JPS605127B2 - Power cable shield grounding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a shield grounding system that prevents accidents such as fire by suppressing heat generation at the cut point when the shield of a power cable is cut.

Generally, power cables are equipped with a metal shield made of copper, aluminum, lead, etc. to protect the inside from the outside world, and one end of the metal shield is grounded. A charging current flows due to the capacitance between the cables, so if the metal shield is cut due to a cable accident, Joule heat will be generated at the cut point.

On the other hand, metal tape is often used as a shielding material in order not to increase the diameter of the cable, but metal tape has low mechanical strength and is easily cut when external force is applied to the cable. The heat generated by cutting the metal shield is regulated to be lower than the allowable heat amount of the cable if the charging current flowing through the shield and the resistance value at the cutting point are small, but if the charging current is large such as in a high-power cable, Alternatively, if the resistance value at the point where the shield is cut is large, this may lead to insulation breakdown of the cable and even cause a cable fire.

Therefore, by dividing the shield into several parts and grounding each shield, the charging current flowing through the shield can be reduced, thereby reducing the amount of heat generated when the shield is disconnected.Alternatively, a protective continuator can be installed to prevent damage caused by insulation breakdown of the cable. A method that detects short circuits is considered as a countermeasure, but the former method has the problem that cable installation work is labor-intensive and expensive, while the latter method cannot detect shield disconnection and may cause insulation breakdown. Since the protection relay is activated only when this happens, it is not a satisfactory measure as a countermeasure when the shield is cut. Note that the charging current flowing through the cut point when the shield is cut can be avoided by grounding both ends of the shield, but if both ends are grounded, an electromagnetic induced voltage will be induced in the shield by the current flowing through the cable conductor. Problems arise with circulating currents flowing and simply connecting any resistor to the open end of the shield in a high power cable.

Although these methods can prevent insulation breakdown and abnormal voltage generation, they are not useful in preventing shield fires due to Joule heat generated by shield cutting, so these two-end grounding methods were not generally adopted. The present invention has been made in view of the above, and an object of the present invention is to minimize the heat generated by the current flowing through the cut point particularly when the shield is cut in a power cable, thereby preventing accidents such as insulation breakdown of the cable and fire. shall be. Therefore, the present invention adopts a method of grounding both ends of the shield to reduce the charging current flowing through the shield cut point, and at the same time, by grounding one end of the shield and grounding the other end of the shield through an appropriate resistor, This minimizes heat generation at the point where the shield is cut.

Hereinafter, a shield grounding system for power cables according to the present invention will be explained based on the accompanying drawings.

FIG. 1 is an equivalent circuit diagram when the shield 3 is cut in a power cable having a conductor 1, an insulator 2 having a capacitance C, and a shield 3 with one end grounded.According to the present invention, the shield The other end (open end) of 3 is grounded via a resistor Rs.

Therefore, both ends of the shield 3 are grounded, and an electromagnetic induction voltage is induced in the shield 3 as described above, causing a circulating current to flow.In this case, by setting the value of the resistance Rs as follows, the shield 3 can be grounded. Joule heat generated at the cutting location can be minimized. That is, as shown in FIG. 1, if the resistance at the cut point of the shield 3 is Rx, and the current flowing through Rx is lx, the Joule heat Wx is expressed by the following equation. Wx=lx2RX......[1'lx=
10 lc (for fail-safe considerations, 1 and lc' are assumed to be in the same phase)......■Here, 1 is the circulating current due to electromagnetic transfer, and the induced electromotive force is expressed as e. if,,
e 1=Wisteria rain......Represented as '3'.

However, shield resistance and ground resistance are ignored. In addition, lc is the shunt of the charging current due to capacitance, and if the total charging current is lco, if the disconnection of the shield 3 occurs near the grounding end opposite to the connection side of Rs, Rs lc = lcoXR It is expressed as Keidanu......{4).

Note that the shield 3 is cut at the middle part or R of the shield 3.
If the charging current occurs near the connection side of s, most of the charging current flows to the opposite side, so it can be set to lc〒0. Furthermore, in the above equation (2), the circulating current 1 and the charging current shunt lc
The reason for making these in phase is that the sources of the circulating current 1 and the shunt lc of the charging current are originally independent, but the reason why the Joule heat Wx generated at the cut point is maximum is due to the resistance Rx of the cut point. This is because the current lx flowing in the current lx reaches its maximum, that is, it reaches its maximum when the circulating current 1 and the branched portion lc of the charging current are assumed to be in phase. {Type 11 has legs and [
4}, then WX = (10) 2RX = (Tomigaue Judo Manaken) 2RX……”■ and Wx becomes Rx:Rs
It is maximum when .

