Understanding the Zed-Meter® Instrument (Draft)
Lightning Impulse Impedance of
Transmission Tower Footings and Ground Electrodes
1015904Understanding the Zed-Meter
®Instrument (Draft)
Lightning Impulse Impedance of Transmission Tower Footings and Ground Electrodes 1015904
Technical Update, December 2008
EPRI Project Manager F. Bologna
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THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:
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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Kinectrics, Inc.
This is an EPRI Technical Update report. A Technical Update report is intended as an informal report of continuing research, a meeting, or a topical study. It is not a final EPRI technical report.
NOTE
For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e-mail [email protected].
CITATIONS
This document was prepared byKinectrics
800 Kipling Avenue
Toronto, Ontario, Canada M8Z 6C4 Principal Investigators
W. A. Chisholm E. Petrache
This document describes research sponsored by the Electric Power Research Institute (EPRI). This publication is a corporate document that should be cited in the literature in the following manner:
Understanding the Zed-Meter® Instrument (Draft): Lightning Impulse Impedance of Transmission Tower Footings and Ground Electrodes. EPRI, Palo Alto, CA: 2008. 1015904.
PRODUCT DESCRIPTION
Most utilities test transmission line tower ground resistance by isolating overhead groundwires; inserting low-frequency, battery-operated test equipment; taking readings; and restoring the connections. The resulting values are useful for power frequency grounding but less
representative for lightning performance. Utilities can improve test productivity by switching to a Zed-Meter® instrument test technique that uses off-the-shelf equipment and injects a safe, lightning-like impulse signal into the tower base.
Results and Findings
The Zed-Meter instrument’s method for testing transmission line grounds relies on the injection of a safe, transient pulse into the tower base. The pulse is similar to lightning, and it delivers the results that utilities need for improving the lightning performance of lines. An additional benefit of this approach is that the surge impedance of the lead wires, rather than the galvanic resistance of driven rods, provides the connection to ground. This means that the Zed-Meter test leads need not be grounded; they need only be laid on the surface of the right of way. In addition, the overhead groundwires need not be isolated from the tower. These are important advantages that save test time, especially in frozen soil or rocky areas.
Challenges and Objectives
The objective of this report is to help users of the Zed-Meter instrument to apply it more effectively and to understand technical issues that might be encountered in the field. Application, Value, and Use
The Zed-Meter instrument was designed to measure the potential rise at the base of a four-footing high-voltage or extrahigh-voltage lattice tower without having to encircle the entire tower in a large, expensive sensor for the tower current. This report shows that the reaction electrode that is used to push current into the tower also provides a value of surge impedance that can be used to establish the soil resistivity near the tower. Other waveform features from this test can yield more information about the tower surge response, transfer impedance to nearby
equipment, and the soil resistivity in the top layer of the soil. EPRI Perspective
EPRI members have collectively used every existing method to measure transmission line grounding. Each has its strengths but none is completely appropriate for transmission line grounding—some require expensive equipment, and others take too long to set up at each tower. What separates the Zed-Meter instrument from the existing technologies is the right combination of test speed and accuracy for its intended purpose—lightning protection.
Approach
The goal of this report is to explain how to use the Zed-Meter instrument. It consists of the following sections:
• Section 1, Overview of the Zed-Meter® Instrument • Section 2, Zed-Meter® Instrument Bench Tests
• Section 3, Zed-Meter® Instrument Dipole Tests of Reaction Leads • Section 4, Zed-Meter® Instrument Tests on Transmission Towers • Section 5, Running the Zed-Meter® System Software
• Section 6, Typical Zed-Meter® Instrument Results • Section 7, Post-Processing
• Section 8, References
• Appendix A, Frequently Asked Questions • Appendix B, Specifications
• Appendix C Troubleshooting Guide • Appendix D, Hardware Evolution • Appendix E, The Current Reaction Lead
• Appendix F, Modeling Zed-Meter® Instrument Leads with NEC-4 Software Keywords Lightning Surge impedance Test methods Transmission lines Zed-Meter instrument
ABSTRACT
The Zed-Meter® instrument’s method for testing transmission line grounds relies on the injection of a safe, transient pulse into the tower base. The pulse is similar to lightning, and it delivers the results that utilities need for improving the lightning performance of lines. An additional benefit of this approach is that the surge impedance of the lead wires, rather than the galvanic resistance of driven rods, provides the connection to ground. This means that the Zed-Meter test leads need not be grounded; they need only be laid on the surface of the right of way. In addition, the overhead groundwires need not be isolated from the tower. These are important advantages that save test time, especially in frozen soil or rocky areas.
CONTENTS
1 OVERVIEW OF THE ZED-METER® INSTRUMENT ...1-1
What is the Zed-Meter® Instrument? ...1-1 How Does the Zed-Meter® Instrument Differ from Conventional Instruments?...1-3 How and Why Do the Zed-Meter® Instrument’s Results Differ from Typical Resistance Measurements? ...1-5 What Is the Basic Principle of Operation?...1-6 Labeled Diagram of Instrument...1-7 Test Lead Arrangement ...1-9 Sequence of Zed-Meter® Operations ...1-9 Operating Temperature Ranges ...1-10 2 ZED-METER® INSTRUMENT BENCH TESTS ...2-1 Operational Test: Charging the Battery...2-1 Operational Test: Short Circuit...2-1 Connections and Process ...2-1 Results for High-Quality Short Circuit...2-2 Results for Poor-Quality Short Circuit ...2-4 Operational Test: Fixed Resistance ...2-5 Reference Result: 54-Ω Calibration Resistor ...2-5 Reference Result: 500-Ω Resistor ...2-7 Practice Makes Perfect ...2-9 3 ZED-METER® INSTRUMENT DIPOLE TESTS OF REACTION LEADS ...3-1 Function of Reaction and Potential Leads ...3-1 Dipole Impedance Test Method ...3-1 Variation of Dipole Impedance with Lead Orientation and Length ...3-2 Dipole Test Results, 90-m Versus 130-m Lead Length ...3-3 Grounding of Reaction and Potential Leads ...3-4 Reasons for Grounding the Current Reaction Lead ...3-4 Reasons for Grounding the Remote Potential Lead ...3-4 Troubleshooting the Zed-Meter® Instrument’s Dipole Test Results ...3-5 Verifying a Safe Work Environment ...3-5 Symptoms of Excessive Noise Level ...3-5 Symptoms of Problems in the Lead Layout ...3-5 Symptoms of Conductors Running in Parallel...3-5 Symptoms of Problems in the Lead Terminations ...3-6 4 ZED-METER® INSTRUMENT TESTS ON TRANSMISSION TOWERS ...4-1 Connection Diagram for Impedance Test ...4-1 Preferred Type of Tower Connection ...4-2
