![Slide 46
Issue A
Restricted Earthfault Protection
(1) Setting voltage
VS = IF (RCT + 2RL)
Assuming “earth” CT saturates,
RCT = 4.8 ohms
2RL = 2 x 100 x 7.41 x 10-3 = 1.482 ohms
∴ Setting voltage = 14.9 (4.8 + 1.482)
= 93.6 Volts
(2) Stabilising Resistor (RS)
RS = {Vs - [VA/(Is^2)]} /Is
Where IS = relay current setting
∴ RS = {93.6 - [1/0.1^2]}/0.1 = 836 ohms](https://image.slidesharecdn.com/05-transformer-241204075955-8e35585e/85/05-Transformer-protection-system-and-pdf-41-320.jpg)
05-Transformer protection system and.pdf
- 2. Slide 2 Issue A Causes of failure: Environment System Mal operation Wrong design Manufacture Material Maintenance
- 3. Slide 3 Transformer failures classification : 1. Internal failure Causes: Winding & terminal faults Core faults Onload tap changer faults Overheating faults Issue A
- 4. 2. External failure Causes: Issue A Slide 4 Abnormal operating condition sustained or unclear faults Transformer failures classification :
- 5. Issue A Slide 5 Vector Groups Phase displacement 0 Group 1 Phase displacement 180 Group 2 Lag phase displacement 30 Group 3 Lead phase displacement 30 Group 4 Yy0 Dd0 Zd0 Yy6 Dd6 Dz6 Yd1 Dy1 Yz1 Yd11 Dy11 Yz11
- 6. Slide 10 Issue A Fault current distribution Earth fault on Transformer winding V2 R T2 T1 V 1 X Fig.3 If Fig.N
- 7. Fault current distribution Slide 11 Issue A Therefore C.T.secondary current ( on primary side of transformer) =, X2 If differential setting =20% For relay operation X2 20% Thus X > 59% ie. 59% of winding is unprotected. Differential relay setting % of winding protected 10% 58% 20% 41% 30% 28% 40% 17% 50%. 7% √3 √3 >
- 8. Slide 13 Issue A Differential Basic Protection Restricted Earthfault Overfluxing Overcurrent & Earthfault
- 9. Slide 14 Issue A Differential Protection Where protection co-ordination is difficult / not possible using time delayed elements For fast fault clearance Applied ∗ Works on Merz-price current comparison principle ∗ Relays with bias characteristic should only be used For zone of protection
- 10. Slide 15 Issue A Differential Protection Consideration for applying differential protection Phase correction Filtering of zero sequence currents Ratio correction Magnetizing inrush during energisation Overfluxing
- 11. Slide 16 Issue A Differential Protection - Principle R I diff = 0 • Nominal current through the protected equipment I Diff = 0 : No tripping
- 12. Slide 17 Issue A Differential Protection - Principle • Through fault current I Diff = 0 : No tripping R I diff = 0
- 13. Slide 18 Issue A Differential Protection - Principle Tripping • Internal Fault I Diff = 0 : R I diff = 0
- 14. Slide 19 Issue A Biased differential protection • Fast operation • Adjustable characteristic • High through fault stability • CT ratio compensation • Magnetising inrush restraint • Overfluxing 5th harmonic restraint
- 15. Slide 20 Issue A Biased differential protection 1 A 100/50 KV 100 / 1 200 / 1 1 A 0 A LOAD = 200 A Why bias characteristic ? OLTC Setting is at mid tap R I1 I2
- 16. Slide 21 Issue A Biased differential protection 100/50 KV 100 / 1 200 / 1 0.9 A 1 A 0.1 A Relay pickup setting = O.2 A, So the Relay restrains LOAD = 200 A OLTC SETTING IS AT 10% Differential current = 0.1 A R
- 17. Slide 22 Issue A Biased differential protection 100/50 KV 100 / 1 200 / 1 9 A 10 A 1 A Relay Pickup Setting is O.2 A OLTC SETTING IS AT 10% 2000 A R Operates So the Relay
