DISTANCE RELAY SETTINGS

DISTANCE RELAY SETTINGS Introduction Commonly used >33kV Determine whether impedance measured is within set characteristic. Non-unit protection (boundaries not defined) During normal operation impedance is combination of line, transformers and load ( >> line impedance) o Determine fault location by impedance measured o Measured impedance < line impedance fault o Example A 132kV Z = 30 ohms B LOAD 100MVA o Load Impedance = V 2 / P = 132000 2 /100x10 6 =174.2Ω o Relay Measures = 30+174 = 204 Ω o Equivalent Circuit A B 30 ohms 174.24 ohms Source o If have fault at B load impedance shorted out and relay measures 30ohm:

A B 30 ohms 174.24 ohms Source o Above is oversimplification (no errors, angle differences) Time Stepped Distance General: Relays have errors as do CTs and VTs allow 5 to 10%. Line impedance calculation errors allow 10%. o total errors 20%. To ensure whole line covered set to 120% but sees past end and could operate for fault on next line. o Solution 1: time delay A B C relay A Reach relay C reach D relay B reach A to grade with B which grades with C similar to overcurrent end up with slowest operation at the source. o Solution 2: set another zone which cant see past end o Set to 80% of line impedance, instantaneous Set to 80%, inst; to 120%, time delayed.

A B C Slowest clearance on any line is time. Zone 3 to provide backup to remote lines, time delayed to grade with. Time Stepped Distance Settings Major requirements: Fault detection: faulted plant is tripped Coordination: only faulted plant is tripped General comment: Impedance generally refers to PPS impedance (covers different fault combinations and earth return via measurement quantities see last week). Settings: So can be set instantaneous (no intentional delay), set as far as possible whilst ensuring doesn t see remote end o = 0.8xZ line...(1) Settings Set to ensure whole line length covered and time delayed to grade with next line protection. Time delay > Remote (say 50msec) + remote CB (say 80msec) + remote trip relay (say 10msec) + local Zone

2 timing errors (say 50msec) + safety margin (say 50 to 100msec). o 300msec is common. So can use 300msec, require no Z2 overlap A B C F Z2 AB Z AB + Z1 BC ignoring errors o 1.1xZ2AB 0.9(Z AB + Z1 BC ) including errors o Z2AB 0.8(Z AB + Z1 BC ) o Z2AB 0.8(Z AB + 0.8xZ BC ) as Z1 BC = 0.8xZBC o Z2AB 0.8(Z AB + 0.8xZ BC ) Also require whole line to be covered: Z2AB 1.2xZ AB o 1.2xZ AB 0.8(Z AB + 0.8xZ BC ) o ZBC 0.625xZAB o if remote line < 62.5% of protected line impedance no Z2 that satisfies both equations 2 and 3 o generally set as long as possible Zone 3 settings Time delay > Remote (say 50msec pickup + 300msec time delay) + remote CB (say 80msec) + remote trip relay (say 10msec) + local Zone 3 timing errors (say 50msec) + safety margin (say 50 to 100msec). o 600msec is common. Assuming wanting to coordinate Z3, and set 600msec, require no Z3 overlap: o Z3 AB Z AB + Z2 BC ignoring errors 1.1xZ3AB 0.9(Z AB + Z2 BC ) with errors

Z3 AB 0.8(Z AB + Z2 BC ) Z3AB 0.8(Z AB + 0.8(Z BC + 0.8xZ CD )) as Z2 AB 0.8(Z AB + 0.8xZ BC ) Z3AB 0.8xZ AB + 0.64xZ BC + 0.512xZCD Assuming wanting to backup line B-C: Z3AB 1.2(Z AB + Z BC ) o 1.2(ZAB + Z BC ) 0.8xZ AB + 0.64xZ BC + 0.512xZCD o 0.4xZAB + 0.56xZ BC 0.512xZCD o ZAB + 1.4xZ BC 1.28xZ CD o Only satisfy if CD much longer than AB or BC. Not generally the case difficult to achieve backup and coordination. If backup preferred Z3TD >> 600msec. Short Line Considerations What if Z BC < 0.625xZ AB ie cant set Z2 to cover line and not overlap next Z2 (on a short line) o Only a problem if short line protected by TSD. Three solutions o Time delay Z2 BC to grade with Z2 AB (ie set 600msec) Slow clearance for remote end faults o Set Z2 BC (300msec) so it doesn t see past Z1 AB and then set Z3 BC to protect 120% of line and to grade with Z2 AB (ie set 600msec) Only worthwhile if 120% reach only just overlaps Z1AB Slow clearance for remote end faults under worse case errors. o Use whole line high speed protection (eg differential, pilot, distance signalling, etc) Expensive as requires comms Short lines often protect by differential as distance doesn t achieve enough fault resistance coverage.

