DC High-speed vacuum circuit breaker and electric motor vehicle equipped with this circuit breaker

Abstract: In the electric rolling stock running in a direct current feeder section, an accidental current has conventionally cut off by the DC high-speed circuit breaker. However, this circuit breaker is low in response to a steep peak of the accidental current and the circuit breaker at the substation is operated earlier, so that the accident in one train affects other trains in the same feeder section. So, attempts have been made to mount a DC high-speed vacuum circuit breaker which operates very quickly. In the conventional DC high-speed vacuum circuit breaker, however, it was impossible to obtain the commutation frequency of higher than 1 kHz, and to obtain such a high commutation frequency, the commutating capacitor and the other devices become very large in size. By this invention, with various inductances taken into consideration, it has been made possible to cut off a current at 2 kHz or higher and it has become possible to provide a DC high-speed vacuum circuit breaker (1) of the reduced size which can be mounted on the electric rolling stock.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a direct current circuit breaker which uses a vacuum valve. DESCRIPTION OF THE PRIOR ARTS Electric cars and electric locomotives (hereinafter referred to as electric rolling stock) have inherent in them a possibility that a failure occurs such as a short circuit due to a breakdown of an element (a thyristor, a GTO thyristor, or a transistor, for example) used in the main circuit of an inverter or a chopper, a ground fault caused by imperfect insulation of some wire in the main circuit, or an abnormal current increase resulting from a failure of the control system. If such a failure is left unattended, the equipment will burn. In order to prevent this accident, the electric rolling stock have been conventionally equipped with a circuit breaker to cut off an excess current. However, with the air circuit breaker heretofore used, for constructional reasons, the breaking speed is slow from when an accidental current flows until the current is cut off, and before the circuit breaker opens itself, it sometimes happens that a circuit breaker in the ground substation of the feeder section where the electric rolling stock is located opens the circuit previously. When the circuit breaker of the ground substation operates, all the electric cars within the feeder section supplied by that substation are unable to receive power and stop. In other words, the accident in one electric car spreads to other electric cars. If such an accident occurs on the line under a congested train schedule, it is easily imaginable that the accident affects not only the electric rolling stock within the feeder section but also the electric rolling stock of the other feeder sections. This is because of the slow breaking speed of the air circuit breaker installed on the electric car mentioned above. Consequently, there has been requirement for a DC vacuum circuit breaker as a circuit breaker with much higher breaking speed. As described in JP-A-54-132776, to cut off a direct current is more difficult than to cut off an alternating current because a direct current has no zero point. As a countermeasure, to facilitate a cutoff of a direct current, current zero points are created artificially by providing a switching valve (hereafter referred to as a valve) with a commutating capacitor in parallel therewith and by forming an oscillation circuit (commutation circuit) in combination with the inductance of the circuit. The methods for this purpose are roughly divided into two groups: the reverse charging methods and the no-charging methods. The reverse charging method is to charge a capacitor and discharge the electric charge stored in the capacitor when opening the valve. In this method, oscillation is produced by the capacitor and the inductance of the circuit. Since this oscillation circuit has a pure resistance component, the amplitude of the oscillation decreases exponentially. As the amplitude of oscillation at its early stage passes through a current zero point, the arc current in the valve is eliminated, thereby completing the cutoff. In the no-charging method, on the other hand, a valve with negative arc characteristics is used, a capacitor is connected in parallel with this valve, and a divergent oscillating current is obtained when opening the valve. When the amplitude of oscillation in the diverging direction passes through a zero point, the current is cut off. This method, however, requires a certain length of time before the oscillation grows and passes through the current zero point. Therefore, it is possible that before this occurs, the circuit breaker of the ground substation operates. For this reason, a circuit breaker of the reverse charging method is more convenient when it is installed on the electric rolling stock. A DC circuit breaker of the reverse charging method disclosed in JP-A-54-132776 is described below. A capacitor is connected in parallel with the valve and a resonance circuit is formed by this capacitor and a stray inductance. By this arrangement, however, the current inclination, which is a time differential (di/dt) when the current flowing through the valve crosses the current zero point is so great that it is difficult to cut off the current. As a solution to this, in order to reduce