Superconducting coil protective system

A superconducting coil protective system comprising a protective resistor connected in parallel to a superconducting coil for dissipating the energy stored on the superconducting coil and a normally-closed power switch for interrupting an electric current flowing from the power source to the superconducting coil in response to a quenching signal from the quenching detector. A fuse circuit, which includes a serially-connected fuse and a closure switch, is connected in parallel to the power switch. The protective system may comprise a current interrupting circuit connected in parallel to the superconducting coil and the protective resistor and a commutation switch connected in series with the protective resistor and the current interrupting circuit for allowing a commutation of a current from the superconducting coil to the current interrupting circuit in response to the quenching signal. In this case, the fuse circuit is connected in parallel to the protective resistor.

BACKGROUND OF THE INVENTION 
This invention relates to a superconducting coil protective system for 
protecting a superconducting coil from being destroyed upon quenching. 
FIGS. 6 to 9 illustrate circuit diagrams of various superconducting coil 
protective systems disclosed in "Improvements in the Parallel Resistor 
Circuit for the Quench Protection of a Superconducting Magnet" by T. 
Nakano, S. Okuma and Y. Amamiya, B-102 No. 12 pp. 73-79, published from 
Japanese Institute of Electrical Engineering, December, 1982. 
In the conventional parallel resistor circuit type protective system 
illustrated in FIG. 6, a cryostat CR comprises a superconducting coil L 
and a resistance R(t) of a normal conduction portion generated in the 
superconducting coil L. This resistance R(t) has a resistance that 
increases as the time lapses. Across the cryostat CR, a power source such 
as a mono-polar electric source E is connected through a power switch S, 
and a protective resistor R.sub.o is connected in parallel to the cryostat 
CR. 
FIG. 7 illustrates a protective circuit in which a diode D is employed in 
place of the protective resistor R.sub.o and two switches S1 and S2 as 
well as three resistors R1, R2 and R3 are used to form a multi-stage 
parallel resistors. 
FIG. 8 illustrates a protective circuit in which series-connected resistors 
Ra and Rb are connected in parallel to the cryostat CR, and a capacitor C 
is connected across the resistor Rb. A protective circuit illustrated in 
FIG. 9 further comprises a series circuit of an inductor Ls and a resistor 
Rs connected in parallel to the protective resistor. 
All of these known protective circuits illustrated in FIGS. 6 to 9, which 
comprise the power switch S connected between the power source E and the 
superconducting coil L, have the basically same disadvantages, so that the 
description of the d.c. current interrupting operation will be made only 
in terms of the protective circuit illustrated in FIG. 6 for simplicity. 
During normal operation, the power switch S of the protective circuit of 
FIG. 6 is closed and a very large current from the power source E flows 
through the superconducting coil L but substantially no current flows 
through the protective resistor R.sub.D because it has a large resistance. 
However, upon the occurrence of quenching in the superconducting coil L, in 
order to quickly remove stored energy within the superconducting coil L, 
as soon as the occurrence of the quenching in the superconducting coil L 
is detected, the voltage of the power source E is decreased and at the 
same time the power switch S is opened. Then a high voltage Vc which 
generates across the power switch S is applied to the protective resistor 
R.sub.D, whereupon an electric current which has been flowing through the 
superconducting coil L initiates to flow as indicated by an arrow i.sub.D. 
Then, the magnetic energy stored in the superconducting coil L is 
converted into heat at the protective resistor R.sub.o to be dissipated to 
the exterior of the cryostat CR, whereby the superconducting coil L can be 
protected. 
With the conventional protective system as above discussed, the power 
switch S must carry an extremely massive current, which also flows through 
the superconducting coil L, during normal operating condition, and also 
the power switch S must interrupt this massive current at a high voltage 
upon the occurrence of the quenching in the superconducting coil L. 
However, it sometimes happens that the power switch S fails to interrupt 
the current when the arc voltage of the arc plasma generated across the 
contacts of the power switch S is low and does not reach the interrupting 
voltage. If the current interruption is failed, the power switch S and the 
superconducting coil L are destroyed. 
