GE D90 Plus Instruction Manual page 238

GROUPED PROTECTION ELEMENTS
228
Figure 200: Example of series compensated system
Assuming 20% security margin, the underreaching zone is set as follows. At the sending
bus, one must consider an external fault at F1, as the 5 Ω capacitor contributes to the
overreaching effect. Any fault behind F1 is less severe as extra inductive line impedance
increases the apparent impedance.
• Reach setting: 0.8 × (10 – 3 – 5) = 1.6 Ω if the line-side (B) VTs are used
• Reach setting: 0.8 × (10 – 4 – 3 – 5) = –1.6 Ω if the bus-side (A) VTs are used
The negative value means that an underreaching zone cannot be used as the circuit
between the potential source of the relay and an external fault for which the relay must
not pickup, since it is overcompensated (that is, capacitive).
At the receiving bus, one must consider a fault at F2.
• Reach setting: 0.8 × (10 – 4 – 2) = 3.2 Ω if the line-side (B) VTs are used
• Reach setting: 0.8 × (10 – 4 – 3 – 2) = 0.8 Ω if the bus-side (A) VTs are used
However, to practically cope with the effect of sub-synchronous oscillations, it can be
necessary to reduce the reach even more. Since the characteristics of sub-synchronous
oscillations are in complex relations with fault and system parameters, no specific setting
recommendations are given with respect to extra security margin for sub-synchronous
oscillations. It is strongly recommended to use a power system simulator to verify the
reach settings or to use an adaptive D90
Ground directional overcurrent guidelines for protecting series compensated lines
The ground directional overcurrent function (negative-sequence overcurrent or neutral
overcurrent) 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:
1. The net inductive reactance from the relay into the local system is –2 Ω + 3 Ω = 1 Ω > 0.
There is no need for offset.
2. The net inductive reactance from relay through far-end busbar is –4 Ω + 10 Ω – 3 Ω = 3
Ω. The offset cannot be greater than 3 Ω.
3. Therefore, it is recommended to use a 1.5 Ω offset impedance.
Plus
feature for dynamic reach control.
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION