BE1-51/27R - General Information
1-1
SECTION 1 • GENERAL INFORMATION
DESCRIPTION
BE1-51/27R Time Overcurrent Relays with voltage restraint are microprocessor-based devices that provide
voltage restraint of the phase time overcurrent function. With voltage restraint, the current pickup decreases
proportionately with decreasing voltage over the rated voltage range. Instantaneous overcurrent element(s),
when supplied, operate independently of the voltage restraint function. Relays are available with one, three,
or four time overcurrent elements. The neutral time overcurrent element, when supplied, operates
independently of the voltage restraint function.
APPLICATION
Voltage restraint provides an added means of discriminating between load and fault conditions. This allows
the time overcurrent pickup to be set below the maximum load (or swing) current. This feature permits the
relay to provide dual protection on a generator. For example, either backing up the differential protection
for generator faults and/or backing up other relays external to the generator zone. As a back-up function,
it must be set with a relatively long delay. Prior to relay time-out, the synchronous impedance of the
generator may be limiting fault current to a level comparable to rated. If the regulator is not in service to
boost excitation, the steady-state fault current, even for a fault on the machine terminals, will usually be less
than rated. The relay pickup must be below generator rated current to insure dependable operation.
Backup Protection
This relay is useful for generator time overcurrent back-up protection for other relaying external to the
system. It also provides primary (first line) phase fault protection for small generators not equipped with
differential protection.
Phase overcurrent units should be supplied on all three phases. Either three single-phase relays or one
three-phase relay when the objective is to protect for phase-phase faults on the other side of a delta-wye
power transformer. Currents at the relay for a three-phase fault are in the proportions of 2:1:1 in the three
phases, so only one phase sees the higher current level. For this application, each phase time-overcurrent
element should be restrained by the phase-to-ground voltage on its phase, rather than by the phase-phase
voltage.
Following fault inception, current varies continuously as the field current decays. In addition, for other than
a bolted fault on the terminals of a generator, the voltage will not be zero and will vary with time as the fault
current decays. If the restraint voltage is between 25 and 100 percent, the time overcurrent element pickup
will also vary with the time because of the changing voltage. Because the pickup varies with time, the
multiples of pickup, and therefore the timing, will also change. These factors must be considered when
coordinating with external protective devices. Section 4 of this manual provides additional coordination
information in the paragraphs on setting the relay.
Instantaneous overcurrent elements would not ordinarily be used for a generator back-up function. They
would not have acceptable operation for faults external to the generator zone.
Because the phase time-overcurrent pickup will be less than the maximum non-fault current, the relay can
misoperate if the voltage signal is interrupted (e.g., a blown voltage transformer fuse). Where two sources
of signal voltage are available, the BE1-60 Voltage Balance relay can prevent such a misoperation. This
relay compares the output of two signal sources to detect an anomaly in one of these sources and block the
operation of those devices connected to that signal source.
Содержание BE1-51/27R
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Страница 36: ...BE1 51 27R General Information 1 31 Figure 1 38 Timing Type E7 BS 142 Extremely Inverse Drawing No 99 1097...
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Страница 69: ...BE1 51 27R Installation 4 7 Figure 4 8 Typical Internal Diagram Sensing Input Type M or N...
Страница 76: ...4 14 BE1 51 27R Installation Figure 4 14 Relay Signals and Current Pickup Example For 3 Phase Fault XE 0 65 X D...