Views:194 Author:Site Editor Publish Time: 2021-07-15 Origin:Site
In order to work and safety in the power system, it is often necessary to connect some parts of the power system and its electrical equipment with the earth, which is called grounding. "Earth" in power system refers to the earth conducting material whose potential at any point is taken as 0 according to convention. Grounding is a conductive connection (metallic connection) between the circuit (equipment) and the earth or some conductive body instead of the earth. The earthing of the power system includes the grounding of the high voltage system and the grounding of the low voltage system. The low voltage grounding system is divided into IT, TT, TN-C, TN-S and TN-C-S5 modes according to the grounding system.
Classification according to grounding mode
(1) Direct grounding system
The neutral point of transformer or generator is connected to the grounding device directly or through small resistance. When single-phase grounding short circuit occurs, the grounding current is very large, so it is also called high current grounding system.
(2) Ungrounded system
The neutral point of transformer or generator is not connected to the grounding device or connected to the grounding device through protection, measurement, signal instrument, arc suppression coil and grounding equipment with high resistance. When single-phase grounding short circuit occurs, the grounding current is very small, so it is also called small current grounding system.
Classification by grounding equipment
(1) The neutral point of transformer or generator is directly grounded or ungrounded without any grounding equipment.
(2) The neutral point of transformer or generator is connected with grounding device through arc suppression coil.
(3) The neutral point of the transformer or generator is connected to the grounding device through a resistor. Resistor for high resistance is called high resistance grounding system, and resistor for low resistance is called low resistance grounding system.
(4) In the grounding system of Reactance Compensation and resistance parallel connection, the neutral point of transformer or generator is grounded in parallel with resistor through reactor, and the reactor should adopt standard arc suppression coil.
IT grounding system
The power supply of IT system is grounded (or ungrounded) through impedance, and the exposed conductive parts of electrical equipment (such as casing) can be directly grounded or connected to the grounding electrode of power supply through protective ground wire. When the first fault occurs in this system, the fault current is limited, and there will be no dangerous contact voltage on the metal shell of the electrical equipment, so the electrical equipment can continue to operate without cutting off the power supply. At this time, the equipment alarms, and the power failure time of electrical equipment can be reduced or eliminated by checking the circuit to eliminate the fault. However, if a second fault occurs when the first fault is not eliminated, the fault point suffers from line voltage and the fault current is very dangerous. Therefore, it is necessary to have reliable and easy to detect the fault point alarm equipment, such as advanced cable fault indicators and accurate electric wire fault indicators.
TT grounding system
TT grounding system must have a direct grounding point, which is usually the neutral point of transformer or generator. The exposed conductive parts of electrical equipment must also be grounded. All exposed conductive parts of the electrical equipment protected by the same protective appliance shall be connected together with protective wires and connected to their common grounding electrode. When several protective appliances are protected by different levels, all exposed conductive parts protected by each protective appliance must also be grounded in this way.
If the neutral line section is smaller than the phase line section, if the phase line protection device of the loop cannot protect the short circuit of the neutral line, or the current flowing through the neutral line during normal operation is not obviously less than the current carrying capacity of the conductor, the over-current detection devices corresponding to the conductor section must be installed on the neutral line, such as fault current indicators and arc fault indicators. When the device is excited, the phase line should be cut off, but the neutral line need not be disconnected.
When a short circuit to earth occurs in the TT grounding system, the short-circuit current is limited by the grounding resistance at the power supply side and the grounding resistance at the electrical equipment side. The current is not large, so it can reduce the risk of ground short circuits, but in most cases, it is not enough to cut off the power supply of general over-current protection equipment, which is easy to cause electric shock accidents. When the electrical load capacity is large, residual current protection must be used.
TN grounding system
The exposed conductive parts of all electrical equipment in TN system are connected to the protection line and connected with the neutral point of distribution system. The protective wire shall be grounded near each substation or substation. When the distribution system is introduced into the building, the protective line is grounded at its entrance. Connect the protective wire to the nearby effective grounding electrode. If necessary, the grounding points can be increased and evenly distributed. The requirements for detection of neutral wires and corresponding cut-off conductors are the same as those for the TT system.
According to the combination of neutral line N and protective line PE, TN grounding system is divided into TN-C, TN-S and TN-C-S.
(1) TN-C grounding system
The protection line and the neutral line are combined into the PEN line system, which has the advantages of simplicity and economy. When a grounding short circuit fault occurs, the fault current is large, and the general over-current protector is used to cut off the power supply. For single phase load or three-phase unbalanced load and harmonic current load line, the PEN line has current, and the resulting voltage drop appears on the metal casing of the electrical equipment and the metal bushing on the line, which is unfavorable to the sensitive equipment. In addition, the weak current on the PEN line may cause an explosion in the explosive dangerous environment, so this system cannot be used in the explosive dangerous environment. Because the PEN lines are electrically connected to each other in the same building, when the PEN line disconnects or the phase line is directly short circuited with the earth, it presents a fairly high voltage to earth fault, which will expand the scope of the accident. Therefore, it is necessary to use advanced electric wire fault indicator and reliable cable fault passage indicator.
(2) TN-S grounding system
TN-S grounding system is a system in which protective wires and neutral lines are separated. Under normal circumstances, the PE line does not pass the load current, so the metal shell of the electrical equipment connected with the PE line is not charged under normal conditions, so it can be used in precision equipment and explosive environment. However, there are still some problems in the TN-S system, such as voltage rise caused by phase line short circuit to ground and fault voltage spreading to ground. Therefore, we need to use digital cable fault indicators and accurate electric wire fault passage indicators.
(3) TN-C-S grounding system
The PEN line is divided into protective lines and neutral lines from one point. The N line after isolation is insulated to the ground. To prevent confusion between PE and N wires. According to GB, PE and PEN lines are painted with yellow green interphase color labels, and N lines are painted with light blue color labels. After the PEN line is separated, it can no longer be merged, otherwise it will lose the characteristics of the TN-C-S grounding system formed after separation.
All the contents of electrical grounding system have been introduced. You must know more about its classification and explanation.