ABB Relion 670 Series Applications Manual page 234

Section 10
Unbalance protection
operates for the cascading fault, but it may be too slow or completely disarmed by the high
level of the unbalanced voltage/current caused by the cascading failure.
IEC19000101 V1 EN-US
Figure 106: Cascading failure within SCB
The cascading failure is characterized by the presence of an unbalanced SCB current and
absence of an unbalance SCB voltage. The absence of the unbalanced SCB voltage is due to the
strength of the connected power system in comparison with the SCB rating.
Shunt capacitor bank impedance
The SCB has a constant impedance determined by the required MVAr rating and the power
system voltage level. For a three-phase SCB, the impedance is calculated as follows:
W =
X [ ]
SCB
IECEQUATION19003 V1 EN-US
It is shown that the SCB positive and the negative zero sequence impedances have exactly the
same value as
SCB star point. If the SCB is solidly earthed, then the zero sequence impedance has value equal
X
to
SCB
Cascading failure protection with sequence components-based method
The theory of symmetrical components states that the source of the negative and the zero
sequence quantities are located at the point of failure. Thus, for all the unbalances external to
the SCB, the resultant SCB negative sequence current and the voltage magnitudes are
correlated by the SCB impedance
=
U X
2
IECEQUATION19004 V1 EN-US
228
Bus
CT
Optional earthing of the SCB
2
U [ kV ]
-
ph ph
Q [ MVAr ]
SCB
X
. The SCB zero sequence impedance depends on the type of earthing of the
SCB
. Thus, all the three sequence impedances are constant for a healthy SCB.
´
I
SCB 2
© Copyright 2017 Hitachi Power Grids. All rights reserved
VT
X
value as as follows:
SCB
1MRK 511 401-UEN Rev. K
670 IED
Cascading failure
IECI19000101-1-en.vsdx
Bay control REC670
Application manual
(Equation 101)
(Equation 102)