Zed-Meter Basic Operation

(1)

Zed-Meter: Basic Operation

Eskom Workshop

July 2014

(2)

Overview of presentation

• What is the Zed-Meter

• Basic Principle of Operation

• Typical Waveform

• Zed-Meter Instrument &

Accessories

• Lead Orientation

• Software

• Calibration Test

(3)

3 What is the Zed-Meter®

The Zed-Meter® is an instrument that

measures the grounding impedance of

transmission line towers

Differs from conventional methods by:

• Not requiring the removal or isolation of shield

wires

• Providing the impedance value of grounding -

most relevant for lightning performance, not

power frequency resistance

(4)

Ground Resistance vs. Ground Impedance

• Conventional instruments for measuring earth

resistance operate at low frequencies (typically

105 to 150 Hz)

• These instruments only provide the potential rise

of the IR term:

(5)

5 Philosophy Behind Zed-Meter

• The lightning performance (number of flashovers) of

transmission 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.

(6)

Philosophy Behind Zed-Meter (cont.)

• Under lightning the peak stress on insulators occurs before

adjacent towers have had a chance to react and help out

by sharing the surge current.

–

2µs -two-way propagation time to the nearest pair of

towers, 300 m or 1000’ away

• Under low frequency the impedance of the grounding

system at a particular structure is determined by the

parallel combination of the impedance of the local ground

electrode and the chain impedance of many towers

connected in parallel via the overhead ground wires.

–

Parallel chain impedance of neighboring structures is

usually much lower than that of the local ground

(7)

7 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

(8)

(9)

9 Basic Principle of Operation

• Inject a “lightning-like” transient current into the

tower base

• Measure the potential rise at the tower base

relative to remote ground

• Compute the ratio of the potential rise to input

current as a function of time

• Impedance measurement taken after effect of the

tower surge response has rung down.

• Impedance measurement taken before the effects

of adjacent towers have time to affect the reading

(10)

Equivalent Circuit

Propagation line

Z

Propagation line

Z

Ground wire

Z

_gw

Ground wire

Z

_gw

Potential Lead

Current Lead

Connection to

the structure

Voltage measurement

Propagation line

Z

Propagation line

Z

Ground wire

Z

_gw

Ground wire

Z

_gw

Potential Lead

Current Lead

Connection to

the structure

Voltage measurement

Current waveform

(11)

11 Schematic Connection Representation

Current Lead

Connection

Potential Lead

to structure

Zed-Meter

90

o

_-180

o

90-125 m

Current Lead

Connection

Potential Lead

to structure

Zed-Meter

90

o

_-180

o

90-125 m

(12)

How the Zed-Meter Works:

Apply Pulse

(13)

13 How the Zed-Meter Works:

Pulse Moves at Speed of Light up Tower

(14)

How the Zed-Meter Works:

Other Three Legs Now Absorb Current

(15)

15 How the Zed-Meter Works: Speed along

Reaction Wire is Less than Speed of Light

(16)

Time to use

Measurement

How the Zed-Meter Works: Overhead

Groundwire Surge Impedance is Constant

(17)

17 Time to use

Measurement

How the Zed-Meter Works:

Situation Stable for “Long” 600-ns Time

(18)

Time to use

Measurement

How the Zed-Meter Works:

Remote Potential Settles to Constant Value

(19)

19

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(21)

21 Resistor Test

• Test instrument before going out into the field

Potential Lead

Connection to the structure

Current Lead

(22)

(23)

23 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

(24)

Zed-Meter Software: Standard

Testing

(25)

25 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, and

should remain relatively constant for at least 300 ns

(26)

Lead arrangements for Tests on Towers with

Buried Counterpoise

• Causes coupling between test leads and counterpoise

wires

–

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 the

counterpoise to reduce coupling.

