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Safety through proper system Grounding and Ground Fault Protection
November 4th, 2015 Presenter: Mr. John Nelson, PE, FIEEE, NEI Electric Power Engineering, Inc.
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 1975. He performed post graduate studies in business administration from 1975-1979. Prior to his retirement, Mr. Nelson held positions with Public Service Company of Colorado, from 1969-1979, Power Line Models, from 1979-1984 and NEI Electric Power Engineering from 1984-2014. In December 2014, Mr. Nelson retired as the CEO and a principle engineer of NEI Electric Power Engineering which he founded in 1984. 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.
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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.
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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
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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?
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System Grounding - The “intentional” ground on the system
Ground Fault Protection - Detection of an “unintentional” ground on the system and taking appropriate action
What is System Grounding and Ground Fault Protection?
“Personnel Safety and Equipment Protection”
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Safety through proper System Grounding and Ground Fault Protection
Intentional System Grounding
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Effect of Impedance Grounding on Phase and Neutral Voltages: Slide 1 System
Safety through proper System Grounding and Ground Fault Protection
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Effect of Impedance Grounding on Phase and Neutral Voltages: Slide 2 (VN = 0)
Safety through proper System Grounding and Ground Fault Protection
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Effect of Impedance Grounding on Phase and Neutral Voltages: Slide 3 (VN and VLL)
Safety through proper System Grounding and Ground Fault Protection
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VA
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
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Three-Wire “Single” Point Grounded System
b) Reactance Grounded
RX
C
B
A
a) Solidly Grounded
B
C
A
c) Resistance GroundedC
A
B
R
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Four-Wire “Single” Point Grounded-Neutral System
RXN
C
B
A
B
C
N
A
b) Reactance Grounded
a) Solidly Grounded
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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)
Symmetrical Components
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Effectively Grounded Definition
Subset of solidly grounded X0/X1 < 3.0 and R0/X1 < 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% VLL
Useful for surge arrester applications
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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
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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
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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
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Positive and Negative Sequence Components
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Zero Sequence Components
Network Connections for Unsymmetrical Faults
Single-to-Line-Ground Fault
Line-to-Line-Fault Line-to-Line-to-Ground Fault
Positive
Negative
Zero
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Positive
Negative
Zero
Positive
Negative
Zero
Zero Sequence Current and Voltage Circuits
Current Circuit - Three Current Transformer adding IA+IB+IC=3I0
Voltage Circuit - Three Voltage Transformers adding VA+VB+VC=3V0
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Safety through proper System Grounding and Ground Fault Protection
Zero Sequence Current Circuit
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Typical Feeder Protection 3Φ-4 Wire High 3Io 3Φ-3 Wire Low 3Io
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Zero Sequence CT
Safety through proper System Grounding and Ground Fault Protection
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Zero Sequence CT & Shield Grounding Wire
Safety through proper System Grounding and Ground Fault Protection
Incorrect Correct
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Zero Sequence Voltage Circuit
Safety through proper System Grounding and Ground Fault Protection
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Use VLL VT’s
Why Ground a Circuit?
Arcing Fault
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“Arcing Fault” Equivalent Circuit for an Ungrounded System
Safety through proper System Grounding and Ground Fault Protection
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Transient Overvoltage from Restriking Ground Fault on “A” Phase
Safety through proper System Grounding and Ground Fault Protection
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Zero Sequence Circuit Showing Capacitance Discharge
Safety through proper System Grounding and Ground Fault Protection
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Grounding
Generator Grounding and Ground Fault Protection
Safety through proper System Grounding and Ground Fault Protection
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VLL
Generator Typical Reactances
• XS Synchronous Reactance: 1.9 P.U. • Xd’ Transient Reactance: 0.20 P.U. • Xd” Sub-transient Reactance: 0.15 P.U. (≈ X1) • X2 Negative Sequence Reactance: 0.15 P.U. • X0 Zero Sequence Reactance: 0.02 P.U.
