Power Automation and Protection Division
I.L. 40-201.9
REL 352 Version 1.00
I-7
I
A
P
P
E
N
D
I
X
The following are external reverse faults.
The system should not trip.
8.2.
Phase-to-phase Unit
To calculate the apparent impedance seen by the relay for phase-to-phase faults the following formula applies:
where x is phase a, b or c and y is the next lagging phase.
The above formula is rigorous and general. However, if a quick approximation of the current required at different
angles (Ø) is desired, the following formula applies:
The phase-to-phase unit is totally directional. For forward Zone 2 trips the following quantities should be ap-
plied:.
V
I
AG
Va = 25
∠
0
Vb = 69
∠
-120
Vc = 69
∠
+120
Ia = 3.75
∠
+105
Ib = 0
Ic = 0
BG
Va = 69
∠
0
Vb = 25
∠
-120
Vc = 69
∠
+120
Ia = 0
Ib = 3.75
∠
-15
Ic = 0
CG
Va = 69
∠
0
Vb = 69
∠
-120
Vc = 25
∠
+120
Ia = 0
Ib = 0
Ic = 3.75
∠
-135
V
I
Z app
AB
Va = 35
∠
0
Vb = 35
∠
-120
Vc = 69
∠
+120
Ia = 10
∠
-45
Ib = 10
∠
+135
Ic = 0
3.08
∠
75
BC
Va = 69
∠
0
Vb = 35
∠
-120
Vc = 35
∠
+120
Ia = 0
Ib = 10
∠
-165
Ic = 10
∠
+15
3.08
∠
75
CA
Va = 35
∠
0
Vb = 69
∠
-120
Vc = 35
∠
+120
Ia = 10
∠
-105
Ib = 0
Ic = 10
∠
+75
3.08
∠
75
Zapp =
Vxy
Ix
Iy
–
--------------
I
V
XY
2 Z 2P
Pang
φ
–
(
)
cos
(
)
-----------------------------------------------------------
=
