AU648864B2 - Apparatus for protecting power distribution line from accidental failure - Google Patents

Description

S48864

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPLICANT(S): NGK Insulators, Ltd.

ADDRESS FOR SERVICE: DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

INVENTION TITLE: Apparatus for protecting power distribution line from accidental failure The following statement is a full description of this invention, ih:!luding the best method of performing it known to me/us:e

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The present invention relates to an apparatus for protecting a power distribution line from accidental failures by cutting off or reducing a fault current which flows through the power distribution line due to the accidental failures.

In a power distribution line, in order to protect the line from a breaking upon the occurrence of a short-circuit fault or a grounding short-circuit fault, a circuit breaker (CB) and an overcurrent relay 10 (OCR) for driving the circuit breaker are hitherto •provided in a distributing substation. However, the operating speed of the overcurrent relay has to be low so that the circuit breaker in the distributing substation is operated only after a consumer's 15 overcurrent relay has been operated. In general, the overcurrent relay is designed such that the circuit breaker is opened after 0.2 seconds from the occurrence of the fault current. Therefore, the power distribution line might be broken or fused during this delay time, because the substantially large fault current flows through the distribution line before th, -Arcuit breaker in the distributing substation is operated. Once the distribution line is down, it takes a relatively long -2-

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time until the electric power supply is recovered.

It has been desired to shorten the power-down period as short as possible.

Further, in the known protecting apparatus for the power distribution line, a current transformer (CT) has been usually utilized as a sensor for detecting the fault current. It should be noted that the current transformer is very sensitive to external noise, so that there is a possibility that a fault judg'-j circuit might generate an erroneous signal and the overcurrent relay might be operated erroneously. It is apparent that this causes an undesired power-down. Moreover, there are problems that when the insulating property of the current transformer is deteriorated or the 15 dielectric breakdown of the current transformer occurs, the power distribution system as a whole could not e operate normally.

STn the known distributing substation, a fault detecting unit such as current transformer (CT) and grounding transformer (GPT) is provided on the secondary side of the circuit breaker, and the circuit breaker is .eee°i controlled by the protective relay which detects a line failure in accordance with the detection signal generated by the fault detecting unit. However, it 26 takes 10 12 cycles in the commercial frequency (in the case of 50/60 Hz system, it takes about 0.2 sec) from the occurrence of a failure to the opening of the -3- I 'N circuit breaker. Therefore, when an arc discharge is generated in the line due to a fault current by lightening surge and the like, not only the aforementioned breaking of the line but also other 06 considerable damages in apparatuses connected to the line occur until the circuit breaker is opened.

The first object of the present invention is to provide a novel and useful apparatus for protecting the distribution line from accidental failures which can solve the aforementioned conventional problems and prevent the breaking or down of the distribution line due to an arc of a fault current, by cutting off or ceasing or decreasing the fault current at a high speed when a short-circuit fault or a grounding short-circuit S" 15 fault is detected in the distribution line.

The second object of the present invention is to provide a distribution line protecting apparatus, in which even when a dielectric breakdown of the fault current sensor occurs, the power distribution system can 20 be operated normally, malfunctions are not caused in the fault judging circuit due to noises coming from outside, and when a failure is occurred, the power distribution line is positively protected frD'm an arc generated by the fault current.

The third object of the present invention is to provide a distribution line protecting apparatus, in which the power distribution line can be protected from -4a fault current by stopping or reducing the fault current at a high speed before the circuit breaker provided in the distributing substation is opened, so that the power-down period due to the opening of the circuit breaker in the power distributing substation can be minimised.

The fourth object of the present invention is to provide a distribution line protecting apparatus, in which the number of the occurrences of the power-down can be reduced and the maintenance of the apparatus can be performed easily and safely by disconnecting the apparatus from the power distribution line at will by operating a protective breaker.

In accordance with the present invention, there is provided an apparatus for protecting a power distribution line from accidental faults, said power distribution line having a plurality of different-phase conductors each being connected to a circuit breaker, comprising: a current sensing meari. for detecting a current passing through the power distribution line to produce a current sensing signal; a fault detecting means for receiving said current sensing signal to detect a fault in the power distribution line due to a short-circuit fault or a grounding short-circuit fault to produce a fault detection signal when it is detected that the current passing through the power distribution line exceeds a predetermined level; 20 a trigger circu'. for responding to said fault detection signal to generate a trigger signal having a predetermined time period which is shorter than a delay time of the circuit breakers; and a thyristor type high speed switching means including a plurality of thyristor type 'switching units each of which is coupled with one of a secondary side of said circuit 25 breakers via one of said plurality of different-phase conductors of the power distribution S.line, said thyristor type switching units being triggered on or off in response to said trigger signal to reduce or cut-off the fault current passing through the power distribution line for said predetermined time period, wherein said predetermined time period of the **trigger signal is set to a value within the range from 1.5 to 5 cycles of an electric power distributed along the power distribution line.

SPreferably, each of said thyristor type switching units is formed by a normally 9apMpeiwAingnk~iUO2.ca4 -6closed type and is connected in series with one of the different-phase conductors of the power distribution line to thereby effect the cut off of the fault current.

Preferably, each of said thyristor type switching units is formed by a normally open type, one ends of the thyristor type switching units are connected in parallel with the different-phase conductors of the power distribution line on the secondary sides of the circuit breakers and the other ends of the thyristor type switching units are connected to each other to thereby effect reduction in the fault current.

