GE 750 Instruction Manual page 193

CHAPTER 5: SETPOINTS
Note
NOTE
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
The Ground Directional feature controls operation of all ground overcurrent elements and
allows them to discriminate between forward or reverse faults. Refer to Phase Directional
Overcurrent on page 5–54 for additional details on directional principles. Ground
directional can be either voltage, current, or dual polarized. The ground current input is
always the operating current.
When voltage polarized, the polarizing quantity is the zero sequence voltage which is
calculated from the bus input voltages. The VT Connection Type must be Wye in this case.
If the polarizing voltage drops below the
defaults to forward. The following table shows the operating current and polarizing
quantities used for ground directional control.
Table 5–13: Ground Directional Characteristics
Quantity Operating
Current
I
Ground
g
On relays with bootware revision 3.00 or newer, the ground directional element may only
be voltage polarized since these relays do not have a polarizing current input. Otherwise,
the polarizing current is input via a dedicated polarizing CT input. See Current Inputs on
page 3–11 for more details.
When current polarized, the 'Polarizing CT Input' is used to determine ground current
direction. The polarizing current comes from a source CT measuring the current flowing
from the ground return into the neutral of a ground fault current source which is usually a
transformer. The direction is Forward when the ground current is within ±90° of the
polarizing current. Otherwise, the direction is Reverse. If the polarizing current is less than
5% of CT nominal then the direction defaults to forward.
Dual polarization provides maximum security and reliability. If the polarizing voltage
magnitude is insufficient then the current polarizing takes control. If the polarizing current
magnitude is insufficient then the voltage polarizing takes control. If neither voltage nor
current polarizing is possible then the direction defaults to forward.
• GND POLARIZING: If ground directional control with both voltage and current
polarized elements is desired, enter "Dual". If ground directional control with only the
voltage polarized element is desired, enter "Voltage". If ground directional control with
only the current polarized element is desired, enter "Current".
• GND DIRECTIONAL MTA: Enter the maximum torque angle by which the operating
current leads the polarizing voltage. This is the angle of maximum sensitivity. This
setpoint affects voltage polarizing only.
• MIN POLARIZING VOLTAGE: This setpoint affects the voltage element only. As the
system zero sequence voltage is used as the polarizing voltage for this element, a
minimum level of voltage must be selected to prevent operation caused by system
unbalanced voltages or VT ratio errors. For well-balanced systems and 1% accuracy
VTs, this setpoint can be as low as 2% of VT nominal voltage. For systems with high-
resistance grounding or floating neutrals, this setpoint can be as high as 20%. The
default value of "0.05 x VT" is appropriate for most solidly grounded systems.
MIN OPERATING VOLTAGE
Polarizing Voltage
(VT connection = Wye)
–V = –(V + V + V ) / 3
o a b c
S5 PROTECTION
value, the direction
Polarizing Current
I (see note below)
pol
5 - 67