Generally zones Z1, Z2, Z3 are taken as forward direction and Z4 is taken as reverse direction with time settings as T1, T2, T3 and T4 respectively.
Settings adopted depends on their respective Regional Power Committees and CBIP recommendation or guidelines.
Transmission Line Zone Protection Setting
Stepped distance relay scheme is followed.
3 Forward Zones + 1 Reverse Zone is kept
Zone 1 Protection
Set to cover 80% of the protected line. There is no intentional time delay for this zone.
Why 80% ?
The under-reach setting is set to avoid overreaching into the next protected line & to avoid unwanted tripping.
Reasons for inaccuracy of distance relay reach
Inaccuracy in CT & PT ratios.
Line parameter errors.
Variation of line parameters according to atmospheric conditions.
It should isolate the fault instantaneously.
Zone 2 Protection
Set to cover (120% of the protected line) Or (Protected line impedance + 50% of adjacent shortest line impedance) or (Protected line + 50% transformer impedance).
150% of the protected line (for Double circuit )
Time delay- 300-500 milli seconds.
Provides Backup protection for a part of the adjacent line.
Should be set in such a way that Zone-2 setting should not encroach into LV side of the transformer as this leads to unwanted tripping of HV side for a fault in LV side.
Trip the faulty line instantaneously using carrier aided tripping (POTT).
Consideration for time delay
Downstream Co-ordination (100ms)
Circuit Breaker Operation(100ms)
Safety margin(100ms)
Zone 3 protection
Set to cover (120% of the protected line + adjacent longest line) or (Protected line impedance + 100% of adjacent longest line impedance + 25% of second adjacent longest line) or (Protected line + 100% transformer impedance).
Time delay- 800-1200milli seconds .
Provides Backup protection for a the next adjacent line.
Should be set in such a way that Zone-3 setting should not encroach into LV side of the transformer as this leads to unwanted tripping of HV side for a fault in LV side.
Zone 4 protection
Reverse Zone
Provides backup protection for station faults. (Acts as Busbar zone backup)
Time delay is normally 1-1.3sec if Bus bar protection available else 450-500ms.
Conversion of primary values into secondary values in relay
Distance relays are fed from the secondary’s of line CT’s & Bus PT’s /Line CVT’s.
The line parameters are to be converted into secondary values Zsec = Zpri / (Impedance ratio) Where Impedance ratio = PT ratio / CT ratio.
The CT and PT ratios are inter related. Hence any changes in CT ratio has to be effected along with revision of relay settings only.