For a better understanding of the invention, reference is taken to the accompanying drawings, in which: Fig. 1 is a circuit diagram showing the

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r e 940302,p\opcMdhngklns ,6 principal construction of the distribution line protecting apparatus according to the invention; Fig. 2 is a circuit diagram showing a first embodiment of the distribution line protecting apparatus 06 according to the invention; Fig. 3 is a graph showing the operation characteristic of a thyristor type high speed switching unit provided in the first embodiment; Fig. 4 is a circuit diagram showing the principal construction of the protecting apparatus according to the invention; Fig. 5 is a schematic front view depicting an embodiment of the actual construction of the first embodiment installed on a distribution line pole; 16 Fig. 6 is a view of another embodiment of the actual construction of the first embodiment; 0* 0 Fig. 7 is a circuit diagram showing a third embodiment of the protecting apparatus according to the invention; Fig. 8 is a block diagram showing the opto- P*9a.S electric current sensor used in the third embodiment; Fig. 9 is a block diagram showing the optical current detecting unit provided in the third embodiment; Fig. 10 is a circuit diagram representing the construction of a fourth embodiment of the protecting apparatus according to the invention; Fig. 11 is a circuit diagram illustrating the detailed construction of the fourth embodiment; Fig. 12 is a schematic front view showing the actual construction of the fourth embodiment arranged on a distribution line pole; Fig. 13 is a circuit diagram showing the principal construction of a fifth embodiment of the protecting apparatus according to the invention; Fig. 14 is a schematic front view depicting the actual construction of the fifth embodiment provided on a distribution line pole; Fig. 15 is a circuit diagram showing an embodiment of the construction of a sixth embodiment of the distribution line protecting apparatus according to the invention; 1 Fig. 16 is a front view showing another ••go embodiment of the construction of the sixth embodinrent; and Figs. 17A nd 17B are graphs showing the operation of the thyristor in the sixth invention.

Fig. 1 shows the principal construction of a first embodiment of the power distribution line protecting apparatus according to the invention.

Numeral 1 denotes a circuit breaker (CB) provided in

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a distributing substation, and numeral 2 indicates a three-phase distribution line connected to a secondary side of the circuit breaker 1. A reference numeral 3 represents a thyristor type high speed switching means i which is connected to the distribution line 2 in parallel therewith. It should be noted that the distribution line 2 includes three-phase conductors and the thyristor type high speed switching means 3 06 comprises three switching units which are connected to respective conductors. One end of each of the switching units of the switching means 3 is connected in parallel with respective one of the three-phase conductors and the other ends of the switching units are connected to each other. The thyristor type switching means 3 may be provided in or near an area of the distributinq substation as will be explained later.

There is further provided a fault detecting S means 4 for detecting a fault current flowing through 16 the distribution line 2. The thyristor type high speed switching means 3 is usually set in the off-state, and when the fault current is detected by the fault detecting means 4, the thyristor type switching means 3 is changed over into the on-state. Then, the three conductors of the power distribution line 2 are connected to each other and the fault current is distributed among theve conductors, so that the amplitudes of the currents transmitting over the conductors are decreased to a safety level and the 26 distribution line can be effectively prevented from being broken.

Fig. 2 is a concrete circuit diagram of the /y first embodiment of the protecting apparatus sh wn in Fig. 1. As shown in Fig. 2, the thyristor type high speed switching means 3 includes three thyristor type switching units 3U, 3V and 3W one ends of which are 06 connected to U-phase conductor 2U, V-phase conductor 2V and W-phase conductor 2W, respectively of the distribution line 2 by means of respective protective breakers 5V and 5W, respectively of a protective breaker unit The other ends of the thyristor type switching units 3U, 3V and 3W are connected to each other. To junction points between the switching units 3U, 3V and 3W and the protective breakers 5U, 5V and 5W, are connected arresting contacts 6U, 6V and 6W, respectively of an arrestor 6 for protecting the apparatus from the 15 lightening surge. Each of the thyristor type switching units 3U, 3V and 3W is formed by a pair of semiconductor controlled rectifiers (SCR) connected in parallel with each other in opposite polarities.

The fault detecting means 4 includes an optelectric current sensing unit 7 having opto-electric Scurrent sensors 7U, 7V and 7W which are coupled with respective conductors 2U, 2V and 2W of the distribution line 2. These opto-electric current sensors 7U, 7V and 7W are optically coupled by means of optical fiber 26 cables 8U, 8V and 8W, respectively to an optical/electrical transducer 10 in a system controller 9. As will been explained later, the optical/electrical t L~p:0 transducer 10 comprises a light source emitting a linearly polarized light beam which is transmitted to a Faraday element or Pockel element provided in the opto-electric current sensor and a photodetector for receiving the light beam emanating "rom the optoelectric current sensor. The polarizing plane of the light beam is modulated in accordance with a magnetic field generated around the conductor of the distribution line 2. In this case, the modulation degree is proportional to the intensity of the magnetic field, i.e. the amplitude of the current passing through the conductor. Therefore, the amplitude of the current transmitting through the distribution line 2 can be measured by detecting the modulation degree of the light 15 beam received by the photodetector in the optical/electrical transducer 10. An electrical signal produced by the optical/electrical transducer represents the amplitude of the current conducting over the distribution line 2 and is supplied to a fault judging circuit 11 provided in the system controller 9, in which the amplitude of the current is compared with a predetermined reference level and when the current •exceeds this level, it is judged that a fault current flows through the distribution line 2. When it is judged that the fault current is generated in any of the three conductors 2U, 2V and 2W of the distribution line 2, the judging circuit 11 sends a command to a first 1) I I control unit 12 to send trigger signals to the thyristor type high speed switching units 3U, 3V and 3W via a thyristor control unit 13. Then, these thyristor type high speed switching units 3U, 3V and 3W are made conductive simultaneously, so that the three-phase conductors 2U, 2V and 2W are connected to each other.

Therefore, the fault current having a large amplitude is distributed among these conductors and thus levels of currents passing through respective conductors become sufficiently low for preventing the conductors from being broken.

The trigger signals are generated for a predetermined time period which is shorter than a delay time of the circuit breaker 1 of the distributing substation, but is sufficiently long enough for protecting the distribution line 2 against the breaking.

After said predetermined time period has elapsed, the generation of the trigger signals from the thyristor control unit 13 is stopped. Then, the thyristor type switching units 3U, 3V and 3W are made non-conductive. In this case, there is a possibility that these switching units could not be turned-off due to malfunction. When said control unit 12 detects such an undesired phenomenon, it sends a signal to the protective breaker unit 5 to open the breakers 5U, and 5W provided therein. In this manner, the thyristor (un A; type high speed switching means 3 can be protected.

Moreover, when it is required to check or repair the thyristor type high speed switching units 3U, 3V and 3W, the protective breakers 5U, 5V and 5W are opened by 06 suitably operating the control unit 12.

