Areva MiCom P120, Technical Manual

Page: 235 / 668

Application Guide P12x/EN AP/A96
MiCOM P120/P121/P122/P123
Page 13/80
The reset timer "t Reset" for MiCOM P122 & P123 can be found in the following menu:
−
If the first phase (earth) threshold is selected with an IDMT IEC or RI curve, the reset
timer "t Reset" with DMT characteristic can be set under the menu:
−
Protection /[50/51] Phase OC/t Reset for the phase.
−
Protection /[50N/51N] E/Gnd/t Reset for the earth.
−
If the first phase (earth) threshold is selected with an IDMT IEEE or CO curve, the
reset timer "t Reset" with a DMT or IDMT characteristic can be set under the menu:
−
Protection /[50/51] Phase OC/Type Tempo Reset for the phase
−
Protection /[50N/51N] E/Gnd/Type Tempo Reset for the earth.
Reset Time "t Reset" with an IDMT characteristic:
The mathematical formula applicable to the five curves is:
( ) ⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
× =
α
s
I I
1
K
T t
Where:
t = Reset time
K = Coefficient (see table)
I = Value of the measured current
I
S
= Value of the programmed threshold (pick-up value)
α
= Coefficient (see table)
T = Reset Time Multiplier (Rtms) between 0.025 and 3.2
Type of curves Standard K factor
α
factor
Short time inverse C02 2.261 2
Moderately Inverse ANSI/IEEE 4.85 2
Long time Inverse C08 5.95 2
Very inverse ANSI/IEEE 21.6 2
Extremely inverse ANSI/IEEE 29.1 2
2.6 Time graded protection
Inverse definite minimum time relays are time graded in such a way that the relay closer to
the fault operates faster than the upstream relays. This is referred to as relay co-ordination
because if the relay nearest to the fault does not operate, the next relay will trip in a slightly
longer time. The time grading steps are typically 400 ms, the operation times becoming
progressively longer with each stage.
When difficulty is experienced in arranging the required time grading steps, the use of a
blocked overcurrent scheme should be considered (described in a later section).
NOTE : The dynamic range of measurement is typically 1000 times minimum
setting.