ABB Relion REG670, Applications Manual

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en06000608.vsd
I
F
U
S
U
’
M
=
H
U
L
H
U
S
With bypassed
capacitor
I
F
H
U
L
U
S
H
U
C
U
M
H
U
S
With inserted
capacitor
IEC06000608 V1 EN
Figure 113: Phasor diagrams of currents and voltages for the bypassed and
inserted series capacitor during current inversion
It is a common practice to call this phenomenon current inversion. Its consequences
on operation of different protections in series compensated networks depend on their
operating principle. The most known effect has current inversion on operation of
distance IEDs (as shown in section
"Distance protection" for more details), which
cannot be used for the protection of series compensated lines with possible current
inversion. Equation
111 shows also big dependence of possible current inversion on
series compensated lines on location of series capacitors. X
L1
= 0 for faults just behind
the capacitor when located at line IED and only the source impedance prevents current
inversion. Current inversion has been considered for many years only a theoretical
possibility due to relatively low values of source impedances (big power plants)
compared to the capacitor reactance. The possibility for current inversion in modern
networks is increasing and must be studied carefully during system preparatory
studies.
The current inversion phenomenon should not be studied only for the purposes of
protection devices measuring phase currents. Directional comparison protections,
based on residual (zero sequence) and negative sequence currents should be
considered in studies as well. Current inversion in zero sequence systems with low
zero sequence source impedance (a number of power transformers connected in
parallel) must be considered as practical possibility in many modern networks.
Location of instrument transformers
Location of instrument transformers relative to the line end series capacitors plays an
important role regarding the dependability and security of a complete protection
scheme. It is on the other hand necessary to point out the particular dependence of
those protection schemes, which need for their operation information on voltage in
IED point.
Protection schemes with their operating principle depending on current measurement
only, like line current differential protection are relatively independent on CT
location. Figure
114 shows schematically the possible locations of instrument
transformers related to the position of line-end series capacitor.
Section 8
1MRK 502 071-UEN -
Impedance protection
234 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual