Transformer impact test
The reasons why new transformers or overhauled transformers need to undergo impact tests before they are officially put into operation are as follows:
(1) Check whether there are weaknesses or defects in the insulation of the transformer and its circuit. When unloading the transformer, operating overvoltage may occur. When the neutral point of the power system is not grounded or is grounded through an arc suppression coil, the overvoltage amplitude can reach 4-4.5 times the phase voltage; when the neutral point is directly grounded, the overvoltage amplitude can reach 3 times the phase voltage. In order to test whether the insulation strength of the transformer can withstand full voltage or operating overvoltage, a no-load full voltage impact test is required before the transformer is put into operation. If the transformer and its circuit have insulation weaknesses, they will be broken down by operating overvoltage and exposed.
(2) Check whether the transformer differential protection malfunctions. When the unloaded transformer is powered on, an excitation inrush current will occur, which can reach 6-8 times the rated current. The excitation inrush current begins to decay quickly. Generally, it can be reduced to 0.25-0.5 times the rated current in 0.5-1s. However, it takes a long time to complete all attenuation. It takes about a few seconds for small and medium-sized transformers and up to 10-20s for large transformers. Therefore, the excitation inrush current In the early stage of inrush current attenuation, the differential protection often malfunctions, causing the transformer to fail to be put into operation. Therefore, during no-load impact closing, under the action of excitation inrush current, the wiring, characteristics, and settings of the differential protection can be actually checked, and an evaluation and conclusion can be made on whether the protection can be put into operation.
(3) Check the mechanical strength of the transformer. Since the excitation inrush current generates a large electric force, in order to assess the mechanical strength of the transformer, a no-load impact test is required.
According to the regulations, the number of full-voltage no-load impact tests should be 5 consecutive times when a new product is put into operation; and 3 times when it is put into operation after overhaul. The interval between each impact should be no less than 5 minutes. Before operation, personnel should be sent to the site to monitor the transformer and check whether there is any abnormality in the transformer. If there is any abnormality, the operation should be stopped immediately.
After the first impact, it must continue to run for more than 10 minutes. For subsequent impacts, it must wait for more than 5 minutes before the next impact.
As for why it is set to 5 times, it is stipulated in the regulations. The specific estimate is that it is a conclusion drawn from the comprehensive consideration of mechanical strength, overvoltage, and excitation inrush current.
