2
I2 = (IL1+alpha IL2+alpha IL3)/3
×
×
IECEQUATION2406 V1 EN
(Equation 52)
where:
IL1, IL2 and IL3 are fundamental frequency phasors of three individual phase currents.
alpha
phasor with an angle of 120 degrees.
The polarizing current is pre-processed by a discrete fourier filter. Thus the phasor of
the fundamental frequency component of the polarizing current is derived. This
phasor is then multiplied with pre-set equivalent zero-sequence source Impedance in
order to calculate equivalent polarizing voltage UIPol in accordance with the
following formula:
0s
UIPol Z
IPol (RNPol j XNPol) IPol
=
×
=
+ ×
×
EQUATION1877 V2 EN
(Equation 53)
which will be then used, together with the phasor of the operating directional current,
in order to determine the direction to the earth fault (Forward/Reverse).
In order to enable current polarizing the magnitude of polarizing current shall be
bigger than a minimum level defined by setting parameter
IPolMin
.
Dual polarizing
When dual polarizing is selected the function will use the vectorial sum of the voltage
based and current based polarizing in accordance with the following formula:
(
)
0
s
UTotPol=UUPol UIPol=UPol Z
IPol UPol
RNPol
jXNPol Ipol
+
+
×
=
+
+
×
IECEQUATION2408 V1 EN
(Equation 54)
Upol and Ipol can be either zero sequence component or negative sequence
component depending upon the user selection.
Then the phasor of the total polarizing voltage UTotPol will be used, together with the
phasor of the operating current, to determine the direction of the earth fault (Forward/
Reverse).
8.2.7.3
External polarizing for earth-fault function
The individual steps within the protection can be set as non-directional. When this
setting is selected it is then possible via function binary input BLKSTx(where x
indicates the relevant step within the protection) to provide external directional
control (that is, torque control) by for example using one of the following functions if
available in the IED:
1. Distance protection directional function.
2. Negative sequence based overcurrent function.
1MRK 502 048-UEN A
Section 8
Current protection
195
Technical manual
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