electrode is not a constant value. The Zed-Meter® instrument’s impedance profile can decline with increasing time, suggesting that the grounding system has long buried wires (counterpoise). In the more common case, Z(t) can increase slightly with time as a result of the changes in resistivity of common earth materials and also because the ground has capacitance that takes some time to charge up. The time window selected for analyzing the impedance profile will affect the result in each case.
Experience has shown that a good sweet spot of 500 to 800 ns can be achieved in most tests, and this is used as a reference in the production Zed-Meter instrument. However, additional
information can gained by evaluating the impedance at different time intervals in post- processing. For this reason, at least one waveform file should be saved for every test. Effects of Overhead Groundwire
The test result should increase by a small amount when the OHGWs are disconnected at the tower top. The high (170–250 Ω) surge impedance of all the OHGWs appears in parallel with the low (5–30 Ω) impedance of the footing. The amount of the increase in the test result depends on the value of footing impedance. For well-grounded towers, the increase should usually be less than 10% when OHGWs are isolated. For this reason, and also to save test time, isolation of the OHGW is not recommended as part of the normal test process. The OHGW heights and
separation should be recorded if a correction for the parallel impedance will be made after the measurement.
On towers with insulated OHGWs, there is often a strong benefit to carrying out conventional low-frequency measurements of resistance, using an oblique (45–60°) orientation of the potential profile in the three-point test. This lead geometry gives the low-frequency tower resistance, R, the local average resistivity, ρ, and effective ground electrode perimeter, ρ/R, with the same effort usually given to obtaining only the resistance value. Tests of this sort formed an important component of the Zed-Meter instrument’s cross-calibration program at several utilities.
Reasons for Steadily Increasing Impedance Values
In many of the test results, there is a slight trend for the impedance profile with a wide sweet spot to increase with time. The median impedance in the time range from 500 ns to 700 ns forms the reference. Nearly all tower and lead configurations produce good results in this window. It takes some extra time to lay out the extra lead length needed to obtain an impedance profile that extends to more than 1000 ns, especially on soil with high resistivity. However, when this is done, processing of the results in two separate time windows will often yield different impedance values.
Low-Frequency Versus High-Frequency Resistivity
The first physical reason for an increase in impedance with increasing time is related to the role of the dielectric response of the soil. Many natural soil materials have relative dielectric
permittivity values, εr, in the range of 5 to 10. Especially for soils with high resistivity
(ρ>1000 Ωm), the initial impedance of the soil will be capacitive, and it will charge up with a time constant to the eventual resistive value observed at lower frequency.
Figure 6-1 gives one indication of how much difference there should be when Zed-Meter instrument values are compared to low-frequency measurements, based on the difference in measured soil resistivity at 100 Hz and 100 kHz.
Figure 6-1
Expected ratio of Zed-Meter® instrument readings to low-frequency readings of compact electrodes
If the soil around the tower is sand, the Zed-Meter instrument’s result should be 70–85% of the low-frequency value. The actual readings will show a large change with moisture content, meaning that readings taken days or hours apart can be quite different if there has been recent rain.
The factors relating Zed-Meter test results to low-frequency values are much larger for other types of soil. Till is soil smeared into place by glaciers, containing a wide range of particle sizes from boulders (>256 mm) and pebbles through sand to silt (0.0625–0.004 mm). Till retains moisture much better than sand and supports hardwood forest or farming. For towers in this common soil type, Zed-Meter instrument readings should be 50–70% of the values found in low-
Clay has the smallest particle size (<0.002–0.004 mm), and its resistivity has the strongest frequency dependence. Zed-Meter instrument readings for towers in clay should be 10–33% of the values obtained with low-frequency tests. Because clay resistivity is very low compared to sand or till, both readings will often be <10 Ω. In practice, most utilities take some care to avoid placing towers in clay soils to ensure mechanical stability of the foundations.
Overall, cross-checks on isolated towers, plotted together in Figure 6-2, confirm that the Zed- Meter instrument’s impedance results tend to be well below the low-frequency measurements.
