Signals - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.1.3.7
356
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 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.

Signals

Table 390: PHPVOC Input signals
Name Type
I3P SIGNAL
U3P SIGNAL
BLOCK BOOLEAN
ENA_MULT BOOLEAN
ENA_LOW_LIM BOOLEAN
Default Description
- Three-phase currents
- Three-phase voltages
0=False Block signal for activating the blocking mode
0=False Enable signal for current multiplier
0=False Enable signal for voltage dependent lower start
value
1MRS759142 C
REX640
Technical Manual