CHAPTER 4: SETPOINTS CONTROL
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL 4–395
FAST PATH:
The ground current input for sequence check must come from a core balance CT or a
transformer neutral point grounding CT. A Residual ground input method should not be
used for sequence check. Refer to the following figure, Sequence Check Ground Inputs.
Differential Check
Under normal load and balanced conditions, the ground current measured from the
ground CT should theoretically be equal to the calculated neutral current (3I0). In such
condition, the total current difference I
CTSdiff
should be zero.
If the CTS Diff. Current goes above the set minimum permitted value Setpoints > Monitoring
>
CT Supervision > Diff. Current PKP (I
CTSdiff
), it can be concluded that the fault is either in
the ground current path CT or in one of the phase current path CTs. A dynamic correction
factor or a Slope
Setpoints > Monitoring > CT Supervision > Slope is used to compensate for
higher CT errors at higher currents. With higher currents, a higher total difference is
tolerated. However, an additional maximum load current supervision check is added to
limit the CT failure detection during real fault conditions.
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