BECKWITH ELECTRIC M-3425A, Instruction Book

Page: 234 / 482

4–103
System Setup and Setpoints – 4
or 3V
0
calculated using V
A
, V
B
and V
C
inputs. The
function provides both definite time and inverse
time elements. The inverse time element provides
several curves. The curves available for use are
shown in
Appendix D
,
Inverse Time Curves
.
They cover a range from 1.5 to 20 times the pickup
setting. An additional one cycle time delay should
be added to these curves in order to obtain the
relay operating time. Inverse time curves saturate
beyond 20 times pickup. For currents in excess of
20 times pickup, operating times are fixed at the 20
time pickup level.
To obtain maximum sensitivity for fault currents,
the directional
element is provided with a maximum
sensitivity angle adjustment (MSA). This setting is
common to both the 67NDT and 67NIT elements.
The pickup sensitivity of the relay remains constant
for 90° either side of the so-called Maximum
Sensitivity Angle (MSA). At angles over 90° from
MSA, the relay operation is blocked. Typical MSA
setting for a generator internal ground fault protector
is approximately 150°.
67N Residual Directional Overcurrent
The Residual Directional Overcurrent function
(67N) provides protection from ground faults.
The 67N function can provide generator ground
fault protection. It can also provide directional
discrimination when multiple generators are bused
together. The 67N Function is subject to the
following configuration limitations:
• V
X
polarization cannot be selected if 25
(Sync) function is enabled.
• 3V
0
polarization can only be used with
Line-Ground VT configuration.
• 67N Function is not available if 87GD is
enabled.
The 67N Function operates on the residual current
either from internal calculation (3I
0
) using I
A
, I
B
and
I
C
or using a residual current input from I
N
input of
the relay (this is preferred compared to 3I
0
). The
relay can be polarized with the neutral voltage
(V
N
), broken delta voltage connected at V
X
input
Figure 4-76 Residual Directional Overcurrent (67N) Trip Characteristics