the current inclination, in addition to the stray inductance, an inductance of more than several millihenries (mH) is connected in series with the capacitor. Let us consider the effect of varying the magnitude of the capacitor and the inductance of the above-mentioned prior art. If the inductance of several millihenries is used, the capacitor will be several thousand to tens of thousand microfarad (µF). Therefore, the capacitor will become very large in size. The electric rolling stock have their equipment mounted under the floor and above the roof. Its space for mounting equipment is very limited, and if some apparatus is too large, it cannot be mounted on the electric rolling stock. DE-A-2 742 965 discloses a DC vacuum circuit breaker with the features included in the first part of claim 1. US-A-4 056 836 describes a similar DC circuit breaker which does not use, however, a vacuum valve included in the DC circuit. Using vacuum valves for interrupting a DC current is known per se from FR-A-1 504 976. SUMMARY OF THE INVENTION The object of the present invention is to provide a DC vacuum circuit breaker of the reverse charging type for mounting on electric stock to permit interrupting high DC currents safely and without requiring a large capacitor. This object is accomplished by the invention defined in claim 1. With this circuit breaker, the oscillation frequency of the communication circuit can be 2 kHz or more. Therefore, the stray inductance serves sufficiently as the commutating reactor, and a substantially smaller commutating capacitor can be used. Such a small element can be mounted in a limited space under the floor of the electric rolling stock. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing an embodiment of this invention; Fig. 2 is a diagram showing the relation among the commutating capacitance, commutating inductance, commutation frequency, and commutating current; Fig. 3 is a diagram showing the commutation principle of this invention; Fig. 4 is a diagram showing a DC high-speed vacuum circuit breaker according to this invention accommodated in a box; Fig. 5 is a diagram showing operation waveforms; Fig. 6 is a diagram showing an application of the DC high-speed vacuum circuit breaker according to this invention to electric rolling stock; Figs. 7 to 10 are diagrams showing other embodiments of this invention; Fig. 11 is a diagram showing an experimental equipment; Fig. 12 is a diagram showing another embodiment of this invention; Fig. 13 is a diagram showing the characteristics of a saturable reactor; Fig. 14 is a diagram showing the effect of the embodiment of Fig. 12; Fig. 15 is a diagram showing a modification of the embodiment of Fig. 12; Fig. 16 is a diagram showing a conventional air circuit breaker mounted in electric rolling stock; and Figs. 17 to 19 are diagrams showing the DC high-speed vacuum circuit breaker mounted in electric rolling stock. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to JP-A-54-132776 mentioned above, in the reverse charging method, the current cannot be cut off securely at a frequency of 1 kHz or above of the oscillating current by a commutating capacitor and an inductance connected in parallel with the valve. The reason is that under the above-mentioned condition, the current inclination becomes so great. The relation among the oscillating frequency, inductance and capacitor of the DC circuit breaker will be described briefly. The breaking capacity of a circuit breaker (a measure of the capacity in terms of how large main current can be cut off) depends on the magnitude of the commutating current flowing from the commutating capacitor, charged in reverse, in the opposite direction from the main current. In other words, to eliminate the arc produced in the valve, it is required that the peak value of the commutating current is larger than the main current. The commutating current i can be given by the following equation. i = 2V 4L C - R 2 e -αt sin βt where α = R 2L , β = 1 LC - R 2L R: pure resistance component of the commutation circuit L: inductance component of the commutation circuit C: capacitance of the commutating capacitor If the commutation circuit is formed of a wire with a sufficient cross-sectional area, the resistance component can be regarded as substantially zero. Therefore, the equation (1) will be: i = V L C sin 1 LC t From the equation (2), the peak value of the commutating current is I p = V L C To increase the commutating current I p , it is only necessary to increase the charged voltage V or the capacitance C of the commutating capacitor or decrease the inductance L. The natural frequency ƒ o of the commutation circuit is given by the following equation. ƒ 0 = 1 2π LC Therefore, if the frequency f o and the maximum current I p are decided, it follows that the inductance L and the commutating capacitance C are decided by the following equations. C = I p 2πfV L = V 2πfI P Therefore, as a way of decreasing the commutating capacitance C without changing the commutating capacity, a possible method is to increase the charged voltage of the commutating capacitor. This method, however, has a problem that the inductance L becomes large. The use of a charged voltage V of the capacitor, which is extremely higher than the circuit voltage, is not advisable from the viewpoint of design of insulating resistance because this increases the size of the circuit and elements used. Fig. 2 shows the relation among