Preferred Location of Tower Connection ...4-4 Orientation of Reaction and Potential Leads...4-7 Dealing with Obstructions When Laying Out Leads...4-10 Twists and Turns: The Meander Line...4-11 Using Shorter Potential Leads ...4-13 Vehicles in Proximity to Leads ...4-15 Considerations for Guyed Towers...4-16 Considerations for Twin Steel-Pole Towers ...4-18 Overhead Groundwire Connections...4-20 Comparison Tests with and Without Overhead Groundwires ...4-20 Calculation of Overhead Groundwire Impedance ...4-22 Unparalleling the Effect of Overhead Groundwires ...4-22 Which Is the Correct Value to Use? ...4-23 Tests on Towers with High Noise Level ...4-23 Tests on Towers with Buried Counterpoise ...4-24 Test Lead Orientation for Lines with Counterpoise ...4-24 Which Leg Has the Counterpoise Connection? ...4-25 5 RUNNING THE ZED-METER® SYSTEM SOFTWARE ...5-1 Overview ...5-1 Main User Interface...5-3 Section 2 Elements ...5-4 Section 3 Elements ...5-7 Setting the Time Windows ...5-12 Setting the Alarm Thresholds ...5-12 Pretrigger Interval...5-12 Demo Mode...5-13 How-To: Start the Measurement Process ...5-14 How-To: Stop the Measurement Process ...5-15 How-To: Save the Measurement Data...5-15 Data File...5-15 Log File ...5-16 How-To: Create Line Reports ...5-16 6 TYPICAL ZED-METER® INSTRUMENT RESULTS...6-1 Effects of Overhead Groundwire...6-1 Reasons for Steadily Increasing Impedance Values...6-1 Low-Frequency Versus High-Frequency Resistivity ...6-2
Reading Past the First Sweet Spot ...6-8 Reasons for High Impedance Values...6-10 What Constitutes a High Reading? ...6-10 Identifying Bad Connection to Current Leads ...6-11 Indications of Local Soil Resistivity from Dipole Test Results ...6-11 Reasons for Low or Negative Impedance Values ...6-11 Identifying a Bad Connection to the Potential Lead ...6-12 Inserting Resistance in Series with Tower to Validate Connections ...6-12 Indications of Local Soil Resistivity from Dipole Test Results ...6-12 7 POST-PROCESSING ...7-1 Organization of Data ...7-1 Converting Waveform Files to Excel Format...7-1 Producing Waveform Graphs in Excel ...7-1 Exporting Waveform Graphs from Excel to Portable Document Format...7-2 Establishing Confidence Level in Test Results ...7-2 Adaptive Filtering to Improve Confidence ...7-2 Offset Subtraction ...7-2 Denoising Digital Artifact ...7-3 Removing Noise Using Pretrigger Record ...7-4 Validating a Novel Test Lead Arrangement ...7-5 Validating Against What? ...7-5 NEC-2 for Simple Coupling Estimates ...7-8 8 REFERENCES ...8-1 A FREQUENTLY ASKED QUESTIONS ... A-1 B SPECIFICATIONS ... B-1 C TROUBLESHOOTING GUIDE ... C-1 D HARDWARE EVOLUTION ... D-1 E THE CURRENT REACTION LEAD ... E-1 Dipole Impedance Test Theory ... E-1 Theoretical Variation of Dipole Impedance with Height ... E-3 Theoretical Variation of Dipole Impedance with Soil Resistivity... E-4 Theoretical Variation of Dipole Impedance with Lead Orientation ... E-5 Conductors Running in Parallel with Test Leads ... E-7 2003 Field Studies at American Electric Power ... E-9 2005 Current Reaction Lead Studies at the CN Tower in Toronto... E-13 Dipole Test Results, 90-m Versus 130-m Lead Length ... E-16 Dipole Test Results, 125-m Coaxial Cable Test Leads... E-16 Dipole Test Results, 300-m, 14-Gauge Solid Copper Wires... E-19
F MODELING ZED-METER® INSTRUMENT LEADS WITH NEC-4 SOFTWARE...F-1 NEC-4 Software for Detailed Calculations Using Inverse Fast Fourier Transform ...F-1 Other Modeling Software ...F-4 Finding an Experienced Practitioner ...F-4
1
OVERVIEW OF THE ZED-METER
®INSTRUMENT
What is the Zed-Meter® Instrument?
The Zed-Meter® instrument is a test instrument that measures the impedance of transmission line grounds by generating an internal, lightning-like signal that is applied to the base of a transmission tower. The lightning impulse response of the local tower grounding system is monitored by tracing the rise and fall of tower base voltage as a function of the injected current for a short time after the signal is triggered. Rather than conducting this test with a single, large-amplitude pulse that has safety and weight issues, the Zed-Meter instrument averages results from repeated applications of lower-amplitude pulses from an electric fence shock generator that has been certified safe (although uncomfortable) for human and animal contact.
The Zed-Meter instrument is a smart instrument that has benefited from improvements in computer technology to reduce its size, weight, and cost. Field trials have been conducted at Public Service Electric and Gas Company, Duke Energy, Tennessee Valley Authority, Georgia Power, Hydro-One, Bonneville Power Administration, Eskom, Manitoba Hydro, and National Grid. Each utility in its turn has seen improved versions of the Zed-Meter instrument that have been smaller, lighter, less expensive, and better at analyzing, presenting, and storing results. The utility input in the development cycle distinguishes the Zed-Meter instrument from the
incremental improvements that have been incorporated in standard earth resistance testers during the same period.
In addition to the instrument itself, the Zed-Meter kit contains the following components: • A short cable and specially adapted clamp that feed the high-frequency test current into the
tower
• A pair of leads, usually coaxial cables of approximately 100-m length, that are used as high-frequency traveling wave antennas
• An optional pair of ground rods and connectors for grounding the traveling wave antennas Table 1-1 shows how the Zed-Meter kit has progressed from a list of suitable parts to a self-contained instrument with full control over pulse application and measurement with a Panasonic Toughbook laptop computer that represents more than half the overall cost.
Table 1-1
Zed-Meter® instrument evolution: prototype to production
2004 2006 2008
Digitizer Tektronix 3054B oscilloscope Tektronix 2000 series oscilloscope, US$6000
Acute DS-1102 200 MS/s USB digitizers, US$900 each Pulse
generator
Lab grade 200 V, 50 Ω with 10-ns rise time
Farm grade, safe (Underwriters Laboratories [UL] approved) with wave shaping circuit Current
sensors
Wideband current transducers, sensitivity 1 V/A, 10-ns rise time
Cable reels 90 m RG58 (2) 90 m RG58 (2) 90–130 m (2)
Power 12 Ah battery, 117 V inverter 12 Ah battery, 117 V inverter Internal rechargeable battery Control and
analysis
Manual setup of oscilloscope; manual control of pulser; comma-separated values (.csv) files on floppy disc to
Macro setup of oscilloscope; manual control of pulser; comma-separated values (.csv) files through network to
National Instruments Labview software control of digitizers, pulser, file transfer through USB, and wave analysis
How Does the Zed-Meter® Instrument Differ from Conventional Instruments? The Zed-Meter instrument works strictly in the time domain, specifically about 100 ns to 2 µs, whereas most conventional earth resistance test instruments use one or more fixed frequencies around 100 Hz, making them low frequency-domain tools.
Conventional, general-purpose, three- or four-terminal earth resistance testers also generate internal signals and perform the same basic functions as the Zed-Meter instrument. However, these testers use a low-frequency signal that gives the resistance of many towers in parallel. This result can indicate the contribution of individual towers to the low overall resistance only when expensive supplemental sensors are placed around each tower leg and guy wire. The low-frequency, three-terminal methods also require the insertion of metal probes into the ground, achieving sufficient depth of penetration to allow injection of test signals. Typically, a probe resistance of less than 10 kΩ must be achieved. This is difficult in frozen soil, because the resistivity is about 100 times higher than unfrozen soil. In winter testing, probes must sometimes be pushed all the way through the frozen soil layer, which can be more than 1 m thick.
