Wave Process Research Of Converter Transformer Winding Based On VLN

Converter transformer is one of the key equipments in HVDC transmission system,and this kind of transformer’s safety and reliable operation is of crucial importance for the whole system.Since the converter transformer shall continuously withstand the AC overvoltage,DC voltage and polarity reversal voltage in operation,the insulation is a key factor to the converter transformer design and has the highest failure proportion among all tests and operations.Because the costs in production and materials are extreme high,it’s not appropriate to verify the insulation via test,and it’s impossible to evaluate whether the design is the optimal one.Based on the analyses mentioned about,the insulation simulation plays a pivotal role in the design of converter transformers.The software known as VLN,used to simulate the transient process of a transformer,is coded based on the calculation of an equivalent electric circuit whose basic units are set according to the properties of each single winding disc.This software can be used to simulate the transient process of a transformer with various types of coil windings and different working status.Besides,low voltage pulse tests have been carried out to verify the accuracy of the simulation.However,because this software was developed about 30 years ago,its functions were limited and could not cover all the new structures and designs that appeared in the past ten years.The calculating method of equivalent capacitance and inductance of transformer windings has been presented in this thesis firstly,then how these parameters can affect the calculation of transient processes in transformer windings has been discussed.A real 110 kV on-load tap changer transformer has been deployed for verification.The transient electric field distributions of three different types of winding types,continuous winding,interleaved continuous winding and inner-shield continuous winding under impulse stress have been calculated and compared.The field gradient of the continuous winding is much higher than the rest due to its low equivalent serially connected inductances.The serially connected equivalent inductances of the inner-shield continuous winding is slightly lower than those of the interleaved continuous winding,however,the maximum of field gradient of the inner-shield continuous winding is lower.Thus,appropriately increase the serially connected inductances in the winding can improve the transient process of a transformer.In this thesis,a Siemens structure converter transformer has been used for analysis.The field distribution in its windings will be used to deduct the equivalent capacitance based on its winding structure.A program based on Excel VBA has been coded to calculate serially connected inductances,capacitances between winding to ground and equivalent capacitances.Because of the complexity of input for the VIT software,another program for automatic date input has been developed to improve the efficiency of the calculation process and shorten the time required for he calculation.Low voltage impulse test is a reliable test to evaluate the transient process of overvoltage in transformer windings.In order to verify the accuracy of the programs and provide further guidance for transformer design and manufacture,the low voltage impulse test has been carried out in the Siemens structure converter transformer.According to the comparison of the test results and calculating results,the error of the program is below 15%,which can satisfy the industrial requirement and provide further guidance for transformer design and manufacture.