Basler Electric BE1-51/27R - Manual (Page 7)

9137200999 Rev F 1
Introduction
BE1-51/27R Time Overcurrent Relays 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) and the
neutral time overcurrent element, when supplied, operate independently of the voltage restraint function.
Each relay is available with one, three, or four time overcurrent elements.
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.
Residually Connected
A neutral (ground) overcurrent element can be applied and connected residually to a set of three current
transformers on solidly grounded applications or on impedance grounded systems that provide ground
fault current approximating rated current level. The neutral element can also be connected to a 10/1
ampere, zero-sequence window current transformer to provide protection on systems producing a
minimum of about 20 amperes primary current. Still another alternative would be connecting this device to
a current transformer in the neutral of a generator.
BE1-51/27R Introduction