Conventional clamp-on earth resistance testers generate internal signals at a medium frequency of approximately 2 kHz. The energy is magnetically coupled into a single ground lead through steel jaws that open and close. The power needed to drive test current into the grounding system can then be measured. If the multigrounded neutral system is in good condition with low
impedance, most of the measured impedance comes from a local (single) ground rod. This method has limitations if there are two or more paths to ground on the same tower or if the connection from local tower to overhead groundwire (OHGW) or neutral is imperfect.
In order to simulate what the Zed-Meter does, a standard earth resistance tester would have to be modified to produce much higher frequencies. This in itself is not unique. One earth resistance tester operated at a single frequency, 26 kHz, and a CIGRE working group recommended that 150 kHz would be a better choice [1]. However, a modified earth resistance tester simulating the Zed-Meter would have to read out a series of measured impedance values, one for every
frequency in a series covering the range from 10 kHz to 4 MHz.
It is possible to convert results between the time and frequency domains using Fourier
transformation, as illustrated in Figure 1-1. A perfect impulse signal contains all frequencies. A lightning impulse has high-frequency and low-frequency roll-off. For the most important waveshape of the first negative downward return stroke, the associated time constants are approximately 1.2 µs on the front and 50 µs on the time to half value. This means that a suitable frequency-domain instrument for grounding tests would measure with several different Nyquist frequencies in the range of 0.5/120 ns = 4 MHz to 0.5/50 µs = 10 kHz. Most of the interesting effects occur at the sine-wave frequency of 124 kHz that has the same peak current (I=31 kA) and peak rate of current rise (dI/dt=24 kA/µs) as a median lightning flash. This characteristic frequency drops to about 80 kHz for very large currents because there is a strong correlation between peak current and rate of current rise.
Zed-Meter® instrument’s test signal in time domain
Zed-Meter® instrument’s test signal in frequency domain
Figure 1-1
Time and frequency domain equivalents for Zed-Meter® test signal
Before committing to the development of the Zed-Meter instrument, EPRI reviewed a wide range of other instruments, including the EPRI Smart Ground Multimeter, which is used mainly for substation grounding tests. The results of this evaluation are given in Table 1-2..
Table 1-2
Suitablity matrix for transmission tower ground impedance (Z) testers Measurement
Approach
Cost Setup
Time
Signal Strengths Limitations
Clamp-on impedance meter
US$1000–2000 1 min 2 kHz Ease of use, size, battery life, wide range of Z
Must fit downlead; needs bond to parallel towers through overhead shield wires
Time-domain reflectometer testers
US$1000–10,000 15 min 1-ns step or pulse
Size, battery life, wide time scale, useful for soil
Wrong readouts; narrow range of Z centers at 50 Ω EPRI Zed-Meter® instrument US$3000–10,000 15 min 1- to 3-μs pulse, 200 V, 50 Ω
Size, noise rejection, wide range of Z, does not require driving rods into frozen soil
125-m leads Four-terminal earth resistance testers US$4000–10,000 30–60 min 100– 140 Hz
Size, battery life, noise rejection, wide range of Z, useful for soil
Needs tower isolation from shield wires; 100-m leads; 100-many intermediate steps ABB 26-kHz meter US$40,000 (estimated)
15 min 26 kHz Size, ease of use Inaccurate for Z>25Ω; 70-m leads; no 0 0.5 1 1.5 2 2.5 0 50 100 150 200 Time [microsecon ds] Inj ec ted I m pul s e [V ] 10-1 100 101 102 0 1 2 x 10-4 FFT So urce Fr equ ency [MHz] |I m p u ls e |
Table 1-2 lists a mixture of frequency-domain and time-domain instruments. With its internal analysis software, the EPRI Smart Ground Multimeter provides impedance readouts in the frequency domain using a Fourier analysis of a time-domain series of step pulses. This is promising, but the analysis extends only from low frequency to 500 Hz; therefore, the Smart Ground Multimeter is not suitable for measuring the lightning surge impedance of transmission tower ground electrodes.
How and Why Do the Zed-Meter® Instrument’s Results Differ from Typical Resistance Measurements?
There is an important physical difference between the lightning impedance of a transmission tower grounding system and the impedance of the same system at power frequency. The lightning surge is so rapid that the peak stress on insulators occurs before adjacent towers have had a chance to react and share the surge current. The two-way propagation time at the speed of light to the nearest pair of towers, 300 m away, is about 2 µs. A test surge can also be injected and measured at a local tower before the signals can detect what is happening far away from the tower under test. This is the basic advantage of the Zed-Meter tester.
In the frequency domain, the effect is described differently. The series inductance of the
overhead groundwires has a higher inductive reactance as frequency increases. For a 300-m span with 300 μH series inductance, the inductive reactance will be 18.8 Ω at 10 kHz, 188 Ω at 100 kHz, and so on. At high frequency, the surge impedance, ZC (300 Ω in this example) appears in
parallel with the local footing (see Figure 1-2 in the following subsection).
The Zed-Meter test result indicates whether the tower under test is well grounded, but it reads a value that is higher than the power-frequency impedance of the OHGW system with multiple grounds or the continuous counterpoise system. In practice, the Zed-Meter test result tends to be a bit lower than the impedance of the isolated tower measured at low frequency.
The lightning performance (number of flashovers) of transmission lines is related to the values of the tower footing resistances along the line length. Low-frequency methods for measuring the resistance of transmission tower ground electrodes lose accuracy when OHGWs are connected to the towers, and they do not provide a result that is related to lightning performance. Accurate measurement of each individual tower resistance, whether tested at low or high frequency, can be obtained only if one or more of the following conditions are true:
• The towers are temporarily isolated from parallel connections to remote earth • Sensitive current sensors can be placed around ground leads
• The measurement frequency is raised to approximately 150 kHz [1]
If the safety of a temporary working ground system relies on having low power-frequency impedance, the Zed-Meter tester is not the right choice for proof tests. When a power system fault occurs, many towers can share the fault current if they are connected together by an OHGW. The 60-Hz impedance of a multigrounded OHGW system can typically be
approximately 2 Ω if each individual footing resistance is approximately 20 Ω. The same effect occurs for continuous counterpoise electrodes. The Zed-Meter tester does not resolve these remote effects, and it is not reasonable to assume that any fixed reduction factor will relate the local Zed-Meter test result to the power frequency impedance for many similar towers in
parallel. This is a case in which a properly applied low-frequency earth resistance test instrument should be used.
What Is the Basic Principle of Operation?
The basic operating principle of the Zed-Meter instrument is the following:
• Two leads, typically 90–125 m long, are laid in straight lines in different directions away from a tower leg. One lead is used for current injection, and the other is used to measure the remote potential rise. The angle between the leads should be at least 90°.
• A lightning-like transient current is injected into the tower base through a current sensor. • The potential rise at the tower base relative to the remote potential is measured.
• A time-varying impedance profile, Z(t), is derived from the potential rise response to the injected current.
• Analysis identifies the desired features of Z(t), which, simply stated, are the median and the standard deviation of the values taken between two programmed time intervals.
The impedance measurement vector, Z(t), is valid after the effects of the tower surge response have rung down and before the effects of the adjacent towers, or the far end of the current injection lead, have time to affect the reading.
As a quality control measure, the current into the reaction lead is also measured and compared with the current into the tower. These are initially different when the measurement is not useful, but they should stabilize after some time (about 200 ns) to the same, constant value if the measurement leads are correct.
