4-18
BE1-51/27R - Installation
Relay 51/27 coordinates with relay 51 over the full range of fault currents with transformer T2 out of service.
With both T1 and T2 in service, relay 51 sees more current and operates faster than the times calculated here.
If the fault is not interrupted by the feeder breaker, the generators will be tripped at a time falling between 0.61
seconds and 9.0 seconds.
Checking Coordination With Relay 67 for Fault 2
67 multiples at transient level with T2 in service:
7256
0.5 ×
3000
5
24
67 time at 24 multiples (B3 curve, 02 time dial): 0.15 seconds.
67 multiples at transient level with T2 out of service:
1552
300
5.2
67 time at 5.2 multiples: 0.19 seconds.
A failure of the differential protection (87T) for fault 2 with T2 out of service is a double contingency.
Coordination of 51/27 relays with 67 relays does occur at the transient level (0.61 versus 0.19 seconds).
However, as the current decays, the times converge and cross because the 67 relays pickup is 300 amperes
versus 84 amperes for the 51/27 relays. (See calculation that follows for the steady state multiple of just 0.86
for the 67 relay.) Coordination may not be achieved for this double contingency with the generator regulators
out of service. Boosting by the regulators will partially arrest the decay of current and any pre-fault load current
will add to the fault current values. For a phase-to-phase fault, the decay will be less.
Steady state level calculation:
67 multiples at steady state level with T2 out of service:
258
300
0.86
For a failure of breaker L to open, the 51 relay on transformer T2 will operate as well as the generator relays to
clear fault 2. For a failure of the T1 differential relay (87T), the T2 51 relay should coordinate with the 67 relay,
so T2 and the generators will remain in service.
Checking Coordination With Relay 67 for Fault 3
67 multiples at transient level with T2 in service:
713
300
2.4
67 time at 2.4 multiples (B3 curve, 02 time dial): 0.26 seconds.
51/27 transient level voltage (reactor and transformer T1 drop): 8 %.
51/27 multiples at transient level:
713
5.7 × 0.25 × 60
8.3
51/27 transient level time at 8.3 multiples: 0.7 seconds.
51/27 relays coordinate with 67 relays for a high-side fault at the transient level. However, the current may drop
below the 67 relays pickup (300 amperes) before this relay can operate. If the 67 relays fail to operate, the 51/27
relays will trip the generators. Figure 4-17 plots these characteristics. If the generator is fully loaded at 194
amperes with a 0.85 power factor, the superposition of this current and the fault current at minus 90 degrees
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.