Application; Setting Guidelines - ABB RET670 Applications Manual

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Section 3
IED application
3.7.1.1
3.7.1.2
396

Application

Long transmission lines often transfer great quantities of electric power from
production to consumption areas. The unbalance of the produced and consumed
electric power at each end of the transmission line is very large. This means that a fault
on the line can easily endanger the stability of a complete system.
The transient stability of a power system depends mostly on three parameters (at
constant amount of transmitted electric power):
The type of the fault. Three-phase faults are the most dangerous, because no power
can be transmitted through the fault point during fault conditions.
The magnitude of the fault current. A high fault current indicates that the decrease
of transmitted power is high.
The total fault clearing time. The phase angles between the EMFs of the
generators on both sides of the transmission line increase over the permitted
stability limits if the total fault clearing time, which consists of the protection
operating time and the breaker opening time, is too long.
The fault current on long transmission lines depends mostly on the fault position and
decreases with the distance from the generation point. For this reason the protection
must operate very quickly for faults very close to the generation (and relay) point, for
which very high fault currents are characteristic.
The instantaneous phase overcurrent protection 3-phase output PHPIOC (50) can
operate in 10 ms for faults characterized by very high currents.

Setting guidelines

The parameters for instantaneous phase overcurrent protection 3-phase output PHPIOC
(50) are set via the local HMI or PCM600.
This protection function must operate only in a selective way. So check all system and
transient conditions that could cause its unwanted operation.
Only detailed network studies can determine the operating conditions under which the
highest possible fault current is expected on the line. In most cases, this current appears
during three-phase fault conditions. But also examine single-phase-to-ground and two-
phase-to-ground conditions.
Also study transients that could cause a high increase of the line current for short times.
A typical example is a transmission line with a power transformer at the remote end,
which can cause high inrush current when connected to the network and can thus also
cause the operation of the built-in, instantaneous, overcurrent protection.
1MRK504116-UUS C
Application manual

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