ABB RET670 Applications Manual page 434

Section 3
IED application
428
Current polarizing is useful when the local source is strong and a high sensitivity is
required. In such cases the polarizing voltage (3V
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-ground voltage is definitely higher than 1% (minimum 3V
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
defined as (ZS -ZS )/3, that is the ground return impedance of the source behind the
1 0
protection. The maximum ground-fault current at the local source can be used to
calculate the value of ZN as V/(√3 · 3I
sequence source) is set. Setting is in primary ohms.
When the dual polarizing method is used it is important that the settingPickupxor the
product 3I · ZNpol is not greater than 3V
0
for faults in the reverse direction.
IPolMin: is the minimum ground-fault current accepted for directional evaluation. For
smaller currents than this value the operation will be blocked. Typical setting is 5-10%
of IBase.
VPolMin: Minimum polarization (reference) residual voltage for the directional
function, given in % of VBase/√3.
IDirPU: Operate residual current release level in % of IBase for directional comparison
scheme. The setting is given in % of IBase and must be set below the lowest INx>
setting, set for the directional measurement. The output signals, PUFW and PUREV
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 harmonic
component. This component can be used to create a restrain signal to prevent this
unwanted function.
At current transformer saturation a false residual current can be measured by the
protection. Also here the 2
) can be below 1% and it is then
0
) Typically, the minimum ZNPol (3 · zero
0
. If so, there is a risk for incorrect operation
0
nd
harmonic restrain can prevent unwanted operation.
1MRK504116-UUS C
>VPolMin setting)
0
· ZNpol. The ZNpol can be
0
Application manual