Protection of an electrical power distribution system

Abstract: An electrical power system protection device has a main circuit including a circuit breaker 1 and a switch 2 connected in series to the circuit breaker 1, the main circuit being connected to a power line 115. The protection device also has a current limiting element 4 connected in parallel to the circuit breaker 1. When the level of electrical current flowing in the power line 115, detected by a current detector 7, has exceeded a predetermined first set level, a first over-current detector 8 delivers immediately a trip command to the circuit breaker. AS a consequence, any fault current such as a short-circuit current is commutated to the current limiting element 4 so as to be limited, whereby any drop of voltage in the power line upstream of the circuit breaker is greatly suppressed. When a state in which a predetermined second set level is exceeded by the detected current level is continued beyond a predetermined period, a second over-current detector 9 delivers an off command to the switch 2, thus protecting upstream portion of the power line against the fault in this power line 115. Preferably, the current level after the limitation is set to fall within a range of current which activates a momentary element of an over-current relay (OCR) 127 of a receiving end breaker 122 of a downstream sub-system, e.g., a consumer's sub-system 120, connected to the power line 115. In the event of a fail;ure in the downstream power line 120, the receiving end breaker 115 is tripped without delay so that the sound portion of the power line is reset to normal state of operation, thus achieving a harmonized protection of the electrical power system.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for protecting an electrical power distribution system, and to an electrical power distribution system in which, in the event of a failure in a part of electrical lines of an electrical power network or system, a suitable measure is taken for ensuring safe operation of other sound lines. 2. Description of the Related Art In general, an electrical power network is constructed to include a plurality of power transmission lines or a plurality of power distributing lines. In the event of a failure such as a short-circuit fault in one of the power lines of the network, the line in which the failure has occurred is disconnected from sound power lines which are on the "upstream" side of the failed line by tripping the circuit breaker of the receiving end of the failed line. In the case of a power distribution system, high voltage such as 77 KV is transformed into a lower distribution voltage, e.g., 6.6 KV, through one or more transformers which are provided in a distributing substation, and the power of the thus reduced voltage is distributed from a substation bus to a plurality of feeders branching off the substation bus. The power is then supplied to consumers through the feeders. In this type of distribution system, when a failure such as short-circuit fault has occurred in the load side of one of the feeders, the circuit breaker provided on the branching end of the feeder adjacent to the substation bus (referred to as "feeder circuit breaker") is made to trip so as to disconnect this feeder, while ensuring supply of electrical power to other feeders which are sound, thus protecting the power distribution system. A protecting method called "harmonic protection" also has been adopted in which circuit breakers in the failed feeder which is most downstream and which is closest to the point of failure is selectively and preferentially opened to enable supply of power to an area as wide as possible. For instance, the timing of tripping of the feeder circuit breaker is delayed a predetermined time, e.g., 0.2 second, after tripping of the circuit breaker of the receiving end of a consumer in which the fault has occurred. In the conventional protecting system as described, when the point at which the fault has occurred is close to the feeder circuit breaker, a large short-circuit current may flow through this feeder during the period of the delay mentioned above. As a consequence, the voltage of the power supplied to other feeders which are sound and which are connected to the same main transformer as the failed feeder may drop. Such a drop of the voltage has not been recognized as a critical problem. In recent years, however, the influence of such voltage drop is often not negligible because the short-circuit current is very large in a current power network having a large capacity. In particular, in the modern information-oriented society in which abundant electronic information devices such as computers are used, a large voltage drop causes a serious situation such as erasure of the information stored in terminal devices of information systems, on-line control systems and computers, even when the period of such voltage drop is very short. On the other hand, an increase in the level of short-circuit current attributable to an increase in the capacity of electrical power network requires that the mechanical durability of equipments in the network, as well as the capacities of the circuit breakers in the network, be increased. In recent years, therefore, a current limiting device has been proposed which suppresses the increase in the level of short-circuit current thereby reducing the breaking capacities required for the circuit breakers, as, for example, in Report SPD-87-43 of The Institute of Electrical Engineers. of Japan, October 27, 1987. In this current limiting device, a current-limit fuse and a current limiting element are connected in parallel with a switching element which has a predetermined breaking capacity. In the event of a short-circuit fault, the switch element is opened first so that