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 trip 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.
4.1.5.5
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
currents by controlling the generator excitation system. If the AVR is out of service
or if there is an internal fault in the operation of AVR, the low fault currents can go
unnoticed and therefore a voltage-depended overcurrent protection should be used
for backup.
Two voltage control characteristics, voltage step and voltage slope, are available
in PHPVOC. The choice is made based on the system conditions and the level of
protection to be provided.
Voltage step characteristic is applied to generators used in industrial systems.
Under close-up fault conditions when the generator terminal voltages drop below
the settable threshold value, a new start value of the current, well below the normal
load current, is selected. The control voltage setting should ensure that PHPVOC
does not trip under the highest loading conditions to which the system can be
Protection functions
1MRS757644 H
332
620 series
Technical Manual
