Zed-Meter Basic Operation

Zed-Meter: Basic Operation Eskom Workshop July 2014 Fabio Bologna

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

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

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

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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: V = L dI/dt + R


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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.


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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 electrode.


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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 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


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Current waveform

Equivalent Circuit Voltage waveform

Ground wire Zgw

Ground wire Zgw

Connection to the structure Voltage measurement

Propagation line Z Potential Lead

Current Lead V

Ground electrode Impedance ZT

I1

Zed-Meter


Propagation line Z

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I2

Current measurement Pulse generator and wave shaping circuit

Schematic Connection Representation

Zed-Meter

Current Lead

Connection to structure

Potential Lead 90o-180o 90-125 m


90-125 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 along Reaction Wire is Less than Speed of Light

Vmeas


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How the Zed-Meter Works: Overhead Groundwire Surge Impedance is Constant

Time to use Measurement

Vmeas


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


Vmeas


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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

Rc

Connection to the structure

Current Lead


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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: Standard Testing


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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, and should 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 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.


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Zed-Meter

Line direction

Line direction

Lead Orientation


Potential lead


Line direction


Line direction

Zed-Meter

Current lead

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Potential lead

Current lead

Line direction

Line direction

Lead Orientation: Zigzag Leads

Potential lead

Potential lead


Zed-Meter


Zed-Meter

Current lead

Line direction

Current lead

Line direction

Line direction

Zed-Meter

ZZ1: In line: Meander Potential lead

Line direction


ZZ2: In line: Meander Both leads

ZZ3: Perpendicular: Meander Potential lead

Current lead Zed-Meter

Current lead



Line direction

Line direction

Potential lead

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Potential lead

ZZ4: Perpendicular: Meander Both leads

Dipole Test: Typical Results (Ice , 1000ft leads)


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Zed-Meter Software: Oblique Shortened Lead Method


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Oblique Shortened Lead Method: Test Features

•Structure Impedance •Soil Resistivity •Electrode Perimeter


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Differences between Standard and Oblique lead Layouts


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Oblique Lead Layout


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Oblique Lead Layout (2)


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Field Guide: Oblique Shortened Lead Method


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Zed-Meter Software: Oblique Shortened 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 30Ω at 1500 ns, the tower is well grounded. © 2014 Electric Power Research Institute, Inc. All rights reserved.

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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 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 • Exception: Reduce the effects of electrostatic pickup


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Connection to Structure • Lattice


Steel Pole

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Conducting objects in Proximity to the ZedMeter® 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.


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Zigzag Leads – Effect of Zigzag (Results [Ω]) 50 Ωm

1000 Ωm

20,000 Ωm

5Ω

46 Ω

261 Ω

ZZ1

5Ω

47 Ω

262 Ω

ZZ2

4Ω

48 Ω

252 Ω

ZZ3

5Ω

47 Ω

261 Ω

ZZ4

5Ω

46 Ω

248 Ω

Soil Resistivity Reference Configuration


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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 [Ω] Config. Reference Configuration

PL75 – Potential lead 75m



50 Ωm

5

5

5

5

1000 Ωm

46

46

45 (-2.2%)

43 (-6.5%)

20000 Ωm

261

257 (-1.5%)

238 (-8.8%)

202 (-22.6%)

Ground


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Typical Waveform Initial transient from structure

Measurement window

1800

Reflection from end of current lead

1600

25

1400 1200

20

1000 15

Tower Voltage Tower Impedance Structure Current Lead current

Actual Measurement interval

10

800 600 400

5

200 0

0 -0.5

0

0.5


1

1.5

Time [µs] 46

2

2.5

3

3.5

Current [mA]

Voltage [V], Impedance[ohm]

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Comparison of Zed-Meter® Impedance with Independent Measurement at Low Frequency

Footing Resistance, Ω (Oblique / Fall of Potential Method)

1000 Field Trial 1 Field Trial 2 Field Trial 3 Field Trial 4

100

Zed < R 10

1 Compact Electrodes (Towers) 0,1 0,1

1

10

100

Footing Impedance, Ω (Zed Meter Result)


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1000

Comparison of Zed-Meter® Results with Low-Frequency Resistance Measurements for Distributed Electrodes with Long Buried Wires 100 Radial Wires <40m Radial Wires >40 m

Footing Resistance, Ω (Reference Method)

Continuous Counterpoise

10

Zed > R Distributed Electrodes 1 1

10 Footing Impedance, Ω (Zed Meter Result)


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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 improving lightning 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 ?


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Zed-Meter Basic Operation

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