ABB RELION 620 Series Technical Manual page 566

Section 4
Protection functions
4.3.4.5
560
Blocking logic
There are three operation modes in the blocking function. The operation modes are
controlled by the BLOCK input and the global setting Configuration/System/
Blocking mode which selects the blocking mode. The BLOCK input can be controlled
by a binary input, a horizontal communication input or an internal signal of the IED
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.
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 F
620 series
Technical Manual