1. 1
A Project Report on
HIGH TEMEPERATURE
SUPERCONDUCTIVITY IN ELECTRICAL
POWER APPLICATIONS; A DETAILED STUDY
OF HTSC SYNCHRONOUS MACHINE &
EXPERIMENTATION ON 1ST
GENERATION
AND 2ND
GENERATION HTSC WIRES
Submitted by
Srinagadatta Srikrishna R. (Reg. No. 011205146)
In partial fulfillment for the award of the degree of
Bachelor of Technology
in
Electrical and Electronics Engineering
Under the esteemed guidance of
Mr. B. V. A. S. Muralidhar Mr. R. Rajesh
Sr. Engineer (EMC) Assistant Professor III
BHEL Corporate R&D SEEE, SASTRA University
School of Electrical & Electronics Engineering
SASTRA University
Thanjavur, India – 613 401
April, 2012
2. 2
BONAFIDE CERTIFICATE
Certified that the project work entitled “HIGH TEMPERATURE
SUPERCONDUCTIVITY IN ELECTRICAL POWER APPLICATIONS;
A DETAILED STUDY OF HTSC SYNCHRONOUS MACHINE &
EXPERIMENTATION ON 1ST
GENERATION AND 2ND
GENERATION
HTSC WIRES” submitted to SASTRA University, Thanjavur by
SRINAGADATTA SRIKRISHNA R (Reg. No:011205146), in partial
fulfillment for the award of the degree of Bachelor of Technology in Electrical
and Electronics Engineering is the work carried out independently under my
guidance during the period Jan 2012 – April 2012.
Project Guide
[Project Guide Name]
[Designation & Affiliation]
External Examiner Internal Examiner
Submitted for the University Exam held on ___________
3. 3
DECLARATION
I submit this project work entitled “HIGH TEMPERATURE SUPERCONDUCTIVITY
IN ELECTRICAL POWER APPLICATIONS; A DETAILED STUDY OF HTSC
SYNCHRONOUS MACHINE & EXPERIMENTATION ON 1ST
GENERATION
AND 2ND
GEENRATION HTSC WIRES” to SASTRA University, Thanjavur in partial
fulfillment of the requirements for the award of the degree of “Bachelor of Technology” in
“Electrical and Electronics Engineering”. I declare that it was carried out independently
by me under the guidance of Mr. B. V. A. S. Muralidhar, Senior Engineer, BHEL Corporate
R&D, Vikasnagar, Hyderabad - 500093
SRINAGADATTA SRIKRISHNA R (Reg. No. :011205146 )
Date: Signature:
Place:
4. 4
ACKNOWLEDGEMENTS
I wish to thank SASTRA University for providing me this opportunity to carry
out my project work at BHEL Corporate R&D and for supporting my work.
I also thank BHEL Corporate R&D, Hyderabad for granting me permission to
carry out my project work under their aegis.
I wish to express my heartfelt gratitude to Mr. M Seetaram, Additional General
Manager (AGM), Electrical Machines Laboratory (EMC), BHEL Corporate
R&D for being with me through the entire tenure of the project providing both
technical and non – technical guidance required to initiate, to progress through
and to complete this project. Without his guidance, this work would not have
taken place.
I thank my guide Mr. B. V. A. S. Muralidhar, Senior Engineer, Electrical
Machines Laboratory (EMC), BHEL Corporate R&D, Hyderabad for giving me
this project and for guiding me throughout the project tenure. I am very grateful
to him for identifying my capabilities and entrusting me with this project. It has
been a very great learning experience under his guidance
I also wish to thank Mr. S. Ramacharyulu, Additional Engineer, Mr. Ramesh,
Draftsman and Mr. Prakash, Junior Engineer, Electrical Machines Laboratory
for patiently entertaining my technical queries and for the invaluable technical
discussions that I had with them.
I wish to thank Dr. J. L. Bhattacharya, Ex – AGM, EMC, BHEL Corporate
R&D for being a source of inspiration to me. The few interactions that I had the
opportunity of having with him gave me a deep insight not only into technical
aspects, but also were enlightening on the aspects of work process, research
methodologies, professionalism and also about life.
I also wish to thank my internal guide Prof. R. Rajesh, Assistant Professor III,
SEEE, SASTRA University for being a great help and encouragement.
I also thank Dr. Umakant Choudhary, GM(PEC & HR), BHEL Corporate R&D
for his kind help and concern.
Finally I wish to thank my parents, friends and all those who associated with me
over the project tenure, inspired me, advised me and helped me at BHEL
Corporate R&D and at SASTRA University.
5. 5
ABSTRACT
This report is a study on High Temperature Superconductivity (HTSC) applications in
electrical power industry with special focus on the HTSC Synchronous Motor. A basic
theoretical treatise on superconductivity is first presented followed by a detailed
discussion on the 1st
generation and 2nd
generation HTSC wires. The manufacturing
procedures and technologies involved have been discussed in detail. Basic
experimentation was conducted on HTSC wires at Electrical Machines Laboratory,
BHEL Corporate R&D, Hyderabad. The data has been presented. Following this, I have
conducted a brief study on various power applications of HTSC i.e. HTSC Power cables,
HTSC Magnetic Energy Storage (MES), HTSC Fault Current Limiter (FCL), HTSC
Transformer and HTSC Machines. Next, an effort was made to understand the concept,
working, conceptual design, design process, the difficulties and complexities involved in
the manufacture and assembly of a HTSC Synchronous Motor currently under research
at the lab. The observations from the above field study at the research lab have been duly
reported in this work.
6. 6
LIST OF FIGURES
Figure No. Description
1.1 Discovery of superconducting materials
3.1 Defining parameters for Superconductivity
3.2 Resistivity graph of Superconductors vs. normal conductors
3.3 Behavior of type 1 superconductors to external magnetic field
3.4 Resistivity, Internal magnetic field and Magnetization of type 1 SC
3.5 Behavior of type 2 SC in an external magnetic field
3.6 Illustration of type 2 SC structure
3.7 Different states in the transition of type 2 SC materials
3.8 Resistivity, Internal magnetic field and Magnetization of type 2 SC
4.1 IBAD
4.2 RABiTS
4.3 Application specific requirements of the Superconducting wire for commercial
applications
4.4 Manufacture of BSCCO wire
4.5 Cross section of BSCCO wire
4.6 Molecular structure of YBCO
4.7 Properties of YBCO superconductors
4.8 Manufacture of YBCO wires
4.9 Structure of YBCO wire
5.1 S.C.C. tests on the prototype transformer
5.2 O.C.C. tests on the prototype transformer
6.1 Cold type HTCS Power cable
6.2 Warm type HTSC Power cable
7. 7
6.3 Difference between AC & DC HTSC cables & conventional cables
7.1 Block diagram of a SMES based system
8.1 Conceptual design of resistive type SFCL
9.1 Reduction in size in transformers with HTSC
9.2 Difference in the conceptual designs of conventional vs. HTSC transformers
9.3 State – of – the Art of HTSC transformer projects
10.1 Loss comparisons of air core (HTSC) and iron – core (conventional) 7.5MW,
3600rpm, 60Hz for full load rated speed operations
11.1. Model of HTSC race track with coil copper encasing
11.2 Race track coils fabricated by Siemens
11.3 Neon based cryo cooling technique
11.4 Illustration of cooling process in Neon based system
11.5 Conceptual design adopted by Electrical Power Research Institute (EPRI)
11.6 Conceptual Design adopted by Siemens AG
11.7 Conventional Synchronous Rotor
11.8 HTSC Synchronous Rotor
11.9 Projected Efficiencies of a HTSC Synchronous Motor compared to a
conventional Synchronous Motor
8. 8
LIST OF TABLES
Table no. Description
4.1 Application specific requirements of the Superconducting wire for
commercial applications
5.1 S.C.C. tests on prototype transformer
5.2 O.C.C. tests on the prototype transformer
6.1 State – of – the Art HTSC Power cable projects
6.2 Present status of Power cables vs required specifications
7.1 Comparison of various energy storage technologies
7.2 State – of – the Art SMES projects
8.1 State – of – the Art HTSC FCL projects
9.1 State – of – the Art HTSC transformers projects
10.1 State – of – the Art HTSC Machine projects
11.1 Current projects on HTSC Synchronous Machines
9. 9
TABLE OF CONTENTS
Acknowledgements 4
Abstract 5
List of Figures 6
List of Tables 8
1. Introduction……………………………………………………………………………………………………….. 12
1.1.Organization of this report……………………………………………………………………….…….15
2. Review of Related Literature………………………………………………………………………………..16
SECTION 1
3. Introduction to Superconductivity…………………………………………………….………………….19
3.1.Superconductivity…………………………………………………………………….……….…………..19
3.2. Superconductors……………………………………………………………………..…………………….19
3.3. Defining parameters of Superconductivity……………………………………………….……19
3.4. Hallmarks of Superconductivity………………………………………………………………..……20
3.5.Theory governing superconductivity (BCS Theory)………………………………………...21
3.6.Classification of Superconductors…………………………………………………………….…….21
3.7.Type 1 Superconductors…………………………………………………………………………………22
3.8.Type 2 Superconductors…………………………………………………………………….…………..23
3.9.Applications of Superconductivity………………………………………………………..…………24
4. High Temperature Superconducting (HTSC) wires………………………………………………..25
4.1.Introduction………………………………………………………………………………………………..….25
4.2.Demands on conductors for coil applications………………………………………………….28
4.3.BSCCO wire…………………………………………………………………………………………………..…29
4.4.Manufacture of BSCCO wire……………………………………………………………………………30
4.5.Disadvantages of BSCCO wire……………………………………………………………………..….31
4.6.YBCO wire………………………………………………………………………………………………..……..31
4.7.Manufacture of YBCO wires…………………………………………………………………………...31
4.8.Properties of HTSC wires………………………………………………………………………..……….33
5. Experimentation on 1st
generation & 2nd
generation HTSC wires………………………..…36
5.1.Testing of 1G HTSC (BSCCO) wire……………………………………………………………….……36
5.2.Testing of 2G HTSC (YBCO) wire……………………………………………………………….………36
5.3.S.C.C. tests on the prototype transformer……………………………………………………...37
5.4.O.C.C. tests on the prototype transformer……………………………………………………..38
SECTION 2
6. High Temperature Superconducting (HTSC) power cables…………………………………….41
6.1.Introduction……………………………………………….…………………………………………………..41
6.2.Types of HTSC power cables……………………………………………………………………….…..41
6.3.Benefits of HTSC power cables…………………………………………………………………………42
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6.4.State – of – the – Art HTSC power cables………………………………………………………….43
7. High Temperature Superconducting (HTSC) Magnetic Energy Storage (MES)………..45
7.1.Introduction……………………………………………………………………………………………………..45
7.2.Concept and working of a SMES…………………………………………………………………….…46
7.3.Advantages of SMES…………………………………………………………………………………………46
7.4.State – of – the – Art SMES projects………………………………………………………………...47
8. High Temperature Superconducting (HTSC) Fault Current Limiter (FCL)………………...48
8.1.Introduction……………………………………………………………………………………………………..48
8.2.Ideal fault current limiter characteristics………………………………………………..………..49
8.3.Types of fault current limiters…………………………………………………………………………..49
8.4.Passive limiters…………………………………………………………………………………………………49
8.5.Solid – state limiters………………………………………………………………………………………...50
8.6.Hybrid limiters………………………………………………………………………………………………….51
8.7.Introduction to Superconducting Fault Current Limiters………………………………….51
