US 3617807 A
Description (OCR text may contain errors)
United States Patent Inventor Fritz Kesselring Kusnacht, Switzerland Appl. No. 39,040 Filed May 20, 1970 Patented Nov. 2, 1971 Assignee Siemens Alttiengesellschaft I Berlin and Munich, Germany Priority May 23, 1969 Germany P 19 26 972.8
CURRENT-LIMITING SWITCH EMPLOYING LOW-TEMPERATURE RESISTOR 6 Claims, 4 Drawing Figs.
U.S. Cl 317/1115, 307/136, 317/1 1 C, 317/4! Int. Cl "02h 7/22 Field oiSearch 317/11 E,
 References Cited UNITED STATES PATENTS 782,134 2/1905 Horry 3l7/l l E 3,454,831 7/1969 Willard..... 3l7/1IC 3,522,472 8/1970 Breitholtz 317/1] E Primary Examiner-James D. Trammell Attorney-Ostrolenk, Faber, Gerb & Soffen ABSTRACT: A circuit interrupter is provided with a parallelconnected resistor which is cooled to a temperature of from 4 K. to 80 l(., but higher than the critical temperature ofthe resistance material. The resistor is connected in parallel with the circuit interrupter contacts at either an arbitrary time or just prior to a current zero value. The heating of the resistor during current-limiting operation increases its resistivity by about I00 times BACKGROUND OF THE INVENTION This invention relates to circuit interrupters, and more specifically relates to circuit interrupters in which a currentlimiting resistance having a positive temperature coefficient of resistance is inserted into the circuit being protected during interruption of the circuit.
The use of current-limiting resistors which are inserted into a circuit to limit the current to be interrupted are well known, where such resistors can be inserted in the vicinity of a current zero in the circuit to be interrupted, Such resistors have been inserted into the circuit and their value increased as by a sliding contact arrangement which gradually inserts additional resistance to limit the current to be interrupted. These resistors have also been made of a material which has a high positive temperature coefficient of resistance, so that their resistance will automatically increase due to the heat generated in the resistor when the resistor is inserted in the circuit, thereby reducing or limiting the current in the circuit to be interrupted; Chemically pure iron has been used for such resistors and these resistors will exhibit an increase in resistance of aboutl2 times. For all other materials known to me the increase in resistance is less than for the pure iron as noted above. U.S. Pat. No. 3,495,056 illustrates an interrupter using a pure iron resistor where the resistor is heated from room temperature to a calculated temperature which the resistor can safely withstand during current-limiting operation.
The principle of the present invention is to cool the resistor which is to be inserted into a circuit which is to be interrupted to a temperature of from 4 K. to 80 K., but above the critical temperature at which the resistor material becomes superconductive and to heat this resistor to a temperature of about 373 K. during current-limiting operation. By starting with the above low temperature for the resistor, it has been found, unexpectedly, that the resistivity of the material changes, depending on the resistor material, with about the third power of -the absolute temperature of the resistor. In particular, when changing temperature from 20 K. to 373 K., the resistivity of a pure iron resistor increased by more than 100 times, although the presently expected linear change in resistivity would anticipate only a several-fold change in resistance.
More specifically, it was commonly thought that change in resistivity is given by a linear relationship of the form:
p po[ +a( To(] where p is resistivity at temperature T, p,, is resistivity at C. and a is the temperature coefficient of the material, typically from 4X 1 0 to 6 l0 per degree.
The present invention is based, in part, on the discovery that the above-noted expression is followed only for temperatures down to about 70 C. Below -70 C., however, the resistivity changes according to a different and nonlinear relationship which is:
P P( o) where T is 273 K. and ,1. is a constant depending on the nature of the material used. For example, for tungsten p. is about equal to 3.
The second discovery leading to the present invention is that the specific heat c decreases below -70 C. with the square of the absolute temperature, and, at very low temperatures, with the third power of absolute temperature.