Note: Generator short circuit strength is typically designed on a combination Xd” and Xd’
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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 / (X1 + X2 + X0) = 3.0/(0.15 + 0.15 + 0.02) = 9.4 P.U.
Note: Generator short circuit (SLGF) current is exceeded by approximately 90% for a solidly grounded generator.
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Generator Neutral Grounding Reactor
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Generator Ground Fault Protection
Safety through proper System Grounding and Ground Fault Protection
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Use VLL for VT’s
Use VP = VLL VS
Generator HRG Example • 5 MVA, 4.16 kV Generator • Assume 3ICO = 5 Amp • Transformer: 4200:120 V (N = 35) • 4200 x 5 Amps = 21 kVA (25 kVA) • RP = 2400V/5 A = 480 Ω • RS = RP/N2 = 480/352 = 0.39 Ω • IS = IP*N = 5*35 = 175 Amps • kW = I2RS/1000= 11.9 kW • VSEC = 2400 Volt/35 = 69 Volts
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Motor Ground Fault Protection
Safety through proper System Grounding and Ground Fault Protection
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Motor Protection – Poor Alternative
Safety through proper System Grounding and Ground Fault Protection
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Transformer Resistance Grounding
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Motor Lead Fault with 15kV with 600 Amp LRG – After Six Ground Faults
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Surge Arresters (SA) • Voltage Ratings
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
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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% VLL rating
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Surge Arrester Duty Cycle Voltage Ratings
Type of Grounding Min Duty Cycle Rating
Solidly Grounded 70 – 100% VLL
Reactance Grounded 80 – 100% VLL
Resistance Grounded 100% VLL
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Surge Arrester Maximum Continuous Operating
Voltage (MCOV) Ratings
Type of Grounding Min MCOV RATING
Solidly Grounded 1.05 x Max V phase
Reactance Grounded 1.4 x Max V phase
Resistance Grounded 1.73 x Max V phase
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System Grounding and Shielding of MV Cables
• None • 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
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Permissible Short Circuit Currents for Copper Tape Shielding (Okonite Engrg Manual)
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Safety through proper System Grounding and Ground Fault Protection
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
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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
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Questions and Comments
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IAS WEBINAR SERIES Questions and Answers
If you have any question for the presenter: Use the Webex Q&A tab to send your question to the moderator
November 4th , 2015 Presenter: John P. Nelson (retired) NEI Electric Power Engrg Title: Safety Through Proper System Grounding and Ground Fault Protection
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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: www.ias.ieee.org
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”
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Backup and Reference Slides
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Grounding Addition to an Ungrounded System
Safety through proper System Grounding and Ground Fault Protection
Grounding of Ungrounded
Power System
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Grounding Addition to an Ungrounded System
Safety through proper System Grounding and Ground Fault Protection
Zigzag Grounding Transformer
Schematic
Winding Arrangement
3I0
I0
I0
I0
I0
3I0
Grounding of Ungrounded
Power System
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Generator Reactor Sizing Calculation Example
• Effectively Grounded: X0 / X1 ≤ 3.0 • X1 = Xd” (Conservative) • X0 = X0G + 3 XR • 3 XR ≤ (3.0 X1 – X0G) • If Xd” = 0.15 PU and XOG = 0.02 PU • Then, 3 XR ≤ (0.45 - 0.02) = 0.43 PU • Note, 3XR ≥ 0.13 P.U. for short circuit withstand • 3 XR ≤ 0.43 P.U. for an effectively grounded generator • XR ≤ 0.143 P.U. Note: Zbase = (KVbase)2/MVAbase Example: 13.8 kV, 10 MVA Gen Zbase = 19 Ω Where, X0G is the generator zero sequence reactance and XR
is the reactance of the neutral reactor
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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: www.ias.ieee.org
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 4 Nov 2015 IAS Webinar Series
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