Question

Question:For a line-to-line fault with a fault impedance ZF, the positive-and negative-sequence networks are to be connected ________ at the fault terminals through the impedance of times ; the zero-sequence current is ________.

Short answer

Answer

For a line-to-line fault with fault impedance , the positive-and negative-sequence networks are to be connected at the fault terminals through the impedance of times ; the zero-sequence current is .

Step-by-step solution

Definition of line-to-line fault.

A line to line fault or unsymmetrical fault occurs when two conductors are short circuited due to breaking of line and falling on each other or on the ground.

Determine the complete and correct statement.

Consider is a zero sequence fault current, is positive sequence fault current and is negative sequence fault current.

Consider is zero sequence fault voltages, is positive sequence fault voltage and is negative sequence fault voltage.

Consider is line to ground voltage at phase, is line to ground voltage at phase and is line to ground voltage at phase.

Consider is fault voltage regarding a bolted three phase fault.

Determine line to line fault current is

,

Consider the expression for the voltage.

The relation between phase current and sequential fault current show in matrix format.

Here, and are phase currents.

Rewrite the expression as,

As reference of equation (8.1.4), (8.1.5) and (8.1.21) in the textbook.

Using above equation in expression

Since,

…… (1)

…… (2)

Simplify above equations as

…… (3)

From above equation (1), (2) and (3) it is clear that the sequence networks are connected parallel through the impedance.

Therefore, for a line-to-line fault with fault impedance , the positive-and negative-sequence networks are to be connected at the fault terminals through the impedance of times ; the zero-sequence current is .