8.8.Conceptual design of Resistive type SFCL…………………………………………………………53
8.9.Basic design aspects…………………………………………………………………………………………53
8.10. State – of – the – Art SFCL projects……………………………………………………………54
8.11. Current status of SFCL technology…………………………………………………….………54
9. High Temperature Superconducting (HTSC) transformer………………………………………56
9.1.Introduction…………………………………………………………………………………………………….56
9.2.Benefits of HTSC transformer………………………………………………………………………..…56
9.3.Design tradeoffs and cost drivers of HTSC Transformers………………………………….57
9.4.Achieving cryogenic temperatures and maintaining it………………………………………59
9.5.Practical issues in the design of HTSC transformers………………………………………….60
9.6.State – of – the – Art HTSC transformer projects……………………………………………..60
10. High Temperature Superconducting (HTSC) Machines…………………………………………..63
10.1. Introduction……………………………………………………………………………………………….63
10.2. Concept behind HTSC machines…………………………………………………………………63
10.3. Efficiency……………………………………………………………………………………………………65
10.4. Introduction to HTSC machine design…………………………………………………………66
10.5. Machine design challenges…………………………………………………………………………67
10.6. State – of – the – Art HTSC machine projects……………………………………………..68
SECTION 3
11. A detailed study of the High Temperature Superconducting (HTSC) synchronous
motor……………………………………………………………………………………………………………………..…71
11.1. Background of this study………………………………………………………………………………71
11.2. Prior experience of the Electrical Machines Laboratory in Superconducting
Applications…………………………………………………………………………………………………..71
11.3. Conceptual design of the HTSC motor…………………………………………………………..71
11.4. Variations in the design followed by R&D organizations worldwide……………..73
11.5. Conceptual design adopted by BHEL R&D………………………………………………………75
11.6. Steps involved in performing the detailed design of a HTSC motor……………….82
11.7. Advance design variations developed by R&D institutions worldwide…………..83
12. Summary and conclusion…………………………………………………………………………………………….90
12. 12
CHAPTER 1
INTRODUCTION
Superconductivity is a phenomenon occurring in certain materials at low
temperatures, characterized by the complete absence of electrical resistance and the damping
of the interior magnetic field (Meissner effect). Superconducting materials have the unique
property of being able to carry current with negligible resistive losses. . The critical
temperature for SCs is the temperature at which the electrical resistivity of a SC drops to
zero. Some critical temperatures of metals are: aluminium (Al) Tc = 1:2 K, tin (Sn) Tc = 3:7
K, mercury (Hg) Tc = 4:2 K, vanadium (V) Tc = 5:3 K, lead (Pb) Tc = 7:2 K, niobium (Nb)
Tc = 9:2 K. Compounds can have higher critical temperatures, e.g., Tc = 92 K for
YBa2Cu3O7 and Tc = 133 K for HgBa2Ca2Cu3O8. Superconductivity was discovered by
Dutch scientist H. Kamerlingh Onnes in 1911 (Nobel Prize in 1913). Onnes was the first
person to liquefy helium (4.2 K) in 1908.
The near no-loss property occurs when the superconducting material is operated below a
critical temperature, magnetic field, and current density level. The SC state is defined by
three factors:
critical temperature Tc;
critical magnetic field Hc;
critical current density Jc.
Maintaining the superconducting state requires that the magnetic field and the current
density, as well as the temperature, remain below the critical values, all of which depend on
the material. For most practical applications, SCs must be able to carry high currents and
withstand high magnetic field without averting to their normal state.
Meissner effect (sometimes called Meissner-Ochsenfeld effect) is the expulsion of a magnetic
field from a SC. When a thin layer of insulator is sandwiched between two SCs, until the
current becomes critical, electrons pass through the insulator as if it does not exists. This
effect is called Josephson effect and can be applied to the switching devices that conduct on-
off operation at high speed.
In type I SCs the superconductivity is ‘quenched’ when the material is exposed to a
sufficiently high magnetic field. This magnetic field, Hc , is called the critical field. In
contrast, type II SCs has two critical fields. The first is a low-intensity field Hc1, which
partially suppresses the superconductivity. The second is a much higher critical field, Hc2,
which totally quenches the superconductivity. The upper critical field of type II SCs tends to
be two orders of magnitude or more above the critical fields of a type I SC.
Some consequences of zero resistance are as follows:
_ When a current is induced in a ring-shaped SC, the current will continue to circulate in the
ring until an external influence causes it to stop. In the 1950s, ‘persistent currents’ in SC rings
immersed in liquid helium were maintained for more than five years without the addition of
any further electrical input.
13. 13
_ A SC cannot be shorted out, e.g., a copper conductor across a SC will have no effect at all.
In fact, by comparison to the SC, copper is a perfect insulator.
_ The diamagnetic effect that causes a magnet to levitate above a SC is a consequence of zero
resistance and of the fact that a SC cannot be shorted out. The act of moving a magnet toward
a SC induces circulating persistent currents in domains in the material. These circulating
currents cannot be sustained in a material of finite electrical resistance. For this reason, the
levitating magnet test is one of the most accurate methods of confirming superconductivity.
_ Circulating persistent currents form an array of electromagnets that are always aligned in
such as way as to oppose the external magnetic field. In effect, a mirror image of the magnet
is formed in the SC with a North pole below a North pole and a South pole below a South
pole.
The main factor limiting the field strength of the conventional (Cu or Al wire) electromagnet
is the I2R power losses in the winding when sufficiently high current is applied. In a SC, in
which R = 0, the I2R power losses practically do not exist. The only way to describe SCs is to
use quantum mechanics. The model used is the BSC theory (named after Bardeen, Cooper
and Schrieffer) was first suggested in 1957 (Nobel Prize in 1973) [5]. It states that:
_ lattice2 vibrations play an important role in SCs;
_ electron-phonon interactions are responsible.
Photons are the quanta of electromagnetic radiation. Phonons are the quanta of acoustic
radiation. They are emitted and absorbed by the vibrating atoms at the lattice points in the
solid. Phonons possess discrete energy (E = hv) where h = 6:626 068 96(33) Js is Planck
constant and propagate through a crystal lattice. Low temperatures minimize the vibrational
energy of individual atoms in the crystal lattice. An electron moving through the material at
low temperature encounters less of the impedance due to vibrational distortions of the lattice.
The Coulomb attraction between the passing electron and the positive ion distorts the crystal
structure. The region of increased positive charge density propagates through the crystal as a
quantized sound wave called a phonon. The phonon exchange neutralizes the strong electric
repulsion between the two electrons due to Coulomb forces. Because the energy of the paired
electrons is lower than that of unpaired electrons, they bind together. This is called Cooper
pairing. Cooper pairs carry the supercurrent relatively unresisted by thermal vibration of the
lattice. Below Tc, pairing energy is sufficiently strong (Cooper pair is more resistant to
vibrations), the electrons retain their paired motion and upon encountering a lattice atom do
not scatter. Thus, the electric resistivity of the solid is zero. As the temperature rises, the
binding energy is reduced and goes to zero when T = Tc. Above Tc a Cooper pair is not
bound. An electron alone scatters (collision interactions) which leads to ordinary resistivity.
Conventional conduction is resisted by thermal vibration within the lattice. In 1986 J. Georg
Bednorz and K. Alex Mueller of IBM Ruschlikon, Switzerland, published results of research
[7] showing indications of superconductivity at about 30 K (Nobel Prize in 1987). In 1987
researchers at the University of Alabama at Huntsville (M. K. Wu) and at the University of
Houston (C. W. Chu) produced ceramic SCs with a critical temperature (Tc = 52:5 K) above
the temperature of liquid nitrogen. As of March 2007, the current world record of
superconductivity is held by a ceramic SC consisting of thallium, mercury, copper, barium,
calcium, strontium and oxygen with Tc = 138 K. There is no widely-accepted temperature
that separates high temperature superconductors (HTS) from low temperature
superconductors (LTS). Most LTS superconduct at the boiling point of liquid helium (4.2 K =
2690C at 1 atm). However, all the SCs known before the 1986 discovery of the
superconducting oxocuprates would be classified LTS. The barium-lanthanum-cuprate Ba-
La-Cu-O fabricated by Mueller and Bednorz, with a Tc = 30 K = 2430C, is generally
14. 14
considered to be the first HTS material. Any compound that will superconduct at and above
this temperature is called HTS. Most modern HTS superconduct at the boiling point of liquid
nitrogen (77 K = 1960C at 1 atm). All HTS are cuprates (copper oxides). Their structure
relates to the perovskite structure (calcium titanium oxide CaTiO3) with the general formula
ABX3. Perovskite CaTiO3 is a relatively rare mineral occurring in orthorhombic
(pseudocubic) crystals.
Economic Considerations
Before 1986, the critical temperature of superconducting materials was in the range where
liquid helium (at 4.2 Kelvin) was the required cooling fluid for superconducting coils. The
cost to cool superconducting coils using these low temperature superconducting (LTS)
materials was prohibitive when considering their use in industrial electric motors. In 1986 the
discovery of high temperature superconducting (HTS) materials raised the interest of rotating
machinery manufacturers as the critical temperature of these materials exceeds the boiling
point of liquid nitrogen (77 Kelvin or 77 K).
Figure 1.1. Progress in the discovery of Superconducting materials.
Source: [Sheahen]
Fig. 1.1 shows the critical temperature of superconducting materials versus their date of
discovery. The discovery of the Yttrium and Bismuth based materials, YBaCuO and
BiSrCaCuO, respectively in Fig. 1, resulted in active development of HTS wire and coils for
industrial electric motor and utility generator applications. Although these materials do
superconduct in liquid nitrogen (77 K), they can carry higher currents at higher magnetic
fields when their operating temperature is dropped. For superconducting motors, these
materials are typically cooled to the 30 to 40 K temperature range. For a 6000 hp (4480 kW)
15. 15
industrial motor, the input power to the cooling system for the superconducting coils
operating at 30 K will be about seven kW or 0.16% of the rated output power of the motor.
