Research On Transformer AC/DC Hybrid Protection Strategy Based On Key Electromagnetic Parameter Identification

Under the conditions of the monopolar ground circuit operation mode of HVDC and geomagnetic storms,the adjacent AC grid neutral grounding transformer and associated electromagnetic equipment would be subject to DC interference.And DC components would be generated by a large number of non-linear components during operation.As a result,a special AC/DC environment is formed in the AC power grid,and transformers and other electromagnetic equipment suffered from the DC bias.Abnormalities and protection problems caused by DC bias in transformers.These issues have a great influence on the safe and stable operation of the power system and its major equipment.This paper is combined with the project funded by the National Natural Science Foundation of China “Study on DC Bias Protection Strategy in transformer based on key parameters identification in AC/DC hybrid environment”.The focus is on the study of transformer AC-DC hybrid simulation methods,key electromagnetic parameter identification and DC disturbance protection strategies.Combining the AC-DC hybrid problem,the nonlinear magnetic field and equivalent circuit model was constructed of transformer.The dynamic inductance is solved by the field model and the time-domain current is calculated using the road model,and dynamic inductance and time-domain current are used as key coupling parameters.Nonlinear magnetic field finite element calculation and time domain differential circuit solution are iterated,and indirect coupling of the field model is achieved.The time-domain field-circuit coupling model was used to analyze the excitation saturation of transformer in the condition of different DC injection modes and different operating modes.The changing laws of current,inductance,harmonics,magnetic fields,and magnetic flux leakage have are investigated under DC disturbances.The effect of different DC injection methods on the AC excitation of transformers was studied.Exciting current distortion,harmonic content and dynamic inductance change and intrinsic connection were analyzed.Magnetic anomaly characteristics characterized by key electromagnetic parameters when transformer suffered from DC disturbance.The correctness of the time-domain field-circuit coupling model and its calculation method was verified by comparing the simulation results with experimental data.The key electromagnetic parameters identification method of single-phase/three-phase transformers is studied based on the simulation of AC/DC hybrid electromagnetic characteristics.And the protection criteria of the transformer is established under DC disturbance.An AC/DC hybrid dynamic experimental platform was set up to study the applicability of different criteria under DC disturbance.An AC/DC hybrid protection strategy based on the identification of key electromagnetic parameters was proposed.The protection strategy based on single-side leakage inductance parameters has higher applicability was verified by simulation and experimental results.