well.
The subject of research which interests me very much is spread spectrum communication systems. Coding theory and combinations is another research subject which arouses my curiosity. The subject Communication Theory which I am studying at present introduces these topics in theory. I am eager to find out more about the applications of coding theory to spread spectrum communication systems.
In addition I have been a student member of the IEEE (Institute of Electrical and Electronics Engineers, Inc.) for the past three years. Through its workshops/seminars and publications like the 'The Spectrum' it has exposed me to a lot of emerging technologies in the field of communications.
It is a strong belief in my family that the American education system has the best to offer in the whole world. This belief arises out of the experience that my parents had when they did their Masters of Science in the University of Pennsylvania during the years 1967-69. If I can get an opportunity to be a part of that intellectually stimulating environment, I am sure my talents will be put to optimal use.
India is a developing country with an enormous potential in the information technology business. To serve the needs of this developing industry and more important its vast population, communications is going to become of utmost importance. Thus conditions here are very conducive to supplement my aspirations when I return after completing my graduate studies.
2
Electrical Engineering
As a graduate student, I will undertake research and coursework in Electrical Engineering to enhance my competencies in this field. I intend to complete my master's degree in order to pursue my doctorate. The research that I am most interested in pursuing at Northeastern University surrounds the optical properties of MEMS devices, and the development of substrate-based fast electro-optical interfaces. My interest in this area stems from my undergraduate study in MEMs development for tri-axial accelerometers.
Engineering has been a key interest of mine since childhood. While still in grade school I enjoyed listening to my father, an electrical engineer, teach me about advances in technology, and was always eager to hear more. I was introduced to my first computer at the age of five, and have loved interacting with them ever since. My decision to study engineering as a career was no surprise to those who knew me.
In college I found that I was always studying something I enjoyed. I believe it is because I enjoy my life and my work that I have been successful. Spending hours in the laboratory is not something that I dread, but instead I take pride in my work and its successful completion. One example of this that is still fresh in my mind is the successful design of a fully functional microprocessor in the Xilinx environment. All told, the project took over 150 hours of each design-team member's time. However, I did not look on it as a drain, but an experience for learning and a focus for my professional and technical development. When we finished the project we felt the sense of worth and pride in completion of a task that was once above our level of knowledge.
Pursuing a graduate degree in the research field I have chosen also feels like a challenge, and I know that study will frustrate me at times. However, I feel that my commitment to learning will not be swayed. I feel confident in my ability to be creative in my perspective, and to persevere. My ultimate goal is to be an innovator in the field I have chosen to study. Professionalism and creativity are my most valued strengths.
At the heart of my interest is the advancement of man in concert with his environment. My personal philosophy of life will matter greatly during my study and after its completion. That is why I devote time to reflection on my goals and their implications. Money has never been a motivator for my work, nor do I think it will be in the future. However, as a professional and a graduate, I realize that my earning potential will be significant. That is why I also commit myself to charity and fairness. In the past I have been a member of the Boy Scouts of America, and have achieved the rank of Eagle Scout. In the course of my experience in that organization, I learned respect and moral value. Now, as a member of the IEEE, I value my professional standing and its commensurate moral implications. Ethics in engineering is as important as technical skill, and as such I intend to uphold my own ethical obligations to the best of my ability.
As a Northeastern University student, I would commit all that I have to offer to my study. I intend to pursue research in MEMS technology. At Rowan University as an undergraduate student I have already conducted some research and development of MEMS sensors for military applications, resulting in publication. An article, written by myself and my project member David Bowen and edited by our advisor Dr. Robert Krchnavek, was published in the NAVSEA Intelligent Ships Symposium Proceedings of 2001. The paper was titled "Designing a 3-Axis, Monolithic, MEMS-Based Accelerometer" and was under review for endorsement by the US Navy's NAVSEA facility in Philadelphia during that year.
Building on my past success in MEMs design, I hope to advance my understanding. Through research at the graduate level, it is my hope to become familiar with, and innovate the design of MEMs Optics in hopes of creating a reliable and practical MEMs Electro-Optical Interface for use in consumer electronics. It is my hope, that through my research, optical waveguides for intradevice communication might be realized.
