Siemens SIPROTEC 7UM62, Manual

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Functions
198
7UM62 Manual
C53000-G1176-C149-3
The conversion factor of the earth resistance is set as
FACTOR R SEF
at address
0275
in Power System Data 1. The general formula for calculation (R
Eprim
/ R
Esec
) is:
This formula is true only for nearly ideal earthing or neutral transformers. If necessary,
the measuring result from the primary tests must be set as
FACTOR R SEF
. For this
the fed fault resistance (trip stage) is put into relation with the secondary fault resis-
tance.
The primary fault resistance should be set to 1 to 2 k
Ω
for the trip stage and to approx.
3 to 8 k
Ω
for the alarm stage. The default times have proven to be good values.
Example:
Loading resistor R
L
10
Ω
(10 A continuously, 50 A for 20 s)
Miniature CT ü
MinCT
200 A / 5 A
The transformation ratio of the miniature CT 400 A:5 A has been reduced to 200 A:5 A
by the primary conductor passing twice through the transformer window.
This yields for FACTOR R SEF a value of:
If for the trip stage R<< a generator-side fault resistance of 1000
Ω
is chosen, a resis-
tance of
R<< SEF TRIP
= 1000
Ω
/8.33 = 120
Ω
is set at address
5303
. For the warn-
ing stage, a primary resistance of 3 k
Ω
yields a setting value of
R< SEF ALARM
= 360
Ω
.
Earth Current Stage The earth current stage has a backup protection function. It is set to a protected zone
of approx. 80 %. Referenced to the maximum secondary fault current, the pickup
threshold is at approx. 20 %, and the setting value is calculated as follows:
The delay time
T SEF TRIP
(address
5305
), which is also relevant for the earth cur-
rent stage, must be less than the 50 A capability time of the loading resistor (in this
example 50 A for 20 s). The overload capability of the earthing or neutral transformer
must also be considered if it lies below that of the loading resistor.
Monitoring At the addresses
5307
,
5308
the monitoring thresholds are set with
U20 MIN
and
I20 MIN
. If the 20 Hz voltage drops below the pickup value without the 20 Hz current
rising, there must be a problem of the 20 Hz connection. The default settings will be
adequate for most applications. Where the loading resistor is less than 1
Ω
, the volt-
age threshold
U20 MIN
must be reduced to 0.5 V. The current threshold
I20 MIN
can be left at 10 mA.
FACTOR R SEF ü
Transf
2
ü
Divider
ü
MinCT
--------------------
⋅
=
10kV
3
--------------
500V
3
--------------
⁄
Earthing transformer
ü
Transf
FACTOR R SEF
10 000 V
3
------------------------
3
500 V
----------------
⋅
è ø
æ ö
2
9
-----------------------------------------------------
500 200
⁄
200 5
⁄
------------------------ 8.33
=
⋅
=
SEF100 I>> 0.2
U
Nsec
R
L
----------------
1
ü
MinCT
------------------- 0.2
500 V
10
Ω
----------------
5 A
200 A
----------------
⋅ ⋅
0.25 A
= =
⋅ ⋅
=