Basler BE1-CDS220, Instruction Manual

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BE1-CDS220 Protection and Control 4-11
1 2 3 4
12
3
D2837-11
01-28-99
5
I
I
RE
STR
OP
Figure 4-10. Percentage Restrained Differential Characteristic
If the target is enabled for the restrained element, the target reporting function will record an 87R target
for the appropriate phases when the 87RT output is TRUE and the fault recording function trip logic
expression is TRUE. See Section 6, Reporting and Alarm Functions, Fault Reporting, for more details on
the target reporting function.
The differential protection function includes a transient monitor to detect the effects of CT saturation
during a through fault. It does this by monitoring the change in restraint current versus the change in
operate current. For an internal fault, the restraint current and operate current will experience a step
increase at the same time. For an external fault, there should be no operate current. If CT saturation
occurs during a through fault, the operate current will increase at some time after the restraint current
increases. In this case, a two-cycle delay is added to the restrained differential output to enhance
security.
The second and fifth harmonic functions check the ratio of the second and fifth harmonic operate current
to the fundamental operate current. Traditional harmonic restraint units operate on the ratio of harmonic
current to total operate current versus the ratio to only the fundamental operate current used by the BE1-
CDS220. For this reason, the relay will provide greater security for inrush and overexcitation with the
same harmonic inhibit ratio settings used with traditional differential relays. When either of these two
comparators is above the threshold, the percentage-restrained output is blocked from setting the 87RT
(87 restrained trip) logic output. If the second or fifth harmonic inhibit comparators are picked up for any of
the three phases, the 2NDHAR and 5THHAR logic outputs respectively are set.
In many cases, the second harmonic content of the inrush current may show up primarily in only one or
two phases, which can cause one or two phases to not be inhibited. The BE1-CDS220 relay allows the
second harmonic currents to be shared between the three phases. When second harmonic sharing is
enabled, the magnitude of the second harmonic operating current is summed from all three phases and
this magnitude is used by the second harmonic comparator for each phase instead of the second
harmonic operate current for only that phase. This is superior to other methods of cross blocking since
each phase element operates independently in its comparison of operating current to harmonic current.
Thus, security is enhanced without sacrificing dependability because a faulted phase will not be
restrained by inrush on unfaulted phases as is the case with cross blocking schemes.
The Unrestrained Element function provides high-speed tripping for high-grade faults inside the zone of
protection. This comparator has a minimum pickup setting. If the operate current is above the threshold,
for any of the three phases, the 87UT (87 unrestrained element trip) logic output is set. The transient
monitor function also enhances security for this function by doubling the pickup threshold when CT
saturation is detected. The minimum setting for the unrestrained trip threshold should be the maximum
inrush current with a small margin.
If the target is enabled for the unrestrained element, the target reporting function will record an 87U target
for the appropriate phases when the 87UT output is TRUE and the fault recording function trip logic
expression is TRUE. See Section 6, Reporting and Alarm Functions, Fault Reporting, for more details on
the target reporting function.