In 2005, a CIGRE working group issued a brochure that reviewed methods for measuring the earth resistance of transmission towers equipped with earth wires [1]. This study considered all the options reviewed for EPRI in the EPRI reports The EPRI Zed-Meter: A New Technique to Evaluate Transmission Line Grounds (1008734), Field Testing of EPRI Zed-Meter: Transient Impedance of Transmission Line Grounds (1010235), and Summary of Zed-Meter Field Tests: Transient Impedance of Transmission Line Grounds (1012314), which are shown in Figure 1-2 [2, 3, 4].
The most promising option for the development of a suitable tower footing resistance test in frozen soil uses impulse test methods, including a pair of propagation lines. These lines have been tried in various configurations such as the reflection method shown in Figure 1-2.
Figure 1-2
Equivalent circuit for voltage impulse test method
A good understanding of the role of the propagation lines in Figure 1-2 is helpful (see Section 3, Zed-Meter® Instrument Dipole Tests of Reaction Leads). This understanding will lead to good decisions about the test lead length, orientation, and configuration in difficult situations on crowded or remote rights of way.
One unique feature of the Zed-Meter tester is that the propagation lines, typically 90–125 m long, need not be terminated in ground rods. The surge impedance, Z, of the lines couples them to ground potential rather than to a remote ground rod.
The lines behave as a surge impedance for only the time that it takes an electromagnetic surge to propagate to the end and to return. The propagation speed is a fraction of the speed of light, c, or 300 m/μs. At a typical speed of 0.6 c, the 125-m lead has a reflection disturbance at about 1.4 μs. If the end of the propagation line has an open circuit, the returning voltage wave will be double the initial value, and the current wave will drop from its initial value to zero.
Labeled Diagram of Instrument
The Zed-Meter tester’s method for measuring the impedance of transmission tower grounds does not require disconnecting the ground wire, and it is ideally suited for evaluating the lightning response or footing impedance of transmission towers, including towers in frozen soil. Photographs and a block diagram of the instrument are shown in Figures 1-3 and 1-4.
External view Internal components
Figure 1-3
Photographs of the Zed-Meter® instrument
Pulse Generator Supervisor Circuit 12-V Power Wave Shaping Circuit 100 MS/s Digitizer Two Channels
USB-2 Interface to Laptop Computer 100 MS/s Digitizer
Two Channels
Figure 1-4
Block diagram of the Zed-Meter instrument
Figure 1-5 shows the Zed-Meter lead arrangement, with the impulse source and measurement equipment at the tower base.
Figure 1-5
Equivalent circuit for Zed-Meter instrument’s impulse test method in frozen soil
Test Lead Arrangement
The test lead arrangement in Figure 1-5 is symmetrical. With suitably short leads, there should be little difference in the results if the connections are reversed so that the current is injected into the left-hand lead and the potential is measured in the right-hand lead. In any test that involves straight leads, the leads should be reversed as a standard practice. Also, when the leads form a dipole antenna on the surface of the earth, a dipole or antenna impedance should be measured between the two wires. Generally, obtaining a constant value in the proof test of the propagation line impedance is a good first step to obtaining a reliable value of tower footing impedance. A proof test of the propagation line impedance should produce a signal that is initially of low amplitude and then rises to a constant level and stays at that level until the reflections from the end of the propagation lines arrive. Practically, there will often be some initial oscillations that damp down in approximately 100 ns if the connections are short and secure. The time after the current waves in each direction are equal and before the reflections arrive from the remote ends is used to establish the current flowing into the tower. The voltage rise in response to this current, measured over the same interval, is divided by the current to establish the value of the dipole surge impedance in the proof test.
Sequence of Zed-Meter® Operations
Under control of the National Instruments Labview program running on the laptop computer, the supervisor circuit initiates a series of high-voltage pulses by controlling the supply voltage of the pulse generator. The wave-shaping circuit flattens the crest of the pulses and sharpens the front time to make a rectangular current pulse (see Figure 1-5) between the two current sensors.
The injected currents, I1 and I2, are fairly constant regardless of the value of the tower footing
resistance, RT, and the parallel OHGW impedance, ZC. The currents in each lead of the pulse
generator are converted to voltage by a pair of current transducers.
If there are no connections linking the “Tower Lead” and “Current Lead” terminals in Figure 1-4, there is no current flow. If there is a resistive connection, the currents should be equal. Two 2-channel digitizer modules running at 100 megasamples per second simultaneously measure the following:
• Current I1 in the tower lead
• Current I2in the current lead
• Voltage rise V relative to the tower lead
Under the control of the Labview program, waveforms from the digitizer are transferred to the laptop computer using the USB interface cable. The waveforms are 8000 points long,
corresponding to a time of 80 μs. The quiet period before each impulse is recorded with the use of a pretrigger. The pretrigger duration is presently fixed at approximately 1.5 μs or 150 points. The waveforms are accumulated by the Labview program, and the process is repeated a specified number of times to collect average values of each of the three parameters.
After the averaging is completed, two vectors are generated: 1) the voltage rise V relative to the tower lead, divided sample by sample by the current I1 in the tower lead, and 2) the voltage rise
V relative to the tower lead, divided sample by sample by the current I2 in the current lead.
Two preprogrammed time intervals are used to calculate median impedance values. The first time interval starts after the tower oscillations have damped down and ends when the reflection from the remote end of the current reaction lead arrives back at the test point. This interval is valid for both grounded and ungrounded lead configurations and also for most practical transmission line span lengths. The second time interval starts after the current in the reaction lead has settled down to a new, stable, large value and normally ends when reflections arrive from adjacent towers, at about 2 to 3 μs. If the span length is short (<100 m), the measured impedance declines continuously in this period and the fitting of an L/Z time constant in post-processing is necessary. The second interval evaluation is meaningful only for grounded test lead configurations that achieve a ground resistance of less than approximately 1000 Ω.
As a quality check, the standard deviation of the impedance over these intervals is reported on the graphic interface of the Labview program. Some user-programmable limit checks are also executed to update a visual display using stoplight (red, yellow, and green) icons.
Operating Temperature Ranges
Figure 1-6
2
ZED-METER® INSTRUMENT BENCH TESTS
As with any instrument, it is best to become familiar with the capabilities and operation of the Zed-Meter® instrument in a comfortable indoor environment before taking the instrument into the field.
Operational Test: Charging the Battery
The Zed-Meter instrument has an internal battery that provides power to the pulse generator and other circuits. This battery is charged by connecting the ac adapter to a suitable source. The light-emitting diode indicators show the state of the charging system and whether the instrument is switched on.
Operational Test: Short Circuit
The Zed-Meterinstrument has internal testing and calibration features to establish that the equipment is in good working order and connected correctly. Because it uses very fast transient signals, the Zed-Meter instrument’s measurements of such common references as a short length of wire or a 500-Ω resistor provide results that correspond to the true inductive or capacitive response.
Connections and Process
The Zed-Meter instrument has three terminals: voltage (potential measurement), tower, and current. The surge energy is applied between the tower and current reaction lead terminals. All three terminals use Bayonet Neill Concelman (BNC) female connectors.
Figure 2-1
The metal collars, not the shielded internal connections, on the three BNC connectors are energized. This means that there is an irritating shock if a finger is placed between the outer metal surfaces of the tower lead and current lead BNC connectors. This arrangement is necessary to take advantage of the large radius of the sheath of a typical coaxial cable to provide
lightweight, flexible cables with low surge impedance.
