ABB RELION 620 Series Technical Manual page 682

Section 4
Protection functions
676
Inverse time
counter
100%
50%
START
GUID-C6AAFC57-08D7-4AF9-99D6-6153D9A7D0DC V1 EN
Figure 341: An example of a delayed reset in the inverse time characteristics.
When the start becomes active during the reset period, the operate
time counter continues from the level corresponding to the drop-off
(reset time = 0.50 · Cooling time)
Overexcitation IDMT curves 1, 2 and 3
The base equation for the IDMT curves "OvExt IDMT Crv1", "OvExt IDMT Crv2"
and "OvExt IDMT Crv3" is:
ak b 100 M
+ −
 
 
c
 
t s ( ) = 60 e
⋅
GUID-FCF33CC8-E4D9-4339-BBAD-593465267F6D V1 EN
t(s) Operate time in seconds
M Excitation level (U/f ratio or volts/hertz) in pu
Time multiplier setting
k
The constant "60" in
seconds.
Table 639: Parameters a, b and c for different IDMT curves
Operating curve type
a
setting
OvExt IDMT Crv1 2.5
OvExt IDMT Crv2 2.5
OvExt IDMT Crv3 2.5
Operate level
Drop-off START Reset
START
resets
reset time
Equation 134 converts time from minutes to
b
115.00
113.50
108.75
1MRS757644 F
25%
time
(Equation 134)
c
4.886
3.040
2.443
620 series
Technical Manual