DISTANCE RELAY SETTINGS

Transcription

1 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 = /100x10 6 =174.2Ω o Relay Measures = = 204 Ω o Equivalent Circuit A B 30 ohms ohms Source o If have fault at B load impedance shorted out and relay measures 30ohm:

2 A B 30 ohms 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.

3 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

4 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

5 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 xZ BC xZCD Assuming wanting to backup line B-C: Z3AB 1.2(Z AB + Z BC ) o 1.2(ZAB + Z BC ) 0.8xZ AB xZ BC xZCD o 0.4xZAB xZ 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.

6 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

7 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 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

8 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

9 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.

10 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.