t s
k A
B
U U
U
C
D
E
=
⋅
×
< −
<
−
+
(Equation 87)
t[s] operate time in seconds
A the set value of
Curve parameter A
B the set value of
Curve parameter B
C the set value of
Curve parameter C
D the set value of
Curve parameter D
E the set value of
Curve parameter E
U measured voltage
U< the set value of
Start value
k the set value of
Time multiplier
11.3.2.3 IDMT curve saturation of undervoltage protection
For the undervoltage IDMT mode of operation, the integration of the operate time
does not start until the voltage falls below the value of
Start value
. To cope with
discontinuity characteristics of the curve, a specific parameter for saturating the
equation to a fixed value is created. The
Curve Sat Relative
setting is the parameter
and it is given in percents compared with
Start value
. For example, due to the curve
equation B, the characteristics equation output is saturated in such a way that when
input voltages are in the range from
Start value
to
Curve Sat Relative
in percents
under
Start value
, the equation uses
Start value
* (1.0 -
Curve Sat Relative
/ 100 )
for the measured voltage. Although, the curve A has no discontinuities when the
ratio U/U> exceeds the unity,
Curve Sat Relative
is set for it as well. The
Curve
Sat Relative
setting for curves A, B and C is 2.0 percent. However, it should be
noted that the user must carefully calculate the curve characteristics concerning
also discontinuities in the curve when the programmable curve equation is used.
Thus, the
Curve Sat Relative
parameter gives another degree of freedom to move
the inverse curve on the voltage ratio axis and it effectively sets the maximum
operate time for the IDMT curve because for the voltage ratio values affecting by
this setting, the operation time is fixed, that is, the definite time, depending on the
parameters but no longer the voltage.
11.4 Frequency measurement and protection
All the function blocks that use frequency quantity as their input signal share the
common features related to the frequency measurement algorithm. The frequency
estimation is done from one phase (phase-to-phase or phase voltage) or from the
positive phase sequence ( PPS). The voltage groups with three-phase inputs use
PPS as the source. The frequency measurement range is 0.6...1.5 × F
n
. The df/dt
measurement range always starts from 0.6 × F
n
. When the frequency exceeds these
limits, it is regarded as out of range and a minimum or maximum value is held as the
measured value respectively with appropriate quality information. The frequency
estimation requires 160 ms to stabilize after a bad quality signal. Therefore, a delay
General function block features
1MRS758755 C
826 REC615 & RER615
Technical Manual
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