54
54
SIF5600 - Manual - 03 - 2008
7.3 cOOrDINAtION
In all applications, the trip and adjustment characteristics must be defined on the basis of a selectiv-
ity study having the aim of protecting each component of the system, obtaining the isolation of the
least number of parts of the system, in the event of a fault, and without any inappropriate tripping
due to transient overcurrents.
By way of example, the method used to determine the adjustment parameters, necessary for protec-
tion against overloading and short-circuiting on the LV side of a distribution transformer, are shown.
Depending on the transformer characteristics (to be supplied from the manufacturer) the trip char-
acteristics and the corresponding settings on the SIF5600 relay will be identified.
transformer data:
- Nominal power:
2000 kVA
- Nominal primary voltage:
20 kV
- Nominal secondary voltage: 6 kV
- Short-circuit voltage:
Vcc% = 6%
the transformers nominal secondary current is:
I
nt
= 2000 / √3 × 6 = 192 A
the transformers secondary short-circuit current (with infinite upstream grid short circuit power)
is:
IccBt= 100 *
I
nt
/ Vcc% = 3210 A
If the transformers significant load is constituted by a motor with nominal current
I
nM
= 100 A, start-
up current of 500 A and start-up time of 2 s, protected against overloading and short-circuiting by a
relay with trip curves shown in the following diagram.
For coordination purposes trip curve of the transformer protective relay must be:
- below the transformers overload curve and short-circuit capacity,
- higher than the motor start-up current curve so as not to trip inappropriately
- selective with the motor protection curves,
- such as to detect short circuit currents on the LV side of the transformer.
From standard Iec curves, it may be observed that better approximation of the overload capacity
curve of the transformer can be achieved with type c curves.
It is necessary to use trip characteristics, having as asymptote, the overload value which the
transformer can permanently support: in the example, it is equal to 120% of the nominal current
I
nt
(192 * 1.2 = 230 A).
Since the asymptotic value of the time-dependent curves corresponds to 1.1 times the value of the
adjustment threshold
I
>
dep
, it is:
I
>
dep
= 230 / 1.1 = 210 A
It is furthermore observed that corresponding to current value 4
I
>
dep
, the desired curve has a trip
time of 10 s.
the operate time setting may be derived from Iec formula (extremely inverse type) with
I
= 4
I
>
dep
and t = 10 s constraint, so:
t
>
dep
=
t
[(
I / I
>
dep
)
2
- 1] / 80 = 10
·
[4
2
- 1] / 80 = 1.87 s
Finally, it is necessary to know the nominal primary current of the current transformers, which it is
assumed in the example under consideration is 250 A. Obviously the nominal secondary current of
the current transformers must coincide with the nominal current set in the SIF5600 relay.
the setting value is computed by dividing the threshold value by the cts nominal current:
I>= 210 / (250)
I
n
= 0.84
I
n
the second threshold, which is time-independent, is obtained by dividing the threshold value by the
nominal current of the cts:
I>>= 1500 / 250
I
n
= 6.0
I
n
50-51
2000 kVA
SIF5600
49-50
20/6 kV
Vcc=6%
M
B
A