In power engineering, superconductivity can be practically applied to synchronous machines,
homopolar machines, transformers, energy storages, transmission cables, fault-current
limiters, linear synchronous motors and magnetic levitation vehicles. The use of
superconductivity in electrical machines reduces the excitation losses, increases the magnetic
flux density, eliminates ferromagnetic cores, and reduces the synchronous reactance (in
synchronous machines).
Research directed at the development of economically viable HTS based industrial motors
has been going on for over 18 years and has included the demonstration of a number of motor
prototypes in ratings up to 5000 hp (3730 kW). Superconducting technology promises
substantial loss reduction for motors in the rating range of 5000 hp and above accompanied
with motor volume reduction. At 60 to 77 K (liquid nitrogen) thermal properties become
friendlier and cryogenics can be 40 times more efficient than at 4.2 K (liquid helium). Loss
reduction of a factor of two compared to energy efficient induction motors of the same rating
appears feasible. Along with loss reduction, significant volume and weight reductions are
also possible, thereby making HTS based motors a technology that will impact large motor
users.
1.1. Organization of this report
This report consists of three sections
1. Section 1
This section consists of three chapters which introduces superconductivity and a
discussion on HTSC wires and presents the results of the experiments conducted on them.
Chapter 3: Introduction to Superconductivity
Chapter 4: High Temperature Superconducting (HTSC) wires
Chapter 5: Experimentation on 1st
generation and 2nd
generation HTSC wires
2. Section 2
This section consists of five chapters focusing on the applications of HTSC in
electrical power industry
Chapter 6: High Temperature Superconducting (HTSC) Power Cables
Chapter 7 High Temperature Superconducting (HTSC) Magnetic Energy Storage
(MES)
Chapter 8: High Temperature Superconducting (HTSC) Fault Current Limiter
(FCL)
Chapter 9: High Temperature Superconducting (HTSC) Transformer
Chapter 10: High Temperature Superconducting (HTSC) Machines (Generators &
Motors)
3. Section 3
This section consists of chapter 11: A Detailed study of High Temperature
Superconducting (HTSC) Synchronous machine.
16. 16
CHAPTER 2
REVIEW OF RELATED LITERATURE
Since the discovery of superconductors, the application of superconductivity has
found its way into many areas. Besides the most popular applications in medical devices
and instruments, such as magnetic resonance imaging (MRI) and superconducting
quantum interference device (SQUID), superconductivity has led to demonstrations of
several electrical power devices including motors, generators, transformers, fault current
limiters, power transmission cables, and superconducting magnetic energy storage
systems (SMES). Past three decades has witnessed a large volume of literature in this
field in the form of research paper published in journals, papers presented at conferences,
reports of the pilot projects undertaken at organizations worldwide, presentations by
companies in this field and the specifications sheet of the superconducting products
being manufactured by industries. All the above mentioned sources have become great
resources in this study.
Each chapter in this report has a different set of literature that was referenced for this
study. In general, some sources have been a great resource for the reading, namely
1. SuperPower Inc
www.superpower-inc.com
It is a world leading developer and producer of second-generation high-temperature
superconducting (2G HTS) wire, providing enormous advantages over conventional
conductors of electric power - high efficiency, smart grid compatible, green, clean, safe
and secure.
The numerous presentations provided by SuperPower Inc.on HTSC wires and on each
of the power applications of HTSC have been very influential in making this report.
2. American Superconductor
www.amsc.com
It is a provider of ideas, technologies and solutions that meet the world’s demand
for smarter, cleaner and better energy.
The information shared by them on the HTSC wires and the various projects;
namely the 5MW HTSC Synchronous Motor Project and the 36.5MW HTSC
Synchronous Motor project have provided deep insight into the subject
17. 17
3. Siemens AG
www.siemens.com
Siemens has been one of the pioneer organizations to experiment the possibility of
HTSC in electrical machines. The 4MVA 3600rpm HTSC generator project and
36.5 MW 120 rpm ship propulsion motor have been a great resource to understand
the HTSC machine design details
4. Nexans
www.nexans.com
It is a worldwide leader in the cable industry, it offers an extensive range of cables
and cabling systems to raise industrial productivity, improve business
performance, enhance security, enrich the quality of life, and assure long-term
network reliability.
The HTSC cable projects and HTSC FCL projects undertaken by Nexans have
been one of the first in the industry providing great information on the art.
5. Electric Power Research Institute
www.epri.com
The Electric Power Research Institute, Inc. (EPRI) conducts research and
development relating to the generation, delivery and use of electricity for the
benefit of the public. The EPRI, through its establishment, Reliance Electric Ltd.
has been the first in the industry to experiment with HTSC motors. The 2MW and
the 5MW HTSC motor projects reports published in the 1997 report titled
‘Electric Motors using High Temperature Superconducting Materials Applied to
Power Generating Station Equipment’ prepared by Reliance Electric for EPRI and
published in January, 1997 has been one of the first project reports on the pioneer
HTSC projects which describes the machine design in detail.
6. A book titled “Introduction to High Temperature Superconductivity” by Thomas
P. Sheahen published by Kluwer Academic Publishers has been a great resource
for all the chapters in this report.
Other projects carried out at General Electric, Zenergy, Wetinghouse, Waukesha
Electric Systems, Converteam and many other companies have made great
contribution to this report through their respective project reports and associated
numerous publications.
Apart from these, many IEEE papers, independent papers, reviews and
publications in various other journals and conferences have assisted in the making
of this report. In general, www.wikipedia.org has been a great source of
information on few topics.
18. 18
SECTION 1
INTRODUCTION TO SUPERCONDUCTIVITY
DETAILED DISCUSSIONS ON 1ST
GENEARTION AND
2ND
GENERATION HTSC WIRES AND EXPERIMENTS
CONDUCTED ON THEM
19. 19
CHAPTER 3
INTRODUCTION TO SUPERCONDUCTIVITY
3.1. Superconductivity
The field of superconductivity, once a mere laboratory curiosity has moved into the
realm of applied science since the last 40 years. Many more applications have become possible
because of the discovery of ceramic superconductors which operate at comparatively at high
temperatures and even more will be possible.
3.2. Superconductors
For most materials which are normal conductors whenever an electrical current flows
there is some resistance to the flow of electrons through the materials. It is necessary to apply a
voltage to keep the current going; to replace the energy dissipated by the resistance in the
material. A superconductor; in contrast is a material with no resistance at all.
A lot of metals show modest resistance at room temperatures but turn into superconductors when
cooled to almost near to absolute zero temperatures. The first superconductor to be discovered
was mercury, soon after the invention of a cryo refrigerator that could attain temperatures below
the boiling point of Helium i.e. 4.2K. In the subsequent years many more materials were found to
be superconducting at these very low temperatures. By the 1960’s certain allows of Niobium
were made which became superconductors in the temperature range of 10 – 25K. It was then
generally believed according to theoretical grounds that there would be no superconductors
above 30K.
Since a superconductor has no resistance, it carries current indefinitely without requiring voltage
or expenditure for electricity. Once the current is started it continues, provided the
superconductor is kept below the critical temperature. The cost of running a superconducting
persistent loop is simply the cost of refrigeration; which for low temperature superconductors
being Helium proved to be very expensive.
3.3. Defining parameters of Superconductivity
1. Critical Current
The ideal physical definition of critical current is the current where a material has a phase
transition from a superconducting phase to a non-superconducting phase. For practical
superconducting wire, the transition is not infinitely sharp but gradual. In this case, the critical
current is defined as the current where the voltage drop across the wire becomes greater than a
specific electric field, usually 1 microvolt/cm. Sometimes, for low-loss magnet applications, a
lower electric field criterion is used. The critical current is represented by the variable Ic.
2. Critical Temperature
It is the temperature below which the transition takes place from normal conducting state to the
superconducting state.
20. 20
3. Critical Field
A superconducting material can tolerate only a certain maximum field over which it loses its
superconducting nature.
So, the domain of superconductivity is restricted within the limits of critical current, critical
temperature and critical field.
Figure 3.1. Defining Parameters for Superconductivity
Source: SuperPower Inc.
3.4.Hallmarks of Superconductivity
A material is said to have transitioned into the superconducting phase when it exhibits certain
characteristic properties. In essence, experimentally when these properties have been identified in
the material, then it can be stated that the material has transitioned into the superconducting state.
1. Zero Resistivity
2. Perfect Diamagnetism
3.4.1. Zero Resistivity
Figure 3.2. Resistivity graph of Superconductors vs. normal conductors
Source: www.wikipedia.org
21. 21
A normal conductor when cooled near to absolute zero temperatures the resistivity decreases
gradually and at absolute zero temperature reaches a finite value which can be found for that material
by analyzing the dependence of resistivity of that material with temperature. Unlike, for a
superconducting material as the temperature is cooled; at a finite temperature called the critical
temperature the resistance abruptly falls to zero.
3.4.2. Perfect Diamagnetism
It is more popularly known by a name i.e. the Meissner effect. This plays a central role in the
magnetic phenomenon associated with superconductivity. Meissner effect is the expulsion of
magnetic field from within the superconductor. This expulsion is very different from not letting any
magnetic field inside the material; any metal with infinite conductivity would do the later. If a
magnetic field is already present and a material is cooled below the critical temperature to become a
superconductor; all the magnetic field within that material is expelled out of it. Hence, it behaves like
a perfect dia – magnet; i.e. all the magnetic field applied over the material simply passes over the
material without entering into it giving the effect of the superconductor behaving like a magnet.
3.5. Theory governing superconductivity (BCS theory)
BCS theory proposed by Bardeen, Cooper and Schrieffer (BCS) was the first microscopic theory
of superconductivity. This theory describes superconductivity as a microscopic effect caused by a
condensation of electrons into a boson – like state.
Roughly speaking the superconducting phenomenon has been given the following explanation by
BCS. An electron moving through a conductor will attract nearby positive charges in the lattice. The
deformation of the lattice causes another electron of opposite spin to move into the region of higher
positive charge density. The two electrons then become correlated. There are many such pairs in a
superconductor so that they overlap very strongly forming a highly collective condensate. Breaking
of one pair results in a change in the energies of the remaining macroscopic number of pairs. If the
required energy is greater than the energy gained by the collisions with the oscillating atoms, then
the electrons will stay paired, thus nit experiencing any resistance. Thus the collective behavior of
the condensate is the crucial ingredient of superconductivity.