short-circuit current flows first through the current limiting fuse. After a while, the current limiting fuse is fused so that the short-circuit current is made to flow through the current limiting element, whereby the level of the short-circuit current is limited. This current limiting device limits the level of the short-circuit current so as to diminish the reduction of voltage in the sound portion of the electrical power network in the event of a fault of the kind described. The art disclosed in the Report mentioned above, however, suffers from the following problem. Namely, the above-mentioned current limiting device, when used as a feeder circuit breaker, is not designed for performing the aforesaid harmonic protection function, i.e., selective and preferential tripping of circuit breaker which is downstream from the feeder and which is close to the point where the fault has occurred. Therefore, when the short-circuit current is limited excessively, the momentary element of an over-current relay which is to trip the circuit breaker closest to the point where the fault has occurred does not operate so that the time required for separation of the fault point is prolonged undesirably. GB-A-2 090 488 describes a method and a system for protecting an electric power distribution system which can readily carry out a current limitation and a selective interruption of a power distributing system. This prior art protection system includes a current limiting circuit which is provided between a main interrupter and a common bus connecting the main power line to feeders of branch systems, wherein each feeder includes a further branch interrupter. The system operates in the following manner: when a short-circuit occurs e.g. in the feeder with the branch interrupter, a permanent fuse is changed to very high resistance. Since the short-circuit current flows through this permanent fuse and a parallel resistor, a current limitation is achieved. However, this current limitation performance affects the voltage of the other feeders connected to the common bus. Therefore, undesired effects in the power subsystems connected to these feeders can occur. Further, US-A-4 354 215 discloses an electric power distribution system having successive power levels with branches, in which each branch includes a current limiting feeder circuit breaking device including a switch and a circuit breaker connected in series, a current limiting element connected in parallel to said circuit breaker and a current detector. Said current detector delivers a trip command to said circuit breaker when the current level detected exceeds a set level, and delivers an off-command to said switch when the state that the current level exceeds the set level has continued beyond a predetermined time. However, since in all power levels the same kind of current limiting feeder circuit breaking device, i.e. in particular a current limiting element is included, a short current occurring in a power branch causes a voltage drop in the other branches by their current limiting elements. Further, the set level of current is fixed and therefore not adaptable to the particular distribution system protected. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method and a system for protecting an electric power distribution system which, in the event of a failure in one of the power lines of the electric power distribution system, effectively suppresses a reduction of voltage in the power lines upstream of the point where the fault has occurred and which can perform harmonic protection of the electric power distribution system, thereby overcoming the above-described problems of the prior art. This object is met by a method according to claim 1 and a system according to claim 4. Preferred embodiments are disclosed in the depending claims. When a fault current is detected in the power line, the current in the power line is commutated from the normal circuit of the power line to a current limiting circuit, thereby limiting the current to a level which is higher than the minimum level of the operation current required for operating a momentary element of an over-current relay which activates a protection equipment of a downstream subsystem connected to the power line. The power line is disconnected if the fault is not eliminated within a predetermined period from the commutation, whereas, when the fault is eliminated within the predetermined period, the current of the power line is switched from the current limiting circuit back to the main circuit. The electrical power distribution system, comprises in each feeder a main circuit including a switch and a circuit breaker connected in series to the switch, the main circuit being connected in series to the feeder; a current limiting element connected in parallel with the circuit breaker; a current detector for detecting electrical current flowing in the feeder; a first over-current detector for delivering to the circuit breaker a trip command when the level of the electrical current detected by the current detector has exceeded a first set level; and a second over-current detector for delivering an off command to the switch when a state in which the level of electrical current exceeds a second set level has continued beyond a predetermined period. The system may be so constructed that a close command is given to the circuit breaker when the detected current level is lowered below the second set level within the predetermined period. In order to minimize the period of voltage drop to, for example, 1/4 cycle time or shorter, the breaker is preferably made of a semiconductor switch such as a GTO thyristor. In operation of the system having the described construction, when an over-current due to a fault such as , for example, a short-circuit is detected, the