Current Differential Relay
GARD 8000 SYS RFL Electronics
June 20, 2014 13-60 973.334.3100
13.5.1.4.3 Calculating the Blocking Current
The HST is essentially a degenerate form of the current differential algorithm that has been tailored for
quick response. Given the HST process begins with the remote relay detecting a high current, the local
(receiving) relay must block if it also measures current equal to the HST setpoint (which would make
the differential current zero).
As with the current differential routines the bias setting is used to desensitize the element. Given the
bias setting an RMS value and the HST routines process instantaneous values the bias setting must be
converted to an equivalent peak value. An additional 1A of desensitization is included to account for
the high currents (as the slope characteristic does for the current differential algorithm) and limitations
of the speed-optimized routine.
The blocking current is further limited to a maximum value of 7.5A
PEAK
. The calculation of the
blocking current is thus:
A5.7
A1414.1
min_
biasimithigh_set_l
currentblock
13.5.1.4.4 Calculating the Trip
When a relay receives a HST message it determines how long ago the HST message was calculated at
the remote end. This process is very similar to the nesting performed by the current differential routine
(section 13.5.1.2.4), however, as mentioned above rather than computing a differential current the local
current is compared to a blocking level.
The normal nesting routines deal only with half-cycle blocks and can thus accommodate modest
nesting errors without any impact. The HST routines work on instantaneous measurements and must
allow for some uncertainties and system errors. As such, the local current is checked over a 12-sample
window—if the local current exceeds the blocking level at any time during this window the unit will
not trip.
If there is insufficient blocking current the local end will trip and send a TCDT message to the remote
relay.
13.5.1.5 OPEN CONDUCTOR (PHASE UNBALANCE) UNIT (46)
The open conductor algorithm is a multi-step algorithm that works only with the three phases. The
only potential involvement of the ground is that it can contribute a FD.
13.5.1.5.1 Prefault Condition
The system must start in a stable condition with all three phases seeing currents of at least 1.5A
RMS
(5A
CTs).
13.5.1.5.2 Phase Loss Detector
A phase is considered lost when there is a phase imbalance and at least two fault detectors while
having been in a valid prefault condition within the past two cycles.
Phase imbalance is defined as at least one phase having greater than or equal to 3A
RMS
(5A CTs) and at
least one phase having less than 0.5A
RMS
(5A CTs).
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