It was further found, in accordance with the invention, that at low temperature the following relation holds:
'lp) 'T= where k is a constant having a characteristic value for different materials. This constant and the relationship above permits the calculation for a resistor for a given application as will be later described. Typical values for the constant K are approximately as follows:
Tungsten: K 0. 1 X10 Iron: 10.09Xl0' Copper: K OJZXIO' It can be shown that the absolute temperature T reached by the resistor after a time t is:
y 1 I I 1.1.1?
where S is the current density in the resistor. The resistor should be dimensioned such that reaches the highest possible value during the current-limiting interval in which current flows in the resistor so that the resistor will be heated to its maximum temperature T (and thus undergo a maximum resistance change). If r is the time permitted for current limiting action, then the resistor should be designed for an effective current density S which is:
The above integral can then be computed from the predicted wave shape of the current in the resistor and the cross section of the resistor wire can be computed.
An important aspect of the present invention is that' useful current-limiting resistors can be designed despite the low specific heat of the resistance materials at the low initial temperature since the heat produced in the low temperature region is slight. In particular, there is a matching between the specific heat c' and resistivity P so that there is a relatively gradual increase in the temperature of the resistor, rather than a rapid increase which could cause an explosive evaporation of the cooling liquid surrounding the resistor.
A few coolants that could be used to cool the resistor are listed in the following table:
Boiling Point Coolant (K.) at l atm.
Nitrogen 77 Neon 27 Hydrogen 20 Helium 4 By varying the pressure, the boiling points can be adapted to specific conditions.
Materials useful for the resistor are preferably extremely pure metals including tungsten, iron and copper.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates an interrupter constructed in accordance with the present invention.
FIG. 2 illustrates the operation of the interrupter of FIG. 1 under short-circuit conditions on the line being protected.
FIG. 3 illustrates the operation of the interrupter of FIG. 1 with the contacts being synchronously operated to insert the resistor at a current zero.
FIG. 4 shows an interrupter arrangement which can be synchronously operated as illustrated in FIG. 3.
DESCRIPTION OF PREFERRED EMBODIMENTS tainer 4 is surrounded in known manner by a covering 7 which reflects the heat rays. The leads 8 and 9 to the resistor 3 may also be cooled so that the heat loss by thermal conduction is as small as possible. The design of resistor 3 can be determined as follows:
Assume, as shown in FIG. 2, that a short-circuit current i with a current slope established by the powerline inductance L occurs at the time t=At. After the very short time At which is a fraction of a millisecond, the auxiliary switch 2 (see FIG. 1) is closed by appropriate operating mechanisms (not shown) and immediately after this, the main switch I is opened by the suitable operating mechanisms. When switch 1 opens, there will be arcing across the open switch, which, even in the case of very small spacing of the contacts, exhibit an arc-drop voltage of about l v. per point of interruption. This are drop voltage causes the current i to switch into the resistor 3. In this connection, the break current I multiplied by the cold resistance r, of resistor 3 must have a value which is smaller than the total are voltage U,,. Therefore:
where l is the length of the resistor wire and q is its cross-sectional area. In the arrangement of FIG. 1, there are two points of interruption so that i U,, is about 20 to 30 v. If the arcs are produced in narrow gaps, then voltage gradients of up to several hundred volts per centimeter per point of interruption can be obtained. This may be of importance in particular when using current limiters in high-voltage direct current systems. The cold resistance r, thus becomes:
r UB/IT In FIG. 1, the peak let-through current i should not be greater than about (2-3) I The time 1' of flow of current in the case of short circuit at 50 cycles is about ms. and at 60 cycles is about 4.5 ms. Furthermore, since the slope of the current i is known, the course of the reduced current i can be approximately determined. Ifq in cm. is the still-unknown cross section of the variable resistor wire, then by squaring and dividing by q it is possible to calculate the value:
s s d 1 d l. aio
from the wave shape of the current.
The design conditions are somewhat different in the case of the so-called reduction switch or switch which inserts the variable resistor in the circuit at a current zero. The wave shape of the current i, which occurs in this type switch is shown in FIG. 3. FIG. 3 shows a short-circuit current i in the circuit, and its wave shape if undisturbed by the resistor. Shortly prior to the time current i,, reaches a current zero N, the auxiliary switch 2 is closed and the main switch 1 is opened by means of a suitable synchronous control. The cold resistor r connected in parallel with the main switch 1 is now so dimensioned that the rise of the recovery voltage (m /d1). is substantially reduced. The value k which is controlling for this is defined by:
limo/J? (ill constant d! 273 In the above relation, N,,=u,,i is the steady are power in the vicinity of the current zero and T is the arc time-constant. Immediately after the current zero N (FIG. 3) the current flows through the cold resistor 3, the resistance of which is thereupon increased by several orders of magnitude so that at the next current zero N the driving voltage u and the very small reduced current i, pass through zero simultaneously. Therefore, the reduced current i, can be easily interrupted. Since in the case of the reduction switch, following the techniques of FIG. 3, the energy conversion is much less than in the case of the switch operating, as shown in FIG. 2, the current limiting resistor 3 will be correspondingly smaller and simpler.