the capacitance, the inductance L, and the peak value I p of the commutating current according to the equations shown above. For instance, supposing a frequency of 1 kHz and a commutating current of 10 kA, the capacitance of the capacitor is 1000 µF and the inductance is 20 µH. The size of the capacitor with this capacitance is generally considered to approximately measure 800 mm (width) x 500 mm (depth) x 500 mm (height). A capacitor of this size is too large to be mounted under the floor of electric rolling stock. To decrease the commutating capacitance C and the commutating inductance L, it is obvious from Fig. 2 that you have only to increase the frequency ƒ o of the commutation circuit. However, in JP-A-54-132776 mentioned above and also in "DC Circuit Breaker for Critical Plasma Test Equipment JT-60" of the Journal of the Institute of Electrical Engineers of Japan, 1978 June issue, Vol. 98, No. 6, page 44, it is stated that the limit of the frequency of the commutating current is about 1 kHz. The reason is that it is said that a large current decrease rate near the current zero point makes it difficult to break the current. Under the present conditions, it is impossible to decrease the commutating capacitance C and the commutating inductance L. The vacuum circuit breaker will now be described briefly. If the valve through which a current is flowing is opened in the air, the atoms existing between the electrodes are ionized. A flow of these atoms is an arc. On the other hand, since there is no atom between the electrodes in vacuum, it follows that in principle, an arc is not formed when the valve is opened in vacuum. A vacuum circuit breaker operates on this principle. And, this is the reason why the vacuum circuit breaker operates at high speed. However, it is very difficult to create a perfect state of vacuum. The fact is that when the valve is opened, an arc is produced by an arc voltage of several tens of volts partly because metallic atoms of the electrodes melted on opening the valve, making a current to continue to flow. To extinguish the arc, the commutation circuit is required. To return to what we discussed before, if the commutation frequency is increased, both the capacitance and the inductance can be decreased. This is considered impossible to realize because there is a limit to the maximum value of the commutation frequency. The inventors of this invention conducted experiment which will be described in the following. Fig. 11A shows a measuring circuit. A current from a DC power source 7 becomes an accidental current as it passes through variable loads 9a, 9b. Initially, a line breaker 16 is put in the OFF state and a vacuum valve 2 is put in the connected state. Main currents I L , I C and a voltage V VCB are the chief items which are to be measured. The experimental procedure progresses as follows. The line breaker 16, which has been turned OFF, is turned ON to cause an accidental current to be produced. If the measured value of a overcurrent detector 8a exceeds a set value, the control section issues a Trip command, so that a reaction coil 2b is excited, and the vacuum valve 2 is opened to cut off the current. Then, the line breaker 16 is turned OFF, and a Reset command is issued to cause the vacuum valve 2 to be closed. And, a subsequent experiment is performed. Since this experiment was conducted in the region where cutting off a current was said to be impossible, a Trip command was issued at times set inside the control section for the initial several sessions (Nos. 1 to 4 in the following table) without using the overcurrent detector 8a. Therefore, the set values to be described later are not shown. Referring to Fig. 11B, the terms will be explained. The solid line in Fig. 11B indicates the current I L , and the alternate long and short dash line indicates the current curve in a case where the accidental current was not cut off and made to flow continuously. The set values are the operating current values, which are the values measured by the overcurrent detector 8a and at which the circuit breaker is operated. The actual breaking current is the operating point of the circuit breaker. The breaking current denotes the breaking capacity of the circuit breaker. The following table shows the result of experiment in which the current was cut off successfully. It has been proved by this table that the current can be cut off at a supply voltage of 1600 V, a set value of 2080 A, and a commutating current of 11.1 kHz. The commutation frequency is about ten times greater than the value believed usable. By making the commutation frequency a high frequency, the commutating reactor can be done away with, and the only inductance component to be utilized is the stray inductance of the wires. Another advantage is that the commutating capacitor can be reduced to as small as 50 µF. The stray inductance of about 1 µH remains even if the wiring is shortened to the shortest possible length. Therefore, the greatest possible commutation frequency is about 30 to 40 kHz. Incidentally, the value of the commutating capacitor in this case is about 30 µH. With reference to Fig. 1, an embodiment of this invention will be described. The main current from the DC power source 7 passes through the vacuum valve 2a and the static overcurrent tripping device 8, 8a, and comes to the load 9. Connected between the poles of the vacuum valve 2a in parallel with the vacuum valve 2a is a series member including the commutating capacitor 4 and a commutating switch 6 as switching means of the valve. A zinc oxide nonlinear resistance 3 included in another loop, different from the circuit with the commutating capacitor 4, etc. is connected in parallel with the vacuum valve 2. The stray inductance of this closed circuit is smaller than that of a closed circuit including the commutating capacitor 4, etc. In other words, the closed circuit formed by the zinc oxide nonlinear resistance 3 and the vacuum valve 2a is shorter in wire length. Though not shown in the figure, a circuit for charging is connected across the commutating capacitor 4. When the static overcurrent tripping device 8 detects an abnormal current, the main repulsion coil 2b is excited to repel the short ring 2c away from the main repulsion coil 2b, so that the vacuum valve 2a is opened. With reference to Fig. 3, the operating principle of this invention will be described. Fig. 3A shows how the main current flows through the closed vacuum valve 2a. The commutating capacitor 4 is charged in the direction as indicated in the figure. When an Open command is issued according to the condition of failure, the vaccum valve 2a is opened as shown in Fig. 3B. After the valve is opened, the main current continues to flow in the form of an arc in vacuum. Then, an ON command is given to the commutating switch 6, which is thereby closed as shown in Fig. 3C. At this moment, a closed circuit is formed which runs through the commutating capacitor 4 → stray inductance 5 → commutating switch 6 → vacuum valve 2a → and back to the commutating capacitor 4, the electric charge stored in the commutating capacitor C starts to flow as an oscillating current in the opposite direction from the main current. When the current in the vacuum valve 2a nears zero (several amperes) in due time, the arc is extinguished. However, the moment the arc goes out, the post-arc current of the current that has existed heretofore (the residues of the current that has flowed as an arc through the vacuum valve 2a) disappears, and emerges as a peak voltage (dv/dt) which is applied across the vacuum valve 2a, thereby causing an arc to be struck again. In this embodiment, the wire length for the zinc oxide nonlinear resistance 3 connected in parallel with the vacuum valve 2a is made shorter than the wire length of the commutation circuit, and therefore, the inductance on the side of the zinc oxide nonlinear resistance 3 is small. Consequently, in contrast to a case where the inductance is large in relation to the varying current, it is easier for the current to flow to the side where there is the zinc oxide nonlinear resistance 3. The zinc oxide nonlinear resistance 3 has a capacitive component, and its magnitude is about 2000 times the capacitance of the vacuum valve 2a when the valve is opened. With this in mind, the phenomenon of restrike prevention of the vacuum valve 2a will be described. The moment the arc is extinguished, the post-arc current flows towards the capacitive component easiest to flow into. In this case, the post-arc current flows into the zinc oxide nonlinear reistance 3, so that a peak voltage is prevented from being applied to the vacuum valve 2a and a restrike of an arc can be prevented. In the above embodiment, the zinc oxide non-linear resistance was dealt with as a typical element. However, in place of this resistance, any other element can be used so long as it is an energy-consuming element with constant-voltage characteristics and some capacitive component. The peak value I p of the commutating current should preferably be more than 1.2 times the actual breaking current. The magnitude of the actual breaking current is determined by the output of the load, such as elastic rolling stock and the DC supply voltage. Given the electric rolling stock output of about 500 to 6000 kW and the DC supply voltage of about 600 to 3000 V, the I p of the commutating current should desirably be 5000 A or more. Thus, the arc is extinguished completely, and the main current charges the commutating capacitor 4 as shown in Fig. 3D. A constant voltage of the zinc oxide nonlinear resistance 3 is selected which is higher than the supply voltage E. When the voltage of the commutating capacitor 4 rises and the commutating switch 6 is opened as shown in Fig. 3E, the energy stored in the inductance of the main circuit is consumed. In this case, the zinc oxide nonlinear resistance 3 acts as a resistance. Referring to Fig. 5, the various waveforms when the circuit breaker trips will be described. In Fig. 5, the axis of abscissa indicates the elapse of time. Suppose that the main current increases due to an accident and exceeds the set value of overcurrent at point (a). The overcurrent is detected, an Open command is issued to the main pole, and the vacuum valve is opened at point (b). The main current continues to flow by arcing across the gap in vacuum. At point (c), the commutating switch 6 is closed, so that a commutating current starts to flow. Canceling each other with the commutating current, the current flowing through the vacuum valve 2a becomes zero in due time at point (d). The post-arc main current flows towards the zinc oxide nonlinear resistance 3, thereby preventing a rise of a peak voltage across the poles of the vacuum valve 2a. After this, the current flowing to the commutating capacitor 4 increases, and in due time, the discharge starting