ABB REC670 Applications Manual page 282

Section 11
Multipurpose protection
276
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 CurrentInput to NegSeq (please note that CVGAPC function measures I2
current and NOT 3I2 current; this is essential for proper OC pickup level setting)
3. Set VoltageInput to -NegSeq (please note that the negative sequence voltage phasor
is intentionally inverted in order to simplify directionality
4. Set base current IBase value equal to the rated primary current of power line CTs
5. Set base voltage UBase value equal to the rated power line phase-to-phase voltage
in kV
6. Set RCADir to value +65 degrees (NegSeq current typically lags the inverted
NegSeq voltage for this angle during the fault)
7. Set ROADir to value 90 degree
8. Set LowVolt_VM to value 2% (NegSeq voltage level above which the directional
element will be enabled)
9. Enable one overcurrent stage (for example, OC1)
10. By parameter CurveType_OC1 select appropriate TOC/IDMT or definite time
delayed curve in accordance with your network protection philosophy
11. Set PickupCurr_OC1 to value between 3-10% (typical values)
12. Set tDef_OC1 or parameter "TD" when TOC/IDMT curves are used to insure
proper time coordination with other ground-fault protections installed in the
vicinity of this power line
13. Set DirMode_OC1 to Forward
14. Set DirPrinc_OC1 to IcosPhi&U
15. Set 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 operate only when NegSeq current is bigger than a certain
percentage (10% is typical value) of measured PosSeq current in the power
line. To do this the following settings within the same function shall be done:
16. Set EnRestrainCurr to On
17. Set RestrCurrInput to PosSeq
18. Set RestrCurrCoeff to value 0.1
1MRK 511 401-UUS A
Bay control REC670 2.2 ANSI
Application manual