Electrical power supply system

Abstract: An electrical power supply system includes an electrical power source system connected to a bus bar (10) and a plurality of electric power transmission systems each connected via circuit breakers (3) to the bus bar (10). The supply system has overvoltage suppression equipments each installed between the transmission system and the ground. The overvoltage suppression equipment includes a switching means (7) and an impedance (8) connected to the switching means (7). The switching means (7) in a sound transmission system is closed when the circuit breaker (3) operates in a faulty transmission system, thereby effectively suppressing an overvoltage occurring on the sound system.

Description: BACKGROUND OF THE INVENTION The present invention relates to an electrical power supply system provided with an overvoltage suppression function, and more particularly it relates to an electrical power supply system capable of suppressing an overvoltage arising when a circuit breaker is operated in said power system. Overvoltages arising when a circuit breaker is operated include a closing switching surge which builds up an overvoltage when a charging voltage in a transmission line at the time of closing undergoes successive reflection as a traveling wave, and an interrupting surge which also builds up an overvoltage when a traveling wave due to a voltage drop in the transmission line at the time of a ground fault interruption undergoes successive reflection. The largest of these overvoltages sometimes reaches three times the phase voltage. Heretofore, against such overvoltages, resistance closing circuit breaker and resistance interrupting circuit breaker have been utilized in conjunction with efforts for designing the insulation level of the transmission system sufficiently to withstand such overvoltages. In order to increase the capacity of electrical power transmission systems in accordance with an increasing demand for a greater power supply, construction plans for a 1000 kV power transmission system have been in progress. In such ultra-high-voltage power transmission systems, it becomes more difficult economically to build a power transmission line which can satisfactorily withstand overvoltages generated when the circuit breaker thereof is operated. Thereby, it is contemplated to design an overvoltage suppression equipment to suppress the overvoltages generated when the circuit breaker is operated to an extent as low as possible so as to enable to construct economical power transmission lines. For example, in a 1000 kV system, it is demanded to hold the overvoltages down less than 1.6 times the phase voltage peak values. By way of example, against the overvoltages arising when a circuit breaker is closed in power transmission systems having a lower voltage than the above, a resistance closing circuit breaker which is closed via a resistance and absorbs the energy of a progressive wave has been employed. This system, however, is not effective for suppressing the overvoltages at the time of interruption. Thereby, a resistance interrupting circuit breaker capable of being interrupted via a resistance also at the time of interruption has been proposed. However, in a puffer-type gas circuit breaker which is most popular nowadays, a resistance current conduction time during interruption is substantially longer as much as about 30 ms. Further, it must withstand the out-of-phase closing and interruption duty which becomes twice as much as the duty voltage for the circuit breaker. Thereby, the resistance thereof becomes inevitably large-sized, and in addition, a complicated delay operation mechanism is required to actuate the resistance closing unit or resistance interrupting unit thereof. Furthermore, a formidable problem of system reliability is always involved therein that a failure of the resistance or the actuating mechanism thereof will result in a bus bar failure then paralyzing the operation of the substation. Further, the inventors of the present application have found that the overvoltages arise on the sound transmission systems, the circuit breakers of which are not operating. However, in the prior art, e.g. Japanese Patent Laid-Open No.Hei-2-146930, Hei-2-106133, the overvoltages arising in the sound transmission systems has not been considered. SUMMARY OF THE INVENTION Objects of the present invention are to solve the problems associated with the prior art, to provide an electrical power supply system which is capable of significantly suppressing overvoltages arising therein, and has an extremely greater system reliability. These objects are accomplished by the power supply system defined in claim 1 and that defined in claim 4. The system of the invention is capable of minimising adverse effect of overvoltage on the other sound transmission system, thus providing a highly reliable electrical power supply system. BRIEF DESCRIPTION OF THE DRAWINGS Further objects, features and advantages of the present invention will become more clear from the following detailed description of preferred embodiments, shown in the accompanying drawings, wherein: FIG.1 is a schematic diagram showing a first embodiment of the invention. FIG.2 is an operational characteristic diagram explaining the operation of the first embodiment of the invention. FIG.3 illustrates voltage characteristic diagrammatical drawings explaining the operation of the first embodiment of the invention. FIG.4 is a characteristic diagram showing changes in overvoltages in terms of multiples when the values of resistance were changed. FIGS.5 to 10 are schematic diagrams showing other embodiments of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.1 shows a first