The maximum value Wm is set as Rx=Rs in the above formula, then wm=
(Exchange + Academic) 2RS = (e + ICOR Takashi)
- Solving for the mountain RSRs gives the following, but since Rs is a real number, Wm and eIC〇,,...,. ,,...must be {8}.

That is, the minimum value of Wm is eICo, and Rs in that case is.

Therefore, in order to minimize the maximum amount of heat generation Wm at the cut point of the shield 3, Rs = e/lco, but the value of e depends on the current flowing through the conductor 1 (current flowing through the cable). Therefore, when the current flowing through conductor 1 changes, the maximum value of the electromagnetic induction voltage is emax, and (
If we set Rs=emax/lco in formula 61, then wm=
e 3, 3, 3 (e dot intersection + 1) 2≦em fence IC...
・・・．・Wm will never exceed ema phantom co.

Therefore, the value of the resistor Rs to be connected to the open end of the shield 3 is the maximum powder voltage emax. due to electromagnetic induction. ．． 、． ．． ，
．． ．． ．． ooRS=set to total charging current ICo due to capacitance.

In the present invention, as described above, one end of the shield 3 is grounded and the end of the shield is grounded via the resistor Rs, thereby minimizing heat generation at the point where the shield is cut. , conductors la and lb, lc and ld, le and l through which common-mode current flows, as shown in Figure 2.
The open ends of the shields 3a and 3b, 3c and 3d, 3e and 3f of
As shown in Figure 3, a new ground wire (N wire, etc.) 4 is laid along the cable from the ground end of the shield 3, and the open end of the ground wire 4 is connected to the open end of the shield 3 with a resistor Rs. When connected, in either case, the electromagnetic induced voltage appearing on the shield is canceled out by the electromagnetic yield voltage appearing on the other in-phase shield (in the case of Figure 2) or the newly laid ground wire (in the case of Figure 3). Therefore, only a slight voltage due to unbalance appears between both terminals of the resistor Rs.

Therefore, it is possible to reduce the electromagnetic induction voltage e and form a circuit in the '51-Tane type, and it is possible to further reduce the amount of heat generation Wx at the shield cutting location. As described above, according to the present invention, in a power cable in which one end of the shield 3 is grounded, the other end of the shield 3 is grounded via the resistor Rs. In addition to being able to suppress
Accidents caused by insulation breakdown can be prevented. On the other hand, when the shield 3 is cut when an external force is applied to the power cable for some reason, the charging current flowing through the cut point (resistance Rx) is reduced, and the electrostatic conduction current and both ends of the shield 3 are grounded. Heat generation at the point where the shield is cut due to the circulating current caused by this can be suppressed to a minimum by appropriately selecting the value of the resistor Rs, so even if the shield 3 is cut, insulation breakdown of the cable and fire can be prevented. .

[Brief explanation of the drawing]

FIG. 1 shows an equivalent circuit when the shield of a power cable is cut. FIG. 2 shows an embodiment in which the present invention is applied to an in-phase multi-strand cable. FIG. 3 shows an embodiment in which the present invention is applied to a single-phase, single-strand cable. 1...
Shield conductor, 2... Insulator, 3... Shield, 4... Ground wire. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. Determine the maximum induced voltage emax induced in the shield 3 due to a change in the current flowing through the conductor 1 of the power cable, and the total charging current Ico flowing through the shield 3 due to the capacitance of the insulator 2. A power cable shield grounding system characterized in that one end of the shield 3 is grounded and the other end is grounded via a resistor Rs of (maximum induced voltage emax)/(total charging current Ico due to capacitance). 2. Shield grounding of a power cable according to claim 1, wherein one end of a plurality of conductors through which in-phase current flows in the in-phase multi-stranded cable is grounded and the other end of the shield is connected via a resistor Rs. method. 3. A patent claim characterized in that in a single-phase single-strand cable, a grounding wire is laid along the cable from the grounding end of the shield, and the open end of the grounding wire is connected to the open end of the shield via a resistor Rs. Scope: Shield grounding type of power cable as described in item 1.