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08_C_3_Basis Stator Earth Fault Protection.ppt
- 1. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 1 Training: Generator protection Basics Stator Earth Fault Protection Presenter: Dr. Hans-Joachim Herrmann E D EA PRO LM1 Phone 0911-433-8266
- 2. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 2 Test Results showing Effects of Arc Burning on Stator Core Laminations during Earth Faults
- 3. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 3 Methods of Neutral-Point Connection (1) Isolated or high ohmic earthed Advantage: Small fault currents Disadvantage: High transient overvoltage for intermittent earth faults (2.5 – 3.5 ) Uph-E Compensated or reactive earthed Advantage: Small fault currents at the fault location Disadvantage: Transient overvoltage (<2.5Uph-E) Higher costs Standard application Very seldom; used in older plants < 10 A < 10 A
- 4. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 4 Methods of Neutral-Point Connection (2) Low ohmic earthed Advantage: Lower transient overvoltage, 95 to 98% protective range of SEF protection Disadvantage: Great damages to generators at longer fault duration Solidly (effective) earthed Advantage: Low transient voltage, better measuring conditions for the protection Disadvantage: Great damages to generators, Leakage zero sequence currents Application in industrial plants Application at low voltage generators G < 200 - 400 A
- 5. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 5 Voltages in Case of an Earth Fault Voltage without earth fault UL3,E UL3,E = UL31 UE = -UL1,E UL1,E = 0 UL2,E = UL21 Voltages at the point of an earth fault at phase L1 ULx,E ... phase-earth-voltage no displacement voltage (UE = 0) UL,E voltage decreases in the faulty phase (min 0) UL,E voltage in the both “healthy” phases are increased (max phase-to-phase) UE displacement-voltage (can be measured at star point to earth) M M UL1,E UL2,E L1 L2 L3
- 6. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 6 Currents in the Case of an Earth Fault Model: Equivalent voltage UE at the point of fault IE =3 IE/3 = 3 UE/ZE IE/3 ... earth fault current in one phase ZE ... earth impedance at one phase Vector diagram: earth fault in phase L1 UL3,E UL2,E IE = 3U0 1 jCE = 3U0 jCE = 3 UE ZE IC,2 IE IC,3 L1 L2 L3 IC 3 3U0 =3UE UE IE/3 IE ZE L1 L2 L3 ~ ~ ~
- 7. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 7 Basics of Stator Earth Fault Protection Fault Location Step-up transformer Auxiliary transformer Possible region of earth faults Region of earth faults Earth fault locations in the stator U0 Low displacement voltage at earth faults close to the star point
- 8. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 8 generator is galvanically isolated under the assumption of an ideal transformer, the displacement voltage caused by an earth fault, can only be measured at the generator G G G Generators Connected via a Unit Transformer to the Grid
- 9. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 9 Earthing transformer üTR = CL CG CTr CK UEO UN 3 (Limb transformation ratio) UGen 3 100 3 500 3 V V R RPrim ü 3 2 TR UR Problem: grid earth faults cause disturbances due to the coupling capacitance between the two transformer windings Solution: attenuation by means of a load resistor Note: At solidly earthed transformer the UE0 is appr. 80% of UN/3 (Safety margin, if solid earthing is open) Unit Connection: Elimination of the Disturbance during a High Voltage Earth Fault
- 10. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 10 Unit Connection - Influence of the Coupling Capacitance UR,prim Rprim CK UC UEO UR,prim Rprim UEO 1 Rprim + jCK Example: CK = 10 nF UEO = 220 kV 3 UR,prim 265V 665 3 R R 2 TR Prim ü UN,G = 10,5 kV R = 5 üTr = 34,6 UR,sek 23V 23V 500V 4,6% disturbance influence CE. equivalent circuit disturbance voltage CG+CL+CTr neglected UEO displacement voltage on the high voltage side CK three phase coupling capacitance Rprim primary load resistor üTr earthing transformer ratio
- 11. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 11 Unit Connection with Neutral Transformer This design is very often used outside Germany, mostly at small generators Generator Unit transformer Rsek= Rprim ü2 TR Design of Rprim so that the fault current is < 10A R UR ÜTR = UGen 3 UR A high secondary nominal voltage UR (250V - 500V) is selected in order to avoid very small load resistors.
- 12. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 12 machines are galvanically connected displacement voltage caused by an earth fault can be measured in all locations G G G M Generator Connected Directly to the Grid
- 13. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 13 Direct Connection - Directional Stator Earth Fault Protection L1 L2 L3 L1 L2 L3 CE IC + IR Ohmic current boost IC IR ICL2 ICL1 3I0 3U0 UL1 UL2 grid 3i0 3u0 DFT 3U0>, 3I0> Direction (3U0, 3I0) Earthing transformer
- 14. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 14 Fault Currents in Case of a Direct Connection G1 G2 IMess IMess
- 15. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 15 Fault Currents in Case of a Direct Connection with Earthing Transformer G1 G2 IMeas IMeas Earthing transformer Ohmic current RB
- 16. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 16 Toroidal Current Transformer IE 3 per phase I´E L1 L3 L2 IE I 3 ~ Holmgreen connection (separate cores) Holmgreen connection (common neutral return connector Sensitivity is limited Problem: Large CT ratio leads to small currents on the secondary side IE 3 per phase L1 L3 L2 I´E (IE = 3 I0) magnetic addition of earth currents, principle is sensitive Earth Current Detection via a Toroidal Current Transformer and Holmgreen Connection
- 17. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 17 Directional Stator Earth Fault Protection Directional Characteristic
- 18. Energy Sector Copyright © Siemens AG 2008. All rights reserved. Siemens PowerAcademy TD 08 Basis Stator Earth Fault Protection Using numerical machine and motor protection Version: C 3_Page 18 Directional Stator Earth Fault Protection - Logic