sepam 20 user manual - Schneider Electric

3
Protection functions Phase overcurrent
ANSI code 50/51
Description
The phase overcurrent function comprises
4 independant elements divided into two groups
of 2 elements called Group A and Group B respectively.
The use of the two groups may be chosen by
parameter setting:
b operation with Group A or Group B exclusively, with
switching from one group to the other dependent on the
state of logic input I13 exclusively, or by remote control
(TC3, TC4),
I13 = 0 group A
l13 = 1 group B
b operation with Group A and Group B active for 4-set
point operation,
b enabling/disabling of each group of 2 elements
(A, B).
Operation
The phase overcurrent protection function is three-pole.
It picks up if one, two or three of the phase currents reach
the operation set point.
It is time-delayed. The time delay may be definite time
(DT) or IDMT according to the curves opposite.
Definite time protection
Is is the operation set point expressed in Amps, and T
is the protection operation time delay.
MT10533
MT10903
t
T
Is
Definite time protection principle.
IDMT protection
IDMT protection operates in accordance with the
IEC (60255-3), BS 142 and IEEE (C-37112) standards.
t
T
1
3/22
type 1
type 1.2
1.2 10 20 I/Is
IDMT protection principle.
I
The Is setting is the vertical asymptote of the curve, and T is the operation time delay
for 10 Is.
The tripping time for I/Is values of less than 1.2 depends on the type of curve chosen.
Name of curve Type
Standard inverse time (SIT) 1.2
Very inverse time (VIT or LTI) 1.2
Extremely inverse time (EIT) 1.2
Ultra inverse time (UIT) 1.2
RI curve 1
IEC standard inverse time SIT / A 1
IEC very inverse time VIT or LTI / B 1
IEC extremely inverse time EIT / C 1
IEEE moderately inverse (IEC / D) 1
IEEE very inverse (IEC / E) 1
IEEE extremely inverse (IEC / F) 1
IAC inverse 1
IAC very inverse 1
IAC extremely inverse 1
The curve equations are given in the chapter entitled "IDMT protection functions".
DE50371
MT10541
The function takes into account current variations during the time delay interval.
For currents with a very large amplitude, the protection function has a definite time
characteristic:
b if I > 20 Is, tripping time is the time that corresponds to 20 Is
b if I > 40 In, tripping time is the time that corresponds to 40 In.
(In: current transformer rated current defined when the general settings are made).
Block diagram
Timer hold delay
The function includes an adjustable timer hold delay T1:
b definite time (timer hold) for all the tripping curves.
T
I > Is time-delayed output
I > Is pick-up signal
value of internal
time delay
counter
T1 T1
T1
tripping
PCRED301005EN - 06/2008