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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.

2 4 Nov 2015 IAS Webinar Series

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.


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


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?


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”


Safety through proper System Grounding and Ground Fault Protection

Intentional System Grounding


Effect of Impedance Grounding on Phase and Neutral Voltages: Slide 1 System



Effect of Impedance Grounding on Phase and Neutral Voltages: Slide 2 (VN = 0)



Effect of Impedance Grounding on Phase and Neutral Voltages: Slide 3 (VN and VLL)



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


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


Four-Wire “Single” Point Grounded-Neutral System

RXN

C

B

A

B

C

N

A

b) Reactance Grounded

a) Solidly Grounded


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


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


Most Common Fault Types

3-Phase (Balanced Fault) - rare Phase-to-Phase Phase-to-Phase-to-Ground Phase-to-Ground – most common


An Analytical Tool for “Unsymmetrical” Fault Calculations


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



“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


Positive and Negative Sequence Components


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


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



Zero Sequence Current Circuit


Typical Feeder Protection 3Φ-4 Wire High 3Io 3Φ-3 Wire Low 3Io


Zero Sequence CT



Zero Sequence CT & Shield Grounding Wire


Incorrect Correct


Zero Sequence Voltage Circuit



Use VLL VT’s

Why Ground a Circuit?

Arcing Fault


“Arcing Fault” Equivalent Circuit for an Ungrounded System



Transient Overvoltage from Restriking Ground Fault on “A” Phase



Zero Sequence Circuit Showing Capacitance Discharge



Grounding

Generator Grounding and Ground Fault Protection



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’


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.


Generator Neutral Grounding Reactor


Generator Ground Fault Protection



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


Motor Ground Fault Protection



Motor Protection – Poor Alternative



Transformer Resistance Grounding


Motor Lead Fault with 15kV with 600 Amp LRG – After Six Ground Faults


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


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


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


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


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


Permissible Short Circuit Currents for Copper Tape Shielding (Okonite Engrg Manual)



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


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


Questions and Comments


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


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”


Backup and Reference Slides


Grounding Addition to an Ungrounded System


Grounding of Ungrounded

Power System


Grounding Addition to an Ungrounded System


Zigzag Grounding Transformer

Schematic

Winding Arrangement

3I0

I0

I0

I0

I0

3I0

Grounding of Ungrounded

Power System


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


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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