Sel 751A - Arc-Flash Protection; Arc-Flash Overcurrent Elements; Table 4.43 Arc-Flash Overcurrent Settings; Figure 4.51 Breaker Failure Logic

4.95
Date Code 20100129 Instruction Manual SEL-751A Relay
Protection and Logic Functions
Arc-Flash Protection
Figure 4.51 Breaker Failure Logic
Arc-Flash Protection
The SEL-751A offers advanced arc-flash protection capability aimed at
minimizing the hazards associated with high energy arc (faults) in metal-
enclosed and metal-clad switchgear. The system supports four fiber-optic light
sensors capable of detecting the high energy arc-flash events and tripping the
breaker within milliseconds of the fault. Light sensors are supervised with an
instantaneous overcurrent element offering enhanced security against false
trips. Each of the four sensors can be routed to multiple tripping outputs using
SEL
OGIC equations, offering ultimate flexibility in creating multiple
protection zones (breaker truck cabinet, bus, PT cubicle etc).
SEL-751A arc-flash protection is exceptionally fast. Typical relay operating
times are in the order of 2–5 ms when equipped with the optional fast hybrid
(high-speed) output card. With standard, electromechanical outputs, tripping
time increases to 7–13 ms. Fault clearing time will typically be longer,
determined by the breaker operating time, which often adds three to five
cycles.
This system supports two distinct types of fiber-optic light sensors. The first
type is the omni-directional point sensor optimized for installation in
individual switchgear compartments. The second sensor is the bare-fiber loop
sensor optimized for protection of long, distributed resources, such as the
switchgear bus compartment. Both types of sensors are supervised by using a
loopback-based attenuation measurement method, and can be used
interchangeably on each of the four light inputs.
Arc-Flash
Overcurrent Elements
(50PAF, 50NAF)
Table 4.43 shows the settings for the arc-flash instantaneous overcurrent
elements. Two elements are provided; the three-phase overcurrent element
50PAF and the neutral overcurrent element 50NAF.
52ABF = Y
BFI
Setting
52A
0.02 • I
NOM
|I1| + |I2|
BFD
0
Relay
Word
Bit
BFT
Relay
Word
Bit
Table 4.43 Arc-Flash Overcurrent Settings
Setting Prompt Setting Range
Setting Name :=
Factory Defau lt
AF PH OC TRP LVL OFF, 0.50-100.00 A
a
a
For I
NOM
= 5 A (Phase and Neutral respectively)
50PAFP := OFF
0.10-20.00 A
b
b
For I
NOM
= 1 A (Phase and Neutral respectively)
AF N OC TRP LVL OFF, 0.50-100.00 A
a
50NAFP := OFF
0.10-20.00 A
b