GE MI-869

CHAPTER 4: SETPOINTS SYSTEM
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–63
Current Sensing
The Current Sensing menu provides the setup menu for the Current Transformers (CTs)
connected to the 869 terminals. The setup of the three-phase CTs, and the Ground CT
r
equires a selection of primary CT ratings. The secondary CT ratings are selected in the
869 Order code. The CT inputs are grouped in banks of four currents on the 869 – three
input
s for phase currents A, B, and C, and one input for ground current. The basic AC card
has two AC banks, one bank currents and one bank voltages.
The 869 basic Motor Protection System has 8 AC inputs, which reside on the card inserted
in slot J
.
There are two options for the 8 AC inputs from card J and they are as follows:
Option 1 (P5G5/P1G1):
• four AC input currents, 3 CTs for phase currents and 1 CT for the ground or residual
connection curr
ent, and
• four AC input voltages, 3 VTs for phase voltages, and 1 VT for auxiliary voltage
Option 2 (P50B/P10B):
• four AC input currents, 3 CTs for phase curr
ents and 1 Sensitive 50:0.025A CT for
ground or residual connection current, and
• four AC input voltages, i.e. 3 VTs for phase voltages, and 1 VT for auxiliary voltage
Additionally, a second card can be inserted into slot K. It has four AC input currents, i.e. 3
CT
s for phase currents and/or 1 CT for the sensitive 50:0.025A ground current.
The Current sensing selection for the 869 is organized in the menu path as shown:
Path:
Setpoints > System > Current Sensing > CT Bank 1-J1
Path: Setpoints > System > Current Sensing > CT Bank 2-K1
CT BANK NAME
This setting allows the user to specify CT bank name which will appear in all menus and
metering screens.
PHASE CT PRIMARY
Range: 1 A to 12000 A
Default: 500 A
Enter the primary rating of the three-phase feeder CTs wired to the relay phase CT
terminals. With the phase CTs connected in wye (star), the calculated phasor sum of the
three phase currents (Ia + Ib + Ic = Neutral Current = 3I0) is used as the input for the
neutral.
GROUND CT PRIMARY
Range: 1 A to 12000 A
Default: 500 A
Enter the primary rating of the ground CT wired to the relay ground CT terminals. When
the ground input is used for measuring the residual 3I0 current, the primary current
must be the same as the one selected for the phase CTs.
NOTE
NOTE:
The cut-off for current measurements is 0.02 x CT. This is the minimum value above which
metering functions.