The following example may serve by way of further explanation of the design of a suitable resistor:
A reductor or switch operating as in FIG. 2 is to be designed for a line-to-line voltage of U,.=l0,000 V. and a three-phase short-circuit power P,, of 1,000 MVA. The short-circuit current is 1000Jfi-1o=ss kA and thus the inductance per phase is L=l0 m 58100.3 1 5'l0 H. The maximum current slope is thus:
2.5 -1 10,000 amp there is obtained a wave shape of the reduced current i such as shown in FIG. 2. By squaring one obtains F=f(!), from which there follows:
I I s(z)=f 5 07 5] M.
with l =4000 amp, the cold resistance is:
r,,=U /I =20/4000 0.005Q
By linear interpolation one finds the temperature T =2l0 K. corresponding to F We must therefore have:
T,=40-e=2 l 0;e=2 l0/40=5.25;x= l .66. From this there follows:
The final temperature T, is:
1: ii 0.67 10* T 4-0 e 40 e I T,,=40-34.8=1400 K.
For the length l of the resistor wire there results:
1 so 225 3700 cm--37 m.
The total resistance ratio is:
" r /r =31/0.005=74iibj Q0 For dependable operation, the variable resistor 3 must have approximately the value r, at the start of an interruption. This result is obtained by closing the auxiliary switch 2 of FIG. I
only when the resistor 3 has cooled to a predetermined temperature T,. In order to obtain short cooling times, the resistor r is preferably made of thin wires or thin strips connected in parallel. With a diameter of the wire of, for instance 0.3 mm., cooling times of fractions of a second can be obtained. The use of thin wires and strips is also advantageous with respect to the skin effect, since the resistivity I in a cooled condition is vary small.
If the interrupter is to be capable of several operations within a very short time interval, it is preferable to provide at least two circuits in parallel to the main switch 1, each consisting of a resistor 3 and auxiliary switch 2 in series therewith. These circuits are alternately connected whereby the resistor being connected on subsequent operations will have been cooled to the desired low temperature.
One illustrative embodiment of such an arrangement is shown in FIG. 4. In FIG. 4 temperature-dependent resistors 11 and 12 are cooled to a low temperature, as described above. Main interrupter switch 13 is then provided with an electrodynamic drive consisting of a stationary coil 14 and a movable metallic plate 15 which is rigidly connected with the movable bridge contact 16. Auxiliary switches 17 and 18 are provided for resistors 11 and 12, respectively, and are each provided with electrodynamic drives consisting of stationary coils l9 and 20, respectively, and the conductive bridge contacts 21 and 22 which simultaneously serve as secondary coils of their electrodynamic drive system. Movable conductive contact discs 23 and 24 are provided which are connected with the pistons 25 and 26 of the delay pumps 27 and 28. Contacts 23 and 24 are also adapted to be accelerated to the right responsive to energization of coils 19 and 20. The contacts 23 and 24 are connected via a balance beam 29 with a freely movable fulcrum 30 to which a lever 31 is rigidly fastened. Contacts 23 and 24 cooperate with stationary contacts 33-36 and 34-35, respectively. An insulating boards, which can be interposed between the contact 33 or contact 34, depending on the position of the pistons 25, 26 respectively, receives lever 31 as shown. Springs 37 and 38 are received in cylinders 27 and 28 and bias the pistons 25 and 26, respectively, to the left. An auxiliary DC power source is then provided (not shown) which charges capacitor 39. A three-electrode spark gap 40 is then provided in series with a capacitor 39 and can be fired in known manner upon the occurrence of a fault current. Springs 41 and 42 are provided to bias open the auxiliary switches 17 and 18, respectively, so that these contacts will reopen some predetermined time after they are closed by their electrodynamic system.
