Series compensated lines - Distance protection

Why series compensation?

1. Increased power transfer capability
2. Better system stability
3. Reduced transmission system losses

Benefits

1.    Optimization of power capabilities

2.    Power sharing

3.    Technical & environmental benefits

Criteria

1.    Strong transmission links

2.    Maximum power transfer within the steady state stability limits

Possible measures

1.    Increase no of parallel feeders

2.    Increase transmission voltage

3.    Decrease the transfer impedance using series compensation

• Spark gap – to bypass C during high current faults
• Circuit breaker – to close during high current faults and also to discharge TRV (Transient Recovery Voltage)
• To control the voltage across the series capacitor

 Problems:

1. Voltage inversion
2. Current inversion
3. Distance estimation

Current Inversion

·         I_R inductive as impedance to fault has X_L which is inductive

·         I_S capacitive as impedance to fault has X_C which is capacitive

·         For an internal fault, I_R  & I_S are in opposite directions

·         Probability of non- operation of relay for a high impedance fault

·         Current inversion affects directional, distance, phase comparison scheme and differential protection

Voltage Inversion

·         If XC > m XL, voltages V & V’ are out of phase

·         Line side VTs detect a forward fault correctly

·         Bus side VTs detect a reverse fault correctly

Distance Estimation

·         Errors in distance estimates

·         Correct estimation when capacitor out of service – High current fault

·         Impedance reduces because of C ( ZL – j XC )

·         The set distance characteristic over reaches for a high impedance fault

·         Mho element reach can be reduced to 90% when C is in service

·         Reach reduces to 50% when C is out of service

·         For a high impedance fault close to the capacitor, relay of line section AB sees it as a reverse fault

·         Relay of adjacent line section may mal operate depending on the location of the fault

·         Hence, series capacitor can cause nuisance tripping

·         Memory polarization for voltage inversion à polarizing memory should be long enough

·         Blocking of zone-1 element for high impedance faults to prevent over-reach when capacitor is in circuit              

·         Reduce the reach of zone-1 when capacitor is in circuit.

·         Using line differential protection à excellent choice for a series compensated line