power system neutral/ground voltages causes, safety
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1PSERC Georgia Tech
Power System Neutral/Ground VoltagesCauses, Safety Concerns and MitigationPower System Neutral/Ground VoltagesCauses, Safety Concerns and Mitigation
A. P. Sakis MeliopoulosGeorgia Institute of Technology
A. P. Sakis MeliopoulosGeorgia Institute of Technology
September 7, 2004PSERC Tele-SeminarSeptember 7, 2004
PSERC Tele-Seminar
©2004 A. P. Sakis Meliopoulos
2PSERC Georgia Tech
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 the papers……Yet, we read in the papers……Yet, we read in the papers……
8PSERC Georgia Tech
Fundamental Facts
Humans are susceptible to even low electric currents
Several Physical Phenomena will Result in Elevated Voltages of the Neutral of Electrical Installations and Interconnected or not Connected Grounds
• Perception: 1 mA, • Let Go 20 mA, • Ventricular Fibrilation 300 mA for 3 seconds
• Perception: 1 mA, • Let Go 20 mA, • Ventricular Fibrilation 300 mA for 3 seconds
• Ground Faults • System Imbalance • High Impedance Ground Faults
• Ground Faults • System Imbalance • High Impedance Ground Faults
9PSERC Georgia Tech
Let-Go CurrentVentricular Fibrillation
SafetyPerception Current
5 10 50 100 500 1000 50000
20
40
60
80
100
Dangerous Current
Let-Go Threshold
Safe Current
Frequency (Hz)
Let-G
o C
urre
nt (M
illiam
pere
s) -
RM
S
99.5%
50% 0.5%
Perc
entil
e R
ank
Perception Currrent, mA (RMS)
99.8
0.2
5
40
80
99
PredictedCurve forWomen
Men
0 1 2
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
10PSERC Georgia Tech
Body Impedance Dependence on Voltage - CEI-1984Total Body Impedance ZT
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
11PSERC Georgia Tech
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 Been Very Serious and Fiercely Litigated
• Continuous Voltages of 30 to 60 Volts Have Resulted in Fatalities of Humans and Animals
12PSERC Georgia Tech
Earth Current / Ground Potential Rise / Safety
P r o g r a m X F M - P a g e 1 o f 1
c : \w m a s te r \ ig s \d a ta u \g p r _ e x 0 1 - M a y 1 4 , 2 0 0 0 , 0 1 :5 1 :4 4 .0 0 0 0 0 0 - 2 0 0 0 0 0 .0 s a m p le s /s e c - 2 4 0 0 0 S a m p le s
4 4 .0 2 0 4 4 .0 4 0 4 4 .0 6 0 4 4 .0 8 0 4 4 .1 0 0
-3 .9 5 2
-3 .1 4 6
-2 .3 3 9
-1 .5 3 2
-7 2 5 .8 m
8 0 .7 6 m
8 8 7 .3 m
1 .6 9 4
2 .5 0 1 P h a s e _ A _ L in e _ C u r r e n t_ _ B U S 1 0 (k A )
-1 .6 2 5
-1 .2 9 1
-9 5 6 .1 m
-6 2 1 .5 m
-2 8 6 .8 m
4 7 .8 1 m
3 8 2 .5 m
7 1 7 .1 m
1 .0 5 2 E a r th _ C u r r e n t_ _ G r o u n d _ a t_ B U S 2 0 (k A )
Important Issues• Ground Potential Rise
Changes Neutral Voltage• Customer Voltage is
Proportional to Phase to Neutral Voltage
• Grounding and Bonding• Single Ground/Multi Ground• Transmission Interconnection
BUS10 BUS20 BUS30 BUS40
G
V
V
V
A
A
A
V
V
V
A
A
A
A
A
A
L R
13PSERC Georgia Tech
Safety Assessment
33--D GraphD Graphofof
Touch Touch VoltagesVoltages
In a In a SubstationSubstation
14PSERC Georgia Tech
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 propagatethrough the neutral and reach residences, pools, offices, etc.
