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350 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
S3 PROTECTION
CHAPTER 6: SETPOINTS
Neutral directional
The Neutral Directional element is used to discriminate between faults that occur in the
forward direction, and faults that occur in the reverse direction. The Neutral Directional
element can be used either individually for control or alarm by energizing the auxiliary
output relays, or as a part of the Neutral Time, or Instantaneous, over-current elements to
define the tripping direction. (See the setup for Neutral TOC, and Neutral IOC elements.)
The polarizing signal for the Neutral Directional element can be set to be either voltage
(zero sequence voltage), current (measured ground current), or dual (both).
Depending on the relay’s order code, the zero sequence voltage used for the Neutral
Directional polarizing voltage, is calculated either when three-phase voltages are available,
or is the measured voltage from the auxiliary Vx voltage input when the three-phase
voltages are not available. For those relays with available phase VTs, the polarizing voltage
for the Neutral directional element is calculated as follows:
Eq. 3
Please note that the phase VT inputs must be connected in Wye.
For those relays with available Vx auxiliary voltage input only, the polarizing voltage for the
Neutral directional element is formed based on voltage measured from the Vx input. The
Vx input should be connected to measure 3V
0
from an open delta VT configuration as
shown in the figure below.
Figure 24: Open Delta VT connection
When “Voltage” polarization is selected, the direction is determined by comparing the
angle between the operating current and the voltage, and the set MTA angle. In cases
where the voltage drops below the setting of the minimum polarizing voltage, the neutral
directional element defaults to the Forward direction.
When “Current” polarizing is selected, the direction of the neutral current is determined
with reference to the direction of the measured ground current . The fault is detected in
the Forward direction when the ground current typically flowing from the ground point into
the neutral current is within ± 90° of the polarizing current. Otherwise the direction is
detected as Reverse. The neutral direction defaults to Forward if the polarizing ground
current drops below 5% of the ground CT.
The diagram below shows the regions for detection of neutral current Forward and
Reverse directions with respect to the zero sequence voltage and the selected Maximum
Torque Angle (MTA).
