where:
Z
Zs Zl Zs Zl
Zs Zl Zs Zl
Z
res
A B B
A a B B
trf
a
=
+
( )
× +
( )
+ + +
+ =198Ω
EQUATION14000039 V1 EN-US (Equation 24)
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 a fault on the LV side
of the transformer, IdMin must be set to:
EQUATION14000038 V1 EN-US (Equation 25)
IdMin
If
I
Base
= × =1 2 11 5
138
. .
EQUATION14000040 V1 EN-US (Equation 26)
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
S
U I
n
n Base
= ×
× ×
=2
3
2 0.
EQUATION14000041 V1 EN-US (Equation 27)
Table 12: 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)
Setting 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 important to achieve a proper back-up protection. The short circuit protection
on the outgoing bays and on the LV side of the transformer are set according to a
prepared selectivity chart. An example in Figure 48 shows that the setting of the
short circuit protection on the LV side is 14 MVA, and Normal Inverse has k=0.12
to give back-up to outgoing bays' relays which are extremely inverse and selective
to remote fuses.
1MRK 505 393-UEN B Section 6
Differential protection
Line differential protection RED650 2.2 IEC 103
Application manual
To Next Page
Loading...
