CHAPTER 5: SETTINGS
GROUPED ELEMENTS
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
5-313
5
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.
Figure 5-177: Transmission line with remote terminal open
Figure 5-178: Phasor diagram
The function approximates the voltage rise at the far end of the transmission line according to the following relationship:
Eq. 5-40
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
nominal
is the phase VT secondary setting in the case of wye VTs and the phase VT secondary setting divided by
in the
case of delta VTs
Z
C_mag
and Z
C_ang
represent an impedance between the local and remote terminals
V_1
C
is the calculated positive-sequence voltage magnitude at the remote terminal
If the magnitude of Z
C
is set to one-half the series impedance of the line (R + jX
L
), the compensated voltage is
approximately equal to the positive-sequence voltage at the remote end of the line. A more accurate setting of Z
C
can be
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:
Eq. 5-41
COMPENSATED OV BLK:
Off
Range: FlexLogic operand
COMPENSATED OV
TARGET: Self-reset
Range: Self-reset, Latched, Disabled
COMPENSATED OV
EVENTS: Disabled
Range: Disabled, Enabled
