ABB Relion 670 Series Applications Manual page 237

1MRK 511 358-UUS A
but the cable negative-sequence impedance is practically constant. It shall be noted that
directional negative sequence OC element offers protection against all unbalance faults (phase-to-
phase faults as well). Care shall be taken that the minimum pickup of such protection function
shall be set above natural system unbalance level.
An example will be given, how sensitive-ground-fault protection for power lines can be achieved by
using negative-sequence directional overcurrent protection elements within a CVGAPC function.
This functionality can be achieved by using one CVGAPC function. The following shall be done to
ensure proper operation of the function:
1. Connect three-phase power line currents and three-phase power line voltages to one CVGAPC
instance (for example, GF04)
2. Set
3I2 current; this is essential for proper OC pickup level setting)
3. Set
intentionally inverted in order to simplify directionality
4. Set base current
5. Set base voltage
6. Set
for this angle during the fault)
7. Set
8. Set
enabled)
9. Enable one overcurrent stage (for example, OC1)
10. By parameter
accordance with your network protection philosophy
11. Set
12. Set
coordination with other ground-fault protections installed in the vicinity of this power line
13. Set
14. Set
15. Set
•
16. Set
17. Set
18. Set
If required, this CVGAPC function can be used in directional comparison protection scheme for the
power line protection if communication channels to the remote end of this power line are
available. In that case typically two
one for reverse direction. As explained before the OC1 stage can be used to detect faults in
forward direction. The built-in OC2 stage can be used to detect faults in reverse direction.
However the following shall be noted for such application:
Application manual
CurrentInput to NegSeq (please note that CVGAPC function measures I2 current and NOT
VoltageInput to -NegSeq (please note that the negative sequence voltage phasor is
IBase value equal to the rated primary current of power line CTs
UBase value equal to the rated power line phase-to-phase voltage in kV
RCADir to value +65 degrees ( NegSeq current typically lags the inverted NegSeq voltage
ROADir to value 90 degree
LowVolt_VM to value 2% ( NegSeq voltage level above which the directional element will be
CurveType_OC1 select appropriate TOC/IDMT or definite time delayed curve in
PickupCurr_OC1 to value between 3-10% (typical values)
tDef_OC1 or parameter "TD" when TOC/IDMT curves are used to insure proper time
DirMode_OC1 to Forward
DirPrinc_OC1 to IcosPhi&U
ActLowVolt1_VM to Block
In order to insure proper restraining of this element for CT saturations during three-
phase faults it is possible to use current restraint feature and enable this element to
NegSeq current is bigger than a certain percentage (10% is typical
operate only when
PosSeq current in the power line. To do this the following settings
value) of measured
within the same function shall be done:
EnRestrainCurr to On
RestrCurrInput to PosSeq
RestrCurrCoeff to value 0.1
NegSeq overcurrent steps are required. One for forward and
Section 10
Multipurpose protection
231