In the circuit illustrated in FIG. 6, since the current must be interrupted 
by the power switch S, the power switch S must have a d.c. current 
interruption capability which can break a current ic at a voltage 
(RD.times.ic). Also, because of an arc plasma generated between the switch 
contacts of the power switch S, the current interruption sometimes fails 
and it is difficult to reliably carry out the interruption. 
FIG. 10 illustrates a further example of a conventional superconducting 
coil protective system in which a reversible polarity power source PS is 
connected across the superconducting coil L through a disconnector DS. The 
system also includes a protective resistor RD connected in parallel to the 
superconducting coil L. A closing switch S3 is connected in series to the 
protective resistor RD and a fuse F is connected in parallel to the the 
resistor RD. The normal conduction portion of the superconducting coil L 
is not illustrated in FIG. 10. 
During normal operation, the disconnector DS is closed so that a 
predetermined current i can flow into the superconducting coil l in the 
direction of the arrow from the reversible polarity power source PS to 
energize the superconducting coil L. 
Upon the occurrence of quenching in the superconducting coil L, the 
quenching is detected and the closing switch S3 is immediately closed and 
the voltage across the reversible polarity power source is reversed or 
made zero. Then, the current from the power source PS rapidly decreases 
and the current in the superconducting coil L is commutated into the fuse 
F as illustrated by the arrow ic, so that the current from the power 
source PS eventually reduces to zero, upon which the disconnector DS can 
be opened. Thereafter, the fuse F melts because of the commutated current. 
Therefore, the commutated current which has been flowing through the fuse 
is interrupted and again commutated to the protective resistor RD where 
the energy in the superconducting coil L is dissipated. 
In this arrangement, it is not possible to control the relationship between 
the time points of the closure of the closing switch S3 and the current 
commutation into the fuse F, so that the arrangement is disadvantageous in 
that a large-sized fuse must be used as the fuse F because the current 
interruption fails if the fuse F melts before the current commutation 
completes, and that it is difficult to simultaneously interrupt the 
current in the system having a plurality of d.c. interrupting units. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide a 
superconducting coil protective system free from the above discussed 
problems of the conventional design. 
Another object of the present invention is to provide a superconducting 
coil protective system which is reliable in operation. 
Another object of the present invention is to provide a superconducting 
coil protective system which is simple and inexpensive. 
A further object of the present invention is to provide a superconducting 
coil protective method which can effectively and efficiently protect the 
superconducting coil. 
With the above objects in view, according to the present invention, the 
superconducting coil protective system for protecting a superconducting 
coil connected in parallel to a power source comprises a quenching 
detector for detecting occurrence of a quenching in the superconducting 
coil and generating a quenching signal indicative of the occurrence of a 
quenching. The system also comprises a power switch which closes in 
response to the quenching signal and a protective resistor for dissipating 
the energy stored in the superconducting coil. The power switch is in 
series with the superconducting coil and the protective load is in 
parallel to the superconducting coil. A series circuit including a fuse 
and a closure switch is connected in parallel to the power switch so that 
the current through the power switch can commutate to the series circuit 
of the fuse and the closure switch when the power switch is opened. 
According to another aspect of the present invention, a normally-open first 
switch is connected in series with the protective resistor instead of the 
power switch and the series circuit including a fuse and a closure switch 
is connected in parallel to the protective resistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a superconducting coil protective system of the present 
invention for protecting a superconducting coil L connected in parallel to 
a power source E. The superconducting coil protective system comprises a 
protective resistor RD adapted to be connected in parallel to the 
superconducting coil L for dissipating the energy stored in the 
superconducting coil L. 
The protective system also comprises a quenching detector QD which may be 
any known quenching detector such as the bridge-type detector of detecting 
occurrence of a quenching in the superconducting coil L and generating a 
quenching signal indicative of the occurrence of a quenching in the 
superconducting coil L. The quenching detector QD is connected to a 
normally-closed power switch S serially connected between the power source 
E and the parallel-circuit of the superconducting coil L and the 
protective resistor RD. The power switch S interrupts an electric current 
flowing from the power source E to the superconducting coil L in response 
to the quenching signal from the quenching detector QD. 
According to the present invention, a fuse circuit FC which includes a fuse 
F and a closure switch S4 serially connected to each other is connected in 
parallel to the power switch S. The closure switch S4 is also connected to 
the quenching detector QD through a delaying circuit .tau.. 
