program. The influence of the
BLOCK
signal activation is preselected with the global
setting
Blocking mode.
The
Blocking mode setting has three blocking methods. In the "Freeze timers"
mode, the operation timer is frozen to the prevailing value. In the "Block all" mode,
the whole function is blocked and the timers are reset. In the "Block OPERATE
output" mode, the function operates normally but the
OPERATE
output is not
activated.
4.3.4.5
Application
In solidly earthed systems, the restricted earth-fault protection is always deployed
as a complement to the normal transformer differential protection. The advantage
of the restricted earth-fault protection is its high sensitivity. Sensitivities of
close to 1.0 percent can be achieved, whereas normal differential IEDs have their
minimum sensitivity in the range of 5 to 10 percent. The level for HREFPDIF is
dependent on the current transformers' magnetizing currents. The restricted earth-
fault protection is also very fast due to the simple measuring principle as it is a unit
type of protection.
The differences in measuring principle limit the biased differential IED's possibility
to detect the earth faults. Such faults are then only detected by the restricted
earth-fault function.
The restricted earth-fault IED is connected across each directly or to low-ohmic
earthed transformer winding. If the same CTs are connected to other IEDs, separate
cores are to be used.
1MRS757644 H
Protection functions
620 series
Technical Manual
571
