IEEE Industry Applications Society – Atlanta Chapter January 19, 2010 Meeting 1 Sakis Meliopoulos Georgia Power Distinguished Professor School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0250, Telephone: 404 894-2926, Fax: 404 894-4641 Email: [email protected] or [email protected]Testing and Evaluation of Grounding Systems: The Revision of the IEEE Std 81
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IEEE Industry Applications Society – Atlanta ChapterJanuary 19, 2010 Meeting
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Sakis MeliopoulosGeorgia Power Distinguished ProfessorSchool of Electrical and Computer Engineering,Georgia Institute of Technology,Atlanta, Georgia 30332-0250,Telephone: 404 894-2926, Fax: 404 894-4641Email: [email protected] or [email protected]
Testing and Evaluation of Grounding Systems:The Revision of the IEEE Std 81
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Purpose of Grounding
Lightning and Surge Protection
Stabilize Circuit Potential and Assist in Proper Operation of:
Grounding and Bonding is Fundamental for a Safe and Reliable Power System
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Grounding, Bonding and Power Quality“Recent studies indicate that as much as 80% of all failures of sensitive electronic equipment attributed to poor power quality may result from inadequate electrical grounding or wiring on the customer’s premises or from interactions with other loads within the premises.”
Wiring and Grounding for Power QualityEPRI CU-2026, March 1990
“However, many power quality problems that occur within customer facilities are related to wiring and grounding practices. Up to 80% of all power quality problems reported by customers are related to wiring and grounding problems within a facility.”
Power Quality Assessment ProcedureEPRI CU-7529, December 1991
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Terms and DefinitionsOver the Years Grounding Design Procedures Have Been Developed as Well as Appropriate Standards, Most Notable:•ANSI/IEEE Std 80-2000, IEEE Guide for Safety in AC Substation Grounding.
•IEEE Std 487-2007, Recommended Practice for the Protection of Wire-Line Communication Facilities Serving Electric Supply Locations.
•IEEE Std 998-1996, IEEE Guide for Direct Lightning Stroke Shielding of Substations.
•IEEE Std 1410-2004, IEEE Guide for Improving the Lightning Performance of Electric Power Overhead Distribution Lines.
•IEEE Std 1243-1997, IEEE Guide for Improving the Lightning Performance of Transmission Lines.
•National Electrical Code.
•National Electrical Safety Code.
•FIPS 94 and Derivatives.
For the Purpose of Verifying Designs, Testing Procedures have Been also Developed. Most Notable:•ANSI/IEEE Std 81-1983, IEEE Guide for Measuring Earth Resistivity, Ground Impedance and Earth Surface Potentials of a Ground System.
•ANSI/IEEE Std 81.2-1991, IEEE Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems.
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The History of IEEE Std 80
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The History of IEEE Std 80
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Basis of Standards: IEEE 80 & IECNon-Fibrillating Body Current as a Function of Shock Duration
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k50 = 0.116 (Non-Fibrillating, 0.5%)
k50 = 0.185 (Fibrillating, 0.5%)
k70 = 0.157 (Non-Fibrillating, 0.5%)
k70 = 0.263 (Fibrillating, 0.5%)
sb tIk =
Value of Constant k for EffectiveRMS Values of IB:
Body Weight (kg)
Fibr
illatin
g C
urre
nt (m
A R
MS)
0
100
200
300
0 10020 40 60 80
MaximumNon-FibrillatingCurrent (0.5%)
MinimumFibrillatingCurrent (0.5%)
Dog
s
shee
pca
lves
pigs
Kise
lev
Dog
s
Ferri
s D
ogs
IEEE Std 80, 1986 Edition
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Earth Current, Ground Potential Rise, Touch & Step 1. Determination of Soil Resistivities2. Computation of Ground Potential Rise3. Computation of Surface Voltages (touch and step)4. Safety Assessment
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Verification - Measurements
Key Fact:Target Values Must be Determined in Design Phase
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The History of the IEEE Std 81First Edition:IEEE Std 81 – 1962
Revision:ANSI/IEEE Std 81-1983IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System
To Address Issues Related to Large Grounding Systems or Systems in Congested Areas:IEEE Std 81.2-1991IEEE Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems
All of Above Standards were sponsored by:
Power System Instrumentation and Measurement CommitteeOf the IEEE Power Engineering Society
In the period 2003-2004, I served as the Chair of the Substations Committee of the IEEE Power Engineering Society. I initiated and succeeded in transferring sponsorship of the standard to the Substations Committee with the plan to combine the two standards into one single standard. The unified standard has been developed in committee (working group E6, Chaired by Dennis DeCosta) and we expect to ballot it within the next 12 months.
