Fundamentals Of Non-Directional Overcurrent & Earth Fault ProtectionOvercurrent Protection: Purpose of Protection X Detect abnormal conditions X Isolate faulty part of the system X Speed Fast operation to minimise damage and danger X Discrimination Isolate only the faulty section X Dependability / reliability X Security / stability X Cost of protection / against cost of potential hazards Overcurrent Protection Co-ordination F1 F2 F3 X Co-ordinate protection so that relay nearest to fault operates first X Minimise system disruption due to the fault Fuses Overcurrent Protection Fuses X Simple X Can provide very fast fault clearance <10ms for large current X Limit fault energy Arcing Time Pre Arc Time Prospective Fault Current Total Operating Time t Overcurrent Protection Fuses .disadvantages X Problematic co-ordination Fuse A Fuse B IFA approx 2 x IFB X Limited sensitivity to earth faults X Single phasing X Fixed characteristic X Need replacing following fault clearance . Tripping Methods . Overcurrent Protection Direct Acting AC Trip 51 Trip Coil IF X AC series trip common for electromechanical O/C relays . Overcurrent Protection Direct Acting AC Trip IF ' + 51 - Sensitive Trip Coil IF X Capacitor discharge trip used with static relays where no secure DC supply is available . Overcurrent Protection DC Shunt Trip IF IF ' 51 DC BATTERY SHUNT TRIP COIL X Requires secure DC auxiliary No trip if DC fails . Overcurrent Protection . Overcurrent Protection Principles X Operating Speed Instantaneous Time delayed X Discrimination Current setting Time setting Current and time X Cost Generally cheapest form of protection relay . Overcurrent Protection Instantaneous Relays B A 50 IF2 50 IF1 X Current settings chosen so that relay closest to fault operates X Problem Relies on there being a difference in fault level between the two relay locations Cannot discriminate if IF1 = IF2 . Overcurrent Protection Definite (Independent) Time Relays TIME TOP IS (Relay Current Setting) Applied Current . Overcurrent Protection Definite (Independent) Time Relays 51 0.5 sec X Operating time is independent of current X Relay closest to fault has shortest operating time X Problem Longest operating time is at the source where fault level is highest .9 sec 51 0. Overcurrent Protection IDMT TIME IS (Relay Current Setting) Applied Current X Inverse Definite Minimum Time characteristic . Overcurrent Protection Disc Type O/C Relays X Current setting via plug bridge X Time multiplier setting via disc movement X Single characteristic X Consider 2 ph & EF or 3 ph plus additional EF relay . Overcurrent Protection Static Relay X Electronic. wide range X Integral instantaneous elements . multi characteristic X Fine settings. Overcurrent Protection Numerical Relay I>1 I>2 Time I>3 I>4 Current X Multiple characteristics and stages X Current settings in primary or secondary values X Additional protection elements . Co-ordination . Overcurrent Protection Co-ordination Principle R1 R2 IF1 T IS2 IS1 Maximum Fault Level I X Relay closest to fault must operate first X Other relays must have adequate additional operating time to prevent them operating X Current setting chosen to allow FLC X Consider worst case conditions. operating modes and current flows . 01 Current (A) FLB FLC FLD .1 0.Overcurrent Protection Co-ordination Example E D C B A 10 Operating time (s) E 1 D C B 0. 02 -1) X VI t = 13.Overcurrent Protection IEC Characteristics 1000 X SI 0.1 1 10 100 Current (Multiples of Is) .14 (I0.5 (I2 -1) X EI t = 80 (I2 -1) X LTI t = 120 (I .1) t = 100 Operating Time (s) 10 LTI SI 1 VI EI 0. g at 10x setting (or PSM of 10) SI curve op time is 3s 1000 Operating Time (s) 100 10 1 0.Overcurrent Protection Operating Time Setting Terms Used X Relay operating times can be calculated using relay characteristic charts X Published characteristcs are drawn against a multiple of current setting or Plug Setting Multiplier X Therefore characteristics can be used for any application regardless of actual relay current setting X e.1 1 100 10 Current (Multiples of Is) . Overcurrent Protection Current Setting X Set just above full load current allow 10% tolerance X Allow relay to reset if fault is cleared by downstream device consider pickup/drop off ratio (reset ratio) relay must fully reset with full load current flowing zPU/DO for static/numerical = 95% zPU/DO for EM relay = 90% X e.1 x IFL/0.95 . Is = 1.g for numerical relay. 