embodiment of the present invention, illustrating an electrical power system 1, in the electrical power source side of which an electrical power transformer 5 connected to a generator 6 is further connected to a bus bar 10 via a circuit breaker 4. To this bus bar 10, two pairs of power transmission lines 2a and 2b are connected through circuit breakers 3a and 3b, respectively. At lead-in portions of the power transmission lines, arrestors 9a and 9b are connected, respectively. Between the ground and respective main circuits connecting circuit breaker 3a or 3b with transmission line 2a or 2b, there are provided resistance 8a or 8b through switching gears 7a or 7b which in conjunction constitute an overvoltage suppressing equipment. If a ground fault 30 occurs on the transmission line 2a, a fault current 11 (shown in FIG.2) is detected by a detector 35a such as a current transformer or the like. In response to a signal 12 (shown in FIG.2) sent from a fault detector 31a such as a relay or the like connected to the detector 35a, first of all, the switch gear 7b of a sound power circuit is closed with a stroke characteristic 14 as shown in FIG.2. Then, circuit breaker 3a on the faulty line is opened with a stroke characteristic 13 as shown in FIG.2. Part of the signal from the fault detector 31a enters a timer 32a which, after an interval of a few cycles, outputs a signal to the switch gear 7b which is designed to open upon reception of the signal. If the ground fault occurs on the transmission line 2b, the switch gear 7a and the circuit breaker 3b are operated by a current transformer 35b, a fault detector 31b and a timer 32b in the same way as described above. Suppose, for example, a ground fault has occurred on one phase of the transmission line 2a near its center. A large ground fault current flows in a direct ground system when there occurs a ground fault, thereby the amplitude of the power source voltage is decreased due to the power source impedance as indicated by a curve 20 in FIG.3, and the terminal voltage of circuit breaker 3a on the faulty line 2a decreases from VO to VT as indicated by a curve 21. At this moment in a healthy transmission line 2b, a further voltage drop by dV down to a level of VM as indicated by a curve 22 in FIG.3 is observed at a location corresponding to the faulty point on the faulty transmission line 2a due to electromagnetic induction. Under such conditions, the grounded current is interrupted by the circuit breaker 3a. Then, the electric power source voltage 20 recovers to its original value through jumping of its spike voltage to VS due to transient oscillation in the electric power source system. On the other hand, the voltage in the interrupted transmission line oscillates reciprocally therein. Also in a sound transmission line in which a further voltage drop by dV has occurred due to electromagnetic induction, the voltage drop thereof oscillates in the transmission line repeating reciprocal reflection. In addition, there occur superposed oscillations induced by the voltage reciprocal oscillations in the faulty transmission line which has been isolated. As a result, a high frequency oscillation extremely greater than dV would be superposed. Thereby, a maximum voltage at a location on the healthy transmission line corresponding to the fault point will increase up to a value Vs+kdV resulting in an excessive surge voltage. According to the present invention, however, when the circuit breaker is operated, resistance 8b is inserted between the healthy line and the ground, and resistance 8a is interconnected to the faulty line. Thereby, an excessive oscillation in the power source side is damped by the resistance 8b connected to the healthy transmission line reducing the amplitude thereof. The reciprocal voltage oscillation within the healthy transmission line is also damped by the same. Further, because the reciprocating reflection oscillation within the faulty transmission line is damped by use of the resistance 8a, electromagnetic induction due to the reciprocating voltage oscillation from the faulty transmission line to the healthy transmission line can be minimized. As a result, the ground interrupting surge becomes extremely smaller compared with that of the prior art. FIG.4 shows findings on overvoltages in terms of multiples at a location on the healthy phase corresponding to the faulty point, when the resistance of the invention was varied in value. In each case, multiples are smaller than 1.6 for which no resistance was provided. In particular, it was found that the overvoltages could have been suppressed effectively and satisfactorily at values of resistance of 1.5 kΩ and below. FIG.5 shows another preferred embodiment of the invention, where the same symbols as in FIG.1 indicate the same components or corresponding parts. In this embodiment of the invention, the current transformer, the fault detector and the timer are provided on the transmission line 2b in the same way as the transmission line 2a. (not shown in FIG.5) In this embodiment of the invention, non-linear resistance was used for the resistance 8a and 8b for constituting an overvoltage suppression equipment. Needless to mention, the same ones as above are installed in a substation at the remote end thereof. As in the above-mentioned first embodiment of the invention, when there occurs a ground fault in one of the phases of a transmission line 2a, thus requiring to isolate the same by interrupting a circuit breaker 3a, a switch gear 7b of an adjacent healthy line is closed prior to opening the circuit breaker 3a so as to insert a non-linear resistance 8b between the ground and the line thereof. A switch gear located at the remote end is operated likewise to