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Tests on a 30 kVA class superconducting transformer
Tests on a 30 k V A class s u p e r c o n d u c t i n g transformer* E.S. Yoneda, I. Tashiro, M. Morohoshi and D. Ito Toshiba R&D Center, Toshiba Corporation, 4-1, Ukishima-cho, Kawasaki-ku, Kawasakishi, Kanagawa 210, Japan To demonstrate the applicability of superconductors to electric power machines, the present authors made and tested a 30 kVA class single-phase superconducting transformer. The aim of the study was to determine the superconducting transformer properties. Therefore the superconducting transformer has a simple structure, i.e. the primary to secondary voltage ratio is 1:1 and the iron core is immersed in liquid helium. The core loss, evaluated from no-load tests, was 13 W and leakage impedance, obtained by short circuit tests, was 0.02 ft in accordance with a calculated value. The superconducting transformer showed the limitation effect of fault currents. The authors succeeded in continuous operation with a 0.5 ~2 load resistance. These results suggest that efficiency can be 98.5%, if the iron core is located outside the cryostat and if high Tc superconductors are used as current leads. Superconducting windings exhibit training quenches in general. The authors also developed a superconducting transformer quench detector with a third winding around the iron core. The quench detector revealed that the secondary winding quenches before the primary winding.
Keywords: superconductors; transformers; stability
Progress achieved in superconductor manufacturing technology had promoted the development of low loss superconductors for a.c. use. The present authors developed and successfully operated a 500 kVA class superconducting coil at 5 0 - 6 4 Hz in collaboration with CRIEPI ] . To demonstrate the applicability of superconductors to electric machines, the authors made and tested a superconducting transformer, which is both the most basic static electric machine and easy to fabricate. A 200 kVA 2 class and a 70 kVA 3 class superconducting transformer have been reported previously. A superconducting transformer has three basic characteristics, light weight, high efficiency and a fault current limitation effect2. Equation (1) shows the nominal voltage, V, for a transformer 4. V = 4.44 fNBS
(1)
where: N is the number of winding turns; B is the maximum flux density in a core; f is the supply frequency; and S is the core cross-sectional area. Usually, a superconductor can be used at higher current densities than a conventional Cu conductor. More turns of superconducting windings than Cu windings can be wound in the same window size. Therefore, to obtain the same nominal voltages as with a conventional transformer the iron core *Paper presented at the S y m p o s i u m on Superconductor Stability, 13 - 15 November 1990, Y o k o h a m a , Japan
weight, which is proportional to S, can be lighter than in the conventional transformer. Simultaneously, the superconducting transformer efficiency, 7, given by Equation (2) 4, can be increased with decreasing iron volume, i.e. iron loss, Q~, if the sum of the superconductor a.c. losses, Qs, in the winding and heat leakage through current leads, QI, is almost the same, or less than, the copper losses = K/(K + Qs + Q~ + Qi)
(2)
where K is a nominal capacity. Conventional large capacity transformers limit fault currents by having a designed per cent impedance of = 10%. On the other hand, a superconducting transformer can limit large fault currents by its critical current. Therefore, for a superconducting transformer, the per cent impedance can be designed to be very small. This paper presents experimental results obtained for a 30 kVA class superconducting transformer.
Superconductor and transformer structure The transformer windings are wound with a 42-strand cable, whose parameters are listed in Table 1. Previously, the same 42-strand cable was used in a 500 kVA class superconducting coil winding and could generate 2 T peak field at 100 A (r.m.s.) ~.