voltage of the zinc oxide nonlinear resistance 3 is reached at point (e). The current flows to the zinc oxide nonlinear resistance 3, which consumes the energy stored in the inductance of the main circuit, so that main current attenuates and a complete cutoff of the current is completed at point (f). With reference to Fig. 4, the arrangement of the circuit described above will be described. Fig. 4 is a diagram showing the layout of the interior of the box 10 mounted under the floor of electric rolling stock. The box 10 of the DC high-speed vacuum circuit breaker contains a vacuum valve 2a, an exciting coil 2b, a commutating capacitor 4, a commutating switch 6, a zinc oxide nonlinear resistance 3, and other elements. The wire length of the closed loop including the commutating capacitor 4 should be shortened insofar as feasible. As is clear from Fig. 4, this is difficult becuase the commutating capacitor 4 is so large. Therefore, the wire of the zinc oxide nonlinear reistance 3 has been shortened. Incidentally, the box 10 measures 550 mm (width) x 600 mm (depth) x 500 mm (height). The reason for the low height of 500 mm is that consideration was given to usability in electric rolling stock for underground railways. The experimental results and the embodiments will be summarized. According to the two known examples mentioned above, it was believed that current could not be cut off at a commutating current frequency of 1 kHz or more. This was because the breaking current inclination (di/dt) was so great that an arc was bound to be struck. By the experiment by the inventors of this invention, however, it has been clarified that current can be cut off at frequencies of 1 kHz or above. In compliance with the experimental result, no reactor is inserted in the commutation circuit in the above-mentioned embodiment. To be more specific, the inductance of the commutation circuit is only the stray inductance of the wire (the commutating reactor 5 is the stray inducatance). Supposing 5 µH for the inductance, the commutating capacitance 4 and the commutating current frequency are calculated. The commutating capacitance is expressed as: C = I p 2 L V 2 Supposing that the charged voltage is 1500 V and the maximum commutating current I p is 6000 A, the capacitance is calculated. C = 80 (µF) At this time, the commutation frequency ƒ is: ƒ 0 = 1 2π LC where ƒ ≒ 8 (kHz) In addition to the elimination of the commutating reactor 5 by reducing the commutating capacitor 4, this embodiment offers an advantage that since the frequency is high, the next zero point comes very quickly even if a cutoff of the current failed at the first zero point for some reason. With reference to Fig. 6, description will now be made of a case where a DC high-speed vacuum circuit breaker is used in electric rolling stock. Normally, the DC high-speed vacuum circuit breaker 1 is in closed state. Then, a pantagraph 15 is raised to contact an electric overhead line, and line breakers 16, 18 are closed. A filter capacitor 21 with large capacitance is charged through a charging resistance 19. After the capacitor has been charged, a line breaker 17 is closed, making the vacuum circuit breaker ready to be operated. When the engineer operates a master controller, not shown, a main motor controller puts a motor, not shown, into motion according to the manipulated variable. When the engineer does a notchoff during power running, the main motor controller (particularly when an inverter is used) reduces the main current, and then, opens the line breakers 16, 17 to cut off the current. This is called current reducing rupture. Description will then be made of operations when an accident occurs. An accident is detected in two cases. The first case is when the overcurrent detector 8a detects the main current exceeding the set value. The second case is when a failure is detected in a device or the like in the main motor controller and an external Trip command is issued. When any of these signals is input into the controller in the DC high-speed vacuum circuit breaker 1 (hereafter referred to as the controller), the controller sends a Trip command to the reaction coil 2b. By the reaction force, the vacuum valve 2a is opened, and the vacuum valve 2a is kept in that opened state by the locking mechanism. Then, about the time when the vacuum valve is opened to the position where the commutating current works effectively (operated by time sequence), the controller sends a Commutation command to the repulsion coil 6a to operate the commutating switch 6. As a result, the previously charged commutating capacitor 4 discharges the commutating current, so that a cutoff is completed as described above. When a cutoff is completed, the main current is zero, with the result that the controller sends an LB Off command, by which the line breakers 16, 17 are opened. When the vacuum circuit breaker is recovered from the accident, the engineer pressed the reset push-button on the engineer's stand, whereby the resetting operation is started. When a Reset command is input into the controller, the controller sends a Reset command to a resetting coil, so that the locking mechanism is released, then the vacuum valve 2a is closed. Then, a charging current is supplied to charge the commutating capacitor 4 to a predetermined value, and the DC high-speed circuit breaker 1 is placed in the standby state. According to this embodiment, it is possible to provide the electric