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Section 12
Multipurpose protection
572
An example will be given, how sensitive-earth-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 StartCurr_OC1 to value between 3-10% (typical values)
12. Set tDef_OC1 or parameter "k" when TOC/IDMT curves are used to insure
proper time coordination with other earth-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 0.10
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 NegSeq
overcurrent steps are required. One for forward and one for reverse direction. As
1MRK 506 369-UEN B
Line distance protection REL670 2.2 IEC
Application manual
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