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ANSI/IEEE Std 81-1983IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System
1. Purpose2. Scope3. Objectives of Tests4. Definitions.5. Safety Precautions While Making Ground Tests 6. General Considerations of the Problems Related to Measurements
6.1 Complexities 6.2 Test Electrodes 6.3 Stray Direct Currents 6.4 Stray Alternating Currents 6.5 Reactive Component of Impedance of a Large Grounding System 6.6 Coupling Between Test Leads 6.7 Buried Metallic Objects
7. Earth Resistivity8. Ground Impedance
8.1 General8.2 Methods of Measuring Ground Impedance8.3 Testing the Integrity of the Ground Grid8.4 Instrumentation
10. Transient Impedance11. Model Tests12. Instrumentation13. Practical Aspects of MeasurementsAnnex A Nonuniform SoilsAnnex B Determination of an Earth ModelAnnex C Theory of the Fall of Potential MethodAnnex D Bibliography
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IEEE Std 81.2-1991IEEE Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems
1. Purpose2. Scope3. References4. Safety Practices5. Factors Effecting Grounding System Measurements6. Preliminary Planning and Procedures7. Earth-Return Mutual Effects When Measuring Grounding-System Impedance
7.1 Introduction7.2 Measurement Error Due to Earth Mutual Resistances7.3 Measurement Error Due to AC Mutual Coupling7.4 Mutual Coupling to Potential Lead From Extended Ground Conductors
8. Measurement of Low-Impedance Grounding Systems by Test-Current Injection8.1 Introduction8.2 Signal Generator and Power Amplifier Source8.3 Portable Power-Generator Source8.4 Power System Low-Voltage Source
9. Measurement of Low-Impedance Grounding Systems by Power System Staged Faults10. Current Distribution in Extended Grounding Systems
10.1 Introduction10.2 Test Considerations10.3 Analysis of Current Distribution in a Grounding System10.4 Induced Current in the Angled Overhead Ground Wire10.5 Current Distribution During a Staged Fault Test
11. Transfer Impedances to Communication or Control Cables12. Step, Touch, and Voltage-Profile Measurements 13. Instrumentation Components
13.5 Fast Fourier Transform Analyzer13.6 Sine Wave Network Analyzer13.7 Staged Fault13.11 Low-Power Random Noise Source13.14 Pulse Generator13.15 Current Transformer (CT)13.16 Resistive Shunt13.17 Inductive Current Pickup13.18 Hall-Effect Probe
It was developed to address the special problems and issues associated with testing large interconnected grounding systems
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Present RevisionIEEE Std 81-XXXXGuide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System
1. Overview1.1 Purpose1.2 Scope
2. References3. Definitions4. Test Objectives5. Safety Precautions While Making Ground Tests
5.1 Station Ground Tests5.2 Special Considerations
6. General Considerations on the Problems Related to Measurement7. Earth Resistivity
7.1 General7.2 Methods of Measuring Earth Resistivity7.3 Interpretation of Measurements7.4 Guidance on performing field measurements
8. Ground Impedance9. Testing Local Potential Differences10. Integrity of Grounding Systems11. Current Splits12. Transient Impedance of Grounding System13. OtherANNEX A (INFORMATIVE) SURFACE MATERIAL RESISTIVITYANNEX B - INSTRUMENTATION
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Grounding System Measurements• Ground Impedance Measurement Methods
The 2-Point MethodThe 3-Point MethodThe Fall of Potential MethodThe 62% RuleThe Ratio MethodThe Tag Slope MethodThe Intersecting Curve MethodStaged Fault TestsDriving Point ImpedanceThe SGM Method
• Continuity/Integrity Testing• Soil Resistivity Measurements• Touch and Step Voltages• Other Tests (Tower/Pole Ground, Transfer V.)
