Single-core cables are widely used in high-voltage power systems, but their grounding methods have special requirements. Direct grounding of both ends will cause the following key problems:
1. Circulation issues
Close circuit formation: When both ends are directly grounded, the metal shielding layer of the cable forms a closed circuit with the ground, and the alternating magnetic field generated by the wire core current will induce the circulation on the shielding layer. According to actual measurement, this circulation can reach 30%-80% of the wire core current, causing significant electrical energy loss.
Thermal effect and current carrying capacity decrease: The circulation will cause the cable to heat up, which not only reduces the actual current carrying capacity, but may also cause the temperature of the insulation layer to rise and accelerate insulation aging.
2. Induction voltage risk
High voltage safety hazard: The wire core of a single-core cable is similar to the transformer windings and the shielding layer, and the induced voltage is proportional to the cable length and current. In the event of a short circuit or lightning impact, the voltage of the ungrounded terminal may rise to twice the system voltage, threatening personal safety and possibly breaking through the outer sheath.
Influence of ground potential difference: When there is potential difference between different ground points, additional current will be generated in the grounding wire, disturbing the operation of the system.
3. Damage to insulation and protection systems
Insulation accelerates aging: Circulation and overheating can damage the molecular structure of the insulation material and shorten the cable life.
Misaction of the protection device: Grounding both ends may cause the relay protection system to misjudge the fault position, resulting in protection failure.
4. Alternative grounding solution
To avoid the above problems, the project is often used:
Single-point grounding: Only one end is directly grounded, and the other end is grounded through a protective cover.
Cross interconnection grounding: Cross-connect the metal sheath after the long cable is segmented to offset the induced voltage (suitable for cables of 66kV and above).
Midpoint Grounding: The midpoint of the metal shield is directly grounded, and both ends are grounded by the protector.
Exception: Short cables (such as length <100 meters) or small load cables can be grounded at both ends because of the small induction voltage and circulation.