Basler Electric BE1-51/27C - Manual (Page 65)

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(7) For a failure of Breaker L to open, the 51 relays on transformer T2 will operate as well as the
generator relays to clear fault 2. For a failure of the T1 differential relays (87T), the T2 51 relays
should coordinate with the 67 relays so T2 and the generators will remain in service.
Checking Coordination with Relay 67 for Fault 3
(1) 67 multiples at transient level with T2 in service:
4 . 2
300
713 =
(2) 67 time at 2.4 multiples: 0.26 s.
(3) 51/27 transient level voltage (reactor and transformer T1 drop): 8%.
(4) 51/27 multiples at transient level:
4 . 7
97
713 =
(5) 51/27 transient level time at 7.4 multiples and TIME DIAL setting of 22: 0.72 s.
(6) 51/27 relays coordinate with 67 relays for a high-side fault at the transient level. However, it
appears that the current may drop below the 67 relays pickup (300 amperes) before this relay can
operate. Should the 67 relays fail to operate, the 51/27 relays will trip the generators. Figure 37
plots these two relay characteristics. If the generator is fully loaded at 194 amperes with a 0.85 PF,
the superposition of this current and the fault current at
−90° yields a total current of 831 amperes
at the transient level and 282 amperes at the steady-state level with no regulator boosting. The
relays are selective down to about 350 amperes.
Figure 37. Relay 51/27 Coordination with Relay 67 for Fault 3 Example
BE1-51/27C Tests and Adjustments