CKT 1
CKT 3
CKT 2
CKT 4
7 A
A
5
.
0
0.5 A
Fault
VCKT 5
3 A
10 A
2 A
D2840-40
01-21-04
Figure 4-14. Virtual Circuit with a CT
Virtual restraint was specifically designed for applications where high per unit current flows through the
primary or secondary breaker current transformers without flowing though the protected transformer.
Using the same example as above, first configure Virtual Circuit 5 for the vector sum of CT Circuits 1 and
2. Next, enable virtual restraint to use Virtual Circuit 5 as one of its restraining inputs. When virtual
restraint is enabled to use Virtual Circuit 5, the BE1-CDS240 automatically excludes Circuits 1 and 2 from
the restraint calculations.
The restraint current is then determined from Virtual Circuit 5 and CT Circuits 3 and 4. The operate
current is determined as before from the vector sum of CT Circuits 1, 2, 3, and 4.
Virtual Circuit 5 becomes the vector sum of Circuits 1 and 2 and is equal to 3 A. Using the maximum
mode, Equation 4-7 applies:
I
RESTRAINT
= 3 A,
I
OPERATE
= 2 A,
%
SLOPE
%
4
.
44
100%
I
I
RESTRAINT
OPERATE
Equation 4-7. Slope Calculation, using VCKT5
Using the typical setting of 45% slope, 87R easily operates for the same fault condition that restrained in
the original example (Figure 4-13
)
. By using virtual restraint, the high current flowing thought CT Circuits 1
and 2 has no impact on the restraint calculations. By using virtual restraint, the restraint current is only
proportional to current actually flowing through the protected transformer. This avoids the need to apply a
separate CT at VCKT5 location (Figure 4-14) and maintains a good balance between sensitivity and
security.
87ND - Neutral Differential Protection
BE1-CDS240 relays can provide sensitive differential protection for ground faults on the grounded side of
a delta/wye transformer. On impedance grounded systems, ground fault levels may be reduced below the
sensitivity of the phase differential protection. The result is that ground faults within the protected zone
have to be cleared by time delayed backup overcurrent protection if sensitive differential protection is not
available.
The function block in Figure 4-15 has two outputs: 87NDPU (pickup) and 87NDT (trip). A
BLK
(block)
logic input is provided to block operation of the differential protection. When this expression is TRUE, the
function is disabled. For example, this may be an input wired to a differential cutoff switch.
4-18
BE1-CDS240 Protection and Control
9365200990 Rev F
Summary of Contents for BE1-CDS240
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Page 40: ...ii BE1 CDS240 Quick Start 9365200990 Rev F This page intentionally left blank ...
Page 152: ...ii BE1 CDS240 Metering 9365200990 Rev F This page intentionally left blank ...
Page 226: ...iv BE1 CDS240 Application 9365200990 Rev F This page intentionally left blank ...
Page 286: ...ii BE1 CDS240 Security 9365200990 Rev F This page intentionally left blank ...
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Page 292: ...ii BE1 CDS240 Human Machine Interface 9365200990 Rev F This page intentionally left blank ...
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Page 308: ...ii BE1 CDS240 ASCII Command Interface 9365200990 Rev F This page intentionally left blank ...
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Page 349: ...Figure 12 5 Horizontal Rack Mount Front View 9365200990 Rev F BE1 CDS240 Installation 12 5 ...
Page 361: ...Figure 12 17 Typical DC Connection Diagrams 9365200990 Rev F BE1 CDS240 Installation 12 17 ...
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Page 544: ...ii BE1 CDS240 Terminal Communication 9365200990 Rev F This page intentionally left blank ...
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