When the protecting apparatus shown in Fig. 2 is applied to the actual distribution line 2 at the distributing substation, the distribution line 2 is branched as shown in Fig. 5, a branch is connected to the protective breaker unit 5 which is provided on an electric pole 14, and electric wires 15a are connected from the protective breaker unit 5 to a thyristor type switching device 16 which is disposed on the ground and includes not only the thyristor type 16 high speed switching means 3 but also the system controller 9.

In Fig. 5, numeral 17 denotes a grounding conductor, and numeral 18 represents a signal cable through which the trigger signal is transmitted from the system controller 9 to the protective breaker unit The arrestor 6 for the lightening surge protection is %to* provided above the protective breaker unit 5 and is •connected to the protective breaker unit by means of wires 15b. The arrestor 6 is connected to the common grounding conductor 17. The opto-electric current sensor 7 is provided on the power distribution line 2 and is coupled with the optical/electrical transducer provided in the system cc.c:roller 9 by means of the optical fiber cable 8. In the present embodiment, since the thyristor type switching means 3 is arranged on the ground, the maintenance and repair thereof can be carried out easily without cutting off the electricity on the distribution line 2, while the protective breaker unit 5 is opened.

Fig. 6 is a schematic front view showing another embodiment of the actual construction of the protecting apparatus according to the invention.

In this embodiment, the protecting apparatus is constructed as a single protection unit 21 and is provided on the electric pole 14. The protection unit 21 includes thyristor type high speed switching means 3, 16 arrestor 6 for the lightening surge protection, system controller 9, opto-electric current sensor 7, protective breaker unit 5, and the like. The arrestor 6 is connected to the ground by means of the common grounding conductor 17. In this embodiment, the opto-electric current sensor 7 and protective breaker unit 5 are connected to a jumper wire 2a connected to the distribution line 2. As a result, the protecting apparatus can be constructed as the single unit 21, and thus the size of the apparatus can be made small.

Further no apparatus is installed on the ground, so that the aesthetic environment can be improved.

Fig. 3 is a graph showing the actuating ^AU^N.iL b I I characteristic of the thyristor type high speed switching means 3 according to the present invention.

In this graph, the vertical axis represents the time, and the horizontal axis represents the current. In the 06 graph, curve A represents the actuating characteristic of the thyristor type high speed switching means 3.

Curve B represents the actuating characteristic of the overcurrent relay (OCR) for the circuit breaker provided in the distributing substation, curres C represent the all wable damage characteristics of conductors having different size, curve D represents the actuating characteristic of an OCR of the circuit breaker provided on the consumer's side, and curve E represents an exciting rush current of a transformer. The allowable damage 16 characteristics of the electric conductors are represented in relating to the arc duration on the vertical axis and the arc current on the horizontal axis. The conductors have the sectional areas of 20 mm 2 32 mm 2 60 mm 2 and 120 mm 2 It should be noted 20 that these conductors are widely used as the distribution line.

As shown in Fig. 3, the actuating charac- .c teristic of the thyristor type high speed switching means 3 according to the present invention, is set lower than the OCR of the circuit breaker provided in the distributing substation and the allowable damage characteristics of the conductors, but is set higher i t5 i/ than the OCR of the circuit breaker provided on the consumer's side. Therefore, when a short-circuit fault or a grounding short-circuit fault occurs at one of the consumers connected with the power distribution line, 06 the OCR of the consumer is first operated. When the fault is in the range between the OCR of the consumer and that of the distributing substation, the thyristor type high speed switching means 3 is operated prior to the opening of the circuit breaker in the distributing substation. Therefore, the distribution line 2 can be protected without being broken by an arc. In this case, since the circuit breaker in the distributing substation is not opened, the power-down does not occur.

In the aforementioned embodiment of the power 15 distribution line protecting apparatus according to the present invention, the thyristor type high speed switching means 3 is usually in the off-state. But when a short-circuit fault or a grounding short-circuit fault is caused in the distribution line 2 as shown in Fig. 1, the opto-electric current sensor 7 detects the fault, and the control unit 11 rapidly turns on the thyristor type high speed switching means 3 to forcibly short- •circuit the distribution line 2 before the distribution line 2 is broken. For that reason, the system voltage 26 of the distribution line 2 is lowered, and the arc of the fault point is extinguished, so that the damage or fusing of the distribution line 2 can be effectively -f-6 prevented. After the arc at the fault point has been extinguished to recover the insulation, as soon as the thyristor type high speed switching means 3 is returned to the off-state, the power distribution system is promptly restored to its initial state. The aforementioned sequential operations are automatically completed in a very short period of time before the OCR for the circuit breaker 1 provided in the distributing substation operates.

Fig. 4 shows a second embodiment of the distribution line protecting apparatus according to the invention. In the present embodiment, the thyristor type high speed switching means 3 which works in the same manner as the first embodiment is inserted in 16 series with the distribution line 2. This thyristor type high speed switching means 3 is always set to onstate, and only when a fault current is detected, it is changed over to off-state so that the fault current can be cut off. The control method and actuating characteristic of the thyristor type high speed switching means 3 are the same as ttbse of the first embodiment.

Fig. 7 shows a third embodiment of the distribution line protecting apparatus according to the invention. The principal construction of the present 26 invention is very similar to that of the first embodiment illustrated in Fig. 1, so that similar po:tions are denoted by the same reference numerals used in Fig. 1. In the present embodiment, the thyristor type high speed switching means 3 is connected in parallel with the distribution line 2 on the secondary side of the circuit breaker 1 of the distributing substation. On the distribution line 2 there is provided the opto-electric current sensor 7 which is optically coupled with the system controller 9 by means of the optical fiber cable 8. When a fault current is detected, the thyristor type switching means 3 is turned on so that the three conductors of the distribution line 2 are connected to each other. Then, the fault current is distributed among these conductors, and therefore the distribution line 2 can be protected from the breaking.

In the present invention, the opto-electric 16 current sensors 7 are respectively provided on the 0** three-phase conductors of the distribution line 2.

As shown in Fig. 8, the opto-electric current sensor 7 comprises a magnetic core 7a having a gap and a sensor head 7b including a Faraday element and polarizing means arranged in the gap of the core which surrounds a conductor 2U, 2V or 2W of the distribution line 2.

