Research And Implementation On The Reliability Technology Of Contactless On-load Voltage Regulating Distribution Transformer

The distribution transformer which has the function of automatic on-load regulating voltage can effectively solve the problem of voltage fluctuation at the end of the power grid under the condition of enough reactive power. Contactless on-load voltage regulating distribution transformer has become an important study direction in the field of power distribution automation system. Experts and scholars at home and abroad have finished a lot of theoretical research and practical exploration within years and our research team has also made breakthrough progress in the early stage of the project. With50kVA distribution transformer as a platform, contactless on-load voltage regulating device prototype based on the parallel thyristor module for tap-changer, single-chip microcomputer as the core of the control unit was developed. It has been verified by laboratory simulation test that the prototype possess basic voltage regulating function. In the operation test with higher proportion of perceptual load, over-current protection action occurred along with puncturing of solid tap-changer. Therefore reliability problem is the current key technology block of the popularization and application of contactless on-load voltage regulating distribution transformer. This topic of contactless on-load voltage regulating distribution transformer reliability technology is the study and implementation of the corresponding theoretical analysis and plan improvements in order to improve the operation reliability. This article begin with the analysis of compositions, working principle and previous experimental results of the current contactless on-load voltage regulating distribution transformer, preliminarily confirm several key factors which may influence the operation reliability, make in-depth study on the cause of these several factors and impact extent to tap-changer, control unit and of overall operation for ruling out the factors which have little impact or almost no influence on operation reliability. According to the analysis, these six factors below likely impact the reliable work of contactless on-load voltage regulating distribution transformer:1) exiting reliability of start agency;2) excitation inrush current and starting current of the motor;3) working reliability of transition branch;4) commutation over-voltage of parallel thyristor;5) drive unit input/output insulating ability;6) program reliability of MCU. The last5factors will threaten the reliable work of contactless on-load voltage regulating device and cause malfunction.There will be three chapters in this article focusing on putting forward the corresponding improvement scheme to weaken or eliminate the identified factors for the reliable operation of contactless on-load voltage regulating distribution transformer. The main circuit structure is improved in the way of combining start agency with transition branch in this topic by comprehensively considering the three factors:excitation inrush current, transition branch and commutation over-voltage of parallel thyristor. Combined startup-transition branch adopts the bidirectional thyristor as the main switch which driving circuit is composed of combining intermediate relay contacts with trigger resistance and connecting with series-opposition zener diode as a gate protection. The accessing and exiting of startup-transition branch is realized by controlling electricity of intermediate relay coil with a control relay. In order to improve the commutation over-voltage absorption ability of parallel thyristor, the appropriate parameters and modularization of resistance capacity absorb circuit are presented by full analysis and calculation. Taking the advantage of optical fiber communication technology in high voltage electrical isolation, this paper designed the parallel fiber thyristor trigger circuit, for implementing the reliable electrical isolation between parallel thyristor in distribution transformer high voltage side and control unit in the low voltage side. For preventing maloperation of tap-changer caused by microcontroller program disorder, this paper designed key control signals blocking circuit based on the watchdog chip as the core. In testing experiments, the control unit fault locking device can hold voltage regulating device steady while MCU is under simulative reset, and make the program of MCU restart from the formal working state before reset. Laboratory simulation test and loading test results show that the improved contactless on-load voltage regulating distribution transformer can basically realize the regulating function, and still reliably work under three special cases of no-load, perceptual load, and suddenly removed all the loads. At present, the improved contactless on-load voltage regulating distribution transformer is commissioning in the company as a factory transformer to further verify the reliability in the process of long-term operation.The solution of reliability problem makes contactless on-load voltage regulating distribution transformer is no longer confined to theoretical analysis and prototype experimental study as well as be helpful to popularization and application. Implementation of reliability technology can eventually promote the improvement of power distribution automation system from the angle of contactless on-load voltage regulating, and let the users at the end of power grid can get stable and reliable electricity.