Analysis and treatment of abnormal conditions in t

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Analysis and treatment of abnormal conditions in distribution transformer testing

I. insulation resistance test

insulation resistance test is a general method to check the insulation condition of distribution transformer. Generally, damp and local defects of distribution insulation (such as broken porcelain parts) can be effectively detected. When the insulation of the winding to the ground is low or zero, it cannot be easily judged to be damp and determined to be unqualified

1. Check whether all the removed external leads are removed (especially the neutral wire)

2. Whether the measurement method is appropriate (whether the meter wiring is correct), whether the rotation speed is reasonable, whether the pointer is stable, whether the lead is spliced with the shell after being chewed soft, etc

3. Check whether the surface of insulating porcelain parts is wet, dirty, damaged and has discharge traces

4. Check whether the transformer body is well sealed and whether there is leakage. If the leakage occurs at the bushing, it is likely that the crimping screw of the guide rod is loose, and the air gap of the rubber pad is infiltrated, causing rain and water in the bushing to be damp, resulting in the decline of the insulation level between the conductive rod and the body shell or the resistance value to zero

5. When checking and disassembling the lead, the conductive rod will shift due to rotation, resulting in a small safe distance or overlap between the live part and the shell, which will also cause the insulation level to drop or the resistance value to zero

after the above inspection and treatment, if the resistance value is still low, it indicates that the winding is indeed damp, which is determined to be unqualified. It should be removed for insulation oil withstand voltage test, oil filter change and drying of the body

II. DC resistance test

the measurement of DC resistance is to check the welding quality of the winding and whether the winding has inter turn short circuit; Whether the tap changer position is good and whether the actual position is consistent with the indication; Whether the outgoing line is broken or loose; Whether the winding wound in parallel has broken strands, etc. The measurement of DC resistance is an essential test item of transformer after overhaul, pre-test and changing the position of tap changer, and it is also an important inspection item after fault. Therefore, the test must be carefully operated to minimize the measurement error

according to the regulations, the phase to phase resistance difference of transformers above 1600KVA is generally not more than 2% of the three-phase average value, and the line to line resistance difference is generally not more than 1% of the three-phase average value. For transformers of 1600KVA and below, the phase to phase resistance difference is generally not more than 4% of the three-phase average value, and the line to line resistance difference is generally not more than 2% of the three-phase average value, The consumption of the measured phase difference during transportation shall not be greater than 2% compared with the phase difference measured at the previous corresponding parts

when the resistance value measured by the DC resistance exceeds the standard:

(1) whether there is measurement error should be considered first (such as whether the external lead is connected, whether the test lead is too long or too thin, whether the contact is good & whether the battery voltage in the bridge is insufficient, etc.)

(2) the value of DC resistance is greatly affected by temperature, so it must be converted to the same temperature (generally 20 ℃, r20= (T + T), t copper = 235) for comparison, and generally based on the oil temperature of the above layer

(3) for the three-phase distribution transformer currently used, the high-voltage winding adopts Y-shaped wiring. When the resistance exceeds the standard, the line resistance can also be converted into phase resistance according to the following formula [ra= (rab+rac-rbc)/2, rb= (rab+rbc-rac)/2, rc= (rbc+rac-rab)/2], so as to find out the defective phase

(4) it is common for the tap changer to have a high resistance value due to poor contact. For example, the switch is not clean, the crossover falls off, the spring pressure is insufficient, the force is uneven, and the contact has carbon deposition during overvoltage, which will cause a high resistance value. At this time, the tap changer cover should be opened and rotated back and forth for several times, which can generally be eliminated

if it still exceeds the standard after the above inspection and treatment, it indicates that the internal fault is likely to be faulty welding, desoldering, broken wire between the winding and the lead, or short circuit between layers, or the winding is burned. It cannot be handled on site, so it needs to be sent to the maintenance room for core overhaul

III. grounding device test

the regulations for the configuration of coolers in pump stations stipulate that the grounding resistance of transformers of 100kVA and above is not greater than 4 Ω; The grounding resistance of transformers below 100kVA shall not be greater than 10 Ω

when the meter is unstable during the test, it is mainly caused by external interference, such as inductive electricity and high-voltage discharge nearby, which will affect the swing of the meter. At this time, the measurement position should be changed or several speeds should be changed to avoid the influence of external interference. When the grounding resistance value is high:

(1) check whether the wiring of the connection meter between the checklist and the fatigue loading experiment control module is correct and whether the contact is good

(2) check whether the insertion of potential probe is reasonable

(3) the wiring of current pole and voltage pole shall be perpendicular to the direction of line or grounding body as far as possible

(4) the lead wire of the grounding machine and the grounding machine body are tested separately (the grounding lead wire is removed from the low-voltage n-wire terminal of the transformer). The clamp wire is polished to make it in good contact. Many high resistance values occur here. Due to years of disrepair and climate change, the contact between the grounding machine and the upper lead wire is corroded and oxidized, and the resistance value is greatly different, which cannot play a role in safety protection. During the pre-test, ensure its good contact. After the above inspection and treatment, if the resistance value is still high, it can be determined to be unqualified, and it should be handled or replaced in time

the operating regulations of power transformer stipulate that the minor repair cycle of distribution transformer is once a year, and the preventive test can also be carried out at the same time. Only in this way can we completely change the long-standing habit of distribution transformer (just push the trolley without reversing), that is, as long as there is oil in the distribution transformer and as long as it can supply power, it can be used safely and boldly, and inspection, identification and replacement can be considered until the fuel injection is burnt out. It must be emphasized that the testers must make records according to the requirements of the procedures when conducting preventive tests and put them into the technical archives of the transformer. (end)

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