Power System Neutral/Ground Voltages Causes, Safety Concerns and Mitigation

Power System Neutral/Ground Voltages Causes, Safety Concerns and Mitigation A. P. Sakis Meliopoulos Georgia Institute of Technology September 7, 2004 Tele-Seminar 2004 A. P. Sakis Meliopoulos 1

Power System Design Principles for Safety Any Individual Should be Safe in the Vicinity of an Electrical Installation An Individual Touching ANY Grounded Structure Should be Safe Under All Foreseeable Adverse Conditions Electrical System Installations Should be Designed so that Meet Above Requirements Yet, we read in in the papers 2

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Fundamental Facts Humans are susceptible to even low electric currents Perception: 1 ma, ma, Let Let Go Go 20 20 ma, ma, Ventricular Fibrilation 300 300 ma mafor for 3 seconds Several Physical Phenomena will Result in Elevated Voltages of the Neutral of Electrical Installations and Interconnected or not Connected Grounds Ground Faults System Imbalance High High Impedance Ground Faults 8

Safety Perception Current Let-Go Current Ventricular Fibrillation 99.8 100 Percentile Rank 99 80 40 5 Predicted Curve for Women Men 0.2 0 1 2 Perception Currrent, ma (RMS) Let-Go Current (Milliamperes) - RMS 80 60 40 20 0 99.5% Dangerous Current 50% 0.5% Let-Go Threshold Safe Current 5 10 50 100 500 1000 5000 Frequency (Hz) Fibrillating Current (ma RMS) 300 200 100 0 Kiselev Dogs Dogs Ferris Dogs sheep calves pigs Minimum Fibrillating Current (0.5%) Maximum Non-Fibrillating Current (0.5%) 0 20 40 60 80 100 ody Weight (kg) 9

ody Impedance Dependence on Voltage - CEI-1984 Total ody Impedance Z T Values for the total body impedance (Ohms) that are not exceeded for a percentage (percentile rank) of Touch Voltage 5% of the population 50% of the population 95% of the population 25 50 75 100 125 220 700 1000 Asymptotic Value 1750 1450 1250 1200 1125 1000 750 700 650 3250 2625 2200 1875 1625 1350 1100 1050 750 6100 4375 3500 3200 2875 2125 1550 1500 850 10

Important Facts Humans: About 2 Volts of Touch Voltage Will Result in Perception of Electrtic Shock. Much Lower for Kids Chicken: About 0.9 Volts of Voltage Will Make Chickens Stay Away Importance??? Claims of Stray Voltage Effects on Cows and Fish Have een Very Serious and Fiercely Litigated Continuous Voltages of 30 to 60 Volts Have Resulted in Fatalities of Humans and Animals 11

Earth Current / Ground Potential Rise / Safety A V A A V A V A A US10 US20 US30 US40 L R G Program XFM - Page 1 of 1 c:\w m aster\igs\datau\gpr_ex01 - May 14, 2000, 01:51:44.000000-200000.0 sam ples/sec - 24000 Sam ples Phase_A_Line_Current US10 (ka) 2.501 1.694 V V V A A A 887.3 m 80.76 m -725.8 m -1.532-2.339-3.146-3.952 Earth_Current Ground_at_US20 (ka) 1.052 717.1 m 382.5 m 47.81 m -286.8 m -621.5 m -956.1 m -1.291-1.625 44.020 44.040 44.060 44.080 44.100 Important Issues Ground Potential Rise Changes Neutral Voltage Customer Voltage is Proportional to Phase to Neutral Voltage Grounding and onding Single Ground/Multi Ground Transmission Interconnection 12

Safety Assessment 3-D D Graph of Touch Voltages In a Substation 13

The IEEE Std 80 Addresses Safety in Utility Substation. In this talk, we will not discuss safety during HV faults. It Is important to mention that because the grounding system is continuous, a high voltage elevation of the substation ground will propagate through the neutral and reach residences, pools, offices, etc. 14

