Let's look at the picture below:
This is a residential area with many homes. Therefore, there are total distribution rooms and primary distribution equipment in the community, as well as intermediate secondary distribution equipment, as well as electricity meter boxes in the home. After passing through these links, the electrical energy is finally connected to our home and connected to the distribution box in our home to complete the home distribution work.
The following figure is a schematic diagram of a residential power distribution system:
This picture is relatively large, let's take a closer look:
On the left side of the figure is the primary distribution equipment. We saw a transformer T whose task was to change the medium voltage of 10kV to a low voltage of 400V/230V.
Note: We call the voltage on the low voltage side of the transformer 400V/230V the system voltage, which is the maximum value of the low voltage distribution system. 400V is the line voltage between the three phases, and 230V is the phase voltage of each relative neutral line, which differs by 1.732 times.
The primary distribution equipment is also called the first-level low-voltage switchgear. It has an incoming circuit, a system bus, and many feeder circuits. Three-pole circuit breakers are generally used for incoming and outgoing lines.
Please pay special attention:
We found that the neutral point of the low-voltage side three-phase winding of the transformer was grounded and then led to the busbar of the primary distribution equipment. This grounding is called working ground or system grounding, and its purpose is to establish ground potential or zero potential.
The wire drawn from the system ground point is called the PEN line, which is the zero line. The three phase lines are sometimes referred to as three-phase live lines, and the three phase lines are labeled L1, L2, and L3.
Such a grounding system is called TN-C according to IEC and national standards. The corresponding standard numbers are: IEC60364-1 and GB16895.1.
Let's continue to look at:
A four-core cable is drawn from a feeder circuit of the primary distribution equipment to deliver the electrical energy to the intermediate secondary distribution equipment.
Why are there secondary distribution equipment? The reason is: Because there are many homes, if the first-level power distribution equipment is used to distribute electric energy, the primary power distribution equipment will be very large and complicated, so the secondary power distribution equipment is used to perform secondary energy distribution.
The secondary distribution equipment has an incoming line switch, a busbar, and a feeder circuit. The incoming switch uses a knife-melt switch, and the feed circuit uses a fuse or a circuit breaker. The illustration shows a fuse.
We see that the right side of the figure consists of three fuses that form a feed loop that delivers electrical energy to the home distribution cable 2.
note:
After the primary distribution equipment and the secondary distribution equipment, the voltage output of the secondary distribution equipment is reduced to 380V/220V due to the voltage drop of the distribution line. This voltage is called the nominal voltage and is the operating voltage of all household electrical distribution and electrical appliances.
In the GB156-2001 (standard voltage) standard, there are regulations on system voltage and nominal voltage.
Let's continue, look at the picture below:
We see that the electrical energy from the home distribution cable 2 from the secondary distribution equipment is sent to the entrance of the metering meter box at the lower left.
Note that according to national standards, the color of the three phase lines is yellow green and red, and the PEN line is blue.
Assume that there are 6 floors in the residential unit building, and there are 2 in each floor, so the L1 phase (A phase) is used for the 1st and 2nd floors, the L2 phase (B phase) is used for the 3rd and 4th layers, and the L3 is used for the 5th and 6th floors. Phase (C phase). As can be seen from the figure, the three phases are sent to 12 watt-hour meters and taken from the watt-hour meter to the households.
Please pay special attention:
We see that the PEN neutral line drawn from the home distribution cable 2 is sent to the right side of the above figure, connected to the ground flat steel MEB, and then separated into N lines and PE lines. The name of the N line is the neutral line, and the name of the PE line is the protection line. Since then, the zero line has disappeared, only the N line and the PE line.
This type of grounding is called protective grounding and its purpose is to protect personal safety.
It is defined as TN-CS grounding system in IEC60364 and national standard GB16895-1.
Notice that the color of the PE line is yellow-green. The yellow-green line can only be used on ground, and in all electrical appliances, as well as in distribution lines, as mandated by IEC60364 and GB16895.