0011 - 2 2 7 5 / 9 1 / 0 7 0 6 5 5 - 0 5 ~) 1991 B u t t e r w o r t h - Heinemann Ltd
Cryogenics 1991 Vol 31 July
655
Tests on 30 kVA class superconducting transformer: E.S. Yoneda et al. 1 Conductor parameters
Table
Strand
Wire diameter (mm) Number of filaments Filament diameter (/zm) Filament twist pitch (ram) Nb - Ti/Cu/Cu - 10%Ni ratio Insulation
0.112 14478 0.49 0.98 1:0.1:2.5 No insulation
7-strand cable
Diameter (ram) Twist pitch (mm)
0.34 3.05
42-strand cable
Diameter (ram) Twist pitch (mm) Centre core wire Core diameter (ram) Core insulation Cable insulation
6.7 Stainless steel 0.37 10/tm poly-vinyl-formal No insulation
700 600 <
\
5OO
f-
400
1.04
a diagram of its structure are shown in Figure 2. The nominal characteristics for the transformer were 30 kVA rating, 100 V primary voltage and 300 A primary current. Since the aim of this work was to investigate superconducting properties, the transformer had a simple structure. For example, the primary to secondary voltage ratio was 1:1 and the iron core was immersed in liquid helium, together with the windings. In order to reduce leakage impedance, the primary and secondary coils have a coaxial winding structure.
\
-6 3OO U
o
200
No-load test
IO 0 I
O
I
I
i
I
I
2 3 4 5 Mognetic field (Teslo)
6
Figure 1 D.c. critical current and load line for the 30 kVA class superconducting transformer
To prevent wire motion, the cable was embedded in grooves on fibre reinforced plastic formers and wound with strong tension. Figure 1 shows d.c. critical currents for the cables measured for short samples. The superconducting transformer parameters are listed in Table 2. A photograph of the transformer and
Table
2
Short circuit test
The leakage impedance, Xl, obtained from the primary voltage to primary current ratio, was 0.02 fl, in accor-
Superconducting transformer parameters ZDKH 6.95 1.7 19.6 22.3
Material Core weight (kg) Maximum flux density (T) Cross-sectional area (cm 2) Window area (cm 2)
Core characteristics
Windings Voltage (V) Current (A) Number of turns Coil in Figure 2
656
The no-load test results are shown in Figure 3. The primary to secondary voltage ratio, obtained from the no-load tests, was l:l, as shown in Figure 3a. This value was in accord with the designed value. The primary voltage versus primary current curve is shown in Figure 3b. The no-load current at 100 V rating voltage, obtained from Figure 3b, was 0.765 A. Figure 3b also shows that the primary voltage was saturated at 120 V. Iron loss was directly measured by a wattmeter. The iron loss, obtained from Figure 3c, was 13 W.
Cryogenics 1991 Vol 31 July
Primary
Secondary
100 300 75x2 P-l, P-2
1O0 300 75x2 S
Tests on 3 0 k VA class s u p e r c o n d u c t i n g transformer: E.S. Yoneda et al.
150
t
,/,/
100
(a) /o
/o
50
0 0 Q) O0
/o
/o
/° e0 I
1
50
lO0
150
Primary voltage (V) 150
O0
0 0
100 (a)
(b)
125
I
0 0
0
E r_ o..
"
0
g 50
0 0 0 0
I
0
Iron c o r e
I
0.5
0
1.5
1.0
Primary current (A)
wir~ing:I[ i FRP
20
(c)
0 0
If) 0 o
i,-
2
o
10
o
o o
ram)
o o
(b) 0
Figure2 (a) Photograph of the transformer and (b) diagram of its
structure
o
I00 5b ' Primary voltage (V)
150
Figure3
dance with the calculated value based on the following equation X ~ = 3 . 9 5 x 10 6 x f m N 2 k [ 6 + ( A ~ + A 2 ) / 3 ] / h
(3)
where: rn is the conductor length per single turn: 6 is the separation between the primary and secondary windings; A~ is the primary winding width; A~ is the secondary winding width; h is the winding height; and k = 1 - (6 + A~ + AO/(hHr ). The impedance voltage was 5.9 V and, therefore, the per cent impedance was 5.9%. In this test, the quench occurred at 296 A, which is less than the short sample d.c. critical current (shown in Figure 1).
