174
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
POWER SYSTEM
CHAPTER 7: PROTECTION
Calculating the power cut-off level
The
Current Cutoff Level
and the
Voltage Cutoff Level
settings are used to determine the
metered power cut-off levels. The power cut-off level is calculated as shown below. For
delta connections, the power cut-off is calculated as follows.
Eq. 4
where VT primary = VT secondary × VT ratio and CT primary = CT secondary × CT ratio.
For wye connections, the three-phase and per-phase power cut-off values are calculated
as follows.
Eq. 5
Eq. 6
where VT primary = VT secondary × VT ratio and CT primary = CT secondary × CT ratio.
For example, given the following settings:
Figure 153: Typical power system settings
We have:
•
CT primary =
Phase CT Ratio
×
Phase CT Secondary
= 20 × 5 A = 100 A.
•
VT primary =
Phase VT Ratio
×
Phase VT Secondary
= 208 × 66.4 V = 13811.2 V.
The power cut-off is therefore:
Eq. 7
Any calculated power value below this cut-off will not be displayed. As well, the three-
phase energy data will not accumulate if the total power from all three phases does not
exceed the power cut-off.
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