rolling stock with a high-performance DC high-speed vacuum circuit breaker reduced in size particularly for mounting in the electric rolling stock, and by this means, it is possible to cut off an accidental current earlier than the circuit breaker at the ground substation. This precludes series effects that the accident would otherwise have on many other electric cars. With reference to Fig. 7, another embodiment of this invention will be described. Referring to Fig. 7, the differences from the circuit configuration of Fig. 1 are that the zinc oxide nonlinear resistance 3 is connected in parallel side by side with the commutation circuit (including the commutating capacitor 4 and the commutating switch 6), and that a surge-absorbing capacitor 30 is connected in parallel close by with a vacuum valve 2a (so that the wire length of the latter branch is shorter than the closed loop of the commutation circuit). In this circuit, as the commutating current flows into the vacuum valve 2a and the arc is extinguished, the greater part of the post-arc current flows to the surge-absorbing capacitor 30, thus suppressing the voltage rise rate, so that a reignition of arc is prevented. It is necessary to select a larger capacitance for the capacitor 30 than the capacitance of the vacuum valve which is opened. The importance thing is that you never select too large a capacitance. This is because a capacitor with large capacitance is too large in size to be mounted on the electric rolling stock. The effect of this embodiment is that you can select the capacitance of the surge-absorbing capacitor 30 according to the purpose of use. For instance, if the zinc oxide nonlinear resistance 3 is large, the stray inductance cannot be sufficiently small. In this case, it is only necessary to select a small surge-absorbing capacitor 30. Referring to Fig. 8, a still another embodiment will be described. The only difference from the circuit configuration of Fig. 7 is that a resistance 31 is connected in parallel with the zinc oxide nonlinear resistance 3. When the vacuum valve 2a is opened and the zinc oxide nonlinear resistance 3 is put into operation, if the energy stored in the stray inductance 5 from the DC power source 7 is large, the resistance 31 participates in the consumption of the energy. This reduces the burden on the zinc oxide nonlinear resistance 3. In this case, however, the main current does not disappear completely, but continues to flow from the DC powder source 7 to resistance 31 to the load 9 in that order, so that it will be necessary to provide a switch to cut off a low current. Referring to Fig. 9, a further embodiment will be described. The difference from the circuit configuration of Fig. 8 is that the zinc oxide non-linear resistance 3 has been done away with. Only the resistance 31 consumes the energy stored in the stray resistance. The reistance 31 does not have constant-voltage characteristics like the zinc oxide nonlinear resistance 3 does, so that the current keeps flowing. Also in this case, it will be necessary to provide another breaker. According to this embodiment, the circuit configuration is so simple and the price is less expensive. Therefore, this embodiment is suitable for cutting off a relatively small current. With reference to Fig. 10, yet another embodiment will be described. The difference from the circuit configuration of Fig. 7 is that the zinc oxide nonlinear resistance 3 is connected in parallel with the surge-absorbing capacitor 30. This embodiment is effective in a case where the zinc oxide nonlinear reistance 3 is not enough to meet the required magnitude of capacitance, leaving a possibility that an arc is struck again. With reference to Figs. 5, 12, 13, 14, and 15, another embodiment will be described. Fig. 5(c) indicates the timing at which the commutating switch is closed, (d) indicates the timing at which the voltage of the vacuum valve becomes zero, (e) indicates the timing at which the zinc oxide nonlinear resistance starts discharging, and (f) indicates the timing at which the main current attenuates completely. V 1 indicates the voltage applied across the vacuum valve just after the vacuum valve recovers its dielectric strength (substantially equal to the commutating capacitor voltage at this time). V 2 indicates the discharge starting voltage of the zinc oxide nonlinear resistance, and V 3 indicates the supply voltage. In the vacuum valve, the arc diffuses very quickly, and the moment the current attenuates to zero, the dielectric strength recovers, so that the current is cut off. However, if the valve current change rate (di/dt) is too large, when the current becomes zero, it sometimes happens that an arc is struck again and the current flows again in the reverse direction (cutoff failure). The reason is considered as follows. In principle, the moment the current in the vacuum valve attenuates to zero, the dielectric strnegth of the valve should recover and from this moment onwards, the valve current should be held at zero. However, the fact is that while the interpole voltage of the valve is zero, the main circuit current on which the oscillating current is superimposed flows. Incidentally, in order to reduce the size of the equipment so as to be mounted on the electric rolling stock, it is necessary to reduce the values of the commutating capacitor and the commutating reactor as mentioned above, decrease other constants, and raise the