1 x IsR2 .g. IsR1 = 1.Overcurrent Protection Current Setting X Current grading ensure that if upstream relay has started downstream relay has also started R1 R2 IF1 Set upstream device current setting greater than downstream relay e. Overcurrent Protection Grading Margin X Operating time difference between two devices to ensure that downstream device will clear fault before upstream device trips X Must include breaker opening time allowance for errors relay overshoot time safety margin GRADING MARGIN . 4s .15 0.05 0. For errors safety margin Total X Calculate using formula 0.1 0.Overcurrent Protection Grading Margin between relays R1 R2 X Traditional breaker op time relay overshoot allow.1 0. 5s 0.375s margin for EM relay. oil CB 0.24s margin for static relay.Overcurrent Protection Grading Margin between relays X Formula t’ = (2Er + Ect) t/100 + tcb + to + ts z Er = relay timing error z Ect = CT measurement error z t = op time of downstream relay z tcb = CB interupting time z to = relay overshoot time z ts = safety margin X Op time of Downstream Relay t = 0. vacuum CB . Overcurrent Protection Grading Margin relay with fuse X Grading Margin = 0.15s over whole characteristic X Assume fuse minimum operating time = 0.4Tf + 0.01s X Use EI or VI curve to grade with fuse X Current setting of relay should be 3-4 x rating of fuse to ensure co-ordination . 33s .175Tr + 0.6Tf Allowance for fuse error (fast) Allowance for CT and relay error or X Tf = 2Tr + 0.1 Safety margin = 0.Overcurrent Protection Grading Margin relay with upstream fuse Tf Tr I FMAX X 1.1 + CB 0. Overcurrent Protection Time Multiplier Setting 100 X Used to adjust the operating time of an inverse characteristic X Not a time setting but a multiplier X Calculate TMS to give desired operating time in accordance with the grading margin Operating Time (s) 10 1 0.1 1 100 10 Current (Multiples of Is) . Overcurrent Protection Time Multiplier Setting . T1 X TMS = Treq /T1 . Treq consider grading margin fault level X Calculate op time of inverse characteristic with TMS = 1.Calculation X Calculate relay operating time required. Overcurrent Protection Co-ordination . check grading over whole curve Grading curves should be drawn to a common voltage base to aid comparison .Procedure X Calculate required operating current X Calculate required grading margin X Calculate required operating time X Select characteristic X Calculate required TMS X Draw characteristic. Overcurrent Protection Co-ordination Example 200/5 100/5 I FMAX = 1400 Amp B Is = 5 Amp A Is = 5 Amp.05. SI X Grade relay B with relay A X Co-ordinate at max fault level seen by both relays = 1400A X Assume grading margin of 0.4s . TMS = 0. 02 -1) (140. TMS = 0.05.13 + 0. SI X Relay B is set to 200A primary.13 + grading margin = 0.4 = 0. 5A secondary X Relay A set to 100A ∴ If (1400A) = PSM of 14 relay A OP time = t = 0.14 x TMS = 0.14 x 0.05 = 0.02 -1) X Relay B Op time = 0.13 (I0.Overcurrent Protection Co-ordination Example 200/5 100/5 I FMAX = 1400 Amp B Is = 5 Amp A Is = 5 Amp.53s X Relay A uses SI curve so relay B should also use SI curve . 13 + grading margin = 0.Overcurrent Protection Co-ordination Example 200/5 100/5 I FMAX = 1400 Amp B Is = 5 Amp A Is = 5 Amp. TMS = 0. SI .14 x TMS = 0.53s X Relay A uses SI curve so relay B should also use SI curve X Relay B set to 200A ∴ If (1400A) = PSM of 7 relay B OP time TMS = 1 = 0.14 = 3.13 + 0.02 -1) (70.15.15 Op time TMS=1 3. SI X Relay B Op time = 0. TMS = 0.02 -1) X Required TMS = Required Op time = 0.4 = 0.05.52 X Set relay B to 200A.53 = 0.52s (I0. 