During normal operation of the superconducting coil apparatus, no quenching 
phenomenon is present in the superconducting coil L and no quenching 
signal is provided from the quenching detector QD. Therefore, the power 
switch S is kept closed and the closure switch S4 in the fuse circuit FC 
is kept opened, so that a predetermined current i is supplied from the 
power source E to the superconducting coil L. Substantially no current 
flows through the protective resistor RD since it has a high resistance. 
When a quenching is detected by the quenching detector QD, a quenching 
signal causes the power switch S to open to interrupt the current i 
flowing therethrough. This causes the current i to commutate to the 
protective resistor RD as a commutated current i2, whereby the energy in 
the superconducting coil L can be dissipated by the protective resistor 
RD. The protective resistor RD has a resistance large enough to covert the 
energy of the commutated current which corresponds to the electromagnetic 
energy stored on the superconducting coil L into heat which can be 
dissipated from the protective resistor RD. 
In order to eliminate the fear that the power switch S fails to interrupt 
the current when the arc voltage of the arc plasma generated across the 
contacts of the power switch S is low and does not reach the interrupting 
voltage, the quenching signal is also applied to the closure switch S4 
through a delay circuit .tau. to close the closure switch S4. Then, the 
current flowing through the power switch S commutates to the fuse circuit 
FC due to its arc voltage Va. The commutation time T1 at this time is 
given by 
EQU T1.apprxeq.Lx/(Va/I-Ry) (1) 
where Lx ia a stray inductance of the fuse circuit FC and the power switch 
S, I is the current to be commutated and Ry is resistance of the fuse 
circuit FC. 
The fuse F is selected so that it does not melt when a current I flows 
therethrough for a time T1 and that it melts when the same current flows 
for a time several times longer than the time T1. Thus the fuse melts only 
after the arc plasma between the contacts of the power switch S has 
completely been disappeared and the insulation between the contacts has 
sufficiently been recovered. Therefore, the fuse circuit FC backs up the 
power switch S which functions as a current interrupter. 
FIG. 2 illustrates another embodiment of the superconducting coil 
protective system of the present invention in which the quenching detector 
QD is connected to a voltage controller VC associated with the power 
source PS so that the signal generated from the quenching detector QD is 
supplied to the voltage controller VC for changing polarity of an output 
voltage from the power source PS in response to the quenching signal. The 
voltage controller VC may be any known controller such as a thyristor or a 
GTO thyristor as long as the voltage controller VC can change the polarity 
of the output voltage to reduce the voltage to a first voltage at which a 
first commutation which will be explained in detail later takes place. 
The protection system also comprises a current interrupter DCM connected in 
parallel to the superconducting coil L through a series connected first 
switch S3 which is usually open but closed in response to the quenching 
signal supplied from the quenching detector QD. The current interrupter 
DCM allows a first commutation in which a current from the superconducting 
coil L is commutated to the current interrupter DCM when the first switch 
S3 is closed and when the output voltage from the power source PS is 
reduced by the voltage controller VC to reduce it to the first voltage at 
which the first commutation occurs and which is determined by the 
relationship between the impedance of the power source PS and the 
impedance of the series circuit composed of the current interrupter DCM 
and the first switch S3. 
The current interrupter DCM comprises a first series circuit including a 
circuit breaker CB and a series circuit including a capacitor C1, and 
inductance L1 and a second switch S5 and connected in parallel to the 
circuit breaker CB. The second switch S5 is normally open and is arranged 
to close upon the occurrence of the quenching in the superconducting coil 
L through a delay circuit .tau. with a predetermined time lag 
corresponding to a very short period of time necessary for completing the 
fist commutation of the current. When the the second switch S5 is closed, 
the electric charge on the capacitor C1 can now flow through the switch S5 
and the inductor L1 into the circuit breaker CB where it is cancelled out 
with the first-commutated current i.sub.2, so that the circuit breaker CB 
is now opened and the current is interrupted. Connected in parallel to the 
current interrupter DCM is a fuse circuit FC similar to that employed in 
the circuit illustrated in FIG. 1. The fuse circuit FC allows the current 
commutated to the current interrupter DCM to further commutate (second 
commutation) thereto when the current interrupter DCM is opened. Connected 
further in parallel to the fuse circuit FC is a protective resistor RD for 
allowing a third commutation in which the second-commutated current is 
commutated to the protective resistor RD upon the melting of the fuse F of 
the fuse circuit FC. The protective resistor RD has a resistance large 
enough to covert the energy of the third-commutated current which 
corresponds to the electromagnetic energy stored on the superconducting 
coil L into heat which can be dissipated from the protective resistor RD. 
