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Development of HTS Transformers and a 10 kVA HTS Transformer Prototype Design Xiao-Yuan Chen and Jian-Xun Jin Abstract⎯With the improvement of high temperature superconductor (HTS) practical performance, research and development concerning the applications of HTS transformers have been progressed actively worldwide. This paper provides a comprehensive summary on various HTS transformers, and studies the design of a single-phase 10 kVA (220V/24V) HTS transformer prototype to verify HTS for practical transformer applications. Index Terms⎯HTS, HTS power transformer, HTS traction transformer, HTS windings.
1. Introduction Transformers represent one of the oldest and most mature elements in power transmission and distribution network. Serious interests in superconducting transformers began in the early 1960s as reliable low temperature superconductors (LTSs) became available. Analysis of the feasibility of LTS transformers concluded that the high refrigeration loads, which required keeping the LTS materials at 4.2 K, made the LTS transformers uneconomical. Immediately following the discovery of HTS materials with higher temperature since 1986, several studies looked into the feasibility of HTS transformers. The benefits of high temperature superconductor (HTS) transformers mainly include lighter and smaller units, higher efficiency, lower life-cycle costs, fewer hazards and reduction of environmental pollution comparing with conventional ones[1],[2]. With the continued development of HTS material performance, HTS transformers have been expected to be one of the most promising HTS device applications.
2. Classification and Structure HTS transformers have three main classification methods: 1) according to windings of different materials, Manuscript received March 1, 2008; revised April 1, 2008. X.-Y. Chen and J.-X. Jin are with the Center of Applied Superconductivity and Electrical Engineering, University of Electronic Science and Technology of China, Chengdu, 610054, China (e-mail: cxy_yjs@ yahoo.com.cn and
[email protected]).
HTS transformers can be divided into HTS conventional transformers with the windings formed by HTS materials and HTS hybrid transformers with the windings formed by HTS and copper materials; 2) according to having a magnetic core or not, HTS transformers can be divided into HTS core-type transformers and HTS air-core transformers; 3) according to different application fields, HTS transformers can be divided into HTS power transformers and HTS traction transformers. Generally, a HTS transformer is primarily consisted of HTS windings, magnetic core, cryostat and refrigerator. 2.1 HTS Winding HTS conductors which are commonly used in HTS transformers can be divided into two types: the first generation bismuth-strontium-copper-oxides (BSCCO) HTS composite conductors, mainly applied in two forms of Bi-2212 and Bi-2223, and the second generation yttrium-barium-copper-oxide (YBCO) HTS coated conductor. Bi-2223 are usually used to make HTS windings. Currently YBCO begin to be considered because YBCO have higher current density and better current magnetic field characteristics than BSCCO. Solenoidal winding, pancake winding and layer winding are common types in HTS transformers. Recently a so-called continuous disk winding (CDW) was developed in Korea[3]. 2.2 Magnetic Core According to the relative position between the core and windings, the core can be divided into core-type and shell-type; according to the operating temperature, the core can be divided into low temperature (LT) and room temperature (RT) cores; according to the core of different materials, the core can be divided into ferrite core, stalloy core, permalloy core, amorphous alloy core and nanocrystalline alloy core. 2.3 Cryostat Cryostat is manufactured by fiber reinforced plastic (FRP) and commonly formed by inner layer, outer layer, and vacuum and super-insulation layer to minimize thermal invasion. There are two main types of cryostat in HTS transformer. One serves as a cryogenic vessel for both LT core and windings, and the other is only for installing HTS windings. The former type has simple structure and can be easily manufactured. The latter type is usually in shape of
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toroid so that RT core can go through in midway and also acts as an insulator between RT core and HTS windings. 2.4 Refrigerator Generally, HTS transformer systems are cooled by saturated liquid nitrogen (LN2) (77 K), but sub-cooled LN2 is more suitable to obtain higher efficiency and better electrical perfor- mances[4]. To produce sub-cooled LN2, a refrigerator is needed and its temperature is required below 63 K. For this purpose, Gifford-McMahon (GM) cryocooler, Stirling cryocooler and pulse tube cryocooler are suitable because of their small size, light weight and easy operation.
