The IED point voltage inverses its direction due to presence of series capacitor and
its dimension. It is a common practice to call this phenomenon voltage inversion.
Its consequences on operation of different protections in series compensated
networks depend on their operating principle. The most known effect has voltage
inversion on directional measurement of distance IEDs (see chapter
"Distance
protection" for more details), which must for this reason comprise special measures
against this phenomenon.
There will be no voltage inversion phenomena for reverse faults in system with
VTs located on the bus side of series capacitor. The allocation of VTs to the line
side does not eliminate the phenomenon, because it appears again for faults on the
bus side of IED point.
Current inversion
GUID-9F7FDF59-D0B4-4972-9CB0-B3D56DACA09E v1
Figure
201 presents part of a series compensated line with corresponding
equivalent voltage source. It is generally anticipated that fault current I
F
flows on
non-compensated lines from power source towards the fault F on the protected line.
Series capacitor may change the situation.
en06000607.vsd
~
Z<
XS
X
L1
IF
U
U
M
Source
Fault voltage
Pre -fault voltage
X
C
Source voltage
U’
M
With bypassed
capacitor
With inserted
capacitor
F
IEC06000607 V1 EN-US
Figure 201: Current inversion on series compensated line
The relative phase position of fault current I
F
compared to the source voltage U
S
depends in general on the character of the resultant reactance between the source
and the fault position. Two possibilities appear:
1
1
0
0
- + >
- + <
S C L
S C L
X X X
X X X
EQUATION1935 V1 EN-US
(Equation 363)
The first case corresponds also to conditions on non compensated lines and in
cases, when the capacitor is bypassed either by spark gap or by the bypass switch,
as shown in phasor diagram in figure 202. The resultant reactance is in this case of
inductive nature and the fault currents lags source voltage by 90 electrical degrees.
Section 8 1MRK 506 369-UEN B
Impedance protection
378 Line distance protection REL670 2.2 IEC
Application manual
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