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The numerical value for
Z
res
input in the formula for
If
138
gives
If
138
= 403 A
To avoid unwanted operation of the differential protection for fault on LV side of the
transformer, the setting of
IdMin must be set to:
>
×
1 2
138
.
If
I
Base
EQUATION14000038 V1 EN-US
(Equation 39)
IdMin
If
I
Base
=
×
=
1 2
11 5
138
.
.
EQUATION14000040 V1 EN-US
(Equation 40)
In order to allow the differential protection to be backup protection for internal faults and
faults on the LV side of the transformer, we activate the function
AddDelay by setting it to On
and calculate suitable setting for the parameter
Imax AddDelay. Differential currents below the
threshold
Imax AddDelay will be time delayed. We choose the value 2 times the rated current of
the transformer on HV side. The setting will be:
ImaxAddDelay
S
U
I
n
n
Base
=
×
×
×
=
2
3
2 0
.
EQUATION14000041 V1 EN-US
(Equation 41)
Table 16: Backup protection example data
Station
Tap transformer 10 MVA
Type of fault
Three phase fault
Object
1 = 10 kV Line A
2 = T1 10 kV 50/51
3 = 130 kV 87L
(ImaxAddDelay)
Settings
2 MVA
14 MVA
20 MVA
Characteristic
IEC Extr.inverse, k=0.5,
A=80, p=2
IEC Norm.inverse k=0.12,
A=0.14, p=0.02
IEC Norm.inverse k=0.18,
A=0.14, p=0.02
High set stage
14 MVA
- - - -
- - - -
It is then important to achieve a proper back-up protection. The Short circuit protection on the
outgoing bays and on the transformer LV side are set according to a prepared selectivity
chart. An example is prepared and shown in Figure
where the setting of the short circuit
protection on the LV side is 14 MVA and Normal Inverse with k=0.12 to give back-up to
outgoing bays relays which are extremely inverse, selective to remote fuses.
1MRK 505 343-UEN B
Section 6
Differential protection
139
Application manual
Содержание Relion 670 series
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