Line Protection( Basics )
Reach:
It is the impedance of the transmission line upto which the distance relay protects the line from fault.
·????????Measured Impedance – Fault Impedance
·????????Actual Impedance – Relay Settings Impedance
Overreach:
·????????Measured Impedance > Actual Impedance
·????????Effective reach of relay increases
·????????When a distance relay is said to overreach, the Impedance seen by the relay due to fault is less than the actual impedance even though the fault is outside the protected region.
Under reach:
·????????Measured Impedance < Actual Impedance
·????????Effective reach of relay decreases
·????????When a distance relay is said to under reach, the Impedance seen by the relay due to fault is more than the actual impedance even though the fault is inside the protected region.
Zone-1 = Under reaching Element
Zone-2 & 3 = Overreaching Element
Distance Protection:
Distance Protection is a stepped zone impedance based protection scheme. It has two directional zones. They are
·????????Forward Zone
o??Zone -1
o??Zone -2
o??Zone -3
·????????Reverse Zone
o??Zone -4
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Zone -1:
·????????Zone -1 is a underreaching element. (T1= 0 ms )
·????????Zone -1 covers 80% of the protected line impedance (or) length
·????????Constraints of 80% for Zone 1
o??CT/PT Error: CT & PT may have limitations in accuracy or during a fault it may undergo a partial or full saturation.
o??Infeed & Outfeed Effects
o??Inaccuracies in line impedance parameters.
·????????In order to avoid overreaching for a Z1 fault, Above constraints are considered.
Zone -2:
·????????It covers 100% of the protected line impedance / length and 20% of the Adjacent short line (or) 50% incase of a double circuit line. (T2= 0.2 - 0.4 s )
·????????It provides backup for the adjacent line/Remote End Busbar.
·????????It should not encroach the next lower voltage level.
·????????Zone -2 set to underreach due to mutual coupling effect incase of a parallel line.
·????????For a fault in parallel line, the healthy line has an adverse effect over it by mutual coupling.
·????????Due to mutual coupling effect, Fault impedance increases & relay starts to underreach. Because of it 150% is set in the parallel line.
·????????In a parallel line, if one circuit is kept out of service the other circuit relay settings need to be altered for proper operation.
Zone -3:
·????????It covers 100% of the protected line impedance/length (+) 100% of the Adjacent longest line (+) Safety Margin(20%) or 10Km of length is considered to clear remote end busbar fault. (T3= 0.6 - 0.8 s)
·????????It should encroach the next voltage level and Time must be coordinated.
Zone -4:
·????????It covers reverse 10% of the protected line impedance/length?incase of long line (or) 20% incase of a short line. (T4= 1.0 - 1.2 s)
·????????It covers close backup non directional fault & safeguards the local system.
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Senior Commissioning Engineer in TATA CONSULTING ENGINEERS LTD, PUNE
3 年nice
Power System, Protection and IEC 61850 Specialist
3 年The terms Underreach and Overreach can be used in three different references so I thought I'd put them all together here to add some clarity: https://ideology.atlassian.net/l/c/wVtUdFa7
Power System Protection Engineer at Schweitzer Engineering Laboratories (SEL)
3 年Apparent impedance is a complex number. A reach setting is also a complex number (reach and maximum torque angle). The two cannot be compared, mathematically speaking. The apparent impedance cannot be less than or greater than the reach impedance. Ask your math professor. A distance element has an operating characteristic. So the apparent impedance can be inside or outside that characteristic. But this is when fun starts. Most characteristics are polarized with a signal that does not fall into the 2D impedance plane. For example, a mho element can be polarized with positive sequence voltage including an element of memory. This means an operating characteristic plotted on the 2D R-X plane is just a simplification for the comfort of humans. An "underreaching application" aka Zone 1 is set short of the remote terminal. It may "overreach" if it operates for a fault beyond the reach point. It may overreach, period. Or it may overreach transiently (momentarily). Coordination of distance elements involves more than just reach settings, especially in blocking applications such as a DCB scheme. For more, refer to this primer: https://selinc.com/api/download/133569/?lang=en
Senior Power System Protection Engineer-Sr. Network Management Engineer
3 年A correction about underreach: A distance relay is said underreach if its measured impedance is greater than the actual impedance.
Power System, Protection and IEC 61850 Specialist
3 年first.. a correction about overreach This is when the measured impedance is LESS THAN the actual impedance such that the relay incorrectly operates thinking the impedance is less that a setting .. i.e. it thinks the impedance is "in zone" Zone-1 is NOT inherently an Under reaching Element Zone-2 & 3 are NOT inherently an Overreaching Element A zone has a certain reach. The effect of parallel lines, multiple sources, weak infeeds ... may cause ANY element to under reach (not operate when it should) or over reach (operate when it should not)