protection. The maximum ground-fault current at the local source can be used to calculate
the value of ZN as V/(√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 Pickupx or the
product 3I
0
· ZNpol is not greater than 3V
0
. If so, there is a risk for incorrect operation 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. A typical setting is 5-10%
of IB.
VPolMin: Minimum polarization (reference) polarizing voltage for the directional
function, given in % of VBase/√3.
IDirPU: 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, PUFW and PUREV 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.
At current transformer saturation a false residual current can be measured by the
protection. Here the 2
nd
harmonic restrain can prevent unwanted operation as well.
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.
9.4.3.3 Parallel transformer inrush current logic
In case of parallel transformers there is a risk of sympathetic inrush current. If one of the
transformers is in operation, and the parallel transformer is switched in, the asymmetric
inrush current of the switched-in transformer will cause partial saturation of the
transformer already in service. This is called transferred saturation. The 2
nd
harmonic of
the inrush currents of the two transformers will be in phase opposition. The summation of
the two currents will thus give a small 2
nd
harmonic current. The residual fundamental
Section 9 1MRK 506 369-UUS -
Current protection
512 Line distance protection REL670 2.2 ANSI
Application manual
To Next Page
Loading...