A coaxial cable is connected across the two current outputs in the short circuit test. A BNC-type tee should be used to provide a parallel path to connect the voltage lead to the current lead terminal.
The connection from the tower lead to the current lead shorts out the pulse generator. The current is limited to about 2.5 A by the internal impedance of the pulse generator and the pulse-shaping circuitry. The high current output also shortens the pulse duration.
The instrument is operated to accept several waveform samples, typically 16, and to store the waveform. The interface should be adjusted to display the test currents in each channel, and the test currents should be compared with the results in Figure 2-2.
Currents into short circuit Long-time voltage response to short circuit
Figure 2-2
Zed-Meter® instrument’s currents and voltage for short circuit: poor operating conditions
Results for High-Quality Short Circuit
The Zed-Meter instrument’s reading of a short-circuit loop of 1 m should settle to a value less than 1 Ω in 1 μs, with a short circuit current level that does not damage the pulse-shaping circuit or overload the current monitoring channels.
In a properly operating Zed-Meter instrument, the currents in Figure 2-2 should be identical, and the measured voltage should be free of noise. Records from a poorly operating Zed-Meter instrument highlight several possible defects. Each current waveform had a dc offset,
The measured voltage had a bias of approximately -5 V, which is easily addressed in processing. However, Figure 2-2 also shows a peak-to-peak noise level of 6 V, which is excessive. The tail of wave also showed evidence of quantization. Post-processing of the short-circuit waveform using Microsoft Excel software can also eliminate some of the internal noise shown these records. The overall result is a negligible change in voltage for a change in current of 2000 mA, which is satisfactory even with these defects; however, the point-by-point values of impedance are meaningless in view of the high noise level on the voltage signal. When the Zed-Meter instrument is set up and running correctly, the averaged currents and voltage for a short circuit across the output should be similar to the waveforms in Figure 2-3.
Current Voltage
Figure 2-3
Zed-Meter® currents and voltage for short circuit: correct operating conditions
The current injected into the short circuit has a well-controlled 10-90% rise time of about 130 ns, with minimal overshoot. The voltage in response to this current has a pulse duration that matches the time that the current is rising. This indicates a nearly pure inductive response. For the
waveform shown, the rate of change of current dI/dt is 2.6 A / 0.16 μs or 16 × 106
A/s. The peak voltage in response to this change in current can be used to compute the inductance of the short-circuit cable system. In this case, the inductance is given by 30 V/ (16 × 106 A/s, or 1.9 μH. This is equivalent to about 2 m of connection length. Figure 2-4 shows the impedance versus time plot from the waveforms in Figure 2-3.
Figure 2-4
Zed-Meter® instrument’s impedance display for short circuit: correct operating conditions
Before the trigger, the ratio of voltage to current has no meaning. Just after the pulse, the
impedance in Figure 2-4 peaks at just greater than 100 Ω, then falls within 200 ns to a value less than 1 Ω. The local standard deviation, taken over a 100-ns sample interval, also falls with time and stabilizes at a level of approximately 0.02 Ω.
The Zed-Meter instrument can sometimes be used to measure transmission towers with low footing impedance of less than 5 Ω. The noise floor of the measurement system is approximately 0.7 Ω in the sweet spot that starts at about 500 ns and continues indefinitely in Figure 2-4.
Results for Poor-Quality Short Circuit
The Zed-Meter instrument produces and measures high-frequency pulse energy that is intended to flow through radio frequency connectors and coaxial cables. One factor in successful
measurements is getting this energy from the instrument into the transmission tower under test without introducing signal losses.
It is useful to conduct the short circuit test using the clamps and cables that will actually be used to attach the tower lead to the tower. If excessive oscillation (>100 ns) on the voltage or current measurements is noted in the results, the connection method should be reevaluated.
Operational Test: Fixed Resistance
The Zed-Meter instrument’s reading of a fixed resistance should reach the low-frequency value in about 10 μs, depending on the characteristics of the resistor. The better the wiring
configuration, the faster the Zed-Meter will settle to this value.
It is recommended that high-quality low-inductance resistors be purchased or constructed for calibration of the Zed-Meter instrument. The value should be in the range of 10–500 Ω. One possibility is standard, matching 50-Ω or 75-Ω resistors, which typically have a constant impedance (or voltage standing wave ratio) at frequencies up to 1 GHz. Alternately, a low-inductance resistor can be constructed by soldering 10 or more metal film resistors in parallel. Generally, wire-wound resistors will have internal inductance that will make them unsuitable for the calibration process. The selected resistor should ideally be mounted in an adapter with a pair of insulated BNC male bulkhead connectors, using only the ground leads.
Reference Result: 54-Ω Calibration Resistor
Bench measurements of a Zed-Meter instrument, using a 1.3-μs pulse width, were conducted on a 54-Ω resistor mounted in a small coaxial box. The setup and test results are shown in
Measured voltage, 105 V after 100 ns Test setup (2006 version)
Measured current, 2 A Calculated Z(t) and standard deviation over 16 samples
Figure 2-5
Zed-Meter® instrument test setup and results for a 54-Ω calibration resistor
In this case, the voltage across the resistor stabilized to a constant value with a time constant, τ, of about 50 ns. This time constant, multiplied by the measured resistance of 54 Ω, gives the inductance of the measurement loop, consisting of two 1-m wire leads, as shown in the test setup. The inductance works out to about 2.7 μH, a little higher than the expected 1 μH/m for straight leads.
Figure 2-5 also shows a plot of the calculated impedance profile, Z(t), from each of the measured currents. Using logarithmic scales, it is also possible to show the standard deviation of the
The time period at which the standard deviation of the reading falls below a threshold and remains relatively constant is called a sweet spot with high signal-to-noise ratio. In original iterations of the Zed-Meter instrument, the sweet spot was identified by eye, sometimes with different results, depending on which eye was used. Modifications to the instrument have focused on making the sweet spot as wide as feasible, consistent with retaining a rapid test time. On the bench, the sweet spot should be quite long.
Reference Result: 500-Ω Resistor
The reference resistor test is set up by fitting coaxial cable–to–banana jack and binding post adapters to each of the two current outputs. The calibration resistor is attached between the ground (black) terminals of the binding posts.
A BNC tee is used to provide a parallel path to connect the potential measurement lead to the current reaction lead terminal using a short length of coaxial cable, terminated in BNC male connectors. Alternately, a third coaxial to banana jack/binding post adapter can be used with a short length of standard wire, connecting the ground (black) end of the potential measurement binding post to the ground (black) binding post of the current reaction lead. The use of a coaxial cable and BNC tee is preferred.
The voltage and current signals should be averaged for 16 impulses. The results can be stored as text (.txt) files for analysis. For example, Figure 2-6 shows the measured voltages and currents for a 500-Ω wirewound resistor.
Figure 2-6
Measured voltage and currents into wirewound 500-Ω resistor showing oscillations
The standard deviation of the measured impedances in Figure 2-7 was approximately 10 Ω when both signals were considered, and the standard deviations reduced to 1–3 Ω when they were computed individually. It took about 500 ns for the oscillations in the currents to damp down sufficiently to start the sweet spot of the record. This is a function of the choice of resistor value
(500 Ω) and its internal capacitance of about 50 pF, giving a resonant frequency of about 17 MHz with the 2-μH wire loop.
