Siemens Siprotec 7SD600 User Manual

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A - 33
Siemens AG ⋅ January 1999
The operational accuracy limit factor for the respecĆ
tive line end must be calculated with formula 1. The
protection is stable when the operational accuracy
limit factor for line end 1 is greater than the necessary
operational accuracy limit factor kn
1
' calculated with
condition 3.1. The same applies for line end 2 with
kn
2
'.
3.3The condition for stability is therefore:
n
1
'>kn
1
' and n
2
'>kn
2
'
The following example explains the situation:
Maximum through fault current
I
kd, max
= 7200 A
Line end 1:
Current transformer 10P20, 10 VA
Rated accuracy limit factor n
1
=20
Rated burden of the current transformer
P
N
=10VA
Internal burden of the current transformer
P
E
= 1.5 VA
Connected burden (summation transformer
+ connecting cable) P
w
=4VA
Primary rated current of the transformer
I
Npr
= 800 A
With (1) the operational accuracy limit factor is
calculated at n
1
' = 41.8
Line end 2:
Current transformer 10P40, 20 VA
Rated accuracy limit factor n
2
=40
Rated burden of the current transformer
P
N
=20VA
Internal burden of the current transformer
P
E
=3VA
Connected burden (summation transformer
+ connecting cable) P
w
=6VA
Primary rated current of the transformer
I
Npr
= 800 A
With (1) the operational accuracy limit factor is
calculated at n
2
' = 102
The ratio of the operational accuracy limit factors
n
1
'/n
2
' = 2,4 violates condition 2 because the current
transformers are too different. Condition 1 is satisĆ
fied, i.e. the maximum steady-state fault current
k
1
'=9istransmitted without saturation. In the clasĆ
sic dimensioning, no statement could be made about
the stability of the protection layout.
Condition 3.3 must be checked additionally to obtain a
statement on the stability of the protection layout.
k
1
'=k
2
'=I
kd, max
/I
Npr
= 7200 A/800A=9
With 3.1/3.2 the necessary operational accuracy limit
factor is given for the saturation-free time and 9 tiĆ
mes the CT rated current flowing for line end 1 and
line end 2:
kn
1
'=kn
2
'=4*9=36
The calculated operational accuracy limit factors
n
1
' = 41 for line end 1 and n
2
' = 102 are respectively
above the necessary accuracy limit overcurrent factor
so that the differential protection layout can provide
stable operation despite very different current transĆ
formers during external short-circuits.

Siemens Siprotec 7SD600 Specifications

General

Brand	Siemens
Model	Siprotec 7SD600
Category	Protection Device
Language	English