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 pickup value of the current, well below the normal load current, is
selected. The control voltage setting should ensure that 51V 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 tripping of the function during wide-area
disturbances. When the terminal voltage of the generator is above the control voltage
value, the normal pickup value is used. This ensures that 51V does not trip 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 pickup value proportional to the terminal voltage.
The current pickup value varies correspondingly with the generator terminal voltages
between the set voltage high limit and voltage low limit, ensuring the operation of 51V
despite the drop in fault current value.
The operation of 51V should be time-graded with respect to the main protection scheme
to ensure that 51V does not trip before the main protection.
4.1.3.6 Signals
Table 259: 51V 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
V_AB SIGNAL 0 Phase-to-phase voltage AB
V_BC SIGNAL 0 Phase-to-phase voltage BC
V_CA SIGNAL 0 Phase-to-phase voltage CA
BLOCK BOOLEAN 0=False Block signal for activating the blocking mode
ENA_MULT BOOLEAN 0=False Enable signal for current multiplier
ENA_LOW_LIM BOOLEAN 0=False Enable signal for voltage dependent lower pickup value
Table 260: 51V Output signals
Name
Type Description
TRIP BOOLEAN Trip
PICKUP BOOLEAN Pickup
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 265
Technical Manual
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