design of a low impedancedesign of a low impedance ... of a low... · grounding system for telecom...
TRANSCRIPT
Design of a Low ImpedanceDesign of a Low Impedance Grounding System for Telecom
A li tiApplications
Mitchell GuthrieIndependent Engineering Consultant
Alain RousseauSEFTIM
ObjectivesObjectives• Discuss typical existing performance yp g p
criteria• Discuss relationship between resistanceDiscuss relationship between resistance
and impedance• Identify broadband response of grounding• Identify broadband response of grounding
system components and systemsRelate gro nding s stem impedance to• Relate grounding system impedance to telecom applications
• Discuss impedance performance criteria
Impulse Current EffectsLow frequency content
energy zone• V(t) = i(t) R + L (di/dt)• High frequency g q y
content is important for overvoltages
• Low frequency content is important f di i t dfor energy dissipated in the earthing system
High frequency content
sparkover zone systemp
Lightning Current PathsLightning Current Paths
Impulse Current PrinciplesImpulse Current Principles• Lightning takes the lowest impedance path to remote
earthearth – lowest resistance and lowest inductance paths preferred
• High inductance path leads to high voltage on LPS– current cannot change instantaneously through an inductance– current cannot change instantaneously through an inductance– overvoltage is created and spark over to a less inductive path
can result • Lightning currents flow radially from the point of injection g g y p j
into earth as a function of earth resistivity or conductors (earth electrodes) embedded in the earth
• Providing multiple paths for current injection into the g p p jearth decreases current density at earth / electrode interface; reducing overvoltages
Grounding System Performance Criteria
• Resistance-to-earth– 5 Ω typical for Telecom applications – Can be as low as 1 - 2 Ω– NFPA 70 – 25 Ω or drive 2nd electrode– DoD / DOE Explosives – <25 Ω
IEC 62305 3 10 Ω d d– IEC 62305-3 - < 10 Ω recommended– NFPA 780 / UL 96A /– no specification
Proposed TIA 607 B• Proposed TIA-607-B– Grounding electrode: conductor … for the purpose of
providing a low impedance path to earthproviding a low impedance path to earth
Circuit Model
LjR ω+ LjRZ ω+=CjG ω+
Impedance vs ResistanceImpedance vs. Resistance• v(t) = i(t) R + L (di/dt) + i(t) / C = i(t) Z• Resistance
– often dominated by earth resistivity– increases with frequency due to skin effect
• Inductance– influenced by geometry of conductors / electrodes– increased by bends in conductor routing
C it• Capacitance– dominated by electrode-to-earth interface
Impedance Measurement MethodImpedance Measurement Method
Vertical Rod in 300 Ωm soilVertical Rod in 300 Ωm soil IMPEDANCE in Ω
vertical rod 300 Ω m
FREQUENCY IN Hz
300 Ω.m
Vertical Rod in 300 Ωm soilVertical Rod in 300 Ωm soil• Impedance decreases with frequency due to
d i t itdominate capacitance• Impedance high even in low resistivity soil (70 to
120 Ω)120 Ω).• Additional experiments (data not shown)
Same behaviour found for horizontal grid in 300 Ωm– Same behaviour found for horizontal grid in 300 Ωm soil
– 3 rods in triangle in 200 Ω.m soil : Z ≈ constant (30 –40 Ω)40 Ω)
– Earth Enhancement Compound decreases local resistivity and improves contact (decreases R and Z)
Crow Foot Arrangement in Rocky Soil
IMPEDANCE in Ω
crow footFREQUENCY IN Hz
crow footrocky soil
Crow Foot Arrangement in Rocky Soil
• 200 Ωm soil • Low increase of impedance (Z)• Less than 20 ohms change across entire• Less than 20 ohms change across entire
frequency spectrumEffi i t li ht i t ti di• Efficient lightning protection grounding system
25 meter long Horizontal Tape25 meter long Horizontal Tape IMPEDANCE in Ω
25 m horizontal tape 110 Ω m
FREQUENCY IN Hz
110 Ω.m
25 meter long Horizontal Tape25 meter long Horizontal Tape• Buried 0.5 meters in 110 Ωm soil • Low increase of impedance (Z)• 20 ohms change from 10 Hz to 1 MHzg• Additional experiments (data not shown)
– Z almost doubles for 50 meters of tapeZ almost doubles for 50 meters of tape– Similar behavior from 20 m to 50 m vertical