3.2 HTS Traction Transformer Traction transformers are stalled in railway locomotive vehicle, the limits of placement and weight lead to serious problems for conventional traction transformers. Current research and development on HTS traction transformer have been well proposed and proceeded in Germany, Japan and China, with the aims to improve the efficiency and lower the size and weight of traction transformer. Major achievements and development are briefly summarized in Table 2. Table 2: Development of HTS traction transformers Country
3. Development Status Recent development of the HTS transformers has two main directions: 1) power transformers; 2) traction transformers, mainly in the United States, Japan and Europe, China and Korea[5],[6]. Some special HTS transformers including HTS air-core transformer, HTS fault current limiting transformer (FCLT) and HTS hybrid transformer are also briefly presented in this section. 3.1 HTS Power Transformer Power transformers are required to have high reliability and high efficiency with the increasing development of power industry. Conventional transformers become more and more unsuitable because of their inherent limitations. Therefore, researches on the HTS transformer are necessary. HTS power transformers have achieved great improvement since the appearance of HTS in 1986 and they are briefly summarized in Table 1. Table 1: Development of HTS power transformers Country
USA
Japan
China
Korea
Switzerland Italy France
Development time ~1998 ~2004 2005~ ~1996 ~2001 ~2004 2005~ ~2003 ~2004 ~2005 ~2004 ~2004 ~2006 ~2007 ~1997 ~2005 ~2004
Rated capacity 1 MVA 5/10 MVA 30/60 MVA 500 kVA 1 MVA 2 MVA 10 MVA 26 kVA 45 kVA 630 kVA 1 MVA 60 MVA 33 MVA 100 MVA 630 kVA 10 kVA 41 kVA
Voltage level 13.8 kV/6.9 kV[7] 24.9 kV/4.2 kV[8] 138 kV / 13.8 kV[9] 6.6 kV/3.3 kV[10] 22 kV/6.9 kV[11] 66 kV/6.9 kV[12] 66 kV/6.9 kV[12] 400 V/16 V[17] 2400 V/160 V[18] 10.5 kV/400 V[19] 22.9 kV/6.6 kV[20] 154 kV/23 kV[21] 154 kV/22.9 kV[22] 154 kV/22.9 kV[23] 13.7 kV/0.4 kV[13, 14] 1 kV/0.231 kV[15] 2.05 kV/0.41 kV[16]
Germany Japan China
Development time ~1999 ~2001 ~2004 ~2005
Rated capacity 100 kVA 1 MVA 4 MVA 300 kVA
Voltage level 5.5 kV/1.1 kV[24] 25 kV/1.4 kV[25] 25 kV/1.2 kV[26] 25 kV/0.86 kV[27]
2007~
3 MVA
25 kV/0.86 kV[28]
3.3 Special HTS Transformers A. HTS Air-Core Transformers Main advantages of HTS air-core transformers comparing with conventional and HTS core-type transformers include no iron loss and no magnetic saturation, reduction of size and weight, increased freedom of design, no insulation care to iron core, no inrush current caused by magnetic saturation, and no higher harmonics induced by magnetic saturation. Also, HTS air-core transformers have the potential of serving not only as power transformers but also as shunt reactors in power transmission systems. However, their major defects include large excitation current and high magnetic flux leakage, which make them unpractical and uncommercial. Little research on HTS air-core transformers has been progressed in the world. A single-phase 2.5 kVA (300 V/150 V) air-core autotransformer using LTS technology was fabricated in Japan[30]. But the research is currently suspended and expected to develop air-core transformer by using HTS technology. In Spain, a HTS air-core prototype transformer was constructed in 2002[31],[32]. All these studies around the world still stayed at laboratory research stage and could not develop practical HTS air-core transformer. B. HTS FCLT In order to improve the total efficiency, controllability and stability in power system, a HTS FCLT was proposed. The HTS FCLT has the functions of HTS transformer since it can reduce leakage impedance of HTS transformer and improve the system stability and capacity in normal operating condition. In the fault condition, it serves as HTS fault current limiter (FCL) because the limiting impedance induced by the quench of HTS FCLT winding will reduce the fault current and bring about the improved dynamic
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stability of the power system . Japan has already designed and fabricated a three-phase 6.25 kVA (275 V/105 V) HTS FCLT in 2004 and verified fundamental characteristics as a transformer and a FCL. Additionally, a small-size HTS FCLT with YBCO conductors was designed in Hungary[35]. C. HTS Hybrid Transformers Many modern industrials need high-voltage or large-current power supply, but conventional transformers can hardly provide sustained and stable high voltage or large current. It is expected to introduce HTS technology for satisfying the requirements of modern devices such as particle accelerator, furnace and welding transformers, etc. Japan has proposed the HTS hybrid transformer since 1980s[36], but little research with regard to the HTS hybrid transformer has been proceeded in the world. Recently, innovation development on HTS special transformers has been achieved, i.e., HTS high-voltage transformer[37] and HTS large-current transformer[38] novel concepts been presented and verified, which are expected to be practical in various industrial applications.
4. A HTS Transformer Prototype Based on the results from the summary of HTS transformers, a single-phase 10 kVA (220 V/24 V) HTS transformer prototype is studied and analyzed to validate the practical HTS transformer. The sketch of the HTS transformer is shown in Fig. 1. Main specifications of the designed HTS transformer are shown in Table 3.
4.1 Specifications of YBCO HTS Conductor BSCCO HTS conductors are usually used to design electrical power devices including HTS transformers, however, YBCO HTS conductors have higher current density and better current magnetic field characteristics. YBCO HTS conductor is chosen to design the 10 kVA HTS transformer. The specifications of YBCO HTS conductor is shown in Table 4. Table 4: Specifications of the YBCO HTS coated conductor Width of HTS conductor
4.0 mm
Thickness of HTS conductor
0.1 mm
Critical current
103 A at 77 K, 0 T
Current density
10 kA/cm2
4.2 Geometrical Configuration Design In this section, practical designs of iron core, HTS windings, and cryostat are presented for the 10 kVA HTS transformer. Main geometries of the 10 kVA HTS transformer are shown in Table 5. Table 5: Main geometries of the HTS transformer Iron core
Windings
Power system
Load
Controller
100 mm
Cross-section area
6340 mm2
Voltage per turn
2.4 V/turn
HV-turns
92
HV-internal radius
90 mm
HV-external radius
94 mm
HV-height
150 mm
LV-turns
10
LV-internal radius
114 mm
LV-external radius
116 mm
Terminal panel
LV-height
150 mm
Fixture
Internal radius
55 mm
External radius
300 mm
Height
350 mm
Magnetic core Cryostat
LN2
Diameter
Cryostat
Primary windings Secondary windings Searching windings
A. Core Design A stalloy core (RT core) with the maximum magnetic density Bm=1.7 T is chosen to design the 10 kVA HTS transformer. The core diameter D can be obtained by B
Cooler
Fig. 1. Sketch of the designed HTS transformer. Table 3: Specifications of the designed HTS transformer Phase
1
Capacity
10 kVA
Frequency
50 Hz
Voltage ratio (prim. / second.)
220 V/24 V
Current (prim. / second.)
45A /416A
Cooling method
LN2 (77 K)
D = K 4 P = 97.8 mm
(1)
where the experience parameter K is 55. In this design, an estimation value D=100 mm is practically chosen. So the cross-section area of the core can be obtained by 2
⎛D⎞ (2) Az = π ⎜ ⎟ K f K l = 6340 mm 2 ⎝2⎠ where the filling factor Kf is 85%, and laminated factor Kl is 95%.
