JP2009293991A - Sheath earth circuit surveillance device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sheath earth circuit surveillance device capable of monitoring whether there is any trouble in a sheath earth circuit disposed on a power cable. <P>SOLUTION: The sheath earth circuit surveillance device includes a first earth current transformer 11<SB>1</SB>disposed near the lead-out point from the power cable C of a first sheath earth line E<SB>1</SB>, a second earth current transformer 11<SB>2</SB>disposed near the earth point of the first sheath earth line E<SB>1</SB>and differentially connected to the first earth current transformer 11<SB>1</SB>, and a surveillance device 12 for determining "there is a trouble in the sheath earth circuit" when the magnitude of the differential current i<SB>3S</SB>between the secondary output current i<SB>1S</SB>of the first earth current transformer 11<SB>1</SB>and the secondary output current i<SB>2S</SB>of the second earth current transformer 11<SB>2</SB>becomes a predetermined value or larger. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a sheath ground circuit monitoring device, and more particularly to a sheath ground circuit monitoring device suitable for monitoring whether or not a failure has occurred in a sheath ground circuit provided in a power cable whose sheath is grounded at both ends.

2. Description of the Related Art Conventionally, a current differential type is used as a power cable protection relay for protecting a power cable whose sheath is grounded at both ends from an internal accident.
In addition, the power cable has a shield earth, but the current transformer for the power cable protection relay is a penetration type, so the sheath ground wire is used to cancel the sheath current flowing through the sheath of the power cable. Pull back in.

For example, as shown in FIG. 5, when the power cable C is connected to the overhead transmission line L, the transmission line current transformer 2 (through-type) installed on the load end side of the overhead transmission line L and the power cable A cable current transformer 3 (through-type) installed on the opposite side to the load end of C is connected to the secondary output current of the transmission line current transformer 2 (hereinafter referred to as “transmission line current transformer secondary current i”). _L ”) and the secondary output current of the cable current transformer 3 (hereinafter referred to as“ cable current transformer secondary current i _C ”), the difference current (= i _L −i _C ) i _in the current differential scheme power cable protection relay 1 (hereinafter, referred to as "power cable protection relay 1".) by entering, when the power cable protection relay 1 detects an internal fault based on the input current i _in The circuit breaker 4 installed on the load end side of the overhead power transmission line L is cut off. .

Further, in the cable current transformer 3, in order to prevent the secondary output current of the sheath current I _S flowing through the sheath of the power cable _{C from} being included in the cable current transformer secondary current i _C , the cable of the power cable C The sheath ground wire on the current transformer 3 side (hereinafter referred to as “first sheath ground wire E ₁ ”). The sheath ground wire on the side opposite to the cable current transformer 3 of the power cable C is referred to as “second sheath ground wire”. Line E ₂ ”) is pulled back into the cable current transformer 3 to cancel the sheath current I _S and the sheath ground current flowing through the first sheath ground line E ₁ .

In Patent Document 1 below, one end of a cable line that is not grounded in the middle is grounded with a capacitor so that the cable line soundness can be confirmed by simultaneously detecting the cutting of the cable shielding layer and the sheath Meg abnormality. The other end is a capacitor or high-resistance ground, and a DC voltage is constantly applied to the shielding layer from the capacitor ground end to measure the sheath leakage current to detect an abnormality in the sheath meg, and the sheath potential is constantly measured at the other end. A cable shielding layer / sheath abnormality monitoring method is disclosed in which a shielding layer disconnection is detected.
Further, in Patent Document 2 below, a current transformer is loosely fitted to the ground line of the power cable so that the insulation state of the power cable operating at a high voltage can be diagnosed and monitored as it is. When an excitation power source is connected to the generator, and excitation is performed from the secondary side of the current transformer with an excitation power source having a frequency different from the commercial frequency, it is possible to superimpose a voltage for insulation diagnosis / monitoring on the ground wire. The zero-phase current transformer is loosely fitted to the power cable, and the insulation monitoring board is connected to the zero-phase current transformer, and insulation monitoring is performed between the output of the zero-phase current transformer and the voltage for insulation diagnosis and monitoring. A power cable insulation diagnosis / monitoring system is disclosed in which a vector operation is performed inside the insulation monitoring board when input to the board is performed, and the insulation resistance and dielectric loss tangent of the power cable are measured.
JP-A-6-289094 JP-A-7-020171

However, as described above, the cable current transformer 3 pulls back the _first sheath ground wire E ₁ of the power cable C and cancels the sheath current I _S and the sheath ground current flowing through the first sheath ground wire E _1. As a result, there is a problem in the sheath ground circuit (earth fault of the sheath ground wire, puncture of the safety arrester connected to the sheath ground wire, damage to the insulation coating of the sheath ground wire, internal deterioration of the cable current transformer 3 Etc.), the sheath current I _S and the sheath ground current flowing through the first sheath ground line E ₁ are not canceled out, and the secondary output current (hereinafter referred to as “secondary sheath difference current Δi _{S) of the} difference between the two currents. ) Is included in the cable current transformer secondary current i _C , there is a problem that the power cable protection relay 1 malfunctions due to an external accident.

