ABB Relion 670 series, Applications Manual

Page: 428 / 760

•
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. 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. Typical setting is 5-10% of
IBase.
UPolMin: Minimum polarization (reference) polarizing 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.
8.4.3.3 2nd harmonic restrain
M15282-90 v5
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.
8.4.3.4 Parallel transformer inrush current logic
M15282-97 v5
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
Section 8 1MRK 505 343-UEN B
Current protection
422
Application manual