Zed-Meter Basic Operation

8/11/2019 Zed-Meter Basic Operation

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Zed-Meter: Basic Operation

Eskom Workshop

July 2014Fabio Bologna


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2 2014 Electric Power Research Institute, Inc. All rights reserved.

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.


7/517 2014 Electric Power Research Institute, Inc. All rights reserved.

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



Zed-Meter & Accessories



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



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



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



How the Zed-Meter Works:Apply Pulse

Vmeas



How the Zed-Meter Works:Pulse Moves at Speed of Light up Tower

Vmeas



How the Zed-Meter Works:Other Three Legs Now Absorb Current

Vmeas



How the Zed-Meter Works: Speed alongReaction Wire is Less than Speed of Light

Vmeas



Time to use

Measurement

How the Zed-Meter Works: OverheadGroundwire Surge Impedance is Constant

Vmeas



Time to use

Measurement

How the Zed-Meter Works:Situation Stable for Long 600-ns Time

Vmeas



Time to use

Measurement

How the Zed-Meter Works:Remote Potential Settles to Constant Value

Vmeas



Zed-Meter Software



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




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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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Questions ?

Zed-Meter Basic Operation

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