Hitachi 670 Series Application Manual page 44

Section 4
Analog and binary signal transfer for line differential protection
138
Z =
S
A
1700
EQUATION14000034 V1 EN-US
138
Z =
S
B
1200
EQUATION14000035 V1 EN-US
Calculating the short circuit impedance of the transformer gives:
e
Z =
trf
100
EQUATION14000036 V1 EN-US
Based on the Thevenin equivalent, it is possible to calculate the fault current on the HV side of the
transformer:
If
=
138
EQUATION14000037 V1 EN-US
where:
(
Zs
Z =
res
EQUATION14000039 V1 EN-US
The numerical value for Z
To avoid unwanted operation of the differential protection for a fault on the LV side of the transformer,
IdMin must be set to:
If
1 2 .
> ×
I
EQUATION14000038 V1 EN-US
IdMin
=
EQUATION14000040 V1 EN-US
To allow the differential protection to act as a backup protection for internal faults and faults on the LV
side of the transformer, AddDelay is set to On, and a suitable setting is calculated for Imax AddDelay.
Differential currents below the set value for Imax AddDelay are time delayed. Value two times the
rated current of the transformer on the HV side is chosen. The setting is calculated as:
ImaxAddDelay
EQUATION14000041 V1 EN-US
38
2
= 11 2 . Ω
2
= 15 9 . Ω
2 2
U 138
k n
× = 0 1 . × = 190 4
S 10
n
138
3 Z
×
res
) ( )
+ Zl × Zs + Zl
a
A B B
+ Z
Zs + Zl + Zs + Zl
A a B B
input in the formula for If
res
138
Base
If
1 2 . 138 11 5 .
× =
I
Base
S
n
2
= ×
3 U I
× ×
n Base
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. Ω
= 198Ω
trf
gives If
138
2 0 .
=
1MRK505382-UEN Rev. K
= 403 A
138
Communication set-up, 670/650 series
Application Guide
(Equation 7)
(Equation 8)
(Equation 9)
(Equation 10)
(Equation 11)
(Equation 12)
(Equation 13)
(Equation 14)