FUNCTION CHARACTERISTICS
267
NVA100X-D - Manual - 02 - 2016
CT supervision - 74CT
Preface
The CT monitoring function is employed to issue an alarm when secondary phase CTs side H and side
L and/or phase input of the 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(H)
/I
LMAX(H)
and I
LMIN(L)
/I
LMAX(L)
.
Operation and settings
The starting of the timer occurs if both the following conditions are fi lled:
I
LMIN(H)
/I
LMAX(H)
< S
(H)
< for CTs side H or II
LMIN(L)
/I
LMAX(L)
< S
(L)
< for
CTs side L;
I
LMAX(H)
> I
(H)
* for CTs side H and I
LMAX(L)
> I
(L)
* for
CTs side L
where
I
(H)
* and I
(L)
*: maximum phase current threshold
S
(H)
< and S
(L)
< : element pickup values
t
S(H)
< and t
S(L)
<
:
operate time
If both conditions are fi lled over the t
S(H)
< and t
S(L)
<adjustable time an alarm is issued.
The output may be assigned to the selected S(H)<TR-K and S(L)<TR-K output relays inside the
Set \ CT supervision-74CT side H and Set \ CT supervision-74CT side L submenu; the same for ad-
dressing the LED indicators S(H)<TR-L and S(L)<TR-L.
Logical block (Block1)
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 side H and Set \ CT supervision-74CT
side L submenu.
All the parameters are common for A and B Profi les.
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
—
A)
B)
Current asimmetry monitoring - 74CT
I
(H)
*
I
(L)
*
α
I
LMIN(H)
I
LMIN(L)
I
LMAX(H)
I
LMAX(L)
tgα=S(H)<
tgα=S(L)<
TRIP
NO TRIP
CT monitoring logic diagram - 74CT
Fun-74CT.ai
&
0
T
t
s
<
t
S<
I
LMIN
/I
LMAX
< S<
S<
TRIPPING MATRIX
(LED+RELAYS)
Trip S<
Start I*
BLK1 S<
I
LMAX
>I*
I*
Enable (ON≡Enable)
Block1 input (ON≡Block)
&
&
S<BLK1
Block1
Block1
Binary input INx
T0
Logic
INx
t
ON
INx
t
ON
INx
t
OFF
T0
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
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