FUNDAMENTAL OF NON DIRECTIONAL OVERCURRENT.pdf

April 3, 2018 | Author: Akinbode Sunday Oluwagbenga | Category: Relay, Fuse (Electrical), Physical Quantities, Electricity, Electrical Engineering


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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 Š Fast operation to minimise damage and danger X Discrimination Š 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 Š <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 Š 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 Š common for electromechanical O/C relays . Overcurrent Protection Direct Acting AC Trip IF ' + 51 - Sensitive Trip Coil IF X Capacitor discharge trip Š 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 Š No trip if DC fails . Overcurrent Protection . Overcurrent Protection Principles X Operating Speed Š Instantaneous Š Time delayed X Discrimination Š Current setting Š Time setting Š Current and time X Cost Š 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 Š Relies on there being a difference in fault level between the two relay locations Š 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 Š 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 Š allow 10% tolerance X Allow relay to reset if fault is cleared by downstream device Š consider pickup/drop off ratio (reset ratio) Š 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 Š ensure that if upstream relay has started downstream relay has also started R1 R2 IF1 Š 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 Š breaker opening time Š allowance for errors Š relay overshoot time Š safety margin GRADING MARGIN . 4s .15 0.05 0. For errors Š safety margin Š Total X Calculate using formula 0.1 0.Overcurrent Protection Grading Margin between relays R1 R2 X Traditional Š breaker op time Š relay overshoot Š allow.1 0. 5s Š 0.375s margin for EM relay. oil CB Š 0.24s margin for static relay.Overcurrent Protection Grading Margin between relays X Formula Š 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 Š consider grading margin Š 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 Š ensure good operation factor. prevents transient faults becoming permanent Š 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 Š Use dedicated EF protection relays X Connect to measure residual (zero sequence) current Š Can be set to values less than full load current X Co-ordinate as for OC elements Š 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 Š special considerations for impedance earthing .Overcurrent Protection Earth Fault Relays Current Setting X Solid earth Š 30% Ifull load adequate X Resistance earth Š 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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