In many superconductors, the attractive interaction between electrons (necessary for pairing) is
brought about indirectly by the interaction between the electrons and the vibrating crystal lattice (the
phonons)
Any more discussion in the field of superconductivity will lead into Quantum Mechanics, hence we
stop here.
3.6. Classification of Superconductors
3.6.1. Based on their response to magnetic field
1. Type 1
They have a single critical magnetic field HC above which superconductivity is lost
22. 22
2. Type 2
They have two critical fields HC1 and HC2 between which there is partial superconductivity. Above
HC2 there is no superconductivity and below HC1 it is a perfect superconductor
3.6.2. Based on their governing theory
1. Conventional
They can be explained by the Bardeen Cooper Schreiffer (BCS) theory
2. Non – conventional
BCS theory fails in explaining the superconductivity in them.
3.6.3. Based on their critical temperature
1. Low temperature
They have a critical temperature below 77K
2. High temperature
They have a critical temperature above 77K
3.7.Type 1 Superconductors
The behavior of the material is described in the figure on the left
In the presence of an external magnetic field, the superconducting material generates surface currents
which oppose the existing magnetic field making it perfectly diamagnetic.
Figure 3.3. Behavior of type 1 superconductors to external magnetic field (left)
Figure 3.4. Resistivity, Internal magnetic field and Magnetization of type 1 SC (right)
Source: Sheahen (left)
Source: SuperPower Inc. (right)
23. 23
3.8.Type 2 Superconductors
The behaviour of type 2 superconductors is very complex and cannot be explained by BCS theory.
They have small gaps within the volume of the material which allows the magnetic field to pass
through it. These gaps are called vortices.
Magnetic field when in between the two critical magnetic fields HC1 and HC2 passes through the
vortices thereby defeating the Meissner effect
Figure 3.5. Behavior of type 2 SC in an external magnetic field (left)
Figure 3.6. Illustration of type 2 SC structure (right)
Source: Introduction to superconductivity, Sheahen
Figure 3.7. Different states in the transition of type 2 SC materials (left)
Figure 3.8. Resistivity, Internal magnetic field and Magnetization of type 2 SC (right)
Source: SuperPower Inc.
24. 24
With this, the discussion on the superconductivity and its theories ends since the main focus of this
report are the applications of superconductivity.
3.9.Applications of Superconductivity
3.9.1. Industrial
a. High Gradient Magnetic Separator
b. In chemical industries, in the synthesis of certain reactions
3.9.2. High Energy Physics
a. High Power magnets
b. SQUID
c. In R&D laboratories
3.9.3. Medical
a. MRI ( Magnetic Resonance Imagery)
b. NMR (Nuclear Magnetic Resonance)
3.9.4. Electronics
a. Antennas
b. Filters
c. Microcontrollers etc.
3.9.5. Automobile
a. MHD (Magneto Hydrodynamic )motors
b. Magnetic Levitation
3.9.6. Electrical
a. Superconducting Power Cables
b. Superconducting Magnetic Energy Storage (SMES)
c. Superconducting Fault Current Limiter (SFCL)
d. Superconducting Transformer
e. Superconducting Synchronous Machine
25. 25
CHAPTER 4
High Temperature Superconducting (HTSC) Wires
4 .1. Introduction
In 1986 two IBM scientists, Georg Bednorz and Alex Müller, announced the discovery of
a material that was superconducting at 34 K, 11 degrees warmer than had ever before been
observed. Within a year, scientists in the U.S. and Japan created new compounds with yet higher
superconducting transition temperatures. In fact, by March 1987 eight new materials were
produced that are superconducting above 77 K, the boiling point of liquid nitrogen at standard
atmospheric pressure. (Liquid nitrogen is an efficient cryogen, inexpensive, easy to insulate,
inexhaustible, readily available, and non-polluting.) One of these, YBa2Cu3Ox (YBCO), has all
the desired characteristics for use in the electronics industry but lacks one feature essential for
use in power applications: ability to be formed into wires by thermo- mechanical means.
In the late 1980’s, scientists turned their attention to the Bi(Pb)SrCaCuO superconductor,
a family of HTS that has plate-like grains that align easily when wire-forming processes are
used. This family of wires is produced by what is commonly referred to as the oxide powder-in-
tube (OPIT or PIT) process. For this, a silver or silver alloy tube is loaded with precursor
powder. The tube is then sealed and drawn into a fine wire. These round wires are cut and re-
stacked into another hollow tube and, after a series of additional drawing, rolling, and heat
treatment steps, multi-filamentary ribbons (or tapes) are produced with the desired
superconducting phase assemblage and texture. Lengths of BSCCO wire as long as 1 km are
now routinely produced by companies in the U.S. and Japan. At liquid nitrogen temperatures,
these wires can have overall engineering current densities in excess of 100 A/mm2 with no
applied magnetic field. This performance degrades by an order of magnitude at 77 K upon
application of just a few tenths of a tesla magnetic field. Thus, in order to use these wires in
electric machinery, such as motors, generators, transformers, and energy storage magnets, the
wires must be cooled to temperatures in the neighborhood of 20-30 K using helium gas or a
closed-cycle cryo cooler. Since superconducting, rotating electric machines may need fields as
high as 5 tesla, and since today’s magnetic resonance imaging machines typically generate fields
of 1 to 4 tesla, new wires are needed that can take advantage of the simpler, less-costly
cryogenics requirements associated with operation at liquid nitrogen temperatures (65-77 K).
The YBCO compound has the unfortunate problem that its grains are difficult to align. In
HTS, electric current doesn’t flow well from grain to grain through high-angle grain boundaries.
Coatings on silver and silver alloys have also proven to make poor superconductors, due to low
superconductor densities and poor grain alignment. So, while YBCO is useful for making thin
films on single-crystal substrates for electronics applications or for small discs for bearings,
something else is needed for wires.
In 1988 Lawrence Berkeley National Laboratory initiated work to form YBCO tape
conductors by depositing films on metal substrates. This was a modest effort, and was regarded
as risky since it seemed likely at the time that a way would be found to make more conventional
wires of the YBCO compound. However the weak link problem, caused by incomplete
alignment of film crystallites, proved highly intractable. It thwarted the conventional approaches,
and nearly prevented success with deposited film conductors as well. Fortunately, YBCO film
growth itself was not a problem; there were literally hundreds of papers reporting the successful
growth of high-current films by epitaxial film growth on single-crystal substrates. Single-crystal
26. 26
substrates are useful for electronic applications. However, for electrical applications (that is, long
wires) strong temperature-resistant nickel-alloy substrates coated with yttria stabilized zirconia
(YSZ) buffer layers took the place of the single-crystal substrates. The films of YBCO and YSZ
were deposited with the pulsed laser deposition (PLD) technique. The YBCO crystallites readily
formed with the correct c-axis orientation normal to the substrate, but the in-plane orientation
was random. As a result the critical current density of YBCO films on metal and polycrystalline
YSZ substrates investigated by Oak Ridge appeared to be limited to about 100 A/mm2 (77 K, 0
T). Thus, in-plane orientation appeared to be necessary. Ion beam assisted deposition (IBAD), as
applied by Lawrence Berkeley National Laboratory, and an independent group at Fujikura in
Japan, proved to be a solution to the texturing problem. This increasingly popular technique
utilizes the bombardment of a growing film with energetic ions, resulting in improved texture.
While a normally incident beam is usually used but, the Berkeley group found that an oblique
ion beam can introduce the needed in-plane orientation in the YSZ buffer layer. Epitaxial growth
of the superconducting YBCO film then resulted in critical current densities up to 6,000 A/mm2,
an enormous improvement. Two new processes have been under development since 1991 that
promise a new way to manufacture flexible, high current density wires made from YBCO,
something that has eluded researchers since the discovery of YBCO in 1987. These wires offer
impressive performance opportunities at liquid nitrogen temperatures. In both cases, the key is to
prepare a textured substrate, or template on which the YBCO may be deposited as a thick film.
Done correctly, the YBCO grains are well-aligned, mimicking the alignment of the underlying
substrate, resulting in the prospect of long-length wires that are strongly-linked. Biaxial-textured
substrates, where the atomic planes of the grains in each layer of the substrate are well-aligned in
the surface of the tape, represent one potential solution to the shortcomings to fabrication of
long-length YBCO wires. The national laboratories attacked the YBCO weak-link problem in
two different ways.
The Los Alamos group worked to improve the IBAD process, refining the quality of the
angular alignment of the YSZ crystallites, and introducing an additional cerium oxide buffer
layer which eliminates the tendency of a few YBCO grains to crystallize with a 45 degree
misalignment angle. The Los Alamos process is illustrated in figure. With this process current
densities reached 8,000 A/mm2 in 1994 and 13,000 A/mm2 in 1995.
27. 27
Figure 4.1. IBAD
Source: [Sheahen]
Oak Ridge National Laboratory researchers turned their attention to developing sharp
biaxial textures in metals, such as nickel and copper, and then depositing on them additional,
chemically benign metal layers with epitaxial orientation similar to that of the underlying metal
strip. In the most recent architecture, Oak Ridge deposits the oxide buffer layers directly on the
nickel tape, with no intervening metal coating on the nickel. Like Los Alamos, the thin oxide
buffer layers are placed on top in order to transfer the alignment to the superconducting layer
while avoiding chemical degradation, but Oak Ridge relies on the alignment of the first metal
strip instead of the IBAD process to provide the template for the superconductor (see figure 4.2).
Oak Ridge calls its substrate technology .RABiTS.,. or rolling-assisted, biaxial-textured
substrates. The Oak Ridge group produced the simplest version of their substrate using dual
metal oxide buffer layer architecture and a common industrial film growth technique, called
electron beam evaporation. For this, extremely thin layers of two ceramic materials are rapidly
deposited sequentially using a laboratory-scale electron beam system. A cerium oxide layer as
thin as 100 angstroms is placed almost instantaneously on the rolled nickel, followed by a 140
nm layer of yttria-stabilized zirconia. In the lab environment, this layer takes about 20 minutes to
grow. The ceramic layers in the RABiTS sandwich are, therefore, remarkably thin.
28. 28
Figure 4.2. RABiTS
Source: [Sheahen]
4.2. Demands on conductors for coil applications
A superconducting material that is being investigated for coil applications must possess
the following properties in general
1. High current densities
2. High mechanical strengths
3. Must be able to fabricate in long lengths
4. Must tolerate high operating currents
5. Low ac losses
Table 4.1. Application specific requirements of the Superconducting wire for commercial
applications
Application Je
(A/mm2
)
77K, self
field
Cost/tape
($/kA-m)
Field
(T)
Op.
