152
NVA100X-D - Manual - 02 - 2016
FUNCTION CHARACTERISTICS
Residual overcurrent - 50N.1/51N.1, 50N.2/51N.2 or high impedance restricted earth fault - 87NHIZ.1, 87NHIZ.2 side 1,2
Preface
The relay can be typically used in radial or meshed MV and LV networks as feeder, power trans-
former protection or motor-generators with neutral connected to ground through an impedance.
Alternatively, by connecting the branch measurement current input to the same CT secondary circuit
measuring the residual current on the generator line side and star point, it is possible to use the
same protective device as a high impedance restricted earth fault to selectively clear all ground
faults within the single area delimited by the measurement CT. This protective element, which re-
quires a suitable stabilization resistor and possibly a non-linear METROSIL resistor in order to limit
over-voltages in the secondary circuit in case of internal faults, is useful when several generators
are connected in parallel on the same busbar, so as to disconnect only the generator with the stator
ground fault.
Three operation thresholds, independently adjustable with adjustable delay are available.
The fi rst one may be programmed with defi nite or inverse time according the IEC and ANSI/IEEE
standard, as well as with EM curve.
The second and third thresholds with defi nite time.
For each threshold a reset time can be set useful to reduce the clearing time for intermittent faults.
The fi rst threshold trip may be inhibited by start of the second and/or third threshold.
Similarly the second threshold trip may be inhibited by start of the third threshold.
Operation and settings
The residual fundamental frequency current is compared with the setting value. Current above the
associated pickup value is detected and a start is issued. After expiry of the associated operate time
a trip command is issued; if instead the current drops below the threshold, the element is restored.
The fi rst threshold may be programmed with defi nite or inverse time according the following char-
acteristic curves
[1]
:
Standard Inverse Time (IEC 255-3/BS142 type A or SIT):
t
= 0.14 ·
t
E1
>
inv
/ [(
I
E1
/
I
E1
>
inv
)
0.02
- 1]
Very Inverse Time (IEC 255-3/BS142 type B or VIT):
t
= 13.5 ·
t
E1
>
inv
/ [(
I
E1
/
I
E1
>
inv
) - 1]
Very Inverse Time (IEC 255-3/BS142 type B or LIT):
t
= 120 ·
t
E1
>
inv
/ [(
I
E1
/
I
E1
>
inv
) - 1]
Extremely Inverse Time (IEC 255-3/BS142 type C or EIT):
t
= 80 ·
t
E1
>
inv
/ [(
I
E1
/
I
E1
>
inv
)
2
- 1]
Moderately Inverse (ANSI/IEEE type MI):
t
=
t
E1
>
inv
· {0.01 / [(
I
E1
/
I
E1
>
inv
)
0.02
- 1] + 0.023}
Very Inverse (ANSI/IEEE type VI):
t
=
t
E1
>
inv
· {3.922 / [(
I
E1
/
I
E1
>
inv
)
2
- 1] + 0.098}
Extremely Inverse (ANSI/IEEE type EI):
t
=
t
E1
>
inv
· {5.64 / [(
I
E1
/
I
E1
>
inv
)
2
- 1] + 0.024}
Electromechanical (EM):
=
t
E1
>
inv
· {0.28 / [-0236 · (
I
E1
/
I
E1
>
inv
)
-1
+ 0.339]}
where:
t
: operate
time
I
E1
>: pickup
value
t
E1
>
inv
: operate
time
setting
Note 1 The formulas concern to the 50N.1/51N.1 protection on the side 1 and similarly for formulas of to the 50N.2/51N.2 protection on the side 2
—
•
•
•
•
•
•
•
•
L1
L2
L3
P2
S2
S1
P1
P2
S2
S1
P1
P2
S2
S1
P1
P2
S2
S1
P1
GS
∼
3
GS
∼
3
I
E
A7
A8
Ru
Rs
P1
S1
S2
P2
87NHIZ
87N-sch.ai
Restricted earth fault (87N) for parallel connection of generators with earthing resistor