We saw that the N-line was led to the meter and finally sent to the home of each home along with the phase line and the PE line. See the bottom right of the figure.
================
From the above description we have seen a few things:
First: We saw how to change from three-phase voltage to single-phase voltage, that is, the relationship between 380V voltage and 220V voltage. This answers the question of the subject: how to change from three-phase voltage to household voltage.
I believe that the subject should get a clear answer from this description.
Second: We saw the grounding method of the system, including system grounding and protective grounding, and clarified the meaning of the TN-C grounding system and the significance of the TN-CS grounding system;
Third: We saw the MEB grounded flat steel. In fact, the MEB is connected to the steel mesh of the building, which ensures that the metal casing of the household appliance remains at ground potential.
Fourth: We have seen the classification and definition of power distribution systems at all levels.
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Finally, let's take a look at the home distribution system.
The picture below is a diagram of the home distribution system I have drawn. Since this picture has been explained N times in the knowledge, it ignores the doubt:
Finally, talk about how the home distribution system is built.
For the community distribution system, it is designed by the engineers of the architectural design institute, and is composed of the construction company and the electric company. The contents include: installation of power transformer, medium voltage 10kV power distribution system and power distribution cabinet, low voltage 400V Level distribution system and power distribution cabinet, secondary power distribution system and power distribution cabinet, laying of various cables, installation and wiring of electricity meter box of each residential building, and matching of the main distribution box inside the home Installation work.
In other words, the construction company and the electric company completed the construction and installation of all distribution lines from the transformer to our home.
The wiring inside the home is completed by the decoration company. Sometimes, the home distribution box will be replaced according to the requirements of the home distribution system.
The design and construction specifications of the residential power distribution system are: GB50054-2011 "Design Specification for Low Voltage Distribution". This specification is written and regulated by the Ministry of Housing and Urban-Rural Development.
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Let us look at the diagram of TN-CS given in IEC60364-1:
At the top left of the figure, we see the three low voltage windings of the power transformer and their common lines. We see that the public line is directly grounded. This is the system grounding, which is the working ground. After the neutral point is grounded, a line is drawn, defined as the PEN line. This PEN line is the zero line. The three phase lines L1/L2/L3 and PEN are always drawn to the right. We see that the PEN line is grounded for the second time. The English in the figure is marked as repeated grounding. Repeated grounding points are typically placed in the incoming loop of the primary distribution equipment.
In actual use, if the transformer is close to the primary power distribution equipment, the system grounding and repeated grounding can be combined, that is, the system grounding is cancelled, and the repeated grounding is retained.
In the middle of the picture above, we see that the zero line PEN starts to separate, and the point of separation is in the meter chart I have drawn. The grounding must be repeated again before the separation, ie the MEB grounding point.
In fact, the PEN line allows multiple grounding and also requires multiple grounding. The advantage of multi-point grounding is to prevent the PEN line at the rear of the breakpoint from taking a higher voltage after the PEN line breaks. This is necessary because the outer casing of each appliance in the home is connected to the PE line. If the grounding is not grounded before entering the home, once the PEN line at the household is broken, the outer casing of all the electrical appliances in the home may be charged.
We see that when the PEN line is divided into N lines and PE lines, the system has 5 lines, which is why the T-S and TN-CS -S parts are called the three-phase five-wire system.
Of course, I don't recommend that you use the three-phase five-wire system. Because the line refers to the wire through which current must flow during normal operation, the PE line has no current flowing during normal operation. Therefore, TN-S is a three-phase four-wire system, and TN-C and TN-CS are also three-phase four-wire systems.
The answer to the question of the subject is actually a combination of a phase line and a PE line and an N line in the right half of the figure.