circuit breaker is interrupted, whereby the fault current is commutated to the current limiting circuit or the current limiting element which is connected in parallel to the circuit breaker. As a consequence, the fault current is significantly limited by the current limiting element, whereby drop of voltage of the power lines upstream of the tripped circuit breaker is significantly limited. Thus, drop of voltage supplied to other power lines connected to the same bus or transformer as the failed power line is remarkably suppressed to realize a highly reliable supply of the electrical power. The level of the limited electrical current falls within the range of current for operating the momentary element of an over-current relay (OCR) provided in the receiving end circuit breaker in a sub-system downstream of the failed power line, e.g., a consumer's sub-system. Therefore, if the fault has taken place in the downstream sub-system,the receiving end circuit breaker of such a sub-system is interrupted in accordance with the performance curve of the OCR, whereby the sound portion of the power line is reset to the normal state of operation,thus meeting the requirement for the harmonic protection of the power system. In the event that the fault has taken place in the power line having the protection device of the invention,the switch is opened in accordance with the timing characteristic of the second over-current relay. The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments when the same is read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of an embodiment of a device used for protecting an electrical power system according to the invention; Fig. 2 is a block diagram of a first over-current detector incorporated in the embodiment shown in Fig. 1; Fig. 3 is a block diagram of a second over-current detector incorporated in the embodiment shown in Fig. 1; Fig. 4 is a graph showing operation characteristics of an over-current relay (OCR) for explaining operation of the embodiment; Fig. 5 is a diagram of a power distributing system of the present invention; Fig. 6 is a time chart illustrative of the manner in which the level of a short-circuit current varies in the embodiment shown in Fig. 1, in comparison with that in conventional device; and Figs. 7 and 8 are illustrations of a circuit breaker in accordance with the present invention and a modification of the circuit breaker, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described with reference to the accompanying drawings. Referring to Fig. 1 which is a block diagram of a protecting device for protecting an electrical power system according to the invention, a main normal circuit has a series circuit including a circuit breaker 1 and a switch 2 connected in series thereto. A current limiting impedance 4 is connected in parallel with the circuit breaker 1 so that a current limiting circuit is formed. In the illustrated embodiment, a GTO thyristor is used as the breaker 1. In order to protect the GTO thyristor from over-voltage, a voltage limiting element 5 is connected in parallel with the GTO thyristor. The voltage limiting element 5 has such a characteristic as to drastically reduce its resistance when the voltage is increased to or above a predetermined set voltage. The load current I flowing through the main normal circuit or the current limiting circuit is detected by a current detector 7. The output from the current detector 7 is delivered to first and second over-current detectors 8 and 9. The first over-current detector 8 is adapted for delivering a trip command to the circuit breaker 1 when the load current I has exceeded a first set level I 1 . The construction of the over-current detector 8 is shown in Fig. 2. As will be seen from this Figure, the first over-current detector 8 has a current monitor section 8a which receives the output from the current detector 7 and conducts a pre-processing such as filter processing, an over-current setting device 8b for variably setting the first set level I 1 , and a comparator 8c which produces a signal when the set level I 1 is exceeded by the output of the current monitor section 8a. The second over-current detector 9 is adapted for delivering an off command to the series switch 2 when a state in which the load current I exceeds a second set level I 2 is maintained for a predetermined period. The construction of the second over-current detector 9 is shown in Fig. 3. As will be seen from this Figure, the second over-current detector 9 includes a current monitor section 9a for receiving the output from the current detector 7 and conducting a pre-processing such as filtering, an over-current setting device 9b for variably setting the second set level I 2 , a comparator 9c for outputting a signal when the output of the current monitor section 9a has exceeded the level I 2 set by the over-current setting device 9b, a fault time monitor section 9d for measuring the duration of the signal from the comparator 9c, a fault time setting device 9e for variably setting a set fault time T 1 , and a comparator 9f for producing a signal when the output from the fault time monitor section 9d has exceeded the set value T 1 of the fault time. The second over-current detector 9 is provided with a resetting function for delivering an on command to the circuit breaker 1 when the load current I has come down below the second set level I 2 within the above-mentioned set time T 1 , although such a function is not illustrated. This function, however, may be provided in the first over-current detector 8. The second over-current detector 9 may be formed by, for example, an induction disk-type detector having a characteristic shown by a curve 8P in Fig. 4. The embodiment having the described construction is used as a feeder circuit breaker of a power