What does it mean that the neutral point of 400v plant electricity is directly grounded

Zero direct grounding of transformers.

First: the upper left corner of Figure 1 is the low-voltage side winding of the power transformer, the first grounding point on the left side of the figure is the neutral point of the power transformer, and the second grounding point on the right is the repeated grounding in the switchgear.

We see that the transformer neutral is directly grounded. And, from the first grounding point, the transformer neutral point is separated into neutral N and ground PE, and they are insulated from each other from then on, not allowing to merge again.

It is noted that the potentials of the neutral and ground wires at the first grounding point are zero. We call this system grounding, and its task is to establish a zero-potential reference point for subsequent circuits.

It can be seen that the zero-potential reference point is important for the system.

Second: The instructions in Wang's book tell us that except for the first grounding point, the neutral line N and PE must not be merged again. If it merges again, it will be affected by stray currents.

Stray currents are single-phase ground fault currents that do not return to the first or second ground point along the PE line, causing system disturbances.

Third: We see that in the TN-S grounding system, the three-phase load needs to be connected to five wires, including three phase wires, neutral wire N and ground wire PE, and note that the PE wire is connected to the housing. Single-phase loads need to be connected to three wires, including phase, neutral N, and ground PE.

Fourth: If a single-phase ground fault occurs at the single-phase load, the leakage current will return to the neutral point of the transformer through the ground PE. Since the ground wire PE and the neutral wire N are connected together at the first grounding point, the value of the leakage current is about the same as the short circuit of the phase wire to the N line, if from the first grounding point, the neutral wire and the ground wire are combined, then this grounding system is called TN-C, and its IEC60364 original diagram is as follows:

Figure 3: Original diagram of the TN-C grounding system for the IEC60364.

Note that the neutral n and ground PE in Figure 3 are merged, and its symbol is pen, which is the zero line.

For TN-S grounding system, if the N line and the PE line are merged again, some of the shells of the electrical equipment are connected to the PE line, and some are connected to the PEN neutral line, and the PEN neutral line is currently flowing through during normal operation, and due to the line resistance, there is a certain voltage in the neutral line, although it is very small. As a result, the housing of the consumer will carry a voltage with a small value.

If the neutral line is broken, the neutral line at the rear of the breaking point will carry a higher voltage due to the three-phase unbalance, and in extreme cases, this voltage is equal to the phase voltage, and the shell of the electrical equipment is connected by the rear neutral wire, which can be imagined how much impact it will have on the human body.

Therefore, in the TN-S grounding system, once the ground PE is separated from the neutral N, it is absolutely not allowed to merge again.

Hello: 1 This 35kV (read as 35 kV is 35000 volts) refers to the voltage level.

2. The transformer capacity is generally measured in kva (kilovolt ampere).

3. It is not necessarily an incoming line, for example, changing the voltage level of 35kV to 10kV level is an incoming line; That 110kv becomes 35kv, and this is the qualification.

4 China generally to the final user (such as motors and lighting) voltage of 400 220V, the transformer used for 10kV to become 400 220V, that is, the incoming line 10kV (triangle connection deliberately not grounded, safer), outgoing star connection, neutral wire lead must be grounded, as the zero line).

5. Imagine if the 4 medium and high voltage (10kV) and low voltage (400 220V) are star-connected on both sides and both neutral wires are grounded, and the phase wire grounding fault occurs on either side, which seriously affects the safety of both sides.

6 Therefore, the 400 220V side must be grounded, and the other side is deliberately arranged not to be grounded.

The key depends on what kind of connection your output is? If it's a star connection. The neutral point must be grounded. If it's triangulated. This is not required.

The neutral grounding of generators and transformers is called protective grounding, and its main function is to strengthen the stability of the potential of the low-voltage system and reduce the risk of overvoltage due to one-phase grounding, high-low voltage short circuit, etc.

The system of 110kV and above in our country generally adopts the neutral point direct grounding system (i.e., the high-current grounding system).

35kV and 10kV systems generally use neutral point non-grounding system or large impedance grounding system (i.e., small current grounding system).

The 380V 220V low-voltage power distribution system can be divided into IT system, TT system and TN system according to the different forms of protective grounding.

The neutral point of the power supply of the IT system is insulated from the ground or grounded with high impedance, while the metal casing of the electrical equipment is directly grounded. That is, in the past, it was called the protective grounding of the three-phase three-wire power supply system.

The neutral point of the power supply of the TT system is directly grounded; The metal casing of the electrical equipment is also directly grounded and has nothing to do with the grounding of the neutral point of the power supply. That is, the protective grounding in the three-phase four-wire power supply system in the past.

TN system, in the 380 220V three-phase four-wire low-voltage power grid where the neutral point of the transformer or generator is directly grounded, the metal shell of the electrical equipment that is not charged during normal operation is directly electrically connected to the neutral point of the power supply through the common protection line. That is, the protection zero connection in the past three-phase four-wire power supply system.

The neutral point of the power supply of the TN system is directly connected to ground, and there is a neutral lead out. According to the form of its protection line, the TN system is divided into three types: TN-C system, TN-S system and TN-C-S system.

1) TN-C system (three-phase four-wire system), the neutral wire (N) and the protection wire (PE) of the system are integrated, and the line is also called the protection neutral wire (PEN) line. It has the advantage of saving one wire, and the disadvantage is that the three-phase load unbalanced or when the protective neutral wire is disconnected, the metal casing of all electrical equipment carries a dangerous voltage.

2) TN-S system is a three-phase five-wire system, the N line and PE line of the system are separated, and the five-wire power supply is used from the transformer. It has the advantage that there is no current passing through the PE line under normal conditions, so there is no electromagnetic interference to other devices connected to the PE line. In addition, since the N-line is separated from the PE line, the disconnection of the N-line will not affect the protection of the PE line.

TN-C-S system (three-phase four-wire and three-phase five-wire hybrid system), the system is from the transformer to the user's distribution box type four-wire system, the neutral wire and the protective ground wire are integrated; From the distribution box to the user's neutral wire and protective ground wire is separated, so it has the characteristics of both TN-C system and TN-S system, and is often used in places with poor environment at the end of the distribution system or strict requirements for electromagnetic anti-interference.

The neutral grounding on the high voltage side of the main transformer of the power plant is to form a "high-current grounding system", and the neutral grounding can prevent the neutral potential displacement and the overvoltage caused by single-phase grounding.

The neutral grounding of the low-voltage side of the user's transformer is to maintain the zero potential of the neutral line (neutral point), which can avoid the unbalance of the three-phase voltage and prevent overvoltage when the three-phase load is unbalanced.

The neutral point direct grounding belongs to the high-current grounding system, the equipment allows single-phase jumping single-phase reclosing, so the insulation of the equipment can be considered according to the phase voltage, saving investment, because more than 80% of the faults of long-distance transmission are mostly single-phase faults, and more than 80% of single-phase faults are instantaneous faults, so the single-phase jump-off single-phase overlap is allowed to improve the reliability of power supply.

If the high voltage side of this transformer is 35kV, the low voltage side is 400 230V.

In our country, 35kV is not grounded as the center point of the system, and the god regrets not requiring its center to be grounded at the spring point. The 400 230V is a direct grounding system for the center point of the blind hand, and its center point requires grounding.