- 18. Slide 23 Issue A Role of Bias Setting range (0.1 - 0.5) Effective bias (x In) = I + I + I + I 1 2 3 4 2 Differential current (x In) = I + I + I + I 1 2 3 4 0 1 2 3 4 1 2 3 Operate Restrain 80% Slope 20% Slope
- 19. Slide 24 Issue A Based on Current operated relay with an external stabilising resistor • Requires matched current transformers of low reactance design, typically class X or equivalent • Equal CT ratios • Non-linear resistor may be required to limit voltage across relay circuit during internal faults • Suitable for zones up to 200 - 300 metres (typically) High Impedance Principle
- 20. Slide 25 Issue A High Impedance Principle TC saturé M RCT ZM RCT 2RL 2RL A M ZM RCT 2RL 2RL RCT
- 21. Slide 26 Issue A High Impedance Principle RCT ZM RCT ZM 2RL 2RL A M M
- 22. Slide 27 Issue A High Impedance Principle RCT ZM RCT ZM 2RL 2RL TC saturé A M M
- 23. Slide 28 Issue A High Impedance Principle RCT ZM RCT ZM 2RL 2RL A M M
- 24. Slide 29 Issue A High Impedance Principle RCT ZM RCT ZM 2RL 2RL TC saturé A M M
- 25. Slide 30 Issue A High Impedance Principle RCT ZM RCT ZM 2RL 2RL A M M
- 26. Slide 31 Issue A High Impedance Principle RCT ZM RCT ZM 2RL 2RL TC saturé A M M
- 27. Slide 32 Issue A High Impedance Principle RCT ZM RCT ZM=0 2RL 2RL TC saturé RCT 2RL 2RL RCT A M M CT Saturation False tripping
- 28. Slide 33 Issue A High Impedance Principle RCT ZM RCT ZM=0 2RL 2RL TC saturé RCT 2RL 2RL RCT A RS M M
- 29. Slide 34 Issue A High Impedance Principle RCT ZM RCT ZM=0 2RL 2RL TC saturé RCT 2RL 2RL RCT A RS M M Stabilising resistor
- 30. Slide 35 Issue A RCT ZM RCT ZM 2RL 2RL A RS M M RCT 2RL 2RL RCT Vset High Impedance Principle
- 31. Slide 36 Issue A RCT ZM RCT ZM=0 2RL 2RL RCT 2RL 2RL RCT A RS M M ZM = 0 (CT "short circuited" ) Vset High Impedance Principle
- 32. Slide 37 Issue A A RCT ZM RCT ZM 2RL 2RL 2RL RCT 2RL RCT RS M M Vset High Impedance Principle
- 33. Slide 38 Issue A High Impedance Principle 2RL RCT 2RL RCT M Vset A RCT ZM RCT ZM 2RL 2RL RS M
- 34. Slide 39 Issue A High Impedance Principle M A RC T ZM RC T ZM 2R L 2R L RS M Vset Metrosil may be required for voltage limitation 2R L RC T 2R L RC T M
- 35. Slide 40 Issue A Restricted Earthfault Protection Increased sensitivity for earth faults REF elements for each transformer winding CTs may be shared with differential element Uses high impedance principle 64 64 64
- 36. Slide 41 Issue A Restricted Earthfault Protection P1 P2 S1 S2 P1 S1 S2 P1 S1 P2 S2 P1 P2 S1 S2 Stability level : usually maximum through fault level of transformer P2 REF Case I : Normal Condition Under normal conditions no current flows thro’ Relay So, No Operation
- 37. Slide 42 Issue A Restricted Earthfault Protection REF Case II : External Earth Fault External earth fault - Current circulates between the phase & neutral CTs; no current thro’ the relay So, No Operation
- 38. Issue A Slide 43 Issue A Restricted Earthfault Protection REF Case III : Internal Earth Fault For an internal earth fault the unbalanced current flows thro’ the relay So, Relay Operates
- 39. Slide 44 Issue A Restricted Earthfault Protection Restricted Earth Fault Protection Setting 1MVA (5%) 11000V 415V 1600/1 RCT = 4.9Ω 80MVA RS 1600/1 RCT = 4.8Ω MCAG14 IS = 0.1 Amp 2 Core 7/0.67mm (7.41Ω/km) 100m Long Setting will require calculation of : 1) Setting stability voltage (VS) 2) Value of stabilising resistor required 3) Peak voltage developed by CT’s for internal fault