Short-term response with standard deviation Detail of long-term response, showing current transformer droop
Figure 2-7
Zed-Meter® instrument measurement of wirewound 500-Ω resistor on two time scales
The detail of the long-term response of the Zed-Meter instrument’s records in Figure 2-7 shows two defects. First, the impedance seems to increase with time. This is a result of the
low-frequency limitations of the selected current transformers. They exhibit a droop, or signal loss, of about 0.1% per microsecond. This means that, compared to the initial value, the measured
current drops by about 8% at a time of 80 μs. For a constant applied source voltage provided by the Zed-Meter instrument, a decrease in measured current corresponds to an increase in
impedance—from 480 to 530 Ω for one current transformer and from 500 to 545 Ω for the other. Each current transformer has a slightly different droop that can be compensated in software. Second, each current transformer indicates a slightly different value of current, leading to a 20-Ω difference in the measured value of the reference resistor. The transient impedance of the tested resistor at late time varied from 480 Ω to 530 Ω using Z1, and it was 3% higher using Z2. It is
expected that these two readings will be within 1% of each other. The software process for correcting the droop and scale factors for the Zed-Meter instrument is under development. When comparing the results in Figure 2-7 with those in Figure 2-5, remember that both setups have approximately the same lead inductance. The L/R time constant is far less of a factor for the 500-Ω resistance than for the 54-Ω resistance, and the lead inductance dominates the response of the short-circuit test.
Practice Makes Perfect
The Zed-Meter instrument can be connected to the tower and current leads in a number of different ways. This section has shown that the type of wire, the type of connectors, and the instrument settings can all affect the quality of the results.
A familiarization period on the bench, testing impedance of resistors with good and bad wiring practice, is helpful to establish good habits in the field. This is especially important when measuring towers with low impedance. If the connection leads are too long, the Zed-Meter instrument will simply report the dynamic impedance of the leads themselves. It is possible to compensate for sloppy wiring practice near the tower by extending the length of reaction leads and analyzing the Z(t) values at later times. However, it is better to start with a good waveform.
3
ZED-METER® INSTRUMENT DIPOLE TESTS OF
REACTION LEADS
Function of Reaction and Potential Leads
The Zed-Meter® instrument relies on the fact that the surge impedance of an insulated wire, laid close to the ground, is constant and has a value in the range of 400–700 Ω. Two propagation lines are used. The first propagation line is a current reaction lead. The impulse source is placed between the tower base and this lead. The current injected into the tower base will be a faithful copy of the current launched down the reaction lead after some initial oscillations up and down the tower have decayed away. If a tower has a resonant structure with guy wires, it can take a bit longer for this to occur, and the propagation lines might have to be extended to 125 m or more. The second propagation line is the potential lead. The tower base potential is measured by this insulated wire, which is also coupled to ground by its surge impedance. This lead impedance plays less of a role in the accuracy of the measurement because the input impedance of the measuring circuit is high. However, any ac or high-frequency noise picked up by this horizontal antenna must be rejected by the measurement circuit. This is done by averaging many impulses, with the expectation that the noise signals are not correlated to the test wave.
If the surge impedance of the current reaction lead is too high (as can be the case over high-resistivity grounds like snow and ice), less current is injected, and the potential rise at the tower base becomes too low. In addition, if there is a high noise level on the potential reference lead, more impulses must be averaged. In severe cases, potentials on the ungrounded leads might be too high (>50 V) to be handled without personal protective equipment.
The configuration and selection of reaction and potential lead layouts has had an extensive period of development. Appendix D, Hardware Evaluation, summarizes the options that have been considered.
Dipole Impedance Test Method
Although it adds to the field test time, it is recommended that a dipole test be conducted on the leads every time, before taking a measurement of the tower footing impedance. The dipole impedance of the two test leads can be measured well, and it should be approximately constant whether the leads are oriented at 90° or 180°. The results make the most sense if both leads run in straight lines and both leads have the same type of termination, either driven thin rods or open circuit.
The dipole test setup connects the Zed-Meter instrument’s voltage (potential measurement) terminal to the current reaction lead using a BNC-type tee and a short coaxial cable. The current lead terminal is also connected to the current reaction lead. The central terminal feeds the potential lead (see Figure 3-1).
90-125 m 90-125 m
Figure 3-1
Connection diagram for dipole impedance test
It should be possible to reverse the red and blue leads in Figure 3-1 and still obtain the same impedance result. This is worth checking, especially in areas in which there is considerable induced noise on the cables or in cases in which it was easier to ground one cable.
Induced noise will probably be measured on the pretrigger record of the digitizers. Ideally, everything before the pulse fires should read zero. In practice, the meter often picks up currents of a few mA resulting from AM radio broadcasts. Averaging multiple test shots should reduce this interference to a low level compared to the 400-mA test signal.
If the dipole impedance values fluctuate from one test series to another, or if there is a large difference when the leads are reversed, it might be appropriate to increase the number of samples being averaged or to terminate the far ends in ground rods if they are floating in the first test series.
Depending on the vegetation, the reaction and potential reference leads can be near the surface of the soil, or they can be suspended a meter or more off the ground. A lead close to the ground will have lower and more constant surge impedance as well a- a slower propagation time. These factors will give a higher quality measurement. For that reason, the coaxial cable should be placed as close to the ground as possible. If possible, it is best to walk back along the lead to force it close to the ground. If this is not feasible, the lead length should be increased. Variation of Dipole Impedance with Lead Orientation and Length
The angle between the two test leads can vary from 180° to 45° without making much change in the dipole impedance. Therefore, the lead orientation will not have a major effect on the results if 3-m leads running parallel to one anohter are avoided.
Lead lengths depend on soil resistivity (see Appendix E, The Current Reaction Lead, for details). In order to have a measurement with a period of constant injected current, lead lengths should be increased in areas with higher soil resistivity or in areas where the lead must be draped in
vegetation well above the earth surface. Observe the following guidance for required length for the current reaction lead:
• Use a minimum 90-m lead length for soil types with resistivity 100 < ρ < 500 Ωm • Lead lengths of approximately 125 m should be considered in any of the following
circumstances:
— High soil resistivity ( ρ >500 Ωm) is anticipated. — Soil is frozen.
— It is not feasible to lay the lead close to the ground. — The tower has guy wires.
— The tower is isolated from the OHGWs by insulators.
• A configuration with two or more possible issues—for example, a guyed tower on frozen soil—might work better with lead lengths of 150 m.
Dipole Test Results, 90-m Versus 130-m Lead Length
The bench tests in Section 2 show some important aspects about Zed-Meter instrument testing. When measuring low values of resistance, the effects of series test lead inductance take some time to damp out before the sweet spot with constant voltage and current enables a valid calculation. Measuring resistances of ≥500 Ω can also lead to high-frequency oscillations. The advantage of a 130-m cable over a 90-m cable was evaluated in a test series in freezing conditions, which would be the worst case, with extremely high surface resistivity in the frozen soil and snow layer.
