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Grounding principle of shield control cable

Information Source:Henan Huadong Cable Co., Ltd  Release date:2014-03-05

Poor balance characteristics of shielded cable, so a good shielding integrity and good grounding of the shielding cable is very important. Shield grounding to prevent electrical equipment due to electromagnetic interference, grounding shielding device and its impact on the job or other equipment caused by electromagnetic interference.

The control cable with shielding layer, and the shielding layer in the switching field and control room at both ends of earth, is the international general an effective secondary circuit two measures against electromagnetic interference. By the IEEE substation of the working group and the relay Environment Committee Working Group proposed "substation control and the selection and installation of" low voltage cable system file, there is a special "metal shielding control cable can reduce the induction of transient voltage" a related issue: "the shield at both ends of the ground is recommended control cable with shielding. We must keep the integrity of the special shielding, broken or separate shielding will greatly reduce the shielding efficiency; if the shield grounded at one end, grounding terminal in non wrapping on the possible transient voltage high appear." Control has the advantages of both ends of the cable shielding layer grounding is:

When the control cable for flux bus transient current generated by the layer will be induced, shielding current shielded cable, the flux is produced by the effect of shielding current, flux will link transient current offset the core cable. Assume that the shielding effect ideal, a combination of the two, will cause by the flux core cable shielding layer completely surrounded in zero, shielding layer formed an ideal Faraday cage. This is the same with two short circuited coil of transformer core flux in the ideal, will be zero. Of course, the shielding effect of shielding layer, due to various reasons, can not be completely ideal, therefore, is the core wire shielding in busbar transient current role, will still induces a certain voltage.

The shield grounded at both ends, can reduce the induction voltage transient ground potential rise produced.

When the lightning arrester grounding through injection, the substation grounding grid in the impact current increases, potential fluctuation will cause transient, while network considered in grounding resistance will rise temporarily. The determination results of substation grounding potential rise that, compared with the normal AC resistance, resistance to increase by more than 10 times. When the low voltage control cables in the ground potential rise of cable laying nearby, potential fluctuation interference. Therefore, the grounding surge current into the ground potential rise will be possible on the low voltage control circuit insulation cooperation brings the serious influence.

In order to quantitatively estimate the quantity injected when the lightning transient induction caused in the control cable core in the net when, in 30 substations in artificial injection grid small impulse current (100 ~ 4000A) when the measured voltage. Transient voltage of two kinds of cable shielding case determined, a cable is no metal shield, two is a metal shielding and grounding cable ends. Experiments prove that shielded cable with two grounded, transient voltage induced inhibition can be below 10% of the original value, is an effective measure to reduce the disturbance voltage.

Therefore: shielding layer shielded cable has two grounding mode, namely the grounded at both ends and connected to ground. One end of the ground, shielding layer voltage is zero, can significantly reduce the electrostatic induction voltage; grounded at both ends of the electromagnetic induction to generate an induction longitudinal currents in the shielding layer, the current generates an opposite the main interference two field, offset the trunk around the field, significantly reduce the magnetic field coupling induction voltage, can the induction voltage drop to less ground induction voltage below 1%.

Shield grounded at both ends of the following two questions of shielded cable:

When the grounding network short circuit current and lightning current, because the potential of two different cable shielding layer, so that the current flowing through the shielding layer, will cause the impact or disturbance voltage extra.

When the current through the shielding layer, will produce interference signals on each core wire, but the influence of interference signals cable core where the loop for the electrical circuit and shielding layer currents smaller.

Making but for application in relay protection and automatic device circuit shielded cable, as its input and output are at one end of the high voltage switch field or ultra high pressure environment, electromagnetic induction interference is the main contradiction, prevent transient over voltage, the relay protection and automatic device for shielding layer should be specified at both ends of the earth.

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