Temp
(K)
IC/tape
(A)
77K,
self
field
AC
losses
(mW/A-
m)
Bend
radius
(m)
Strain Wire
lenght
(m)
Fault
current
limiter
10 - 100 30 - 100 .3 - 3 40 - >65 100 0.4 0.15 –
0.05
0.2 –
0.4
200 –
1000
Motor 100 10 4 >25 300 NA 0.05 0.2 –
0.3
1000
29. 29
Generator
(100MVA)
10 10 4 – 5 20 - >65 100 –
200
NA 0.1 <0.2 500 –
1000
Cable 10 – 100 10 -100 <0.2 >65 >30 0.15 0.01 >0.4 100 –
1000
Transform
er
10 – 100 20 - >5 0.15 20 – 65 200 0.25 0.1 –
0.2
0.1 250 –
3000
High field
Magnet
10 – 100 5 - >1 >20 4.2 - >65 300 –
500
NA 0.01 0.5 500 –
1000
Magnetic
seperator
1 10 2 - 3 77 500 NA 0.5 0.2 1000
Source: DOE, US
4.3. BSCCO wire
The structure of the BSCCO molecule is shown in figure 4.1.
Figure 4.3. Molecular structure of BSCCO
Bi2Sr2Ca2Cu3O8
Source: www.wikipedia.org
30. 30
Covalent Cu-O bonding and Cu3+ valence states leads to partly filled energy bands
Oxidation to Cu3+ leaves a hole in the conduction band for p-type conductivity
Weak localization of valence electrons (low ionic character)
Alkaline and rare earth metals act as charge reservoirs
4.4. Manufacture of BSCCO wire
Figure 4.4. Manufacture of BSCCO wire
Source: SuperPower Inc.
Figure 4.5. Cross section of BSCCO wire
Source: [DOE 1997]
31. 31
4.5. Disadvantages of BSCCO wire
The first generation HTSC wires are made from BSCCO. They are very delicate and
expensive.
BSCCO is by no means an ideal material since it carries very little current at 77K in
high magnetic fields.
To get appreciable current densities in BSCCO, it must be cooled to 20 -30K.
This necessitated the need for developing new SC materials
4.6. YBCO wire
Figure 4.6. Molecular structure of YBCO (left)
Source: www.wikipedia.org
Figure 4.7. Properties of YBCO superconductors (right)
4.7. Manufacture of YBCO wires
Figure 4.8. Manufacture of YBCO wires
33. 33
4.8. Properties of HTSC wires
4.8.1. Performance
The word performance applied to superconducting wire refers to the critical current
density Jc of the wire. The early elemental (Type I) superconductors were never of practical
interest, mainly because they carried very little current; and a magnetic field of a few hundred
gauss (0.03 tesla) would quench superconductivity completely.
Type II superconductors have the very important property of having high Jc even in
magnetic fields of several tesla. All the HTS materials fall within the Type II category. Samples
of YBCO, BSCCO, etc. from 1988-1990 were plagued with crystal imperfections and
mechanical irregularities, and showed Jc values below 10 A/mm2. The advantage of
contemporary BSCCO wire is that it has Jc > 1,000 A/mm2 (at sufficiently low temperatures and
modest magnetic fields). YBCO coated conductors made via IBAD techniques have Jc > 10,000
A/mm2 in short samples. RABiTS technology is equivalent in Jc in short samples, and may offer
other advantages, which are further discussed below. The high performance of these samples is
due to very good grain-alignment, which in turn is due to the substrate conditioning achieved by
IBAD and RABiTS. If the alignment of consecutive grains deteriorates (i.e., disorientation of
adjacent grains by more than 5 to 10 deg.), the value of Jc drops sharply, and the material is no
longer useful for high-current applications. One objective of the second-generation wire program
is to extend these coated conductors to very long lengths ( > 1 km) while still preserving high Jc
values. YBCO coated conductors require a completely new type of production equipment and
"thin film" processing techniques (common in the metalized can label, snack food bag, and
recording tape industries but quite different from much of the equipment used to make BSCCO-
2223 .OPIT. wires). This capitalization represents a barrier to the YBCO coated conductor
development business that few companies can afford to overcome without strategic partnerships
with other companies, the national laboratories, and universities. For example, estimates of the
capital cost to install a pilot line for coated conductors range from as low as $5 million to as high
as $50 million.
4.8.2. Real wire Considerations
Increasing the thickness of the conductor film is an important issue. Total current, as
contrasted to current density, is what is needed in practical applications, and so a high Jc must be
accompanied by a large film cross sectional area in order to deliver the total current. Typically,
thin films are perhaps 0.4m in thickness, so even a 1m film borders on the category of .thick.
These conductors may require thicknesses total) of 5 or 10 µm to achieve the total current
needed (unless values of Jc can be increased substantially), and this introduces a new worry. As
the YBCO layer thickens, is there a possibility that mis-oriented grain growth will occur,
defeating the purpose of the original textured substrate? Also, will film mechanical properties
(cracks in YBCO film over 3-5 µm thick) similarly limit the overall thickness? It may turn out
that 1 or 2 :m is the maximum practical thickness. Any real wire includes some .overhead. for
insulation, etc., and therefore we distinguish between the critical current in the superconducting
material itself Jc , and the .engineering. critical current Je. For practical applications, the figure of
merit is Je , not Jc, because Je relates to how much actual current flows through a real conductor
with a certain cross- ectional area. In the case of BSCCO made by the Powder-in-Tube (PIT)
method, the amount of silver surrounding the BSCCO reduces Je compared to Jc. In the case of
YBCO coated conductors, the thickness of the substrate and buffer may be ten times the
34. 34
thickness of the YBCO itself, in which case the reduction from Jc to Je will exceed a factor of 10 -
- the penalty for .overhead. is very severe.
4.8.3. Magnetic Properties
In the first generation of wire, the sheathing material (silver) is non-magnetic. Using
RABiTs, the first thing to note is that the substrates are often magnetic materials (e.g., nickel).
Therefore, it is important to investigate the interactions between substrate and HTS in a magnetic
field. Alternative choices of substrate (Hastelloy, stainless steel) may be used to minimize
adverse effects of external magnetic fields, although these alloys may be difficult to align by
rolling.
4.8.4. AC Losses
The subject of AC losses is complex and depends on the application. Experimental
measurements are usually needed to verify theoretical expectations. Often differences between
theory and experiment are interpreted in terms of conductor non-uniformity. Generally, AC
losses are associated with changing magnetic fields. Self-field losses are those which occur due
to the magnetic fields produced by the conductor acting on itself. Other losses are caused by the
interactions of the different components of a system. For a tape conductor, the orientation of the
magnetic field is important; the losses are usually larger when the field has a significant
component normal to the plane of the conductor.
Eddy current losses are due to currents induced in normal metal as the result of time-
varying magnetic fields. In conventional motors, generators, and transformers, for example,
these losses are reduced by the use of laminated steel for the magnetic circuit and the use of thin
conductor strands for the copper conductors. The steel is formulated with high electrical
resistivity and minimum magnetic hysteresis in mind. Transposition of the copper conductors
also can be used to reduce eddy currents. Second generation coated conductor technology offers
reduced eddy current losses relative to BSCCO powder-in-silver tube technology due to the
higher resistivity of the nickel or nickel alloys (relative to silver) used to support the
superconductor. However, the ferromagnetism of pure nickel may lead to hysteretic losses if it is
used as a substrate. The physical picture is that changes in the externally imposed magnetic field
cause flux penetration into the superconductor. The losses are proportional to the frequency,
since the loss per cycle is fixed, and are also proportional to the inverse of the critical current
density. Since second generation conductors are expected to have high critical current densities,
the inverse dependence of loss on Jc is a beneficial aspect. Initial measurement of self field losses
on YBCO coated conductors seem to be encouragingly low.
4.8.5. Geometry
The geometrical considerations have mostly to do with the matter of flexibility. Truly
useful wire will be bent in most applications. The parameters of layer thickness, bend radius,
bending strain, and tensile/compressive strain all come together under the umbrella of
.geometry.. At first, it seems desirable to coat the substrate with a very thick film of YBCO.
Doing so increases Je, hopefully without sacrificing Jc. But this is not assured. The total current
flowing in the full conductor is the key figure of merit; if very thick films accumulate defects
and then succumb to poor grain alignment, for example, the anticipated Je will not be realized.
Maximizing the useful film thickness is a key goal.
35. 35
Bending: When a layer is 5µm thick, and bent on a radius of 5cm, the strain is of the
order of 1 part in 104. However, these coated conductors including substrate may have total
thicknesses as large as 100 µm, and it is the total conductor that will be bent around the specified
radius in each application.
Following this discussion on HTSC wires, basic experimentation on 1st
generation and 2nd
generation HTSC wires were carried out, the results of which are presented in the next chapter.
36. 36
CHAPTER 5
EXPERIMENTATION ON 1ST
GENEARTION & 2ND
GENERATION
HTSC WIRES
In this chapter, the results of the tests performed on the HTSC winding are presented.
5.1. Testing of 1G HTSC (BSCCO) wire
A BSCCO (1G wire) of 10 cm length was taken and 150 current was passed in it with a
potential difference of 1µV/cm. A separate test apparatus was prepared for this application.
5.2. Testing of 2G HTSC (YBCO) wire
A prototype transformer has been manufactured and tested. This test was done to study the
behavior of high temperature superconducting winding in superconducting stage
particularly for ac operation like a coil in transformer.
A rectangular frame was fabricated with the help of 0.5mm CRGO steel. One limb of the
transformer was wound with normal air cooled copper winding acting as a primary
winding. Second limb is having a 2G YBCO superconducting winding.
In this case core is air cooled and the winding is liquid nitrogen cooled. A double wall
vacuum insulated FRP container has been used for accommodating two coils of HTSC
winding immersed in liquid nitrogen. After the winding attained a temperature of 77K,
primary winding is excited with a 50 Hz supply. OCC tests were done and SCC test has
been carried for a rated current of 135 Amps.
Special type of connectors and soldering of copper connector and HTSC tape is required
for taking out the connection from HSC coil at 77K to outside at room temperature.
