use of sequence quantities are preferred as polarizing quantities for the earth
directional elements. Optionally six modes are available:
• Zero-sequence voltage polarized (-U
0
)
• Negative-sequence voltage polarized (-U2)
• Zero-sequence current (I
0
)
• Dual polarization (-U
0
/I
0
)
• Zero-sequence voltage with zero-sequence current compensation (-U0Comp)
• Negative-sequence voltage with negative-sequence current compensation (-
U2Comp)
The zero-sequence voltage polarized earth directional unit compares the phase
angles of zero sequence current I
0
with zero sequence voltage -U
0
at the location of
the protection.
The negative-sequence voltage polarized earth directional unit compares
correspondingly I
2
with -U2.
In general the zero sequence voltage is higher than the negative sequence voltage at
the fault, but decreases more rapidly the further away from the fault it is measured.
This makes the -U
0
polarization preferable in short line applications, where no
mutual coupling problems exist.
Negative sequence polarization has the following advantages compared to zero
sequence polarization:
• on solidly earthed systems U2 may be larger than U
0
. If the bus behind the
IED location is a strong zero-sequence source, the negative sequence voltage
available at the IED location is higher than the zero-sequence voltage.
• negative sequence polarization is not affected by zero sequence mutual
coupling (zero sequence polarized directional elements may misoperate in
parallel lines with high zero-sequence mutual coupling and isolated zero
sequence sources).
• negative sequence polarization is less affected by the effects of VT neutral
shift (possible caused by unearthed or multiple earths on the supplying VT
neutral)
• no open-delta winding is needed in VTs as only 2 VTs are required (U
2
=
(U
L12
- a · U
L23
)/3)
The zero sequence current polarized earth directional unit compares zero sequence
current I0 of the line with some reference zero-sequence current, for example the
current in the neutral of a power transformer. The relay characteristic AngleRCA is
fixed and equals 0 degrees. Care must be taken to ensure that neutral current
direction remains unchanged during all network configurations and faults, and
therefore all transformer configurations/constructions are not suitable for
polarization.
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 283
Application manual
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