9-6 D30 Line Distance Protection System GE Multilin9.3 SERIES COMPENSATED LINES 9 APPLICATION OF SETTINGS99.3SERIES COMPENSATED LINES 9.3.1 INTRODUCTIONFor reasons described in Chapter 8: Theory of Operation, it is recommended to apply a combination of distance, grounddirectional overcurrent and high-set overcurrent functions for protection of series compensated lines.The setting rules described below must take into account variety of system configurations, particularly a status of seriescapacitors (in-service, by-passed). Either the worst-case topology shall be considered or - if possible - adaptive settingsshall be applied though the multiple settings groups mechanism.A line compensating capacitor is a bank of three physical capacitors and their overvoltage protecting devices (air gaps and/or MOVs). If none of the MOV/gaps conducts any significant current, the positive-, negative- and zero-sequence reactanceof the three-phase bank equal the reactance of the actual (phase) capacitors. Under asymmetrical conditions, however,such as a single line to ground fault, when only one MOV/gap may operate, the series capacitor bank would create extra(series) asymmetry in addition to the fault (shunt) asymmetry. The positive-, negative- and zero-sequence impedances willdiffer from each other and will not equal the impedance of the phase capacitors. Moreover, there may be mutual couplingbetween the sequence networks representing the series capacitor bank. This makes analytical analysis of fault conditionsvery burdensome. For setting calculations, however, it is justified to assume the zero-, positive-, and negative-sequencereactance of the capacitor bank equal the reactance of the actual (phase) capacitors. This represents a worst-case low-cur-rent fault scenario, when the steady-state effects of series compensation are most weighty.9.3.2 DISTANCETraditionally, the reach setting of an underreaching distance function shall be set based on the net inductive impedancebetween the potential source of the relay and the far-end busbar, or location for which the zone must not overreach. Faultsbehind series capacitors on the protected and adjacent lines need to be considered for this purpose. For further illustrationa sample system shown in the figure below is considered.Figure 9–2: SAMPLE SERIES COMPENSATED SYSTEMAssuming 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 overreachingeffect. 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 usedReach Setting: 0.8 x (10 – 4 – 3 – 5) = –1.6 Ω if the bus-side (A) VTs are usedThe negative value means that an underreaching zone cannot be used as the circuit between the potential source of therelay 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 usedReach Setting: 0.8 x (10 – 4 – 3 – 2) = 0.8 Ω if the bus-side (A) VTs are usedPractically, 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 solidsetting 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 D30 feature for dynamicreach control.If the adaptive reach control feature is used, the PHS DIST Z1 VOLT LEVEL setting shall be set accordingly.10 Ω-4 Ω -3 Ω -5 Ω7 Ω-2 Ω3 ΩSENDINGBUSRECEIVINGBUSProtected LineA B B AINFINITEBUSINFINITEBUSF1F20.5 pu 0.6 pu 0.5 pu 0.7 pureactancevoltageprotectionlevel
