ABB RET670 - Page 313

wideness of the operating sector. The sector is mirror-symmetric along the MTA
(Maximum Torque Axis).
Directional elements for ground-faults must operate at fault current values below the
magnitude of load currents. As phase quantities are adversely affected by load, the use
of sequence quantities are preferred as polarizing quantities for ground directional
elements. Optionally six possibilities are available:
• Zero-sequence voltage polarized (-V
0
)
• Negative-sequence voltage polarized (-V2)
• Zero-sequence current (I
0
)
• Dual polarization (-V
0
/I
0
)
• Zero-sequence voltage with zero-sequence current compensation (-V0Comp)
• Negative-sequence voltage with negative-sequence current compensation (-
V2Comp)
The zero-sequence voltage polarized ground directional unit compares the phase angles
of zero sequence current I
0
with zero sequence voltage -V
0
at the location of the
protection.
The negative-sequence voltage polarized ground directional unit compares
correspondingly I
2
with -V2.
In general 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 -V
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 grounded systems V2 may be larger than V
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 ungrounded or multiple grounds on the supplying VT neutral)
• no open-delta winding is needed in VTs as only 2 VTs are required (V
2
= (V
L12
- a
· V
L23
)/3)
The zero sequence current polarized ground directional unit compares zero sequence
current I0 of the line with some reference zero-sequence current, for example the
1MRK504116-UUS C Section 3
IED application
307
Application manual