Finally, my intent to pursue graduate study is laid plain. Study of MEMs optics is my intended focus, and I am committed to my goal. In pursuing a doctoral degree, I have closely analyzed myself to determine the reasons for my previous successes and my goals for the future. I have found that I do and have always enjoyed engineering, and that I have a strong desire to pursue my study further. I am prepared to commit myself to that study, and achieve what I have set out to do.
3
I Wish to Pursue an MS Degree in Electrical Engineering
During my senior year at Purdue University, I made a decision that has impacted the entire course of my education. While my classmates were making definite decisions about their career paths, I chose to implement a five-year plan of development and growth for myself. I designed this plan in order to examine various careers that I thought might interest me, as well as to expand upon my abilities at the time. As I was attaining a BS degree in Electrical Engineering, I decided to focus primarily on fields related to the VLSI (Very Large-Scale Integrated) circuits area. My main goals were either to gain work experience or to further my education by pursuing an MS degree in Electrical Engineering (MSEE). I saw an opportunity to both work and learn through employment at Xilinx Inc. Operating as a product engineer at a successful, high-tech semiconductor company has enabled me to utilize my technical and interpersonal skills in new and challenging ways. The position has also allowed me to interact with a multitude of departments including marketing, integrated circuit (IC) design, software/CAD development, manufacturing, reliability, accounting, and sales. I thus have gained an array of experience that extended beyond the parameters of my own responsibilities. In the workplace, I rely heavily upon the interpersonal techniques I developed as a counselor in a Purdue residence hall, as well as the organizational skills I had acquired through holding various leadership positions in cultural and engineering societies. I have also cultivated an interest in high-technology marketing that has continued to grow throughout my career.
My experiences with Xilinx have heightened my hunger for knowledge in the VLSI field. Two months after joining the corporation, I applied to several part-time programs in the vicinity that would allow me to acquire an MSEE degree within two to three years. San Jose State seemed an ideal choice, for its evening MSEE courses would allow me to pursue two independent, full-time positions concurrently. The San Jose program has complimented my Xilinx duties well; both demand large levels of energy and enthusiasm while guiding me to my ultimate goal a high degree of education in VLSI sciences. The resources that I poured into both endeavors have reaped many gains. I have been promoted to a Product-Yield Engineering position within Xilinx's Coarse Grain Static Memory (CGSM) Product Engineering division. My extensive coursework plays a key role in my continued success at Xilinx. Relevant classes in advanced digital and analog VLSI design, as well as sub-micron ULSI technology, have allowed me to understand more completely the workings of Xilinx, a fab-less semiconductor company that also functions as a software and hardware design, testing, and marketing center. The gains in knowledge I have made through the combination of work experience and education have indeed been exponential.
The academic records of my senior year at Purdue, coupled with my MSEE coursework, are ample proof of my dedication to learning. I feel I have overcome through hard work and dedication the brief "dry phase" I underwent at Purdue during the close of my sophomore and the first semester of my junior years. My performance at that time is in no way indicative of my usual achievements; they are instead the result of urgent family difficulties that required much foreign travel and serious attention to resolve. In May, I shall graduate with an MSEE degree from San Jose well ahead of my original estimates. This early graduation with Dean's Honors is the result of my firm belief in the value of diligence, as well as my renewed determination to strive for perfection in both work and school.
I am now embarking on another five-year plan, during which I hope to fulfill several specific career goals. For instance, being part of a very dynamic and results-oriented Yield team at Xilinx calls for continuous development of computational and statistical techniques. The Yield team is divided to focus on specific process/fabrication issues and process (manufacturing) optimization. My own position is an integral part of the optimization group. Speed and cost issues continue to press high technology atmospheres towards optimization, probability and stochastic processes and systems, and rigorous simulations of mathematical models. The MS in EES&OR offered at your university will grant me the statistical knowledge that is crucial for process and production optimization in a fab-less environment. In addition, product engineering requires fundamental research on mathematical models for linear and non-linear programming, as well as the utilization of efficient computer software. I continuously employ the knowledge I gained at Purdue in Operations Research and advanced mathematics courses. Yet despite the value of these classes and my high performance in them, I now require further education to best fulfill my duties. An MS in the EES&OR field, will give me knowledge that is invaluable to a career in product development, project management and strategic planning. The program will allow me to improve decision-making skills in operations, strategy, and policy issues. I will strengthen my theory and application in countless areas:continuous, discrete, numerical optimization; probabilistic and stochastic processes; dynamic systems and simulation; economics, finance, and investment; decision analysis; dynamic programming and planning under uncertainty; operations and service; corporate and individual strategy; and private and public policy issues.Thus, the EES&OR program will not only help me to excel at Xilinx but will also further any future career. My commitment to work and education over the last three years proves that I will pursue this MS with enthusiasm and zeal.The technical edge that the MS would provide is incomparable.Since I will be working while attending Stanford, I shall mingle education with practical application, and bring to the table interesting problems from my experience and past education.
