JPH01320480A - Monitoring equipment for shield breaking of single core cable and its method - Google Patents

Abstract

PURPOSE:To easily discover at early stage by equipping with an alarm equipment for an abnormal voltage detection being connected between one side of shield terminal of an activated power cable and earth, an electrostatic capacity being connected with a shield earth changeover switch in parallel and a resistance meter. CONSTITUTION:A down current side terminal part of shields 1R, 1S, 1T in each lines of a activated power cable truck consisting of three-lines of a single core cable is earthed through an alarm equipment for the abnormal voltage detection 4 being high impedance collectively. Also, as for an up current side terminal part of the shields, shield earth changeover switches 5R, 5S, 5T are earthed by every shields 1R, 1S, 1T of each lines, and earthed via respective contacts 'b'. Shield earth changeover switch contacts 'a' of each lines are connected collectively with the one side terminal wherein an electrostatic capacity 6, resistance meter 3 and arrester 7 are connected in parallel, and parallel contact points of the other side terminal are earthed. By this method, even in case the shield breaking of the cable is happened, mostly, a possibility leading directly to an electric destructive accident is fully controlled, a degree of safety for a truck driving is large and also, by measuring the resistance of a shield loop, it is able to easily detect which shield in which phase takes an abnormal resistance value.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to an apparatus and method for monitoring breaks in the shield of a single-core cable.

(b) Conventional technology If the shielding metal body of a single-core cable breaks for some reason, electrical discharge may occur at the breakage point, which is likely to directly lead to an electrical breakdown accident. In order to prevent this accident, the monitoring method shown in FIGS. 2 and 3 has conventionally been used.

In FIGS. 2 and 3, IR, Is, and IT indicate the shielding of each strip of a live electric cable line consisting of three single-core cables at the R-phase, S-phase, and T-phase portions. Usually, the shielding of a single-core cable is done at its upstream (power supply side) end, A in the diagram.
Three strips are connected together at the end and grounded. On the other hand, each shield terminal on the downstream side (load side) of the shield, end B in the figure, is used with 70 points. The reason for this is to avoid the inflow of stray current from the ground into the shield and the generation of circulating current between the shields. In order to monitor the shield disconnection under such a grounding system, bring an AC voltmeter 2 and an ohmmeter 3' as portable measuring instruments to the B end where the shield terminal is connected to 70°. This method measures the AC voltage or loop resistance of the shield to see if there is an abnormality.

When measuring with AC voltmeter 2, one end is connected to the ground as shown in Figure 2, and the other terminal is connected to the shielded terminal of each strip connected to 70-meters. The AC voltage is measured, and the measurements are performed three times by sequentially switching connections for each phase. Also, when measuring with resistance meter 3', the second
Connect one terminal to ground and connect the other terminal to 7 as shown in the figure.
The loop resistance between the shield terminal and the ground is measured by connecting it to the shield terminal of each strip that is connected to the ground, and the measurement is performed three times by sequentially changing the connection. In this case, the measured value includes the common ground return resistance, but this value is usually treated as small and negligible.

In FIG. 3, a resistance meter 3' is used at the B end to measure the resistance without positioning it relative to the ground. In this case, the shielding IR,Is
, if the resistances of IT are Rr, Rs, and Rt, respectively, then Rr
+Rs-r, , Rs + Rt=r, , Rt+Rr-r3, three combinations of series resistances are made and the loop resistance is measured three times.

The measurement results are decomposed using the following formula to obtain the shielding resistance value for each phase. Rr--! ll-nu, +fyu・R5-r
2-r, +r,, Rt=r3-r, +r.

(C) Problems to be Solved by the Invention The above-mentioned conventional technology has the following problems.

(i) Connection for measurement is not easy. Since the shielded terminal at the B end is intended to be insulated from the ground and from other phases, there are many obstacles and troubles in approaching people and connecting measurement lead wires. Furthermore, it is inconvenient that the B ends of each line are separated from each other.

(ii) There is a risk of electric shock. If a disconnection has already occurred, high voltage may be induced at the shield terminal. At this time, if there is no lead coming out from the shielded terminal, or if there is a lead but the tip is insulated with tape, etc., expose the lead tip metallically and connect the measurement lead wire to it. This work is difficult and poses a risk of electric shock.

