ABB RELION REX640, Техническое руководство, Страница: 375 / 1960

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
subjected. Choosing too high a value for the control voltage may allow an undesired
operation of the function during wide-area disturbances. When the terminal voltage
of the generator is above the control voltage value, the normal start value is used.
This ensures that PHPVOC does not operate during normal overloads when the
generator terminal voltages are maintained near the normal levels.
Voltage slope characteristic is often used as an alternative to impedance protection
on small to medium (5...150 MVA) size generators to provide backup to the
differential protection. Other applications of the voltage slope characteristic
protection exist in networks to provide better coordination and fault detection
than plain overcurrent protection. The voltage slope method provides an improved
sensitivity of overcurrent operation by making the overcurrent start value
proportional to the terminal voltage. The current start value varies correspondingly
with the generator terminal voltages between the set voltage high limit and voltage
low limit, ensuring the operation of PHPVOC despite the drop in fault current value.
The operation of PHPVOC should be time-graded with respect to the main
protection scheme to ensure that PHPVOC does not operate before the main
protection.
4.1.3.7 Signals
PHPVOC Input signals
Table 416: PHPVOC Input signals
Name Type Default Description
I3P SIGNAL - Three-phase currents
U3P SIGNAL - Three-phase voltages
BLOCK BOOLEAN 0=False Block signal for acti-
vating the blocking
mode
ENA_MULT BOOLEAN 0=False Enable signal for cur-
rent multiplier
ENA_LOW_LIM BOOLEAN 0=False Enable signal for volt-
age dependent lower
start value
1MRS759142 F Protection functions
REX640
Technical Manual
375