4-26 BE1-951 Protection and Control 9328900990 Rev L
angle is used during the zero-sequence directional test. Angle compensation is not required for current
polarization since the polarizing quantity IG is inherently compensated.
Internally, the BE1-951 also uses several constant limits to determine if the system levels are adequate to
perform reliable directional tests and set directional bits. See
Table 4-14.
Table 4-14. Internal Constants
Internal Constant Purpose Value
Positive-Sequence
Current
Minimum 3I1 current threshold for Positive-Sequence test. 0.50 A
Zero-Sequence Current Minimum 3I0 current threshold for Current Polarization test. 0.25 A
Ground current (IG)
Minimum Ground (IG) current threshold for Current
Polarization test.
0.50A
Negative-Sequence
Voltage
Minimum V2 voltage threshold for Negative-Sequence test.
3.33% of V
nominal
Zero-Sequence Voltage Minimum V0 voltage threshold for Zero-Sequence test.
3.33% of V
nominal
External Zero-Sequence
Voltage (VX Input)
Minimum external 3V0 voltage threshold for Zero-Sequence
test.
10% of V
nominal
Negative-Sequence Ratio
Minimum ratio between 3I1 and 3I2 for Negative-Sequence
test.
9%
Zero-Sequence Ratio
Minimum ratio between 3I1 and 3I0 for Zero-Sequence test
is 9%.
9%
If the minimum levels are not met for a particular directional test, then the test is not run and the
directional bits are clear for that test. For instance, if 3I1 is less than 0.50 A, the positive-sequence test is
skipped and the positive-sequence directional bits are cleared.
The Sequence Ratio refers to the minimum ratio required between the positive-sequence current and
either the negative or zero-sequence current. A Negative-Sequence Directional test would be allowed if
the negative current were greater than 9% of the positive-sequence current. The same applies for the
Zero-Sequence Directional test.
The directional tests are also supervised by the loss of potential function 60FL. If the 60FL bit is TRUE,
then voltage sensing was lost or is unreliable. Under this condition positive, Negative, and Zero-
Sequence Directional tests are disabled and their bits are cleared. Current polarization is not affected by
the 60FL since it does not rely on voltage sensing.
The direction bits are updated once per ½ cycle. Under sudden reversal conditions, depending on the
change in magnitude of the forward current to reverse current, the DFT could require 1 cycle to determine
the polarity of the fault. Beyond this, the 50 element adds an additional ½ cycle delay when operating in
direction mode for security, for an overall response time of a 50 element to a sudden current reversal of
approximately 2 cycles.
Negative-Sequence Overcurrent Protection
For years, protection engineers have enjoyed increased sensitivity to phase-to-ground unbalances with
the application of ground relays. Ground relays can be set more sensitively than phase relays because a
balanced load has no ground (3
I0
) current component. The negative-sequence elements can provide
similar increased sensitivity to phase-to-phase faults because a balanced load has no negative-sequence
(I
2
) current component.
Negative-Sequence Pickup Settings
A typical setting for the negative-sequence elements might be one-half the phase pickup setting in order
to achieve equal sensitivity to phase-to-phase faults as three-phase faults. This number comes from the
fact that the magnitude of the current for a phase-to-phase fault is √3/2 (87%) of the three-phase fault at
the same location. This is illustrated in
Figure 4-20.
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