1% 4 350MVA CTZ61 3 ACB ACB CTZ61 2 1 3 (Open) MCCB 27MVA 1 2 3 4 F Relay 1 Relay 2 Relay 3 Relay 4 Fuse Fuse Load F K 20MVA .Overcurrent Protection LV Protection Co-ordination 11kV MCGG 4 CB 2 x 1.5MVA 11kV/433V 5. 01S 0. 1kA 10kA 1000kA Very inverse TX damage .Overcurrent Protection LV Protection Co-ordination 1000S 100S 10S 1.0S 0.1S 0. 5MVA 11kV/433V 5.Overcurrent Protection LV Protection Co-ordination 11kV KCGG 142 4 CB 2 x 1.1% 4 350MVA KCEG 142 3 ACB 2 3 (Open) ACB 1 1 2 3 4 F MCCB 27MVA Relay 1 Relay 2 Relay 3 Relay 4 Fuse Fuse Load F K 20MVA . 01S 0.0S 0.Overcurrent Protection LV Protection Co-ordination 1000S Long time inverse 100S 10S 1.1S 0. 1kA TX damage 10kA 1000kA . Overcurrent Protection Blocked OC Schemes Graded protection R3 R2 IF2 Block t > I > Start Blocked protection R1 IF1 M (Transient backfeed ?) . Delta/Star Transformers . Overcurrent Protection Transformer Protection .866 If3∅ .2-1-1 Fault Current Turns Ratio = √3 :1 X A phase-phase fault on one side of transformer produces 2-1-1 distribution on other side X Use an overcurrent element in each phase (cover the 2x phase) X 2∅ & EF relays can be used provided fault current > 4x setting Iline Idelta 0. 866 E∅-n/Xt X Istar = 0.2-1-1 Fault Current Turns Ratio = √3 :1 X Istar = E∅-∅/2Xt = √3 E∅-n/2Xt X Istar = 0.866 If3∅ X Idelta = Istar/√3 = If3∅ /2 X Iline = If3∅ .866 If3∅ Iline Idelta 0.Overcurrent Protection Transformer Protection . 2-1-1 Fault Current 51 HV 51 LV Ø/Ø X Grade HV relay with respect to 21-1 for ∅-∅ fault X Not only at max fault level 86.Overcurrent Protection Transformer Protection .6%If3∅ If3∅ . Use of High Sets . If >> Is (5 x ?) X Current setting must be co-ordinated to prevent overtripping X Used to provide fast tripping on HV side of transformers X Used on feeders with Auto Reclose.Overcurrent Protection Instantaneous Protection X Fast clearance of faults ensure good operation factor. prevents transient faults becoming permanent AR ensures healthy feeders are re-energised X Consider operation due to DC offset .transient overreach . 3IF(LV) .Overcurrent Protection Instantaneous OC on Transformer Feeders HV2 HV1 LV HV2 TIME HV1 LV IF(LV) IF(HV) CURRENT X Set HV inst 130% IfLV X Stable for inrush X No operation for LV fault X Fast operation for HV fault X Reduces op times required of upstream relays 1. Earthfault Protection . Overcurrent Protection Earth Fault Protection X Earth fault current may be limited X Sensitivity and speed requirements may not be met by overcurrent relays Use dedicated EF protection relays X Connect to measure residual (zero sequence) current Can be set to values less than full load current X Co-ordinate as for OC elements May not be possible to provide co-ordination with fuses . Overcurrent Protection Earth Fault Relay Connection .3 Wire System E/F OC OC OC E/F OC OC X Combined with OC relays X Economise using 2x OC relays . Overcurrent Protection Earth Fault Relay Connection .4 Wire System E/F OC OC OC E/F OC OC OC X EF relay setting must be greater than normal neutral current X Independent of neutral current but must use 3 OC relays for phase to neutral faults . t earth fault level special considerations for impedance earthing .Overcurrent Protection Earth Fault Relays Current Setting X Solid earth 30% Ifull load adequate X Resistance earth setting w.directional? .r. 2% possible E/F X Isolated/high impedance earth networks X For low settings cannot use residual connection.Overcurrent Protection Sensitive Earth Fault Relays A B C X Settings down to 0. use dedicated CT X Advisable to use core balance CT X CT ratio related to earth fault current not line current X Relays tuned to system frequency to reject 3rd harmonic . Overcurrent Protection Core Balance CT Connections NO OPERATION OPERATION X Need to take care with core balance CT and armoured cables X Sheath acts as earth return path X Must account for earth current path in connections .insulate cable gland CABLE GLAND CABLE BOX E/F CABLE GLAND/SHEATH EARTH CONNECTION .
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