40. 40
SECTION 2
HIGH TEMPERATURE SUPERCONDUCTIVITY IN
ELECTRICAL POWER APPLICATIONS
HTSC POWER CABLES
HTSC MES
HTSC FCL
HTSC TRANSFORMER
HTSC MACHINES
41. 41
CHAPTER 6
High Temperature Superconducting (HTSC) Power Cables
6.1. Introduction
Projects to demonstrate superconducting power cable in utility power grids have increased
internationally as the technology improves and the need to ease issues related to power congestion in
densely populated urban centers is realized by power system operators. Superconducting power
cables are a possible solution to these congestion issues because they ca provide three to five times
more capacity than conventional underground power cables in the same physical space. One
application touted for High Temperature Superconducting (HTSC) power cables is underground
cable retrofits, where the cost of expanding existing tunnels or digging new ones outweighs the
initial cost of superconducting system. Additionally HTSC Power Cables may be an excellent option
where rights of ways (ROW) are difficult or impossible to obtain.
6.2. Types of HTSC Power Cables
6.2.1. Cold
A cold dielectric superconducting power cable employs concentric layers of HTS wire separated by
the high voltage insulation material, commonly referred to as the dielectric. Superconducting tapes
(cooled by liquid nitrogen) are both inside and outside the dielectric, and consequently the dielectric
itself is also immersed in liquid nitrogen. This ‘cold dielectric’ gives the cable design its name.
The inner, high voltage layer(s) of superconductor tapes are transmitting power while the outer
layer(s) are grounded. In the outer layers, currents equal in magnitude but opposite in phase to the
inner layers are being induced. These induced currents completely cancel the electromagnetic fields
of the inner layers, so that a cold dielectric HTS power cable has no stray electromagnetic fields
outside the cable, no matter how high its current (and thus transmission power) rating. This is one of
the key benefits of the cold dielectric design. The fact that the electromagnetic field is contained
inside the superconducting screen also significantly reduces the cable inductance, another important
benefit of HTS power cables.
6.2.2. Warm
This simpler design of HTS power cables is the ideal choice when electromagnetic stray fields can
be tolerated and a slightly lower transmission capacity than that of a cold dielectric cable is
acceptable. Its high voltage phase layer(s), consisting of superconducting tapes, are stranded around
a core that also serves as the channel for the liquid nitrogen coolant. Unlike in the cold dielectric
design, there are no superconducting screen layers requiring cooling, and consequently the dielectric
is kept at ambient temperature, or warm.
As this cable designs has higher electrical losses and a higher inductance when compared to a cold
dielectric design, it has its place in applications where conventional cables have reached their limits
but not all the features of a cold dielectric design are necessary. In such situations, it can be the
choice that makes the best economical sense, owing to its simpler overall design, cheaper
manufacturing cost, and reduced superconductor length.
42. 42
6.3. Benefits of HTSC Power Cables
Lower voltages
Because of the higher capacity of VLI (Very Low Impedance) cable – approximately three to five
times higher ampere carrying capacity than conventional cables – utilities may employ lower voltage
equipment, avoiding both the electrical (I²R) losses typical of high current operation and the capital
costs of step up and step down transformers. High current VLI cables at 115 kV or even 69 kV may
solve problems that would ordinarily require a 230 kV or 345 kV conventional solution.
Easier installation
HTS cables are actively cooled and thermally independent of the surrounding environment.
Life extension and improved asset utilization
Over time, thermal overload ages and degrades cable insulation. By drawing flow away from
overtaxed cables and lines, strategic insertions of VLI cable can „take the heat off“ urban power
delivery networks.
Reduced electrical losses
In optimized designs, lower net energy losses occur in VLI cables, than in either conventional lines
and cables or unshielded HTS cables with a single conductor per phase, offering a transmission path
with high electrical efficiency. Because VLI circuits tend to attract power flow, they will naturally
operate at a high capacity factor, reducing the losses on other circuits and further magnifying their
efficiency advantage.
Indirect and non monetary savings
In addition to these “hard cost“ savings, VLI cables may result in other “soft cost“ savings. For
example, time to install may be shortened because of reduced siting obstacles associated with
compact underground installations and less burdensome siting requirements for lower voltage
facilities. VLI cables might be routed through existing, retired underground gas, oil or water pipes,
through existing (active or inactive) electrical conduit, along highway or railway rights-of-way, or
through other existing corridors.
Reduced regional congestion costs
Finally, and perhaps most significantly, the ability to complete grid upgrade projects more quickly
will translate into the earlier elimination or relaxation of grid bottlenecks. Solving physical
bottleneck problems will sharply reduce the grid congestion costs that, in today‘s unsettled,
imperfectly competitive marketplace, can impose huge penalties on consumers and the economy at
large.
Underground installation
The underground installation of VLI cable eliminates the visual impact of overhead lines.
Environment friendly dielectric
Liquid nitrogen, the coolant/dielectric of choice for VLI cables, is inexpensive, abundant and
environmentally benign.
Elimination of EMF
The coaxial design of VLI cold dielectric cables completely suppresses electromagnetic fields
(EMF).
Refer figure 6.3. for a detailed comparison between the HTSC cables and conventional
cables
43. 43
6.4. State – Art – of - Art HTSC Power Cables
Table 6.1. State – of – the Art HTSC Power cable projects
Manufacturer Place/Country/Year Type Data HTSC
Innopower Yunnan, CN, 2004 WD 35kV, 2kA,
33m, 3Φ
Bi – 2223
Sumitomo Albany, US, 2006 CD 34.5kV, 800A,
350m, 3Φ
Bi – 2223
Ultera Columbus, US, 2006 Triax 13.2kV, 3kA,
200m, 3Φ
Bi – 2223
Sumitomo Gochang, KR, 2006 CD 22.9kV, 1.25kA,
100m, 3Φ
Bi – 2223
LS Cable Gochang, KR,2007 CD 22.9kV, 1.26kA,
100m, 3Φ
Bi – 2223
Sumitomo Albany, US, 2007 CD 34.5kV, 800A,
30m, 3Φ
YBCO
Nexans Hannover, D, 2007 CD 138kV, 1.8kA,
30m, 1Φ
YBCO
Nexans Long Island, US,
2008
CD 138kV, 1.8kA,
600m, 3Φ
Bi – 2223
Nexans Spain, 2008 CD 10kV, 1kA,
30m, 1Φ
YBCO
Ultera New York, US, 2010 Triax 13.8kV, 4kA,
240m, 3Φ
YBCO
Ultera Amsterdam, NL Triax 50kV, 2.9kA,
6000m, 3Φ
YBCO
Nexans Long Island, US,
2011
CD 138kV, 2.4kA,
600m, 1Φ
YBCO
LS Cable Gochang, KR, 2011 CD 154kV, 1GVA,
100m, 3Φ
YBCO
LS Cable Seoul, KR, 2011 CD 22.9kV,
50MVA, 500m,
3Φ
YBCO
Sumitomo Yokohama, JP, 2012 CD 66kV, 200MVA,
200m, 3Φ
Bi – 2223
Sumitomo TEPCO, JP CD 66kV, 5kA TBD
Furukawa TEPCO, JP CD 275kV, 3kA Bi – 2223
Sumitomo Chubu U., JP, 2010 CD 10kV, 3kA DC,
20m, 200m
Bi – 2223
VNIIKP Moscow, RU, 2010 CD 20kV, 200m Bi – 2223
Nexans Spain CD 10kV, 3.2kA,
30m, 1Φ
Bi – 2223
Table 6.2. Present status of Power cables vs required specifications
Present Value Required Value
Cryostat losses 1.5 – 2 W/m 0.5 W/m
AC losses ( at 2.9kA) 1.4 W/m/phase 0.2 W/m/phase
44. 44
Figure 6.1.Cold type HTCS Power cable
Figure 6.2.Warm type HTSC Power cable
Figure 6.3. Difference between AC & DC HTSC & conventional cables
Source: Nexans Superconductors
45. 45
CHAPTER 7
High Temperature Superconducting (HTSC) Magnetic Energy Storage
(MES)
7.1. Introduction
The desirability of electric energy storage is by now a given, and a number of recent studies
have examined the economics associated with various methods of storage. Some are
conventional, such as charging and discharging lead-acid batteries; other methods are more
innovative. In the storage method known as pumped hydro, electricity is generated at night
and used to pump water uphill to a basin above a hydroelectric dam; later on, during peak
demand hours, the water flows downward through turbines and generates electricity at the
time it is needed. In all cases, the figure of merit by which competing methods of storage are
evaluated is the round-trip efficiency, which means simply the ratio of power delivered upon
exit to the power input at the start.
The round-trip efficiently is weighed along with both initial capital cost and annual
operating costs to perform a cost/benefit analysis of any particular energy storage pathway.
In the case of pumped hydro, for example, Virginia Electric Power has obtained3 a round-
trip efficiency over 80%, but they incurred capital costs in acquiring land and building dams
and hydroelectric generators; and, of course, there are finite operating costs of their system.
A lifecycle cost analysis incorporates some expected-use profile, and amortizes capital costs
over the lifetime of the equipment, so as to arrive at a net cost per kilowatt figure. That can
then be compared with cost estimates for other forms of storage, and with the option of
having no storage at all. Such factors as the estimated future price of coal and natural gas
enter into the calculation.
The options available to a utility are many. Although a blackout is to be avoided through
astute advanced planning, gentle reductions in line voltage are not entirely out of the
question. Clearly, however, it is better to actually meet the full demand. Doing so may or
may not require electricity to be stored. One variation of the no-storage option is to buy
power from other utilities to meet peak demand. Not everyone can do that.
In any case, storage of electricity has a place in the utility sector. SMES is attractive because
it has a round-trip efficiency of over 90% under the right circumstances.
46. 46
Table 7.1. Comparison of various energy storage technologies
Source: KIT
7.2. Concept and Working of a SMES
SMES is a device for efficiently storing energy in the magnetic field associated with a
circulating current. An inverter/convertor is used to transform AC power to direct current,
which is used to charge a large solenoid or toroidal magnet. Upon discharge, energy is
withdrawn from the magnet and converted to AC power. Figure is a schematic diagram of a
SMES system. The components include a DC coil, a power conditioning system (PCS)
required to convert between DC and AC, and a refrigeration system to hold the
superconductor at low temperature. The inverter/converter accounts for about 2–3% energy
loss in each direction.
Figure 7.1. Block diagram of a SMES based system
Source: Sheahen
7.3. Advantages of SMES
Rapid response for either charge or discharge
Power is available when needed, not only when generated
Minimal resistive energy losses in the superconducting coil and solid state power
conditioning
Ability to go to high fields i.e. allow high power density
High hoop strength of 2G HTSC
47. 47
Continued price improvements in HTSC materials
Development in enabling technologies like cryocoolers, insulation etc
Safe – no chemical reactions, no toxins produced.
There is a further economic advantage associated with larger SMES units. Denoting the
magnetic induction by B, the energy stored in a magnetic field is proportional to the
dimensions of the SMES unit go up only linearly with B, and the refrigeration requirement is
proportional to size. Therefore, larger SMES units have the economic advantage of less
refrigeration need per stored megawatt.