- 40. Slide 45 Issue A Restricted Earthfault Protection Example : Earth fault calculation :- Using 80MVA base Source impedance = 1 p.u. Transformer impedance = 0.05 x 80 = 4 p.u. 1 Total impedance = 14 p.u. ∴ I1 = 1 = 0.0714 p.u. 14 Base current = 80 x 106 √3 x 415 = 111296 Amps ∴ IF = 3 x 0.0714 x 111296 = 23840 Amps (primary) = 14.9 Amps (secondary) 1 P.U. 1 4 I1 4 I2 4 I0 1 1
- 41. Slide 46 Issue A Restricted Earthfault Protection (1) Setting voltage VS = IF (RCT + 2RL) Assuming “earth” CT saturates, RCT = 4.8 ohms 2RL = 2 x 100 x 7.41 x 10-3 = 1.482 ohms ∴ Setting voltage = 14.9 (4.8 + 1.482) = 93.6 Volts (2) Stabilising Resistor (RS) RS = {Vs - [VA/(Is^2)]} /Is Where IS = relay current setting ∴ RS = {93.6 - [1/0.1^2]}/0.1 = 836 ohms
- 42. Slide 47 Issue A Restricted Earthfault Protection 3) Peak voltage = 2√2 √VK (VF - VK) VF = 14.9 x VS = 14.9 x 936 = 13946 Volts IS For ‘Earth’ CT, VK = 1.4 x 236 = 330 Volts (from graph) ∴ VPEAK = 2√2 √330 (13946 - 330) = 6kV Thus, metrosil voltage limiter will be required.
- 43. Slide 48 Issue A Magnetising Inrush • Transient condition - occurs when a transformer is energised • Normal operating flux of a transformer is close to saturation level • Residual flux can increase the mag-current • In the case of three phase transformer, the point-on-wave at switch-on differs for each phase and hence, also the inrush currents
- 44. Slide 49 Issue A Transformer Magnetising Characteristic Twice Normal Flux Normal Flux Normal No Load Current No Load Current at Twice Normal Flux Magnetising Inrush
- 45. Slide 50 Issue A Magnetising Inrush m Φ + SWITCH ON AT VOLTAGE ZERO - NO RESIDUAL FLUX m Φ - m Φ 2 STEADY STATE V Φ m I m I V Φ Inrush Current
- 46. Slide 51 Issue A Magnetising Inrush
- 47. Slide 52 Issue A Magnetising Inrush • Appears on one side of transformer only - Seen as fault by differential relay - Transient magnetising inrush could cause relay to operate • Makes CT transient saturation - Can make mal-operation of Zero sequence relay at primary Effect of magnetising current
- 48. Slide 53 Issue A P1 S1 P2 S2 P1 S1 P2 S2 P1 S1 P2 S2 IR IS IT IR + IS + IT = 3Io = 0 Magnetising Inrush
- 49. Slide 54 Issue A Effect of magnetising current Example of disurbance records with detail Magnetising Inrush
- 50. Slide 55 Issue A 2nd (and 5th) harmonic restraint • Makes relay immune to magnetising inrush • Slow operation may result for genuine transformer faults if CT saturation occurs Magnetising Inrush Restrain
- 51. Slide 57 Issue A Overfluxing - Basic Theory Low frequency High voltage Geomagnetic disturbances Causes Overfluxing = V/F
- 52. Slide 58 Issue A Overfluxing - Basic Theory Transient Overfluxing - Tripping of differential element Prolonged Overfluxing - Damage to transformers Effects m Φ 2 Ie m Φ V = kfΦ
- 53. Slide 59 Issue A Overfluxing - Condition Differential element should be blocked for transient overfluxing-+ 25% OVERVOLTAGE CONDITION 43% 5TH HARMONIC CONTENT Overfluxing waveform contains very high 5th Harmonic content
- 54. Slide 60 Issue A Φ V α K f • Trip and alarm outputs for clearing prolonged overfluxing • Alarm : Definite time characteristic to initiate corrective action • Trip : IT or DT characteristic to clear overfluxing condition Overfluxing - Protection
- 55. Slide 60 Issue A Oil conservator Bucholz Relay BUCCHOLZ PROTECTION
- 56. Slide 60 Issue A Buchholz Relay Installation 5 x internal pipe diameter (minimum) 3 x internal pipe diameter (minimum) Transformer 76 mm typical To oil conservator BUCCHOLZ PROTECTION
