How to ground high voltage cables?

Published on:

2022-09-29

High-voltage three-core cable grounding: voltage levels of 35kV and below are grounded at both ends of the cable. This is because most of these cables are three-core cables. In normal operation, the total current flowing through the three cores is zero, there is basically no magnetic chain outside the aluminum cladding or metal shield, and there is basically no induced voltage at both ends of the aluminum cladding and metal shield, so the induced current will not flow through the aluminum cladding or metal shield after grounding at both ends.

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

1. High-voltage three-core cable grounding: voltage levels of 35kV and below are grounded at both ends of the cable. This is because most of these cables are three-core cables. In normal operation, the total current flowing through the three cores is zero, there is basically no magnetic chain outside the aluminum cladding or metal shield, and there is basically no induced voltage at both ends of the aluminum cladding and metal shield, so the induced current will not flow through the aluminum cladding or metal shield after grounding at both ends.

2. Grounding of high-voltage single-core cable: When the cable is running with a load, an induced voltage will be formed on the shield. If both ends of the shield are grounded at the same time, a circuit will be formed between the shield and ground, and an induced current will be generated. Thus, the cable shield will generate heat and lose a lot of power, affecting the normal operation of the line. When a short-circuit fault occurs in the cable line, when it suffers a lightning shock or an operating overvoltage, a very high induced voltage will form on the shield. This endangers personal safety and may even puncture the cable's outer sheath. To avoid this phenomenon, grounding at one end is usually used, or when the line is long, midpoint grounding and cross interconnection can be used.

When manufacturers make cable heads for high-voltage single-core cables, the steel armor and copper shield must be welded and grounded separately. The purpose is to facilitate the detection of the quality of the sheath inside the cable. When testing the cable sheath, apply voltage between the steel armor and copper shield. If it can withstand a certain voltage, the inner sheath is proved to be in good condition. If you don't need to test the inner sheath of the cable, the steel armor and copper shield can be connected together and grounded (it is recommended to lead out separately and then grounded).

Second, the single-core cable grounding several methods:

1. Direct grounding at both ends: This grounding method can reduce the workload, but there is a circulating current on the metal sheath, the applicable conditions are harsh. It requires a short cable line, small transmission power, transmission power has a large margin. In this case, there is no need to install cable sheath protector, which can reduce the operation and maintenance workload. However, there is circulating current on the metal sheath, so it is not recommended to use this method.

2. Metal shield is directly grounded at one end and the other end is grounded through the sheath protector: When the cable line is short (within 500m), the metal sheath is generally grounded directly at one end of the sheath and the other end is grounded through the sheath protector. Grounding insulation does not form a circuit, which can reduce and eliminate the loop current, which is conducive to improving the transmission capacity of the cable and the safe operation of the cable. According to the code requirements, the induced voltage of the metal sheath part not directly grounded shall not exceed 50 V. If connected to the overhead line, direct grounding is generally installed at one end of the overhead line, and the protector is installed at the other end.

3. Cross interconnection ground: When the cable line is long (generally greater than 1km), the metal sheath of the cable can be cross interconnected installation. Cross interconnection is to divide the cable line into three equal sections (deviation not more than 5%) and install insulation joints between each section. The metal sheath is led from the coaxial cable insulation joint, crossed and interconnected through the interconnection box, and then grounded through the cable sheath protector, and the metal sheath at both ends of the cable is grounded directly to form the interconnected section. If the cable line is long, a multi-segment interconnection can be formed by connecting several interconnecting segments.

After grounding through the cross interconnection, the potential difference between the two grounding points is zero, so it is not possible to generate circulating current in the sheath. At this point, the line is very high sheath voltage, i.e., the induced voltage determined according to the length of each small section, which can be limited to 50 V. The cross-interconnection method can reduce the induced voltage and circulation of the metal sheath, which helps to increase the transmission capacity of the cable.

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图片ALT信息： Yunnan Qianlie Cable Co., Ltd.

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