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How are high-voltage cables grounded?

2022-09-29

Grounding of High-Voltage Three-Core Cables: For cables with voltage ratings of 35 kV and below, both ends must be grounded. This is because most of these cables are three-core designs. During normal operation, the total current flowing through the three cores remains zero, resulting in virtually no magnetic flux linkage outside the aluminum sheath or metallic shielding layer. Consequently, there is essentially no induced voltage at either end of the sheath or shielding layer. Therefore, grounding both ends ensures that any induced currents will not flow through the aluminum sheath or metal shielding.

　　1. Why do high-voltage cables need to be grounded?

　　1. Grounding of High-Voltage Three-Core Cables: For cables with voltage ratings of 35 kV and below, both ends must be grounded. This is because most of these cables are three-core designs. Under normal operating conditions, the total current flowing through the three cores remains zero, resulting in virtually no magnetic flux linkage outside the aluminum sheath or metallic shielding layer. Consequently, there is essentially no induced voltage at either end of the sheath or shielding. Therefore, grounding both ends ensures that any induced currents will not flow through the aluminum sheath or metal shielding.

　　2. Grounding of High-Voltage Single-Core Cables: When a cable is operating under load, an induced voltage develops across the shield layer. If both ends of the shield are grounded simultaneously, a loop forms between the shield and ground, leading to the flow of induced current. This, in turn, causes the cable shield to heat up, resulting in significant power losses and potentially disrupting the normal operation of the circuit. Moreover, during short-circuit faults, lightning strikes, or overvoltage events caused by switching operations, extremely high induced voltages can appear on the shield. Such conditions pose serious risks to personal safety and could even lead to insulation breakdown, compromising the integrity of the cable’s outer sheath. To prevent these issues, one common practice is to ground only one end of the cable; for longer cable runs, midpoint grounding combined with cross-interconnection may also be employed.

　　When manufacturers are producing cable terminations for high-voltage single-core cables, the steel armor and copper shield must be welded and grounded separately. This practice facilitates the inspection of the cable's inner sheath quality. During the sheath test, a voltage is applied between the steel armor and the copper shield. If the cable can withstand a specific voltage level, it confirms that the inner sheath is intact. However, if testing the inner sheath isn’t required, the steel armor and copper shield can be connected together and grounded—though it’s recommended to ground them via a separate dedicated path instead.

　　II. Several Methods for Grounding Single-Core Cables:

　　1. Direct grounding at both ends: This grounding method reduces workload but results in circulating currents in the metallic sheath, making it suitable only under stringent conditions. It requires short cable lengths and low transmission power—with significant power margins. In such cases, installing a cable sheath protector is unnecessary, further minimizing operational and maintenance efforts. However, since circulating currents still exist in the metallic sheath, this approach is generally not recommended.

　　2. The metal shield is directly grounded at one end, while the other end is grounded via a sheath protector. When the cable length is short (within 500 meters), the metallic sheath is typically grounded directly at one end, with the other end connected through the sheath protector. This grounding configuration prevents the formation of a ground loop, thereby reducing and eliminating circulating currents—conditions that help enhance the cable's transmission capacity and ensure safe operation. According to regulatory requirements, the induced voltage on the non-directly grounded portion of the metal sheath must not exceed 50V. If the cable is connected to an overhead line, direct grounding is usually installed at one end of the overhead line, while the protector is placed at the opposite end.

　　3. Cross-Interconnected Grounding: When cable lines are relatively long (typically over 1 km), the metallic sheaths of the cables can be installed in a cross-interconnected configuration. This involves dividing the cable line into three equal-length segments (with a deviation no greater than 5%), placing insulated joints between each segment. The metallic sheath is then routed from the coaxial cable insulation joint, passes through an interconnection box for cross-connecting, and finally connects to ground via a cable sheath protector. Meanwhile, the metallic sheaths at both ends of the cable are directly grounded, creating the interconnected segments. If the cable line is even longer, multiple interconnected segments can be linked together to form a multi-segment interconnection system.

　　After cross-bonding the grounding connections, the potential difference between the two grounding points becomes zero, eliminating the possibility of circulating currents in the cable sheath. At this point, the sheath experiences a relatively high induced voltage—specifically, the induced voltage determined by each short segment's length—which can be kept below 50V. The cross-bonding method effectively reduces both the induced voltage and circulating currents in the metallic sheath, thereby enhancing the cable's transmission capacity.

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