(Mountain) Often practically impossible to measure. Due to the above-mentioned complexity, it is practically impossible to repeat measurements frequently, and as a result, there is a high possibility that an accidental electric breakdown will occur.

(iv) Measurement errors are likely to occur. The ground AC voltage generated at the shield terminal is not only caused by the shield disconnection, but also by the electromagnetic induction voltage from the conductor current of the own line, the electromagnetic induction voltage from other parallel cables, and the shield length due to the self-capacitance charging current. This is a composite voltage due to multiple factors such as voltage drop in the horizontal direction. In particular, the electromagnetic induction voltage changes in proportion to the magnitude of the induced current, so it is not constant, and even if there is an initial state of disconnection, it is often masked by the fluctuations and cannot be detected.

In addition to the above, the loop resistance measurement using the earth as the return path shown in FIG. 2 has a flaw in that accurate measurements cannot be made because the earth return resistance is not zero and is variable and not constant. Third
When measuring the loop resistance consisting of two shields in series as shown in the figure, the AC voltage entering the resistance meter is large, so the resistance meter is
If the internal filter circuit is not strengthened, there is a risk of false indications being obtained.

The present invention provides a single-core cable shield disconnection monitoring device and its equipment that can easily and safely detect shield disconnections at an early stage, and can greatly reduce the possibility that even if a disconnection occurs, it will directly lead to an electrical breakdown accident. The purpose is to provide a method.

(d) Means for Solving the Problems The device of the present invention is a high impedance abnormal voltage detection and notification device connected between the collective shielding terminal of each strip at one end of a live power cable and the earth. and a shielding terminal provided for each strip of the other end of the live power cable, having a grounding position and a measurement position, and the shielding terminal is normally grounded via the grounding position. and a capacitance and resistance meter connected in parallel between the collective measurement position of each of the shielded grounding switches and the ground.

Further, in the method of the present invention, one end of the single-core cable is grounded at all times through a high-impedance abnormal voltage detection device through a high-impedance abnormal voltage detection device, and the other end of the single-core cable is grounded at all times through a high-impedance abnormal voltage detection device. the step of constantly grounding through the shield earthing switch provided for each shielded terminal; and the step of constantly grounding the shield earthing switch provided from the grounding position to the measurement position when receiving notification from the abnormal voltage detection device or periodically. At each switching step, the loop resistance between the shielding terminal and the ground is measured to obtain measurement values at three points, and from each measurement value, calculations are made for each strip. The method includes the step of obtaining a shielding resistance value.

(E) Function This invention provides that each strip at one end of a live electric cable line is grounded all at once via an abnormal voltage detection device, and each strip at the other end is always connected to each cut-off terminal switch. Since the terminal is grounded through the terminal, the voltage generated when there is no abnormality at the one end can be suppressed to be lower than in the past. In addition, unless the three-phase shields are all disconnected, they are not lifted from the ground, so abnormal voltage is unlikely to occur at the one end, and even if it occurs, there is little possibility that it will directly lead to an electrical accident. Furthermore, since the shield grounding terminal switch is switched to the measurement position for each strip, each strip's shield is grounded via capacitance, and the loop resistance of the shield is measured three times using an ohmmeter, so there is no danger during the measurement. Measurements can be made safely and easily without generating excessive voltage.

(D) Embodiment Figure 1 shows an embodiment of the present invention, in which the shielding IR of each strip of a live electric cable line consisting of three single-core cables is shown.
, Is, the downstream (load side) end of IT, ie, the B end, connects the shield terminals of each strip and is grounded through a high impedance abnormal voltage detection and notification device 4. In addition, at the upstream side (power supply side) end of the shield, that is, at the A end, each strip of shield IR, 15
, IT are connected to shielded grounding changeover switches 5R, 5S, and 5T, and are grounded via their respective b (normally closed) contacts. The a (normally open) contacts of each shield grounding switch are connected to one end where the capacitance 6, resistance meter 38, and arrester 7 are connected in parallel, and the parallel connection point at the other end is grounded. .