distributing system shown in Fig. 5. This power distributing system, as will be seen from Fig. 5, has a transformer bank 111 which is connected to an upstream 77 kV power system through a primary circuit breaker 110, a bus 113 connected to the transformer bank 111 through a secondary circuit breaker 112, a plurality of feeders 115A to 115D branching from the bus 113, and consumer's sub-systems 120 connected to the respective feeders 115. The feeders 115A to 115D respectively have feeder circuit breakers 100A to 100D and section switches 116. Each sub-system 120 includes a section switch 121, a receiving end circuit breaker 122, a transformer 123, a secondary circuit breaker 124, a bus 125 and circuit breakers 126. The receiving end circuit breaker 122 is adapted to be tripped by an over-current relay 127 for detecting a fault occurring in the sub-system 120. The operation of the protecting device of Fig. 1, applied to the power distributing system shown in Fig. 5, will be described with specific reference to Fig. 4 which shows the operation characteristic of an over-current relay(OCR) and Fig. 4 which is a time chart showing the manner in which the load current varies. It is assumed here that a fault has occurred in one of the consumers sub-systems 120 belonging to one 115A of the feeders. In such a case, a large short-circuit current I s , e.g., 12.5 kA, which is determined by factors such as the capacity of the transformer bank 111, is caused to flow into the feeder 115A through the series switch 2 and the circuit breaker 1. This short-circuit current I s activates the over-current detector 8, so that the circuit breaker 1 is made to interrupt immediately, e.g., within half cycle period in the case of the GTO thyristor, so that the short-circuit current Is is commutated to flow through the line having the current limiting impedance 4. The level of the current flowing through this line is varied by the value of the impedance 4. The impedance value is determined, taking into account also the capacity of the main transformer, to the greatest value which does not cause the voltage supplied to other sound feeders to be lowered to an unacceptable level. In the embodiment shown in Fig. 4, the impedance is determined to be,for example, 2 K Ohms. The impedance value of the limiting impedance 4 also is determined such that the current is not lowered below the minimum value of the operation current range for the momentary element of the over-current relay 127 whose operation characteristic is shown by a curve 127P in Fig. 4, in order to ensure that the consumers receiving end circuit breaker 122 is tripped preferentially. In the embodiment shown in Fig. 4, this minimum value of the operation current range for the momentary element of the over-current relay 127 is 400 A. It is assumed here that the time limiting characteristic of the second over-current detector 9 is set in accordance with the characteristic curve 8P shown in Fig. 4. In such a case, the series switch 2 is opened after elapse of a predetermined time in accordance with the time limiting characteristic of the over-current detector 9, whereby the feeder 115A having the fault is disconnected. When the momentary element of the consumer's receiving end over-current relay (OCR) operates before the elapse of the above-mentioned time, the receiving end circuit breaker 122 trips to extinguish the short-circuit current I s , so that the circuit breaker 1 is reset to allow the power supply to be continued to the sound consumers sub-systems other than the failed sub-system. As a consequence, the short-circuit current I s varies in a manner shown by the solid-line curve in Fig. 6, which should be contrasted to the manner of change of the short-circuit current in a conventional system shown by the broken line in the same Figure. As will be seen from this Figure, in the conventional system, a large short-circuit current flows for several cycles between the moment t 1 at which the fault has occurred and the moment t 2 at which the receiving end over-current relay (OCR) operates, whereas, in the system employing the protecting device of the invention, the current limiting function is put into effect within the period of as half cycle of the current,thereby remarkably limiting the level of the short-circuit current. As will be understood from the foregoing description, the described embodiment offers the following advantages. First of all, it is to be pointed out that the drop of voltage applied to the feeders 115 connected to the same transformer bank 111 is remarkably decreased. In particular, a very short tripping time is attained by virtue of use of a GTO thyristor as the circuit breaker 1. In fact, it is possible to limit the fault current within a very short time of 1 ms or so. It is therefore possible to remarkably shorten the duration of the voltage drop in the power-distributing substation, whereby undesirable effects on sound consumer's substations can be greatly suppressed. In contrast,when a known circuit breaker is used, a large short-circuit current of 12.5 kA or so flows over the period of several cycles as shown in Fig. 6, causing a risk that the drop of the voltage supplied to other feeders 115B to 115D connected to the same transformer 111 is continued for a considerably long time. Another advantage offered by the described embodiment is that thermal and mechanical stresses in the conductors and load equipments can be reduced. More specifically, in the power distributing system having the conventional protection device, the conductors and the load equipments are required to undergo a stress produced by a large short-circuit current of 12.5 KA for a period of 0.2 second, whereas, in the system having the protection device of the present invention, the conductors and equipments are required