Figure 6-2
Comparison of the Zed-Meter® instrument’s impedance with independent measurement at low frequency
The graph of results from a total of six weeks of cross-calibration tests in Figure 6-2 confirms the expectations raised from fixed-frequency tests of soil samples in Figure 6-1. The majority of the test points lie above the y=x line. For compact electrodes, such as tower foundations without radial counterpoise, the Zed-Meter instrument’s impedance is less than that found at low frequency.
Effect of the Time Window, 500 ns Versus 1000 ns
Some of the test results in the cross-calibration program had Z(t) records that allowed analysis at two different periods in a wide sweet spot, extending all the way from 500 ns to 1000 ns. Figure 6-3 shows that, for towers with impedance <3 Ω, there was little change in Z(t) with time. Above this level, the impedance at 500 ns increased by a factor of two when the Z(t) profile was
Figure 6-3
Comparison of Zed-Meter® instrument results at 500 ns and 1000 ns for compact electrodes with no extensive buried wire
The Z(t) results for 1000 ns are much closer numerically to the low-frequency measurements. The power-law fit has an exponent that is nearly linear, and the regression coefficient is better than the relation for the 500-ns values. The change in impedance from 500 ns to 1000 ns is also interesting because models based on material tests predict that this change should occur much more slowly.
Two points are circled in Figure 6-3. For these two towers, reference tests showed that one of the tower legs had an intact counterpoise connection to a nearby substation. For these distributed grounding systems, the Zed-Meter instrument’s impedance was considerably higher than the low-frequency value at both 500 ns and 1000 ns. Valid Zed-Meter instrument results at both 500 ns and 1000 ns can usually be obtained by laying out current reaction leads of suitable length. This is now recognized as a desirable goal in the test procedure.
Special Case: Towers with Isolated Overhead Groundwires and High Resistivity
At Tennessee Valley Authority, the 500-kV transmission system is operated with insulated OHGWs to reduce power loss. This simplifies the measurement of tower resistance using low- frequency methods, and in fact, Tennessee Valley Authority maintains an extensive database of these results. During cross-calibration, some of the towers with the highest values of resistance were re-measured with standard and Zed-Meter instrument methods.
Figure 6-4
Schematic Zed-Meter® instrument circuit for transmission lattice tower with insulated overhead groundwires
Figure 6-5 shows the Zed-Meter instrument’s Z(t) profiles for the two towers with the highest impedances in the test program. For these isolated and poorly grounded towers, both show increasing impedance with time in the range of 500 to 1000 ns.
Tower 307: low-frequency resistance 163 Ω Tower 309: low-frequency resistance 208 Ω
Figure 6-5
Impedance profiles for isolated 500-kV towers in area of high soil resistivity
The sweet spot for these measurements starts rather early, at about 200 ns. The R-C time constant of the tower capacitance (1 nF) and the Zed-Meter instrument’s impedance is about 100 ns, and this effect is seen early in the wave. However, the rise in impedance from 500 ns to 1000 ns is a characteristic of the soil response, not the tower.
The sweet spot of the Z(t) profile for tower 309 also ends rather early, at 500 ns. The reason that the standard deviation of the measurement in tower 307 stays low after the reflection from the current reaction lead is that the termination resistance in this case provided a reasonable match to the current reaction lead surge impedance.
Reasons for Steadily Decreasing Impedance Values
In the bench tests for the Zed-Meter instrument, it was noted that the meter responds to a short circuit cable across its terminals with a reading that is initially high and then falls to a low value. It was possible to estimate the lead inductance (typically L= 1 or 2 μH) from the time constant, t=L/Z.
The Zed-Meter instrument uses this capability to measure the effect of the inductive response of ground electrodes, which will typically be 1 μH per meter of length, in addition to the resistive response. This is reported as an impedance profile, Z(t), that starts with a high reading and falls with time.
Typical Responses of Buried Horizontal Wires
It is practical and useful to measure the impedance of newly installed buried wires with the Zed-Meter instrument. The wiring connection that is normally used for the dipole test is adapted in Figure 6-6 for this purpose.