The high impedance value of the abnormal voltage detection device is required to be at least IMΩ or more. This high impedance also has the purpose of blocking stray current from flowing into the shield from the ground.
It is important to maximize the AC voltage detection sensitivity of the abnormal voltage detection device 4 itself. Therefore, in addition to high DC resistance, AC impedance must also be high, and it is impossible to insert a large capacitor in parallel with the abnormal voltage detection device 4. By the way, the performance required of the abnormal voltage detection and notification device 4 is to have both the detection of abnormal AC voltage and the notification function to make this detection easily recognized.

Moreover, since the abnormal voltage detection device 4 is always installed at each B end of a large number of lines, its price is limited, and a complex, sophisticated, and expensive device is impractical and difficult to use. Therefore, the first
The abnormal voltage detection device 4 illustrated in the figure is a simple and inexpensive device such as a neon lamp with a stabilizing resistor connected in series. It is sufficient that the lighting start voltage of this neon lamp is higher than the induced voltage normally generated at the B terminal. The lighting starting voltage of several tens of volts for an ordinary 100V neon lamp satisfies this condition, and the occurrence of an abnormality can be easily notified by the lighting light.

The above-mentioned normal induced voltage can be measured using an AC voltmeter 2 shown in Fig. 2.
This is not the electromagnetic induced voltage from the conductor current of the own circuit that is measured when there is no abnormality in the shielding, but is much smaller than that. In the shielded grounding circuit shown in Figure 1, the electromagnetic induced voltage from the conductor chamber and flow of the own line disappears, so the normal induced voltage is generated because the arrangement of the power cables on each line is not a pure triangle. It is a combination of residual zero-sequence voltage and electromagnetic induction voltage from other parallel cables, and it is highly unlikely that its value will reach 50V or more. Therefore, if the neon lamp lights up, it can be determined that the shielding wire is definitely broken. Furthermore, since the shield terminals of each strip are grouped together at the B end, even if a shield breakage occurs, the possibility of it directly leading to an electrical breakdown is much less than in the conventional grounding type.

The following table compares the conventional grounding system and the grounding system of the present invention with respect to prediction of the presence or absence of abnormal AC voltage at terminal B for various occurrences of shield breakage.

(Table) The above table shows that the grounding type, which is part of the present invention, which does not completely float from the ground unless all three phase shields are disconnected, has a much lower proportion of B than the conventional shielding grounding type. This shows that no abnormal voltage alternating current occurs at the end. That is, even if a shield breakage occurs, the AC voltage generated across the breakage point is low, so that discharge at the breakage point is unlikely to occur, and the possibility of a direct electrical breakdown accident is drastically reduced. In the most dangerous situation where the three-phase shields IR, IS, and IT are all disconnected, abnormal voltage will occur and will be detected and reported.

However, if all three-phase shielding breaks happen to occur at the same location on the line, no voltage will be generated at the B end, and therefore no detection will be reported, but there will be no abnormal voltage generation at the breakage location, so it is safe.

With the shielded grounding method shown in Figure 1, even if a wire breakage occurs, the possibility of an immediate electrical breakdown is greatly reduced; however,
This is not always the case, but it is especially dangerous if the shield is broken at multiple locations within the same phase. This grounding method does not prevent the occurrence of shield breakage, but the probability of shield breakage occurring is the same as in the conventional method.

Therefore, the method of the present invention is such that as soon as the abnormal voltage generation at the B terminal is detected, or periodically even without receiving the detection alarm, the shielded grounding changeover switch installed in advance at the A terminal is sent to one line. Switch from the normal ground position to the measurement position. For example, as shown in FIG.
When R is switched from the B contact to the A contact, the terminal potential of the shielding IR is led to the capacitance 6 through the A contact, and the capacitance 6
Becomes grounded through. At the time of this switching, a temporary depression mechanism between the a and b contacts is required to ensure the safety of the measurer. Even if the shield IR is disconnected, dangerous voltage will not be induced at the terminal of the shield IR during operation, and it can be several tens of V.
A value of capacitance 6 is used so that the voltage is suppressed to about 10 to 10-odd volts.

Furthermore, the loop resistance of the shield is measured using the resistance meter 3.