frequency of the oscillating current with fixed peak values. In consequence, the current change rate becomes large at the time when the vacuum valve current attenuates to zero, thus offering a possibility that an arc is struck again. A prior-art example of a solution to this problem is disclosed in JP-A-59-163722 which suggests that a resistance is connected in series with the commutating capacitor. In this technique, however, part of the commutating energy is consumed by the resistance, the peak value of the commutating current decreases, and the maximum breaking current becomes small. A solution according to the present embodiment is to insert a saturable reactor 32 in series with the vacuum valve 21 in the closed commutating circuit as shown in Fig. 12. The saturable reactor 32 has a characteristic shown in Fig. 13 that ideally, its inductance is very large when the current flowing through the valve is small and as the current increases more than a certain value, the inductance decreases rapidly. Fig. 14 shows the waveforms when the current is cut off in this embodiment. Generally speaking, the waveforms are almost the same as before, but it is obvious from Fig. 14 that the current change rate decreases notably at timing d, or just before the current zero point of the vacuum valve. This is a phenomenon that occurs because the current is prevented from changing as the inductance increases very large when the current decreases as it comes close to a zero point. According to this embodiment, without changing the constant values of the resonance circuit, by increasing the frequency of the oscillating current and securing high peak values of the current, it is possible to decrease the current change rate near current zero points and ensure a recovery of the dielectric strength of the vauum valve. With reference to Fig. 15, an additional embodiment will be described. In the preceding embodiment, the saturable reactor 32 was inserted to reduce the current change rate in the vicinity of a current zero point of the vacuum valve 2a. Besides the current change rate, another factor which impedes the recovery of the dielectric strength of the vacuum valve is the voltage change rate in the process of recovery of the dielectric strength. A high voltage change rate induces dielectric breakdown in the process of recovery of the dielectric strnegth, so that an arc is struck again and the current flows again. In order to prevent this phenomenon, by connecting capacitor 33 between the poles of the vacuum valve 2a and in parallel with the vacuum valve 2a, a sharp change in the voltage can be suppressed. According to this embodiment, it is possible to reduce a possibility of restrike of the switching circuit using a vacuum valve. Description will be made of how the vacuum circuit breaker described above is mounted in the electric rolling stock. As described in Japanese Utility Model Application Laid-open No. 61-65640, the conventional line breaker is so constructed as to be mounted on the electric rolling stock with the whole line breaker box insulated by double insulators. Description will be made briefly using Fig. 16. The main circuit current flows through the overhead wire 14 and current collector 15 into the line breaker box. Arranged in series in the line breaker box are the line breakers 16, 17, and a high-speed circuit breaker 40. The main circuit current that has passed through the line breaker box 41 flows through a filter reactor 20 to the control equipment box 42. (Those parts are mounted to the car body 50 with fixtures 45.) The main motor 43 is driven by a control current from the control equipment box 42. Then, the current flows through a car body 50 and wheels 23 to rails 24 and returns to the substation, not shown. Incidentally, the line breakers 16, 17 and the high-speed circuit breaker 40 (different from the above-mentioned DC high-speed circuit breaker 1) are all air circuit breakers, and therefore, an arc is struck when the current is cut off. When the arc is caught on the line breaker box 41, because this box is insulated by the insulators 44 from the car body, a ground fault does not occur. Nor is the circuit breaker at the substation tripped. Under the above arrangement, if a DC high-speed vacuum circuit breaker 1 shown in Fig. 6 is used to replace the high-speed circuit breaker 40, the following problem will result. The DC high-speed vacuum circuit breaker 1 does not emits an arc to the outside. However, the line breakers 16, 17, which are air circuit breakers, give off an arc and the arc flies to the parts at low potential. Meanwhile, in a DC high-speed vacuum circuit breaker 1 shown in Fig. 6, there is provided a controller 12 to send various commands. This controller 12 is connected through a terminal, not shown, to a main motor controller (to a gate controller in the case of the inverter electric rolling stock and the chopper electric rolling stock, or a notch step advance controller and an engineer's stand in the case of the camshaft rolling stock). For the control power source connected to the above terminal, 100 VDC, 24 VDC or 15 VDC, for lower than the main circuit voltage, is used. When the current is cut off by opening the line breakers 16, 17, part of the arc produced flows into the line breaker box 41, and flies to a moving contact spring, terminals, and a fixed contact carrier, not shown, which are exposed. This is because the control voltage is very low