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Development of superconductors for a 70 MW superconducting generator
Development of superconductors for a 70 MW superconducting generator* Y. Matsunobu, R. Shiobara $
K.
Yamaguchi,
N.
Maki,
N.
Tada,
Y.
Yagi t and
Hitachi Research Laboratory, Hitachi Limited, 4026 Kuji-cho, Hitachi-shi, Ibaraki-Ken 319-12, Japan tHitachi Works, Hitachi Limited, 3-1-1 Saiwai-cho, Hitachi-shi, Ibaraki-Ken 317, Japan tSuper-GM, 5-14-10 Nishitenman, Kita-ku, Osaka-shi, Osaka-fu 530, Japan A national project in which 70 MW superconducting generators are to be constructed has been running in Japan. In the project t w o types of superconductors have been developed for the field winding of the generator. The A-type has been designed to have properties giving low a,c. losses. The B-type has been designed with high stability. The a.c. losses and stability of both types were measured, Then it was evaluated which was the more suitable superconductor with respect to the energy margin to quenching.
Keywords: superconductors; stability; generators; a.c. losses
To cope with the steady growth of user demand for electricity, a reduction in power loss and an improvement in power system stability are needed. Superconducting generators using superconductors as the field windings have many merits compared with conventional generators, including reductions in size and weight, and improvements in efficiency and stability of the transmission line due to low reactance of the generator 1-4. In Japan, 70 MW class model machines aiming at a 200 MW class superconducting generator (pilot machine) has been researched and developed since 19885-8. When the superconductor for the 70 MW superconducting generator was developed, two types of superconductors were designed using two methods for improvement of stability. One method decreases a.c. losses, the other increases the stabilizer characteristic. These two methods oppose one another, however. It was then suggested that an evaluation method be made for determining the more suitable superconductor for the 70 MW superconducting generator.
Load characteristics of superconductor The load characteristics showing the relation between the operating load line of the superconducting field windings and the short sample characteristics of the superconductor are shown in Figure 1. The rated operating point is decided with a magnetic flux density of 4 T and a field current of 3.0 kA. The transient operating point is decided with a magnetic flux density of 4.8 T and a field current of 3.6 kA. Changes of current and magnetic field, as in the case of a transmission line fault, are shown in Figure 2. An asynchronous *Paper presented at the Symposium on Superconductor Stability, 13 - 15 November 1990, Yokohama, Japan 0011 - 2 2 7 5 / 9 1 / 0 7 0 6 6 0 - 04 © 1991 B u t t e r w o r t h - Heinemann Ltd
660
Cryogenics 1991 Vol 31 July
10
Short sample ~teristics
,---, $
The rated
.
,~ 4
2
Load line . ~ The transient operating point
J 0
0
4
2
6
Flux Density(T) Figure
1
Load
line
of
field
winding
and
short
sample
characteristics
magnetic field due to the armature current and a high frequency fault torque cause severe conditions in the field winding.
Structures and specifications of superconductors Figure 3 shows the cross-sectional structures of the respective superconductors and Table 1 lists their specifications. The A-type superconductor is a kind of conventional pulse-use superconductor which stabilizes the superconductor by decreasing a.c. losses to ensure a sufficient temperature margin. In addition it must have a high rigidity so as not to move. The B-type supercon-
Superconductors for 70 MW generator: Y. Matsunobu et al. 4.8
3.6
Field current
e,
3.4
ClearingPoint (Most Severe P o i ~ , j , " "
4.4 ."~
I ~ " ~ i m u l a t i o n of ,~ A.C. Field
3.2
L) o
---¢
.~.,q
3.0
~
-o.]