Such an opto-electric current sensor head is known in the art and has been described in, for instance U.S. Patent Specification No. 4,894,609 issued to 26 T. Fujiki et al. The sensor head 7b is optically coupled with a light source 10a and a photodetector provided in the optical/electrical transducer 10 by means of optical fiber cables 8a and 8b, respectively.

The light source 10a includes a light emitting element such as a laser diode for emitting a linearly polarized light beam, which is transmitted to the opto-electric current sensor head 7b via the optical fiber cable 8a.

The Faraday element in the sensor head 7b is provided in the gap of the magnetic core 7a and thus is subjected to the magnetic field generated in the gap in accordance with the current passing through the conductor 2U, 2V or 2W. Then, the polarizing plane of the light beam is modulated in accordance with the magnitude of the magnetic field and thus the amplitude of the electric current. The light beam having the thus modulated polarizing plane is transmitted through the polarizing means, so that the intensity of the light transmitted through the polarizing means is changed in accordance with the amplitude of the current. This light beam is transmitted to the photodetector lOb by means of the optical fiber cable 8b. In Fig. 8, there are shown the incident light beam emitted from the light source and the modulated light beam transmitted through the optical fiber cable 8b. The incident light beam has S* a constant intensity and the modulated light beam includes an AC component.

Fig. 9 is a block diagram showing the detailed construction of the optical/electrical transducer 10 and judging circuit 11. The photodetector 10b includes a phototransistor 25 which generates an output signal whose amplitude is modulated in accordance with the intensity of the light beam impinging upon the phototransistor. Then, a DC component of the output signal is removed by a coupling capacitance 26, and is amplified by an amolifier 27. Then, the signal is passed through a rectifying circuit 28 and a smoothing circuit 29 provided in the judging circuit 11.

The signal generated by the smoothing circuit 29 is supplied to a comparing circuit 30 and is compared therein with a predetermined.reference voltage.

The comparing circuit 30 produces a signal having a logic level or according to whether the signal supplied from the smoothing circuit 29 exceeds the 16 predetermined reference value or not. The output signal

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of the comparing circuit 30 is supplied to a logic circuit 31. When a current having an amplitude larger than a predetermined level passes through one or two of the three-phase conductors 2U, 2V and 2W, the signal supplied from the smoothing circuit 29 to the comparing circuit 30 exceeds the predetermined reference value and

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the logic signal is supplied from the comparing circuit to the logic circuit 31. Then, the logic circuit 31 judges that a fault current has transmitted 26 thro he distribution line 2 and sends a trigger command signal to the control unit 12 (see Fig. and then the control unit generates the trigger signal for _^AU^-a4B-.

i^ I all of the thyristor type high speed switching units 3U, 3V and 3W. In this manner, the three-phase conductors 2U, 2V and 2W of the distribution line 2 are connected to each other and the fault current is distributed among these conductors, so that the conductors can be effectively prevented from being fused.

The output signal of the photodetector 10b is also supplied to a comparing circuit 32 and is compared with a predetermined standard value. An output signal of the comparing circuit 32 is then supplied to a laser diode driving circuit 33 provided in the light source such that the intensity of the light beam emitted by a laser diode 34 can be always maintained at a reference leve.. By providing the feedback control loop in the 16 manner mentioned above, the intensity of the modulated light beam received by the phototransistor 25 in the 4 photodetector 10b can be maintained always constant as long as the electric current passing through the distribution line 2 has the nominal value, so that the 4* fault current can be detected accurately without being affected by the secular variations of various elements.

In the aforementioned embodiment of the power •distribution line protecting apparatus according to the *o present invention, when the fault current is detected by the opto-electric current sensor 7, the logic circuit 31 of the fault jue-ging circuit 11 judges the occurrence of the fault current and the thyristor type high speed Y switching means 3 is triggered to be conductive prior to the opening of the circuit breaker of the distributing substation. Therefore, the fault current is distributed among the three-phase conductors and the arc is 06 distinguished. In this manner, the damage or fusing of the distribution line 2 can be effectively prevented.

As described above, the light source 10a of the optical/electrical transducer 10 is provided with the feedback loop for maintaining constant the intensity of the light beam impinging upon the photoelectric current sensor 7, so that the output signal of the photodetector l0b can be maintained at the reference level.

Accordingly, even when the light source 10a is o do deteriorated after it has been used over a long period 15 of time, the opto-electric current sensor 7 is stably

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operated and the fault current can be positively and accurately detected.

Further, in the present embodiment, the fault current is detected by the opto-electric current sensor 7, so that the fault current can be detected precisely without being affected by external electric noise.

Furthermore, since the opto,-electric current sensor 7 is a coupled with the optical/electrical transducer 10 by means of the optical fiber cable which is nonmetallic and has an excellent insulating property, there is no possibility that a trouble occurs in the operation of the power distribution system in a case of an emergency "t( Kl /a f::s in which the insulation of the photoelectric current sensor 7 is broken.

Next, a fourth embodiment of the distribution line protecting apparatus according to the present 06 invention will be explained. Fig. 10 shows the principal construction of the present embodiment, which is similar to that of the previous embodiment illustrated in Fig. 7. Therefore, portions of the present embodiment similar to those of the previous embodiment are denoted by the same reference numerals as far as possible. A high voltage distribution line L is connected to a distributing substation H which includes transformer 41, circuit breaker 1, zero-phase current transformer 42, ground transformer 43 and protective 15 relay 44 which drives the circuit breaker 1 in accordance with signals detected by the transformers 43 and 44. The construction of the distributing substation H is well known in the art, so that more detailed explanation is dispensed with.

To the secondary side of the circuit breaker 1 ,ee is connected a power distribution line 2 including three-phase conductors. The power distribution line 2 is divided into a plurality of sections by means of normally-closed line switches Swl SwN. Further, the 26 free end of the distribution line 2 is structured in such a manner that the distribution line can be 8electively connected to an adjacent distribution line 2 1 through normally-opened line switch SwO.

As shown in Fig. 10, a current sensing means 7 includes three current sensors which are respectively suspended from the three-phase conductors of the distribution line 2 and are connected to a system controller 9 by means of a signal cable 8. Further, a thyristor type high speed switching means 3 is connected in parallel with the distribution line 2.

