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Power Automation and Protection Division
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:
Vxy
------------- -
Zapp =
–
Ix
Iy
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:
V
XY
I
=
---------------------------------------------------------- -
(
(
Pang φ
2 Z 2P
cos
The phase-to-phase unit is totally directional. For forward Zone 2 trips the following quantities should be ap-
plied:.
REL 352 Version 1.00
V
AG
Va = 25 ∠0
Vb = 69 ∠-120
Vc = 69 ∠+120
BG
Va = 69 ∠0
Vb = 25 ∠-120
Vc = 69 ∠+120
CG
Va = 69 ∠0
Vb = 69 ∠-120
Vc = 25 ∠+120
)
)
–
V
AB
Va = 35 ∠0
Ia = 10 ∠-45
Vb = 35 ∠-120
Ib = 10∠+135
Vc = 69 ∠+120
Ic = 0
BC
Va = 69 ∠0
Ia = 0
Vb = 35 ∠-120
Ib = 10∠-165
Vc = 35 ∠+120
Ic = 10∠+15
CA
Va = 35 ∠0
Ia = 10∠-105
Vb = 69 ∠-120
Ib = 0
Vc = 35 ∠+120
Ic = 10∠+75
I
Ia = 3.75 ∠+105
Ib = 0
Ic = 0
Ia = 0
Ib = 3.75∠-15
Ic = 0
Ia = 0
Ib = 0
Ic = 3.75∠-135
I
Z app
3.08∠75
3.08∠75
3.08∠75
I.L. 40-201.9
A
P
P
E
N
D
I
X
I
I-7
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