MTA: Line to Ground Fault

MTA for Line-Ground Fault

Consider R-Ph to ground fault. During this fault R-Ph voltage will decrease and R-Ph current will be lagging to R-Ph voltage nearly by 90⁰ as shown in the figure-1.

However Earth fault relay current I_n will be 180⁰ out of phase with respect to R-Ph current as shown in figure-2

Thus depending up on fault impedance R-Phase current may be anywhere as shown by red shaded area and thus corresponding earth fault relay current may be anywhere as shown by green shaded area for figure-3.

As there may be the fault on any phase and as during fault; volatge of faulty phase reduces nearly to zero it is not desirable to consider volatge of faulty phase for deciding the direction of fault current. It is obvious choice to use residual voltage V₀ (vector sum of Vr, Vy and Vb) for deciding the direction of fault current as shown in the figure.

To include all this probable area for earth fault current it is necessary to redefine zone of forward direction with respect to polarizing voltage V₀. 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 lags V₀. For electromagnetic relays this angle use to be -45⁰. Now for numerical relay this angle is settable still recommended value is -45⁰.