Stray Voltages in Overhead Distribution Circuits Generation Mechanism 15

Neutral Voltages Under Normal Operation G Vn = 281.6 mv SOURCE1 115 kv Source Z1=Z2=j0.084 pu @500MVA Z0=j0.066 pu @500MVA Yellowjacket Substation 30 MVA XFMR Z=8.5% @30MVA Three 5 kw Loads on Phase A-N Soil Resistivity: 100 Ohm-Meters Transformer Grounding Resistor: 20 Ohms 1 2 Vn = 180.2 mv Vn = 5.418 V Vn = 3.397 V SU1 SU2 POLE1 Vn = 3.637 V POLE2 Vnn = 4.126 V POLE3 Vn = 4.141 V POLE4 Vnn = 4.139 V POLE5 Vnn = 4.141 V HOUSE2 Grounding Model G Vn = 77.57 mv SOURCE2 115 kv Source Z1=Z2=j0.102 pu @500MVA Z0=j0.093 pu @500MVA 16

Shock Hazard 17

Pool Electrical Equipment Wiring 18

Line Touching Case Permanent Ground Fault 19

Can These Phenomena e Simulated? Grounding, Safety and Neutral Voltage Analysis Requires: Physically-ased Detailed-Models 20

Why Physically ased Models? Consider Actual Wiring and Grounding ~ I sky Sky Wire HA H HC Neutral LA L LC Circuits are Asymmetric Phase Voltages Vary Ground Mat Counterpoise Ground Rod Ground Rod ~ I counterpoise ~ I earth ~ I neutral CATV Circuits Are Unbalanced Phase Voltages Vary Finite Ground Impedances NonZero Neutral Voltages Customer Phase to Neutral Voltage 21

Physically ased Models Example: Three Phase Power Line Physically ased Model Neutral is Represented Asymmetry is Represented 1 3.5' A1 C1 N1 Sequence Parameter Model Neutral is Lost Asymmetry is Lost Transmission Line Sequence Networks Close Positive Sequence Network 3.495 + j 5.004 All Values in Ohms 0.582 - j 121921.3 0.582 - j 121921.3 38.4 feet Negative Sequence Network 0.582 - j 121921.3 3.495 + j 5.004 0.582 - j 121921.3 Zero Sequence Network 5.688 + j 11.231 0.948 - j 295626.5 0.948 - j 295626.5 S. POLE DISTRI. LINE (TRIANGLE)/ 12 KV Program WinIGS - Form OHL_REP2 22

Ground Modeling The Method of Images (Two Layer Soil) Reflection Coefficient K = σ σ 1 1 σ + σ 2 2 V = + I n K + 4πσ 2 2 2 2 1 n= 0 d + (2nD + z + zc ) d + (2nD + z zc ) I 1 n K + 4πσ 2 2 2 2 1 n= 1 d + (2nD z + zc ) d + (2nD z zc ) 1 1 1 23

G Computer Generated Example 1 Neutral Voltages Under Normal Operation Vn = 281.9 mv SOURCE1 115 kv Source Z1=Z2=j0.084 pu @500MVA Z0=j0.066 pu @500MVA Yellowjacket Substation 30 MVA XFMR Z=8.5% @30MVA Three 5 kw Loads on Phase A-N Soil Resistivity: 100 Ohm-Meters Transformer Grounding Resistor: 200 Ohms 1 2 Vn = 31.28 mv Vn = 5.891 V Vn = 3.265 V SU1 SU2 POLE1 Vn = 3.507 V POLE2 Vnn = 4.001 V POLE3 Vn = 4.016 V POLE4 Vnn = 4.015 V POLE5 Vnn = 4.016 V HOUSE2 Grounding Model G Vn = 77.58 mv SOURCE2 115 kv Source Z1=Z2=j0.102 pu @500MVA Z0=j0.093 pu @500MVA 24

Earth Surface Voltages Under Normal Operation 25

Computer Generated Example 2 Faults on Low Voltage Underground Distribution Circuit 26