No-load test results. (a) Secondary voltage versus primary voRage; (b) primary voltage versus primary current; (c) iron loss versus primary voltage
Load test and sudden short circuit test Load tests were conducted with a 0.5 ~2 load resistance. In this test, quench occurred at low current and the maximum continuous current was 248 A (r.m.s.), which is less than the nominal primary current. Figure 4 shows both voltage and current variations for a sudden short circuit test following a continuous operation state at 248 A and 138 V. The primary and secondary current show almost the same variations. After a short circuit, the primary current should increase to 9758 A, the value determined by 138 V (r.m.s.) and
Cryogenics 1991 Vol 31 July
657
Tests on 30 kVA class superconducting transformer: E.S. Yoneda et al.
g 6oo~
~2000
I~
I
~
.-
"r-
a_ -6001
~ --~'TOl
,
j
i
i
,
I
I
'
i
{~1 enc~l
0
O3
,
i
I
i
1-2000
i
,
'
=5o
a.
I O0
03
T i m e (ms)
Figure 4 Voltage ( ) and current ( - - - ) variation at sudden short circuit condition from continuous operation
where: i is the current; R is the resistance; E is the induced electromotive force at the tertiary winding around the core; I2 is the terminal voltage of a winding; X is the leakage impedance; and subscripts 1 and 2 indicate primary and secondary windings, respectively. Figure 6 shows the results of measurements using the quench detector. Figure 6a shows that the secondary current decreases, after reaching a peak current, with the rapidly increasing voltage, 12R2. This result suggests that the secondary winding quenches at the peak current. On the other hand, Figure 6b shows that, in spite of having almost the same current behaviour, i.e. the primary current decreased after reaching a peak current, the voltage i~RI, did not increase for --- 100 ms following the peak current. This result indicates that the primary winding does not quench at the peak current. The voltage increases slowly from = 100 ms after the peak current. From these results, it is revealed that the secondary winding quenched first, after which the
0.02 ft. However, as soon as the current reaches 389 A, 0.5 ms after the short circuit condition, it begins to decrease rapidly from the current peak to 82 A in 1 ms. It is shown that the short circuit current, 9758 A, was limited to 389 A, which corresponds to the superconductor quench current.
600
0 H
Quench characteristics Figure 5 shows the training quenches during the short circuit test. The quenching currents gradually increased. However, the maximum quenching current is below the critical current shown in Figure 1. Acoustic emission (AE) measurement showed that AE occurred after the quench. Primary and secondary currents couple so strongly that it is impossible to distinguish which winding originated the entire transformer winding quench from the current variation chart, as shown in Figure 4. In order to find the quench origin, the authors developed a quench detector for transformers. The quench detector can distinguish which winding quenches first, based on vector representation of a transformer. The detector monitors the voltage which appears in each winding after quench initiation, as explained by Equation (4). /l X Rl = 12, - E - il x Xl
-6000. 0 (a) >
I
2.0
1.0 Time (s)
2.O
60.0
tr
0
H
- 60.00. 0
600
(4a) 0
/z X R 2 = E -- 122 - - / 2 X X 2
I
1.0 Time (s)
l--I
(4b)
lllllBltlilJllUlJllW/ ,, ..........:.................... {f,,m mlllmlflll l llF' .............................
- 600 I
700
0.0
600
(b)
~500 = 400 D
-8 300
0
OOD
D 0
0
O
D
0
O000
0
_> E.
•
t 2.0
6.00
O
0
i l.O Time (s)
.
...........................................t#l................l ,dllllll
O . - . . . , . . - . , , , . . . , , , , ... . , , . . . . . . .. .
, ..
0 I00 I
0
"
I
4
I
I
I
I
I
I
8 12 16 Quench number
I
I
20
Figure 5 Training of transformer windings. The broken line corresponds to (he current at the crossing point of the t w o lines in
Figure 1
658
Cryogenics 1991 Vol 31 July
I
-6"000'.0
1.0
2.0
Time (s) Figure 6 Results of measurements of current, I, and I multiplied by resistance, R, using the quench detector: (a) in the secondary winding; (b) in the primary winding
Tests on 30 kVA class superconducting transformer: E.S. Yoneda et al.
primary winding quenched due to heat conduction from the quenched secondary winding.
