High voltage current limiting circuit breaker utilizing a super conductive resistance element

Abstract

A current limiting circuit breaker which is particularly suitable for use on high voltage direct current systems is provided. The current limiting circuit breaker utilizes a super conductive resistance element connected in parallel with a fixed resistor. During normal circuit operation almost all of the current flows through the superconductive resistance element. When the current flowing through the superconductive resistant element exceeds some known predetermined value the superconductive resistant element loses its superconducting property and has a resistance value which is much larger than the fixed resistor. This forces most of the current through the fixed resistor which limits maximum current flow. This limited current flow can then be interrupted by a conventional circuit interrupting device. When used as a direct current circuit breaker inductance of the fixed resistor and the superconductive resistor can be chosen to minimize the current to be interrupted.

Description

BACKGROUND OF THE INVENTION

This invention relates to high voltage circuit breakers and more particularly to a current limiting circuit breaker which utilizes a superconductive resistance element.

Present breakers are designed to not only open or close the circuit at the design voltage level; but also to be able to break the circuit in the event of a short circuit, with the maximum short circuit current flowing. The circuit breaker must be constructed so as to withstand the large resulting mechanical forces caused by the short circuit currents. These large resulting short circuit forces cause difficulty in the design and operation of the circuit breaker devices and results in massive mechanical bulk of the high power structures.

Current limiting devices are especially useful in power distribution systems for reducing the current to be interrupted during circuit breaker operation and for limiting any resulting short circuit fault current. If the short circuit fault current is not limited, the current may become so intense that the resulting excessive mechanical forces will damage circuit components. In some prior art circuit breaker choke coils have been inserted into the load circuit and alternating current distribution systems to limit current. These coils however produce a voltage drop and cause considerable power loss during normal continuous operation of the system. This causes undesirable inefficiency in the distribution system. For continuous operation, it is desirable that the current limiting elements only be inserted in the electrical circuit during overcurrent conditions when current limiting is required.

Most of the major developments on high voltage circuit interruption has been on alternating current (AC) circuit breakers where the cyclic current zero each half cycle provides an opportunity for the breaker to regain control once it opens. With direct current (DC) systems no current zero occurs naturally and recourse must be had to alternate methods for current interruption. One method has been to use auxiliary circuitry which will transiently reverse the current through the arc which has been struck between the breaker contacts when they open. For example, the current can be brought to zero in an artificial way by discharging a charged capacitor across the contacts of a DC breaker. For high voltages this method requires extremely large capacitors which make it unpractical due to the high cost of the necessary capacitors. Another method is to suitably shape the length and area of the arc so that the arc voltage produced will exceed the supply voltage and force the current down to zero. This approach while suitable for relatively low circuit voltages is not suitable when it is considered for use on higher voltage systems. For example, for a DC voltage level in the range of 100Kv to 1000Kv the required arc length will have to be in the order of 10 feet to 100 feet. It can be easily appreciated that the length of the arc required to make the arc voltage equal to the network voltage makes devices using this principal very bulky and impractical. In practice, of course, multiple gap contactors are used with individual gap ratings of around 30Kv. These multiple contactors are further complicated by air or gas blast nozzles necessary to disperse the arc built up when opening under fault conditions.

SUMMARY OF THE INVENTION

It is well known that certain materials when cooled to the region of less than 20.degree.K exhibit the property of being virtually resistanceless. For example, a current once initiated in a superconducting loop will circulate virtually forever without any driving EMF. Probably not so well known is that the transition from superconductive to merely low resistance, called normalization, is very abrupt. This abrupt transition or normalization can be initiated by exceeding a certain critical current level, by being exposed to an external magnetic field, by a combination of a given current and external field, by a shock or by a heat pulse. In some applications it is very sensitive to the rate of change of current.

A high voltage current limiting circuit breaker utilizing a superconductive resistance element is provided. The superconductive resistance element is connected in series with a circuit interrupter which is capable of carrying continuous full load current. The superconductive resistance element is constructed so that as the load current rises above a predetermined value the superconductive resistance element will switch from a virtually zero resistance condition to a high resistance condition, thereby limiting the current which can flow through the circuit breaker.

In another embodiment of the invention a parallel path around the superconductive resistance element and the circuit interrupter is provided. The additional parallel path contains another circuit interrupter and a fixed resistance in series. Under normal operating conditions when the load current is less than the predetermined current limiting value virtually all of the load current will flow through the first circuit interrupter and the superconductive element. If a fault develops in the load circuit as soon as the load current rises past the predetermined current limiting value, then the superconductive resistance element will normalize and switch from approximately zero resistance to a relatively high resistance. The resistance of the normalized superconductive resistance element is made much larger than the resistance of the fixed resistor which is connected in parallel. The resistance of the fixed resistor exceeds the normalized resistance of the superconductive element by about 100 times. Thus approximately 99% of the fault current will be transferred from the path containing the superconductive element to the path containing the fixed resistance. The fixed resistance is selected to limit the fault current to a value less than the breaker's rated current. A short time after the superconductive resistance element is normalized and the current is limited, both circuit interrupters will be opened by a trip circuit. Thus neither curcuit interrupter will have to interrupt more current than the breaker full load rating.

