Safety through proper system Grounding and Ground Fault Protection

Transcription

1 Safety through proper system Grounding and Ground Fault Protection November 4 th, 2015 Presenter: Mr. John Nelson, PE, FIEEE, NEI Electric Power Engineering, Inc. Event to start shortly Scheduled time: 11:00 USA Eastern Standard Time 1

2 Webinar Presenter: Mr. John Nelson John P. Nelson graduated from the University of Illinois Champaign-Urbana, in 1970, with a Bachelor of Science in Electrical Engineering, and a Master of Science in Electrical Engineering, from the University of Colorado Boulder, in He performed post graduate studies in business administration from Prior to his retirement, Mr. Nelson held positions with Public Service Company of Colorado, from , Power Line Models, from and NEI Electric Power Engineering from In December 2014, Mr. Nelson retired as the CEO and a principle engineer of NEI Electric Power Engineering which he founded in Mr. Nelson has also been active in the IAS Petroleum and Chemical Industry Committee since 1980 where he received the Russel W. Mills award for outstanding contributions to PCIC. Mr. Nelson was elevated to IEEE Fellow in 1999 and is the recipient of the 2012 Harold Kaufman award. Mr. Nelson is a registered professional engineer in the state of Colorado, as well as eight other states. 2 IAS Webinar Series

3 Webinar Abstract Abstract: The webinar on Safety Through Proper System Grounding and Ground Fault Protection is intended for the practicing electric power engineer whether a recent graduate or a seasoned engineer. The webinar will begin with a brief discussion on electrical safety and ground faults. The term system grounding should not be confused with the requirements for equipment grounding. The fundamentals of system grounding will be covered which will include solidly, ungrounded and impedance grounded systems. The use of symmetrical components will be briefly discussed as a tool for better understanding ground fault currents and ground fault protection. A brief discussion will be held on generator and motor protection which is a little more complex that standard feeder protection. Finally, a brief discussion will be held concerning the application of surge arresters and power cables based on the type of system grounding used. 3 IAS Webinar Series

4 Safety through Proper System Grounding and Ground Fault Protection (Outline) Electrical safety, system grounding and ground faults Symmetrical components: zero sequence network and ground faults Example: Generator neutral grounding and ground fault protection Example: Motor ground fault protection System grounding and surge protection System grounding and MV cable shielding Conclusions 4 IAS Webinar Series

5 Ground Faults and Safey Industrial Plant: > 95% faults originate as a ground fault Trans and Distribution lines: >80% are ground faults Quick sensing and tripping improves safety Good system grounding practices improves safety Good System grounding techniques can reduce serious and fatal injuries significantly Why? 5 IAS Webinar Series

6 6 IAS Webinar Series

7 What is System Grounding and Ground Fault Protection? System Grounding - The intentional ground on the system Ground Fault Protection - Detection of an unintentional ground on the system and taking appropriate action Personnel Safety and Equipment Protection 7 IAS Webinar Series

8 Intentional System Grounding Safety through proper System Grounding and Ground Fault Protection 8 IAS Webinar Series

9 Effect of Impedance Grounding on Phase and Neutral Voltages: Slide 1 System Safety through proper System Grounding and Ground Fault Protection 9 IAS Webinar Series

10 Effect of Impedance Grounding on Phase and Neutral Voltages: Slide 2 (V N = 0) Safety through proper System Grounding and Ground Fault Protection 10 IAS Webinar Series

11 Effect of Impedance Grounding on Phase and Neutral Voltages: Slide 3 (V N and V LL ) V A Safety through proper System Grounding and Ground Fault Protection 11 IAS Webinar Series

12 MV Power Cable Ratings On Resistance Grounded Systems Phase Conductor Normal Operating Voltage: Cable KV/ 3 (Example: 5 kv / 3 = 2.9 kv) o 100% Insulation Fault Cleared in 1 minute or less o 133% Insulation Fault Cleared in 1 hour or less o 173% Insulation Fault left on system > 1 hour Neutral Conductor Normal Operating Voltage: 0 KV o 2.4 kv non-shielded cable for 4.16 kv LRG systems o 8 kv non-shielded cable for 13.8 kv LRG systems 12 IAS Webinar Series

13 Three-Wire Single Point Grounded System A B a) Solidly Grounded C A X R B b) Reactance Grounded C A B R c) Resistance Grounded C 13 IAS Webinar Series

14 Four-Wire Single Point Grounded-Neutral System A B N a) Solidly Grounded C A X R b) Reactance Grounded B N C 14 IAS Webinar Series

15 Simple Examples of Resistance Grounding LRG 2400/4160 Volt system 6 Ohm Resistor 2400 Volts/6 Ohms = 400 Amps (Compare to 10,000A 3 phase fault) HRG 277/480 Volt System 100 Ohm Resistor 277 Volts/100 Ohms = 2.7 Amps (Compare to 50,000A 3 phase fault) 15 Symmetrical IAS Webinar Components Series

