176 D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
POWER SYSTEM CHAPTER 7: PROTECTION
Ground CT Secondary
Range: 1 A, 5 A
Default: 5 A
This setting selects the ground CT secondary value. It is used to derive secondary current
values from per-unit settings used in protection elements.
Current Cutoff Level
Range: 0.002 to 0.020 pu in steps of 0.001
Default: 0.020 pu
This setting modifies the current cut-off threshold. Very low currents (1 to 2% of the
rated value) are very susceptible to noise. Some users prefer very low currents to display
as zero, while others prefer the current be displayed even when the value reflects noise
rather than the actual signal. The D90
Plus
applies a cut-off value to the magnitudes and
angles of the measured currents. If the magnitude is below the cut-off level, it is
substituted with zero. This applies to phase and ground current phasors as well as true
RMS values and symmetrical components. The cut-off operation applies to quantities
used for metering, protection, and control, as well as those used by communications
protocols. Raw current samples available via oscillography are not subject to cut-off.
NOTE:
Lower the Current Cutoff Level setting with care because the D90
Plus
accepts lower signals
as valid measurements. Unless dictated otherwise by a specific application, the default
setting of “0.020 pu” is recommended.
Voltage inputs
Select the Settings > Protection > Power System > AC Inputs - Voltage menu to open the
voltage input configuration window.
Figure 156: Voltage input configuration settings
This menu sets the parameters for each voltage input. The
Phase VT Ratio and Auxiliary VT
Ratio
settings are used to calculate primary metering values or matching primary voltage
in a synchrocheck application. The
Phase VT Secondary and Auxiliary VT Secondary
settings are used to derive secondary voltage values from the per-unit settings used in
protection elements. Protection functions that require phase-to-neutral or zero-sequence
quantities do not function correctly for delta VT configurations.
The following voltage input settings are available.
Phase VT Connection
Range: Wye, Delta
Default: Wye
This setting specifies the phase VT connection type for the corresponding voltage input.
Enter an open-delta source VT connection as “Delta”.
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