Transformer efficiency Transformer efficiency is calculated from iron and copper losses, heat leakage through the current leads and nominal capacity. The iron loss was 13 W, obtained from the no-load tests. Copper losses correspond to a.c. losses in superconducting windings and a.c. losses in a superconductor consist of hysteresis losses and coupling losses. Hysteresis losses, Ph, are described by Equations (5) and (6) 5. Equations (5) and (6) give the hysteresis losses at the maximum field below the penetration field and above the penetration field, respectively. The penetration field for the superconductor, obtained from a magnetization measurement, was 0.13 T.
Ph = JoBodfAV In [(B + Bo)/Bo],
for B > Bp
(5)
Ph = 2AB3( B + Bo)fXV/(#:oJoBod),
for B < Bp
(6)
where: Jo is the critical current density at zero field; Be is a field where Jc equals Jo/2; d is the filament diameter; B is the maximum magnetic field; X is a volumetric proportion for the superconductor in a composite; V is the volume of the composite; and A is a constant. From Equation (7), the leakage flux density value, B~, was calculated as 0.38 T at 300 A (r.m.s.) B~ = 1.414 izoNl/2h
(7)
Considering field distribution, the hysteresis losses obtained by Equations (5) and (6) totalled 0.77 W. The coupling losses, Pc, are given by Equation (8) 6
Pc = B2°°27"V/[#o( 1 +
(8)
6027"2)]
where
Conclusions The authors made a 30 kVA class superconducting transformer using ultrafine multi filamentary superconductors. The transformer was evaluated using no-load test, short circuit test and 0.5 fl load test conditions. Leakage impedance, obtained from the short circuit tests, was 0.02 fi, which was in accordance with the calculated value. In spite of such a small leakage impedance, the fault current under sudden short circuit test conditions was limited by the superconductor critical current. Iron loss, obtained from the no-load tests, was 13 W. The efficiency for the transformer is estimated as 98.5 %, based on the assumptions that the iron core is located outside the cryostat, that high Tc superconductors are used as current leads and that the refrigerator efficiency is 500. The maximum quench current after training could not reach the short sample d.c. critical current. It was revealed that the secondary winding quench occurred prior to the primary winding quench, by measurements using a newly-developed quench detector for superconducting transformers.
(9)
7" =/zo(lp/27r J..12p
(10)
p = Pm(1 -- X')/(I + X')
and where: lp is the twist pitch length; Pm is the matrix resistivity; and X' is the volumetric proportion of super-
Table 3
Comparison of superconducting transformer and conventional transformer losses
Capacity (kVA) Iron loss (W) Copper losses (W) Other losses (W) Transformer efficiency (%)
conductor and copper in a composite. The total coupling losses, obtained from Equation (8), were 0.02 W. Therefore, the total a.c. losses were 0.79 W. If the refrigerator efficiency is 500, the total a.c. loss at room temperature becomes 395 W. Heat leakage through the current leads is not small 7. However, if high Tc superconductors are used as current leads, the heat leakage will fall to ~ 10% of the value for copper current leads. The total heat leakage at room temperature then becomes 63.6 W. These losses are lower than those for a conventional transformer of the same capacity, as shown in Table 3. The superconducting transformer efficiency can be estimated as 98.5%, assuming the iron core is located outside the cryostat. This value is higher than the efficiency for a typical conventional 30 kVA transformer.