I
o.o
,1
\
~. A transmission line fault 2.8
'~
I
"= o qbTi
\,
% /
,0 CuNi
X
V
V i
I
I
0.2
0.3
0.4
Time(sec) Figure 2
Changes of current and magnetic field as in the case of a transmission line fault
A-type
ductor has an aluminium stabilizer which is better than copper with respect to thermal and electrical conductivities around 4 K. Wh_ile the A-~pe is a single-stranded superconductor, the B-type is a double-stranded superconductor which has an aluminium stabilizer in the centre of the primary strand and has twelve wires soldered around the aluminium stabilizer. The aluminium stabilizer is divided by a copper-nickel barrier to decrease a.c. losses and to give mechanical reinforcement.
t E E ox -o-
CuNi
oo
B - I c characteristics and resistance To obtain a common basis for the heating test results, the critical current characteristics of the strand for the Atype and the primary strand for the B-type superconductor were measured. The A-type strand has Ic of 1630 A at 4 T and 920 A at 7 T. The B-type strand has Ic of 1950 A at 4 T and 1050 A at 7 T. Both superconductors satisfy one-sixth of the demand specification of the superconductors. The A-type superconductor has a resistance of 23.9/zfl m- t at 10 K and 0 T, and that of the B-type superconductor is 3.6/x9 m-l. Both superconductors meet the desired specification.
Characteristics of a.c. losses As shown in Figure 2, when a transmission line fault occurs and is cleared four cycles after the fault, a low frequency, 3 - 4 Hz, asynchronous magnetic flux increases monotonously on the rotor during the fault and
Solder
Cu
t
~
~
NbTi
B-type Figure 3
Cross-section of the conductors
changes slowly after it is cleared. A.c. losses should be evaluated by using the a.c. field of 3.75 Hz with a onefourth period of 0.067 s. The amplitude of the a.c. field is 0.33 T due to the 5 T s -~ field changing rate during the fault. The clearing point (most severe slope point of field current in Figure 2) was selected as the test point.
Table 1 Specifications of conductors Item
A-type
B-type
Cross-section (mm) Critical current (A)
3.6 x 6.0 > 8500 at 4 T > 4 5 0 0 at 7 T < 2 5 at 10 K and 0 T < 1 5 at 5 T s -1 and 4 T Copper
3.6 × 6.0 > 8500 at 4 T > 4 5 0 0 at 7 T < 5 at 10 K and 0 T < 3 0 at 5 T s-1 and 4 T Aluminium
Resistance (/xfl m - 1) A.c. loss (kW m -3) Stabilizer
Cryogenics 1991 Vol 31 July
661
Superconductors for 70 MW generator: Y. Matsunobu et al.
FLI .'.
If
Epoxy ~ resin ~
~
:
:
,
~
Heate
Density
~
~
I 1/::Helium
DC Pow---~susupply 7 ::i:i:): :::1:
:[IN,~:iili:i:iiil|magnet
ii:iiii:i:.ii! ~
Figure 4 A.c. losses measuring equipment
H
holder
iii::::)i iii:i i::::Heli~ iiii!::!i!::!:i!::ii:!:.!! Figure 5 Heatingtest equipment
The a.c. losses measuring equipment is shown in Figure 4. When the a.c. flux density is generated by a copper magnet, a.c. losses are caused in the superconductor. Then the heat from a.c. losses vaporizes liquid helium around the superconductor. As helium gas was measured by a flow meter, the a.c. loss characteristic was obtained. The A-type superconductor has an a.c. loss of 15 kW m -3 at 4 T and 5 T s -l, while the Btype superconductor has an a.c. loss of 20 kW m -3. These superconductors both satisfy the desired specification.