As shown in Fig. 11, between the three-phase conductors 2U, 2V and 2W of the distribution line 2 and switching units 3U, 3V and 3W of the aforementioned thyristor type high speed switching means 3 are provided protective breakers 5U, 5V and 5W of a protective breaker unit 5, end arrestor contacts 6U, 6V and 6W of 15 an arrestor unit 5 are connected to junction points between the protective breakers and the thyristor switching units. The output signals generated by the current sensors 7U, 7V and 7W provided on the conductors 2U, 2V and 2W are supplied to a judging circuit provided in the system controller 9 via signal cables 80, 8V and 8W, respectively. When the fault current is detected, the judging circuit 45 sends a command to a trigger circuit 46 which then supplies trigger signals 4 to the gates of the thyristor type high speed switching units 3U, 3V and 3W co make them conductive. After the switching units 3U, 3V and 3W have been made conductive for a predetermined time period, the generation of the trigger signals is stopped, and the switching units are returned to the initial non-conductive state.

The system controller 9 further includes a protection circuit 47 and a protective switch 48 which is normally opened. When the thyristor type switching means 3 is not turned-off after the elapse of the predetermined time period, the protective switch 48 is closed and the protection circuit 47 sends an operation command to the protective breaker unit 5 so that the protective breakers 5U, 5V and 5W are opened.

Also in the present embodiment, when a shortcircuit current or a grounding short-circuit current is flowed through the power distribution line 2, the 6 judging circuit 45 detects the fault current and .4* 1 supplies the signal to the trigger circuit 46. Then, the trigger circuit 46 sends the trigger signal to the thyristor type high speed switching means 3.

As described before, the high speed switching means 3 is usually opened, and when it receives the aforementioned 20 trigger signal, it is rapidly turned-on for the o predetermined time. After that the switching means 3 is turned-off again.

In the present embodiment, for example, wher the high speed switching means 3 is out of order due to the fault current and it can not operate, the trigger circuit 46 closes the protective switch 48 and the protection circuit 47 is operated to open the protective beaker unit Fig. 12 is a schematic front view depicting the actual construction of the present embodiment of the protecting apparatus according to the invention.

The power distribution line 2 is supported by an insulator 51 mounted on an electric pole 14.

The distribution line 2 is branched at a point 52 and a branch line 2b is connected to the protective breaker unit 5 through a bushing 53. The protective breaker 1I unit 5 is connected to the thyristor type high speed switching means 3 by means of a conductor 54 which extends through a bushing 55 secured to the protective t breaker unit and a bushing 56 provided on the switching means 3.

16 The aforementioned system controller 9 is provided in a housing 57 in which the thyristor switching means 3 is installed and is connected to the switching means via a trigger signal conductor 58.

The current sensor 7 mounted on the power distribution line 2 is coupled with the system controller 9 by means *AUU. *of the signal cable 8. The system controller 9 is further connected to the protective breaker unit 5 by means of a signal conductor 59 which extends along the outer surface of the electric pole 14. The arrestor 26 unit 6 is provided below the protective breaker unit and is connected thereto by means of the bushing In order to increase the signal transmission speed and to avoid the influence of the external noise, it is preferable to construct the signal cables 8, 58 and 59 by the optical fiber cables.

Function of the fourth embodiment of the power distribution line protecting apparatus according to the invention constructed in the manner described above will be explained.

The current sensors 7U, 7V and 7W mounted on the three-phase conductors 2U, 2V and 2W of the power distribution line 2 always detect the currents passing therethrough. When an accidental fault such as lightening surge, short-circuit fault and grounding short-circuit fault occurs, an abnormally large current transmits over any one or more of the conductors 2U, 2V S. 15 and 2W and the judging circuit 45 detects the increase of the current of each phase as well as the zero-phase current, and sends a command signal to the trigger circuit 46. Then, the trigger circuit 46 immediately supplies the trigger signal to the thyristor type high speed switching means 3. The thyristor switching means .3 is instantaneously made conductive. This conduction **e state lasts for a predetermined time period and after that the generation of the trigger signal is stopped and ee the thyristor switching means 3 is made non-conductive 26 again. Consequently, the arc current caused by shortcircuit or grounding short-circuit on the distribution line 2 can be cut off and the power distribution line 2 .C7 can be effectively protected against the breaking.

As described before, the protective relay 44 of distributing substation H has the delay time of about 0.2 seconds upon the occurrence of a fault on the 06 distribution line 2 until the circuit breaker 1 is opened. Therefore, according to the invention the thyristor type high speed switching means 3 is made conductive for the predetermined time period which is shorter than the aforementioned delay time of the protective relay 44 for driving the circuit breaker 1.

When the fault current is removed or decreased to a safety level during the conduction time of the thyristor switching means 3, the circuit breaker 1 is no more opened. Consequently, the power-down in the power 15 distribution system can be avoided.

%see In a case even if the thyristor type high speed switching means 3 is once made conductive, the fault current can not be cut off, the fault current is existent in the distributing substation H, so that the circuit breaker 1 is opened after the predeterined delay time has elapsed, and the power supply to the power distribution line 2 is stopped.

In the aforementioned embodiment, when the thyristor type high speed switching means 3 could not be made non-conductive although the trigger signal is stopped, the protective breaker unit 5 is opened by the protecting circuit 47 provided in the system controller 9. Therefore, the power distribution line 2 is not short-circuited via the conductive switching means 3.

In this embodiment, the protective breaker unit 5 and thyristor type high speed switching means 3 are connected to the branch lines 2a connected to the power distribution line 2. Therefore, if the protective breaker unit 5 is opened, repair and maintenance of the protecting apparatus according to the invention can be easily performed without causing the power-down in the power distribution system.

Further, the present invention also can be executed by arranging the thyristor type high speed switching means 3 in series with the power distribution line 2 as shown in a fifth embodiment illustrated in Figs. 13 and 14.

In this fifth embodiment, the power distribution line 2 is supported by strain insulators 61, and jumper cables 62 and 63 are connected to a housing 21 of the protecting apparatus including the thyristor type high speed switching means 3 and system controller 9 9 which is connected to the current sensor 7 arranged on the distribution line 2 by means of the signal cable 8.