7.4. State – of – the – Art SMES projects
Table 7.2. State – of – the Art SMES projects
Institution Country Year Data SC Application
KIT Germany 1997 320kVA, 203kJ NbTi Flicker
compensation
AMSC US 2MW, 2.6MJ NbTi Grid Stability
KIT Germany 2004 25MW, 237kJ NbTi Power Modulator
Chubu Japan 2004 5MVA, 5MJ NbTi Voltage stability
Chubu Japan 2004 1MVA, 1MJ Bi 2212 Voltage Stability
KERI Korea 2005 750kVA, 3MJ NbTi Power stability
Ansaldo Itlay 2005 1MVA, 1MJ NbTi Voltage stability
Chubu Japan 2007 10MVA, 19MJ NbTi Load compensation
CAS China 2007 0.5MVA, 1MJ Bi - 2223 -
KERI Korea 2007 600kJ Bi - 2223 Power, Voltage
quality
CNRS France 2008 800kJ Bi - 2212 Military
Application
KERI Korea 2011 2.5MJ YBCO Power quality
ABB/SP US 2013 2.5MJ, 20kW YBCO -
48. 48
CHAPTER 8
High Temperature Superconducting (HTSC) Fault Current Limiter
(FCL)
8.1. Introduction
Damage from short circuit currents is a constant threat to any electric power system, since it
threatens the integrity of its generators, bus-bars, transformers, switchgears, and transmission and
distribution lines . Building on this statement, the FCL is described below.
The role of the FCL is to limit prospective fault current levels to a more manageable level without a
significant impact on the distribution system. Consider a simple power system model, as shown in
figure, consisting of a source with voltage VS, internal impedance ZS, load Zload, and fault impedance
Zfault.
In steady state.
Iline = VS / ( Zs + Zload ) Eq 1
When a fault occurs in a system,
Iline = VS / ( ZS + Zfault ) Eq 2
Where Zfault << Zload
Since the supply impedance ZS is much smaller than the load impedance, Equation (2) shows that the
short circuiting of the load will substantially increase the current flow. However, if a FCL is placed
in series, as shown in the modified circuit, Equation (3) will hold true;
Iline = VS / ( ZS + ZFCL + Zfault ) Eq 3
Equation (3) tells that, with an insertion of a FCL, the fault current will now be a function of not only
the source ZS and fault impedance Zfault, but also the impedance of the FCL ZFCL. Hence, for a
given source voltage VS and increasing will decrease the fault current Iline.
49. 49
8.2. Ideal fault current limiter characteristics
Before discussing any further, it is important that some of the ideal characteristics be laid out for an
FCL.
An ideal FCL should meet the following operational requirements:-
1. Virtually inexistent during steady state. This implies almost zero voltage drop across the FCL
itself
2. Detection of the fault current within the first cycle (less than 16.667ms for 60Hz and 20ms for
50Hz) and reduction to a desirable percentage in the next few cycles.
3. Capable of repeated operations for multiple faults in a short period of time
4. Automatic recovery of the FCL to pre-fault state without human intervention
5. No impact on voltage and angle stability
6. Ability to work up to the distribution voltage level class
7. No impact on the normal operation of relays and circuit breakers
8. Finally, small-size device that is relatively portable, lightweight and maintenance free
In reality, one would like to have an FCL that would satisfy all of the foregoing characteristics.
However, certain trade-offs and compromises have been made in nearly all categories and types.
8.3. Types of fault current limiters
This section presents a brief review of the various kinds of FCL that has been implemented or
proposed. FCL(s) can generally be categorized into three broad types:
1. Passive limiters
2. Solid state type limiters, and
3. Hybrid limiters
In the past, many approaches to the FCL design have been conducted ranging from the very simple
to complex designs. A brief description of each category of limiter is given below.
8.4. Passive limiters
Fault limiters that do not require an external trigger for activation are called passive limiters. The
current limiting task is achieved by the physics involved in the FCL itself. The simplest of all kinds
of fault current limiter is the inductor. The current limiting strategy is achieved by inserting
impedance Z = jωL. Since current cannot change instantaneously in an inductor, current is therefore
limited at the moment of a fault. Figure shows an inductor in series with the load and source.
There are a few pros and cons in using an inductor for FCL application:
1. Technique has been well known, installed, field tested and commissioned for many years
50. 50
2. Relatively low cost and maintenance, but
3. Bulky to handle and replace
4. Produces a voltage drop in steady state and causes lagging power factors
Another kind of passive limiter that is gaining attention is the superconducting fault current limiter
(SFCL). SFCL(s) work on the principle that under steady state, it allows for the load current to flow
through it without appreciable voltage drop across it. During a fault, an increase in the current leads
to a temperature rise and a sharp increase in the impedance of the superconducting material. Below
are a few advantages and disadvantages of using an SFCL:
1. Virtually no voltage drop in steady state
2. Quick response times and effective current limiting, but
3. Cooling technologies still at infancy, leading to frequent break downs
4. Commercial deployment is still to be witnessed
5. Superconducting coils can saturate and lead to harmonics
8.5. Solid-state limiters
Recent developments in power switching technology have made solid state limiters suitable for
voltage and power levels necessary for distribution system applications. Solid state limiters use a
combination of inductors, capacitors and thyristors or gate turn off thyristors (GTO) to achieve fault
limiting functionality.
An example of a solid state limiter is shown in Figure. In this type of limiter, a capacitor is placed in
parallel with an inductor and a pair of thyristors.
In steady state, the thyristors are turned off and all current flows through the capacitor. The
placement of the capacitor is also useful by nature because it provides series compensation for the
inductive transmission line. Hence, equation (2.4) holds true:
ZFCL (NORMAL) = -j / ωC
However, when a fault occurs the thyristors are switched on, which forces most of the current to
flow through the inductor branch. The net FCL impedance seen by the circuit is as follows.
Z FCL (FLT) = jωL / ( 1 - ω2
LC )
Below are a few advantages and limitations of solid state limiters in general:-
1. Provide significant fault current limiting impedance
2. Low steady state impedance as capacitors and inductors can be tuned for a particular frequency to
show virtually no impedance and voltage drops.
3. Harmonics introduced due to switching devices
4. Voltage drop introduced during faults
51. 51
8.6. Hybrid limiters
As the name implies, hybrid limiters use a combination of mechanical switches, solid state FCL(s),
superconducting and other technologies to create current mitigation. It is a well know fact that circuit
breakers and mechanical based switches suffer from delays in the few cycles range. Power electronic
switches are fast in response and can open during a zero voltage crossing hence commutating the
voltage across its contacts in a cycle.
In 2001, Shi et al proposed a novel Triggered Vacuum Switch (TVS) based FCL. Figure shows the
circuit arrangement of one such device.
In their work, they state that the reactance of the capacitor C1 and reactor L is about zero at nominal
power frequencies. In steady state, the TVS and SW2 are in the off state. SW2 is a quick permanent
magnetism vacuum contactor with a 3-10ms closure delay, which prevents TVS from long-time arc
erosion. When a fault occurs, a trigger signal is sent to both TVS and the contactor turning on the
bypass capacitor C1. This creates a situation where the reactor L will limit the fault current
immediately. The ZnO arrestor is used for over voltage protection and capacitor C2 and switch SW1
are set-up as a conventional series compensation.
8.7. Introduction to Superconducting Fault Current Limiters (SFCL)
8.7.1. Types of SFCLs
8.7.1.1.Resistive
The operation of this type of SCFCL is based on the quench of the superconducting material, which
describes its transition from the superconducting state to the normal conducting state. The quench
occurs rapidly when the short circuit current flowing through the SCFCL exceeds the
superconductor’s critical current.
This variation of the SCFCL utilizes a resistor in parallel with the superconducting material that
protects the superconductor from hotspots that may develop during the quench, as well as avoiding
overvoltage over the SCFCL that may damage it.
These SCFCLs are considered fail safe and can be built to exhibit negligible impedance during
normal system operation. A recovery time is however required following a quench, which can range
52. 52
from one second to under one minute, depending on the material employed. One present
disadvantage is that there is energy loss caused by the current leads passing from room temperature
to cryogenic temperature that will result in a loss of approximately 40-50 W/kA heat loss per current
lead at cold temperature (Noe and Steurer, 2007, p. 17). This would equate to a maximum operating
loss of approximately 80kW for a three phase SCFCL operating in series with a 10MW generator
connected at 11kV.
8.7.1.2. Resistive Magnetic
This variation of the SCFCL utilizes a parallel inductance with the superconducting material. Their
paper describes how the increasing magnetic field, caused by the growing current flowing in the
inductor under fault conditions, accelerates the quench and mitigates the hot spot phenomenon in the
superconducting material.
8.7.1.3. Bridge Type SCFCL
This SCFCL employs solid state technology to control the flow of current through a superconducting
inductance. The disadvantages of this Bridge Type SCFCL are that it is not considered to be fail-safe
device, and it exhibits relatively high total energy losses.
8.7.1.4. DC biased Iron core SCFCL
These devices incorporate two iron-core coils that are driven into saturation by introducing a DC
bias current under normal operating conditions. These two cores are placed in the series path of the
potential fault current. While these two cores are in operating in saturation mode, their (and hence
the SCFCL) inductances are low. When fault current flows, these coils will be driven out of
saturation resulting in an increase in the apparent coil inductance. This concept has the advantage of
requiring relatively less superconductor material, and a smaller cryogenic system is required to cool
the device. The requirement for the iron cores does however make the device bulky when compared
to other SCFCL devices
8.7.1.5. Power Electronics
Power electronic components may be used to interrupt the fault current and direct it through limiting
superconducting impedance, thereby controlling the magnitude of the fault current along the
particular path. Once again, these devices will not be considered fail-safe as the failure of one power
electronic device can lead to mal – operation of the fault current limiting device.
53. 53
8.8 Conceptual Design of Resistive type SFCL
Figure 8.1. Conceptual design of resistive type SFCL
Source: Converteam
8.9. Basic Design Aspects
SCFCL passively limits a fault current by intrinsically developing resistance under over-current.
The rated power of a SCFCL is defined by P = IN UN; where IN is the nominal current (current in
normal operation) and UN is the voltage of the system protected by SCFCL, which approximates the
total voltage developed across the conductor during a fault. IN is given by Ajc / √2, with A being the
cross-section of HTS and jc its critical current density. UN is given by Lemax / √2, where L is the
length of HTS and Emax is the designed maximum electric field.
