GE 750 Instruction Manual page 188

S5 PROTECTION
Note
NOTE
5 - 62
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 voltage
used for neutral directional control
Table 5–12: Neutral Directional Characteristics
Quantity Operating
Current
3I = I + I
Neutral
o a b
On relays with bootware revision 3.00 or newer, the polarizing current is input via the
Ground CT input. Otherwise, the polarizing current is input via a dedicated polarizing CT
input. See Current Inputs on page 3–11 for additional details.
When current polarized the Ground CT Input (Terminals G10 and H10) is used to determine
neutral 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 neutral 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 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.
FIGURE 5–13: Neutral Directional Voltage Polarization
The Neutral Directional specific setpoints are described below.
• NEUTRAL POLARIZING: If neutral directional control with both voltage and current
polarized elements is desired, enter "Dual". If neutral directional control with only the
MIN OPERATING VOLTAGE
Polarizing Voltage
(VT connection = Wye)
+ I –V = –(V + V + V
c o a b
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS
value, the direction
Polarizing Current
) / 3 I (see note below)
c g