Technical challenges encountered through projects in the EES&OR program will provide motivation and opportunity for methodological innovation.The data collection, processing and presentation issues presented are integral to my future goals, and the management challenges raised will provide invaluable experience for professional practice. This will in turn build a solid foundation for a life-long career that can overcome any problem in decision-making. In addition, taking courses in economics, finance, and investment analysis will allow much growth of knowledge in investment issues in different industries. The EES&OR program thus appeals not only to my engineering, economics, science and mathematical background, but will compliment my technical abilities with the conceptual frameworks needed to analyze problems in operations, production, strategic planning, and marketing in the realm of emiconductor/IC/engineering systems. I feel that I am prepared to meet the challenges of the curriculum. My coursework in intermediate microeconomics and macroeconomics, international trade, operations research, linear algebra, and probabilistic methods, along with my extensive calculus background, will allow me to function well within the program.
My long-term career goals include a move into marketing and product management. I believe that attaining this MS degree is the cornerstone to achieving my goals. It will give me the academic background necessary to succeed in product development, project management, and strategic planning. It will improve decision-making skills necessary for optimizing performance. The integration of two excellent programs in Economics Systems and Operations Research thus suits my current position and ties in with future goals perfectly by improving decision making in operations, strategy and policy. At present I desire to continue at Xilinx; attending a program that provides the flexibility and convenience of the SITN, is therefore imperative. Hence, being at Stanford as an HCP student alsoattracts me. I believe that Stanford is the best environment for me to achieve my goals while gaining exposure to and experience with a diverse student body and faculty. It is my belief that one continues to learn throughout one's life, and the most effective method of learning is through interaction with others.Stanford's diversity offers an environment for learning, both inside and outside the classroom. I hope to share my varied knowledge with my classmates and to take from them a new understanding of topics that are foreign to me. I believe that no other school provides students with the combination of education and environment offered by Stanford. Its outstanding academic reputation, mingled with its diverse environment and thriving Bay Area location, creates an opportunity for growth that is second to none. I have many ambitions for myself as I embark on this stage of my life. I believe that an education from Stanford will provide invaluable experiences and skills that will allow me to become a successful and innovative business leader in the new millennium.
4
Research Department of Biomedical Engineering is designed to research on and solve the bio-electrical and biomagnetic engineering problems in the field of biology and medicine with the aid of engineering principles and methods. Its main task is to explain, from perspective view of engineering, the biological and pathologic processes of the living organisms, especially human beings, and research on and develop the related medical devices and life science devices. Its research directions mainly include the modeling and emulation of the biological system, testing and analysis of biomedical signals, the biomedical imaging and processing , the biological effects of electromagnetic field and the development of artificial organs and medical devices, etc.