Power application is most practically realized both by a high Emax and a long length L. As a
practical approach, long length can be achieved by structuring a plate into a long meander. For
YBCO thin film, an Emax value of around 25 V/cm has been reported. However, in reality a much
more compromised value is taken because designs with high Emax are more prone to hot spot.
SCFCL with distinctively different limitation behaviors can be tailored by simply varying the Emax.
For economical HTS conductors, a current carrying capability, expressed as Ampere per width,
higher than 100 A/cm would be required. This can be achieved either by high jc and/or large cross
section.
The exploitation of cross-section, A, has its limitation firstly, because SCFCL components usually
take the form of plates where a compact design calls for a minimized width and secondly, the
thickness is limited because of AC-losses.
For Bi-2212 with a typical jc in the range of 1000–10,000 A/cm2 at 77 K, sufficient current
capability can be achieved with bulk conductor (thickness in millimeter range, which can still be
tolerated from the AC-losses point of view).
54. 54
8.10. State – of – the – Art SFCL projects
Table 8.1. State – of – the Art HTSC FCL projects
8.11. Current status of SFCL Technology
In 2001 ABB reported the successful test of an 8kV, 6.4 MVA resistive SCFCL. No new information
regarding this development was available, with ABB concluding that the widespread application of
such devices would only be achieved with the realization of low cost superconductors and cost
effective and reliable cooling.
Nexans Superconductors have developed a 3Φ, 10MVA, 10kV resistive SCFCL that was field tested
in Germany for one year from 2003. It was named CURL 10 and the test was deemed successful for
MV applications. The device is currently undergoing further testing in Germany.
Following on from the Nexans CURL 10 resistive SCFCL development described in the paragraph
above, the company have moved to develop a resistive type SCFCL with magnetic field assisted
quench (i.e. resistive magnetic ).The aim of this project is to develop a 110kV, 1.8kA demonstrator.
Following earlier successful research relating to the Matrix Fault Current Limiter Project, an
American based project is developing a 138kV SCFCL using the pure resistive SCFCL concept and
the latest second generation (2G) superconducting components (Superpower, 2006). This project
forms part of the US Department of Energy’s Superconductivity Partnership Initiative program and
the use of the 2G components promise to make this development more cost effective and
commercially viable.
A national project is currently underway in Japan to develop and demonstrate a 6.6kV, 600A
resistive SCFCL application. In Korea, the ten-year “Dream of Advanced Power Systems by Applied
55. 55
Superconductivity (DAPAS) Technology Program” is aiming to commercialize superconducting
power equipment. During the first phase of the program they have successfully built and tested a
6.6kV
SCFCL.
Innopower in China are developing a 35kV prototype DC biased iron core SCFCL.
Many challenges lay ahead for developers and manufacturers of SCFCLs. As utility (substation)
based solutions will be required to have a life in excess of 30 years, the ageing and long term
behavior of the superconducting material needs to be understood.
As this is relatively new and unexploited technology, such information is not available at this stage.
As a result of the relatively high cost of these superconducting devices, research and development is
currently focused on the MV and HV applications where large technical and economic benefits are
to be achieved.
56. 56
CHAPTER 9
High Temperature Superconducting (HTSC) Transformer
9.1. Introduction
Transformers utilizing High Temperature Superconductors are perceived as a
“breakthrough” technology coming at an “opportune time”.
High Temperature Superconductor (HTSC) properties, improved refrigeration reliability and
lower refrigeration costs make it possible to overcome the limitations experienced in the
Low Temperature Superconducting (LTSC) designs of the 70’s and the 80’s. But
commercial success will depend on demonstrated reliability of operation and the scale up of
HTSC manufacturing.
9.2. Benefits of a HTSC Transformer
9.2.1. Greater Effective Capacity
One major advantage of HTSC transformer is reduced size and weight. Another is a
distinct environmental plus – in the conventional transformer, oil is a fire hazard and a
potential contaminant, whereas in the HTSC Transformer, the only substance present in
large volume is the non – inflammable and environmentally benign liquid nitrogen. But
perhaps the key advantage is the capability for over – capacity operation, due in part to the
low temperatures at which the HTSC windings operate.
Heat is the principle enemy of the paper oil electrical insulation system of conventional
power transformers. In order to meet the desired life of 30 or more years, transformer
capacity ratings are based on holding the temperature of the hottest part of the insulation
under 1100
C. Thermal damage is cumulative, so that operation at only 200
C over the limit
for a total of 100 days – less than 1% of 30 years – will reduce the transformers life by 25%.
In view of this sensitivity, the thermal management of conventional transformers has
received much attention in the recent years. This is also because utility customers are
making much heavier use of air – conditioning systems, even in colder climates, giving rise
to peak loading conditions that can last 10 hours or more on the hottest days of the year.
Loss of insulation life can be significant under these conditions. So transformers are
increasingly being purchased with excess capacity, just to meet maximum temperature limits
that may occur only on a few days. The upshot is that they operate well below an optimal
level most of the time.
In contrast, HTSC Transformers, their windings and insulations necessarily operate in the
ultra – cold range of 20K to 77K, where insulations will not degrade. HTSC units can run at
rated power continuously and efficiently. In fact, at up to twice rated power, they can run for
57. 57
indefinite periods of time without any loss of operating life, albeit at greatly reduced
efficiency because of a disproportionate increase in the use of liquid nitrogen or an increased
refrigeration load. Thus one HTSC transformer can in emergencies carry the loads normally
handled by two, and HTSC Transformer lifetime can be greatly extended.
Refer figure 9.1. for a better visualization
9.2.2. Low impedance with immunity
HTS Transformers will normally be designed to operate as one – for – one replacements
for conventional transformers, complete with an ability – limited only by their own internal
impedance – to operate through a fault current of 10 – 12 times the rated current. But they
also can be configured to provide additional power system advantages.
Preliminary analyses done by labs worldwide indicate that they can be built to have very
low internal impedance and still, through an alternative fault current limiting transformer
design, be self protecting against the higher fault currents that could result. It may be
possible, if needed, to limit the low – voltage side current to the rating of existing breakers.
Low impedance makes the transformer better at maintaining output voltage levels over a
wide range of operating power levels and better able to transmit power downstream through
the power system. Utilization of this feature will involve consideration of transformer
interfaces with the grid and the load in each situation, and may especially apply to a new
power construction where a complete system of compatible components can be installed in
an economical way.
Conventional transformers are efficient (typically 99.3 – 99.7 % for the 30MVA class,
depending upon loading), but there is considerable room for improvement. About 25% of
the 7 – 10% losses in transmission and distribution systems occur in power transformers.
The transformer loss costs more than $2 billion annually in United States alone.
Most of the conventional transformer losses are due to resistive heating in its windings – and
HTSC transformers have zero winding resistance. Admittedly, the HTSC versions still have
ac losses in the iron core and low levels of other kinds of ac losses in the windings that
require refrigeration power. Nonetheless, they can be substantially higher in efficiency than
conventional transformers, to the extent that the reduced loss in each HTSC unit can more
than pay for its initial capital cost over its lifetime.
9.3. Design tradeoffs and cost drivers of HTSC Transformers
Zero resistance and 10 – 100 times greater current density promise striking advantages in
transformer size and performance. Classical resistive losses are eliminated, and the quantity
of conductor in the HTSC Transformer windings can be reduced to tens as against thousands
of kilograms for the conventional transformer. Since the windings in principle require little
space and generate little resistive heat, it should be possible to make superconducting
transformers inexpensively, with greatly reduced power capacity, much increased efficiency,
58. 58
and very much smaller size. While these advantages can be realized in large part, they
cannot all be achieved to the same degree in the same transformer. As always, there are
practical limitations and tradeoffs
Ultimately, reductions in size will be limited by dielectric design considerations. The
transformer must meet the international standard dielectric tests for system voltages and the
basic impulse insulation test levels that are specified. For example, a 138kV winding may
need to withstand impulse voltages of 650kV. The design of the transformer winding must
include sufficient space for insulation if it is to accommodate these high voltages with
commercially available dielectric materials and proven design approaches.
Iron core size, which is related to winding size, mainly determines overall transformer size
and weight. Eddy current and magnetic hysteresis losses are produced in the core in direct
proportion to the core volume. These losses tend to be on the order of tens of kilowatts,
much too large to be economically removed by low – temperature refrigerators. HTSC
transformers are consequently designed to operate with cores near ambient temperature and
isolated thermally from the windings. If the core is too large, its losses occur regardless of
whether current (power) is drawn from the transformer, they contribute strongly to the total
owning costs. So there are strong incentives to reduce core and winding size.
But reducing core diameter adds to the number of turns and so to the total length and cost of
the HTSC conductor. Though the superconductor winding has no classical resistive losses,
there are several forms of eddy current and hysteresis losses, which depend on the
magnitude of the ac magnetic flux density in the transformer windings, typically a maximum
of 0.1 – 0.3T. Compared to conventional resistive and eddy current losses, ac losses in the
HTSC transformer winding are small; but because they occur at low temperatures, it takes
many times their value in refrigeration power to extract the heat produced. The multiplier is
20 at 77K, increasing to over 100 at 20K.
Great care is therefore given to the design of low-loss conductor and winding
configurations. At a fixed transformer power rating, the ac flux density in the windings is
increased as the size of the transformer core (and windings) is reduced. Dielectric ac losses
in insulating materials also tend to increase as the volume of the winding is reduced. HTSC
transformer are therefore made with the core large enough so that the conductor quantity and
the cost are reasonably low and the fields on the windings are low enough to keep ac losses
within reasonable limits.
Another trade off involves the current density of superconductors, which increases as their
operating temperatures are increased. Clearly, the lower the operating temperature, the less
HTSC material is needed to provide the ampere-turns of the transformer windings, and the
lower its cost becomes. But, as noted earlier, the lower the operating temperature, the higher
are both refrigeration capital costs and the refrigeration power needed to remove the ac
losses that are generated.
![2
BONAFIDE CERTIFICATE
Certified that the project work entitled “HIGH TEMPERATURE
SUPERCONDUCTIVITY IN ELECTRICAL POWER APPLICATIONS;
A DETAILED STUDY OF HTSC SYNCHRONOUS MACHINE &
EXPERIMENTATION ON 1ST
GENERATION AND 2ND
GENERATION
HTSC WIRES” submitted to SASTRA University, Thanjavur by
SRINAGADATTA SRIKRISHNA R (Reg. No:011205146), in partial
fulfillment for the award of the degree of Bachelor of Technology in Electrical
and Electronics Engineering is the work carried out independently under my
guidance during the period Jan 2012 – April 2012.
Project Guide
[Project Guide Name]
[Designation & Affiliation]
External Examiner Internal Examiner
Submitted for the University Exam held on ___________](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)