16PSERC Georgia Tech
Neutral Voltages Under Normal Operation
• Three 5 kW Loads on Phase A-N• Soil Resistivity: 100 Ohm-Meters• Transformer Grounding Resistor: 20 Ohms
G
1 2
Yellowjacket Substation
G
115 kV SourceZ1=Z2=j0.084 pu @500MVAZ0=j0.066 pu @500MVA
115 kV SourceZ1=Z2=j0.102 pu @500MVAZ0=j0.093 pu @500MVA
30 MVA XFMRZ=8.5% @30MVA
B
B
B
Grounding Model
SOURCE1
SUB1 SUB2 POLE1
SOURCE2
POLE3 POLE4
HOUSE2
POLE5POLE2
Vn = 281.6 mV
Vn = 180.2 mV Vn = 5.418 V Vn = 3.397 V
Vn = 77.57 mV
Vnn = 4.126 V Vn = 4.141 V
Vnn = 4.141 V
Vnn = 4.139 VVn = 3.637 V
20PSERC Georgia Tech
Can These Phenomena Be Simulated?
Grounding, Safety and Neutral Voltage Analysis Requires:
Physically-BasedDetailed-Models
21PSERC Georgia Tech
Why Physically Based Models? Consider Actual Wiring and Grounding
• Circuits are AsymmetricPhase Voltages Vary
• Circuits Are UnbalancedPhase Voltages Vary
• Finite Ground ImpedancesNonZero Neutral Voltages
• Customer Phase to Neutral Voltage
Sky Wire
HA
HB
HC
I sky
neutralI
Neutral
Counterpoise Ground Rod Ground Rod
I earth~
LA
LB
LC
Ground Mat
~
~
counterpoiseI~
CATV
22PSERC Georgia Tech
Physically Based Models Example: Three Phase Power Line
• Physically Based Model• Neutral is Represented• Asymmetry is Represented
Transmission Line Sequence Networks Close
3.495 + j 5.004
0.582 - j 121921.3 0.582 - j 121921.3
Positive Sequence Network
Negative Sequence Network
Zero Sequence Network
3.495 + j 5.004
0.582 - j 121921.3
5.688 + j 11.231
0.948 - j 295626.5
0.582 - j 121921.3
0.948 - j 295626.5
All Values in Ohms
Program WinIGS - Form OHL_REP2
• Sequence Parameter Model• Neutral is “Lost”• Asymmetry is Lost
A1B1 C1
N1
38.4 feet
3.5'
S. POLE DISTRIB. LINE (TRIANGLE)/ 12 KV
23PSERC Georgia Tech
Ground ModelingThe Method of Images
(Two Layer Soil)
21
21
σσσσ
+−
=K
Reflection Coefficient
∑∞
= ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−+++
+++=
02222
1 )2(1
)2(1
4 n CC
nB
zznDdzznDdKIV
πσ
∑∞
= ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−−++
+−++
12222
1 )2(1
)2(1
4 n CC
n
zznDdzznDdKI
πσ
24PSERC Georgia Tech
• Three 5 kW Loads on Phase A-N• Soil Resistivity: 100 Ohm-Meters• Transformer Grounding Resistor: 200 Ohms
Computer Generated Example 1Neutral Voltages Under Normal Operation
G
1 2
Yellowjacket Substation
G
115 kV SourceZ1=Z2=j0.084 pu @500MVAZ0=j0.066 pu @500MVA
115 kV SourceZ1=Z2=j0.102 pu @500MVAZ0=j0.093 pu @500MVA
30 MVA XFMRZ=8.5% @30MVA
B
B
B
Grounding Model
SOURCE1
SUB1 SUB2 POLE1
SOURCE2
POLE3 POLE4
HOUSE2
POLE5POLE2
Vn = 281.9 mV
Vn = 31.28 mV Vn = 5.891 V Vn = 3.265 V
Vn = 77.58 mV
Vnn = 4.001 V Vn = 4.016 V
Vnn = 4.016 V
Vnn = 4.015 VVn = 3.507 V