In this embodiment, the thyristor type high speed switching means 3 is usually made conductive, and when the fault current is detected, the thyristor switching means 3 is instantaneously made cut-off for a predetermined time period shorter than the delay time of the protective relay 44 provided in the distributing substation H. After the predetermined time period has elapsed, the thyristor switching means 3 is made conductive again.

Figs. 15 and 16 show a sixth embodiment of the distribution line protecting apparatus according to the present invention. Also in the present embodiment, portions similar to those of the previous embodiments are denoted by the same reference numerals used in the previous embodiments.

As shown in Fig. 15, first current sensors 7U, 7V and 7W are arranged on the three-phase conductors 2U, 2V and 2W of the power distribution line 2, and signal cables 8U, 8V and 8W are connected to a judging circuit 16 45 provided in a system controller 9. A current transformer, an optical sensor of magnetic field detection type or the like may be utilized as the first current sensors 7U, 7V and 7W. In the case when the S.optical sensor is used, an optical fiber cable is utilized as each of the aforementioned signal cable 8U, 8V and 8W. Then, the light source and photodetector such as a light emitting diode (LED), a photodiode (PD) and the like are provided in the judging circuit The judging circuit 45 is connected to a trigger circuit 46 which generates a trigger signal to be supplied to a thyristo type high speed switching means 3 comprising thyristor switching units 3U, 3V and 3W which are connected in parallel with the conductors 2U, 2V and 2W, respectively of the power distribution line 2. The amplitudes of the currents passing through the conductors 2U, 2V and 2W detected by the current sensors 7U, 7V and 7W are always monitored and when the judging circuit 45 judges the occurrence of the fault current which exceeds a predetermined current level, a command signal is supplied from the judging circuit to the trigger circuit 46. Then, the trigger signal is supplied from the trigger circuit 46 to the thyristor type switching means 3 for a predetermined time period.

Therefore, the thyristor type switching units 3U, 3V and 3W are made conductive for the predetermined time and after that are made non-conductive again. That is to 15 say, during the time interval in which the trigger *fee signal is supplied to the thyristor type switching means 3, the switching means is kept to be conductive, and after the predetermined time has elapsed, the thyristor type switching means is brought back into the initial 20 non-conductive state.

S

Further, in the sixth embodiment shown in Fig. 15, a protective breaker unit 5 is connected .0 between the power distribution line 2 and the thyristor e type switching means 3. The protective breakers 5U, and 5W in the unit 5 are normally made conductive.

Moreover, second current sensors 61U, 61V and 61W in the form of the current transformer are connected between ao..

3 1 the protective breakers 5U, 5V and 5W and the thyristor type switching units 3U, 3V and 3W, respectively.

The second current sensors 61U, 61V and 61W are connected to a control unit 62, and the control unit is connected to a locking device 63 including a closing coil (CC) 64 and a trip coil (TC) 65. The protective breaker unit 5 is controlled by the locking device 63.

Numeral 67 is a low volcage power source line to which electric power is supplied from the power distribution line 2 through a transformer 68. The low voltage power source line 67 is parallelly connected to the judging circuit 45, trigger circuit 46 and control unit 62. The control unit 62 may be constructed such that it has a built-in battery which is not shown, and 15 is charged from the low voltage power source line 67 so eo..

that the on and off states of the protective breaker unit 5 can be indicated by a lamp and the like. There is further provided an arrestor unit 6.

Fig. 16 shows the actual construction of the 20 power distribution line protecting apparatus of the present embodiment.

The power distribution line 2 is supported by an insulator 51 mounted on an arm 14a of an electric pole 14. A branch line 2a is connected to the power distribution line 2 through a connector 52, and is connected to the protective breaker unit 5 suspended from the aforementioned arm 14a through a bushing 53.

The protective breaker unit 5 is connected through a bushing 55 to the thyristor type high speed switching means 3 provided in a housing 57 via a bushing 56.

The housing 57 further includes the system controller 9 and is suspende" from a lower arm 14b of the electrir pole 14.

The aforementioned protective breaker unit 5 as well as the arrestor unit 6, second current sensors 61 and locking device 63 are installed in the housing 51, and the second current sensors 61 are connected to the control unit 6? by means of the signal cable 69.

The control unit 62 is provided under the housing 57 and is suspended from a lower arm 14c. The low voltage source line 67 is connected to the control unit 62 by 16 means of a wire 67a. The control unit 62 is connected to the locking device 63 provided in the upper housing 21 by means of a signal cable 70 which extends along the outer surface of the electric pole 14.

Also in the present embodiment, when the fault current flowing through the power distribution line 2 is detected by any one or more of the first current sensors 7U, 7V and 7W, the judging circuit 45 sends the command to the trigger circuit 46 to generate the trigger signal for a predetermined time period, so that the thyristor 26 type switching units 3U, 3V and 3W are all made conductive and the conductors 2U, 2V and 2W of the power distribution line 2 are connected to each other. After ThK^ 1 the above time period has passed, the generation of the trigger signal from the trigger circuit 46 is stopped, and thus the switching units are brought into the nonconductive state. The fault current which flows in the power distribution line 2 is reduced, then it returns again to the on-state due to the breaking of the control signal. Therefore, while the thyristor type switching means 3 is made conductive, the fault current is distributed among the conductors 2U, 2V and 2W and the arc discharge is extinguished in the power distribution line 2 before the switching means 3 is made non-

V

conduct4.ve again. In this manner, the distribution line 2 can be protected without causing the undesired powerdown, because the circuit breaker 1 in the distributing 15 substation H is not opened.

If the fault current is not stopped or reduced during the time interval during which the thyristor type switching means 3 is remained to be made conductive, then the circuit breaker 1 provided in the distributing 20 substation H is opened after the delay of the protective relay 44. In this manner, the power distribution line 2 can be protectod effectively.

