5-12
Applications
Date Code 20010515
SEL-2BFR, -2/BFR Instruction Manual
Select 50LD Overcurrent Element Setting
The 50LD overcurrent element is used in the failure to trip load current breaker protection
scheme. The 50LD element should pick up when the protected breaker is closed. This scheme
detects failures of the breaker to open when breaker current is lower than the 50FT setting, such
as Zone 2 faults and load breaking operations.
If the protected breaker is in a ring-bus or breaker-and-a-half arrangement, set 50LD to pick up
for line-charging current of the shortest line serviced by that breaker.
Charging current for a given line can be calculated using the equation:
I
c
= Vg • Bc A primary
where Vg is the line-to-ground voltage and Bc is the line capacitive susceptance.
For the example application, line-charging current, I
c
, is 350 A primary or 0.58 A secondary.
The 50LD element accuracy specification is ±0.025 A secondary ±5% of setting. The error figure
is calculated with the method used for 50FT. The maximum error for a setting of 0.58 A
secondary is ±0.054 A secondary. To ensure that 50LD picks up any time the protected breaker
is closed, we set the 50LD element:
50LD = 0.58 A – 0.054 A = 0.53 A secondary
The relay also uses the 50LD element in the flashover protection logic. If you intend to use the
breaker pole flashover protection logic, please refer to the logic descriptions before finalizing
your 50LD setting selection.
When the protected breaker is part of a ring-bus or breaker-and-a-half installation, load current
may be very low due to unequal current distribution between the two breakers. Failure to trip
load current logic may still be used to protect the breaker. Because current application may be
sequential, there is an uncertainty in scheme timing when used in ring-bus or breaker-and-a-half
installations. Scheme timing is discussed below.
Select 62LP and 62LD Settings
The 62LP and 62LD timers are breaker pending failure and failure timers for the failure to trip
load current breaker protection scheme.
It may be acceptable to extend the time delay of this protection scheme due to lower current
duties associated with this type of breaker failure operation. Extending the time delay allows
more time for a slow but operative breaker to clear a low-current fault. A disadvantage in the
extended time delay is that a fault does remain on longer if the breaker actually fails. These
considerations must be weighed when selecting time delays for this scheme. Please note that
some breakers actually take more time to break low amounts of current. Consult the
manufacturer of the protected breaker for details.
The timing of this protection scheme is nearly identical to that shown in Figure 5.5. The
difference is that pending failure bit, LPF, is provided with this scheme. Consider using the
pending failure bit to retrip the protected breaker through a second trip coil.
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Страница 234: ...C 8 Appendix C Date Code 990714 SEL 2BFR 2 BFR Instruction Manual Figure C 2 Horizontal Front and Real Panel Drawings ...
Страница 235: ...Date Code 990714 Appendix C C 9 SEL 2BFR 2 BFR Instruction Manual Figure C 3 Vertical Front and Rear Panel Drawings ...
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