In addition, please note the N-line connection of the two loads in the figure:
In the middle of the load, we see that the PEN line is first connected to the load of the shell terminal, and then to the N-line terminal, which shows that the PEN line is also the function of the zero line protection priority; the load on the right is clear, Three phase lines, N lines and PE lines are respectively connected to the load. Of course, for single-phase loads, such as refrigerators and air conditioners in the home, only one of the three phase lines is used.
Another explanation of User A
First of all: after the high-voltage three-phase electricity to the residential transformer, it leads to our household (single-phase) electricity.
Generally, the voltage on the high voltage side is 10kV. Then, after the transformer is stepped down, the 220V abc three-phase low voltage power is obtained.
(As shown below)
The picture above shows a three-phase four-wire power supply system.
The so-called three-phase four-wire system is like the above picture, a, b, c are all so-called fire lines, and any one of them and the zero line N with the power supply is the fire and neutral lines that our resident users need.
Of course, in order not to cause waste, let the three-fire line be used, our zero line N will generally split two with the fire line b, c. That is to say, after the above lead, three sets of "fire line + zero line" can be directly led out.
Therefore, simply speaking, after passing through the cell transformer, we obtained three sets of household electricity, namely “fire line a+zero lineâ€, “fire line b+zero line†and “fire line c+zero lineâ€.
2. After getting the three sets of electricity, we still need to allocate:
Assuming that there are only three buildings in this community, each group can lead a group of "fire line + zero line" in the past.
If we simply think of each family as a resistor, then we can see the following picture:
3, on the issue of power balance: the teacher mentioned in the class before. It is mainly to pay attention to even distribution when distributing load.
Just like the one just in the cell, if the power consumption of one building is several times that of 2 and 3, then the load of phase a is several backs of phase b and c, which will cause a large load and current imbalance.
Therefore, in the allocation, first estimate the overall load situation, and then evenly distribute.
4. About leakage:
We often say "leakage", the so-called leakage is the direct contact between the metal casing of your household appliance and the live wire, so that the metal casing is charged. (Most of the insulation of the wire inside the appliance is aged so that the wire directly touches the casing)
When we manually touch the metal casing of the electrical appliance, it is equivalent to our direct contact with the fire line, that is, you have come into close contact with the 220V voltage, what?
When we put the shell grounding, leakage current through the ground wire directly through into the ground, like this:
And how is the outer casing of the appliance connected to the ground?
Plug wiring as above;
The socket wiring is as above;
Most parts of China are powered by TN-S system. The TN-S system is as follows:
This kind of power supply is called three-phase five-wire system , that is, the line that connects to your home is a combination of "hot line + neutral line + ground line" , which has one more ground line than the three-phase four-wire system to prevent leakage accidents. Therefore, the third grounding wire PE is also called the protection line.
Then the metal casing of the appliance is grounded through the socket and then connected to the PE (grounding wire).
Extrapolating a short story:
Last year, I encountered a problem of “leakage†during the renovation of an old block in Guangzhou.
An old man took the rice cooker that he estimated to have used for seven or eight years to say: "After the birth, help me kneel down, often leaking electricity."
I was plugged in for the late life test, and it was leaking, okay, repair. Disassemble the rice cooker:
Inside, actually ran out two small strong! ! ! (Aber, you don't have to buy meat at home.)
Later, it was discovered that the wire of the rice cooker's metal casing to the plug ground wire was broken, and the three pieces were removed.
Tested with an electric pen first, no leakage. At this time, Arbor deeply suspected that he would ask his brother to touch it with his hand.
At that time, thousands of grass mud horses were rushing in my heart!
But I believe in my own technology, um, touch it, nothing! ! Hahaha
This story is actually intended to tell everyone:
Home appliances are too old, the elimination must be eliminated, not to mention a little bit of small problems are still okay to use, before the electric shock accident is a lucky, but a serious electric shock accident, it is regrettable!
5. Finally, the expansion of the protection measures for low-voltage systems (for friends in the electrical profession):
Protection measures for low voltage power systems:
TT system: Need to spend extra money, this grounding, the power grid will generally not help you install.
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