That is, when there is no defect in the sheath ground circuit, the transmission line current transformer secondary current i _L and the cable current transformer secondary current i _C are caused to flow through the overhead power transmission line L and the power cable C in the event of an external accident. Since this is the secondary output current of the current I _g (hereinafter referred to as “secondary accident current i _g ”), the input current i _in of the power cable protection relay 1 is “0” as shown in FIG. The power cable protection relay 1 does not operate.

However, for example, if the first ground fault sheath grounding wire cable current transformer 3 load end than the E ₁ and on the opposite side as shown in FIG. 6 occurs, the sheath grounding current flowing through the first sheath grounding wire E ₁ Is 1/2 of the sheath current I _S , so that the reciprocal of the current transformation ratio n of the cable current transformer 3 is “k”, the cable current transformer secondary current i _C is expressed by the equation (1). Is obtained by subtracting k (I _S / 2) from the secondary fault current i _g .
i _C = i _g -Δi _{S
= I g -k (I S /} 2) (1)
Therefore, the input current i _in of the power cable protection relay 1 is k (I _S / 2) as shown in the equation (2).
i _in = i _L −i _C = i _g − {i _g − (k (I _S / 2)} = k (I _S / 2) (2)
As a result, the power cable protection relay 1 malfunctions.

  In particular, when the sheath wire is grounded in the cable current transformer 3 due to internal deterioration of the cable current transformer 3 among the defects of the sheath ground circuit, it is very difficult to visually monitor the power cable. There was a problem that it was very difficult to prevent malfunction of the protective relay 1 due to an external accident.

  An object of the present invention is to provide a sheath ground circuit monitoring device capable of monitoring whether or not there is a defect in a sheath ground circuit provided in a power cable.

The sheath earth circuit monitoring device of the present invention is a sheath earth circuit monitoring device for monitoring whether or not a failure has occurred in a sheath earth circuit provided in a power cable (C) having both ends grounded. A difference current detecting means for detecting a difference current between a current flowing in the vicinity of the drawing point of the ground wire (E ₁ ) from the power cable and a current flowing in the vicinity of the grounding point of the sheath ground wire; And monitoring means for monitoring whether or not a failure has occurred in the sheath earth circuit based on the difference current.
Here, the differential current detecting means is provided in the vicinity of the grounding point of the first ground current transformer (11 ₁ ) provided near the drawing point of the sheath grounding wire from the power cable. And a second earth current transformer (11 ₂ ) connected to the first earth current transformer and connected to the first earth current transformer, and the monitoring means includes a secondary output current of the first earth current transformer ( If the difference current (i _3S ) between i _1S ) and the secondary output current (i _2S ) of the second earth current transformer exceeds a predetermined value, it is determined that “the sheath earth circuit has failed”. May be.
The differential current detecting means is provided in the vicinity of the first clamp-type current transformer (21 ₁ ) provided in the vicinity of the drawing point of the sheath ground wire from the power cable, and in the vicinity of the grounding point of the sheath ground wire. A second clamp-type current transformer (21 ₂ ), and the monitoring means obtains a difference current between currents input from the first and second clamp-type current transformers, and the obtained difference current It may be determined that “a failure has occurred in the sheath earth circuit” when is equal to or greater than a predetermined value.
The predetermined value may be set so that the monitoring means operates even at an accident current (I _g ) at the time of an external ground fault when an adjacent substation is the fault point.

The sheath earth circuit monitoring device of the present invention has the following effects.
(1) When the sheath ground circuit malfunctions, the secondary sheath current included in the secondary output current of the cable current transformer is the current flowing in the vicinity of the extraction point of the sheath ground wire from the power cable and the current flowing in the vicinity of the ground point of the sheath ground wire. Therefore, by detecting the magnitude of this difference current, it is possible to monitor whether or not there is a defect in the sheath earth circuit.
(2) If there is a malfunction in the sheath ground circuit, it is possible to prevent malfunction of the current differential power cable protection relay by locking the current differential power cable protection relay. The trouble can be eliminated.
(3) By using a clamp-type current transformer to detect the magnitude of the difference current, it is possible to easily install a sheath earth circuit monitoring device for existing equipment.

