MTA: Line to Line Fault

MTA for Line-Line Fault

Consider a part of the power system as shown in the figure. Let there be Line-Line fault at point A and let direction decision to be made by the relay at point B.

  

When there is no fault; vector diagram representing voltage and current is as shown in figure.

For R-Y fault R-Ph and Y-Ph source voltage will decrease and will approach close to each other and R-Ph current will lag approximately 90⁰ with respect to R-Y Phase voltage thus during fault vector diagram will be as shown in figure.

Thus depending up on nature of load and fault impedance R-Phase current may be anywhere as shown by shaded area for figure.

Obviously voltage selected for this direction decision making will be of healthy phases. That means if we are considering direction decision making in respect of R-Ph current then we have to choose V_YB voltage as our reference voltage as shown in figure.

To include all this probable area for R-Ph current it is necessary to redefine zone of forward direction with respect to polarizing voltage V_YB. Thus line AB is selected as new dividing line for operating direction (Forward) and Non-Operating direction (Reverse). Perpendicular line to this dividing line is line C-D. Now we can see that line CD leads V_YB. For electromagnetic relays this angle use to be 45⁰. Now for numerical relay this angle is settable still recommended value is +45⁰.