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Neutral Grounding & Burning Damage Considerations for Industrial Generators

Louie J. Powell, PEEngineering Consultant

Saratoga Springs, NYmonophoto@nycap.rr.com

© IEEE 2006

Acknowledgement

The presentation is based in part on work done by the Working Group on Generator Grounding of the IEEE Industry Application Society Protection Committee. Membership in the Working Group included:

Eastman ChemicalRalph YoungBeckwith ElectricCharles Mozina

Eaton ElectricalDavid D. ShippConsultantDaniel J. Love

Artwell ElectricNorman T. StringerUniversity of AlabamaJames R. Jones

GE Energy/ConsultantLouie PowellParsons EnergyJay D. Fischer

Kellog-Brown & RootPrafulla PillaiBruce Douglas

Factory Mutual EngineeringAlan PierceBasler ElectricGerald Dalke

Padden EngineeringLorraine PaddenPowell ElectricJim Bowen

International PaperClifford NormandSiemens/ConsultantBruce Bailey

Kocher & Sherra/ConsultantNeil NicholsKBR/ConsultantDavid S. Baker

AffiliationNameAffiliationName

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Introduction• “Grounding” encompasses all matters involved in the

connections between an electrical system and earth.• Grounding practices establish critical system design

objectives– Maximum steady-state and transient voltages imposed on

insulation– Maximum currents in conductors and interrupting devices– Maximum potential gradients between components and

structures– Maximum flash energy at the point of an arc

Grounding Decisions Affect Both System Performance and Electrical Safety

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System Grounding vs. Equipment Grounding• System Grounding – How is

the electrical neutral connected to earth?

• Equipment Grounding – How is the electrical frame or enclosure connected to earth?

Focused on Different Objectives –Inextricably Linked

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Central Station Practice: High-Resistance Grounding

• Generator rating 100 MVA or larger• Dedicated generator step-up transformer• Generator breaker at high-voltage terminals of GSU• Generator neutral grounded through distribution transformer• Ground fault current limited to about 5-7 amperes

Generator

Simple Application – Widely Used

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Critical Differences in Industrial Applications

Generator

• Generator rating 10-50MVA • Generator connected to a distribution bus• Distribution feeders at generator terminal

voltage• Supply transformer connected delta at higher

voltage, wye at lower voltage• Generator breaker at generator terminal voltage

Grounding Must Address Application Requirements

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• Transient overvoltages• Workplace safety

– Arc-flash (incident) energy– Potential gradients

• Selective ground fault detection and clearing • Burning damage at the point of the fault• Circulation of third-harmonic current • Stator winding mechanical bracing• Unbalanced phase-to-neutral loading

Factors for consideration

Complex Array of Concerns

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Traditional Industrial Practice:Low-Resistance Grounding

75+ Year History of Applications

Generator

• Resistor rating: 100 – 1200 a• 400a. Rating is most typical

• Rationale -Enough current for selective protectionMinimizes potential gradientsMinimizes fault-point arcing“Acceptable compromise” between

competing objectives

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Reality Strikes: Recent Failures

• Challenged traditional thinking about resistance grounding

• Serious burning damage for internal stator ground faults

• Required extensive stator rebuild

• Generator out of service for months

• Significant replacement energy cost

Stimulated New Thinking About Grounding Practices

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The Physics of Stator Ground Faults:• For a ground fault inside the stator winding of a generator

• Two components of fault current– Supplied from the “system” - Is

• Interrupted by opening the generator breaker• Typically interrupted in about 0.1 seconds (6 cycles)

– Supplied by the generator itself - Ig• Opening the generator breaker has no effect• Requires full demagnetization of the generator field

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Energy Released at Fault Point

•

• Two critical parameters:1. Magnitude of ground fault current2. Time to interrupt fault current

• Value of k– 2 for resistive heating– 1.5 suggested for low voltage arcing– Subject for further research

dtIt

k∫=0

arc in Energy

Analyze Two Components SeparatelySum Results for Total Energy Released

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Case Study – System Current

400A. Low Resistance Grounding Provides Reasonable Damage ControlGenerators on Effectively Grounded Systems Exposed to Serious Jeopardy

0.01 0.1 1 100

5000

1 .104

1.5 .104

2 .104

2.5 .104

3 .104

3.5 .104

4 .104

400 A. Lo-R system5kA Effect. Grd system

Time, seconds

Ener

gy re

leas

ed, w

att-s

econ

ds

© IEEE 2006

Case Study – Generator CurrentTypical medium voltage generator reactances and time constants

