Module 2 : Current and Voltage Transformers
Lecture 7 : CT Saturation and DC Offset Current
7.1

Origin of DC Off-set Current

  Typically, fault current consists of symmetrical ac component and a dc offset current. To understand this issue, consider an unloaded transmission line excited by a voltage source. The fault strikes at time . This can be simulated by closing the switch at in fig 7.1.
If  or models the line impedance, then the fault current in the line is given by the following expressions.
 

   
   
where is the time constant of the line = Lline/Rline. Fig 7.2 shows a typical wave form of fault current containing DC offset current. The fault current can be decomposed into two components. The first component models the steady state sinusoidal ac response while the second component is the dc offset current due to the presence of inductance in the circuit. Recall that current in an inductance can not change instantaneously. DC offset current is a consequence of maintaining initial condition . While the dc offset current in theory, would persist till infinity, it's trace in the actual wave form would not be seen beyond a few time constants. Table - 1 illustrates the values of up to 10 time constants.

   
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