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Section 9
Current protection
9.4.3.2
478
polMethod: Defines if the directional polarization is from
•
Voltage (3U
or U
)
0
2
•
Current (3I
· ZNpol or 3I
0
•
both currents and voltage, Dual (dual polarizing, (3U
I
· ZNpol)).
2
Normally voltage polarizing from the internally calculated residual sum or an
external open delta is used.
Current polarizing is useful when the local source is strong and a high sensitivity is
required. In such cases the polarizing voltage (3U
necessary to use current polarizing or dual polarizing. Multiply the required set
current (primary) with the minimum impedance (ZNpol) and check that the
percentage of the phase-to-earth voltage is definitely higher than 1% (minimum
3U
>UPolMin setting) as a verification.
0
RNPol, XNPol: The zero-sequence source is set in primary ohms as base for the
current polarizing. The polarizing voltage is then achieved as 3I
ZNpol can be defined as (ZS
source behind the protection. The maximum earth-fault current at the local source
can be used to calculate the value of ZN as U/(√3 · 3I
ZNPol (3 · zero sequence source) is set. The setting is in primary ohms.
When the dual polarizing method is used, it is important that the setting INx> or
the product 3I
· ZNpol is not greater than 3U
0
operation for faults in the reverse direction.
IPolMin: is the minimum earth-fault current accepted for directional evaluation.
For smaller currents than this value, the operation will be blocked. A typical setting
is 5-10% of IB.
UPolMin: Minimum polarization (reference) polarizing voltage for the directional
function, given in % of UBase/√3.
IN>Dir: Operate residual current release level in % of IB for directional
comparison scheme. The setting is given in % of IB and must be set below the
lowest INx> setting, set for the directional measurement. The output signals,
STFW and STRV can be used in a teleprotection scheme. The appropriate signal
should be configured to the communication scheme block.
2nd harmonic restrain
If a power transformer is energized there is a risk that the current transformer core
will saturate during part of the period, resulting in a transformer inrush current.
This will give a declining residual current in the network, as the inrush current is
deviating between the phases. There is a risk that the residual overcurrent function
will give an unwanted trip. The inrush current has a relatively large ratio of 2nd
·ZNpol where ZNpol is RNpol + jXNpol), or
2
) can be below 1% and it is then
0
-ZS
)/3, that is the earth return impedance of the
1
0
. If so, there is a risk for incorrect
0
Line distance protection REL670 2.2 IEC
1MRK 506 369-UEN B
+ 3I
· ZNpol) or (U
0
0
· ZNpol. The
0
) Typically, the minimum
0
Application manual
+
2
M15282-90 v6
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