The two inputs I3P — Three phase group signal for current and
U3P — Three phase group signal for voltage, must be connected to
non-adaptive SMAI blocks if ANY OF THE ZONES are set for
directional operation. That is, the parameter DFTReference in used
SMAI must be set to InternalDFTRef. If adaptive SMAI block is
used this might result in a wrong directional and reach evaluation.
8.3.2.1 System earthing
M17048-9 v4
The type of system earthing plays an important role when designing the protection
system. Some hints with respect to distance protection are highlighted below.
Solidly earthed networks
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In solidly earthed systems, the transformer neutrals are connected directly to earth
without any impedance between the transformer neutral and earth.
IEC05000215 V2 EN-US
Figure 107: Solidly earthed network
The earth-fault current is as high or even higher than the short-circuit current. The
series impedances determine the magnitude of the fault current. The shunt
admittance has very limited influence on the earth-fault current. The shunt
admittance may, however, have some marginal influence on the earth-fault current
in networks with long transmission lines.
The earth-fault current at single phase-to-earth in phase L1 can be calculated as
equation
101:
L1 L1
0
1 2 0 f 1 N f
3 U U
3I
Z Z Z 3Z Z ZZ
×
= =
+ + + + +
EQUATION1267 V3 EN-US (Equation 101)
Where:
U
L1
is the phase-to-earth voltage (kV) in the faulty phase before fault
Z
1
is the positive sequence impedance (Ω/phase)
Z
2
is the negative sequence impedance (Ω/phase), is considered to be
equal to Z
1
Table continues on next page
Section 8 1MRK 506 369-UEN B
Impedance protection
210 Line distance protection REL670 2.2 IEC
Application manual
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