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Characteristic of idmt curves for overcurrent relays
1. characteristic of IDMT curves for overcurrent
relays
โข There are many types of IDMT
curve as shown in the figure
below.
โข Long time inverse type
โข IEC inverse type
โข IEC very inverse type
โข IEC extremely inverse
2. Equations of IDMT curves
โขT= ๐๐๐(
๐พ
(
๐ผ๐ .๐
๐ผ๐ ๐๐ก
) ๐ธโ1
)
โข These standard according to IEC
60255 standard characteristics for
constant parameters (K and E)
โขIsetis pickup setting of the current.
โขIs.cis a value of short circuit current
No Relay characteristics K E
1 Standard inverse 0.14 0.02
2 Very inverse 13.5 1
3 Extremely inverse 80 2
4 Long time inverse 120 1
3. Calculate and Comparison between times
according to curve, TMS, Iset, and Is.c.
โข If the TMS is equal to 0.1 and CT is
equal to 300/5 and Iset is equal to
5A as a secondary value.
โข Is.c = 2 times of Iset
โขIs.c = 4 times of Iset
โขIs.c = 6 times of Iset
โขIs.c = 10 times of Iset
โขIs.c = 20 times of Iset
4. Type of curve Iset
value
Is.c value K & E
value
equations Result of
time
Standard inverse
300
300
300
300
300
2T=600
4T=1200
6T=1800
10T=3000
20T=6000
K=0.14
E=0.02
T= ๐๐๐(
๐พ
(
๐ผ๐ ๐๐ก
๐ผ๐ .๐
) ๐ธโ1
) 1.002
0.4979
0.3837
0.2970
0.2267
Very inverse
300
300
300
300
300
2T=600
4T=1200
6T=1800
10T=3000
20T=6000
K=13.5
E=1
T= ๐๐๐(
๐พ
(
๐ผ๐ ๐๐ก
๐ผ๐ .๐
) ๐ธโ1
) 1.35
0.45
0.27
0.15
0.0710
Extremely inverse
300
300
300
300
300
2T=600
4T=1200
6T=1800
10T=3000
20T=6000
K=80
E=2
T= ๐๐๐(
๐พ
(
๐ผ๐ ๐๐ก
๐ผ๐ .๐
) ๐ธโ1
) 2.666
0.5333
0.2285
0.0808
0.020
Long time inverse
300
300
300
300
300
2T=600
4T=1200
6T=1800
10T=3000
20T=6000
K=120
E=1
T= ๐๐๐(
๐พ
(
๐ผ๐ ๐๐ก
๐ผ๐ .๐
) ๐ธโ1
) 12
4
2.4
1.3333
0.6315