insert a non-linear resistance between the line thereof and the ground. As a result, transient oscillations of a power source voltage between this side substation and the remote end is suppressed to a lower level by means of the non-linear resistances, and an interrupting surge in the center portion of the healthy line can be suppressed sufficiently lower. Then, after a predetermined period of time, the non-linear resistances are cut off by the switch gears to prevent their thermal destruction. The smaller the limiting voltage of the non-linear resistance, the more effective the overvoltage suppression, however, because a progressive wave current in the transmission line is usually less than 10 kA, if a limiting voltage at 10 kA is permitted to be set at 1.4 times or less the peak value of a phase voltage, it becomes certain for the overvoltage to be suppressed under 1.4 times thereof, assuring the effects and advantages of the invention to be achieved for certain. In the above embodiment of the invention, closing instructions to the switching gears may be given utilizing a tripping instruction to the circuit breaker, to close the same at a high speed such that it may complete its interrupting operation within a minimum period of time required for interrupting the circuit breaker. Thus, without lowering the stability of the electrical power system, overvoltages are capable of being suppressed. On the other hand, operation of the switch gears is capable of being interlocked with an actuator of the circuit breaker, and the switch gear itself can be installed within a housing of the circuit breaker. Further, in the above embodiment of the invention, in place of the switch gear, a gap device, in particular, a trigger gap device can be utilized. Still further, a semiconductor switch is also capable of being utilized as well. In particular, when a trigger gap device or semiconductor switch which has a fast switching capability such that it permits a resistance or non-linear resistance to be inserted between the line and the ground at rise time of a transient recovery voltage immediately after the interruption of a circuit breaker, is utilized to open the circuit at a high speed, the thermal resistance strength of the resistance or non-linear resistance can be lessened, thus enabling to make the resistance and non-linear resistance more compact-sized. Still further, for the switch gears, a movable type gap device may be utilized the gap of which is shortened to allow discharging only when required. Further, operation of the above-mentioned switch function devices or switch function elements constituting the switching means may be limited to the case of a ground fault or short-circuit fault of the transmission line. By such arrangement, superfluous operation thereof can be eliminated thus capable of prolonging their service life. Still further, if a fault occurs only in one phase and only the circuit breakers on the same phase are operated, all of the switch gears or switch function devices on the related three phases can be operated concurrently. Further, operation thereof can be limited only to the phase on which the circuit breaker has been operated. Furthermore, operation thereof can be effectuated at both ends of the transmission line concurrently. FIG.6 shows still another embodiment of the invention, in which the same symbols as in FIG.1 indicate the same components or corresponding parts thereof. In this embodiment of the invention, the current transformer, the fault detector and the timer are provided on the transmission line 2b in the same way as the transmission line 2a. (not shown in FiG.6) In this embodiment of the invention, an overvoltage suppression equipment comprising a switch gear 7 and a resistance 8 is installed to the side of a bus bar as shown in the drawing. According to this embodiment of the invention, the number of the overvoltage suppression equipment required for installation can be minimized. On the other hand, with respect to the non-linear resistance, it may be provided in a series connection or a parallel connection of a linear resistance and a zinc oxide non-linear resistance. Through such arrangement a more compact-sized non-linear resistance will be provided. Further, these resistances or non-linear resistances may be designed to have a positive temperature coefficient. In particular, when a thermistor having a positive temperature property is utilized, as its electrical resistance value increases with temperatures thereby limiting the input of energy, down-sizing of the resistance or non-linear resistance can be realized. Hereinabove, the operation of the circuit breaker during the interruption has been described. However, it is also possible to perform closing of the switch gears 7a and 7b prior to the interruption of the circuit breaker, and opening the switch gears immediately after the circuit breaker has completed its opening action. With such arrangements, overvoltages arising at the time of interruption can be also suppressed effectively. FIG.7 shows still another embodiment of the invention, in which the same symbols as in FIG.1 indicate the same components or corresponding parts thereof. In this embodiment of the invention, the current transformer, the fault detector and the timer are provided on the transmission line 2b in the same way as the transmission line 2a. (not shown in FIG.7) According to this embodiment of the invention, in a substation provided with no arrestors, at a lead-in of a power transmission line, an overvoltage suppression equipment comprising a switch gear 7a or 7b and a resistance 8a or 8b is provided to the side of the power transmission line viewed from a circuit breaker 3a or 3b. This embodiment