If the parallel path containing the fixed resistor is eliminated then when the superconductive element is normalized current flowing through the breaker will be limited to a value much less than the rated load current. Thus, the circuit interrupter will only have to break a current less than the critical value. Inductance of the electrical circuit will influence the practicality of using the superconductive element without a parallel fixed resistance.

In another embodiment the inductance of the superconductive resistance element and inductance of the fixed resistor are utilized to further reduce the current to be interrupted. Inductance of the linear resistor and the superconductor can be minimized but non-eliminated. In this embodiment of the invention the breaker parameters are selected to optimize the performance of the circuit breaker. That is, the instantaneous load current magnitude when the superconductive resistance element normalizes is equal to the sum of a constant plus a decaying exponential term plus an increasing exponential term. Therefore, an instantaneous current minimum will occur at some finite time after the superconductive resistant element normalizes. If the parameters of the circuit breaker are properly selected the instantaneous current minimum can be made to occur when the breaker interrupter contacts open. Thus, the circuit interrupter will have to break only a minimum circuit current. The magnitude and the time of the minimum instantaneous current can be varied by varying the resistance parameters of the circuit interrupter. If the time of contact separation is the same as the time of minimum instantaneous current, by proper selection of circuit breaker parameters, the circuit breaker can open at a minimum current point which is substantially less than steady state value of the circuit current.

This invention discloses a circuit breaker construction and assembly which allows the use of a conventional circuit breaker with a rating sufficient only to be able to carry and interrupt full load current. The required circuit breaker does not have to be able to interrupt the potential short circuit current since this is limited by the superconductive elements. In addition the proposed device having current limiting properties will allow simplification and reduction in size of interconnecting links in the power distribution system, because they will not be required now to withstand the short circuit capability of the generating system.

It is an object of this invention to provide a quick response current activated, current limiting device which does not effect the load circuit during rated current operation and has substantially no power loss during normal operation. It is also an object of this invention to provide a high voltage DC current limiting circuit breaker which uses only conventional breaker contacts, rated sufficient to be able to carry full load current and to break the circuit at full load current.

It is a further object of this invention to provide a current limiting device which utilizes a superconductive element that is current sensitive to limit short circuit and overload current to a safe predetermined value.

It is further object of this invention to provide a circuit breaker having inductance and resistance parameters selected so that when a short circuit current is to be interrupted the value of the current to be interrupted is significantly less than the final steady state current which could flow through the circuit breaker.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be had to the preferred embodiment exemplary of the invention shown in the accompanying drawings in which:

FIG. 1 is a schematic diagram of a circuit breaker having a superconductive resistance device in series with a circuit interrupter;

FIG. 2 is a schematic diagram similar to FIG. 1, with the circuit breaker having a parallel path including a specific resistance;

FIG. 3 is a schematic diagram similar to FIG. 2, illustrating inductance parameters of the circuit breaker;

FIG. 4 is a graphical representation of current versus time for the circuit schematic shown in FIG. 3;

FIG. 5 is a graph similar to FIG. 4 for a different load parameter; and

FIG. 6 if a graph similar to FIG. 4 for yet another load circuit parameter.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, and FIG. 1 in particular, there is shown a voltage source 10 feeding a load 12 through a circuit breaker 14. Circuit breaker 14 comprises a circuit switch 16 in series with a superconductive resistance element 18.

Resistance element 18 is current sensitive and passes from a superconductive state with virtually zero resistance to a normal conductive state with a known resistance value after a predetermined current limit is passed. When element 18 is in a superconductive state, with virtually zero resistance, breaker 14 carries the entire load current with very little loss.

It is well known that certain materials when cooled to the region of 0.degree.K to 20.degree.K exhibit the property of being virtually resistantless. That is, a current once initiated in a superconducting loop will circulate virtually forever without any driving electro motive force (EMF). Not so well known is that the transition called normalization from superconducting to merely low resistance is very abrupt. This abrupt transition can be initiated by exceeding a certain critical level. The critical current level which causes superconducting element 18 to normalize can be predetermined so that element 18 will normalize at a desired value and limit the current which can pass through breaker 14. Inductance in the rest of the circuit influence the practicality of the rapid introduction of a normalized resistance of element 18 into the circuit.

