power system neutral/ground voltages causes, safety concerns … · power system neutral/ground...

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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

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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……

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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

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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

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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



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


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


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


Safety Assessment

33--D GraphD Graphofof

Touch Touch VoltagesVoltages

In a In a SubstationSubstation


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.


Stray Voltages in Overhead Distribution

CircuitsGeneration Mechanism


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


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


Shock Hazard


Pool Electrical Equipment Wiring


Line Touching CasePermanent Ground Fault


Can These Phenomena Be Simulated?

Grounding, Safety and Neutral Voltage Analysis Requires:

Physically-BasedDetailed-Models


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


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


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


• 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


G




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


Earth Surface Voltages Under Normal Operation


Computer Generated Example 2Faults on Low Voltage Underground Distribution Circuit


Line Touching Case


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 = 436.2 mVVgg = 343.8 mVVL2-NN = 123.1 VVL1-NN = 122.9 V



Vnn = 5.704 VVgg = 6.629 mVVL2-NN = 128.3 VVL1-NN = 114.3 V

Vgg = 120.0 V


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


Earth Surface Voltages During Fault


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


• 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


G




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


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


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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power system neutral/ground voltages causes, safety concerns … · power system neutral/ground...

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