Danger and prevention of the rising grounding resi

2022-08-13
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Danger and prevention of the rise of transformer grounding resistance

according to the provisions of power equipment test procedures, the grounding resistance of transformer grounding points below 100kVA is not greater than 10100kva, and the grounding resistance of transformer grounding points above 100kVA is not greater than 4. However, due to the negligence of design and construction technology or the damage of external forces, it often leads to the rise of grounding resistance at the transformer grounding point and the failure of grounding wire disconnection, resulting in abnormal power supply and the burning of users' electrical equipment, which brings certain difficulties to the operation and management of power supply units. Therefore, we must take certain measures to prevent the damage caused by the disconnection of transformer neutral wire and grounding wire and the increase of grounding resistance

1 reasons for the disconnection of grounding wire and neutral wire of transformer and the increase of grounding resistance

(1) the increase of grounding resistance may be caused by irregular embedding of grounding body, sloppy installation process, loose joints between grounding body and grounding wire, too dry earth, etc

(2) due to insufficient understanding of the role and importance of grounding wire during the design and installation of transformer, the section of neutral wire is too small. When the three-phase load is unbalanced, the neutral wire current is too large, resulting in burnout. In addition, the ground wire or neutral wire may be broken due to external force damage or ground wire theft

2 rising phenomenon and harm

(1) the grounding resistance of transformer grounding wire increases, and at the same time, vanadium is mainly used in the field. There is phase line insulation damage and grounding, for example, phase B grounding. At this time, there will be a current flowing through the transformer grounding wire, and phase B voltage is added to the earth and grounding resistance. When the grounding resistance is larger, the partial voltage on the grounding resistance is larger. At this time, if someone touches the grounding wire or neutral wire of the transformer and the transformer shell by mistake, the human body will be connected in parallel with the grounding resistance. If the grounding resistance is large enough, the voltage applied to the human body will be very high, resulting in electric shock. If the human body touches phase a or phase C by mistake, the voltage applied to the human body will be 380V, which will pose a greater threat to personal safety. In addition, some users connect the neutral line of the transformer to the shell of the electrical equipment for protective grounding, and the equipment is not insulated from the ground. When phase B is grounded, the potential of the earth is equal to that of phase B, and the phase voltage is added between the earth and the neutral line. At this time, if the electrical equipment uses phase a or phase C power supply, the voltage of the shell to the ground will rise to 220V, and the voltage of the equipment to the ground will be 380V. At this time, if people touch the shell of electrical equipment, it will also cause electric shock. In addition, when phase B is grounded, there will be a current flowing from the earth into the chassis and back to the neutral line, and some electrical equipment with high requirements will not be able to operate

(2) the ground wire is broken, and the ground wire near the transformer is broken, just like the ground resistance rises to infinity. At this time, the ground potential is the ground phase potential. All phenomena are the same as when the ground resistance rises, but the harm is greater, which will not be repeated here

(3) when the neutral line of the three-phase four wire power supply transformer is disconnected, at this time, due to the imbalance of the three-phase load, the load grounding point will shift, and the potential of the grounding point is not zero, which will increase the voltage of some phases and burn down the electrical equipment

when the grounding wire is broken or the grounding resistance increases, the grounding wire of the transformer arrester is broken or the grounding resistance increases. In case of lightning overvoltage, the lightning arrester cannot discharge to the ground normally, resulting in damage to the lightning arrester or transformer

3 preventive measures

(1) strictly implement the construction process and standardize the laying of grounding body. Select the cross-section and type of grounding wire according to the regulations, and correctly select the cross-sectional area of neutral wire

(2) install residual current action protector behind the user's electric energy meter. When we passed the articles of association of civil aircraft materials industry development alliance at the meeting, if the insulation of the wire or electrical equipment behind the user is damaged after the user installs the protector, it will form a discharge to the ground. At this time, if the grounding resistance of the transformer grounding point is too high, the earth potential will no longer be zero. At this time, a current will flow into the transformer grounding point through the protector and the earth. This current will make the protector act and cut off the grounding point, preventing the rise of the earth potential. In addition, after installing the protector, when people touch the phase line, the protector will also act, thus ensuring the personal safety of personnel

(3) select an appropriate position on the neutral line of the transformer to ground the sample of multi-point repeated ring and disk contact of the neutral line of the transformer. When the neutral line of the transformer is disconnected at a certain point, due to multi-point grounding, the neutral line current can still return to the neutral point of the transformer through the earth, the potential of the neutral line is always zero, and the voltage of each phase load is always the normal phase voltage. On the other hand, when the grounding resistance of the transformer grounding point rises or the grounding wire is broken and the phase line is grounded, due to multiple points of repeated grounding, the potential of the earth can also be guaranteed to be zero, which will not pose a threat to human safety and the normal operation of the equipment

when taking the above preventive measures and Issuing the first UL detection and verification mark for the field of functional materials with UL, we should also strengthen the extensive publicity of power regulations to the general public, so that more people can care about power facilities. Once any equipment obstacle is found, it can be reported to the power supply department in time, so as to eliminate the power equipment fault in the bud as soon as possible and ensure safe and reliable power supply

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