A disconnector DS is connected between the power source PS and the 
parallel-connected commutation switch S3 and the superconducting coil L. 
During normal operation of the superconducting coil apparatus, no quenching 
phenomenon is present in the superconducting coil L and no quenching 
signal is provided from the quenching detector QD. Therefore, the first 
switch S3 is opened and the voltage controller VC allows a predetermined 
current i.sub.1 to be supplied from the power source PS to the 
superconducting coil L. The circuit breaker CB in the current interrupter 
DCM is closed and the second switch S5 is opened so that an electrical 
charge on the previously charged capacitor bank C1 is maintained. The 
disconnector DS is closed and the commutation switch S4 is opened. 
When a quenching is detected by the quenching detector QD, a quenching 
signal is generated. The quenching signal causes the voltage controller VC 
associated with the power source PS to operate to decrease the output 
voltage from the power source PS and causes the first switch S3 to close. 
The decrease of the output voltage from the power source PS is continued 
until the polarity of the output voltage is reversed. This can be done by 
means of GTO thyristor. As the current i.sub.1 flowing through the 
superconducting coil L begins to decrease slightly, the current from the 
power source PS decreases rapidly and a current i.sub.2 flowing through 
the current interrupter DCM increases rapidly. When the output voltage 
from the power source PS decreases to a first voltage at which the current 
flowing through the power source PS becomes zero and all the current 
i.sub.2 from the superconducting coil L flows through the current 
interrupter DCM, the power source PS is electrically isolated from the 
superconducting coil L and the current commutation completes. The 
disconnector DC can now be opened. This current commutation as above 
described is referred to as a first commutation. 
Then, the second switch S5 is closed in response to the quenching signal 
from the quenching detector QD but with a certain time lag .tau..sub.I 
which corresponds to a period of time necessary for the first commutation 
to complete after the occurrence of the quenching in the superconducting 
coil L. Then, the electrical charge on the capacitor bank C1 discharges as 
a current indicated by an arrow i.sub.3 through the circuit breaker CB. 
Since the commutated current i.sub.2 and the current i.sub.3 are opposite 
in the direction relative to the circuit breaker CB, and since the 
capacitor bank C1 is selected to have an amount of discharge current 
sufficient to reduce the current i.sub.2 to zero, the current flowing 
through the circuit breaker CB is reduced to zero. 
Therefore, the circuit breaker CB can now be opened in response to the 
quenching signal with a suitable time lag .tau. without generating an 
electric arc across the separating contact and the current flowing through 
the current interrupter DCM is interrupted. The, the current i.sub.2 
flowing through the current interrupter DCM is commutated as a 
second-commutated current i.sub.4 flowing through the fuse circuit FC when 
the closure switch S4 of the fuse circuit FC is closed by the quenching 
signal. The second-commutated current i.sub.4 eventually causes the fuse F 
to melt, whereupon the current i.sub.4 is again commutated to the 
protective resistor RD connected in parallel to the fuse circuit FC as 
indicated by i.sub.5, whereby the energy of this current i.sub.5 is 
dissipated as heat generated at the protective resistor RD. 
If some trouble occurs in the circuit of the capacitor C1, the inductor L1 
and the switch S5 or in the control system of the circuit and the current 
flowing through the circuit breaker CB cannot be reduced to zero, the 
normal interruption of the second-commutated current cannot be achieved. 
Even in this case, the circuit breaker CB is opened and at the same time 
the switch S4 is closed as in the previously-described operation, so that 
the second-commutated current i.sub.2 through the circuit breaker CB is 
commutated again to the the fuse circuit FC due to the arc voltage at the 
circuit breaker CB. 
FIG. 3 illustrates another embodiment of the superconducting coil 
protective system in which the d.c. current interrupter DCM comprises a 
commutation resistor RX connected in series to the circuit breaker CB. 
