Protection Functions; Broken Conductor Protection - Toshiba GRD110 Manual

GRD110

PROTECTION FUNCTIONS

Phase Fault Overcurrent Protection
Models GRD110-400, 420, 500 and -520 can provide
two or three phase overcurrent protection. Each provides
four independent overcurrent thresholds. The first
and second thresholds may be set for inverse time or
definite time operation. If inverse time is selected,
then any one of nine curves may be chosen,
including IEC and
characteristics. Further, user configurable curve is
available, if required. See Figure 1.
The first and second
programmable reset
instantaneous, definite time or dependent time reset.
This feature can be used to protect against flashing
fault conditions, or
electromechanical overcurrent relays.
The other overcurrent thresholds may be set for
definite time, or instantaneous operation. These
elements are immune to the effects of transformer
magnetising inrush and dc offset transient over-reach.
All elements can be inhibited by binary input signals
for operation in blocked overcurrent schemes and
busbar zone blocking protection.
Earth Fault Protection
The standard earth fault protection is available in all
models, and provides four independent overcurrent
thresholds. Protection functionality is the same as for
the phase fault elements, only with more sensitive
current thresholds.
For models GRD110-110, 400 and -500 the earth
fault quantity is measured directly, either by
connecting the input in the residual circuit of the phase
CTs, or, as is recommended for more sensitive
settings, using a dedicated core balance earth fault
CT. For model GRD110-420 and -520, the standard
earth fault quantity is derived internally from the
residual sum of the three phases.
Sensitive Earth Fault Protection (SEF)
GRD110-110, 420 and -520 provide earth fault
protection with more sensitive settings for use in
applications where the fault current magnitude may
be very low. A 2-stage overcurrent function is
provided, with the first stage programmable for
inverse time or definite time operation. The second
stage provides definite time operation and runs after
operation of the first stage. Three additional
overcurrent thresholds are provided, each with a
definite time delay.
The sensitive earth fault element includes a digital
filter which rejects all harmonics other than the
fundamental power system frequency.
The sensitive earth fault quantity is measured directly,
IEEE/ ANSI standard
thresholds have
feature, selectable
to grade correctly
using a dedicated core balance earth fault CT.
This input can also be used in transformer restricted
earth fault applications, by the use of external metrosils
and setting resistors.
Phase Undercurrent Protection
Protection against loss of load is provided by the phase
undercurrent protection. Two independent thresholds
are provided, each with a programmable definite time
delay.
Thermal Overload Protection
The thermal overload feature provides protection for
a
cables and other plant against the effects of prolonged
for
operation under excess load conditions. A thermal replica
algorithm is applied to create a model for the thermal
characteristics of the protected plant. Tripping times
with
depend not only on the level of overload current, but
also on the level of prior load current, the thermal
replica providing 'memory' of previous conditions.
The thermal characteristics of the system are defined
by entering settings for full load current and thermal
time constant. The GRD110 issues a trip according to
the 'cold' and 'hot' curves specified in IEC60255-8
(see Figure 2), to prevent the protected system from
exceeding its thermal capacity. The cold curve tripping
times are applicable when the system is first energised,
while the hot curves are relevant when the system
has already been carrying some prior load for a
period of time. An alarm output is also available to
give early warning of high load current, set as a
percentage of thermal capacity.
Negative Phase Sequence Overcurrent
Protection (NPS)
NPS protection can be used in applications where
certain fault conditions may not be detected by the
normal phase and earth overcurrent protections, for
example, in the case of a relay applied on the delta
side of a delta-star transformer, to detect an earth
fault on the star side. Alternatively, NPS can be used
to protect a three phase motor against the severe
overheating which results from operating with an
unbalanced supply.
Two independent thresholds are provided, each with
a programmable definite time delay.

Broken Conductor Protection

The unbalance condition caused by an open circuited
conductor is detected by the broken conductor protection.
An unbalance threshold with programmable definite
time delay is provided.
Circuit Breaker Fail Protection (CBF)
Two stage CBF protection provides outputs for re-
tripping of the local circuit breaker and/or back-
tripping to upstream circuit breakers. The CBF
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