Line Touching Case 27

Neutral Voltages Under Normal Operation G US0001 G US0002 US0003 VL1-NN = 125.0 V VL2-NN = 125.0 V Vnn = 298.6 mv US0005 VL1-NN = 121.1 V VL2-NN = 121.2 V Vnn = 302.6 mv US0007 VL1-NN = 125.0 V VL2-NN = 125.0 V Vnn = 289.8 mv US0006 US0004 VL1-NN = 121.1 V VL2-NN = 121.1 V Vnn = 344.4 mv US0014 VL1-NN = 117.5 V VL2-NN = 117.5 V Vnn = 345.9 mv US0012 VL1-NN = 121.1 V VL2-NN = 121.2 V Vnn = 327.8 mv US0015 VL1-NN = 122.9 V VL1-NN = 125.0 V VL2-NN = 123.1 V VL1-NN = 125.0 V VL2-NN = 125.0 V Vgg = 343.8 mv VL2-NN = 125.0 V Vnn = 418.5 mv Vnn = 436.2 mv Vnn = 380.3 mv US0008 US0009 US0013 US0011 US0010 Ground Model VL1-NN = 114.3 V VL2-NN = 128.3 V Fault Model Vgg = 6.629 mv Vgg = 120.0 V Vnn = 5.704 V US0017 US0016 1Ph 28

Neutral Voltages During Permanent Fault G US0001 G US0002 US0003 VL1-NN = 125.0 V VL2-NN = 125.0 V Vnn = 2.223 V US0005 VL1-NN = 121.0 V VL2-NN = 121.2 V Vnn = 2.203 V US0007 VL1-NN = 125.0 V VL2-NN = 125.0 V Vnn = 2.201 V US0006 US0004 VL1-NN = 121.0 V VL2-NN = 121.2 V Vnn = 2.382 V US0014 VL1-NN = 117.2 V VL2-NN = 117.6 V Vnn = 2.562 V US0012 VL1-NN = 121.0 V VL2-NN = 121.2 V Vnn = 2.340 V US0015 VL1-NN = 122.0 V VL1-NN = 125.0 V VL2-NN = 123.1 V VL1-NN = 125.0 V VL2-NN = 125.0 V Vgg = 3.794 V VL2-NN = 125.0 V Vnn = 2.662 V Vnn = 4.609 V Vnn = 2.562 V US0008 US0009 US0013 US0011 US0010 Ground Model VL1-NN = 76.23 V VL2-NN = 126.5 V Fault Model Vgg = 64.03 V Vgg = 64.03 V Vnn = 16.70 V US0017 US0016 1Ph 29

Earth Surface Voltages During Fault 30

Detection of Stray Voltages Many approaches are being pursued Mitigation of Stray Voltages Minimize system imbalance Improve Grounds/neutrals Avoidance of Neutral/Ground Voltages from Permanent Faults Improve grounds Provide ground fault protection Need to re-design some old systems Avoid same design mistakes in new systems 31

G Vn = 280.9 mv SOURCE1 115 kv Source Example of Neutral Voltages Mitigation Electric Loads Line-to-Line Z1=Z2=j0.084 pu @500MVA Z0=j0.066 pu @500MVA Yellowjacket Substation 30 MVA XFMR Z=8.5% @30MVA Three 5 kw Loads on Phase A- Soil Resistivity: 100 Ohm-Meters Transformer Grounding Resistor: 200 Ohms 1 2 Vn = 19.94 mv Vn = 56.34 mv Vn = 33.00 mv SU1 SU2 POLE1 Vn = 35.49 mv POLE2 Vnn = 38.06 mv POLE3 Vn = 38.77 mv POLE4 Vnn = 38.76 mv POLE5 Vnn = 38.77 mv HOUSE2 Grounding Model G Vn = 78.80 mv SOURCE2 115 kv Source Z1=Z2=j0.102 pu @500MVA Z0=j0.093 pu @500MVA 32

For more in-depth information Integrated Grounding System Design and Testing Grounding, Harmonics, & Electromagnetic Influence Design Practices Power Distribution System Grounding and Transients March 22-25, 2005 May 16-18, 2005 September 21-23, 2004 33

Conclusions Elevated voltages in neutrals and grounds is reality. Proper design practices can mitigate these voltages. Physically based modeling provides the basis to study simultaneously grounding, neutral voltages and safety. Disadvantage: More Complex Models. Observation: Electric power installations can be designed to be safe at low cost. Retrofitting is relatively expensive. However, there is a substantial percentage of the industry that does not pay attention to this issue at the design phase. 34

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