High voltage power supply and low voltage power supply system's differences

Differences between H.V system and L.V system

The first is the neutral point grounding mode. The 10kv high voltage system for EAF belongs to the neutral point ungrounded system, while the neutral point of the low-voltage 380V / 220V power supply system must be directly grounded; The second is the power supply mode. The 10kv High-voltage power supply system adopts three-phase three wire system, and the low-voltage 380V / 220V power supply system adopts three-phase four wire system; In addition, the main incoming cabinet in the 10kv High-voltage Distribution Cabinet usually adopts the lower incoming line, commonly known as the inverted fire, that is, "the knife is charged". 

At present, China mostly adopts three-phase four wire low-voltage power supply system, that is, 380V / 220V neutral directly grounded low-voltage power supply system. The power supply system has three live lines and one zero line. In the three-phase four wire low-voltage power supply system, it is not only the working ground wire, but also the protective zero wire. Now it is called pen wire, commonly known as "zero ground in one".

By the way, the direct grounding of neutral point at the secondary side of transformer is called working grounding, and its grounding resistance shall not be greater than 4 Ω according to the regulations. We know that 10kv High-voltage system adopts neutral ungrounded power supply system, so why does 380V / 220V low-voltage power supply system have to be grounded directly?

In the 380V / 220V low-voltage power supply system with neutral point directly grounded, because the neutral point is directly grounded, the voltage of any live line to the ground is 220V. If any live wire is grounded, it will cause a short circuit, which will cause the switch to trip or the fuse to fuse.

If the neutral point is not directly grounded, if there is a live wire grounded, because the neutral point is not directly grounded, it will not form a loop, and will not cause switch tripping or fuse fusing, thus electrifying the earth. At this time, the two ungrounded phase to ground voltages are not 220V, but 380V (equivalent to the line voltage between live wire and live wire). At this time, standing on the ground is equivalent to stepping on a live wire. If you accidentally encounter any live wire in the other two phases, you will bear 380V voltage, which is very dangerous.

Another significance of direct grounding of secondary neutral point is that it can reduce the insulation level of low-voltage electrical equipment. We know that the difference between the primary and secondary voltages of the distribution transformer is 25 times. If the neutral point is not directly grounded, once the insulation of the transformer is damaged and the primary high voltage is connected into the secondary low voltage, the low-voltage electrical equipment is difficult to bear such a high voltage, which is bound to cause insulation breakdown and burn down the low-voltage electrical equipment; If the insulation level of low-voltage electrical equipment is improved, the cost of electrical equipment will inevitably increase. If the neutral point is directly grounded, even if the insulation of the transformer is damaged and the primary high voltage is connected in series to the secondary low voltage side, the current can also flow to the earth through the neutral point grounding (working grounding), so as to reduce the insulation level of low-voltage electrical equipment.

10kV High voltage power supply system adopts neutral ungrounded system. Even if the neutral point is grounded through arc suppression coil, it still belongs to ungrounded system, that is, as long as it is not directly grounded, it is called ungrounded system. The use of ungrounded system for power supply can avoid the fault tripping caused by one phase grounding in the system, which will affect the normal operation of the power supply system.