ABB REL670 2.2 IEC Applications Manual page 737

1MRK 506 369-UEN B
Line distance protection REL670 2.2 IEC
Application manual
to the length of the line and the properties of the transmission line. The Ferranti
effect is symmetrical between all three phases for normal balanced load condition.
The overvoltage caused by Ferranti effect can be reduced by drawing larger load
through the line or switching in the shunt reactor (connected either to line or to
remote bus) at the remote end. The calculated compensated voltage at the local end
can detect such overvoltage phenomenon.
The vector representation of local end and remote end voltages are shown below:
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IEC09000774 V1 EN-US
Figure 380: Vector diagram for local end and remote end voltage at no power
transfer conditions
Where:
OM Remote end voltage Ur
OP Local end voltage Us
OC Current drawn by capacitance (Ic)
MN Resistance drop (IcR)
NP Inductive reactance drop (IcX)
If there is a transmission line that is opened at the remote end or radial or remote
end source is weak, then a fault anywhere on the line can result into undervoltage
at the remote end. There can be undervoltage at remote end also due to heavy
loading or poor power factor on lagging side. A fault in a line connected beyond
the remote end bus can also produce undervoltage at remote end. The compensated
voltage calculated at the local end can detect such undervoltages. The undervoltage
caused by a fault can be asymmetrical while that due to overloading is
symmetrical.
The trip signal issued by compensated over and under voltage function should be
accompanied by a transfer trip signal received from the remote end. The trip signal
should be used as a release signal which can permit a remote transfer trip to be
used to trip the local circuit breaker.
Setting of over voltage and under voltage levels for compensated voltage should be
same as the remote end over and under voltage levels. This will ensure proper
operation of voltage protection of the transmission line.
The definite delay time for compensated over and under voltage can be shorter than
that at remote end, but not too short. A short delay time would result in frequent
Scheme communication
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Section 16
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IEC09000774-1-en.vsd
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