• Voltage (3U
0
or U
2
)
• Current (3I
0
· ZNpol or 3I
2
·ZNpol where ZNpol is RNpol + jXNpol), or
• both currents and voltage, Dual (dual polarizing, (3U
0
+ 3I
0
· ZNpol) or (U
2
+ I
2
· ZNpol)).
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
0
) can be below 1% and it is then
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
0
>UPolMin setting) as a verification.
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
0
· ZNpol. The ZNpol
can be defined as (ZS
1
-ZS
0
)/3, that is the earth return impedance of the 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
0
) Typically, the minimum 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
0
· ZNpol is not greater than 3U
0
. If so, there is a risk for incorrect 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.
9.4.3.2 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 harmonic
component. This component can be used to create a restrain signal to prevent this
unwanted function.
Section 9 1MRK 502 071-UEN -
Current protection
342 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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