In order to operate the circuit of FIG. 4 in the mode shown in FIG. 3, the trigger electrode of spark gap 40 is energized from a suitable zero current anticipating circuit 40a, which is suitably coupled to the line being protected, as shown by dotted lines. Circuit 40a may be any conventional type in which an output pulse is generated at some predetermined time prior to a current zero. Note, however, that the circuit of FIG. 4 could be operated outside of the current region, as described in FIG. 2.
The manner of operation of the arrangement is as follows:
In response to an overcurrent and in the vicinity of a current zero, the spark gap 40 fires and the capacitor 39 discharges through coil 14 of the main switch 13 and through coil of the auxiliary switch 18. Auxiliary switch 18 closes and main switch 13 opens, whereby the current i is switched to the resistor 12. The sequence of the closing of the auxiliary switch 18 and of the subsequent opening of the main switch 13 can be controlled by proper design of the movable masses of the switches. With the excitation of the coil 20, the contact 24 is accelerated toward the right and the piston 26 is also pushed to the right in opposition to the spring 38. Auxiliary switch 18 then opens under the influence of the spring 42 and after a predetermined period of time, whereupon the resistor current is interrupted at the subsequent current zero.
In FIG. 4 assume that an interruption occurred prior to the interruption described above, using resistor 11 and auxiliary switch 17. A second switching operation using resistor 11 is possible only after the resistor 11 has cooled down approximately to the cold value r,,. This could be done by providing a conventional temperature-measuring device which prevents operation of switch 17 (and switch 18) until the temperature T, is reached. However, it is simpler to block the closing of the auxiliary switch during a known cooling time, as measured by conventional means such as a clock or damped pump.
FIG. 4 illustrates a lockout arrangement for switches 17 and 18 which uses damping pumps. In FIG. 4, after excitation of the coil 20, the auxiliary switch 18 is closed and the piston 26 moves to the right together with the disc 24. The balance beam 29 rotates to a perpendicular position and the lever 31 reaches a horizontal position. During this time, the piston 25 moves toward the left and-since the piston 26 is still approximately in the right-hand position-pushes the lever 31 and the insulating disc 32 downward. In this way the contact 23 engages contacts 33, 36, whereby the auxiliary switch 17 is again ready for operation. The closing of the auxiliary switch 18, however, is prevented by the downwardly displaced insulating board 32 so that only auxiliary switch 17 can be actuated upon the next disconnection.
At very low temperatures, but above the critical temperature, the resistivity of resistors is slightly increased by the magnetic field traversing the resistor material. This slight increase in resistance is generally unimportant to the use of the resistors in accordance with the invention. The magnetic field strength can, however, be reduced to harmless values without difficulty by known magnetic shielding screening.
Although there has been described a preferred embodiment of this novel invention, many variations and modifications will now be apparent to those skilled in the art. Therefore, this invention is to be limited, not by the specific disclosure herein, but only by the appending claims.
The embodiments of the invention in which an exclusive privilege orproperty is claimed are defined as follows:
1. An arc interrupting device for an electrical circuit comprising at least one main switch and at least one resistor electrically connectable in parallel circuit therewith whose resistance increases with temperature and to which current flowing through said main switch is shifted upon the opening of said switch, characterized by means being present to cool said resistor under normal operating conditions to a temperature between 4 K. and K. and higher than the critical temperature of the material from which said resistor is constructed, and that as said circuit resistance material there is used a material having a resistivity which increases by at least times with a temperature increase'over the range of 20 K. to 373 K.
2. The arc interrupting device of claim 1 having at least one auxiliary switch in parallel connection to said main switch and in series connection with said resistor, characterized by means for opening said auxiliary switch under normal operating conditions when said main switch is closed and for closing said auxiliary switch upon directed opening of said main switch, and just prior to said main switch opening.
3. The arc interrupting device of claim 2 further characterized by means for closing said auxiliary switch substantially only when said resistor has cooled to a predetermined temperature.
4. The are interrupting device of claim 2 particularly for use where rapid reconnection is directed, characterized by the fact that at least two circuits are connected in parallel with said main switch, with each such circuit including one of said resistors and one of said auxiliary switches serially connected across said main switch and wherein means are included to close only that auxiliary switch whose corresponding resistor has cooled to a predetermined temperature at the time direction is generated for opening said main switch.
5. The arc interrupting device of claim I particularly for use where high magnetic fields are present, characterized by magnetic screening means being present to shield said resistor from the effects of said fields.