ABB REL650 series, Applications Manual

Page: 157 / 516

• 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 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.
In dual polarization, zero sequence voltage polarization and zero sequence current
polarization elements function in an “OR-mode”. Typically when zero sequence current
is high, then zero sequence voltage is low and vice versa. Thus combining a zero sequence
voltage polarized and a zero sequence current polarized (neutral current polarized)
directional element into one element, the IED can benefit from both elements as the two
polarization measurements function in an OR mode complementing each other.
Flexibility is also increased as zero sequence voltage polarization can be used, if the zero
sequence current polarizing source is switched out of service. When the zero sequence
polarizing current exceeds the set value for startPolCurrLevel, zero sequence current
polarizing is used. For values of zero sequence polarizing current less than the set value
for startPolCurrLevel, zero sequence voltage polarizing is used.
Zero-sequence voltage polarization with zero-sequence current compensation (-
V0Comp) compares the phase angles of zero sequence current I
0
with zero-sequence
voltage added by a phase-shifted portion of zero-sequence current (see equation 72 ) at the
location of the protection. The factor k = setting K
mag
. This type of polarization in intended
for use in applications where the zero sequence voltage can be too small to be used as the
polarizing quantity, and there is no zero sequence polarizing current (transformer neutral
current) available. The zero sequence voltage is “boosted” by a portion of the measured
line zero sequence current to form the polarizing quantity. This method requires that a
significant difference must exist in the magnitudes of the zero sequence currents for close-
up forward and reverse faults, that is, it is a requirement that |V0| >> |k · I0| for reverse
faults, otherwise there is a risk that reverse faults can be seen as forward.
1MRK 506 334-UUS A
Section 6
Impedance protection
151
Application manual