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B. Windings Design Referring to (2), a suitable voltage per turn can be obtained by Et = 4.44 fBm Az = 2.4 V/turn (3) For the voltage of primary windings V1=220 V and voltage of secondary windings V2 = 24 V, we get the turns of primary windings (N1) and secondary windings (N2) by V 220 N1 = 1 = = 91.7 (4) Et 2.4 V2 24 = = 20 . (5) Et 1.2 Practically, the N1=92 is chosen for primary windings in the design. Considering the effects from cryostat and practical windings with a reasonable filling factor, the internal radius, external radius and height of primary windings are designed to be 90 mm, 94 mm and 150 mm, respectively. The internal radius, external radius and height of secondary windings are 114 mm, 116 mm and 150 mm, respectively. C. Cryostat Construction A suitable cryostat is designed to match with the designed core and HTS windings. Its internal radius, external radius and height are 55 mm, 300 mm, and 350 mm, respectively. The designed cryostat serves as a cryogenic vessel for HTS windings, and it is in shape of toroid so that the designed core can go through in midway and also acts as an insulator between core and HTS windings.
of total windings (B1), B of high-voltage windings only (B1H), and B of low-voltage windings only (B1L) in different cases along the line a are shown in Fig. 3. It can be seen that the magnetic flux leakage is very high comparing with the maximum magnetic flux (about 0.03 T). Practically it is a HTS air-core transformer and should be studied to lower the high magnetic flux leakage. B
B
0.025
0.020
N2 =
4.3 Magnetic Field Analysis QuickField software is the most easy-to-use tool for finite element analysis in the world, and can be applied in various projects, such as, capacitors, sensors, transformers, transmission lines, heat transfer systems, etc. In this design, QuickField is used to analyze the magnetic distribution of the transformer. A. Analysis of HTS Windings without a Core Assume that the center coordinates are (0, 0) and the coils are symmetrically placed around (0, 0), then we draw line a from (−150, 76) to (150, 76) to analyze flux density B distributions.
B
B1 B1L B1H
B (T)
0.015
0.010 0.005 0 −150
−100
−50
0 X (mm)
50
100
150
Fig. 3. B distributions of HTS windings without a core in different cases.
B. Analysis of HTS Windings with a Core In the design, an iron core was introduced to design a HTS core-type transformer. Magnetic field distribution of HTS windings with a core is shown in Fig. 4. B distributions of total windings (B), B of high-voltage windings only (BH), and B of low-voltage windings only (BL) in different cases along the line a are shown in Fig. 5. Parallel flux density Bx(//c) will affect the HTS conductor critical current Ic, i.e., the Ic decreases while Bx increases. Quench occurs when the current value through HTS windings reach or exceed the critical current value. It is essential to reduce Bx so that the operating current of practical windings can reach a larger value and HTS transformer may provide a better performance. From Fig. 6, Bx1 represents Bx of the windings without a core and is larger than Bx of the windings with a core. Bx decreases while a core is added, which is favorite for the HTS windings. B
B
B
B
B
B
B
B
B
0.044 Line a 0.037
1.8
0.029
1.5
0.022
1.2
0.015 0.007
Line a
0.9 0.6
0
Fig. 2. Magnetic field distribution of HTS windings without a core.
The total steady magnetic field distribution of HTS windings without a core is shown in Fig. 2. B distributions
0.3 0
Fig. 4. Magnetic field distribution of HTS windings with a core.