of the invention has the advantage that the resistance 8a and 8b can be utilized as lightning arrestors by closing the switch gears 7a and 7b when lightning is anticipated. FIG.8 shows still another embodiment of the invention, where the same symbols as in FIG.1 indicate the same comor corresponding parts thereof. According to this embodiment of the invention, a tap is provided at a lighting arrestor connected to a lead-in of each power transmission line, and in parallel with part of a section defined by the tap, there is provided switch gear 7a or 7b, thus providing a combined use serving as an arrestor and a non-linear resistance as well. FIG.9 shows a still further embodiment of the invention, where the same symbols as in FIG.1 denote the same components or corresponding parts. This embodiment is an example of the embodiment of FIG.8 as applied at a location toward the bus bar side, where a tap and a switch gear are installed at a low voltage side. Thereby, there is an advantage that a lower class of voltage is capable of being utilized for the switch gears. FIG.10 shows still another embodiment of the invention, where the same symbols as in FIG.1 indicate the same components or corresponding parts thereof. This embodiment of the invention is another example of application for the embodiment of FIG.9, where a switch gear 47 is connected in series with a section defined by a tap, while a gap 48 is connected in parallel with the section defined by the tap, and the switch gear 47, further in a case of a fault, a trip instruction to the circuit breaker 3 is divided, and part thereof is given to the switch gear 47 as an interruption instruction. Because a current to flow through the switch gear 47 is as small as a few amperes, the same is capable of being interrupted with a shorter duration of arc than that of and prior to the circuit breaker 3. As a result, a high voltage applied across the gap 48 is discharged as an arc, which means that the gap 48 has been closed, providing the function of the switch gear 7 of FIG.9. According to this embodiment of the invention, there arises an advantage that, in place of the switch gear the closing operation of which is performed at an extremely high speed, a circuit breaker which has a long record of services and a high reliability can be utilized as a switch gear for this purpose. In particular, a vacuum switch which can interrupt a current with a minimum duration of arc is most suitable for this type of circuit breakers. In the above-described example of the invention, if anything such as failure of the resistance or non-linear resistance should happen, a continued transmission of electricity should still be possible by isolating the failed component by means of a switch function device or switch function element. Meanwhile, the failed resistance or non-linear resistance can be replaced with a spare component quickly enough not to retard restoration. Further, it is also possible to install two pairs of combination of a switch function device or switch function element and a resistance or non-linear resistance, which are to be utilized by switching therebetween in case of a failure. Through the use of this embodiment of the invention, as hereinabove described, an electrical power system in which an extremely high reliability of the system has been realized is capable of being provided. As hereinabove set forth, according to the embodiments of the present invention, in an electrical power supply system connected to transmission lines via electrical power circuit breakers, because a linear resistance or non-linear resistance is connected via switching means between the ground and at least the bus bar side of the circuit breaker or the transmission line side thereof, or both sides thereof on main circuit, it becomes possible to interconnect a compact-sized linear resistance having a low electrical resistance or a non-linear resistance capable of lowering its limit voltage near the peak value of the phase voltage, with the main power circuit only when the circuit breaker is operated. Thereby, an overvoltage suppression level of the overvoltage suppression equipment is capable of being reduced significantly, and the down-sizing thereof is also possible because the same is connected to the main circuit only for an extremely short duration of time thus requiring only a small energy resistance. Further, in case the linear resistance or non-linear resistance should fail to operate, it could be isolated with the switch function device or switch function element preventing any troubles to occur in the transmission of electricity, thus capable of essentially increasing the reliability of the electrical power supply system. Because the overvoltage suppression equipment according to the present invention installed between the ground and the bus bar side of the circuit breaker, or the transmission line side thereof, or both sides thereof on the main circuit, is arranged to be interconnected to the main circuit only when the circuit breaker is operated, the overvoltage suppression level in the system is capable of being reduced significantly by means of the overvoltage suppression equipment. As a result, adverse effects of overvoltages in a faulty line can be prevented from propagating into other healthy transmission lines, thus an electrical power supply system having a high system reliability is capable of being provided. While preferred embodiments along with variations and modifications have been set forth for disclosing the best mode and important details, further embodiments, variations and modifications are contemplated according to the broader aspects of the present invention, all within the scope of the following claims.

