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For fault conditions, the DC offset is caused by where the fault occurs on the voltage waveform.
When the fault initially occurs, the voltage can be looked at as an AC waveform superimposed onto an impulse response (A DC wave). The magnitude of the impulse is equivalent to the voltage when the fault initiated (flash over usually occurs near a voltage peak, providing a potentially large DC component).
The inductance of the system (j*w*L) appears as 0 impedance to this DC voltage.
Z (impedance) = R (resistance) + X (inductance), usually X >> R, X = j*w*L
Since X = 0 for the impulse, there is an initial large current since V = I / Z, and Z = R initially.
This current slowly dies off based on the time constant of the circuit, 1/(L*R).
A lot of times you will hear talk of the the DC component being caused by the "X/R ratio". The higher this ratio, the larger the DC component (generally) and the longer the time constant generally is (although this is not always the case).
Similar analysis applies to other causes of DC offset.
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∙ 14y ago
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