Hazard of excessive grounding resistance
(1) The grounding resistance of the transformer grounding wire is too high. If it is accompanied by low-voltage phase line insulation damage and grounding, for example, phase A is grounded. At this time, there will be a current flowing in the transformer grounding wire, and the phase A voltage is applied to the earth and On the grounding resistance, if the grounding resistance is larger, the partial pressure on the grounding resistance is larger. At this time, if someone accidentally touches the transformer grounding wire or the neutral wire and the transformer casing, the human body will be connected in parallel with the grounding resistance, and the voltage applied to the human body will be high, resulting in electric shock.
(2) When the neutral line grounding resistance of the three-phase four-wire power supply transformer is too high or disconnected, the neutral point of the transformer will be offset due to the imbalance of the three-phase load, and the grounding point potential is not zero, making Some phase voltages rise and burn down electrical equipment.
(3) When the grounding resistance value is too high, the grounding resistance of the transformer arrester is too high. When the lightning strikes the voltage, the arrester cannot discharge to the ground normally, causing the arrester or transformer to burn out.
Reasons why the transformer grounding resistance is too high
(1) The material of the grounding device is not specified. Because the grounding body is buried in a non-standard manner, the installation process is sloppy, the grounding body and the grounding wire connector are loose, and the earth is too dry, etc., which may cause the grounding resistance value to be too high.
(2) Due to the lack of understanding of the importance of the grounding wire during the design and installation of the transformer, the selection of the neutral line section is too small. In addition, due to damage of external force or theft of the grounding wire, the grounding wire may be broken, and the grounding resistance value is too high.
Precaution
(1) Strict construction process, standardizing the burying of the grounding body:
1 Grounding device is generally made of steel pipe, angle steel, steel strip and steel strand. The buried depth should be no less than 0.5 to 0.8 m.
2 Construction of the grounding device. The construction of the grounding device should generally be carried out simultaneously with the foundation construction. Electrician's house
a. The depth of the grounding groove should meet the design requirements, generally 0.5~0.8m, and the arable land should be laid below the depth of the cultivated land. The width of the grounding groove is generally 0.3-0.4m, and all the objects in the groove that affect the contact between the grounding body and the soil should be removed.
b. The specification of the steel pipe and the depth into the soil should meet the design requirements. The grounding body should be driven vertically into the ground and fixed to avoid increasing the grounding resistance. In areas with high mountain and soil resistance, use tubular grounding devices as little as possible, and grounding devices with surface embedding.
c. The grounding down conductor should be laid down along the pole, as short and straight as possible to reduce its impact reactance. The grounding down conductor is fixed on the pole with the support. The distance between the supports is usually 1.0 in the straight part. ~1.5m, 1.0m in the turning part.
d. The grounding down conductor shall not have a joint except for the disconnection reserved for measuring the grounding resistance. The connection of the grounding device shall ensure reliable contact. The connection between the grounding down conductor and the grounding body and the connection between the grounding bodies themselves are all welded. The connection between the grounding down conductor and the disconnection reserved for measuring the grounding resistance is screwed, and the connecting screw should be galvanized and rustproof.
e. After the grounding body is laid, the earth should be backfilled, and the debris such as stones that affect the grounding body and the soil should not be buried.
(2) Select the appropriate position on the neutral line of the transformer to repeat the grounding 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 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 normal. Phase voltage.
(3) Install a residual current action protector after the user's energy meter. When we install the protector in the user, if the grounding resistance of the transformer grounding point is too high, the earth potential will no longer be zero. At this time, there will be a current flowing through the protector and the earth into the transformer grounding point. The protector is actuated and the grounding point is cut off to prevent the earth potential from rising. In addition, after the protector is installed, when the person touches the phase line, the protector will also act, thereby ensuring the personal safety of the person.
Product introduction
1000kVA and above Oil Immersed Transformers must be equipped with outdoor signal thermometers and can be connected to remote signals. Oil immersed transformers of 800kVA and above shall be equipped with gas relay and pressure protection device. Oil immersed transformers of 800kVA and below can also be equipped with gas relay according to the use requirements and through consultation with the manufacturer. Dry type transformers shall be equipped with temperature measuring devices according to the manufacturer's regulations, generally 630kVA and above transformers.
According to the phase number of a single transformer, it can be divided into three-phase transformer and single-phase transformer. In three-phase power systems, three-phase transformers are generally used. When the capacity is too large and limited by transportation conditions, three single-phase transformers can also be used to form a transformer bank in three-phase power systems.
According to the number of windings, it can be divided into two winding transformers and three winding transformers. Generally, transformers are double winding transformers, that is, there are two windings on the iron core, one is the primary winding and the other is the secondary winding. The three winding transformer is a transformer with large capacity (above 5600 KVA), which is used to connect three different voltage transmission lines. In special cases, there are also transformers with more windings.
According to the structure, it can be divided into core type transformer and shell type transformer. If the winding is wrapped around the iron core, it is an iron core transformer; If the iron core is wrapped around the winding, it is a shell type transformer. However, they are slightly different in structure and have no essential difference in principle. All power transformers are iron core type.
According to insulation and cooling conditions, it can be divided into oil immersed transformer and Dry Type Transformer. In order to strengthen the insulation and cooling conditions, the core and winding of the transformer are immersed together in the oil tank filled with transformer oil. In special cases, such as street lamps and mine lighting, dry-type transformers are also used.
In addition, there are various special transformers for special purposes. For example, high-voltage transformer for test, transformer for electric furnace, transformer for electric welding and transformer for SCR circuit, voltage transformer and current transformer for measuring instrument.
Conservator Type Transformer,380v transformer,66KV Transformer,medium voltage transformer,High voltage transformer
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