Application - ABB RELION RET670 Applications Manual

1MRK 504 163-UUS A
8.19.2
Transformer protection RET670 2.2 ANSI
Application manual

Application

Phase preference logic function PPLPHIZ is an auxiliary function to Distance
protection zone, quadrilateral characteristic ZMQPDIS (21) and Phase selection with
load encroachment, quadrilateral characteristic function FDPSPDIS (21). The purpose
is to create the logic in resonance or high resistive grounded systems (normally sub-
transmission) to achieve the correct phase selective tripping during two simultaneous
single-phase ground-faults in different phases on different line sections.
Due to the resonance/high resistive grounding principle, the ground faults in the system
gives very low fault currents, typically below 25 A. At the same time, the occurring
system voltages on the healthy phases will increase to line voltage level as the neutral
displacement is equal to the phase voltage level at a fully developed ground fault. This
increase of the healthy phase voltage, together with slow tripping, gives a considerable
increase of the risk of a second fault in a healthy phase and the second fault can occur
at any location. When it occurs on another feeder, the fault is commonly called cross-
country fault.
Different practices for tripping is used by different utilities. The main use of this logic
is in systems where single phase-to-ground faults are not automatically cleared, only
alarm is given and the fault is left on until a suitable time to send people to track down
and repair the fault. When cross-country faults occur, the practice is to trip only one of
the faulty lines. In other cases, a sensitive, directional ground-fault protection is
provided to trip, but due to the low fault currents long tripping times are utilized.
Figure 245 shows an occurring cross-country fault. Figure
of line voltage on healthy phases and an occurring cross-country fault.
ANSI06000550 V2 EN-US
Figure 245: An occurring cross-country fault on different feeders in a sub-
transmission network, high impedance (resistance, reactance)
grounded
CG
AG
Section 8
Impedance protection
SEMOD167856-4 v5
246 shows the achievement
Load
Y
Load
Y
ANSI06000550_2_en.vsd
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