only to withstand a stress corresponding to 2 KA for the same period. Furthermore, damage produced by an arc generated in the feeder having the fault is diminished. The level of the current after the limitation falls within the range which enables operation of the momentary element of the receiving end over-current relay (OCR) provided in the consumer's receiving end circuit breaker 122 which is downstream from the failed feeder 115A. Therefore, the receiving end circuit breaker 122 is tripped without delay when the accident has occurred in this consumer's sub-system. On the other hand, the feeder which is sound is reset to normal state of operation, thus satisfying the requirement for the harmonic protection of the power system. In the event that the fault has occurred in the feeder 115A itself, the switch 2 is opened in accordance with the time-limit characteristic of the second over-current detector 9, while other sound feeders are reset to the normal state of operation. According to the invention, it is possible to reduce the breaking capacity of the receiving end circuit breaker 122, by virtue of the above-described current limiting function. More specifically, the breaking capacity of the receiving end circuit breaker 122 is as small as 2 KA in the system incorporating the described embodiment, whereas, in the system having the conventional protection device, the receiving end circuit breaker 122 is required to have a breaking capacity which is as large as 12.5 kA. Although the described embodiment employs a GTO thyristor as the circuit breaker 1, this is only illustrative and the circuit breaker 1 may be constituted by other types of circuit breakers such as, for example, semiconductor switches incorporating a thyristor switch having a commutating function or an electrostatic induction thyristor, as well as ordinary circuit breakers and vacuum circuit breakers. It is also possible to use a switch made from a superconducting material as the breaker 1 in the protecting device of the present invention. In the use of the device shown in Fig. 1, it is preferred that the series switch 2 is closed first, followed by closing of the circuit breaker 1. This manner of operation effectively prevents any rush current from flowing through the series switch 2 because, when the series switch is closed, the current limiting impedance already has been put into the circuit, whereby application of an over-voltage to the circuit breaker 1 is interrupted. A higher reliability is obtained when a semiconductor switch is used as the circuit breaker 1. A practical example of the circuit breaker 1 shown in Fig. 1, employing a GTO thyristor, will be described in reference to Fig. 7. The circuit breaker 1 has a main breaking portion composed of inverse parallel circuits of GTO thyristors, employing a GTO 10 and a GTO 20. In general, a GTO thyristor has a small backward dielectric strength. Therefore, series diodes 11, 21 and inverse parallel diodes 12, 22 are connected to each GTO. In addition, a snubber circuit, which has a capacitor 13, a resistor 14 connected in series to the capacitor 13 and a diode 15 connected in parallel with the resistor 14, is connected to the GTO thyristor 10. Similarly, a snubber circuit, which has a capacitor 23, a resistor 24 connected in series to the capacitor 23 and a diode 25 connected in parallel with the resistor 24, is connected to the GTO thyristor 20. In order to prevent over-voltages from being established across the diodes 11 and 21, snubber circuits which respectively include a series connection of a capacitor 16 and a resistor 17 and a series connection of a capacitor 26 and a resistor 27 are connected in parallel with the diodes 11 and 21. An example of the practical construction of the circuit breaker 1 has been described. The portion of this circuit breaker 1 shown in the area demarcated by broken line 30 is installed in a hermetic tank as a GTO stack container and is immersed in an incombustible cooling liquid for self-cooling, in order to prevent overheating of the semiconductor elements which may otherwise be caused by high temperature established in the circuit breaker during conduction of the load current, thereby ensuring safe functioning of the semiconductor elements. In order to protect the main breaking portion against any over-voltage, a voltage limiting device 5 and a current limiting impedance 4 of a low resistance are connected in parallel with the main breaking portion, as explained before in connection with Fig. 1. A control circuit system associated with the circuit breaker 1 includes a gate drive circuit 31 for controlling the turn-on and turn-off operations of the GTO thyristors,and an over-current detection circuit 32 including the current detector 7 for detecting the level of the load current and adapted for detecting over-current while transmitting a signal for the gate drive. This over-current detection circuit 32 has functions of both the first over-current detector 8 and the second over-current detector 9 which were described before in connection with Fig. 1. Fig. 8 shows another example of the construction of the circuit breaker 1. In this modification, diodes 41, 42, 43 and 44 are connected to form a bridge and a GTO thyristor 45 is connected between the junction of the diodes 41 and 42 and the junction of the diodes 43 and 44. This embodiment employs only one GTO thyristor which is rather expensive, so that the cost is reduced as compared with the circuit shown in Fig. 7. Although the invention has been described through its preferred forms, it is to be understood that the described embodiments are only illustrative and various changes and modifications may be imparted thereto without departing from the scope of the present invention which is limited solely by the appended claims.

