Research On Relay Protection Of Transformer Converter Equipment In AC/DC Hybrid Distribution Network

In recent years,more and more distributed power sources represented by wind power and photovoltaic have been connected to the urban power distribution network,effectively increasing the utilization rate of various new energy sources.But on the other hand,the access to a large number of distributed power sources has also made harmonic pollution and voltage fluctuations in traditional distribution networks increasingly serious.Under this background,scholars have proposed the concept of AC-DC hybrid distribution network.The AC-DC hybrid distribution network fully retains the advantages of the existing AC distribution network while taking advantage of the DC distribution network's fast response,high compatibility,and saving power supply corridors.It is a future distribution method with great potential and high economy.This paper takes DC transformer as the research object,analyzes its various fault characteristics,proposes a new sub-module open circuit fault diagnosis method,and configures a protection scheme for its DC side fault,hoping to achieve the purpose of ensuring stable operation of the equipment.DC transformer is known as the "heart" of AC-DC hybrid distribution network,which plays the role of DC voltage transformation and AC-DC converter in the system,and its reliability directly affects the stable operation of the whole distribution network.The structure of DC transformer is diverse,and the topology based on modular design is the focus of current research.In view of the open circuit fault of the submodule inside the DC transformer,this paper proposes to apply wavelet analysis to the fault diagnosis.The specific detection capability of wavelet analysis can be used to efficiently diagnose the location of the faulty submodule.In this paper,Pearson correlation coefficient is introduced to determine the optimal wavelet function for fault diagnosis by comparing the similarity between fault detection signals and different wavelet functions.Finally,a DC transformer model is built on the MATLAB/Simulink simulation platform to verify the effectiveness and feasibility of the proposed fault diagnosis method.Aiming at the external faults of the modularized DC transformer,this paper analyzes the fault characteristics of the unipolar ground fault and bipolar short circuit fault on the DC side,and establishes the mathematical model of the fault.Through analysis,it can be concluded that the system can maintain stable operation for a period of time in the event of single pole ground fault of DC transformer,but there will be obvious DC bias in its high-frequency transformer,which will bring negative impact on theequipment;in the event of bipolar short-circuit fault,the main component of transient impulse current is the discharge current of sub module capacitance,and the main component of steady-state short-circuit current is the non fault side Feed in current through transformer coupling.The simulation results obtained on MATLAB / Simulink simulation platform verify the correctness of the theoretical analysis and provide a theoretical basis for the engineering application of DC transformer.