Inductance of the circuit may necessitate a parallel circuit as shown in FIG. 2. A parallel path around interrupter 16 and superconducting element 18 is provided by a fixed resistive element 20 in series with a switch 22. Under normal operating conditions, switches 16 and 22 are both closed and element 18 is in the superconducting state, with a very low resistance. Since the resistance of element 18 is virtually zero and much less than the fixed resistance of element 20, virtually no current will flow through the parallel path comprising switch 22 and fixed linear resistor 20. Element 18 can be constructed so that as the load current rises to a predetermined value, 125% of full load current, then element 18 will change from a superconductive state to a normal resistance state, with a normal resistance value approximately 100 times greater than the resistance of fixed resistor 20.

If a fault develops in the load circuit as the load current passes the critical predetermined value then superconducting element 18 normalizes and approximately 99% of the fault current will be transferred from the path containing components 16 and 18 through the path containing components 20 and 22. The fault current will be forced to a finite value dependent upon the values of fixed resistor 20 and the magnitude of the supply voltage 10. The fault current will be limited to a value less than or equal to the breakers rated current and interrupting element 16 and 22 will be opened by a trip circuit a short time later. Circuit switch 22 must break the load current limited by fixed resistor 20. Circuit switch 16 has only to break, at most, full rated current and never more than a fraction of full load current when superconductive element 18 normalizes. With a suitable mechanical construction to give some delay in operating sequence both circuit switches 16 and 22 can be controlled by a single mechanical activator.

A circuit breaker 14 capable of interrupting 100Kv DC with a full load capacity of 1000 amps, that is 100 megawatt circuit breakers can be constructed from readily available material. With 100Kv DC supply voltage and a full load current of 1000 amps then fixed resistor 20 must have a value of 100 ohms to limit the fault current to a value less than or equal to the full load current. The normalized resistance of element 18 must be approximately 100 times greater than the resistance of fixed resistor 20 to force approximately 99% of the current through the path containing fixed resistor 20. Thus, the normalized resistance of element 18 must be equal to approximately 10,000 ohms. A suitable element 18 can be constructed using a single strand of niobium titanium (NbTi) wire of approximately 27 awg and 10,000 meters long. Other experimental materials which are suitable for superconductive elements offer resistance values two orders of magnitude higher. Using these materials the length of the required wire would necessarily be much shorter.

The size of the NbTi wire required can be calculated as follows. Standard NbTi wire would typically have resistance of the order of 10 micro ohm-centimeters. At a current density of 10.sup.6 amps per square centimeter which represents a fairly reasonable current density for superconductive elements a wire having a cross sectional area of ##EQU1## which is approximately equivalent to 27 awg. is required. The length of the wire required equals the required resistivity of 10,000 ohms multiplied by the cross sectional area of the wire used which is approximately 10.sup.-.sup.3 sq. centimeters divided by the resistivity of the wire which is approximately 10 micro ohms centimeter.sup.2 yields a wire length of 10,000 meters. A length of 10,000 meters using 27 awg wire fault current will be forced to split 99% through elements 20 and 22 and 1% through elements 16 and 18. Since the supply voltage is fixed at 100Kv DC the current will be forced down to less than 1000 amps, from some maximum value slightly above 1250 amps, at some rate fixed by the inductance of the circuit.

The amount of refrigerant required can be estimated by assuming a maximum possible operating time of 100 milliseconds for the main breaker. Under these conditions something nearer 10 Joules/Meter will be dissipated in the superconductor element 18 after it goes normal. This will cause some boiling off of the refrigerant used in superconductive element 18. The refrigerant used will probably be liquid helium. Assuming that it will take 100 miliseconds for the main breaker contacts to open after the superconductive element 18 goes normal than the heat dissipated in the normal superconductor element 18 can be calculated using the formula I.sup.2 R .times. t which yields 10.sup.5 Joules. The amount of helium boiled off can be determined using the following formulas: ##EQU2##

Latent heat of vaporization for helium = 0.021 K Cal/Mole.

Molecular weight of helium = 4; that is, one mole of helium weights 4 grams. Latent heater of vaporization for helium = 5.25 Cal/gm, which is equivalent to approximately 22 joules per gram. ##EQU3##

It is believed that this combination of a fixed linear resistor 14 paralleling a superconducting resistive element 18 for a current limiting device is novel. Breaker 14 also has a fail safe characteristic in that loss of the required refrigerant or an open circuit in the superconductor element 18 will always initiate an opening of circuit breaker 14.

