ABB REL 551-C1*2.5, Technical Reference Manual

Page: 81 / 170

72
Time delayed residual overcurrent protection
(TEF) Chapter 5
Current
3 Time delayed residual overcurrent protection
(TEF)
3.1 Application
Use the inverse and definite time delayed residual overcurrent functions in solidly earthed sys-
tems to get a sensitive and fast fault clearance of phase to earth faults.
The nondirectional protection can be used when high sensitivity for earth fault protection is re-
quired. It offers also a very fast back-up earth fault protection for the part of a transmission line,
closest to the substation with the protection.
The nondirectional residual overcurrent protection can be given a relatively low current pick-up
setting. Thus the protection will be sensitive, in order to detect high resistive phase to earth
faults.
3.2 Functionality
The residual overcurrent protection measures the residual current of the protected line. This cur-
rent is compared to the current settings of the function. If the residual current is larger than the
setting value a trip signal will be sent to the output after a set delay time. The time delay can be
selected between the definite or inverse possibility.
In order to avoid unwanted trip for transformer inrush currents, the function is blocked if the sec-
ond harmonic content of the residual current is larger than 20% of the measured residual current.
3.3 Function block
Figure 29: Function block, nondirectional
xx00000701.vsd
TEF--
TEF
BLOCK
BLKTR
BC TRIP
TRSOTF
START