2 differential protect the case study that uses by accident

2.1 excitation current causes transformer differential protection to be moved by accident

The characteristic of transformer excitation current is normal moving circumstance next its values are very little, do not exceed transformer commonly of rated electric current 3% one 5% , transformer job is in the linear of magnetic flux paragraph OS, if pursue 1. Core is not saturated, its guide magnetism leads μ relatively very big, the excitation inductance of transformer winding is very big also. When producing exterior short circuit, because voltage drops, excitation electric current is smaller, because this is opposite below these circumstances,the influence of excitation electric current need not consider commonly [3] .

Graph the relation curve of 1 Φ= F (I) and U = F (I)

Fig. 1 Function Of Φ= F (I) And U = F (I)

When the voltage after transformer airdrop or breakdown are excised restores, because the magnetic flux in transformer core is urgent leap is great, make core instant saturated,

Guide magnetism rate is close to relatively 1, transformer winding inductance is reduced, accompany the excitation current with occurrence very great numerical value, what the bag contains very large part is aperiodic heft and heft of Gao Cixie wave, give priority to with 2 harmonic, the 6 ～ that its numerical value can achieve rated electric current 8 times above, the excitation current of occurrence fastigium appearance, if pursue 2, in attenuation of current of initiative instant excitation very fast, to general medium or small transformer, after classics 0.5 ~1s, its are worth the 0.25~0.5 that does not exceed rated electric current times, the attenuation rate of current of large transformer excitation is slower, want 2~3s to afore-mentioned values damply, already the capacity of transformer is jumped over damply greatly more slow, at the same time excitation current weaveform appears discontinuous, have discontinuous part, this electric current flows into differential relay, the likelihood causes protector to be moved by accident [4] .

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