It is also possible to utilize the inherent inductance of superconductive element 18 and fixed resistor 20 to further reduce the current to be interrupted. Referring now to FIG. 3 there is shown a circuit breaker 14 in which inductance of element 18 is indicated by component 24 and inductance of fixed resistor 20 is indicated by component 26. By the proper choice of circuit parameters the value of the current to be interrupted can be lowered substantially. It is believed that the utilization of the inductive-resistant time constants of the circuit breaker to reduce the current to be interrupted is novel.

Inductance 24 and 26 of elements 18 and 20 can be minimized but not eliminated. It is possible to use inductance 24 and 26 to optimize performance of circuit breaker 14. For example, consider the following analysis: R.sub.S and L.sub.S are the resistance 18 and inductance 24 of the superconductor; V.sub.o is the DC voltage source 10; R.sub.1 is load resistance 12 of the external circuit; and R and L are the resistance 20 and the inductance 26 of the fixed resistance path. Thus the full load steady state current is equal to V.sub.o /R.sub.L and the resistance R.sub.s of superconductor 18 equals 0. When the resistance of the superconductive path goes normal, R.sub.s the resistance of element 18 is raised in a step to a value 100 times R, the resistance 20. Final current in the circuit after the transient has faded away is ##EQU4##

For a transient analysis of circuit breaker 14, the following calculations are necessary. Equations for the current i through R.sub.L load resistant 12 and current is through R.sub.s 18 and L.sub.s 24 are given as: ##EQU5## Take the Laplace transforms of (1) and (2) with respect to time, noting that i (0.sup.+) = V.sub.o /R.sub.L and i (o.sup.+) = V.sub.o R.sub.L, to obtain ##EQU6## equations (3) and (4) are combined to yield the load current I (s) which is ##EQU7## by the initial and final value theorems of Laplace transform theory. Equation 5 is multiplied by R.sub.2 divided by V.sub.o and rearranged to yield ##EQU8## The inverse Laplace transform of (6) is given by ##EQU9## where

As shown by equation (7) instantaneous current through circuit interrupter 14 is equal to a constant plus a decaying exponential current plus an increasing exponential current. As shown in FIGS. 4, 5, and 6 the instantaneous current first decreases to the final steady state value of current. The minimum instantaneous value of current 40 attained is a function of the circuit parameters R, R.sub.s, L, and L.sub.s and the load R.sub.1. By varying the circuit parameters R.sub.L, L.sub.s, L, R.sub.s, and R it is possible to vary the magnitude of the minimum instantaneous current 40 and time of occurance of minimum instantaneous current 40. Shown schematically in FIG. 3, switches 16 and 22 are controlled mechanically by an operator means 30. From FIGS. 4, 5, and 6, and from equation (7) it is clear that the magnitude and time of a occurance of the minimum current can be controlled by astute choice of circuit parameters. If the parameters are properly selected switches 16 and 22 will open when the current is at a minimum 40. This system is particularly suitable for limiting the fault current to be interrupted. Current through switches 16 and 22, during interruption, can be limited to a value which is considerably less than the full load trip current. Since the maximum fault current to be interrupted can be preset the switches 16 and 22 can be considerably derated over prior art switches.

The values of the parameters selected for FIG. 3, to give the graphs shown in FIGS. 4, 5, and 6 are:

For the graph in FIG. 4 R.sub.L = 0.01.OMEGA.

For the graph in FIG. 5 R.sub.L = 0.1.OMEGA.

For the graph in FIG. 6 R.sub.L = 0.001.OMEGA.

As can be seen the minimum current 40, in all cases, is significantly less then the final steady current. Thus it can be seen that by properly coordinating the opening of switches, 16 and 22, with the occurrence of the minimum current 40, the value of current to be interrupted can be substantially reduced.

Claims

What is claimed is:

1. A direct current high voltage current limiting circuit breaker comprising:

a first current path having a first circuit disconnection means in series with a circuit element, which can be actuated from a superconductive state to a normal conductive state;

said circuit element when in the normal conductive state having a first fixed resistance value and a first fixed inductive value;

a second current path in parallel with said first current path having a second circuit disconnecting means in series with a second fixed resistance and a second fixed inductance;

an actuator means mechanically connected to said first circuit disconnecting means and said second circuit disconnecting means to open said first circuit disconnecting means and said second disconnecting means together a fixed time after being activated; and said first fixed resistance value, said second fixed resistance value, said first fixed inductance value, and said second fixed inductance value being selected so that a current minimum occurs in the circuit to be interrupted at the fixed time when said actuator opens said first circuit disconnecting means and said second circuit disconnecting means.

2. A direct current high voltage current limiting circuit breaker as claimed in claim 1 wherein:

the current flow through the circuit breaker after the circuit element is actuated from a superconductive state to a normal conductive state is equal to a constant plus a decaying exponential current plus an increasing exponential current; and,

said actuator means opens the circuit breaker near the point of minimum current.