4-24 BE1-951 Protection and Control 9328900990 Rev L
Negative-Sequence Polarization. This is used to test directionally for all fault types except three-phase
faults. Negative-sequence bits are used to supervise phase, neutral and negative-sequence over current
elements. With load flow and low fault currents, it is possible for the positive-sequence bits to be set at
the same time negative-sequence bits are TRUE. Under these conditions, the negative-sequence bits
have priority and the positive-sequence bits are cleared.
Zero-Sequence Voltage Polarization. This is used to test directionally for ground faults and is used to
supervise only the neutral overcurrent elements (V0IN, V0IG, VXIN, and VXIG). The neutral overcurrent
elements can be set to operate on either calculated 3I0 or independent ground input IG. When an
external source of 3V0 is used as the polarizing reference (VX input), the same “angle adjustment factor”
as described for V0 and V2 is applied. Typical AC connections for external sources of 3V0 are provided in
Section 12, Installation.
Zero-Sequence Current Polarization. This is also used to test directionally for ground faults and is used to
supervise the neutral overcurrent elements.
Polarization Summary for tripping elements is as follows:
• Phase Element: Positive-Sequence; Negative-Sequence
• Negative-Sequence Element: Negative-Sequence
• Neutral Element: Negative-Sequence; Zero-Sequence Volt; Zero-Sequence Current
The neutral overcurrent elements can be supervised by various polarization methods using either or both
zero-sequence and negative-sequence quantities. This is necessary depending on the application and
fault conditions applied to the relay. For example, negative-sequence polarizing can be used when zero-
sequence mutual coupling effects cause zero-sequence polarizing elements to lose directionality. Also,
high Z ground faults may cause values of zero-sequence voltage to be too low to measure during a fault,
making zero-sequence polarization unreliable. A similar condition can occur with the negative-sequence
voltage, although it is less likely. Under these conditions a user may need to use current polarization or
dual polarization to provide reliable directional tripping.
Polarizing Settings for 67N Directional Overcurrent Element
Polarization settings are made using BESTCOMS.
Figure 4-19 illustrates the BESTCOMS screen used to
select operational settings for the 67N element. To open the screen, select Overcurrent Protection from
the S
creens pull-down menu, and select the 67N Polarization tab. Alternately, settings may be made
using the S#-67N ASCII command. To the right of the Logic pull-down menu is a pull-down menu labeled
Settings. The Settings menu is used to select the setting group that the element’s settings apply to.
Figure 4-19. Overcurrent Protection Screen, 67N Polarization Tab
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