Electromagnetic Bioengineering
With the development and integration of electromagnetism, biology and medicine, biological electromagnetism exercises more and more influence on human life and health, environment protection and biological engineering. The research on electromagnetic bioengineering is a new research direction for IEECAS, mainly including research on rules of mutual influence between electromagnetic field and life matter, biological electromagnetic effect and its application in biology, medicine and medical equipment. At present, the research team has set up labs such as biological electromagnetic environment lab, biological electromagnetic signals & electromagnetic property testing lab, electromagnetic biological effect testing lab and biological electromagnetic simulation lab. It is equipped with various electrical and magnetic fields for experiments of biological electromagnetic effects, simulation software and biochemical experiment equipment. With such equipments, it can do biological electromagnetic experiments on live animals and detached live cells, detect, analyze and process the very weak biological electromagnetic signals, analyze and test live organism or detached cell under electromagnetic interaction with biochemical quantitative methods. The recent research work focuses on the effects
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Electric Power Systems 电力系统
The modern society depends on the electricity supply more heavily than ever before. 现代社会的电力供应依赖于更多地比以往任何时候。 It can not be imagined what the world should be if the electricity supply were interrupted all over the world. 它无法想象的世界应该是什么，如果电力供应中断了世界各地。 Electric power systems (or electric energy systems), providing electricity to the modern society, have become indispensable components of the industrial world. 电力系统（或电力能源系统），提供电力到现代社会，已成为不可缺少的组成部分产业界的。
The first complete electric power system (comprising a generator, cable, fuse, meter, and loads) was built by Thomas Edison – the historic Pearl Street Station in New York City which began operation in September 1882. 第一个完整的电力系统（包括发电机，电缆，熔断器，计量，并加载）的托马斯爱迪生所建-站纽约市珍珠街的历史始于1882年9月运作。 This was a DC system consisting of a steam-engine-driven DC generator supplying power to 59 customers within an area roughly 1.5 km in radius. The load, which consisted entirely of incandescent lamps, was supplied at 110 V through an underground cable system. 这是一个半径直流系统组成的一个蒸汽发动机驱动的直流发电机面积约1.5公里至59供电范围内的客户。负载，其中包括完全的白炽灯，为V提供110通过地下电缆系统。 Within a few years similar systems were in operation in most large cities throughout the world. With the development of motors by Frank Sprague in 1884, motor loads were added to such systems. This was the beginning of what would develop into one of the largest industries in the world. In spite of the initial widespread use of DC systems, they were almost completely superseded by AC systems. By 1886, the limitations of DC systems were becoming increasingly apparent. They could deliver power only a short distance from generators.
在一个类似的系统在大多数大城市在世界各地运行数年。随着马达的弗兰克斯普拉格发展在1884年，电机负载被添加到这些系统。这是什么开始发展成为世界上最大的产业之一。在最初的直流系统广泛使用尽管如此，他们几乎完全被空调系统所取代。到1886年，直流系统的局限性也日益明显。他们可以提供功率只有很短的距离从发电机。
To keep transmission power losses ( I 2 R ) and voltage drops to acceptable levels, voltage levels had to be high for long-distance power transmission. Such high voltages were not acceptable for generation and consumption of power; therefore, a convenient means for voltage transformation became a necessity. 为了保持发射功率损失（我2 R）和电压下降到可接受的水平，电压等级，必须长途输电高。如此高的电压不发电和电力消耗可以接受的，因此，电压转换成为一个方便的手段的必要性。
The development of the transformer and AC transmission by L. Gaulard and JD Gibbs of Paris, France, led to AC electric power systems. 在发展的变压器，法国和交流输电由L.巴黎戈拉尔和JD吉布斯导致交流电力系统。
In 1889, the first AC transmission line in North America was put into operation in Oregon between Willamette Falls and Portland. 1889年，第一次在北美交流传输线将在俄勒冈州波特兰之间威拉梅特大瀑布和实施。
It was a single-phase line transmitting power at 4,000 V over a distance of 21 km. With the development of polyphase systems by Nikola Tesla, the AC system became even more attractive. By 1888, Tesla held several patents on AC motors, generators, transformers, and transmission systems. Westinghouse bought the patents to these early inventions, and they formed the basis of the present-day AC systems.这是一个单相线路传输功率为4,000公里，超过21 V系统的距离。随着交流的发展多相系统由尼古拉特斯拉，成为更具吸引力的。通过1888年，特斯拉举行交流多项专利电动机，发电机，变压器和输电系统。西屋公司购买了这些早期的发明专利，并形成了系统的基础，现在的交流。
