How should the grounding welding of high voltage power cables be done hope for details ?

There are two ways to ground the cable shield, namely two-point grounding and one-point grounding. From the point of view of preventing transient overvoltage, it is better to use two-point grounding for the shielding layer. The two-point grounding causes electromagnetic induction to produce an induced longitudinal current on the shield, which generates a secondary field opposite to the main interference, cancels out the action of the main interference field, and reduces the interference voltage.

However, there are two problems with two-point grounding: first, when there is a short-circuit current or lightning current on the grounding network, due to the different potentials of the two points of the cable shield, the current flows in the shield, and the shield may be burned. Second, when an electric current flows through the shield, an interference signal will be generated for each core.

For relay protection and automatic devices, because one end of its input and output is in the high-voltage or ultra-high-voltage environment of the switching field, electromagnetic induction interference is the main contradiction, and the circuit where the cable core is located is a strong current circuit, so the interference signal generated by the shield current has little impact, so the relay protection and automatic device regulations stipulate that the shielding layer should be grounded at both ends; For thermal professional cables, the contradiction is not prominent in the comparison of electromagnetic induction interference, and the shielding current generated by the two-point grounding may interfere with the core wire, which may make the device malfunction, so it is advisable to use a point grounding. Therefore, there is no contradiction between the two-point grounding specified in relay protection and automatic devices and the one-point grounding specified in thermal engineering. Ground both ends of the shield of the control cable.

The ability of the shield to reduce the induced overvoltage is mainly based on the cancellation effect of the magnetic field generated by the shield current on the magnetic field generated by the interfering current. The grounding of both ends of the shield is because when the short-circuit current and lightning current pass, it is not easy to burn the shielding layer because the action time of the large short-circuit current and lightning current is very short. If one end of the shield is grounded, there is no current loop, but its ability to prevent overvoltage and anti-interference is very low, so the shield cannot achieve a good shielding effect.

Rectification measures: First, the control cable is shielded, and the shielding layer is grounded at the same time in the switch field and the control room, and the shielding layer of the communication cable should also be connected correctly and reliably; Second, lay special grounding copper bars for secondary equipment and secondary cables to eliminate ground potential difference interference as much as possible; Third, all switching input and output contacts of the substation are specially photoisolated. The induced current flowing in the shield is generated by the induction of the external electromagnetic field, and its actual function is to cancel out the interference of the external electromagnetic field.

Therefore, the two ends of the cable shield are grounded, which can effectively suppress electromagnetic induction. An ungrounded shield has no shielding effect on the electric field, while a shield that is grounded at one end and grounded at both ends has the same shielding effect on the electric field. If the shield is well grounded, the electric field terminates at the shield and couples directly to ground.

The metal shield of the shielded cable has an electrostatic shielding effect, so that the strong power line of the primary high-voltage power supply terminates in the metal shield, and the internal electric field strength is zero, so that the core wire in the shielding layer is protected from the interference of the external strong electric field. From the point of view of electrostatic shielding, in order for the surface of the shield to be a fixed equipotential surface, one end of the shield should be grounded.