Research On Ferroresonance In Distribution Network Based On Electric Field

Ferro-resonance overvoltage is one of the most common innerover-voltage in the small current grounding system，which is a seriousthreat to the stable operation of the distribution network．In this paper，ferro-resonance caused by voltage transformer in ungrounded neutralsystem is mainly studied and a new method to detect and identify theferromagnetic resonance phenomenon is raised．This paper first introduces the current condition and the existingdrawbacks of ferro-resonance research at home and abroad．On the basis ofexisting theory， it analyzes the basic principle and the conditions ofresonant circuit．Besides，the ferro-resonance influencing factors are alsodiscussed．For distribution network with neutral unearthed，this paper doesthe further analysis with symmetrical component method．Supposing Aphase is excited to ferro-resonance，analyzes quantitatively the neutral pointvoltage，the positive，negative and zero sequence components of voltage andcurrent．Besides，how the voltage transformer saturation degree influenceson the neutral point voltage is studied．At the same time，this paper also studies the electric field quantityaround transmission lines and the quasi static approximation method is usedto decouple electromagnetic field．This article regards the electric fieldaround three-phase transmission line under the normal operation andferro-resonance fault as electrostatic field and utilizes the equivalent chargemethod to analyze the vertical and horizontal component of the electricfield and then decomposes symmetrically．In addition to， this paper deeplystudies the relationship between the electric field around transmission linesand the neutral point voltage．Based on the above theoretical research，this paper suggests take theelectric field signal that is just below the line and near the ground as thedetected signal to identify ferro-resonance， which is a new method toresearch ferro-resonance phenomena. Comparing the advantages anddisadvantages of the wavelet transform and Fast Fourier Transform (FFT)， a comprehensive algorithm of wavelet analysis and FFT is proposed．Thewavelet analysis is to detect the resonant occurrence time，and then FFTanalyzes energy spectrum of resonance signal to determine the resonantfrequency．Finally，the ferromagnetic resonance simulation model with neutralungrounded is built by Matlab/Simulink．The relationship between neutralpoint voltage and electric field is verified by comparing theirwaveforms．The wavelet toolbox and FFT function in Matlab can test thedetection algorithm and accurately identify the resonance occurring timeand frequency， that is， division， fundamental and high frequencyresonance． The simulation results proved the effectiveness andpracticability of the proposed method in this paper．...