Research On Condition Monitoring Technology Of Distribution Transformer In Single-phase Grounding Operation Of System

The 35 k V and below distribution system in China usually adopts the neutral point non-effective grounding operation mode.After single-phase grounding occurs on the high-voltage side of the distribution transformer,the phase voltage of the phase that does not have a fault will rise to the line voltage,and the distribution transformer can continue to operate for 1 ～ 2 hours when the insulation meets the requirements.The distribution transformer is connected to the distribution line,and the single-phase grounding operation is allowed to operate for 1 ～ 2 hours according to the operation requirements.In this period,the insulation weak link of the distribution transformer is prone to failure.Therefore,it is necessary to monitor the state of the distribution transformer in the single-phase grounding operation of the system.In view of the current state monitoring method based on the voltage and current port data of the high and low voltage sides of the distribution transformer,when the single-phase grounding occurs in the system,due to the change of the voltage and current of the high voltage side of the distribution transformer,the monitoring method is no longer applicable within several hours of the single-phase grounding operation of the system.Therefore,this paper proposes a distribution transformer condition monitoring method suitable for normal operation and single-phase ground operation of the system.In this paper,Ansoft Maxwell software was used to analyze the electromagnetic and loss operation characteristics of the distribution transformer under single-phase ground operation,the influence of single-phase grounding short circuit at high voltage side of distribution transformer on operation transformer was analyzed,and the relationship between the winding insulation failure of the distribution transformer and the change of its electrical performance parameters was analyzed,which provides a certain reference value for the establishment of the state evaluation index system of the distribution transformer.Secondly,the parameter identification method of fundamental wave inductance and resistance of distribution transformer winding based on the positive and negative sequence components of fundamental wave of voltage and current was proposed.This identification method is suitable for the normal operation of the system and the single-phase-to-ground operation of the system.The fundamental wave sequence component of the operating voltage and current was extracted by using 90° phase-shift circuit with the second-order generalized integrator as the core.Taking the distribution transformer circuit equation as the model,the least square identification model equation based on the positive and negative sequence components of fundamental wave of voltage and current was established.The parameter identification results are of high precision,and the defects of the high-voltage side winding or the low-voltage side winding could be correctly identified.Then the calculation formula of harmonic no-load loss was obtained according to the relationship between no-load loss and operation frequency and the internal magnetic induction intensity of distribution transformer core.According to the Γ-type equivalent circuit and superposition principle of transformer,the winding resistance loss model under harmonic conditions was established.Based on the proportional relationship between winding resistance loss and winding eddy current loss and stray loss in the improved IEEE method,the online acquisition method of harmonic load loss of distribution transformers with Dyn11 and Yyn0 winding connection modes was obtained.The simulation verified the calculation model of fundamental and harmonic losses of distribution transformer,and analyzed the influence of the change of operation conditions of distribution network on fundamental and harmonic losses.Finally,in order to facilitate the proposed method of on-line identification of winding parameters and calculation of loss parameters to monitor the operation status of distribution transformers applied to practice,this paper designed a condition monitoring scheme suitable for distribution transformers,and studied the condition assessment system and method based on the electrical performance parameters of on-line monitoring,combined with auxiliary family defects and operation inspection records.