BE1-51/27R - Installation
4-15
Figure 4-15 shows an example similar to Figure 4-14, but the higher external reactance (X
E
= 2X"
D
) to the fault
develops a higher restraint voltage. In Figure 4-15, the shaded area of the graph where pickup exceeds 25
percent of nominal is much larger than the shaded area in Figure 4-14. (This is the area where the relay
operates slower.) If we assume a fixed 25 percent of nominal pickup current, a significant margin in the time
coordination with external protection is added, although the log scale tends to exaggerate this effect.
Figure 4-15. Relay Signals and Current Pickup Example For 3-Phase Fault (X
E
= 2 X"
D
)
Setting Example
Figure 4-16 illustrates generator relay elements that are to be set in this example. Plant 11 kV feeders are
supplied by a 115 kV system that is supplemented by two local 3700 kVA generators. Line 1 is illustrated
completely; line 2 is the same as line 1 and only shows major components. Settings are provided for the BE1-
51/27R Time Overcurrent Relay with voltage restraint, BE1-51 Time Overcurrent Relay, and BE1-67 Phase
Directional Overcurrent Relay.
Generator Backup
BE1-51/27R (51/27), relay style number, U3E B6S B2C0F
(Breaker A)
3-phase and neutral overcurrent
1.5 to 12A phase (instantaneous trip is out of service)
0.5 to 4A neutral (instantaneous is in service)
Very inverse timing
300/5 CTs