16 Effectively Grounded Definition Subset of solidly grounded X 0 /X 1 < 3.0 and R 0 /X 1 < 1.0 3ϕ, 4 wire multi-grounded systems Neutral grounded at source and at regular intervals along neutral conductor Maintains voltages on unfaulted phases to less than 80% V LL Useful for surge arrester applications 16 IAS Webinar Series

17 Most Common Fault Types 3-Phase (Balanced Fault) - rare Phase-to-Phase Phase-to-Phase-to-Ground Phase-to-Ground most common Symmetrical Components An Analytical Tool for Unsymmetrical Fault Calculations 17 IAS Webinar Series

18 Ground Fault Modeling Three Phase Loop Equations - Kirchoff s Loop Equations Symmetrical Components - Zero Sequence Circuit/Zero Sequence Plane Computer Programs - Mathematical Algorithms Utilizing Symmetrical Components 18 IAS Webinar Series

19 Symmetrical Components Positive Sequence Network: Balanced Three-Phase, Phase-to-Phase, Phase-to- Phase-to-Ground and Phase-to-Ground Negative Sequence Network: Phase-to- Phase, Phase-to-Phase-to-Ground and Phase-to-Ground Zero Sequence Network: Phase-to-Ground and Phase-to-Phase-to-Ground 19 IAS Webinar Series

20 Positive and Negative Sequence Components 20 IAS Webinar Series

21 Zero Sequence Components

22 Network Connections for Unsymmetrical Faults Positive Positive Positive Negative Negative Negative Zero Single-to-Line- Ground Fault Zero Line-to-Line-Fault Zero Line-to-Line-to- Ground Fault 22 IAS Webinar Series

23 Zero Sequence Current and Voltage Circuits Current Circuit - Three Current Transformer adding I A +I B +I C =3I 0 Voltage Circuit - Three Voltage Transformers adding V A +V B +V C =3V 0 23 IAS Webinar Series

24 Zero Sequence Current Circuit Safety through proper System Grounding and Ground Fault Protection 24 IAS Webinar Series

25 Typical Feeder Protection 3Φ-4 Wire High 3I o 3Φ-3 Wire Low 3I o 25 IAS Webinar Series

26 Zero Sequence CT Safety through proper System Grounding and Ground Fault Protection 26 IAS Webinar Series

27 Zero Sequence CT & Shield Grounding Wire Incorrect Correct Safety through proper System Grounding and Ground Fault Protection 27 IAS Webinar Series

28 Zero Sequence Voltage Circuit Use V LL VT s Safety through proper System Grounding and Ground Fault Protection 28 IAS Webinar Series

29 Why Ground a Circuit? Arcing Fault 29 IAS Webinar Series

30 Arcing Fault Equivalent Circuit for an Ungrounded System Safety through proper System Grounding and Ground Fault Protection 30 IAS Webinar Series

31 Transient Overvoltage from Restriking Ground Fault on A Phase Safety through proper System Grounding and Ground Fault Protection 31 IAS Webinar Series

32 Zero Sequence Circuit Showing Capacitance Discharge Grounding Safety through proper System Grounding and Ground Fault Protection 32 IAS Webinar Series

33 Generator Grounding and Ground Fault Protection V LL Safety through proper System Grounding and Ground Fault Protection 33 IAS Webinar Series

34 Generator Typical Reactances X S Synchronous Reactance: 1.9 P.U. X d Transient Reactance: 0.20 P.U. X d Sub-transient Reactance: 0.15 P.U. ( X 1 ) X 2 Negative Sequence Reactance: 0.15 P.U. X 0 Zero Sequence Reactance: 0.02 P.U. Note: Generator short circuit strength is typically designed on a combination X d and X d 34 IAS Webinar Series

35 Typical Generator Terminal Fault Currents Transient Current: 1 P.U. V / 0.20 P.U. X = 5.0 P.U. Sub-transient Current: 1 P.U. V / 0.15 P.U X = 6.7 P.U. Ground Fault Current: 3.0 x 1 P.U. V / (X 1 + X 2 + X 0 ) = 3.0/( ) = 9.4 P.U. Note: Generator short circuit (SLGF) current is exceeded by approximately 90% for a solidly grounded generator. 35 IAS Webinar Series

36 Generator Neutral Grounding Reactor 36 IAS Webinar Series

37 Generator Ground Fault Protection Use V LL for VT s V S Use V P = V LL Safety through proper System Grounding and Ground Fault Protection 37 IAS Webinar Series

38 Generator HRG Example 5 MVA, 4.16 kv Generator Assume 3I CO = 5 Amp Transformer: 4200:120 V (N = 35) 4200 x 5 Amps = 21 kva (25 kva) R P = 2400V/5 A = 480 Ω R S = R P /N 2 = 480/35 2 = 0.39 Ω I S = I P *N = 5*35 = 175 Amps kw = I 2 R S /1000= 11.9 kw V SEC = 2400 Volt/35 = 69 Volts 38 IAS Webinar Series

