10.4 SERIES COMPENSATED LINES
10.4SERIES COMPENSATED LINES
Traditionally, the reach setting of an underreaching distance function shall be set based on the net inductive impedance
between the potential source of the relay and the far-end busbar, or location for which the zone must not overreach. Faults
behind series capacitors on the protected and adjacent lines need to be considered for this purpose. For further illustration
a sample system shown in the figure below is considered.
Assuming 20% security margin, the underreaching zone shall be set as follows.
At the Sending Bus, one must consider an external fault at F1 as the 5 Ω capacitor would contribute to the overreaching
effect. Any fault behind F1 is less severe as extra inductive line impedance increases the apparent impedance:
Reach Setting: 0.8 x (10 – 3 – 5) = 1.6 Ω if the line-side (B) VTs are used
Reach Setting: 0.8 x (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 pick-up, is overcompensated, i.e. capacitive.
At the Receiving Bus, one must consider a fault at F2:
Reach Setting: 0.8 x (10 – 4 – 2) = 3.2 Ω if the line-side (B) VTs are used
Reach Setting: 0.8 x (10 – 4 – 3 – 2) = 0.8 Ω if the bus-side (A) VTs are used
Practically, however, to cope with the effect of sub-synchronous oscillations, one may need to reduce the reach even more.
As the characteristics of sub-synchronous oscillations are in complex relations with fault and system parameters, no solid
setting recommendations are given with respect to extra security margin for sub-synchronous oscillations. It is strongly rec-
ommended to use a power system simulator to verify the reach settings or to use an adaptive L60 feature for dynamic
If the adaptive reach control feature is used, the
This setting is a sum of the overvoltage protection levels for all the series capacitors located between the relay potential
source and the far-end busbar, or location for which the zone must not overreach. The setting is entered in pu of the phase
VT nominal voltage (RMS, not peak value).
If a minimum fault current level (phase current) is causing a voltage drop across a given capacitor that prompts its air gap to
flash over or its MOV to carry practically all the current, then the series capacitor shall be excluded from the calculations
(the capacitor is immediately by-passed by its overvoltage protection system and does not cause any overreach problems).
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, shall be used
(RMS, not peak value).
Assuming none of the series capacitors in the sample system is guaranteed to get by-passed, the following calculations
For the Sending Bus:
For the Receiving Bus:
Figure 10–9: SAMPLE SERIES COMPENSATED SYSTEM
PHS DIST Z1 VOLT LEVEL
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
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
L60 Line Phase Comparison System
setting shall be set accordingly.
10 APPLICATION OF SETTINGS
10.4.1 DISTANCE SETTINGS