Stability characteristics The heating test equipment is shown in Figure 5. The superconductors were wound with resistance heater wire and were fixed using epoxy resin. The reason for insulating with epoxy resin is that more heater power is needed to cause quenching if the superconductor is not insulated. As this experiment was intended to make a relative comparison of the A-type and B-type, it was not affected by the presence or absence of insulator on the superconductor. When a magnetic field was applied and a current was conducted, the heating power was increased gradually and the minimum power to quenching was found. The pulse duration was 67 ms. The heating test was done on the load line of the field winding. Typically, the heating test uses a constant field condition and several currents. However, in these heating tests both flux density and current were varied. Therefore the concept of a temperature margin was introduced. In Figure 1 there is a point on the load line corresponding uniquely to the temperature margin. The present evaluation method, including the temperature margin, could determine a value of total stability including the influence ofa.c, losses, because a.c. losses can be substituted for a rise in temperature by calcula-
662
Cryogenics 1991 Vol 3~ July
tion of the heat produced. Generally a more accurate picture of the stability of the superconductor is obtained from the heating test than from the temperature margin. However, in Figure 2, for the transmission line fault, the rise in temperature resulting from a.c. losses caused by a change of field current could not be ignored. Consequently one must consider the influence of a.c. losses. To complete the experiments relating to rise in temperature, the sample must be put in a highly rotating field. The required experiment is very hard to do, so a calculation was substituted for this. The heater test was done in the normal gravity condition. Therefore the evaluation method that includes the concept of the temperature margin for comparing A-type and B-type superconductors was relative. Cooling condition could be considered in various forms. In the evaluation method including the concept of temperature margin, an actual state was considered in which superconductors were surrounded partially by a spacer and partially by liquid helium and the temperature rise due to a.c. losses was calculated. The results of the heater test are shown in Figure 6. The horizontal axis indicates the temperature margin and the vertical axis indicates the heater input power per unit capacity. For the rated point, the A-type heater input power is 3700 kW m -3 and the B-type heater input power is 6900 kW m-3. For the most severe point, the A-type heater input power is 3520 kW m -3 and the Btype heater input power is 6280 kW m -3. The power of the temperature rise from a.c. losses is W[. The power of the most severe point is W2 and that at total stability is W3. The B-type superconductor has a value of W3 1.8 times higher than that of the A-type superconductor. This result was not a good example of an evaluation of a low a.c. loss and high stability superconductor. As
Superconductors for 70 MW generator: Y. Matsunobu et al. 8O00
with low a.c. losses and good stability, was developed. A stability evaluation method has been suggested that can include the influence of a.c. losses.
Acknowledgements
O.0
1.0
2.0
Temperau~cMargin(K) Figure 6
Curves showing the stability of the conductors
the wires were not insulated, the rise in temperature due to a.c. losses is lower, giving good cooling in the superconducting generator. In the case of insulated wires, one would obtain a different result, because in cases of poor cooling, there is a very different rise in temperature between the two types of superconductors from a.c. losses. The authors believe the present evaluation is suitable. One can carry out an experiment and obtain an actual state for the cooling condition, and if one can calculate accurately the temperature rise due to a.c. losses, one should be able to obtain an accurate value for quenching energy in this actual state.
Conclusions A 70 MW class model superconducting generator, onethird of the scale of a 200 MW class generator, has been designed and studied. The superconductors to be used in the generator have been developed and tested. An aluminium-stabilized superconductor for the generator,
The authors wish to thank M. Ohi, M. Furuyama, S. Ohasi and T. Ohkawa of Hitachi Limited for their support. They would also like to thank the personnel at the Furukawa Electric Co. Limited, Sumitomo Electric Industries Limited and Hitachi Cable Limited for their valuable manufacturing work. This work was performed as part of R&D on Superconducting Technology for Electric Power Apparatus as a subject of the Engineering Research Association for Superconductive Generation Equipment and Materials (Super-GM) under the Moonlight Project of the Agency of Industrial Science and Technology, MITI, being consigned by the New Energy and Industrial Technology Development Organization (NEDO).
References I Smith, J.L. Overview of the development of superconducting synchronous generator IEEE Trans Magn (1983) MAG-19 (3) 2 Fomin, B.L. etal., Main stage of manufacturing a 300 MW superconducting generator Cryogenics (1987) 27 243-248 3 Lambreeht, D. Superconducting turbogenerator: status and trends Cryogenics (1985) 25 619-627 4 Keim, T.A. etal. Design and manufacture of a 20 MVA superconducting generator IEEE Trans Power Apparatus Systems (1985) PAS-104 1475 - 1483 5 Yamaguchi, K. et al. Superconducting rotor development for a 50 MVA generator IEEE Trans Power Apparatus Systems (1984) PAS-103 (7) 6 Ashkin, M. et al. Stability criteria for superconducting generators - electrical system and cryostability considerations IEEE Trans Power Apparatus Systems (1982) PAS-1 (12) 7 Sabrie, J.L. and Coyer, J. Technical overview of the French program 1EEE Trans Magn (1983) MAG-19 (3) 8 0 h a r a , T. et al. Development of 70 MW class superconducting generators, paper presented at Applied Superconductivity Conference, Snowmass, Colorado, USA (September 1990) 9 Yamaguchi, K. et al. Development of superconductors for a 70 MW class superconducting generator, paper presented at Applied Superconductity Conference, Snowmass, Colorado, USA (September 1990)
Cryogenics 1991 Vol 31 July
663
K.
Yamaguchi,
N.
Maki,
N.
Tada,
Y.
Yagi t and
Hitachi Research Laboratory, Hitachi Limited, 4026 Kuji-cho, Hitachi-shi, Ibaraki-Ken 319-12, Japan tHitachi Works, Hitachi Limited, 3-1-1 Saiwai-cho, Hitachi-shi, Ibaraki-Ken 317, Japan tSuper-GM, 5-14-10 Nishitenman, Kita-ku, Osaka-shi, Osaka-fu 530, Japan A national project in which 70 MW superconducting generators are to be constructed has been running in Japan. In the project t w o types of superconductors have been developed for the field winding of the generator. The A-type has been designed to have properties giving low a,c. losses. The B-type has been designed with high stability. The a.c. losses and stability of both types were measured, Then it was evaluated which was the more suitable superconductor with respect to the energy margin to quenching.
Keywords: superconductors; stability; generators; a.c. losses
To cope with the steady growth of user demand for electricity, a reduction in power loss and an improvement in power system stability are needed. Superconducting generators using superconductors as the field windings have many merits compared with conventional generators, including reductions in size and weight, and improvements in efficiency and stability of the transmission line due to low reactance of the generator 1-4. In Japan, 70 MW class model machines aiming at a 200 MW class superconducting generator (pilot machine) has been researched and developed since 19885-8. When the superconductor for the 70 MW superconducting generator was developed, two types of superconductors were designed using two methods for improvement of stability. One method decreases a.c. losses, the other increases the stabilizer characteristic. These two methods oppose one another, however. It was then suggested that an evaluation method be made for determining the more suitable superconductor for the 70 MW superconducting generator.
Load characteristics of superconductor The load characteristics showing the relation between the operating load line of the superconducting field windings and the short sample characteristics of the superconductor are shown in Figure 1. The rated operating point is decided with a magnetic flux density of 4 T and a field current of 3.0 kA. The transient operating point is decided with a magnetic flux density of 4.8 T and a field current of 3.6 kA. Changes of current and magnetic field, as in the case of a transmission line fault, are shown in Figure 2. An asynchronous *Paper presented at the Symposium on Superconductor Stability, 13 - 15 November 1990, Yokohama, Japan 0011 - 2 2 7 5 / 9 1 / 0 7 0 6 6 0 - 04 © 1991 B u t t e r w o r t h - Heinemann Ltd
660
Cryogenics 1991 Vol 31 July
10
Short sample ~teristics
,---, $
The rated
.
,~ 4
2
Load line . ~ The transient operating point
J 0
0
4
2
6
Flux Density(T) Figure
1
Load
line
of
field
winding
and
short
sample
characteristics
magnetic field due to the armature current and a high frequency fault torque cause severe conditions in the field winding.
Structures and specifications of superconductors Figure 3 shows the cross-sectional structures of the respective superconductors and Table 1 lists their specifications. The A-type superconductor is a kind of conventional pulse-use superconductor which stabilizes the superconductor by decreasing a.c. losses to ensure a sufficient temperature margin. In addition it must have a high rigidity so as not to move. The B-type supercon-
Superconductors for 70 MW generator: Y. Matsunobu et al. 4.8
3.6
Field current
e,
3.4
ClearingPoint (Most Severe P o i ~ , j , " "
4.4 ."~
I ~ " ~ i m u l a t i o n of ,~ A.C. Field
3.2
L) o
---¢
.~.,q
3.0
~
-o.]
I
o.o
,1
\
~. A transmission line fault 2.8
'~
I
"= o qbTi
\,
% /
,0 CuNi
X
V
V i
I
I
0.2
0.3
0.4
Time(sec) Figure 2
Changes of current and magnetic field as in the case of a transmission line fault
A-type
ductor has an aluminium stabilizer which is better than copper with respect to thermal and electrical conductivities around 4 K. Wh_ile the A-~pe is a single-stranded superconductor, the B-type is a double-stranded superconductor which has an aluminium stabilizer in the centre of the primary strand and has twelve wires soldered around the aluminium stabilizer. The aluminium stabilizer is divided by a copper-nickel barrier to decrease a.c. losses and to give mechanical reinforcement.
t E E ox -o-
CuNi
oo
B - I c characteristics and resistance To obtain a common basis for the heating test results, the critical current characteristics of the strand for the Atype and the primary strand for the B-type superconductor were measured. The A-type strand has Ic of 1630 A at 4 T and 920 A at 7 T. The B-type strand has Ic of 1950 A at 4 T and 1050 A at 7 T. Both superconductors satisfy one-sixth of the demand specification of the superconductors. The A-type superconductor has a resistance of 23.9/zfl m- t at 10 K and 0 T, and that of the B-type superconductor is 3.6/x9 m-l. Both superconductors meet the desired specification.
Characteristics of a.c. losses As shown in Figure 2, when a transmission line fault occurs and is cleared four cycles after the fault, a low frequency, 3 - 4 Hz, asynchronous magnetic flux increases monotonously on the rotor during the fault and
Solder
Cu
t
~
~
NbTi
B-type Figure 3
Cross-section of the conductors
changes slowly after it is cleared. A.c. losses should be evaluated by using the a.c. field of 3.75 Hz with a onefourth period of 0.067 s. The amplitude of the a.c. field is 0.33 T due to the 5 T s -~ field changing rate during the fault. The clearing point (most severe slope point of field current in Figure 2) was selected as the test point.
Table 1 Specifications of conductors Item
A-type
B-type
Cross-section (mm) Critical current (A)
3.6 x 6.0 > 8500 at 4 T > 4 5 0 0 at 7 T < 2 5 at 10 K and 0 T < 1 5 at 5 T s -1 and 4 T Copper
3.6 × 6.0 > 8500 at 4 T > 4 5 0 0 at 7 T < 5 at 10 K and 0 T < 3 0 at 5 T s-1 and 4 T Aluminium
Resistance (/xfl m - 1) A.c. loss (kW m -3) Stabilizer
Cryogenics 1991 Vol 31 July
661
Superconductors for 70 MW generator: Y. Matsunobu et al.
FLI .'.
If
Epoxy ~ resin ~
~
:
:
,
~
Heate
Density
~
~
I 1/::Helium
DC Pow---~susupply 7 ::i:i:): :::1:
:[IN,~:iili:i:iiil|magnet
ii:iiii:i:.ii! ~
Figure 4 A.c. losses measuring equipment
H
holder
iii::::)i iii:i i::::Heli~ iiii!::!i!::!:i!::ii:!:.!! Figure 5 Heatingtest equipment
The a.c. losses measuring equipment is shown in Figure 4. When the a.c. flux density is generated by a copper magnet, a.c. losses are caused in the superconductor. Then the heat from a.c. losses vaporizes liquid helium around the superconductor. As helium gas was measured by a flow meter, the a.c. loss characteristic was obtained. The A-type superconductor has an a.c. loss of 15 kW m -3 at 4 T and 5 T s -l, while the Btype superconductor has an a.c. loss of 20 kW m -3. These superconductors both satisfy the desired specification.