  For the above purpose, the secondary output current of the first earth current transformer provided near the drawing point of the sheath ground wire from the power cable and the second earth current transformation provided near the ground point of the sheath earth wire. This was realized by determining that “a failure occurred in the sheath earth circuit” when the magnitude of the difference current from the secondary output current of the device exceeded a predetermined value.

Hereinafter, embodiments of the sheath earth circuit monitoring device of the present invention will be described with reference to the drawings.
As shown in FIG. 1, a sheath earth circuit monitoring device according to an embodiment of the present invention includes first and second earth current transformers 11 ₁ and 11 ₂ (through-type) connected to each other, a monitoring device 12, and It comprises.

Here, the first earth current transformer 11 ₁ is provided in the vicinity of the drawing point of the first sheath earth wire E _{1 from} the power cable C.
Further, as shown in FIGS. 2 and 3, the first earth current transformer 11 ₁ has a current transformation ratio of 1: 1, and from the secondary side of the _first earth current transformer 11 _1. The first secondary sheath ground current i _1S having the same magnitude as the sheath ground current flowing in the vicinity of the drawing point of the first sheath ground wire E ₁ (primary side of the first ground current transformer 11 ₁ ) is output. Is done.

The second ground current transformer 11 ₂ is provided in the vicinity of the ground point of the first sheath ground wire E ₁ .
Further, the second earth current transformer 11 ₂ has a current transformation ratio of 1: 1 as shown in FIGS. 2 and 3, and from the secondary side of the second earth current transformer 11 _2. A second secondary sheath ground current i _2S having the same magnitude as the sheath ground current flowing near the ground point of the first sheath ground wire E ₁ (the primary side of the _second ground current transformer 11 ₂ ) is output. Is done.

Since the first and second earth current transformers 11 ₁ and 11 ₂ are differentially connected, the secondary sheath earth, which is the difference current between the first and second secondary sheath earth currents i _1S and i _2S. The difference current i _3S is input to the monitoring device 12.

Monitoring device 12 operates based on the secondary sheath grounding difference current i _3S, determines the magnitude of the secondary sheath grounding difference current i _3S becomes a predetermined value or more as "failure occurs in the sheath grounding circuit" Then, an alarm for notifying the malfunction of the sheath ground circuit is displayed and a lock signal S _LOCK for locking the power cable protection relay 1 is output to the power cable protection relay 1.

The predetermined value is set as follows.
For example, if the fault current I _g (1-wire current) at the time of an external ground fault when the adjacent substation is the fault point is 500 A, the fault current _Ig is The monitoring device 12 is made to operate even at a half current value of 250A.
Therefore, assuming that the sheath current I _S is 15% of the accident current I _g (one-line current), the secondary sheath ground differential current i _3S when a failure occurs in the sheath ground circuit as described later is the sheath current I _The predetermined value is smaller than (250A × 0.15) /2=18.5A (for example, for example) so that the monitoring device 12 operates when ½ of _S (= I _S / 2). 15A).

  Next, the operation of the sheath ground circuit monitoring device will be described with reference to FIGS. 2 and 3 for a case where there is no defect in the sheath ground circuit and a case where there is a defect in the sheath ground circuit.

(1) When there is no defect in the sheath ground circuit When there is no defect in the sheath ground circuit, the first and second secondary sheath ground currents i _1S and i _2S are both the sheath current I as shown in FIG. _{Since S} , the secondary sheath grounding difference current i _3S (= i _1S −i _2S ) is “0”. As a result, since the monitoring device 12 does not operate, an alarm for notifying the malfunction of the sheath earth circuit is not displayed, and the lock signal S _LOCK is not output to the power cable protection relay 1.

(2) ground fault occurs between the first ground current transformer 11 ₁ and a cable current transformer 3 of the sheath grounding circuit as shown in FIG. 3, for example if there is a problem the first sheath grounding wire E ₁ Since the sheath ground current flowing on the primary side of the cable current transformer 3 is ½ of the sheath current I _S , the cable current transformer secondary current electromagnetically induced on the secondary side of the cable current transformer 3 i _C = i _g −k (I _S / 2) (see equation (1)).

Further, since the sheath earth current flowing through the primary side of the first earth current transformer 11 ₁ becomes the sheath current I _S , the first secondary sheath earth current i _1S = I _S becomes, but the second earth Since the sheath earth current flowing through the primary side of the current transformer 11 ₂ is ½ of the sheath current I _S , the second secondary sheath earth current i _2S = I _S / 2.
Accordingly, the secondary sheath grounding difference current i _3S = I _S / 2 (= i _1S −i _2S = I _S −I _S / 2) having a magnitude greater than or equal to a predetermined value (= 15 A) is input to the monitoring device 12. Therefore, the monitoring device 12 operates to display an alarm for notifying the malfunction of the sheath earth circuit, and the lock signal S _LOCK is output to the power cable protection relay 1.
Thereby, it is possible to easily monitor the failure of the sheath ground circuit (in particular, it is possible to easily monitor the grounding of the first sheath ground wire E ₁ in the cable current transformer 3 that is difficult to monitor visually). In addition, it is possible to prevent the circuit breaker 4 from being interrupted due to a malfunction due to an external accident of the power cable protection relay 1 when the sheath ground circuit malfunctions.