First, when the shield earthing selector switch 5R is switched to the a contact, measure the loop resistance which becomes Rr + (Rs and Rt in parallel)'' Rr. Next, the shield earthing selector switch 5R is switched to the a contact
Return the shielding grounding changeover switch 5S to the contact point, switch it to the a contact, and measure the loop resistance R2 which becomes Rs + (parallel of R1 and R') - R2. Next, the shielding grounding changeover switch 5
Return S to the b contact, switch the shield grounding changeover switch 5T to the a contact, and measure the loop resistance R1 where Rs+(parallel of Rt and Rr)=R1. Shield grounding switch 5
Return T to B contact to complete the measurement, and perform similar measurements on other lines. In this measurement, the capacitance 6 and the arrester 7 may be combined in one set regardless of how many single-core cable lines there are to be measured.

In addition, in the above measurement, the resistance meter 3 is measured by dropping one end to the ground, but the ground is not included in the loop resistance measurement circuit, and the potential of the ground is used as one potential. It's for a reason.

The loop resistance values R1, R2, and R1 obtained from the above measurements are calculated and decomposed as follows to obtain the shielding resistance value Rr.

Obtain Rs and Rt. That is, Rs=P4r Note that instead of measuring with the above-mentioned combination of loop resistances, for example, simultaneously switch the shielding grounding switch 5R and 55 to the a contact at the measurement position and measure the resistance of the series loop of the shielding IR and is. In order to ensure safety, the combination of capacitance 6 and arrester 7 is required for each strip, which is at least 3 times as many, and if there are n single-core cable lines, 3R times is required. This is not a good idea and is not adopted because the resistance meter 3 must be used without positioning and the incoming AC voltage will also be high.

Moreover, it is desirable that the frequency of carrying out the present invention on an actual track is once a day. Therefore, it is desirable to automate all of the aforementioned measurement operations, calculation and decomposition of measured values, and the operation of determining whether or not there is an abnormality in the shielding resistance based on the results and issuing an alarm, so that the process can proceed without the need for human intervention. The invention is suitable for such automatic measurements.

(G) Effect In the present invention, since the shield terminals of each strip at one end of the live cable cable are collectively grounded via the abnormal voltage detection and alarm device, even if a shield disconnection occurs, most of the shield terminals are grounded. The possibility of direct connection to electrical breakdown accidents is greatly reduced, resulting in a high level of safety for track operation.

Furthermore, in the event that all three-phase shields are disconnected, an abnormal voltage alarm device will perform the alarm function, but this is a simple device that can be installed economically and is therefore practical. Further, it is possible to easily determine which phase of the shield has an abnormal resistance value by periodically measuring the loop resistance of the shield, either with or without notification from the abnormal voltage detection and notification device. Furthermore, the measurement operation is simple and safe, and it is easy to automate, allowing early detection of single-core cable shield breaks.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a circuit diagram illustrating a conventional single-core cable shield disconnection monitoring method, and Fig. 3 is a circuit diagram illustrating another conventional monitoring method. FIG. IR, IS, IT...Single electrocardiogram cable shielding, 3.
...Resistance meter, 4...Abnormal voltage detection alarm device, 5R. 5S, 5T... Shield grounding switch, 6... Capacitance, 7... Arrester. Figure 3

Claims (2)

[Claims] (1) In a live power cable line consisting of three single-core cables, detection of high impedance abnormal voltage connected between the collective shielding terminal of each wire at one end of the live power cable and the ground. an alarm device, provided for each shielded terminal of each strip at the other end of the live power cable, having a grounding position and a measurement position, and the shielding terminal is normally grounded via the grounding position; A shield breakage monitoring device for a single-core cable, comprising: a shielded grounding changeover switch; and a capacitance and resistance meter connected in parallel between the collective measurement position of each shielded grounding changeover switch and the ground. (2) A method for monitoring disconnection of a live power cable shield consisting of three single-core cables, wherein one end of the single-core cable detects a high impedance abnormal voltage by collectively detecting the shield terminals of each strip. a step in which the single-core cable is always grounded through the device, and the other end of the single-core cable is always grounded through a shield grounding switch provided at each shield terminal of each strip, and upon receiving a notification from the abnormal voltage detection device; Periodically, turn the shield earthing selector switch 1.
The step of switching from the line grounding position to the measurement position and grounding through capacitance, and each time the switching is made, the loop resistance between the shielding terminal and the ground is measured to obtain three measured values, and each of these measured values A method for monitoring single-core cable shield breakage, comprising: obtaining a shielding resistance value for each line by calculation.