Further, in the present embodiment, the protective breaker unit 5 is connected in series with the thyristor type switching means 3 and is normally made conductive. When a leak current flowing through the thyristor switching means 3 is detected by the second current sensors 61U, 61V and 61W, the control unit 62 controls the locking device 63 to open the protective breakers 5U, 5V and 5W. In this manner the thyristor type switching means 3 can be disconnected from the power distribution line 2. This operation of the protective breaker unit'5 is also performed when the thyristor type switching means is remained to be conductive even after the switching means has been made conductive for the predetermined time of, for instance ten cycles, by the trigger circuit 46. The control unit 62 also detects that there is no voltage in the system applied from the low voltage source line 67 to which the electrical power is applied from the power distribution line 2, and when the low voltage is dropped to zero, the S• 15 control unit opens the protective breaker unit immediately, so that the power distribution line 2 is not subjected to the fault. Since the thyristor type switching means 3 is disconnected from the power distribution line 2 when a failure occurs in the protecting apparatus in the manner described above, there is an advantage that the electric power supply to the user is not interrupted and the repair and maintenance of the thyristor type switching unit 3 can be performed at any time without causing the power-down.

Next; the operation of the present embodiment will be explained in detail also with reference to Figs. 17A and 17B.

W f The thyristor trigger circuit 46 is actuated by the command signal generated by the judging circuit and is structured such that it generates the trigger signal having the predetermined number of cycles of the A.C. power distributed along the power distribution line 2. Fig. 17A shows the variation of the arc current while the thyristor type switching means 3 is transferred from the off-state to the on-state, and further returned to the off-state. Tl represents a time from a time instant at which the arc current occurs to a time instance at which the switching means 3 is transferred from the off-state to the on-state when the arc current is detected on the power distribution line 2. T2 represents a time during which the arc current is 15 attenuated after the thyristor type switching meane 3 has been made fully conductive. T3 denotes a time interval during which the trigger circuit 46 generates the trigger signe In order to extinguish the arc current effectively, te above mentioned time T3 is set to T3 T2 a as shown in Fig. 17B. It should be noted that usually the thyristor type switching means 3 is remained to be conductive even after the trigger signal is stopped, so that a time T5 during which the thyristor type switching means 3 is kept in the on-state is 26 represented by T5 T3 T4 as shown in Fig. 17B, where T4 is not more than 0.5 cycle.

In the present embodiment, the time T3 during which the trigger circuit 46 generates the trigger signal is set to not less than 1.5 cycles of the electric power distributed along the power distribution line 2. Then, the arc current is completely distinguished during this time interval T3.

The inventors have conducted experiments in which the above mentioned time interval T3 was changed in various ways.

TABLE 1 Cointrol Timle of hy :istor 1 Cycle 1.5 5 Cycles Not less than 6 Cycles 9 9.

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Stit e of Not Arc Extnui d Extinguished Extinguished Arcd Extinguished Aluminum alloy Aluminum alloy Aluminum alloy Scconductors were conductors were State of slightly fused.

-ab6 'lo Disconnected slightly fused. C e Cable Copper conductors opper wire Copper conductors conductors hardly were hardly fused. fused fused.

There was a strong influence due to Although voltage in ence u instantaneous Power-down was was instantaneously vltage o Other voltage drop.

other caused due to lowered, theit breakers States Circuit breakers in disconnection, influence was i substrate were small.

opened and powerdown occurred.

s. S bo*.: 0666 5 SSSS9*

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As it is obvious by the Table 1, when the time duration T3 of the trigger signal was set to one cycle, the arc discharge was not extinguished and the power distribution conductor was fused. When the time interval T3 was set to 1.5 to S cycles, the arc discharge was extinguished and the conductor was hardly damaged. When the thyristor type switching means 3 was made conductive, an instantaneous voltage drop of the power distribution line 2 was occurred. However, it has been confirmed that there was no practically harmful effect when the control time of the thyristor was in the range of 1.5 5 cycles.

When the time period T3 was set to more than cycles, there was a remarkable influence of instantaneous voltage drop even when the arc was extinguished. Further, in the case of 10 12 cycles, 9* the circuit breaker 1 provided in the distributing substation H was opened. Therefore, according to the present embodiment it has been confirmed that the time 15 duration of the trigger signal T3 is preferably set to to 5 cycles of the electric power distributed along the line 2. That is to say, when the electric power has 50 Hz, the time pTeriod T3 is preferably set to 0.03 to 0.1 seconds. Ii: other words, according to the 20 invention, the trigger signal has preferably the time duration not more than a half of the delay time of the 0 protective relay for driving the circuit breaker provided in the distributing substation, but longer than cycles of the electric power distributed along the power distribution line.

According to the present invention, the thyristor type high speed switching means is provided in parallel or in series with the power distribution line, and is operated by detecting the fault current on the power distribution line. Accordingly, when a ground fault or a ground short-circuit fault occurs, the fault current is rapidly cut off or reduced to a low level before the conductors of the power distr-bution line are fused by an arc. so that a disconnection fault can be prevented effectively. Further, when the opto-electric current sensor is used as the current sensor, the fault current can be detected accurately without being influenced by the deterioration or breakdown of the insulation and external noise.

Furthermore, When the thyristor type high speed switching means is made conductive for a time period 16 which is shorter than the delay time of the circ' .t breaker provided in the distributing substation.

In almost all cases, the fault current can be distinguished without opening the circuit breaker, so that the number of the power-down can be reduced.

eoe 20 Moreover, when the protective breaker unit is connected in series with the thyristor type switching means which is connected in parallel with the power distribution line, if the switching means is not returned into the initial non-conductive state even after the predetermined time period has elapsed, the protective breakers are opened to disconnect the switching meats from the power distribution line.

Further by opening the protective breakers it is possible to repair the switching means without causing the undesired power-down.

