not cause the overcurrent step start signal. Start signal will only come if all of the
enabled built-in features in the overcurrent step are fulfilled at the same time.
Second harmonic feature
The overcurrent protection step can be restrained by a second harmonic component in
the measured current quantity (see table
332). However it shall be noted that this
feature is not applicable when one of the following measured currents is selected:
• PosSeq (positive sequence current)
• NegSeq (negative sequence current)
• UnbalancePh (unbalance phase current)
• UnbalancePh-Ph (unbalance ph-ph current)
This feature will simple prevent overcurrent step start if the second-to-first harmonic
ratio in the measured current exceeds the set level.
Directional feature
The overcurrent protection step operation can be made dependent on the relevant
phase angle between measured current phasor (see table 332) and measured voltage
phasor (see table 333). In protection terminology it means that the General currrent
and voltage protection (CVGAPC) function can be made directional by enabling this
built-in feature. In that case overcurrent protection step will only operate if the current
flow is in accordance with the set direction (Forward, which means towards the
protected object, or Reverse, which means from the protected object). For this feature
it is of the outmost importance to understand that the measured voltage phasor (see
table
333) and measured current phasor (see table 332) will be used for directional
decision. Therefore it is the sole responsibility of the end user to select the appropriate
current and voltage signals in order to get a proper directional decision. CVGAPC
function will NOT do this automatically. It will simply use the current and voltage
phasors selected by the end user to check for the directional criteria.
Table
335 gives an overview of the typical choices (but not the only possible ones) for
these two quantities from traditional directional relays.
Table 335: Typical current and voltage choices for directional feature
Set value for the
parameter
CurrentInput
Set value for the
parameter
VoltageInput
Comment
PosSeq PosSeq Directional positive sequence overcurrent function is
obtained. Typical setting for
RCADir
is from -45° to
-90° depending on the power system voltage level (X/
R ratio)
NegSeq
-NegSeq Directional negative sequence overcurrent function is
obtained. Typical setting for
RCADir
is from -45° to
-90° depending on the power system voltage level (X/
R ratio)
3ZeroSeq
-3ZeroSeq Directional zero sequence overcurrent function is
obtained. Typical setting for
RCADir
is from 0° to -90°
depending on the power system earthing (that is,
solidly earthed, earthed via resistor)
Table continues on next page
1MRK502052-UEN B Section 11
Multipurpose protection
579
Technical manual
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