In the 1890s, there was considerable controversy over whether the electric utility industry should be standardized on DC or AC. By the turn of the century, the AC system had won out over the DC system for the following reasons: 在19世纪90年代，有很大的争议或交流电力行业是否应该统一于直流。到了世纪之交的，在交流系统赢得了原因出在下面的直流系统为：
（1）Voltage levels can be easily transformed in AC systems, thus providing the flexibility for use of different voltages for generation, transmission, and consumption. （1）电压水平可以很容易地改变了空调系统，从而提供了传输的灵活性，发电用不同的电压和消费。
（2）AC generators are much simpler than DC generators. （2）交流发电机简单得多比直流发电机。
（3）AC motors are much simpler and cheaper than DC motors. （三）交流电机和电机便宜简单得多，比直流。
The first three-phase line in North America went into operation in 1893——a 2,300 V, 12 km line in southern California. 前三个阶段的美国北线投产于1893年- 1 2300五，南加州12公里路线研究。 In the early period of AC power transmission, frequency was not standardized. 在电力传输初期交流，频率不规范。 Many different frequencies were in use: 25, 50, 60, 125, and 133 Hz. 有许多不同频率的使用：25，50，60，125，和133赫兹。 This poses a problem for interconnection. Eventually 60 Hz was adopted as standard in North America, although 50 Hz was used in many other countries. 这对互连的问题。最后60赫兹标准获得通过，成为美国在北美，虽然是50赫兹在许多其他国家使用。
The increasing need for transmitting large amounts of power over longer distance created an incentive to use progressively high voltage levels. To avoid the proliferation of an unlimited number of voltages, the industry has standardized voltage levels. In USA, the standards are 115, 138, 161, and 230 kV for the high voltage (HV) class, and 345, 500 and 765 kV for the extra-high voltage (EHV) class. In China, the voltage levels in use are 10, 35, 110 for HV class, and 220, 330 (only in Northwest China) and 500 kV for EHV class . 较长的距离越来越需要大量的电力传输多激励他们逐步使用高压的水平。为了避免电压增殖数量无限，业界标准电压水平。在美国，标准是115，138， 161，和230千伏的高电压（高压）类，345，500和765千伏级的特高电压（超高压）。在中国，各级使用电压为10，35，110级高压， 220，中国330（仅在西北）和500千伏超高压类。
The first 750 kVtransmission line will be built in the near future in Northwest China. 第一个750 kVtransmission线将建在不久的将来在中国西北地区。
With the development of the AC/DC converting equipment, high voltage DC (HVDC) transmission systems have become more attractive and economical in special situations. 随着交流的发展/直流转换设备，高压直流高压直流（HVDC）传输系统已经成为更具吸引力的经济和情况特殊。 The HVDC transmission can be used for transmission of large blocks of power over long distance, and providing an asynchronous link between systems where AC interconnection would be impractical because of system stability consideration or because nominal frequencies of the systems are different. 在高压直流输电可用于输电块以上的大长途电话，并提供不同系统间的异步连接在AC联网系统将是不切实际的，因为稳定考虑，或因标称频率的系统。
The basic requirement to a power system is to provide an uninterrupted energy supply to customers with acceptable voltages and frequency. 基本要求到电源系统是提供一个不间断的能源供应，以客户可接受的电压和频率。 Because electricity can not be massively stored under a simple and economic way, the production and consumption of electricity must be done simultaneously. A fault or misoperation in any stages of a power system may possibly result in interruption of electricity supply to the customers. 由于电力无法大量储存在一个简单的方法和经济，电力的生产和消费必须同时进行。系统的故障或误操作的权力在任何阶段可能导致电力供应中断给客户。 Therefore, a normal continuous operation of the power system to provide a reliable power supply to the customers is of paramount importance. 因此，一个正常的电力系统连续运行的，提供可靠的电力供应给客户的重要性是至关重要的。
Power system stability may be broadly defined as the property of a power system that enables it to remain in a state of operating equilibrium under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a disturbance. 电力系统稳定，可广泛定义为干扰财产的权力系统，可继续经营的状态下正常运行的平衡条件和后向遭受恢复一个可以接受的平衡状态。
Instability in a power system may be manifested in many different ways depending on the system configuration and operating mode. 在电力系统的不稳定可能会表现在经营方式和多种不同的方式取决于系统配置。
Traditionally, the stability problem has been one of maintaining synchronous operation. Since power systems rely on synchronous machines for generation of electrical power, a necessary condition for satisfactory system operation is that all synchronous machines remain in synchronism or, colloquially "in step". This aspect of stability is influenced by the dynamics of generator rotor angles and power-angle relationships, and then referred to " rotor angle stability ". 传统上，稳定性问题一直是一个保持同步运行。由于电力系统的发电电力，一个令人满意的系统运行的必要条件是，依靠同步电机同步电机都留在同步或通俗的“步骤”。这一方面是受稳定的发电机转子的动态角度和功角的关系，然后提到“转子角稳定”。
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