Pulse G enerator
Me asure s curr ent to the right wire Measures current to the left wire
Insul ated 90-m Wire Insulated 90 m Wire
Snow and/o r Frozen S oil La yer Pulse G enerator
Me asure s cur rent to the right wire Measures current to the l eft wire
Insulated 1 30-m Wire Insulate d 130 m Wire
Snow and/or Frozen Soil L ayer
Figure 3-2
Because it is pointless to drive ground rods into frozen soil, the leads were left floating. This meant that the traveling wave reflections from the ends of the cables were marked by clear reductions or reversals of current. In Figure 3-2, the sweet spot of constant and equal current in each leg starts at about 500 ns in all cases. The suitable evaluation time continues to 800 ns for the 90-m leads, corresponding to a propagation velocity of 0.75 c. The sweet spot increases to 1100 ns for the 130-m leads, also corresponding to 0.75 c. This is a worthwhile gain for an extra minute or two of walking time in each direction.
Grounding of Reaction and Potential Leads
Test experience has shown that, in most cases, there is no need to ground the ends of either the current reaction lead or the reference potential lead. Both wires are “earthed” through their surge impedance to ground, which is typically about 500 Ω.
It can be fairly difficult to drive a temporary rod into the local soil to obtain a resistance lower than 500 Ω. If the termination resistance is higher or lower than the wire surge impedance, a negative or positive current reflection coefficient will appear at the end of the current reaction lead. This means that, after two-way travel time along the wire, the current will drop or increase to a new value. Several reflections can occur before the current settles to a new, constant value given by the pulse generator output voltage divided by the ground termination resistance.
Reasons for Grounding the Current Reaction Lead
The initial reading of impedance is taken while the test current is constant and limited by the surge impedance of the current reaction lead. If the current reaction lead is terminated in a ground rod, the test current will settle down to a new value after a few round trips of the traveling waves. It is then possible to take a second reading of impedance in a different, slower time window. The degree of agreement between the two values, one in the 0.5–1 μs range and another in the 2–5 μs range, will help to characterize the soil better and thus improve modeling. In order to obtain a useful second reading, the resistance of the remote termination of the current reaction lead must typically be <1000 Ω. It can be useful to use a standard or clamp-on earth resistance tester to ensure that the driven rod meets this specification.
Reasons for Grounding the Remote Potential Lead
The main reason for grounding the remote end of the remote potential lead is to reduce the effects of electrostatic pickup. In some cases, continuous applications of the surge pulse can also cause the lead to charge up, leading to an offset in the measured signal that must be corrected in software. If the static charge builds up to an excessive level, it can overload the voltage channel and cause an over-range indication. The remote end of the potential lead should be grounded to preserve signal integrity if the measured potential relative to the tower exceeds about 10 Vrms or
Troubleshooting the Zed-Meter® Instrument’s Dipole Test Results
Verifying a Safe Work Environment
The first problem that can occur when setting up a dipole test beneath a transmission line is that there can be excessive voltage on the leads. Good practice is to treat the coaxial cables as live until they are proven safe with a voltage reading. If the ac or dc voltage exceeds local safety limits (such as 50 Vrms), the test will require special care, including the use of insulated rubber
gloves and covers rated for grounding tests by the local utility safety practices.
Symptoms of Excessive Noise Level
If the induced noise on the cables is a considerable fraction of the test voltage (200 V) or the circulating currents are on the order of 50 mA, there are two possible problems. The first
problem is that the sum of the signal and noise will exceed the automatically selected voltage or current scale. This over-range condition should be recognized by the instrument software, but waveforms should be examined carefully for clipping. Another symptom of clipping is a very low standard deviation in the measurement results. If clipping is noted, it might be necessary to restart the instrument’s auto-range function (see “How-To: Start Measurement Process” in Section 5) in order to establish the most appropriate internal measurement scales.
The second problem with noise on the cables is easier to fix. If there is considerable noise on the leads but it is not correlated with the Zed-Meter instrument’s free-running impulse source, taking multiple records and averaging them will reduce the noise level within seconds. General
experience has shown that 16 waveform averages are sufficient for reducing noise beneath transmission lines ≤765 kV. The presence of a nonzero signal amplitude in the pretrigger period before the step of voltage and current is an indication that the averaging is not sufficient for the local noise level.
Symptoms of Problems in the Lead Layout
The dipole test should indicate the same current in each leg. The currents and voltage should rise quickly, stabilize within at most 500 ns, and remain relatively constant for at least another 300 ns. This will result in a measured surge impedance value with a low relative standard deviation from the instrument readout.
Symptoms of Conductors Running in Parallel
If one or both of the leads cross areas of different resistivity, there can be one or more
perturbations in the current records. For example, leads that run in low-resistivity soil for some distance and then follow gravel construction roads have had changes in impedance of more than 100 Ω. Running the lead over obstructions can also introduce bumps in the dipole results. The reasons for these problems relate to changes in the surge impedance of the cable as a function of high gravel compared to typical soils or extra height above ground. The remediation is fairly simple: reroute the lead so that it has consistent height and resistivity. Rerouting the lead should introduce fairly gentle curves, if possible. However, if a 90° bend is necessary to maintain constant height and soil type, the numerical modeling for this project suggests that the effect of the bend will be much less than the effect of different height or soil.
Symptoms of Problems in the Lead Terminations
It is unlikely that the driven rods at each end of the dipole will have the same resistance values. This means that some asymmetry in the leg currents can occur after the return of reflections from the terminated ends. Because this is beyond the sweet spot, the differences can be ignored. In fact, it can be useful to exploit this effect deliberately by leaving one of the two ends open to ensure that the return time is marked clearly in the analysis.
4
ZED-METER® INSTRUMENT TESTS ON
TRANSMISSION TOWERS
Although it is not strictly necessary to conduct a dipole test before testing a tower, the leads must always be checked with a voltmeter for standing potential before connecting the Zed-Meter® instrument. This is done by measuring the voltage potential between the tower and the leads using a handheld voltmeter as shown in Figure 4-1.
>90 Meter Lead
Voltmeter
Figure 4-1
Connections to measure voltage potential on leads
If the potential exceeds 50 Vrms, most utilities call for the use of insulating gloves or other
countermeasures. The Zed-Meter instrument itself can generate good results even if the induced pickup exceeds 100 V, because the current transducers are dielectrically isolated from the leads. Connection Diagram for Impedance Test
The basic layout of the current reaction and remote potential leads was described in Section 3, Zed-Meter® Instrument Dipole Tests of Reaction Leads, along with a recommended method for validating the impedance of the configuration. Deviations from the recommended practice, 125-m leads in two directions, 180° apart along the right of way, are described in “Orientation of Reaction and Potential Leads” in this section.
When a good-quality result is obtained in the dipole test for the leads, say 400–700 Ω with 20-Ω standard deviation, the Zed-Meter is ready for connection to the tower (see Figure 4-2).
90-125 m 90-125 m
Figure 4-2
Connection of Zed-Meter® to reaction lead, remote potential lead, and tower leg
Some interesting things should happen when the dipole test is changed to a tower test. First, the current injected into the tower should be about twice the current into the dipole. The surge impedance of one leg of the dipole is twice that of the full dipole. This impedance is typically much greater than the impedance of the tower under test. Second, the potential rise of the tower base relative to the ground reference potential lead can initially be quite high, but it soon settles down to a relatively constant value. The Zed-Meter instrument is fast enough to see the effects of currents traveling up and down tower legs and guy wires. This also means that the connection of the tower lead to the base of the tower steel should be as short as possible, and it should make a good electromagnetic connection at high frequency.
It might be necessary to initiate the auto-range function of the Zed-Meter to adjust to the new signal levels (see “How-To: Start Measurement Process” Section 5). This is done by changing the line name or structure number when starting a test. The software uses a change in either field to initiate the auto-range sequence.