Superconducting transformer
Conventional transformer
30 13 395 63.6 98.5
30 99 496 98.06
References 1 Ito, D., Shimizu, E., Fujioka, T., Ogiwara, H., Akita, S., Ishikawa, T. and Tanaka, T. Development of 500 kVA A.C 50 Hz superconducting coil Proc ICEC 12 Butterworths, Guildford, UK (1988) 719-721 2 Fevrier, A. Superconducting Application Technology CMC Press, Tokyo, Japan (1988) 22 (in Japanese) 3 Funaki, K., Iwakuma, M., Takeo, M. and Yamafuji, K. Preliminary test and quench analysis of a 72 kVA superconducting four-winding power transformer Proc ICEC 12 Butterworths, Guildford, UK (1988) 729-733 4 Connelly, F.C. Transformers Pitman & Sons Ltd, London, UK (1950) 53-174 5 Brechna, H. Superconducting Magnet Systems Springer-Verlag, Berlin, Germany (1973) 241-246 6 Wilson, N.M. Superconducting Magnets Clarendon Press, Oxford, UK (1983) 174-181 7 Wilson, N.M, Superconducting Magnets Clarendon Press, Oxford, UK (1983) 257-260 8 Fujioka, T. Personal communication at Superconducting Transformers meeting, Toshiba Keihin Works, Japan
Cryogenics 1991 Vol 31 July
659
Keywords: superconductors; transformers; stability
Progress achieved in superconductor manufacturing technology had promoted the development of low loss superconductors for a.c. use. The present authors developed and successfully operated a 500 kVA class superconducting coil at 5 0 - 6 4 Hz in collaboration with CRIEPI ] . To demonstrate the applicability of superconductors to electric machines, the authors made and tested a superconducting transformer, which is both the most basic static electric machine and easy to fabricate. A 200 kVA 2 class and a 70 kVA 3 class superconducting transformer have been reported previously. A superconducting transformer has three basic characteristics, light weight, high efficiency and a fault current limitation effect2. Equation (1) shows the nominal voltage, V, for a transformer 4. V = 4.44 fNBS
(1)
where: N is the number of winding turns; B is the maximum flux density in a core; f is the supply frequency; and S is the core cross-sectional area. Usually, a superconductor can be used at higher current densities than a conventional Cu conductor. More turns of superconducting windings than Cu windings can be wound in the same window size. Therefore, to obtain the same nominal voltages as with a conventional transformer the iron core *Paper presented at the S y m p o s i u m on Superconductor Stability, 13 - 15 November 1990, Y o k o h a m a , Japan
weight, which is proportional to S, can be lighter than in the conventional transformer. Simultaneously, the superconducting transformer efficiency, 7, given by Equation (2) 4, can be increased with decreasing iron volume, i.e. iron loss, Q~, if the sum of the superconductor a.c. losses, Qs, in the winding and heat leakage through current leads, QI, is almost the same, or less than, the copper losses = K/(K + Qs + Q~ + Qi)
(2)
where K is a nominal capacity. Conventional large capacity transformers limit fault currents by having a designed per cent impedance of = 10%. On the other hand, a superconducting transformer can limit large fault currents by its critical current. Therefore, for a superconducting transformer, the per cent impedance can be designed to be very small. This paper presents experimental results obtained for a 30 kVA class superconducting transformer.
Superconductor and transformer structure The transformer windings are wound with a 42-strand cable, whose parameters are listed in Table 1. Previously, the same 42-strand cable was used in a 500 kVA class superconducting coil winding and could generate 2 T peak field at 100 A (r.m.s.) ~.
0011 - 2 2 7 5 / 9 1 / 0 7 0 6 5 5 - 0 5 ~) 1991 B u t t e r w o r t h - Heinemann Ltd
Cryogenics 1991 Vol 31 July
655
Tests on 30 kVA class superconducting transformer: E.S. Yoneda et al. 1 Conductor parameters
Table
Strand
Wire diameter (mm) Number of filaments Filament diameter (/zm) Filament twist pitch (ram) Nb - Ti/Cu/Cu - 10%Ni ratio Insulation
0.112 14478 0.49 0.98 1:0.1:2.5 No insulation
7-strand cable
Diameter (ram) Twist pitch (mm)
0.34 3.05
42-strand cable
Diameter (ram) Twist pitch (mm) Centre core wire Core diameter (ram) Core insulation Cable insulation
6.7 Stainless steel 0.37 10/tm poly-vinyl-formal No insulation
700 600 <
\
5OO
f-
400
1.04
a diagram of its structure are shown in Figure 2. The nominal characteristics for the transformer were 30 kVA rating, 100 V primary voltage and 300 A primary current. Since the aim of this work was to investigate superconducting properties, the transformer had a simple structure. For example, the primary to secondary voltage ratio was 1:1 and the iron core was immersed in liquid helium, together with the windings. In order to reduce leakage impedance, the primary and secondary coils have a coaxial winding structure.
