FUNCTION CHARACTERISTICS
122
NA30 - Manual - 04 - 2022
CT supervision - 74CT
Preface
The CT monitoring function is employed to issue an alarm when secondary phase CTs and/or phase
input of the NA30 relay failure are detected.
Interruptions are detected by means of a symmetry criterion of the
I
L1
,
I
L2
,
I
L3
input currents.
The symmetry factor is calculated comparing the minimum and maximum of the fundamental compo-
nents of the three phase currents (
I
LMIN
/
I
LMAX
).
Operation and settings
The starting of the timer occurs if both the following conditions are filled:
A) (
I
LMIN
/
I
LMAX
) < S
<
that is the symmetry factor is lower than the S
<
adjustable threshold;
B)
I
LMAX
> I*
Where
I*:
maximum phase current threshold
S<:
element pickup value
t
S<:
operate time
If both conditions are filled over the
t
S<
adjustable time an alarm is issued.
The output may be assigned to the selected
S<TR-K
output relays inside the
Set\CT supervision-
74CT
submenu; the same for addressing the LED indicators
S<TR-L
.
If the
S<-BLK1
parameter is set to
ON
, and a binary input is designed for logical block (Block1), the
CT supervision function is blocked off whenever the given input is active. The trip timer is held in re-
set condition, so the operate time counting starts when the input block goes down.
[1]
The
S<-BLK1
parameter is available inside the
Set \ CT supervision-74CT
submenu.
All the parameters are common for A and B Profiles.
Note 1 The exhaustive treatment of the logic block (Block 1) function may be found in the “Logic Block” paragraph inside
CONTROL AND MONITORING
section
char74CT.ai
I
*
α
I
LMIN
I
LMAX
tg
α
=S<
Current asimmetry monitoring - 74CT
TRIP
NO TRIP
CT monitoring logic diagram - 74CT
Fun-74CT.ai
&
0
T
t
s
<
t
S<
I
LMIN
/
I
LMAX
<
S
<
S
<
TR
IP
PIN
G
M
AT
RIX
(
LE
D+R
EL
AY
S)
Trip S<
Start I*
BLK1 S<
I
LMAX
>
I*
I*
Enable (ON
≡
Enable)
Block1 input (ON
≡
Block)
&
&
S<BLK1
Block1
Block1
Binary input INx
T
0
Logic
INx
t
ON
INx
t
ON
INx
t
OFF
T
0
n.o.
n.c.
INx
t
OFF
I
L1
I
LMAX
I
L2
I
L3
I
LMIN
I
LMIN
I
LMAX
S<TR-K
S<TR-L
RESET