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1.8 1.5
0.03 0.02 0.01
0. 9
B (T)
B (T)
1.2
0. 6
−0.02
0 −150
0.018
−100
−50
0 X (mm) (a)
50
100
150
0.009 0.006 0.003 0 −150
−100 X (mm) (b)
−50
1.73 B BL BH
1.71
−50
0 X (mm)
50
100
150
4.4 LabVIEW-Based Test and Control System Design LabVIEW is a graphical programming language based on the virtual instrument software, widely used in the field of measurement and control system. It provides almost the entire classic signal processing function and plenty of advanced signal analysis tools, not only can be easily integrated with hardware to achieve various data acquisition, but also can be used to communicate with a variety of standard buses and thus control of the instruments to achieve more accurate data access, analysis and processing. By using LabVIEW, a new system to test accurate and precise performance measurements of the 10 kVA HTS transformer is proposed, such as I-V characteristics test, AC losses test, critical current test, short circuit test, and so on. A block diagram of LabVIEW test system is shown in Fig. 7.
1.70 Flow meter
1.69 1.68
Temperature sensor
1.67 −50 −40
−30
−20 −10 0 10 X (mm) (c)
20
30
40 50
HTS transformer
B BL BH
0.018 0.015
Data collection module
Current detector Quench detector
0.012 B (T)
−100
B
0.012
1.72
−0.03 −150
Fig. 6. Bx (//c) distributions in different cases.
B BL BH
0.015
B (T)
0 −0.01
0. 3
B (T)
Bx Bx1
Data process
0.009 0.006
Main control module
0.003
Data record Data display
LabVIEW-Based Test & Control System Quench protection
0 50
100 X (mm) (d)
150
Fig. 5. B distributions of HTS windings with a core in different cases: (a) B distribution of HTS windings; (b), (c), (d) enlargement of B distribution.
Control output
AC power
Fig. 7. A block diagram of LabVIEW-based test and control system.
CHEN et al.: Development of HTS Transformers and a 10 kVA HTS Transformer Prototype Design
Additionally, a LabVIEW-based control system is designed to control and protect the designed HTS transformer: a controllable AC power is proposed to measure more precise data based on different conditions, the proposed controllable AC power can provide adjustable current output, adjustable frequent output and arbitrary signal wave including sine, triangle and square wave; a quench detection and protection circuit is also design by using LabVIEW, which can detect the occurrence of quench and protect the HTS transformer quickly and precisely. The block diagram of LabVIEW-based control system and principle of controllable AC power and quench protection are shown in Fig. 7 and Fig. 8, respectively.
HTS transformer
Power control
[5]
[6]
[7]
[8]
[9] Load
Quench protection
[10]
LabVIEW control system
[11] Fig. 8. Principle of controllable AC power and quench protection.
5. Conclusions After the discovery of HTSs and the availability of the first HTS wire for use in a winding, a number of groups all over the world began working on the development of HTS transformers. Investigation and summary on various HTS transformers developed has been presented in the paper. A 10 kVA HTS transformer prototype has been designed and studied to verify the HTS technology for practical transformer applications. The physical design of the prototype HTS transformer and its steady magnetic field analysis have been studied and presented. A LabVIEWbased test and control system has also been developed to control, protect and test the performance of the 10 kVA HTS transformer forming a whole HTS transformer system. Optimization of the 10 kVA transformer will be carried and presented in the future.
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Xiao-Yuan Chen was born in Jiangxi Province, China, in 1986. In 2007, he received the B.S. degree from the Chengdu University of Technology. He is currently pursuing the M.S. degree with the University of Electronic Science and Technology of China (UESTC). His research interest is in the high temperature superconductivity. Jian-Xun Jin was born in Beijing, in 1962. He received B.S. degree from Beijing University of Science and Technology in 1985, M.S. degree from University of New South Wales, Australia in 1994, and Ph. D. degree from University of Wollongong, Australia in 1997. He was a research fellow and Australian ARC project chief investigator and senior research fellow with Australian University of Wollongong from 1997 to 2003. He is currently a professor and the Director of the Center of Applied Superconductivity and Electrical Engineering, UESTC. His research interests include applied high temperature superconductivity, measurement, control and energy efficiency.