\
-6 3OO U
o
200
No-load test
IO 0 I
O
I
I
i
I
I
2 3 4 5 Mognetic field (Teslo)
6
Figure 1 D.c. critical current and load line for the 30 kVA class superconducting transformer
To prevent wire motion, the cable was embedded in grooves on fibre reinforced plastic formers and wound with strong tension. Figure 1 shows d.c. critical currents for the cables measured for short samples. The superconducting transformer parameters are listed in Table 2. A photograph of the transformer and
Table
2
Short circuit test
The leakage impedance, Xl, obtained from the primary voltage to primary current ratio, was 0.02 fl, in accor-
Superconducting transformer parameters ZDKH 6.95 1.7 19.6 22.3
Material Core weight (kg) Maximum flux density (T) Cross-sectional area (cm 2) Window area (cm 2)
Core characteristics
Windings Voltage (V) Current (A) Number of turns Coil in Figure 2
656
The no-load test results are shown in Figure 3. The primary to secondary voltage ratio, obtained from the no-load tests, was l:l, as shown in Figure 3a. This value was in accord with the designed value. The primary voltage versus primary current curve is shown in Figure 3b. The no-load current at 100 V rating voltage, obtained from Figure 3b, was 0.765 A. Figure 3b also shows that the primary voltage was saturated at 120 V. Iron loss was directly measured by a wattmeter. The iron loss, obtained from Figure 3c, was 13 W.
Cryogenics 1991 Vol 31 July
Primary
Secondary
100 300 75x2 P-l, P-2
1O0 300 75x2 S
Tests on 3 0 k VA class s u p e r c o n d u c t i n g transformer: E.S. Yoneda et al.
150
t
,/,/
100
(a) /o
/o
50
0 0 Q) O0
/o
/o
/° e0 I
1
50
lO0
150
Primary voltage (V) 150
O0
0 0
100 (a)
(b)
125
I
0 0
0
E r_ o..
"
0
g 50
0 0 0 0
I
0
Iron c o r e
I
0.5
0
1.5
1.0
Primary current (A)
wir~ing:I[ i FRP
20
(c)
0 0
If) 0 o
i,-
2
o
10
o
o o
ram)
o o
(b) 0
Figure2 (a) Photograph of the transformer and (b) diagram of its
structure
o
I00 5b ' Primary voltage (V)
150
Figure3
dance with the calculated value based on the following equation X ~ = 3 . 9 5 x 10 6 x f m N 2 k [ 6 + ( A ~ + A 2 ) / 3 ] / h
(3)
where: rn is the conductor length per single turn: 6 is the separation between the primary and secondary windings; A~ is the primary winding width; A~ is the secondary winding width; h is the winding height; and k = 1 - (6 + A~ + AO/(hHr ). The impedance voltage was 5.9 V and, therefore, the per cent impedance was 5.9%. In this test, the quench occurred at 296 A, which is less than the short sample d.c. critical current (shown in Figure 1).
No-load test results. (a) Secondary voltage versus primary voRage; (b) primary voltage versus primary current; (c) iron loss versus primary voltage
Load test and sudden short circuit test Load tests were conducted with a 0.5 ~2 load resistance. In this test, quench occurred at low current and the maximum continuous current was 248 A (r.m.s.), which is less than the nominal primary current. Figure 4 shows both voltage and current variations for a sudden short circuit test following a continuous operation state at 248 A and 138 V. The primary and secondary current show almost the same variations. After a short circuit, the primary current should increase to 9758 A, the value determined by 138 V (r.m.s.) and
Cryogenics 1991 Vol 31 July
657
Tests on 30 kVA class superconducting transformer: E.S. Yoneda et al.
g 6oo~
~2000
I~
I
~
.-
"r-
a_ -6001
~ --~'TOl
,
j
i
i
,
I
I
'
i
{~1 enc~l
0
O3
,
i
I
i
1-2000
i
,
'
=5o
a.
