Application - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.3.5.6
674
Timer
Once activated, the timer activates the START output. The time characteristic is
according to DT. When the operation timer has reached the value set by Minimum
operate time, the OPERATE output is activated. If the fault disappears before the
module operates, the reset timer is activated. If the reset timer reaches the value set by
Reset delay time, the operation timer resets and the START output is deactivated.
The timer calculates the start duration value START_DUR, which indicates the ratio
of the start situation and the set operation time. The value is available in the monitored
data view.
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
protection relay's 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 protection relays 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 protection relay's
possibility to detect the earth faults. Such faults are then only detected by the restricted
earth-fault function.
The restricted earth-fault protection relay is connected across each directly or to low-
ohmic earthed transformer winding. If the same CTs are connected to other protection
relays, separate cores are to be used.
1MRS759142 C
REX640
Technical Manual