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RaaaaR
aa
ππρ 221
4
2222 4
≅
+−
++
=
llI
VR =
ρ
ρ
1
2
ha a a
VoltMeter
Source CurrentMeter
Earth Surface
l
Four Point – Wenner Method
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Limitations of the Wenner Method
aIjxaIrRIV eem −−= ajxaraI
Vee
m −−=πρ
2
Example: Soil of 10 Ohm.meter, separation 300 feet, measurements at 150 Hz. Compute error
ρ
ρ
1
2
ha a a
VoltMeter
Source CurrentMeter
Earth Surface
l
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Basic PrinciplesBasic Arrangement
IVRg =
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The Fall of Potential MethodThe “62%” Rule
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Optimal Voltage Probe Location – The 62% Rule
⎟⎠⎞
⎜⎝⎛
−−=
rDrIVp
112πρ
⎟⎠⎞
⎜⎝⎛
−+−−=
−=
rDrDaIVV
R paa
11112πρ
⎟⎠⎞
⎜⎝⎛ −=
DaIVa
112πρ
IEEE Industry Applications Society – Atlanta ChapterJanuary 19, 2010 Meeting
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Optimal Voltage Probe Location – The 62% Rule
618034.02
51=
±−=
Dr
0111=
−−+
rDrD
⎟⎠⎞
⎜⎝⎛
−+−−=
−=
rDrDaIVV
R paa
11112πρ
CompareaI
VR ag π
ρ2
==
Ra= Rg requires that:
Solving for r/D yields:
IEEE Industry Applications Society – Atlanta ChapterJanuary 19, 2010 Meeting
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The Fall of Potential Method – 62% Rule and Two Layer Soil
D x
h ρ1
ρ2
12
12
ρρρρ
+−
=K
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Ground Impedance MeasurementsThe Fall of Potential Method – Measurement Process
THEORY
Flat Curve Portion
CurrentReturnElectrode
Current Source
VoltmeterVoltageProbe
App
aren
t Res
ista
nce
GroundUnderTest
True Ground Resistance
Distance from Ground Under Test
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 500 1000 1500 2000 2500Distance (feet)
Res
ista
nce
(Ohm
s)
The Fall of Potential MethodEarth Voltage Distribution - Actual Measurements
Contributor: American Electric Power
276 x 276 ft 2160 ft
CurrentElectrodeVoltmeter
I
R = V / I
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Factors Affecting Ground Impedance Measurement
• Difficulty reaching true remote earth reference voltage
• Effect of Auxiliary Electrode Location (Earth Current Return)
• Size and location of voltage probes
• Interaction Between Instrumentation Wires
• Interference from Overhead Lines and their Grounding
• Background 60 Hz Voltage and Harmonics
• Ground Impedance Magnitude
The Fall of Potential Method
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Fall of Potential Method ErrorsInteraction between Instrumentation Wires
⎟⎠⎞
⎜⎝⎛=
dDlM eln
20
πμ
fDeρ2160=
(d,De in feet)
where
CurrentReturnElectrode
VoltageProbe
Ground SystemUnder Test
M
Current Source I(ω)
Voltmeter d
D
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Induced Voltage Computation Example:Length 500.00 feetCurrent 1.00 Amperes
Rho 100.00 Ohm-metersFrequency 90.00 Hz
d 10.00 feetDe 2276.84 feetM 0.0001437 Henries
Voltage 0.081 Volts
Ground Mat Voltage:
Induced Voltage on Lead:
Measured Voltage:
Measured Impedance:
Measurement Error:
)(ωRIV =)(ωωMIjVmd =
indm VVV +=
)(ωIVZ m
m =
%100×−
RRZm
The Fall of Potential MethodInteraction between Instrumentation Wires
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Induced Voltage Computation Example:Length 500.00 feetCurrent 1.00 Amperes
Rho 100.00 Ohm-metersFrequency 90.00 Hz
d 10.00 feetDe 2276.84 feetM 0.0001437 Henries
Voltage 0.081 Volts
The Fall of Potential MethodInteraction between Instrumentation Wires