*oat* 0:

Claims (1)

1. 41- THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. An apparatus for protecting a power distribution line from accidental faults, said power distribution line having a plurality of different-phase conductors each being connected to a circuit breaker, comprising: a current sensing means for detecting a current passing through the power distribution line to produce a current sensing signal; a fault detecting means for receiving said current sensing signal to detect a fault in the power distribution line due to a short-circuit fault or a grounding short-circuit fault to produce a fault detection signal when it is detected that the current passing through the power distribution line exceeds a predetermined level; a trigger circuit for responding to said fault detection signal to generate a trigger signal having a predetermined time period which is shorter than a delay time of the circuit breakers; and a thyristor type high speed switching means including a plurality of thyristor type switching units each of which is coupled with one of a secondary side of said circuit breakers via one of said plurality of different-phase conductors of the power distribution line, said thyristor type switching units being triggered on or off in response to said trigger signal to reduce or cut-off the fault current passing through the power distribution line for said predetermined time period, wherein said predetermined time period of the trigger signal is set to a value within the range from 1.5 to 5 cycles of an electric power i distributed along the power distribution line. 2. An apparatus according to claim 1, wherein each of said thyristor type switching 25 units is formed by a normally closed type and is connected in series with one of the S" different-phase conductors of the power distribution line to thereby effect the cut off in the fault current. 3. An apparatus according to claim 2, wherein said current sensing means comprises opto-electric current sensors each being arranged on respective different-phase conductors of the power distribution line, a first set of optical fiber cables and a second set of optical fiber cables, and said fault detecting means comprises a light source for 94=0 p:\eop1nabnm2.r ",41 42 emitting light beams which are transmitted to said opto-electric curreirl sensors by means of said first set of optical fiber cables, photodetectors for receiving the light beams whose intensity is modulated in accordance with the magnitude of the current by the opto- electric current sensors and are transmitted through said second set of optical fiber cables, a judging circuit for processing output signals supplied from the photodetectors to produce a trigger command signal representing the occurrence of the fault current, said trigger command signal being supplied to said trigger circuit to produce said trigger signal so as to render said thyristor type switching units non-conductive for said predetermined time period. 4. An apparatus according to claim 1, wherein each of said thyristor type switching units is formed by a normally open type, one ends of the thyristor type switching units are connected in parallel with the different-phase conductors of the power distribution line on the secondary sides of the circuit breakers and the other ends of the thyristor type switching units are connected to each other to thereby effect reduction in the fault current. An apparatus according to claim 4, wherein said current sensing means compri'res opto-electric current sensors each being arranged on respective different-type conductors of the power distribution line, a first set of optical fiber cables and a second set of optical fiber cables, and said fault detecting means comprises a light source for emitting light beams which are transmitted to said opto-electric current sensors by means of said first set of optical fiber cables, photodetectors for receiving the light beams whose intensity is modulated in accordance with a magnitude of the current by the opto-electric current sensors and are transmitted through said second set of optical fiber cables, a judging circuit for processing output signals supplied from the photodetectors to produce a trigger command signal representing the occurrence of the fault current, said trigger command :signal being supplied to said trigger circuit to produce said trigger signal so as to render said thyristor type switching units conductive for said predetermined time period. 6. An apparatus according to claim 1, wherein said thyristor type high speed switching means is constructed such that its characteristic represented on a graph whose vertical axis denotes a time and whose horizontal axis shows an amplitude of a current, 94=ov3op.\oPMcangknaO.x42 43 is set lower than an allowable damage characteristic range of an overcurrent relay of the circuit breaker provided in the distributing substation and that of the conductors of the power distribution line, and is set higher than that of an overcurrent relay of a circuit breaker provided on a consumer's side. 7. An apparatus according to claim 5, wherein said fault detecting means further comprises a feedback control circuit for maintaining the intensity of the light beams transmitted to the opto-electric current sensors to be constant. 8. An apparatus according to claim 4, further comprising a plurality of protective breakers which are connected between the respective different-phase conductors and the respective thyristor type switching units, and a control device for opening said protective breakers. 9. An appratus according to claim 8, wherein said control device is constructed such that said protective breakers are opened when the thyristor type switching units are not brought into the initial non-conductive state even after the generation of the trigger signal is stopped. t e 10. An apparatus according to claim 8, wherein said control device comprises additional current sensors provided on lines for connecting the thyristor type switching units to the different-phase conductors of the power distribution line, a control unit for processing output signals supplied by said additional current sensors to detect an abnormal condition of the thyristor type switching units to produce a driving signal, and 25 a locking device responding to said driving signal to open the protective breakers. C 11. An apparatus according to claim 10, wherein said control unit is constructed such that said driving signal is produced only when the electric power distributed along the power distribution line is not existent. 12. An apparatus according to claim 11, wherein said control unit is energized with a low voltage which is obtained from the electric power distributed along the power 940302,OZpi\opA 2gk ru,43 -44- distribution line by means of a current transformer. 13. An apparatus according to claim 1, wherein said fault detecting means and thyristor type switching units are provided on the ground near an electric pole on which the distributing substation is arranged. 14. An apparatus according to claim 1, wherein said fault detecting means and thyristor type switching units are provided on an electric pole on which said distributing substation is arranged. An apparatus for protecting a power distribution line from accidental faults, substantially as hereinbefore described with reference to the drawings. DATED this 2nd day of March, 1993 NGK INSULATORS, LTD. by DAVIES COLLISON CAVE S 20 Patent Attorneys for the applicant **0 *5* o *o* 94=3Op:opcAdz&UaaksMa2jc*,44 APPARATUS FOR PROTECTING POWER DISTRIBUTION LINE FROM ACCIDENTAL FAILURE ABSTRACT OF THE DISCLOSURE In a power distribution system for distributing an electric power along a power distribution line consisting of different-phase conductors, a distributing substation includes circuit breakers, protective relays for driving the circuit breakers, and an apparatus for protecting the power distribution line from accidental faults. The protecting apparatus includes a current :"sensor for detecting an amplitude of a current passing through the power distribution line, a judging circuit for processing a signal supplied from the current sensor to judge a fault current, a trigger circuit for respond- ing to a command signal generated by the judging circuit when the fault current is detected to produce a trigger signal, and thyristor type high speed switching units connected in series or parallel with the power distribution line and being operable in response to the ooooo trigger signal generated by the trigger circuit. When the switching units are connected in parallel with the power distribution line, the switching units are made conductive when the fault current is detected, so that the different-phase conductors of the power distribution line are connected to each other, so that the fault current is distributed among these conductors. Therefore, the conductors can be effectively prevented from being fused or broken by the fault current. The switching units are made conductive for a predetermined time period which is shorter than a delay time of the protective relays for driving the circuit breakers, and thus the fault current can be reduced or cut-off without opening the circuit breakers. e* a°