•
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.
In dual polarization, zero sequence voltage polarization and zero sequence current polarization
elements function in a “one-out-of-two mode”. Typically when the zero sequence current is
high, then the 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 a “one-out-of-two mode” complementing each other. In this mode,
if IPOL is greater than IPOL> setting, then only IPOL based direction is detected and UPOL
based direction will be blocked. 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 IPOL>, 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 (-U0Comp)
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
) at the location of the
protection. The factor k = setting K
mag
. This type of polarization is 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
1MRK 505 343-UEN B
Section 7
Impedance protection
261
Application manual
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