well in 200 Ωm soil 50 m deep well – Z = 10 Ω at low frequency / 70 Ω at 1MHz
Telecom ApplicationTelecom Application
• 3 legged tower• Equipment cabinet• Equipment cabinet• Driven rod per leg• Driven rod for cabinet• Driven rod for cabinet• Ground loop
conductor tyingconductor tying together driven rods
Tower Leg ImpedancesTower Leg ImpedancesSouth Tower Leg East Tower Leg
Tower Test PointsTower Test Points
South Tower Leg East Tower Legg
Cabinet Ground ImpedanceCabinet Ground Impedance
Summary of ObservationsSummary of Observations• Data from tower legs similar (w/in 3 Ω)g ( )
Effect of ground rods noted @ 202 kHz
• R (tower leg – cabinet bus) = 1 mΩ( g )• Bend in solid wire between copper bus bar
and grounding electrode shows up as Land grounding electrode shows up as L• Rgnd Cabinet = South tower leg = 9 Ω
R d (1MH ) C bi t 79 2 t 52Ω• Rgnd (1MHz): Cabinet =79.2; tower = 52Ω• All 3 less than 10 Ω @ low frequency but
test “bad” given HF criteria
Impedance Acceptance Criteriap p
Case StudiesCase Studies• A: Building with large grounding systemg g g g y• B: Factory Extension• C: Metal Silos• C: Metal Silos• D: Metallic-framed Shed• E: Group of Chimneys• F: Metallic Tanks
ES 1002 Mesure 14
Nombre de points 20
80
60
20
40
)
Z
0
20
100 1000 10000 100000 1000000
Z R
X(O
HM
)
R
X
-20
-60
-40
Fréquence (Hz)
Case A: Building with large earthing systemFréquence (Hz)
Case B: Factory Extension
• Extension of existing• Extension of existingfactory
• Bad soil, mainly rocks• Crow foot system• R = 150 Ω• Clearly a bad earthing• Clearly a bad earthing
system
Case B: Building with a crow foot in bad soilCase B: Building with a crow foot in bad soil
Case C: Metallic Silos
• Small contact surface between metal and soil
Case C: Metallic Silos
• Small contact surface between metal and soil• R = 15 Ω
130
ohms
8090
100110120
4050607080
R
Z
-100
102030
R
X
-40-30-2010
10 100 1000 10000 100000 1000000
X
Case C: Silo in contact with soil
Frequency (Hz)
Case C: Silo in contact with soil
Case D: Large shed with metallic frame
• R = 4 Ω
g
• R = 4 Ω
130
ohms
8090
100110120
304050607080
R
Z
-100
102030 R
X
-50-40-30-20
10 100 1000 10000 100000 1000000
X
Case D: Shed with metallic frame
Frequency (Hz)
Case D: Shed with metallic frame
Case E: Group of Chimneys
• Earthing on each chimney
Case E: Group of Chimneys
Earthing on each chimney• Connected together
by copper tapey pp p• R = 5 Ω• Good low frequencyresistance but gets very high at frequencies above 100 kHz• bad lightning
thi tearthing system
120130
ohms
708090
100110120
102030405060
R
Z
50-40-30-20-10
010
X
-80-70-60-50
10 100 1000 10000 100000 1000000Frequency (Hz)
Case E: Group of chimneys
Frequency (Hz)
Case F: Metallic Tanks
• Tank diameter 6 meters
Case F: Metallic Tanks
• Tank diameter 6 meters• Near the sea• Concrete base
immersed insand/watermixture
• No dedicatedearthing systemg y
• R = 1 Ω
130
ohms
8090
100110120
203040506070
R
Z
-30-20-10
01020
X
-80-70-60-50-40
10 100 1000 10000 100000 1000000
Case F: Metallic tank near the sea
10 100 1000 10000 100000 1000000Frequency (Hz)
Proposed Criteria Development• Difficult to establish a single criteria for quality of
Proposed Criteria Development
earthing system to dissipate lightning current while minimizing overvoltagesE l f id ti• Examples for consideration:– 30 Ω from low frequency to 1 MHz
10 Ω at low frequency and 100 Ω at high frequencies ?– 10 Ω at low frequency and 100 Ω at high frequencies ?• To evaluate the earthing systems we will calculate
the voltage generated by the injection of a 10 kAthe voltage generated by the injection of a 10 kA 1/20 current pulse into the earthing impedance measurements reported earlier.p
• Determine equivalent resistance (RHF)
Summary of ResultsSummary of ResultsZ Impedance value (Ω)
Case A Case B Case C Case D Case E Case F
RLF (Ω) 4 150 15 4 5 1
RHF (Ω) 47 203 35 22 47 16
( ) 46 184 40 22 9 4 1 9Mean Z (Ω) 46.7 184 40.7 22.9 45.5 17.9
The mean value of the measured impedance between 63 kHz and 1 MHz(Mean Z) computed from data provided by high frequency earth impedancetest equipment.
