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Measures to prevent the burning out of distribution transformers

in the power system, distribution transformers occupy an extremely important position. Once burned out, it will cause a large area of power failure, directly or indirectly affecting the production and life of the majority of power users. Through the analysis of several transformer burnout accidents, the author believes that a considerable number of distribution transformer burnout accidents can be avoided or eliminated in the bud by taking the following preventive measures

1 detection of transformer before operation

in order to ensure that the distribution transformer has operating conditions and prevent burning, the transformer must be tested on site before operation from overseas market. The main contents are:

(1) check whether the oil level gauge on the conservator is intact and whether the oil level is appropriate. The oil level is too high. When the transformer is put into operation and loaded, the oil temperature rises, and the oil expansion is likely to overflow the oil from the breather connecting pipe on the top of the conservator; If the oil level is too low, the oil level may drop to the position where the oil level cannot be seen during light load or shutdown in a short time in winter, which will reduce the insulation and cooling effect of the transformer and affect the operation of the transformer

(2) check whether the cover plate, bushing, oil level gauge, oil drain valve, etc. are well sealed and whether there is oil leakage. Otherwise, when the transformer is loaded, more serious leakage will occur in the hot state

(3) check whether the explosion-proof membrane of the explosion-proof tube (safety airway) is intact by actively mastering the professional knowledge of the product, such as technology, price, technical indicators, etc

(4) check whether the casing is in good condition and whether there is crack and discharge

(5) check whether the hygroscopic agent in the respirator (silicone tank) is invalid

(6) check whether the shell grounding of the transformer is firm and reliable

(7) check whether the primary and secondary outlet bushings of the transformer and their connection with the conductor are good, and whether the phase color is correct

(8) check whether the nameplate on the transformer is consistent with the required transformer specification. For example, the voltage level at each side, the wiring group of the transformer, the capacity of the transformer and the position of the tap changer

(9) measure the insulation of the transformer, but there is no one at home who can not put out the fire in time. Use a 1000 ~ 2500 m Ω meter to measure the insulation resistance of the primary and secondary windings of the transformer to the ground, as well as the insulation resistance between the primary and secondary windings, and record the ambient temperature at the time of measurement. The allowable value of insulation resistance has no rigid provisions, but it should be compared with historical conditions or original data, and should not be less than 70% of the factory value

(10) measure the DC resistance of transformer group and bushing. The mutual difference of DC resistance of each phase of distribution transformer shall be less than 4% of the average value, and the mutual difference of DC resistance between lines shall be less than 2% of the average value

(11) check whether the fuse selection is reasonable. The selection standard of the fuse on the primary side of the transformer is 1.5 ~ 2 times of the rated current, and the selection standard of the fuse on the secondary side is usually the rated current on the secondary side of the transformer

if all the above checks are qualified, first drop the transformer (without load), check whether the electromagnetic sound is abnormal, and measure whether the secondary voltage is balanced. If the voltage is balanced, the transformer ratio is normal, there is no inter turn short circuit, and the transformer can operate normally with load

2 precautions in transformer operation

(1) during the use of distribution transformer, it is necessary to regularly check whether the three-phase voltage is balanced. If it is seriously unbalanced, measures should be taken to adjust it in time. At the same time, the oil level and color of the transformer should be checked frequently, and the shell should be checked for leakage. If any defect is found, it should be eliminated in time to avoid the tap changer and coil being burned out due to moisture

(2) regularly clean the dirt on the distribution transformer, check whether the bushing has flashover discharge, whether the grounding is good, and whether there are wire breaks, desoldering, and fractures. Regularly telemetry the grounding resistance is not greater than 4 (capacity ≥ 100 KVA) or 10 (capacity < 100 KVA), or take anti fouling measures and install the bushing anti fouling cap

(3) when connecting and removing the outgoing line of distribution transformer, strictly follow the detection process to avoid internal fracture of the outgoing line, and reasonably select the wiring mode of the secondary side conductor

(4) promote the use of S11 series new lightning protection and energy-saving transformers, or when installing on the primary and secondary sides of distribution transformers, we should also pay attention to the normal protection of the experimental machine. When installing lightning arresters, the grounding downlead of the lightning arrester, the shell of the transformer and the neutral point on the secondary side should be grounded together. Carry out preventive tests regularly, and replace unqualified lightning arresters in time to reduce overvoltage damage to the transformer caused by lightning resonance

(5) when switching the no-load voltage regulating switch, after each switching, first measure the DC resistance value of the last two times, make records, and compare whether the three-phase DC resistance is balanced. It can be put into use only after it is confirmed that the switching is normal. When measuring in each gear, in addition to making records separately, pay attention to putting the DC resistance of the operating gear on the last measurement

(6) it is necessary to do a good job in the monitoring and management of the load in the substation area, and replace the transformer in time for the overloaded substation area to prevent the transformer from burning out due to overload operation

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