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Zed-Meter: Basic Operation
Eskom Workshop
July 2014Fabio Bologna
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Overview of presentation
What is the Zed-Meter
Basic Principle of Operation
Typical Waveform
Zed-Meter Instrument &
Accessories
Lead Orientation
Software
Calibration Test Comparison between High
and Low Frequency results
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What is the Zed-Meter
The Zed-Meter is an instrument thatmeasures the grounding impedanceoftransmission line towers
Differs from conventional methods by:
Not requiring the removal or isolation of shieldwires
Providing the impedance value of grounding -most relevant for lightning performance, notpower frequency resistance
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Ground Resistance vs. Ground Impedance
Conventional instruments for measuring earthresistance operate at low frequencies (typically
105 to 150 Hz)
These instruments only provide the potential rise
of the IR term:
V = L dI/dt + R I
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Philosophy Behind Zed-Meter
The lightning performance (number of flashovers) oftransmission lines is related to the values of the tower
grounding impedance along the line length
High-frequency response of the ground electrode is
important
Difference between the lightning impedance of a
transmission tower grounding system and the impedance of
the same system at power frequency.
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Philosophy Behind Zed-Meter (cont.)
Under lightning the peak stress on insulators occurs beforeadjacent towers have had a chance to react and help outby sharing the surge current.
2s -two-way propagation time to the nearest pair oftowers, 300 m or 1000 away
Under low frequency the impedance of the groundingsystem at a particular structure is determined by theparallel combination of the impedance of the local groundelectrode and the chain impedance of many towersconnected in parallel via the overhead ground wires.
Parallel chain impedance of neighboring structures isusually much lower than that of the local groundelectrode.
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Important Features
Does not require shield (static) wires to be removed
Provides an indication of lightning performance
Faster to implement i.e. Lower Cost
Non-Lethal Voltages for operator and public safety Small / Flexible Lead Foot Print
limited to ROW
Portable does not require large power supply
Low cost equipment
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Zed-Meter & Accessories
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Basic Principle of Operation
Inject a lightning-like transient current into thetower base
Measure the potential rise at the tower baserelative to remote ground
Compute the ratio of the potential rise to inputcurrent as a function of time
Impedance measurement taken aftereffect of thetower surge response has rung down.
Impedance measurement taken beforethe effectsof adjacent towers have time to affect the reading
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Equivalent Circuit
I1 I2
V
Zed-Meter
Propagation line
Z
Propagation line
Z
Ground wire
Zgw
Ground wire
Zgw
Ground electrode
Impedance ZT
Potential Lead Current Lead
Connection to
the structure
Pulse generator and
wave shaping circuit
Current measurement
Voltage measurement
I1 I2
V
Zed-Meter
I1 I2
V
Zed-Meter
Propagation line
Z
Propagation line
Z
Ground wire
Zgw
Ground wire
Zgw
Ground electrode
Impedance ZT
Potential Lead Current Lead
Connection to
the structure
Pulse generator and
wave shaping circuit
Current measurement
Voltage measurement
Current waveform
Voltage waveform
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Schematic Connection Representation
Current Lead Potential LeadConnection
to structure
Zed-Meter
90o-180o
90-125 m90-125 m
Current Lead Potential LeadConnection
to structure
Zed-Meter
90o-180o
90-125 m90-125 m
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How the Zed-Meter Works:Apply Pulse
Vmeas
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How the Zed-Meter Works:Pulse Moves at Speed of Light up Tower
Vmeas
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How the Zed-Meter Works:Other Three Legs Now Absorb Current
Vmeas
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How the Zed-Meter Works: Speed alongReaction Wire is Less than Speed of Light
Vmeas
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Time to use
Measurement
How the Zed-Meter Works: OverheadGroundwire Surge Impedance is Constant
Vmeas
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Time to use
Measurement
How the Zed-Meter Works:Situation Stable for Long 600-ns Time
Vmeas
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Time to use
Measurement
How the Zed-Meter Works:Remote Potential Settles to Constant Value
Vmeas
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Zed-Meter Software
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Zed-Meter Software: Resistor Test
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Resistor Test
Test instrument before going out into the field
Potential Lead
Connection to the structure
Current LeadR
c
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10 0hm Resistor Test: Typical Results
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Open circuit testing of leads
Measure voltage on leads Voltage on the leads caused by electromagnetic coupling to the
energized phase conductors of the line
Potential > 50 Vrms, most utilities call for the use of insulating
gloves or other countermeasures
Zed-Meter can generate good results even if the induced pickup
exceeds 100 V because the current transducers are dielectrically
isolated from the leads
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Zed-Meter Software: StandardTesting
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Dipole Test
Test integrity of lead layout
Impedance of the two leads are measured.
Provides information on the condition of the current and potential leads
and the adequacy laid out pattern utilized
Current in both leads should be the same
Currents and voltage should rise quickly, stabilize within 500 ns, andshould remain relatively constant for at least 300 ns
Test is prompted by Zed-Meter software
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Lead arrangements for Tests on Towers with
Buried Counterpoise
Causes coupling between test leads and counterpoisewires
Tends to reduce the measured impedance, resulting in a
low estimate of the actual impedance
In such cases orientate test leads at right angles to thecounterpoise to reduce coupling.