(27)

27 Lead Orientation

Current lead

Potential lead

Li

ne d

ir

ec

ti

o

n

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

Current lead

Potential lead

Li

ne d

ir

ec

ti

o

n

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

Current lead

Potential lead

Li

ne d

ir

ec

ti

o

n

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

Current lead

Potential lead

Li

ne d

ir

ec

ti

o

n

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

(28)

Lead Orientation: Zigzag Leads

Current lead

Potential lead

Li

ne d

ir

ec

ti

o

n

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

Current lead

Potential lead

Li

ne d

ir

ec

ti

o

n

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

ZZ1: In line:

Meander Potential

lead

Current lead

Potential lead

Li

ne d

ir

ec

ti

o

n

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

Current lead

Potential lead

Li

ne d

ir

ec

ti

o

n

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

ZZ2: In line:

Meander Both

leads

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Connection to

structure

ZZ3:

Perpendicular:

Meander Potential

lead

Current lead

Li

ne d

ir

ec

ti

o

n

Zed-Meter

Current lead

Li

ne d

ir

ec

ti

o

n

Zed-Meter

ZZ4:

Perpendicular:

Meander Both

(29)

29

(30)

Zed-Meter Software: Oblique

Shortened Lead Method

(31)

31 Oblique Shortened Lead Method: Test

Features

• Structure Impedance

• Soil Resistivity

(32)

Differences between Standard and Oblique lead

Layouts

(33)

33

(34)

(35)

35 Field Guide: Oblique Shortened

Lead Method

(36)

Zed-Meter Software: Oblique

Shortened Lead Method

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37

(38)

Tower to Counterpoise Measurement:

Lead Connection

(39)

39

(40)

Grounding of Current and Potential Leads

• In most cases it is not necessary to ground

(unless you are

using the Oblique method)

–

Impedance measurement is usually performed in the

time before the reflection from the end of the lead returns

back to the measuring point

–

Both wires are essentially “grounded” through their

capacitance to ground

(41)

41 Connection to Structure

(42)

Conducting objects in Proximity to the

Zed-Meter® Leads

• Presence of conducting objects, such as fences, vehicles

will tend to reduce the measured potential rise

–

Coupling

• Recommended that the lead be separated from any large

conducting objects by at least 1 m.

(43)

43 Zigzag Leads – Effect of Zigzag (Results [

Ω])

Soil Resistivity

50 Ωm

1000

Ωm

20,000

Ωm

Reference

Configuration

5 Ω

46 Ω

261 Ω

ZZ1

5 Ω

47 Ω

262 Ω

ZZ2

4 Ω

48 Ω

252 Ω

ZZ3

5 Ω

47 Ω

261 Ω

ZZ4

5 Ω

46 Ω

248 Ω

(44)

Lead Length: Shorter Potential Leads

Studied Configurations:

PL75 – Potential lead: 75 m

PL50 – Potential lead: 50 m

PL25 – Potential lead: 25 m

(45)

45 Effect of shorter Potential Leads –Results [

Ω]

Config.

Ground

Reference

Configuration

PL75 –

Potential lead

75m

PL50 –

Potential lead

50m

PL25 –

Potential lead

25m

50 Ωm

5

5 1000

Ωm

46

46 45 (-2.2%)

43 (-6.5%)

20000

Ωm

261 257 (-1.5%)

238 (-8.8%)

202 (-22.6%)

(46)

Typical Waveform

Initial transient

from structure

Reflection from end

of current lead

Measurement

window

Actual

Measurement

interval

5

10

15

20

25

30 V

ol

tage [

V

],

I

m

pedanc

e[

ohm

]

400

600

800 1000

1200

1400

1600

1800

C

u

rre

n

t [

mA]

Tower Voltage

Tower Impedance

Structure Current

Lead current

(47)

47 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,

Ω (Zed Meter Result)

F

o

ti

n

g

R

esi

st

an

ce,

Ω

(O

bl

ique

/

Fa

ll

of

P

ot

e

nt

ia

l M

e

thod)

Field Trial 1

Field Trial 2

Field Trial 3

Field Trial 4

Compact Electrodes (Towers)

Zed < R

(48)

Comparison of Zed-Meter® Results with Low-Frequency

Resistance Measurements for Distributed Electrodes with

Long Buried Wires

10

100 F

o

ti

n

g

R

esi

st

an

ce,

Ω

(R

ef

er

en

ce M

et

h

o

d

)

Radial Wires <40m

Radial Wires >40 m

Continuous Counterpoise

Distributed Electrodes

Zed > R

(49)

49 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 improving

lightning performance

(50)

Reference Material

• Zed-Meter Application Guide (#1020243)

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51