ABB RET670 - Page 220

Voltage and current inversion
Series capacitors influence the magnitude and the direction of fault currents in series
compensated networks. They consequently influence phase angles of voltages
measured in different points of series compensated networks and this performances of
different protection functions, which have their operation based on properties of
measured voltage and current phasors.
Voltage inversion
Figure 84 presents a part of series compensated line with reactance X
L1
between the
IED point and the fault in point F of series compensated line. The voltage measurement
is supposed to be on the bus side, so that series capacitor appears between the IED
point and fault on the protected line. Figure
85 presents the corresponding phasor
diagrams for the cases with bypassed and fully inserted series capacitor.
Voltage distribution on faulty lossless serial compensated line from fault point F to the
bus is linearly dependent on distance from the bus, if there is no capacitor included in
scheme (as shown in figure 85). Voltage V
M
measured at the bus is equal to voltage
drop D V
L
on the faulty line and lags the current I
F
by 90 electrical degrees.
The situation changes with series capacitor included in circuit between the IED point
and the fault position. The fault current I
F
(see figure
85) is increased due to the series
capacitor, generally decreases total impedance between the sources and the fault. The
reactive voltage drop D V
L
on X
L1
line impedance leads the current by 90 degrees.
Voltage drop DV
C
on series capacitor lags the fault current by 90 degrees. Note that
line impedance X
L1
could be divided into two parts: one between the IED point and the
capacitor and one between the capacitor and the fault position. The resulting voltage
V
M
in IED point is this way proportional to sum of voltage drops on partial
impedances between the IED point and the fault position F, as presented by
( )
M F L1 C
V I j X X= × -
EQUATION1995-ANSI V1 EN
(Equation 126)
Section 3 1MRK504116-UUS C
IED application
214
Application manual