Claims: An electrical power supply system (1) having an electrical power source system (4, 5, 6) connected to a bus bar (10) and a plurality of electric power transmission systems (2a, 2b) each connected via a respective associated circuit breaker (3a; 3b) to said bus bar (10), characterised in that overvoltage suppression equipment (7a, 8a, 7b, 8b; 7a, 9a; 7b, 9b) is installed between each of said electric power transmission systems and ground, each said overvoltage suppression equipment including a switching means (7a; 7b), an impedance (8a; 8b; 9a; 9b) connected to said switching means, and control means (35b, 31b, 32b; 35a, 31a, 32a) for closing said switching means when the circuit breaker of any electric power transmission system which is not associated with the overvoltage suppression equipment operates. The system of claim 1, wherein said electric power transmission systems are three-phase systems, each overvoltage suppression equipment includes switching means for each of the three phases of the respective associated electric power transmission system, and the control means is arranged for simultaneously closing said switching means provided on each of three phases before a circuit breaker on one of the three phases of any electrical power transmission systems, which is not associated with the overvoltage suppression equipment, has completed its current interruption, and opening said switching means provided on the three phases after a predetermined period of time has elapsed. The system of claim 1, wherein said electric power transmission systems are three-phase systems, each overvoltage suppression equipment includes switching means for each of the three phases of the respective associated electric power transmission system, and the control means is arranged for closing, before a circuit breaker on one of three phases of any electric power transmission system, which is not associated with the overvoltage suppression equipment, has completed its current interruption, the switching means associated with the same phase line in response to a signal indicating the occurrence of a short circuit or ground fault at said electric power transmission system which is not associated with the overvoltage suppression equipment, and opening said switching means after a predetermined period of time has elapsed. An electrical power supply system (1) having an electrical power source system (4, 5, 6) connected to a bus bar (10) and a plurality of electric power transmission systems (2a, 2b) each connected via a respective associated circuit breaker (3a; 3b) to said bus bar (10), characterised in that an overvoltage suppression equipment (7, 8; 7, 9; 47, 48, 9) is installed between said bus bar (10) and ground, said overvoltage suppression equipment including a switching means (7; 47,48), an impedance (8; 9) connected to said switching means, and control means (35a, 35b, 31a, 31b, 32) for closing said switching means when any one of said circuit breakers operates. The system of claim 1 or 4, wherein said control means is arranged for closing said switching means before the circuit breaker has completed its current interruption, and opening said switching means after a predetermined period of time. The system of claim 1 or 4, wherein said control means is arranged for closing said switching means in response to a trip instruction from the circuit breaker, and completing the closing of said switching means within a minimum interrupting period of time, i.e. contact parting time plus minimum arcing time, of the circuit breaker. The system of claim 1 or 4, wherein said switching means includes a switching gear (7a; 7b) capable of closing during the rise time of a transient voltage immediately after a current interruption by the circuit breaker. The system of claim 5, wherein said control means is arranged for closing said switching means in response to a signal indicating the occurrence of a short circuit or ground fault on said electric power transmission system. The system of claim 1 or 4, wherein said switching means interlocks with an actuating mechanism of said circuit breaker such that said switching means is closed before said circuit breaker has completed its current interruption, and is opened after a predetermined period of time has elapsed. The system of claim 1 or 4, wherein said switching means is installed in a container of said circuit breaker. The system of claim 1 or 4, wherein said impedance is a linear resistance or a non-linear resistance. The system of claim 1 or 4, wherein said switching means includes a switching gear (47) connected to said impedance (9) and a discharge gap (48) connected to a tap provided on said impedance (9), said discharge gap (48) connecting in parallel with said switching gear (47) and a portion of the impedance (9) separated by the tap, the control means comprising means (31a, 31b, 32) for opening said switching gear in response to a trip instruction from said circuit breaker.

Inventors: ASAI YOSHIHITO, KUROSAWA YUKIO, NISHIDA ISAO, OHSHITA YOUICHI, WATANABE MASARU, YAMAGIWA TOKIO, ASAI, YOSHIHITO, KUROSAWA, YUKIO, NISHIDA, ISAO, OHSHITA, YOUICHI, WATANABE, MASARU, YAMAGIWA, TOKIO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1992
Main classification: H02H7
IPC codes: H02H9, H02H7