26PSERC Georgia Tech
Computer Generated Example 2Faults on Low Voltage Underground Distribution Circuit
28PSERC Georgia Tech
Neutral Voltages Under Normal OperationG G
B
B
B B
B
Fault Model
1Ph
Ground Model
BUS0001 BUS0002
BUS0003 BUS0004BUS0005 BUS0006BUS0007
BUS0008 BUS0009 BUS0010BUS0011
BUS0012
BUS0013
BUS0014 BUS0015
BUS0016BUS0017
Vnn = 298.6 mVVL2-NN = 125.0 VVL1-NN = 125.0 V
Vnn = 289.8 mVVL2-NN = 125.0 VVL1-NN = 125.0 V
Vnn = 302.6 mVVL2-NN = 121.2 VVL1-NN = 121.1 V
Vnn = 418.5 mVVL2-NN = 125.0 VVL1-NN = 125.0 V
Vnn = 380.3 mVVL2-NN = 125.0 VVL1-NN = 125.0 V
Vnn = 345.9 mVVL2-NN = 117.5 VVL1-NN = 117.5 V
Vnn = 436.2 mVVgg = 343.8 mVVL2-NN = 123.1 VVL1-NN = 122.9 V
Vnn = 344.4 mVVL2-NN = 121.1 VVL1-NN = 121.1 V
Vnn = 327.8 mVVL2-NN = 121.2 VVL1-NN = 121.1 V
Vnn = 5.704 VVgg = 6.629 mVVL2-NN = 128.3 VVL1-NN = 114.3 V
Vgg = 120.0 V
29PSERC Georgia Tech
Neutral Voltages During Permanent FaultG G
B
B
B B
B
Fault Model
1Ph
Ground Model
BUS0001 BUS0002
BUS0003 BUS0004BUS0005 BUS0006BUS0007
BUS0008 BUS0009 BUS0010BUS0011
BUS0012
BUS0013
BUS0014 BUS0015
BUS0016BUS0017
Vnn = 2.223 VVL2-NN = 125.0 VVL1-NN = 125.0 V
Vnn = 2.201 VVL2-NN = 125.0 VVL1-NN = 125.0 V
Vnn = 2.203 VVL2-NN = 121.2 VVL1-NN = 121.0 V
Vnn = 2.662 VVL2-NN = 125.0 VVL1-NN = 125.0 V
Vnn = 2.562 VVL2-NN = 125.0 VVL1-NN = 125.0 V
Vnn = 2.562 VVL2-NN = 117.6 VVL1-NN = 117.2 V
Vnn = 4.609 VVgg = 3.794 VVL2-NN = 123.1 VVL1-NN = 122.0 V
Vnn = 2.382 VVL2-NN = 121.2 VVL1-NN = 121.0 V
Vnn = 2.340 VVL2-NN = 121.2 VVL1-NN = 121.0 V
Vnn = 16.70 VVgg = 64.03 VVL2-NN = 126.5 VVL1-NN = 76.23 V
Vgg = 64.03 V
31PSERC Georgia Tech
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
32PSERC Georgia Tech
• Three 5 kW Loads on Phase A-B• Soil Resistivity: 100 Ohm-Meters• Transformer Grounding Resistor: 200 Ohms
Example of Neutral Voltages MitigationElectric Loads Line-to-Line
G
1 2
Yellowjacket Substation
G
115 kV SourceZ1=Z2=j0.084 pu @500MVAZ0=j0.066 pu @500MVA
115 kV SourceZ1=Z2=j0.102 pu @500MVAZ0=j0.093 pu @500MVA
30 MVA XFMRZ=8.5% @30MVA
B
B
B
Grounding Model
SOURCE1
SUB1 SUB2 POLE1
SOURCE2
POLE3 POLE4
HOUSE2
POLE5POLE2
Vn = 280.9 mV
Vn = 19.94 mV Vn = 56.34 mV Vn = 33.00 mV
Vn = 78.80 mV
Vnn = 38.06 mV Vn = 38.77 mV
Vnn = 38.77 mV
Vnn = 38.76 mVVn = 35.49 mV
33PSERC Georgia Tech
For more in-depth information
Integrated Grounding System Design and Testing
March 22-25,2005
Grounding, Harmonics, & Electromagnetic Influence Design Practices
May 16-18,2005
Power Distribution System Grounding and Transients
September 21-23,2004
34PSERC Georgia Tech
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.
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