Application - ABB RELION REX640 Technical Manual

1MRS759142 C
4.3.4.6
REX640
Technical Manual
The second harmonic blocking is disabled when Restraint mode is set to "None" and
enabled when set to "Harmonic2".
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 reset timer resets and the START output is deactivated.
The Timer calculates the start duration value START_DUR which indicates the
percentage ratio of the start situation and the set operate time. The value is available
through 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 in 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 operate timer is frozen to the prevailing value, but the OPERATE output is not
deactivated when blocking is activated. 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. The activation of the
output of the second harmonic blocking signal BLK2H deactivates the OPERATE
output.

Application

An earth-fault protection using an overcurrent element does not adequately protect the
transformer winding in general and the star-connected winding in particular.
The restricted earth-fault protection is mainly used as a unit protection for the
transformer windings. LREFPNDF is a sensitive protection applied to protect the star-
connected winding of a transformer. This protection system remains stable for all the
faults outside the protected zone.
LREFPNDF provides higher sensitivity for the detection of earth faults than the
overall transformer differential protection. This is a high-speed unit protection
scheme applied to the star-connected winding of the transformer. LREFPNDF is
normally applied when the transformer is earthed solidly or through low-impedance
resistor (NER). LREFPNDF can be also applied on the delta side of the transformer if
an earthing transformer (zig-zag transformer) is used there. In LREFPNDF, the
difference of the fundamental component of all three phase currents and the neutral
Section 4
Protection functions
665