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Grounding methods for underground high voltage cables

The electrical safety regulations stipulate that all non-charged metal shells of electrical equipment must be grounded, so the aluminum cladding or metal shielding layer of the cable must be grounded. Usually, cables with voltage levels of 35kV and below are grounded at both ends. This is because most of these cables are three core cables. In normal operation, the total current flowing through the three cores is zero, and there is basically no magnetic flux outside the aluminum or metal shielding layer. Therefore, there is basically no induced voltage at both ends of the aluminum or metal shielding layer, so there will be no induced current flowing through the aluminum or metal shielding layer after grounding at both ends. However, when the voltage exceeds 35kV, most underground power cables are in a single core conductor. The relationship between the core of an underground high voltage cable and the metal shield can be seen as the primary winding of a transformer. When the conductor of this underground high voltage cable passes through an electric current, there will be a magnetic field wire cross-linked with aluminum or metal shielding layer, causing induced voltage to appear at both ends. The magnitude of the induced voltage is proportional to the length of the cable line and the current flowing through the conductor. When this underground power cable is very long, the induced voltage on the sheath can be combined to endanger personal safety. In the event of a short circuit fault, operating overvoltage, or lightning impact on the line, a high induced voltage will form on the shield, and may even break through the insulation of the sheath. At this point, if the two ends of the aluminum package or metal shielding layer are still interconnected and grounded, there will be a large circulating current in the aluminum package or metal shielding layer, which can reach 50% -95% of the core current, forming losses and causing the aluminum package or metal shielding layer to heat up. This not only wastes a lot of electricity, but also reduces the current carrying capacity of the cable and accelerates the insulation aging of the cable. Therefore, an underground high voltage cable should not be grounded at both ends. Except for individual cases, such as short cables or light load operation, the aluminum cladding or metal shielding layer can only be interconnected and grounded at both ends of the three-phase connection.

However, when one end of the aluminum package or metal shielding layer is not grounded, the following problems arise: when lightning current or overvoltage waves flow along the core, the ungrounded end of the aluminum package or metal shielding layer of the cable will experience a high impulse voltage; When a short circuit occurs in the system, when the short-circuit current flows through the wire core, the ungrounded end of the aluminum or metal shielding layer of the cable will also experience a high power frequency induced voltage. When the insulation of the cable outer protective layer cannot withstand this overvoltage and is damaged, it will lead to multiple grounding points and form a loop current. Therefore, when using one end interconnected grounding, measures must be taken to limit overvoltage on the protective layer. During installation, special connection and grounding methods should be adopted at a certain position of the aluminum clad or metal shielding layer according to the economic and reasonable principles of the line, and protective layer protectors should be installed at the same time to prevent the insulation of the cable protective layer from being broken down.

Based on this, during the installation of underground high voltage cables, according to relevant standards, when only one point of the metal sheath of a single core cable line is grounded, the induced voltage at any point of the metal sheath should not exceed 50-100V (if no safety measures are taken to prevent arbitrary contact with the metal sheath, it should not exceed 50V; if effective measures are taken, it should not exceed 100V), and should be insulated to the ground. If the voltage is greater than this specified value, metal sheaths should be used for segmented insulation or connected to interconnected wiring after insulation. In order to reduce the induced voltage of single core cable lines on adjacent auxiliary and communication cables, AC * interconnection wiring should be used as much as possible. For cases where the cable length is not long, a single point grounding method can be used. To protect the insulation of the cable sheath, a sheath protector should be installed at the ungrounded end.

It can be seen that there are several grounding methods for underground high voltage cables:

1. One end of the protective layer is directly grounded, and the other end is protected and grounded through the protective layer – methods can be used;
2. The midpoint of the protective layer is directly grounded, and the shielding at both ends is grounded through the protective layer – commonly used method;
3. Interconnection of protective layers – commonly used methods;
4. Cable transposition and metal sheath interconnection – the most effective grounding method;
5. Grounding at both ends of the sheath – not commonly used, only suitable for extremely short cables and small load cable lines.