This value is consistent with RHF and is suggested as criterion
Proposed Criteria• Based on RHF (or Zmean)
Proposed Criteria
HF ( )
RHF ≤ 10 Ω : very good earthing10 Ω < R ≤ 30 Ω : good earthing10 Ω < RHF ≤ 30 Ω : good earthing30 Ω < RHF ≤ 40 Ω : acceptable earthing40 Ω < RHF : bad earthing
• Quality of earthing is based on Q y gexperience and has been presented to the scientific community (ICLP SIPDA)the scientific community (ICLP, SIPDA)
Application of the Proposed C i iCriteria
Z Impedance value (Ω)
Case A Case B Case C Case D Case E Case FRLF (Ω) 4 150 15 4 5 1RLF (Ω) 4 150 15 4 5 1RHF (Ω) 47 203 35 22 47 16
Mean Z (Ω) 46.7 184 40.7 22.9 45.5 17.9
• Earthing systems for A, B and E are considered bad
eq. length (m) 24 102 18 11 24 8
Earthing systems for A, B and E are considered bad earthing systems
• Case C is acceptable• Cases D and F are good earthing systems• Cases D and F are good earthing systems.
Summary
Earthing Impedance FactorsEarthing Impedance Factors
LjRZ ω+
CjGLjRZ ω+
=• R is function of material used for grounding system
ft li ibl b t fl t d t b tt th d t hi h
CjG
– often negligible but flat conductor better than round at high frequency (skin effect) given same cross sectional area
• G - earth conductance: related to earth resistivity and t t i t b t th l t d d ilcontact resistance between earth electrode and soil
– may be increased through use of additives to increase contact between soil and earth conductors.
Earthing Impedance FactorsEarthing Impedance FactorsLjRZ ω+=
• L – Inductance of earthing system hardware
CjGZ ω+
L Inductance of earthing system hardware– reduced by use of shorter multiple conductors instead of single
one of equivalent total length
C C it b t th d thi• C – Capacitance between earth and earthing electrodes– reduced by increasing earth contact area (plates and flat– reduced by increasing earth contact area (plates and flat
conductors better than round conductors)– reduced by increasing contact between electrode and earth such
as through earth enhancing materialas through earth enhancing material
Summary and ConclusionsSummary and Conclusions• High frequency behavior of an earthing g q y g
system is different from low frequencyZ ≠ R !Z ≠ R !
• Microsecond rise time of lightning current pulse leads to high frequency componentspulse leads to high frequency components
• High frequency lightning current content l d l blleads to overvoltage problems
Summary and ConclusionsSummary and ConclusionsEarthing system impedance reduced by:
– Lowering resistance of electrode(s) and associated conductors (typically negligible)Lowering inductance of electrode(s) and associated– Lowering inductance of electrode(s) and associated conductors
– Lowering resistance-to-earthg– Increasing capacitance of earth-electrode interface
Increasing length of electrodes can reduce g gresistance-to-earth but resulting effectiveness offset by increased inductance (impedance)
Summary and ConclusionsSummary and Conclusions• Long earthing conductors good for R but g g g
not Z– Multiple paths better for high frequency p p g q y
response• Flat conductors and plates increase p
capacitance and reduce impedance• Earthing enhancing compounds can helpEarthing enhancing compounds can help
reduce R and Z
Summary and ConclusionsSummary and Conclusions• Engineering criteria in standards are better than
citing earthing resistance req irementciting earthing resistance requirement• Devices exist for measurement of high
frequency response of grounding systemsfrequency response of grounding systems• Recommendations available for standards /
specifications incorporating impedance values p p g pfor earthing systems– Average value of impedance between 60 kHz and 1
MHz (5 kHz not high enough to identify critical value)MHz (5 kHz not high enough to identify critical value)– High Frequency Resistance based on 10 kA impulse(RHF < 40 Ω)
Thank You
QUESTIONS ?