Preferred Type of Tower Connection
A number of methods have been explored for making a good high-frequency connection from the Zed-Meter instrument to the transmission tower. Figure 4-3 shows the following three examples:
• An alligator clip with 0.5-m wire lead and banana plug
• A citizens band (CB) radio antenna clamp and coaxial cable (intended for truck mirror mount)
2003 (American Electric Power), alligator clip to lattice tower leg—defect: overshoot
2004 (Public Service Electric and Gas), CB radio clamp-on antenna mount
2008 (Manitoba Hydro), modified welding clamp
Figure 4-3
Generally, the best results are obtained when the two measured currents are in close agreement and not oscillating too much after the initial rise. Figure 4-3 shows that the alligator clip is poor in this respect, with 100% overshoot; the CB radio antenna clamp is good, and the modified welding clamp is quite good.
Although the high-frequency performance of the CB antenna radio mount is theoretically better than the modified welding clamp and its cost is much lower, in practice the device could be loosened and tightened only about 50 times before it broke in half. The welding clamp (see Figure 4-4) is likely to give longer service life and is recommended as the best choice at this time.
Connection to lattice tower Connection to stud on steel pole
Figure 4-4
Close-ups of modified welding clamp with swaged BNC female connector
Preferred Location of Tower Connection
Numerical simulations have suggested that the height of current reaction plays a role in the quality of the results. The effect of lead height on the propagation velocity and impedance of the reaction lead was detailed in Section 3, Zed-Meter® Instrument Dipole Tests of Reaction Leads. A dipole test establishes whether the leads are long enough to compensate for this effect.
The numerical simulations also found a second factor. The height and length of the connection from the instrument’s tower lead to the tower plays a considerable role in the quality of the results. This seems to be more important than the height of the reaction lead further away from the tower. An example of a desirable, tight connection, with short leads and close to the ground, is shown in Figure 4-5.
Figure 4-5
Preferred Zed-Meter® instrument connection to lattice tower
Figure 4-6 shows that, when the wiring is tight, there is little difference in the results with a connection to the tower 0.1 m off the ground.
ρ=50 Ωm h=0.1 m: 5 Ω h=1 m: 4 Ω ρ=1000 Ωm h=0.1 m: 46 Ω h=1 m: 43 Ω
Reaction lead 0.1 m above ground Reaction lead 1 m above ground, except 0.1 m at tower
Figure 4-6
Effect of reaction lead height on measured impedance with 90-m leads using good (tight) wiring
practice as a function of soil resistivity ρ and height above ground h
0 0.5 1 1.5 2 2.5 0 10 20 30 40 50 60 70 80 90 100 Sim# I1-C-L01-GR-GW-1000 Time [microseconds] M easur ed I m pe dance : V me a s / I m eas [O h m ] V meas / Imeas Median 450-650 ns = 46 Ω 0 0.5 1 1.5 2 2.5 0 5 10 15 20 25 30 35 40 Sim# I1-C-L01-GR-GW-50 Time [microseconds] M eas ur ed I m pedan ce: Vm eas / Ime a s [O h m ] Vmeas / Imeas Median 450-650 ns = 5 Ω 0 0.5 1 1.5 2 2.5 0 5 10 15 20 25 30 35 40
Sim# I1-C-L1-GR-GW-50 BIS
Time [microseconds] M eas ur ed I m pedan ce: Vm eas / Ime a s [O h m ] Vmeas / Imeas Median 450-650 ns = 4 Ω 0 0.5 1 1.5 2 2.5 0 10 20 30 40 50 60 70 80 90 100
Sim# I1-C-L1-GR-GW-1000 BIS
Time [microseconds] M eas ur ed I m pedan ce: Vm eas / Ime a s [O h m ] V meas / Imeas Median 450-650 ns = 43 Ω
In contrast, Figure 4-7 shows simulations for a detailed electromagnetic model of a lattice tower. These simulations were performed for three values of soil resistivity—extremely low (50 Ωm), high (1000 Ωm), and extremely high (20,000 Ωm).
Bottom leg of lattice tower geometry in NEC-4 model ρ=50 Ωm Good: 5 Ω Loose: 9 Ω ρ=1000 Ωm Good: 46 Ω Loose: 43 Ω ρ=20000 Ωm Good: 261 Ω Loose: 219 Ω 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -100 0 100 200 300 400 500 Sim# I1-C-L1-GR-GW-20000 Time [microseconds] M eas ur ed I m peda nce : V m eas / Ime a s [O h m ] Vmeas / Imeas Median 450-650 ns = 219 Ω 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -100 0 100 200 300 400 500 Sim# I1-C-L01-GR-GW-20000 Time [microseconds] M eas ur ed I m peda nce : V m eas / Ime a s [O h m ] Vmeas / Imeas Median 450-650 ns = 261 Ω 0 0.5 1 1.5 2 2.5 0 10 20 30 40 50 60 70 80 90 100 Sim# I1-C-L1-GR-GW-1000 Time [microseconds] M eas ur ed I m peda nce : Vm eas / Imeas [O h m ] V meas / Imeas Median 450-650 ns = 43 Ω 0 0.5 1 1.5 2 2.5 0 10 20 30 40 50 60 70 80 90 100 Sim# I1-C-L01-GR-GW-1000 Time [microseconds] M eas ur ed I m peda nce : Vm eas / Imeas [O h m ] V meas / Imeas Median 450-650 ns = 46 Ω 0 0.5 1 1.5 2 2.5 0 5 10 15 20 25 30 35 40 Sim# I1-C-L1-GR-GW-50 Time [microseconds] M ea s ured I m pe da nc e : V me a s / I me a s [O h m ] Vmeas / Imeas Median 450-650 ns = 9 Ω 0 0.5 1 1.5 2 2.5 0 5 10 15 20 25 30 35 40 Sim# I1-C-L01-GR-GW-50 Time [microseconds] M ea s ured I m pe da nc e : V me a s / I me a s [O h m ] Vmeas / Imeas Median 450-650 ns = 5 Ω
As Figure 4-7 shows, there is an extensive and large disturbance in the Z(t) profiles during the sweet spot (about 450–650 ns) when using poor wiring practice, with a connection 1 m above the ground. This effect is greatest when the soil resistivity is low; the variation in Z(t) seems to have a constant amplitude.
In practice, the differences between clamp locations are somewhat less than those suggested by simulations. Even so, minimizing the length of the tower lead is probably as important as using a high-quality clamp with low impedance at high frequency for obtaining satisfactory results. Orientation of Reaction and Potential Leads
The normal procedure is to run the current reaction lead in one direction along the right of way and to run the remote potential lead in the other direction (see the right side of Figure 4-8). The calculation of tower base voltage with this configuration gives an estimate of tower-base potential that is closest to the true situation of a vertical lighting flash to the tower top. It is also the most practical orientation in a typical right of way because permission to run the leads through adjacent properties is not necessary.
If access to the right of way along one direction is restricted, perhaps by a road, the alternative of running the current reaction lead at right angles to the line direction, 90° to the potential lead, can also be used (see the left side of Figure 4-8). This configuration is more common for grounding tests at a single, fixed frequency because it reduces steady-state mutual coupling effects among leads.
The effect of lead orientation is most noticeable when testing towers with low footing
impedance. The differences amount to 1–2 Ω, which is a considerable fraction of a 5-Ω result but can be neglected if the tower impedance is measured to be 25 Ω.
