In my country, most of the high-voltage cables use three-phase three-wire systems, while HVDC transmission uses two-phase Phase two-wire system. A three-phase system requires three wires for transmission because of three-phase power, while DC transmission requires only two wires for transmission because of two-phase power.

The home appliances we usually come into contact with are AC. This is due to the performance competition between AC and DC, and the survival of the fittest results. However, that doesn't mean DC is useless. In some long-distance transmission projects, DC still has a place.

High Voltage CableThree cables lead out the three-phase power from the step-up transformer, which can easily drive the three-phase motor to rotate, and can also provide three lighting circuits. The lighting circuit also needs a neutral wire. The shape connection is obtained, that is, the three-phase high voltage is connected to the head of the three-phase transformer winding, and the three tails are connected together to become the neutral line.

Domestic 10KV, 110KV, 220KV. The 500kV high-voltage transmission line has no neutral wire, because these voltage levels cannot be directly used for equipment, so our daily electricity use is actually a three-phase four-wire system, that is, three live wires and one neutral wire. The function of the neutral wire is to be the neutral wire, the high voltage cable is connected to the live wire to form a voltage, and then another function is to ground the neutral point of some equipment.

Another question is why power transformers need to be gas shielded. In fact, in grid transformers, differential protection and gas protection together constitute the main protection of the transformer. It protects the windings and bushings of the transformer to the current transformer side, and the differential protection belongs to the power protection.

In order to ensure that the electrical parameters of the High Voltage Cable stress tube meet the values ​​specified in the above standard, it can be Works reliably. In addition, attention should be paid to filling the air gap at the break of the insulating semiconducting layer of the cable with silicone grease to remove gas and reduce partial discharge. The cross-linked cable will shrink greatly due to poor internal stress treatment during operation. Therefore, when installing accessories, pay attention to the overlap between the stress tube and the insulation shield of not less than 20 mm. When the high-voltage cable prevents shrinkage, the stress tube and the insulation Shield separation. Due to the low elasticity of heat-shrinkable accessories, air gaps may appear at the interface during thermal expansion and cold contraction during operation, so sealing technology is critical to prevent moisture from entering.