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.
4.1.5.6 Signals
Table 315: PHPVOC Input signals
Name Type Default Description
I_A SIGNAL 0 Phase A current
I_B SIGNAL 0 Phase B current
I_C SIGNAL 0 Phase C current
U_AB SIGNAL 0 Phase-to-phase volt-
age AB
U_BC SIGNAL 0 Phase-to-phase volt-
age BC
U_CA SIGNAL 0 Phase-to-phase volt-
age CA
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
Table 316: PHPVOC Output signals
Name
Type Description
OPERATE BOOLEAN Operate
START BOOLEAN Start
1MRS757644 H Protection functions
620 series
Technical Manual
333
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