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Lead Orientation
Current lead
Potential lead
Line
direction
Line
direc
tion
Zed-Meter
Connection to
structure
Current lead
Potential lead
Line
direction
Line
direc
tion
Zed-Meter
Connection to
structure
Current lead
Potential lead
L
inedirection
Linedirection
Zed-Meter
Connection to
structure
Current lead
Potential lead
L
inedirection
Linedirection
Zed-Meter
Connection to
structure
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Lead Orientation: Zigzag Leads
Current lead
Potential lead
Linedirect
ion
Linedirec
tion
Zed-Meter
Connection to
structure
Current lead
Potential lead
Linedirect
ion
Linedirec
tion
Zed-Meter
Connection to
structure
ZZ1: In line:
Meander Potential
lead Current lead
Potential lead
Linedirec
tion
Linedirection
Zed-Meter
Connection to
structure
Current lead
Potential lead
Linedirec
tion
Linedirection
Zed-Meter
Connection to
structure
ZZ2: In line:
Meander Both
leads
Current lead
Potential lead
Line
direction
Line
direction
Zed-Meter
Connection to
structure
Current lead
Potential lead
Line
direction
Line
direction
Zed-Meter
Connection to
structure
ZZ3:
Perpendicular:
Meander Potential
lead
Current lead
Potential lead
Li
ne
direction
L
ine
direction
Zed-Meter
Connection to
structure
Current lead
Potential lead
Li
ne
direction
L
ine
direction
Zed-Meter
Connection to
structure
ZZ4:Perpendicular:
Meander Both
leads
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Dipole Test: Typical Results (Ice , 1000ft leads)
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Zed-Meter Software: ObliqueShortened Lead Method
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Oblique Shortened Lead Method: TestFeatures
Structure ImpedanceSoil Resistivity
Electrode Perimeter
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Differences between Standard and Oblique leadLayouts
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Oblique Lead Layout
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Oblique Lead Layout (2)
Fi ld G id Obli Sh t d
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Field Guide: Oblique ShortenedLead Method
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Zed-Meter Software: ObliqueShortened Lead Method
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Tower to Counterpoise Measurement
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Tower to Counterpoise Measurement:Lead Connection
Generally, if the measured series impedance is
less than 30 at 1500 ns, the tower is well
grounded.
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Live Demo
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Grounding of Current and Potential Leads
In most cases it is not necessary to ground (unless you areusing the Oblique method)
Impedance measurement is usually performed in the
time beforethe reflection from the end of the lead returns
back to the measuring point
Both wires are essentially grounded through their
capacitance to ground
Exception: Reduce the effects of electrostatic pickup
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Connection to Structure
Lattice Steel Pole
C d ti bj t i P i i t t th Z d
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Conducting objects in Proximity to the Zed-
Meter Leads
Presence of conducting objects, such as fences, vehicleswill tend to reduce the measured potential rise
Coupling
Recommended that the lead be separated from any large
conducting objects by at least 1 m.
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Zigzag Leads Effect of Zigzag (Results [])
Soil Resist ivity 50 m 1000 m 20,000 m
Reference
Configuration
5 46 261
ZZ1 5 47 262
ZZ2 4 48 252
ZZ3 5 47 261
ZZ4 5 46 248
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Lead Length: Shorter Potential Leads
Studied Configurations:
PL75 Potential lead: 75 m
PL50 Potential lead: 50 m
PL25 Potential lead: 25 m
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Effect of shorter Potential Leads Results []
Config.
Ground
ReferenceConfiguration
PL75 Potential lead
75m
PL50 Potential lead
50m
PL25 Potential lead
25m
50 m 5 5 5 5
1000 m 46 46 45 (-2.2%) 43 (-6.5%)
20000 m 261 257 (-1.5%) 238 (-8.8%) 202 (-22.6%)
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Typical Waveform
Initial transientfrom structure Reflection from endof current leadMeasurementwindow
Actual
Measurement
interval
0
5
10
15
20
25
30
-0.5 0 0.5 1 1.5 2 2.5 3 3.5
Time [s]
Volt
age[V],Impeda
nce[ohm]
0
200
400
600
800
1000
1200
1400
1600
1800
Current[mA
]
Tower Voltage
Tower Impedance
Structure Current
Lead current
Comparison of Zed Meter Impedance with
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Comparison of Zed-Meter Impedance with
Independent Measurement at Low Frequency
0,1
1
10
100
1000
0,1 1 10 100 1000
Footing Impedance, (Zed Meter Resul t)
FootingResista
nce,
(O
blique/FallofPotentialMethod)
Field Trial 1
Field Trial 2
Field Trial 3
Field Trial 4
Compact Electrodes (Towers)
Zed < R
Comparison of Zed-Meter Results with Low-Frequency
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Comparison of Zed Meter Results with Low Frequency
Resistance Measurements for Distributed Electrodes with
Long Buried Wires
1
10
100
1 10 100
Footing Impedance, (Zed Meter Resul t)
FootingResistance,
(ReferenceMet
hod)
Radial Wires 40 m
Continuous Counterpoise
Distributed Electrodes
Zed > R
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Low Frequency/High Frequency Summary
Zed-Meter works in the correct frequency / time range for
lightning.
Results for concentrated electrodes (20 tower legs) track
low-frequency results
Results for distributed electrodes (counterpoise) will be
quite different.
Some grounding improvements that are effective for 60 Hz
(counterpoise near stations) are less effective for improvinglightning performance
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Reference Material
Zed-Meter Application Guide (#1020243)
Field Guide: Zed-Meter Oblique Method Testing (#3002000956)
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