Wideband Modeling Method And Its Application Of High-Voltage High-Frequency Transformers

The high-power DC-DC converter is key equipment to establish and develop the high-voltage DC networks.The magnetically coupled DC-DC converters based on high-voltage high-frequency transformers(HV-HFTs)have obvious advantages on realization of galvanic isolation and high reliabilities,which are important development directions of future power system applications.The high-voltage high-frequency transformers,as main technological bottlenecks restricting development and commercial applications of high-power DC-DC converters,are extremely lacking researches on fundamental theories and key technologies.This dissertation,focusing on parasitic effects of the HV-HFT,is to investigate wideband modeling and parameter extracting method of the HV-HFT by the mean of combining theory analysis and experiment measurement.A wideband mechanism model is proposed for the HV-HFT.This model is characterized by elaborate considerations on magnetic and capacitive effects of the HV-HFT,including electrostatic couplings between windings,magnetic core and the enclosure,with parameters having explicit physical meanings.In terms of large number of turns and racetrack windings of the HV-HFT,a finite-element method based on weighted algorithm is used to compute model parameters.Impedance characteristics of a HV-HFT prototype under different load conditions and under different winding grounding conditions are measured and simulated over a wide frequency band,by which the proposed model and corresponding parameter computing method are verified,and impacts of grounding windings on resonant frequencies of the HV-HFT are revealed.The proposed wideband mechanism model and parameter computing method provide a powerful tool to recognize and control parasitic effects of the HV-HFT in design phases.A terminal capacitance equivalent method for the HV-HFT is proposed.Procced from the concept of partial capacitances of the multi-conductor system,partial capacitances corresponding to turns are equivalent to terminal capacitances corresponding to windings by defining coefficient matrix accounting for potential distribution of each winding,and the complexity of the model due to partial capacitances is significantly reduced.Simultaneously,quantificational links between partial capacitances and terminal capacitances are established,revealing that terminal capacitance are essentially linear weighted sum of partial capacitances,where the weighted factors are corresponding to potential distribution condition of the windings.The terminal capacitance equivalent method is verified by an air-core reactor and the HV-HFT prototype.The proposed terminal capacitance equivalent method provides an effective access to equalize distributed capacitance parameters between conductors inside the transformer as lumped capacitance parameters between winding terminals,and equivalence mechanisms of these lumped capacitance parameters are revealed,which have some guiding significances for analyzing and controlling parasitic effects of the HV-HFT.An experimental method for extracting capacitance parameters of the HV-HFT is proposed.By analyzing internal relationships between impedance resonant frequencies,the reasons for some impedance resonant frequencies are too high to be measured when the transformer has a large turn ratio are revealed,and the drawbacks of traditional capacitance extraction method based on impedance measurements when apply to HV-HFT are pointed out.Simultaneously,a new capacitance extraction method based on voltage transfer characteristic is proposed through analyzing resonance features of the transfer characteristics.By using the frequency corresponding to zero of transfer characteristics,which accounts for parallel resonance between leakage inductance and mutual capacitance of the two windings,the missing equation due to unobtainable impedance resonant frequency is added.By measuring impedance and transfer characteristics of several HF transformers,the two methods for extracting capacitances are compared.The results show that the proposed method based on voltage transfer characteristic measurement effectively solves the problem of extracting capacitances of the HV-HFT with large turn ratio.As an application of the wideband modeling method,change rules of the voltage and current transfer characteristics of the HV-HFT with different loads are investigated,and the relationships among parasitic parameters,loads and transfer resonant frequencies are revealed.On this basis,the method for controlling the voltage and current transfer ratio at fixed frequency is proposed,and specific controlling measures are discussed.The HV-HFT prototype is used to verify the method for analyzing and controlling transfer characteristics.The proposed controlling method is helpful to ensure the HV-HFT achieve expected voltage and current conversion.