2-46 Relay Element Settings
Date Code 20020206 SEL-300G Instruction Manual
You can determine the lower boundary of stator winding coverage provided by this element
using the equation:
()
%100•
CTRN
N
•In
51NP•2
%x
ú
ú
ú
ú
û
ù
ê
ê
ê
ê
ë
é
÷
ø
ö
ç
è
æ
=
Where
In = generator ground fault current, A primary
N = grounding transformer voltage ratio to 1,
(use 1 in solidly grounded or resistance grounded applications)
CTRN = neutral current transformer ratio to 1
Ground faults occurring above the winding point calculated above are detected and a trip
generated by the 51N time-overcurrent element in under 6.5 seconds, using the settings above.
The 50N1P element can provide a definite-time delayed trip for ground faults occurring higher in
the stator winding, between the generator and the step-up transformer, or within the step-up
transformer delta windings. A 50N1P setting of four to five times the 51NP setting will reduce
the 50N1P element sensitivity to system ground faults, while still providing detection of ground
faults high in the stator winding.
timeclearingfault ground systemon depending second, 0.5 to1.1050N1D
51NP times5 to450N1P
=
=
Set the 50N1D delay time greater than the maximum clearing time for system ground faults that
can be detected by the 50N1P element.
Neutral Overcurrent Tripping
Because the neutral overcurrent elements detect generator faults of a serious nature, tripping
generally is applied to the generator main breaker, the field breaker, and the prime mover. Refer
to Section 4: SEL
OGIC
®
Control Equations for more detail and examples of tripping SELOGIC
Control Equations.
Other Overcurrent Elements
Enable and apply those phase, neutral, and residual overcurrent elements that are required for
your specific control scheme. Pickup settings and time delays are governed by the requirements
of the application. Set the pickup of unused elements to OFF.
Element Operating Characteristics
Pickup and Reset Time Curves
Figure 2.21 and Figure 2.22 show pickup and reset time curves applicable to all instantaneous
overcurrent elements in the SEL-300G Relay. These times do not
include output contact
operating time and are accurate for determining element operation time for use in internal
SEL
OGIC Control Equations. Output contact pickup/dropout time is less than 5 ms.
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