I O0
03
T i m e (ms)
Figure 4 Voltage ( ) and current ( - - - ) variation at sudden short circuit condition from continuous operation
where: i is the current; R is the resistance; E is the induced electromotive force at the tertiary winding around the core; I2 is the terminal voltage of a winding; X is the leakage impedance; and subscripts 1 and 2 indicate primary and secondary windings, respectively. Figure 6 shows the results of measurements using the quench detector. Figure 6a shows that the secondary current decreases, after reaching a peak current, with the rapidly increasing voltage, 12R2. This result suggests that the secondary winding quenches at the peak current. On the other hand, Figure 6b shows that, in spite of having almost the same current behaviour, i.e. the primary current decreased after reaching a peak current, the voltage i~RI, did not increase for --- 100 ms following the peak current. This result indicates that the primary winding does not quench at the peak current. The voltage increases slowly from = 100 ms after the peak current. From these results, it is revealed that the secondary winding quenched first, after which the
0.02 ft. However, as soon as the current reaches 389 A, 0.5 ms after the short circuit condition, it begins to decrease rapidly from the current peak to 82 A in 1 ms. It is shown that the short circuit current, 9758 A, was limited to 389 A, which corresponds to the superconductor quench current.
600
0 H
Quench characteristics Figure 5 shows the training quenches during the short circuit test. The quenching currents gradually increased. However, the maximum quenching current is below the critical current shown in Figure 1. Acoustic emission (AE) measurement showed that AE occurred after the quench. Primary and secondary currents couple so strongly that it is impossible to distinguish which winding originated the entire transformer winding quench from the current variation chart, as shown in Figure 4. In order to find the quench origin, the authors developed a quench detector for transformers. The quench detector can distinguish which winding quenches first, based on vector representation of a transformer. The detector monitors the voltage which appears in each winding after quench initiation, as explained by Equation (4). /l X Rl = 12, - E - il x Xl
-6000. 0 (a) >
I
2.0
1.0 Time (s)
2.O
60.0
tr
0
H
- 60.00. 0
600
(4a) 0
/z X R 2 = E -- 122 - - / 2 X X 2
I
1.0 Time (s)
l--I
(4b)
lllllBltlilJllUlJllW/ ,, ..........:.................... {f,,m mlllmlflll l llF' .............................
- 600 I
700
0.0
600
(b)
~500 = 400 D
-8 300
0
OOD
D 0
0
O
D
0
O000
0
_> E.
•
t 2.0
6.00
O
0
i l.O Time (s)
.
...........................................t#l................l ,dllllll
O . - . . . , . . - . , , , . . . , , , , ... . , , . . . . . . .. .
, ..
0 I00 I
0
"
I
4
I
I
I
I
I
I
8 12 16 Quench number
I
I
20
Figure 5 Training of transformer windings. The broken line corresponds to (he current at the crossing point of the t w o lines in
Figure 1
658
Cryogenics 1991 Vol 31 July
I
-6"000'.0
1.0
2.0
Time (s) Figure 6 Results of measurements of current, I, and I multiplied by resistance, R, using the quench detector: (a) in the secondary winding; (b) in the primary winding
Tests on 30 kVA class superconducting transformer: E.S. Yoneda et al.
primary winding quenched due to heat conduction from the quenched secondary winding.
Transformer efficiency Transformer efficiency is calculated from iron and copper losses, heat leakage through the current leads and nominal capacity. The iron loss was 13 W, obtained from the no-load tests. Copper losses correspond to a.c. losses in superconducting windings and a.c. losses in a superconductor consist of hysteresis losses and coupling losses. Hysteresis losses, Ph, are described by Equations (5) and (6) 5. Equations (5) and (6) give the hysteresis losses at the maximum field below the penetration field and above the penetration field, respectively. The penetration field for the superconductor, obtained from a magnetization measurement, was 0.13 T.
