In a drop-off situation, that is, when a fault suddenly disappears before the
operating delay is exceeded, the timer reset state is activated. The functionality
of the Timer in the reset state depends on the combination of the
Operating curve
type
,
Type of reset curve
and
Reset delay time
settings. When the DT characteristic
is selected, the reset timer runs until the set
Reset delay time
value is exceeded.
When the IDMT curves are selected, the
Type of reset curve
setting can be set to
"Immediate", "Def time reset" or "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 and the
value of START_DUR. 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.
4.1.3.6 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 voltage regulator AVR can help to maintain high fault
Protection functions
1MRS759142 F
374 REX640
Technical Manual
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