A B C D short line with high speed protection Need to ensure that the zone 2 of line AB does not see into the zone 2 of line CD. 1.1xZ2 AB 0.9(Z AB + Z BC + Z1 CD ) Z2 AB 0.8(Z AB + Z BC + Z1 CD ) This can be satisfied provided: 1.2xZ AB 0.8(Z AB + Z BC + 0.8xZ CD ) 0.4xZ AB 0.8(Z BC + 0.8xZ CD ) Z AB 2xZ BC + 1.6xZ CD )...(8) In an interconnected system this becomes easier due to throttling (infeeds from other lines cause adjacent lines to appear to have larger impedances). Affects of Power Transformers

Transformers > 33kV generally use high speed protection grade with Z2 OK. o Check Z2 and Z3 don t see through TX to plant protected by slow protections: Z2 Z Line + ZTX 1.1xZ2 0.9(ZLine + Z TX ) with errors Z2 0.8(Z Line + Z TX ) o What TX impedance to use due to tap position effecting impedance (nominal, worse case) o Also consider TXs run in parallel. o If no TX high speed protection or have to set Z2/3 so can see through, grade Z2 and Z3 with these slower protections. Considerations of Load Load appears as an impedance to a distance relay. Line angle 70-80 deg, load impedance +/- 40 deg (ie pf of 0.8). On heavily loaded, long lines load impedance will approach line impedance load encroachment. Worse case is minimum voltage conditions and maximum load on line (Z = V/I). X Load R

Throttling Change in relay measured impedance due to multiple remote infeeds to fault A ZL1 I1 I2 B ZL2 Fault With no infeed Zr = ZL1 + ZL2. With infeed at B: V A = I 1 Z L1 + (I 1 + I 2 )Z L2 V A Z L1 + Z L2 + I 2 Z L2 = I 1 I1 Potentially large increase in measured impedance; o need to allow for this if setting Z3 to give remote backup, ie extend Z3 but then need to be careful with grading if remote infeeds removed/reduced o Can assist with grading between lines (eg short lines look longer). o Need to be careful of parallel paths and effects of ends opening before others. o Can show we can set Z2 AB so doesn t see past Z1BC ZAB (1+ I 2 /I 1 )1.6xZ BC (compared with Z BC 0.625xZ AB or Z AB 1.6xZ BC previously) Settings for Teed Lines

B A C T Throttling at tee point reduces effectiveness of distance. set to 80% of shortest distance to remote ends ignore throttling here as one end could be open. o If external low impedance connection between remote ends may need to pull back Z1. has to protect whole line and thus set to 120% of largest distance to remote ends, plus consider throttling. o Eg Z2 at A = 120% x Z AT + Z tee max where Ztee max is the max of: (1 + ICB/I AB )Z TB For the impedance from T to B (1 + IBC/I AC )Z TC For the impedance from T to C where Ixy is the current from x to y o When checking coordination, assume one end open o Effected by changes to system. o If coordination not possible, use differential or distance signalling.

TUTORIAL WEEK 4 1) For the following power system determine, 2 and 3 reach and time delay settings. A G B C F H I D E Impedances, loads a) Generator 0.1pu, 1200A, 11kV b) Line A-G 0.3pu, 1000A c) Line B-C 0.4pu, 800A d) Line D-E 0.35pu, 800A e) Line F-H 0.2pu, 400A f) I has impedance of 0.5pu Assume I operates instantaneously. Allow 300msec safety margin between relays. 2) Check reaches determined in Question 1 do not result in any load encroachment problems. Assume all lines have a line angle of 70 degrees and relays are mho relays. 3) Consider options for short line F-H.