Stability characteristics The heating test equipment is shown in Figure 5. The superconductors were wound with resistance heater wire and were fixed using epoxy resin. The reason for insulating with epoxy resin is that more heater power is needed to cause quenching if the superconductor is not insulated. As this experiment was intended to make a relative comparison of the A-type and B-type, it was not affected by the presence or absence of insulator on the superconductor. When a magnetic field was applied and a current was conducted, the heating power was increased gradually and the minimum power to quenching was found. The pulse duration was 67 ms. The heating test was done on the load line of the field winding. Typically, the heating test uses a constant field condition and several currents. However, in these heating tests both flux density and current were varied. Therefore the concept of a temperature margin was introduced. In Figure 1 there is a point on the load line corresponding uniquely to the temperature margin. The present evaluation method, including the temperature margin, could determine a value of total stability including the influence ofa.c, losses, because a.c. losses can be substituted for a rise in temperature by calcula-
662
Cryogenics 1991 Vol 3~ July
tion of the heat produced. Generally a more accurate picture of the stability of the superconductor is obtained from the heating test than from the temperature margin. However, in Figure 2, for the transmission line fault, the rise in temperature resulting from a.c. losses caused by a change of field current could not be ignored. Consequently one must consider the influence of a.c. losses. To complete the experiments relating to rise in temperature, the sample must be put in a highly rotating field. The required experiment is very hard to do, so a calculation was substituted for this. The heater test was done in the normal gravity condition. Therefore the evaluation method that includes the concept of the temperature margin for comparing A-type and B-type superconductors was relative. Cooling condition could be considered in various forms. In the evaluation method including the concept of temperature margin, an actual state was considered in which superconductors were surrounded partially by a spacer and partially by liquid helium and the temperature rise due to a.c. losses was calculated. The results of the heater test are shown in Figure 6. The horizontal axis indicates the temperature margin and the vertical axis indicates the heater input power per unit capacity. For the rated point, the A-type heater input power is 3700 kW m -3 and the B-type heater input power is 6900 kW m-3. For the most severe point, the A-type heater input power is 3520 kW m -3 and the Btype heater input power is 6280 kW m -3. The power of the temperature rise from a.c. losses is W[. The power of the most severe point is W2 and that at total stability is W3. The B-type superconductor has a value of W3 1.8 times higher than that of the A-type superconductor. This result was not a good example of an evaluation of a low a.c. loss and high stability superconductor. As
Superconductors for 70 MW generator: Y. Matsunobu et al. 8O00
with low a.c. losses and good stability, was developed. A stability evaluation method has been suggested that can include the influence of a.c. losses.
Acknowledgements
O.0
1.0
2.0
Temperau~cMargin(K) Figure 6
Curves showing the stability of the conductors
the wires were not insulated, the rise in temperature due to a.c. losses is lower, giving good cooling in the superconducting generator. In the case of insulated wires, one would obtain a different result, because in cases of poor cooling, there is a very different rise in temperature between the two types of superconductors from a.c. losses. The authors believe the present evaluation is suitable. One can carry out an experiment and obtain an actual state for the cooling condition, and if one can calculate accurately the temperature rise due to a.c. losses, one should be able to obtain an accurate value for quenching energy in this actual state.
Conclusions A 70 MW class model superconducting generator, onethird of the scale of a 200 MW class generator, has been designed and studied. The superconductors to be used in the generator have been developed and tested. An aluminium-stabilized superconductor for the generator,
The authors wish to thank M. Ohi, M. Furuyama, S. Ohasi and T. Ohkawa of Hitachi Limited for their support. They would also like to thank the personnel at the Furukawa Electric Co. Limited, Sumitomo Electric Industries Limited and Hitachi Cable Limited for their valuable manufacturing work. This work was performed as part of R&D on Superconducting Technology for Electric Power Apparatus as a subject of the Engineering Research Association for Superconductive Generation Equipment and Materials (Super-GM) under the Moonlight Project of the Agency of Industrial Science and Technology, MITI, being consigned by the New Energy and Industrial Technology Development Organization (NEDO).
References I Smith, J.L. Overview of the development of superconducting synchronous generator IEEE Trans Magn (1983) MAG-19 (3) 2 Fomin, B.L. etal., Main stage of manufacturing a 300 MW superconducting generator Cryogenics (1987) 27 243-248 3 Lambreeht, D. Superconducting turbogenerator: status and trends Cryogenics (1985) 25 619-627 4 Keim, T.A. etal. Design and manufacture of a 20 MVA superconducting generator IEEE Trans Power Apparatus Systems (1985) PAS-104 1475 - 1483 5 Yamaguchi, K. et al. Superconducting rotor development for a 50 MVA generator IEEE Trans Power Apparatus Systems (1984) PAS-103 (7) 6 Ashkin, M. et al. Stability criteria for superconducting generators - electrical system and cryostability considerations IEEE Trans Power Apparatus Systems (1982) PAS-1 (12) 7 Sabrie, J.L. and Coyer, J. Technical overview of the French program 1EEE Trans Magn (1983) MAG-19 (3) 8 0 h a r a , T. et al. Development of 70 MW class superconducting generators, paper presented at Applied Superconductivity Conference, Snowmass, Colorado, USA (September 1990) 9 Yamaguchi, K. et al. Development of superconductors for a 70 MW class superconducting generator, paper presented at Applied Superconductity Conference, Snowmass, Colorado, USA (September 1990)
Cryogenics 1991 Vol 31 July
663