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Cause Analysis of Abnormal Grounding Circulation of High Voltage Cable
Release time:
2021-12-15
The cable above 10 kV adopts single-core structure, and the metal protective layer produces grounding circulation. If the circulating current exceeds the standard (the circulating current value is above 50A or 20% of the load current or the maximum/minimum value between phases is above 3), it will not only affect the current carrying capacity and service life of the cable, but also the alternating magnetic field generated by the working current will generate induced potential in the metal protective layer. If the sheath passes through the earth to form a channel, it may also cause malignant power grid accidents. Serious heating caused by circulating current will also burn the grounding wire or grounding, let's understand the cause analysis of the abnormal grounding circulation of the lower high-voltage cable!
The cable above 10 kV adopts single-core structure, and the metal protective layer produces grounding circulation. If the circulating current exceeds the standard (the circulating current value is above 50A or 20% of the load current or the maximum/minimum value between phases is above 3), it will not only affect the current carrying capacity and service life of the cable, but also the alternating magnetic field generated by the working current will generate induced potential in the metal protective layer. If the sheath passes through the earth to form a channel, it may also cause malignant power grid accidents. Serious heating caused by circulating current will also burn the grounding wire or grounding, let's understand the cause analysis of the abnormal grounding circulation of the lower high-voltage cable!
The main factors affecting the grounding circulation of high-voltage cables are as follows.
1. Contact resistance of high voltage cable
If there is poor welding or poor contact, the contact resistance of the phase increases, with the increase of resistance, the grounding circulation of the phase will be significantly smaller, but the grounding circulation of the other two phases does not necessarily become smaller. The total ground current is also not necessarily reduced.
2. Grounding resistance
As the sum of the ground resistance and the ground circuit resistance increases, heating and losses are caused, and the circulating current of each ground is reduced. However, the ground resistance is too large can lead to poor contact at the ground point.
3. Grounding mode of high voltage cable
In order to limit the induced potential of the metal protective layer of high-voltage cables, for long high-voltage cable lines, high-voltage cables usually use one end of the sheath or shielding layer to ground, both ends to ground, cross interconnection and other grounding methods. It is the cross-interconnection grounding method that can effectively limit the grounding circulation.
Here, Ia, Ib and Ic are the current values flowing through the metal sheaths of A, B and C three-phase high voltage cables respectively. Ie is the current value passing through the earth circuit. Generally, the operating current values of the three-phase cable can be consistent by default. Rd is the equivalent resistance of the earth circuit through the phase difference between the three-phase currents, and Rd1 and Rd2 are the grounding resistances at both ends of the cable sheath; the voltages induced by the metal protection layer of the cable in the fully cross-connected section also cancel each other to reduce the ground circulation current.
4. Length of each cable section, arrangement of cables, inter-phase distance, etc.
High-voltage cables generally use cross-connected grounding methods to reduce grounding circulation, but in the engineering practice of cable pipeline laying, the cross-connected sections of the sheath often have different lengths and different arrangements. This is because, therefore, in the unequal-length cable, the long cable adopts a triangular arrangement with a small induced voltage, and in the horizontal or vertical arrangement of the unit length cable under the same core current, the induced voltage of the metal sheath is greater than the induced voltage of the right-angled triangular arrangement. The horizontal or vertical arrangement of short cables with large induced voltage is beneficial to reduce the sheath induced voltage of large sections, and the arrangement of each section should be appropriately selected.
The above is the analysis of the causes of abnormal grounding circulation of high-voltage cables. If you need to know more, please feel free to contact us!
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