ABB Relion REQ650, Applications Manual

Page: 127 / 340

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. Setting is in primary ohms.
When the dual polarizing method is used it is important that the product INx> · 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. Typical setting is 5-10%
of
IBase .
UPolMin : Minimum polarization (reference) residual voltage for the directional
function, given in % of UBase /√3.
I>Dir : 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, STFW and STRV can
be used in a teleprotection scheme. The appropriate signal should be configured to the
communication scheme block.
6.6.3.3 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
nd
harmonic restrain can prevent unwanted operation.
2ndHarmStab : The rate of 2nd harmonic current content for activation of the 2nd
harmonic restrain signal. The setting is given in % of the fundamental frequency
residual current.
HarmRestrainx : Enable block of step x from the harmonic restrain function.
1MRK 505 291-UEN A
Section 6
Current protection
121
Application manual