39 Motor Ground Fault Protection Safety through proper System Grounding and Ground Fault Protection 39 IAS Webinar Series

40 Motor Protection Poor Alternative Safety through proper System Grounding and Ground Fault Protection 40 IAS Webinar Series

41 Transformer Resistance Grounding 41 IAS Webinar Series

42 Motor Lead Fault with 15kV with 600 Amp LRG After Six Ground Faults 42 IAS Webinar Series

43 Voltage Ratings Surge Arresters (SA) o Duty Cycle (Older Rating) o MCOV Maximum Continuous Operating Voltage Protection inversely proportional to SA voltage rating Cost: Proportional to SA voltage rating Reliability: Increases with SA rating 43 IAS Webinar Series

44 Lightning Arresters Voltage Ratings Determine maximum phase-to-ground voltage Faulted phase voltage Decreases Unfaulted phase voltage Increases Solidly grounded: minimum arrester voltage ratings Impedance grounded: higher voltage rating LRG and HRG system: 100% V LL rating 44 IAS Webinar Series

45 Surge Arrester Duty Cycle Voltage Ratings Type of Grounding Solidly Grounded Reactance Grounded Resistance Grounded Min Duty Cycle Rating % V LL % V LL 100% V LL 45 IAS Webinar Series

46 Surge Arrester Maximum Continuous Operating Voltage (MCOV) Ratings Type of Grounding Solidly Grounded Reactance Grounded Resistance Grounded Min MCOV RATING 1.05 x Max V phase 1.4 x Max V phase 1.73 x Max V phase 46 IAS Webinar Series

47 None System Grounding and Shielding of MV Cables 5 mil tape shield Full Concentric Neutral 1/3 Concentric Neutral 1/6 Concentric Neutral Others 15 kv Power Cable with 5 mil Tape Shield 47 IAS Webinar Series

48 Permissible Short Circuit Currents for Copper Tape Shielding (Okonite Engrg Manual) 48 IAS Webinar Series

49 15kV Cable with 1/3 Concentric Neutral Power 15kV Power Cable with 1/3 Concentric Flat Strap Neutral 15kV Power Cable with 5 mil Tape Shield and Drain Wires Safety through proper System Grounding and Ground Fault Protection 49 IAS Webinar Series

50 CONCLUSIONS This webinar covered safety aspects of Solidly grounded systems Resistance (High and Low) grounded systems Reactance ground systems Ungrounded grounded system Importance of system grounding and ground fault protection for proper safety of Personnel Equipment 50 IAS Webinar Series

51 Questions and Comments 51 IAS Webinar Series

52 IAS WEBINAR SERIES Questions and Answers November 4 th, 2015 Presenter: John P. Nelson (retired) NEI Electric Power Engrg Title: Safety Through Proper System Grounding and Ground Fault Protection If you have any question for the presenter: Use the Webex Q&A tab to send your question to the moderator 52 IAS Webinar Series

53 IAS WEBINAR SERIES CONCLUSION We thank the presenter, John Nelson, and we thank you for your attention This session was recorded and will be posted on line at: Next webinar: December 2nd, 9:00 am USA EST Prof. Sang-Bin Lee Electrical Testing and Diagnostics of Medium-High Voltage Induction Machines in an Industrial Environment 53 IAS Webinar Series

54 Backup and Reference Slides 54 IAS Webinar Series

55 Grounding Addition to an Ungrounded System Grounding of Ungrounded Power System Safety through proper System Grounding and Ground Fault Protection 55 IAS Webinar Series

56 Grounding Addition to an Ungrounded System Zigzag Grounding Transformer I 0 I 0 Grounding of Ungrounded Power System 3I 0 I 0 Schematic I 0 3I 0 Safety through proper System Grounding and Ground Fault Protection Winding Arrangement 56 IAS Webinar Series

57 Generator Reactor Sizing Calculation Example Effectively Grounded: X 0 / X X 1 = X d (Conservative) X 0 = X 0G + 3 X R 3 X R (3.0 X 1 X 0G ) If X d = 0.15 PU and X OG = 0.02 PU Then, 3 X R ( ) = 0.43 PU Note, 3X R 0.13 P.U. for short circuit withstand 3 X R 0.43 P.U. for an effectively grounded generator X R P.U. Note: Z base = (KV base ) 2 /MVA base Example: 13.8 kv, 10 MVA Gen Zbase = 19 Ω Where, X 0G is the generator zero sequence reactance and X R is the reactance of the neutral reactor 57 IAS Webinar Series

58 IAS WEBINAR SERIES CONCLUSION We thank the presenter, John Nelson, and we thank you for your attention This session was recorded and will be posted on line at: Next webinar: December 2nd, 9:00 am USA EST Prof. Sang-Bin Lee Electrical Testing and Diagnostics of Medium-High Voltage Induction Machines in an Industrial Environment 58 IAS Webinar Series