Module 3 : Sequence Components and Fault Analysis
Lecture 10 : Sequence Components
10.5 Fault Current Calculation in Sequence Domain
  Consider a transposed transmission line connected to an ideal voltage source E. The fault appears at the remote end of transmission line. We now derive sequence network interconnections for different fault types. We begin with a three phase fault.
10.5.1 Three phase fault: Three phase faults are considered to be symmetrical and hence sequence components are not
  necessary for their calculation.
    It can be easily shown that for a three phase fault, fault currents are balanced with,
I2 = I0 = 0 and I1 = Ia
(Hint : I012 = T -1 Iabc with Ib = a2Ia and Ic = aIa).
Thus, for a Three Phase Fault only Positive Sequence Network is considered. The fault currents are given by the following equations
(solid fault)  
(Fault through impedance Zf)  
  10.5.2 Single Line to Ground Fault (SLG):  
On an unloaded system (fig 10.7), let there be 'a' phase to ground fault with a fault impedance Zf. Then, the faulted system is described by,
Ia = If, Ib = 0 and Ic = 0. Applying sequence transformation, we get
Thus, I0 = I1 = I2 =Ia/3. Let Vf represent the voltage of the transmission line at the receiving end of the line where fault is created.
Further, from equation,
  (10)
  Equivalent in the sequence domain we get by premultiplying (9) by T -1
i.e.
   
  or  
      (11)
      (12)
      (13)
 
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