GE D90 Plus Instruction Manual page 181

CHAPTER 7: PROTECTION
Current inputs
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
For three-phase channel groups, the number of the lowest numbered channel identifies
the group. For example, J1 represents the three-phase channel set of J1, J2, and J3, where
J is the slot letter and 1 is the first channel of the set of three channels. The first channel in
the group is identified as phase A, the second channel as phase B, and the third channel as
phase C.
Select the Settings > Protection > Power System > AC Inputs - Current menu item to
open the current inputs configuration window.
Figure 151: Current inputs configuration settings
The current inputs configuration window allows the user to specify the parameters for
each current input (represented by the module slot position letter). The
Ground CT Ratio settings are used for calculation of primary metering values and for ratio
matching when multiple CT inputs are added within a source. The
settings are used to derive secondary current values from per-unit
Ground CT Secondary
settings used in protection elements. This setting will be "1 A" or "5 A", depending on the
hardware.
Since the phase CTs are connected in a wye (star) configuration, the calculated phasor
sum of the three phase currents (IA + IB + IC = neutral current = 3I0) is used as the input for
the neutral overcurrent elements. In addition, a zero-sequence (core-balance) CT which
senses current in all of the circuit primary conductors, or a CT in a neutral grounding
conductor may also be used.
The following current input settings are available.
Phase CT Ratio
Range: 1 to 65000 in steps of 1
Default: 1
This setting specifies the phase CT ratio for the corresponding current input. It is used for
calculation of primary metering values and for ratio matching when multiple CT inputs
are added within a source.
Phase CT Secondary
Range: 1 A, 5 A
Default: 5 A
This setting selects the phase CT secondary value. It is sued to derive secondary current
values from per-unit settings used in protection elements.
Ground CT Ratio
Range: 1 to 65000 in steps of 1
Default: 1
This setting specifies the ground CT ratio for the corresponding current input. It is used
for calculation of primary metering values and for ratio matching when multiple CT
inputs are added within a source.
POWER SYSTEM
Phase CT Ratio and
and
Phase CT Secondary
171