In the above description, the sheath ground currents flowing near the lead point and the ground point of the first sheath ground wire E ₁ are detected using the first and second ground current transformers 11 ₁ and 11 ₂ , respectively. However, when the sheath earth circuit monitoring device of the present invention is installed in the existing equipment, the first and second clamp-type current transformers 21 ₁ and 21 ₂ (current transformation ratio 1: 1) as shown in FIG. ) May be used to detect the sheath ground current flowing near the lead point and the ground point of the first sheath ground line E ₁ .
At this time, the monitoring device 12 obtains a difference current between the currents input from the first and second clamp-type current transformers 21 ₁ and 21 _2, and the magnitude of the obtained difference current exceeds a predetermined value. It may be determined that “a failure has occurred in the sheath earth circuit”.

It is a figure which shows the structure of the sheath earth circuit monitoring apparatus by one Example of this invention. It is a figure for demonstrating operation | movement when there is no malfunction in the sheath earth circuit of the sheath earth circuit monitoring apparatus shown in FIG. It is a figure for demonstrating operation | movement when there exists a malfunction in the sheath earth circuit of the sheath earth circuit monitoring apparatus shown in FIG. A sheath earth circuit monitoring device in which the first and second clamp-type current transformers 21 ₁ and 21 ₂ are used instead of the first and _second earth current transformers 11 ₁ and 11 ₂ shown in FIG. It is a figure which shows a structure. It is a figure for demonstrating the power cable protection relay 1 for protecting the power cable by which the sheath was earth | grounded at both ends from an internal accident. It is a figure for demonstrating the malfunctioning by the external accident of the power cable protection relay 1 shown in FIG. 5 at the time of a sheath earth circuit malfunction.

Explanation of symbols

1 power cable protection relay 2 transmission line current transformer 3 cable current transformer 4 breaker 11 _1, 11 ₂ first and second ground current transformer 12 monitoring device 21 _1, 21 ₂ first and second clamp-type current transformer C power cable L overhead lines E _1, E ₂ the first and second sheath grounding wire I _g fault current I _S sheath current i _L transmission line current transformer secondary current i _C cable current transformer secondary Current i _in input current i _g secondary fault current i _1S , i _2S first and second secondary sheath ground current i _3S secondary sheath ground differential current Δi _S secondary sheath differential current k coefficient n current transformation ratio S _LOCK Lock signal

Claims (4)

A sheath earth circuit monitoring device for monitoring whether or not a failure has occurred in a sheath earth circuit provided in a power cable (C) whose sheath is grounded at both ends,
Differential current detection means for detecting a differential current between a current flowing near the drawing point of the sheath ground wire (E ₁ ) from the power cable and a current flowing near the ground point of the sheath ground wire;
Monitoring means for monitoring whether or not a failure has occurred in the sheath ground circuit based on the difference current detected by the difference current detection means;
A sheath earth circuit monitoring device comprising: The differential current detection means is provided near the grounding point of the first ground current transformer (11 ₁ ) provided near the drawing point of the sheath ground wire from the power cable, and A second ground current transformer (11 ₂ ) differentially connected to the first ground current transformer;
The monitoring means determines a difference current (i _3S ) between a secondary output current (i _1S ) of the first earth current transformer and a secondary output current (i _2S ) of the second earth current transformer. When the value is equal to or greater than that, it is determined that "the sheath earth circuit has failed"
The sheath earth circuit monitoring apparatus according to claim 1, wherein: The differential current detecting means is provided in the vicinity of the first clamp-type current transformer (21 ₁ ) provided in the vicinity of the drawing point of the sheath ground wire from the power cable, and in the vicinity of the grounding point of the sheath ground wire. A second clamp-type current transformer (21 ₂ ),
The monitoring means obtains a difference current between the currents input from the first and second clamp-type current transformers, and when the obtained difference current exceeds a predetermined value, “the sheath earth circuit has a problem. "
The sheath earth circuit monitoring apparatus according to claim 1, wherein: 3. The predetermined value is set so that the monitoring means operates even in the case of an accident current (I _g ) at the time of an external ground fault when an adjacent substation is the fault point. Or the sheath earth circuit monitoring apparatus of 3 description.