GE Multilin G650 Manual page 193

5.4 PROTECTION ELEMENTS
Figure 5–8: shows the Operation of the directional element for a Phase A to Ground fault, where the Phase A current grows
in magnitude and is delayed with respect to its voltage by an angle similar to the protected line. Va voltage decreases or
can even disappear if the fault is close and the fault resistance is very low.
5
The voltage polarization algorithm uses –Vn, -(Va+Vb+Vc) = -3·V
magnitude can be rotated by the desired angle to fix the MTA line and to define the operative semi plane of the relay,
following the rule that positive angles are in counter clockwise direction. A typical setting is –45º, as shown on the figure.
The operative semi plane is delimited to ± 85º of the MTA line. Every time the operation magnitude, In, is inside this semi
plane, the element will consider that the direction is forward. If the Direction setting is set as Forward, the operation signal
of the neutral directional element (NEUTRAL DIR OP) will be activated.
Minimum acceptable values, both for the polarization magnitude and the operation magnitude are as follows: minimum In
current for the element to operate is 50 mA. Minimum polarization voltage for the element to operate is set in the
Polarization Voltage Threshold setting. Minimum polarization current (Ip) is 5 mA.
The voltage polarized directional element needs a typical time of 1 cycle (20ms @ 50Hz) to polarize. This time must be
considered when setting the overcurrent elements with the Block Logic setting as Permission. This may cause, especially
in testing processes, the relay to trip with counter direction faults when voltage and current are applied at the same time
starting from zero. As there is no previous polarization voltage, the overcurrent element is ready to trip under any
overcurrent (as set in the Block Logic setting), while the directional element will need a complete cycle to polarize and give
the correct direction. If the current is high enough to pickup the overcurrent element and there is no set time delay, the
element will trip before the directional element blocks the trip. In cases where this situation is foreseen, it is recommended
to program the Block Logic setting as Block, or else to add a small time delay to the overcurrent element to allow the
directional element to polarize and block the trip.
5-50
Cone 5º
Vc
Figure 5–8: VOLTAGE POLARIZATION
G650 Generator Protection & Control System
-3V0
Maximum
-45º
torque angle
IA
-85º = 90º - 5º cone
-90º
Forward
Vb
3V0 = Va +Vb+Vc
, as a substitute for the faulted phase voltage. This
0
5 SETTINGS
Fault Ia In
GEK-113285A