Ground Directional Overcurrent; High-Set Phase Overcurrent - GE D60 Instruction Manual

SERIES COMPENSATED LINES
If a minimum fault current does not guarantee an immediate capacitor by-pass, then the capacitor must be included in
the calculation: its overvoltage protection level, either air gap flash-over voltage or MOV knee-point voltage, is used (RMS,
not peak value).
Assuming none of the series capacitors in the sample system is guaranteed to get by-passed, the following calculations
apply.
For the Sending Bus:
0.5 + 0.7 = 1.2 pu if the line-side (B) VTs are used
0.6 + 0.5 + 0.7 = 1.8 pu if the bus-side (A) VTs are used
For the Receiving Bus:
0.6 + 0.5 = 1.1 pu if the line-side (B) VTs are used
0.6 + 0.5 + 0.5 = 1.6 pu if the bus-side (A) VTs are used

8.4.3 Ground directional overcurrent

Ground directional overcurrent function (negative-sequence or neutral) uses an offset impedance to guarantee correct
fault direction discrimination. The following setting rules apply:
• If the net impedance between the potential source and the local equivalent system is inductive, then there is no need
for an offset. Otherwise, the offset impedance shall be at least the net capacitive reactance.
• The offset cannot be higher than the net inductive reactance between the potential source and the remote equivalent
system. For simplicity and extra security, the far-end busbar can be used rather than the remote equivalent system.
As the ground directional functions are meant to provide maximum fault resistance coverage, it is justified to assume that
the fault current is very low and none of the series capacitors is guaranteed to get by-passed. Consider settings of the
negative-sequence directional overcurrent protection element for the Sample Series Compensated System.
For the Sending Bus relay, bus-side VTs:
Net inductive reactance from the relay into the local system = –2 + 3 = 1 Ω > 0; there is no need for offset
•
• Net inductive reactance from relay through far-end busbar = –4 + 10 – 3 = 3 Ω; the offset cannot be higher than 3 Ω
It is recommended to use 1.5 Ω offset impedance
•
For the Sending Bus relay, line-side VTs:
• Net inductive reactance from relay into local system = –2 + 3 – 4 = –3 Ω < 0; an offset impedance ≥3 Ω must be used
• Net inductive reactance from relay through far-end busbar = 10 – 3 = 7 Ω; the offset cannot be higher than 7 Ω
• It is recommended to use 5 Ω offset impedance
For the Receiving Bus relay, bus-side VTs:
• Net inductive reactance from relay into local system = –5 + 7 = 2 Ω > 0; there is no need for offset
• Net inductive reactance from relay through far-end busbar = –3 + 10 – 4 = 3 Ω; the offset cannot be higher than 3 Ω
• It is recommended to use 1.5 Ω offset impedance
For the Receiving Bus relay, line-side VTs:
8
• Net inductive reactance from relay into local system = –3 – 5 + 7 = –1 Ω < 0; an offset impedance ≥1 Ω must be used
• Net inductive reactance from relay through far-end busbar = 10 – 4 = 6 Ω; the offset cannot be higher than 6 Ω
• It is recommended to use 3.5 Ω offset impedance

8.4.4 High-set phase overcurrent

The setting rules for high-set overcurrent protection are explained in the High-Set Overcurrent Elements section.
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CHAPTER 8: APPLICATION OF SETTINGS
D60 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL