Basler BE1-1051, Инструкция по применению

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Fig. 10 shows an example of generator current
decay for a 3 phase fault and a phase-phase
fault. For a 3 phase fault, the fault current
decays below the pickup level of the 51 relay in
approximately one second. If the time delay of
the 51 can be selectively set to operate before
the current drops to pickup, the relay will provide
3 phase fault protection. The current does not
decay as fast for a phase-phase or a phase-
ground fault and, thereby, allows the 51 relay
more time to trip before current drops below
pickup. Fig. 10 assumes no voltage regulator
boosting, although the excitation system re-
sponse time is unlikely to provide significant
fault current boosting in the first second of the
fault. It also assumes no voltage regulator
dropout due to loss of excitation power during the
fault. If the generator is loaded prior to the fault,
prefault load current and the associated higher
excitation levels will provide the fault with a
higher level of current than indicated by the Fig.
10 curves. An estimate of the net fault current of
a pre-loaded generator is a superposition of load
current and fault current without pre-loading. For
example, assuming a pre-fault 1pu rated load at
30 degree lag, at one second the 3 phase fault
value would be 2.4 times rated, rather than 1.75
times rated (1@30°+1.75@90°=2.4@69°). Under
these circumstances, the 51 relay has more time
to operate before current decays below pickup.
FIGURE 10. GENERATOR FAULT CURRENT DECAY
EXAMPLE FOR 3 PHASE AND PHASE-PHASE FAULTS
AT GENERATOR TERMINALS - WITH NO REGULATOR
BOOSTING OR DROPOUT DURING FAULT AND NO
PREFAULT LOAD.
FIGURE 8. MEDIUM-LEVEL GROUNDING WITH 87N
GROUND DIFFERENTIAL PROTECTION.
Phase-Fault Protection
Fig. 9 shows a simple means of detecting
phase faults, but clearing is delayed, since the
51 relay must be delayed to coordinate with
external devices. Since the 51 relay operates for
external faults, it is not generator zone selective.
It will operate for abnormal external operating
conditions such as remote faults that are not
properly cleared by remote breakers. The 51
pickup should be set at about 175% of rated
current to override swings due to a slow-clearing
external fault, the starting of a large motor, or the
re-acceleration current of a group of motors.
Energization of a transformer may also subject
the generator to higher than rated current flow.
FIGURE 9. PHASE-OVERCURRENT PROTECTION (51)
MUST BE DELAYED TO COORDINATE WITH
EXTERNAL RELAYS.
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