compared with the main circuit voltage. Though a cover is applied over the parts which would be subject to arcing, the ionized air can easily enter. When a potential difference becomes large between the cover and the line breaker box 41, the cover's insulation is broken, so that the arc is grounded to the exposed parts. The control power line connected to the terminal are bundled together with other control lines and connected to the control equipment box. Such being the situation, when the arc current flows, a large induced current flows not merely in the equipment connected with the control power line but also in the other control lines, resulting in the destruction of the devices connected to these lines, such as the master controller and the inverter control device. Incidentally, the insulation of the power lines can be reinforced so as to prevent these lines from being contacted by a high voltage of the arc when the arc flows to the box. However, it is difficult to reinforce the insulation of the exposed parts. An embodiment for solving this problem will be described with reference to Figs. 17, 18, and 19. In Fig. 17, the circuit configuration different from Fig. 16 is that while the line breakers 16, 17 are contained in the line breaker box 41, a circuit breaker box 51 added to contain the DC high-speed vacuum circuit breaker 1. The circuit breaker box 51 for the DC high-speed vacuum circuit breaker 1 is directly attached to the car body 50 with fixtures 45. This is because in principle, the DC high-speed vacuum circuit breaker 1 does not give off an arc and it is not necessary to separate it from the car body 50. In this case, the line breakers 16, 17 should preferably be made in double insulation construction, but the circuit breaker 51 need not. According to this embodiment, there is no arcing to the circuit breaker box 51, so that it is not necessary to reinforce the insulation of the various control lines. Referring to Fig. 18, a still another embodiment will be described. As described earlier, an external Trip command to the DC high-speed vacuum circuit breaker, or the like, is given by motor control devices, such as an inverter controller. Referring to Fig. 18, the DC high-speed vacuum circuit breaker 1 is accommodated in the inverter control device box 22 and forms an integral body with the box 22. In this case, the wire length of the Trip-command line from the inverter control device is short, so that the interfacing can be done easily. According to this embodiment, there is very little possiblity that the controller of the DC high-speed vacuum circuit breaker 1 malfunctions due to inductive interference. Under the above arrangement, however, the filter reactor 20 is not protected completely, which is one of the objects to be protected on that side of the DC high-speed vacuum circuit breaker 1 closer to the overhead wire 4. This is because the filter reactor 20 is located closer to the power source than the DC high-speed vacuum circuit breaker 1. There has been no ground fault of the reactor at all. However, it is necessary to protect the reactor against emergency which hardly occurs. An embodiment which has solved this problem is shown in Fig. 19. The filter reactor 20 is located on the load side of the DC high-speed vacuum circuit breaker 1, and accommodated integrally in the control box 53 including the inverter control device. By this construction, not only the external appearance is made neat but the protected range is increased.

Claims: A DC vacuum circuit breaker for mounting on an electric motor vehicle, comprising a vacuum valve (2) included in the DC circuit to interrupt the same upon detection of a current fault, a series combination of a capacitor (4) and switching means (6) and connected in parallel with said vacuum valve (2), control means (12) for charging said capacitor (4) with a voltage opposite to the voltage across said vacuum valve (2), and resistance means (3) connected in parallel with said vacuum valve (2) for consuming energy stored in the inductance of the circuit in which direct current flows, characterised in that the oscillation frequency f C , the oscillation current amplitude I CP and the inductance L C of the closed circuit constituted by said vacuum valve (2), said capacitor (4) and said switching means (6) simultaneously satisfy the following conditions: f C ≥ 2 kHz, I CP ≥ 5 kA, L C ≥ µH, and that said control means (12) closes said switching means (6) after opening said vacuum valve (2). The circuit breaker of claim 1, further including a capacitive element (30) connected in parallel with said vacuum valve (2). The circuit breaker of claim 1 or 2, wherein the loop length of the closed circuit constituted by said vacuum valve (2) and said resistance means (3) is shorter than the loop length of the closed circuit constituted by said vacuum valve (2), said capacitor (4) and said switching means (6). The circuit breaker of any preceding claim, wherein said resistance means (3) includes a non-linear resistance element. The circuit breaker of claim 4, including a resistance element (31) connected in parallel with said non-linear resistance element (3).

Inventors: AKIYAMA HIROYUKI, HASEGAWA MITSUYOSHI, KAMADA TADASHI, TSUBOI TAKASHI, UCHII TARO, AKIYAMA, HIROYUKI, HASEGAWA, MITSUYOSHI, KAMADA, TADASHI, TSUBOI, TAKASHI, UCHII, TARO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1990
Main classification: H01H33
IPC codes: H01H33