Electrical Engineering Interview Questions and answers:Job interview or viva Topics for Electrical Engineers (Part-7).
In this topics Fault analysis procedure is described.
27. The symmetrical designed generator only generates the voltage of positive phase sequence. Whether, the system is faulted or not there is only positive sequence voltage.28. Fault analysis procedure:
Basic fault analysis procedures are as follows,• Circuit diagram.
• Boundary condition.
• Transformation.
• Sequence currents and sequence voltages.
• Interconnections of sequence networks.
• Information from sequence networks.
• Circuit diagram:
If all the fault and phase connection are shows in the circuit diagram from that the fault analysis is possible.• Boundary condition:
The relation between the known phase current and Voltage at the fault point of the line is known as the boundary condition.• Transformation:
From the boundary condition we found symmetrical components of voltages and currents by solving the equation. This process is called Transformation from the a-b-c to 0-1-2 system. The relation for the transformation is,V012=Aˉ¹Vabc
In the transformation step we found that, for single line to ground fault the sequence currents are equal.
I012=Aˉ¹Iabc
Ia0=Ia1=Ia2=(Ia/3)
Here,
Ia0 = zero sequence current.
Ia1= positive sequence current.
Ia2= negative sequence current.
• Sequence currents and sequence voltages:
For determining the interconnection of sequence networks, the sequence currents and sequence voltages which is found in the transformation step is examined. The impedances which are found may be added in the sequence network.• Interconnections of sequence networks:
The sequence networks are interconnected in such a way by this the equation describing the fault conditions are satisfied. The interconnection shows the constraints impressed on the system by the fault.• Information from sequence networks:
From the known angular relation of the balance sheet the voltages of phases and currents are found sequentially.-- - - - -- --- - -- - - -- - - - -- - -- - - - - - -- - - - -- - - - - - - - -
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