GE L90 Instruction Manual page 537

CHAPTER 5: SETTINGS
The compensated overvoltage function provides protection against an overvoltage due to the opening of the remote
terminal of a transmission line—the so called the Ferranti effect. This can be achieved using a transfer-tripping scheme.
However, with high voltage, more corona can exist on the line and inhibit the proper reception of a carrier-transfer-trip
signal. Also, the presence of a line with an open terminal in weak systems can raise the voltage level of the local bus.
Detecting and tripping a line with an open terminal can prevent tripping at the local bus in this case.
The function approximates the voltage rise at the far end of the transmission line according to the following relationship:
where
V_1 is the positive-sequence voltage (phasor quantity) in secondary volts measured at the local terminal
I_1 is the positive-sequence current (phasor quantity) in secondary amps measured at the local terminal
V is the phase VT secondary setting in the case of wye VTs and the phase VT secondary setting divided by
nominal
case of delta VTs
Z and Z represent an impedance between the local and remote terminals
C_mag C_ang
V_1 is the calculated positive-sequence voltage magnitude at the remote terminal
C
If the magnitude of Z is set to one-half the series impedance of the line (R + jX
C
approximately equal to the positive-sequence voltage at the remote end of the line. A more accurate setting of Z
made if the positive-sequence charging current and the voltages at the local and remote line ends resulting from an open
breaker are known. In this case, the required reach setting is:
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
COMPENSATED OV BLK:

Off
COMPENSATED OV

TARGET: Self-reset
COMPENSATED OV

EVENTS: Disabled
Figure 5-177: Transmission line with remote terminal open
Figure 5-178: Phasor diagram
GROUPED ELEMENTS
Range: FlexLogic operand
Range: Self-reset, Latched, Disabled
Range: Disabled, Enabled
), the compensated voltage is
L
5
Eq. 5-40
in the
can be
C
Eq. 5-41
5-313