This commutation resistor RX functions to enable the second current 
commutation from the current interrupter DCM into the fuse circuit FC to 
be achieved more smoothly. If the arc voltage of the circuit breaker CB is 
high enough, the resistor Rx is not necessary since the necessity of the 
resistor Rx depends on the characteristics of the arc plasma which is 
generated between the contracts of the circuit breaker CB. That is, when 
the commutation switch S4 is closed, the current commutates to the fuse 
circuit FC by an arc voltage Va across the contacts of the circuit breaker 
CB and a voltage (Rx.multidot.ic) generated by the commutation resistor 
Rx. At this time, the commutation time T2 is expressed by 
EQU T2.apprxeq.Lx/(Rx+Va/ic-Ry) (2) 
where, Lx is a stay inductance of the fuse circuit FC, the circuit breaker 
CB and the commutation resistor Rx, Rx is a resistance value of the 
commutation resistor, and the other items are the same as those of 
equation (1). 
FIG. 4 illustrates another embodiment of the superconducting coil 
protective system of the present invention, in which it is seen that the 
series circuit composed of capacitor C1, the inductor L1 and the switch S5 
is eliminated from the circuit illustrated in FIG. 3. 
During the normal operation, the disconnector DS and the circuit breaker CB 
are closed and the switches S3 and S4 are opened. When the quenching 
occurs, the commutation switch S3 is closed and the polarity of the 
voltage at the power source PS is reversed. Simultaneously with the 
reduction of the current flowing through the disconnector DS toward zero, 
the current commutates to the circuit including the first closure switch 
S3, the circuit breaker CB and the commutation resistor Rx. The 
disconnector DS is opened when the current flowing through the 
disconnector DS becomes zero. Then, the circuit breaker CB is opened and 
the closure switch S4 is closed, so that the current is commutated from 
the circuit breaker CB to the fuse circuit FC. The commutation time T2 is 
given by equation (2). 
The fuse F is selected to have such characteristics that it does not melt 
within the commutation time T2, so that the fuse F melts only after the 
commutation to the fuse circuit FC has been completed and the insulation 
between the contacts of the circuit breaker DB has been recovered, 
whereupon the current commutates to the protective resistor RD. 
If the closure switch S4 were not provided, the current commutation to the 
fuse F is initiated simultaneously with the closure of the closure switch 
S3. With the closure switch S4 as illustrated in FIGS. 3 and 4, the timing 
relationship between the operation of the closure switch S4 and the 
commutation switch S3 as well as the circuit breaker CB can be controlled. 
Therefore, a suitable relationship between these operation and the melting 
characteristics of the fuse F can suitably be obtained. 
FIG. 5 illustrates a further embodiment of the superconducting coil 
protective system of the present invention, in which a saturable reactor 
SR connected to a control power source PSl is employed in place of the 
closure switch S4 in the circuit arrangement illustrated in FIG. 4. The 
time interval during which no current flows through the fuse F is 
expressed by 
EQU T3.apprxeq..DELTA.W/(Va+ic.multidot.Rx) (3) 
Since .DELTA.W can be controlled by the power source PS1, the time T3 with 
no current can be controlled, resulting in advantageous results similar to 
those of the embodiment previously discussed in conjunction with FIG. 4. 
Also, the surge voltages which are generated upon the current interruption 
by the fuse F is advantageously absorbed by the saturable reactor SR to 
protect the superconducting coil L. 
As has been described, the superconducting coil protective system comprises 
a fuse circuit including a serially-connected fuse and a closure switch 
and connected in parallel to a power switch which is serially connected 
between the power source and the superconducting coil. Alternatively, the 
protective system comprises a current interrupting means connected in 
parallel to the superconducting coil and a protective resistor, and a 
commutation switch is connected in series with the protective resistor and 
the current interrupting means for allowing a commutation of a current 
from the superconducting coil to the current interrupting means in 
response to the quenching signal. In this case, the fuse circuit is 
connected in parallel to the protective resistor. 
Therefore, according to the present invention, the fuse circuit functions 
to back up the power switch or enables to control the timing relationship 
between the operation of the switch and the fuse, whereby the current 
interruption can be reliably and easily achieved.