Application - ABB RELION REX640 Technical Manual

1MRS759142 C
4.1.3.6
REX640
Technical Manual
"Inverse reset". The reset curve type "Immediate" causes an immediate reset. With the
reset curve type "Def time reset", the reset time depends on the Reset delay time
setting. With the reset curve type "Inverse reset", the reset time depends on the current
during the drop-off situation. The START output is deactivated when the reset timer
has elapsed.
The "Inverse reset" selection is only supported with ANSI or user
programmable types of the IDMT operating curves. If another
operating curve type is selected, an immediate reset occurs during the
drop-off situation.
The Time multiplier is used for scaling the IDMT operate and reset times.
The Minimum operate time setting defines the minimum desired operating time for
IDMT operation. The setting is applicable only when the IDMT curves are used.
Though the Time multiplier and Minimum operate time settings are
common for different IDMT curves, the operating time essentially
depends upon the type of IDMT curve chosen.
The Timer calculates the start duration value START_DUR which indicates the
percentage ratio of the start situation and the set operating time. This output 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

The three-phase voltage-dependent overcurrent protection is used as a backup
protection for the generators and system from damage due to the phase faults which
are not cleared by primary protection and associated breakers.
In case of a short circuit, the sustained fault current of the generator, determined by the
machine synchronous reactance, could be below the full-load current. If the generator
excitation power is fed from the generator terminals, a voltage drop caused by a short
circuit also leads to low fault current. The primary protection, like normal overcurrent
protection, might not detect this kind of fault situation. In some cases, the automatic
Section 4
Protection functions
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