Application Guide - NP900 Series
327 (504
)
A996A
changer is in maximum position thus causing the absolute measurement to be 1 xIn +
TCE
IMEAS
UNC
=
TCE + CTEsec × TCE
1 + TCE
× 100
With our example configuration the calculation would be:
IMEAS
UNC
=
0.1 + 0.05 + 0.125 + (0.05 × 0.125)
1 + 0.125
× 100 =
25%
Now this value presents the transformer properties maximum caused differential current to
nominal. Other uncertainties known now may be added to this got value and after this
operation can be said that:
I
db>Pick
-
up
> IMEAS
UNC
+ 2 × REm + AUTE + TME
→ 25%
+ 2 × 0.
5%
+ 2.
5%
+
3%
=
31%
This means that in worst case this differential current will flow while the operation is still
normal in the transformer. Therefore normally this got combined result is increased with
safety marginal to ensure stabile operation of the differential protection and to negate
possible calculation errors.
With these values following base sensitivity (e.g. the minimum setting for the differential
current required to operate the relay), is given for the differential protection characteristics:
I
db>Pick
-
up
=
�
TCE + CTEsec × TCE
1 + TCE
× 100
�
+ 2 × REm + AUTE + TME + SME =
36%
Now this basic sensitivity is taking account the starting situation with no load to Turnpoint
1 in the characteristics. Next thing is to decide where to set the Turnpoint 1. In most of
differential relays this point is either fixed or automatically defined based on base
sensitivity and slope 1. In NP900 differential relay it is settable. For high sensitivity it may
be set to 1 xIn since the calculated base sensitivity takes into account already the tap
changer effect and all of the other normal operating caused differential current sources.
For coarse settings this Turnpoint 1 can be set to for example 0.5xIn or even 0.01xIn. How
to determine this limit is the sum of the protection principle wanted. Smaller value leads to
conservative and stabile operation, while bigger means highly sensitive and possibly
unstable protection.
Also there if Turnpoint 1 is set to 0.01xIn, the Slope 1 will start directly from the
I
db>Pick
-
up
setting. In such case there is no unbiased sensitive section available. This could
be used in case when the tap changer effect is not wanted to be taken into account for
basic sensitivity and the effect is wanted to be taken into account in the first slope directly.
Содержание NP900 Series
Страница 1: ......
Страница 49: ...Application Guide NP900 Series 49 504 A996A Figure 3 2 1 3 25 Definite time operating characteristics...
Страница 50: ...Application Guide NP900 Series 50 504 A996A Figure 3 2 1 3 26 IEC predefined characteristics NI VI LTI and EI...
Страница 52: ...Application Guide NP900 Series 52 504 A996A Figure 3 2 1 3 28 IEEE predefined characteristics EI MI and VI...
Страница 243: ...Application Guide NP900 Series 243 504 A996A Figure 3 3 2 1 122 NPS biased thermal trip curves with kNPS values 7 and 10...
Страница 491: ...Application Guide NP900 Series 491 504 A996A 3 7 5 6 SETTING EXAMPLE...
Страница 504: ...Application Guide NP900 Series 504 504 A996A A996A...