Why should the distribution transformer or the transformer after overhaul be subjected to five withstand voltage and impact tests before being officially put into operation?
Writer： Hengfeng you electric Time：2023-04-18 views：times
Why should the distribution transformer or the transformer after overhaul be subjected to five withstand voltage and impact tests before being officially put into operation?
Let us hengfengyou Transformer Engineer Madison explain to you: why do the withstand voltage and impulse test for the new distribution transformer or the transformer after overhaul:
1. Check the insulation of the transformer and its circuit for weaknesses or defects
Switching overvoltage may occur when the no-load transformer is opened. When the neutral point of power system is not grounded or grounded through arc suppression coil, the overvoltage amplitude can reach 4-4.5 times of phase voltage; When the neutral point is directly grounded, the overvoltage amplitude can reach 3 times of phase voltage. In order to test whether the insulation strength of the transformer can withstand the effect of full voltage or operating overvoltage, it is necessary to conduct no-load full voltage impulse test before the transformer is put into operation. If the transformer and its circuit have insulation weakness, it will be exposed by operation overvoltage breakdown.
2. Check the mechanical strength of the transformer
Because the inrush current produces great electric power, in order to assess the mechanical strength of the transformer, it is necessary to conduct no-load impact test. According to IEC 60076 regulations, the number of full voltage no-load impulse tests shall be 5 times continuously when new products are put into operation; After overhaul, it shall be put into operation for 3 consecutive times. The interval between each impact shall not be less than 5min. Before operation, personnel shall be sent to the site to monitor the transformer and check whether there is any abnormality in the turn changer. If there is any abnormality, the operation shall be stopped immediately. Continue to run for more than 10 minutes after the first impact, and wait for more than 5 minutes for the next impact. As for why it is specified in the regulations to set it to 5 times, the specific estimation is the conclusion based on the comprehensive consideration of mechanical strength, overvoltage and inrush current.
3. Check whether the transformer differential protection operates wrongly.
When the no-load transformer is put into live operation, the magnetizing inrush current will be generated, and its value can reach 6-8 times the rated current. The magnetizing inrush current begins to decay rapidly, which can generally be reduced to 0.25-0.5 times the rated current after 0.5-1s, but the complete attenuation takes a long time. For small and medium-sized transformers, it takes about a few seconds, and for large transformers, it can reach 10-20s. Therefore, in the early stage of magnetizing inrush current attenuation, the differential protection often malfunctions, resulting in the failure of the transformer to be put into operation. Therefore, during no-load impulse closing, under the action of inrush current, the wiring, characteristics and setting value of differential protection can be actually checked, and the evaluation and conclusion of whether the protection can be put into operation can be made.
4. Operation method of transformer impulse test:
The impulse closing test of transformer under rated voltage shall be conducted for 5 times with an interval of more than 10 minutes, and there shall be no abnormality. The impulse closing test shall be conducted at the high voltage side. For the power system with neutral grounding, the neutral point of the transformer must be grounded during the test. The dry-type transformer without current differential protection can be impacted for 3 times. The method is: disconnect the low-voltage side switch, close the side circuit breaker, pay attention to the excitation current during the test, and listen to whether the sound of the transformer is normal. On the one hand, check the end insulation of the transformer, on the other hand, monitor whether there is abnormal noise inside the transformer. The method is to lean against the transformer shell with a wooden rod or insulating rod to carefully listen to whether the sound of the transformer is uniform. In case of intermittent explosion or sudden violent sound, stop the operation immediately, and put into normal operation only after passing the five impact tests.
