The current 10kV, 110kV, 220kV, 500kV high-voltage transmission lines in China (the State grid already has 1000kV) have no zero lines, because these voltage levels cannot be directly accepted by the equipment (except a few ultra-high-voltage equipment).But we usually use more 3-phase 4-wire system (TN-C system), 3 fire lines +1 zero line.The main functions of the zero line are as follows:
1. Neutral line (N line) is connected with the fire line to form a phase voltage.
2. Act as neutral ground (working ground) for some operating equipment.
3. Connect with the equipment shell for protection (P line).However, these voltage levels above 10kV are not required. There are two overhead zero lines (or overhead lightning conductor and overhead ground wire) above the transmission lines above 110kV, which play the role of lightning protection (preventing lightning waves).So the daily high - pressure line is not zero line.Generally, there is one set of protection for 110kV, while more than 220kV requires two sets of protection with different principles and from different manufacturers. Fiber longitudinal difference and high-frequency protection are widely used.Tripping will occur when a phase is connected to the ground, because the lines have reclosing (single weight, 3 weight, and total weight), no coincidence will be performed after the fault is judged to be permanent.So: short circuit - coincidence - tripping.As for the large and small current grounding system, the large current grounding system refers to the neutral direct grounding system, such as our 3-phase 4-wire system, because the ground current will be relatively large in case of fault.Small current grounding system includes: neutral ungrounded system, neutral grounded system by arc suppression coil, neutral grounded system by large resistance.The ground current is low when a fault occurs.