Basler BE1-1051, Инструкция по применению

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winding from the neutral, the 51N current will be
0.5A, with a 1000/5 CT.
Fig. 3 shows multiple generators with the trans-
former providing the system grounding. This
arrangement applies if the generators will not be
operated with the transformer out of service. The
scheme will lack ground fault protection before
generator breakers are closed. The transformer
could serve as a step-up as well as a grounding
transformer function. An overcurrent relay 51N or
a differential relay 87G provides the protection
for each generator. The transformer should
produce a ground current of at least 50% of
generator rated current to provide about 95% or
more winding coverage.
faults, then the 51N provides the primary protec-
tion for the generator. The advantage of the 87G
is that it does not need to be delayed to coordi-
nate with external protection; however, delay is
required for the 51N. One must be aware of the
effects of transient DC offset induced saturation
on CTs during transformer or load energization
with respect to the high speed operation of 87G
relays. Transient DC offset may induce CT
saturation for many cycles (likely not more than
10), which may cause false operation of an 87G
relay. This may be addressed by not block load-
ing the generator, avoiding sudden energization
of large transformers, providing substantiallly
overrated CTs, adding a very small time delay to
the 87G trip circuit, or setting the relay fairly
insensitively.
FIGURE 2. GROUND-FAULT RELAYING -
GENERATOR LOW-IMPEDANCE GROUNDING.
The neutral CT should be selected to produce a
secondary current of at least 5A for a solid
generator terminal fault, providing sufficient
current for a fault near the generator neutral. For
example, if a terminal fault produces 1000A in
the generator neutral, the neutral CT ratio should
not exceed 1000/5. For a fault 10% from the
neutral and assuming
I
1
is proportional to percent
FIGURE 3. SYSTEM GROUNDED EXTERNALLY WITH
MULTIPLE GENERATORS.
Fig. 4 shows a unit-connected arrangement
(generator and step-up transformer directly
connected with no low-side breaker), using high-
resistance grounding. The grounding resistor and
voltage relays are connected to the secondary of
a distribution transformer. The resistance is
normally selected so that the reflected primary
resistance is approximately equal to one-third of
the single phase line-ground capacitive reactance
of the generator, bus, and step-up transformer.
This will limit fault current to 5-10A primary.
Sufficient resistor damping prevents ratcheting up
of the sound-phase voltages in the presence of an
intermittent ground. The low current level mini-
mizes the possibility of sufficient iron damage to
require re-stacking. Because of the low current
level, the 87G relay will not operate for single-
phase ground faults.
FIGURE 4. UNIT-CONNECTED CASE WITH HIGH-
RESISTANCE GROUNDING.
Protection in Fig. 4 consists of a 59N overvoltage
relay and a 27-3N third-harmonic undervoltage
relay (e.g., Basler relays per Table 2). As shown
3