0.01 0.1 1 100.1

1

10

100

1 .103

1 .104

400A. LoR ground10A. HiR ground

Time, seconds

Ener

gy re

leas

ed, w

att s

econ

ds

10A. High Resistance Grounding Provides Reasonable Damage Control400A. Grounding Exposes Machines to Jeopardy

τεt

g Ii−

=

© IEEE 2006

The Issue: Fault Current Decrement

• Tripping the generator breaker DOES NOT remove fault current (IG)

• Current persists until field has collapsed

• Single-line-to-ground fault time constant ~ 1 second

0 1 2 3 4 50.01

0.1

1

10

100

1 .103

Time - seconds

Ener

gy re

leas

e, jo

ules

:

Damage is Mainly Self-Inflicted

τεt

g Ii−

=

© IEEE 2006

What has changed?Technical considerations: well, duh?

Economic considerations:

• Generators returned to manufacturer for repair

• Higher cost of replacement energy

New Economic Framework Drives Different Technical Solutions

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Evolving New Practices• Dedicated generation systems (with GSU transformer)

– High resistance grounding (< 10a)• Generators embedded in resistance grounded industrial

distribution systems– 10a. High-resistance grounding on generator, 400 a. grounding on

systemIdeal – but not practical in all casesRequires careful consideration of all potential operating modes

– Hybrid grounding

• Predictive diagnostics (eg. partial discharge monitoring)

N:1

R400a.

Neutral switch10a. High resistance package

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Further Work: Energy Release Comparison

0

1000

2000

3000

4000

5000

6000

High-resist ance(10a)

Hybrid (2 cycleswit ching)

Hybr id (6 cycleswit ching)

Low resist ance(400a)

Joul

es

• Evaluate potential for switching transients with hybrid grounding

• Enhance understanding of arc-flash burning mechanism– Understand how arc damage occurs– Quantify threshold of safety

• Enhance on-line predictive & diagnostic capabilities

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Discussion

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References1. IEEE Guide for Generator Ground Protection, IEEE Standard C37.101, 1993. 2. IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems (The Green

Book), IEEE Standard 142, 1991.3. IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power

Systems (The Buff Book), IEEE Standard 242, 2001.4. American National Standard General Requirements for Synchronous Generators, ANSI C50.10, 1990.5. NEMA Standard for Motors and Generators, NEMA Standard MG1-1998 (Rev. 1, 2000).6. P. G. Brown, “Generator Neutral Grounding,” General Electric Co., Schenectady, NY, Application

Engineering Information GET 1941A, p. 5.7. L. J. Powell, “The impact of system grounding practices on generator fault damage,” IEEE Transactions on

Industry Applications, vol. IA-34, , Sept./Oct. 1998, pp. 923-927.8. L. J. Powell, “Stator Fault Damage Considerations for Generators on Solidly Grounded Systems” IEEE

Transactions on Industry Applications, vol. IA-37, Jan/Feb 2001, pp. 218-222.9. IEEE Working Group Report, P. Pillai, Chair, “Grounding and Ground Fault Protection of Multiple-

Generator Installations in Industrial and Commercial Power Systems” (a four-part paper), IEEE Transactions on Industry Applications, vol IA-40-1, Jan/Feb 2004, pp 11-32

10. Moody, D,. V. Beachum, T. Natali, W. Vilcheck and DD Shipp, “Application of a hybrid grounding scheme to a paper mill 13.8kV generator”, Conference Record, 2003 Pulp & Paper Industry Technical Conference, pp 107-116.

11. M. Zielichowski,, “Uszkodzenia zelaza czynnego w stojanie turbogeneratora przy zwarciach doziemnych(Stator core damage due to grounds in a turbine-generator),” Energetika, No 8, pp. 263-267, 1970.

12. L. J. Powell, “Influence of third harmonic circulating currents in selecting neutral grounding devices,” IEEE Transactions on Industry Applications, vol. IA-9, Nov./Dec. 1973, pp. 672-679.

13. E. M. Gulachenski and E. W. Courville, “New England Electric’s 39 years of experience with resonant neutral grounding of unit-connected generators,” IEEE Transactions on Power Delivery, vol. 6, Jul. 1991, pp. 1016-1024.