The Research On Inductive Filtering Converter Transformer’s Windings

In the period with booming Power Industries, to coordinated development ofresources and environment is a tough challenge we faced with. For China’s powerindustry, it not only needs to provide reliable electrical energy for economicdevelopment, but also to ensure that the power transmission project environment-friendly, which is a realistic subject power industry concerned about. DC transmissionis of great significance for achieving the optimal allocation of resources and energyoptimizing supply nationwide, and it has been widely used in China’s electricitysystem. However, due to the non-linear behavior of the power electronic devices of DCtransmission as well as the DC magnetic bias phenomenon occurred, resulting in alarge number of harmonic currents and reactive power, which not only pollutes thepower quality of the power grid，but also do harm to the respective devices of theconverter stations, especially converter transformer. The traditional filtering scheme ofDC power transmission only take power grid into consideration, the characteristicharmonic currents generated by commutating valve still free flow in the transformerwindings, then the electromagnetic forces of windings will be increased, which causeelectromagnetic vibration of the windings intensified. On the one hand, convertertransformer windings vibration makes the audible noise beyond the standards seriously;the other hand, it may damage the transformer insulation structure, shortening theservice life of the converter transformer.Inductive filtering converter transformer of this thesis is a new convertertransformer integrated inductive filtering technology. It can perform the harmonicsuppression nearby harmonic source, making the most of the characteristics harmonicshield in valve winding, and almost no harmonic currents flow through the primarywinding. By this way, the winding electromagnetic vibration can be suppressed. Basedon the inductive filtering converter transformer and its winding electromagneticvibration, the fundamental research and the related innovative achievement in thisthesis can be summarized as follows:(1) Inductive Filtering wiring scheme of converter transformer and its filteringmechanism are researched, derived voltage and current transfer equations of inductivefiltering converter transformer and its winding turns-ratio. In ideal conditions, current calculation method of inductive filtering converter transformer winding are derived, bycomparing with the measured waveform, the correctness of the calculation method isverified. Calculated current and measured current shows the inductive filters greatlysuppressed the windings harmonic currents of converter transformer, which also laidthe foundation for the winding electromagnetic forces calculation consideringharmonics currents;(2) By detailed comparing of Biot-Savart’s law and Lagrange’s theorem,Lagrange’s theorem is determined to calculate windings electromagnetic forces ofinductive filtering converter transformer. Based on Lagrange’s theorem and themulti-winding transformer theory, winding electromagnetic forces on the harmonicconditions in both radial and axial directions are computed. The calculation resultsshow that the inductive filters effectively reduced the amplitude of the radial forces ofthe primary winding and the common winding;(3) Based on the single-phase inductive filtering converter Transformers of DCtransmission system research platform, a finite element model of the transformerwindings and core are established. Modal analysis method and the basic ideas ofinductive filtering converter transformer vibration modal analysis are determined. Bychecking modes, determined the winding boundary condition is free boundary, and thecore boundary condition is bottom with constraints. Modal calculation results showthat the natural frequencies of the windings avoid the occurrence of resonance withtwice harmonic frequency, and the primary winding is the most easily vibrationportion of all windings. In order to avoid the resonance of the core, a measure that loadprestressing force on core is put forward. By recalculating the core vibration modal,the feasibility of the measure is verified;(4) The winding vibration finite element model under harmonic conditions isestablished. Sweep mesh is adopted to ensure the radial and axial symmetry ofwinding node distribution, which is consistent with force and vibration directions ofwindings. The effect of core vibration is reasonably simplified and the boundarycondition of the transient dynamics calculation is determined. As for irregularwaveform, the electromagnetic forces are divided into a large number of equal loadsteps. The finite element calculation results show the vibration characteristics in fourdifferent conditions, it reveals inductive filtering technology significantly suppressedthe vibration of windings, and5,7order inductive filters are more effectively inwinding vibration suppression than11,13order inductive filters;(5) The harmonic suppression mechanism of inductive filtering converter transformer under DC magnetic bias is researched, and the inductive filteringconverter transformer DC magnetic bias experimental matlab simulation model isestablished. Transformer exciting current and winding current are obtained in two DCbias introduced way that verified the inductive filtering converter transformer canprevent harmonic current spread to the primary winding. By the finite elementcalculating, the vibration characteristics of all windings are obtained in the two DCbias introduced way. Analysis of the winding vibration characteristics showed that,without inductive filters, the vibration of the windings are more severe when DC biasintroduced from primary winding; with inductive filters, the vibration of the windingsare significantly reduced in two DC bias introduced way, indicating that the inductivefiltering technique can effectively suppress windings vibration under DC magneticbias.This thesis makes a systematic study of the inductive filtering convertertransformer’s winding vibration characteristics, and a relatively complete theorysystem of winding vibration is founded. By in-depth studying of winding vibrationmodes, the windings vibration in harmonic conditions revealed the winding vibrationsuppression effect of the inductive filtering technique, which indicates that theinductive filtering converter transformer has a good engineering application prospect.