ABB Relion 670 series, Applications Manual

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Z Z Z
I I
I
Z
U
U
C Trf CT
A C
C
TF
= + +
+
⋅





 ⋅






2
1
2
DOCUMENT11524-IMG3510 V3 EN-US
(Equation 146)
Where:
Z
AT
and Z
CT
is the line impedance from the A respective C station to the T point.
I
A
and I
C
is fault current from A respective C station for fault between T and B.
U2/U1 Transformation ratio for transformation of impedance at U1 side of the
transformer to the measuring side U2 (it is assumed that current and voltage
distance function is taken from U2 side of the transformer).
Z
TF
is the line impedance from the T point to the fault (F).
Z
Trf
Transformer impedance
For this example with a fault between T and B, the measured impedance from the T point to
the fault will be increased by a factor defined as the sum of the currents from T point to the
fault divided by the IED current. For the IED at C, the impedance on the high voltage side U1
has to be transferred to the measuring voltage level by the transformer ratio.
Another complication that might occur depending on the topology is that the current from one
end can have a reverse direction for fault on the protected line. For example, for faults at T the
current from B might go in reverse direction from B to C depending on the system parameters
(see the dotted line in figure 133 ), given that the distance protection in B to T will measure
wrong direction.
In three-end application, depending on the source impedance behind the IEDs, the impedances
of the protected object and the fault location, it might be necessary to accept zone 2 trip in
one end or sequential trip in one end.
Generally for this type of application it is difficult to select settings of zone 1 that both gives
overlapping of the zones with enough sensitivity without interference with other zone 1
settings, that is, without selectivity conflicts. Careful fault calculations are necessary to
determine suitable settings and selection of proper scheme communication.
Fault resistance
M17048-597 v8
The performance of distance protection for single phase-to-earth faults is very important,
because normally more than 70% of the faults on transmission lines are single phase-to-earth
faults. At these faults, the fault resistance is composed of three parts: arc resistance,
resistance of a tower construction, and tower-footing resistance. The arc resistance can be
calculated according to Warrington's formula:
1.4
28707 L
Rarc
I
×
=
EQUATION1456 V1 EN-US
(Equation 147)
where:
L represents the length of the arc (in meters). This equation applies for the distance protection zone 1.
Consider approximately three times arc foot spacing for the zone 2 and to give extra margin to the
influence of wind speed and temperature.
I is the actual fault current in A.
Section 7 1MRK 505 343-UEN B
Impedance protection
210
Application manual