Feasibility Analysis Of Transformer Renergize With Neutral Point Ungrounded

For the sake of seeking shorten the complex operation time and improving power supply reliability, a certain power supply company establish the project to analyze the feasibility of transformer fast reenergize after the external fault being removed with neutral point ungrounded. Taking this as the background, the transformer recovery model for analyzing the transient of transformer switch-in with load is established by calculating overvoltage, discriminating sympathetic inrush and magnetizing inrush. It analyses four ways of transformer reenergize with neutral point ungrounded and chooses the optimal strategy which is safe and time-saving. The main research results are as follows:1) Through the analysis of the operation mode, code requirements and existing problems, it establishes the transformer recovery model. The model considers the inter-phase winding electromagnetic coupling and other various factors. After calculating the overvoltage and insulation standard of transformer in different recovery scheme, the optimal strategy is selected which is safe and time-saving.2) Compared to the protection of transmission line, the correct operation ratio of the transformer differential protection is very low in a long time. One of the main reason for the transformer differential protection is the mal-operation arising from the transformer close without load.In this thesis, it gives the presentation of the transformer differential protection principal and analysis the cause for the magnetizing inrush and the affection upon transformer differential protection. It also presents the methods for detecting the magnetizing inrush of the transformer differential protection and gives the analysis and evaluation.The result shows that in the case of transformer neutral point ungrounded, closing the low-voltage circuit side breaker before the high-voltage, it can effectively limit over-voltage in a safety range. The second harmonic characteristic of magnetizing waveform may dis-appear, while intermission angle characteristics is still significantly. With the help of intermission angle principle, transformer differential protection may not misused. Through this strategy, the average time for each reenergizing is reduced from40minutes to10minutes and save1hour for engineer on the way of back and forth. It will greatly improving the power supply reliability. At last it concludes the study and gives the prospect for the latter work.