Ph = JoBodfAV In [(B + Bo)/Bo],
for B > Bp
(5)
Ph = 2AB3( B + Bo)fXV/(#:oJoBod),
for B < Bp
(6)
where: Jo is the critical current density at zero field; Be is a field where Jc equals Jo/2; d is the filament diameter; B is the maximum magnetic field; X is a volumetric proportion for the superconductor in a composite; V is the volume of the composite; and A is a constant. From Equation (7), the leakage flux density value, B~, was calculated as 0.38 T at 300 A (r.m.s.) B~ = 1.414 izoNl/2h
(7)
Considering field distribution, the hysteresis losses obtained by Equations (5) and (6) totalled 0.77 W. The coupling losses, Pc, are given by Equation (8) 6
Pc = B2°°27"V/[#o( 1 +
(8)
6027"2)]
where
Conclusions The authors made a 30 kVA class superconducting transformer using ultrafine multi filamentary superconductors. The transformer was evaluated using no-load test, short circuit test and 0.5 fl load test conditions. Leakage impedance, obtained from the short circuit tests, was 0.02 fi, which was in accordance with the calculated value. In spite of such a small leakage impedance, the fault current under sudden short circuit test conditions was limited by the superconductor critical current. Iron loss, obtained from the no-load tests, was 13 W. The efficiency for the transformer is estimated as 98.5 %, based on the assumptions that the iron core is located outside the cryostat, that high Tc superconductors are used as current leads and that the refrigerator efficiency is 500. The maximum quench current after training could not reach the short sample d.c. critical current. It was revealed that the secondary winding quench occurred prior to the primary winding quench, by measurements using a newly-developed quench detector for superconducting transformers.
(9)
7" =/zo(lp/27r J..12p
(10)
p = Pm(1 -- X')/(I + X')
and where: lp is the twist pitch length; Pm is the matrix resistivity; and X' is the volumetric proportion of super-
Table 3
Comparison of superconducting transformer and conventional transformer losses
Capacity (kVA) Iron loss (W) Copper losses (W) Other losses (W) Transformer efficiency (%)
conductor and copper in a composite. The total coupling losses, obtained from Equation (8), were 0.02 W. Therefore, the total a.c. losses were 0.79 W. If the refrigerator efficiency is 500, the total a.c. loss at room temperature becomes 395 W. Heat leakage through the current leads is not small 7. However, if high Tc superconductors are used as current leads, the heat leakage will fall to ~ 10% of the value for copper current leads. The total heat leakage at room temperature then becomes 63.6 W. These losses are lower than those for a conventional transformer of the same capacity, as shown in Table 3. The superconducting transformer efficiency can be estimated as 98.5%, assuming the iron core is located outside the cryostat. This value is higher than the efficiency for a typical conventional 30 kVA transformer.
Superconducting transformer
Conventional transformer
30 13 395 63.6 98.5
30 99 496 98.06
References 1 Ito, D., Shimizu, E., Fujioka, T., Ogiwara, H., Akita, S., Ishikawa, T. and Tanaka, T. Development of 500 kVA A.C 50 Hz superconducting coil Proc ICEC 12 Butterworths, Guildford, UK (1988) 719-721 2 Fevrier, A. Superconducting Application Technology CMC Press, Tokyo, Japan (1988) 22 (in Japanese) 3 Funaki, K., Iwakuma, M., Takeo, M. and Yamafuji, K. Preliminary test and quench analysis of a 72 kVA superconducting four-winding power transformer Proc ICEC 12 Butterworths, Guildford, UK (1988) 729-733 4 Connelly, F.C. Transformers Pitman & Sons Ltd, London, UK (1950) 53-174 5 Brechna, H. Superconducting Magnet Systems Springer-Verlag, Berlin, Germany (1973) 241-246 6 Wilson, N.M. Superconducting Magnets Clarendon Press, Oxford, UK (1983) 174-181 7 Wilson, N.M, Superconducting Magnets Clarendon Press, Oxford, UK (1983) 257-260 8 Fujioka, T. Personal communication at Superconducting Transformers meeting, Toshiba Keihin Works, Japan
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