- 57. Slide 60 Issue A Buchholz Relay Petcock From transformer Deflector plate Trip bucket To oil conservato r Mercury switch Alarm bucket BUCCHOLZ PROTECTION
- 58. Slide 60 Issue A Accumulation of gaz Oil Leakage Severe winding faults Buccholz Protection Application BUCCHOLZ PROTECTION
- 59. Slide 60 Issue A Interturn faults Winding faults to earth with low power (fault close to neutral for example) Accumulation of Gaz BUCCHOLZ PROTECTION
- 60. Slide 60 Issue A Inter-Turn Fault Nominal turns ratio Fault turns ratio Current ratio : 11,000 / 240 : 11,000 / 1 : 1 / 11,000 Shorted turn Load Primary Secondary CT E BUCCHOLZ PROTECTION
- 61. Slide 60 Issue A Nominal turns ratio Fault turns ratio Current ratio : 11,000 / 240 : 11,000 / 1 : 1 / 11,000 CT E Shorted turn Primary Secondary Inter-Turn Fault BUCCHOLZ PROTECTION
- 62. Slide 60 Issue A Interturn Fault Current / Number of Turns Short Circuited 5 10 15 20 25 Turn short- circuited (percentage of winding) Primary current (multiples of rated current) Fault current (multiples of rated current) 100 80 60 40 20 BUCCHOLZ PROTECTION
- 63. Slide 60 Issue A Interturn Fault Current / Number of Turns Short Circuited 5 10 15 20 25 Primary current (multiples of rated current) Fault current (multiples of rated current) 100 80 60 40 20 Fault current very high Primary phase current very low Detected by Bucholz relay Not detected by current operated relays BUCCHOLZ PROTECTION
- 64. Slide 60 Issue A Interturn faults Winding faults to earth with low power (fault close to neutral for example) Accumulation of Gaz BUCCHOLZ PROTECTION
- 65. Slide 60 Issue A Earth Fault Current / Number of Turns Short Circuited 5 10 15 20 25 Turn short- circuited (percentage of winding) Primary current Fault current 100 80 60 40 20 multiples of max fault current BUCCHOLZ PROTECTION
- 66. Slide 60 Issue A Operating principle Accumulation of Gaz BUCCHOLZ PROTECTION
- 67. Slide 60 Issue A Buchholz Relay Accumulation of gaz BUCCHOLZ PROTECTION
- 68. Slide 60 Issue A Accumulation of gaz Buchholz Relay BUCCHOLZ PROTECTION
- 69. Slide 60 Issue A Accumulation of gaz Buchholz Relay BUCCHOLZ PROTECTION
- 70. Slide 60 Issue A Color of gaz indicates the type of fault White or Yellow : Insulation burnt Grey : Dissociated oil Accumulation of gaz BUCCHOLZ PROTECTION
- 71. Slide 60 Issue A Accumulation of gaz Gaz can be extracted for detailled analysis Buchholz Relay BUCCHOLZ PROTECTION
- 72. Slide 60 Issue A • After oil maintenance, false tripping may occur because Oil aeration Effects of Oil Maintenance Bucholz relay tripping inhibited during suitable period Need of electrical protection BUCCHOLZ PROTECTION
- 73. Slide 60 Issue A Accumulation of gaz Oil Leakage Severe winding faults Bucholtz Protection Application BUCCHOLZ PROTECTION
- 74. Slide 60 Issue A Oil Leakage Buchholz Relay BUCCHOLZ PROTECTION
- 75. Slide 60 Issue A Oil Leakage Buchholz Relay BUCCHOLZ PROTECTION
- 76. Slide 60 Issue A Oil Leakage Buchholz Relay BUCCHOLZ PROTECTION
- 77. Slide 60 Issue A Oil Leakage Buchholz Relay BUCCHOLZ PROTECTION
- 78. Slide 60 Issue A Accumulation of gaz Oil Leakage Severe winding faults Buccholz Protection Application BUCCHOLZ PROTECTION
- 79. Slide 60 Issue A Severe winding fault Buchholz Relay BUCCHOLZ PROTECTION
- 80. Slide 60 Issue A Severe winding fault Buchholz Relay BUCCHOLZ PROTECTION
- 81. Slide 60 Issue A Severe winding fault Buchholz Relay BUCCHOLZ PROTECTION