Claims: A method for protecting an electric power distribution system including a plurality of power subsystems (120), each comprising a circuit breaking device (122, 127) which trips when the current in the corresponding power subsystem exceeds a minimum level, a plurality of feeders (115A, ..., 115D), each connecting one of said power subsystems (120) to a common bus (113) and comprising a current limiting feeder circuit breaking device (100A, ...100D), and a transformer (111) whose secondary is connected to said plurality of power subsystems via a circuit breaker (112), said common bus (113) and said feeders (115A, ..., 115D), said method including the steps of detecting the current flowing in each feeder (115A, ..., 115D), delivering a trip command to the current limiting feeder circuit breaking device (100A, ..., 100D) included in a feeder (115A, ..., 115D) when the level of the current detected in the corresponding feeder has exceeded a first set level, and thereby limit the current in the corresponding feeder to a level higher than the minimum level of the current required for operation of the current breaking device (122, 127) of the corresponding power subsystem, and disconnecting the feeder from said common bus (113), when a state in which the current level exceeds a second set level has continued beyond a predetermined period, wherein said first and second set levels are set independently from each other. The method according to claim 1, wherein a close command is delivered to said current limiting feeder circuit breaking device (100A, ..., 100D) to cancel the current limiting operation when the current level has come down below said second set level within said predetermined period. The method according to claim 1 or 2, wherein said predetermined period is variably set. An electric power distribution system including a plurality of power subsystems (120), each comprising a circuit breaking device (122, 127) having an element which operates to trip the circuit breaking device when the current in the corresponding power subsystem exceeds a minimum level, a plurality of feeders (115A, ..., 115D), each connecting one of said power subsystems (120) to a common bus (113) and comprising a current limiting feeder circuit breaking device (100A, ..., 100D), and a transformer (111) whose secondary is connected to said plurality of power subsystems (120) via a circuit breaker (112), said common bus (113) and said feeders (115A,..., 115D), each current limiting feeder circuit breaking device (100A,... 100D) including a main circuit connected in series to the feeder, wherein said main circuit includes a switch (2) and a circuit breaker (1) connected in series to said switch, a current limiting element (4) connected in parallel to said circuit breaker (1), wherein the current level limited by said current limiting element is higher than the minimum level of the current required for operation of said circuit breaking device (122, 127) of the subsystem (120) connected to the feeder, a current detector (7) for detecting current flowing in the feeder, a first over-current detector (8) for delivering to said circuit breaker (1) a trip command when the level of the current detected by said current detector has exceeded a first set level, and a second over-current detector (9) for delivering an off command to said switch (2) when a state in which the level of current exceeds a second set level has continued beyond a predetermined period, said first and second over-current detectors (8,9) having over-current setting means (8b,9b) for setting said first and second set levels independently from each other. The system according to claim 4, wherein each current limiting feeder circuit breaking device (100A, ..., 100D) further comprises means for delivering a close command to said circuit breaker (1) when the current level detected by said current detector (7) has come down below said second set level within said predetermined period. The system according to claim 4 or 5, wherein said circuit breaker (1) is a semiconductor switch. The system according to any of claims 4 to 6, wherein said second over-current detector (9) includes fault time setting means (9e) for variably setting said predetermined period. The system according to any of claims 4 to 7, wherein said second over-current detector (9) includes a comparator means (9c) for comparing the current level detected with said second set level and producing a signal when said second set level is exceeded by the detected current level and a delay means (9d, 9f) for delivering said off command to said switch (2) when said signal from said comparator means has lasted for said predetermined period.

Inventors: ARITA HIROSHI, KUROSAWA YUKIO, MORITA MASAO, UEDA TOSHIAKI, YAMAGIWA TOKIO, ARITA, HIROSHI, KUROSAWA, YUKIO, MORITA, MASAO, UEDA, TOSHIAKI, YAMAGIWA, TOKIO
Applicants: CHUBU ELECTRIC POWER, HITACHI LTD, CHUBU ELECTRIC POWER COMPANY INC., HITACHI, LTD.
Application year: 1991
Main classification: H02H3
IPC codes: H02H7, H02H3, H02H9, F02B75