Distributed
Figure 6-6
Wiring layout for Zed-Meter® instrument measurement of distributed electrode impedance
A test of this sort was carried out on a set of four 38-m buried wires, radiating outward from the tower base. The impedance profile is shown in Figure 6-7.
Figure 6-7
Typical Zed-Meter® instrument measurement of 38-m buried wire with 26-Ω low-frequency
resistance
The initial impedance of about 160±20 Ω is typical of the surge impedance of counterpoise systems. In this case, the impedance of the four wires in parallel falls relatively quickly, reaching a value of 47 Ω at 500 ns and 29 Ω at 1000 ns. The low-frequency resistance between this four- radial-crowfoot electrode and the tower was measured to be 26 Ω with an earth resistance tester. The impedance result can be modeled as an L/R time constant of 300 ns to give an effective inductance of 8 μH. This is a good match to a rough estimate given by the inductance of each 32-m wire (32 μH) divided by the number of wires (4) in parallel.
In 10 of 15 cases in which Zed-Meter instrument tests were carried out on towers that were known or proven to have a distributed electrode, the impedance at 500 ns was greater than the impedance at 800 or 1000 ns. Figure 6-8 also shows that the Zed-Meter instrument result at 500 ns was greater than the low-frequency value in 12 of the 15 cases, by as much as a factor of four.
Figure 6-8
Comparison of Zed-Meter® instrument results with low-frequency resistance measurements for distributed electrodes with long buried wires
As a reminder, results for compact electrodes in Figure 6-2 were consistently above the y=x line, corresponding to Zed-Meter instrument impedances that were less than the low-frequency values, and the results for a later time (see Figure 6-3) showed an increasing impedance with time.
Thus, a declining impedance profile in a Zed-Meter instrument result is a characteristic of a ground electrode with considerable inductance in series with the low-frequency resistance. Again, it is stressed that there is some advantage to taking the extra time to lay out sufficient reaction and remote potential cable lengths, oriented at right angles to the run of the counterpoise wires, to obtain a wide sweet spot of 500–1000 ns.
It may be tempting to use one or both of the counterpoise leads as alternatives to the coaxial cables laid above the ground. Some advice: Do not try this unless you are willing to accept the slow convergence of your Sommerfeld integral terms when you do your post-processing analysis.
Reading Past the First Sweet Spot
The two impedance profiles in Figure 6-5 showed that it can be feasible to obtain impedance readings after the return of the reflection from the terminated end of the current reaction lead. In these measurements, the question about what happens at a later time could not be answered because the test pulse width was 1200 ns. Since these tests were performed, the Zed-Meter instrument’s hardware has been modified to provide a much longer pulse width.
Although the initial Zed-Meter instrument reading is not influenced by the termination (open or grounded) of the current reaction lead, this is not true for the readings taken at later times. The potential at the tower base must be adjusted for the influence of the test pulser voltage minus the tower base potential rise at a later time. The steps to this process for an isolated tower include the following:
1. Estimation or measurement of the potential applied to the remote ground rod. For a high resistance relative to the tower, this will be approximately 200V.
2. Estimation of the effective radius of the temporary grounding probe for the current reaction lead. For example, a driven rod with a 0.005 m radius and a 0.3 m length is equivalent to a hemisphere of radius a= 0.055 m in uniform soil.
3. Calculation of the influence voltage, in this case 200 V⋅ (a/d), where d is the distance from tower base to the temporary ground probe. This is about 90 mV when d=125 m. Several factors can lead to a poor-quality low-frequency (late-time) measurement with the Zed- Meter instrument. If the tower current falls much below 100 mA, the typical tower base potential rise will be on the order of 2 V. It can require extra averaging to obtain a suitable waveform if the noise on the test leads is considerable. It can be desirable to reduce the probe resistance by wetting the soil in order to increase test current if this technique is to be exploited. However, this changes the shape of the electrode and spoils the estimation of resistivity. Finally, the soil
resistivity estimate is quite sensitive to changes in the humidity of the upper soil layer. All of these factors argue for using the correct tool—a three-point low-frequency earth resistance tester with an oblique probe geometry, rather than the Zed-Meter instrument—to obtain a late-time (low-frequency) impedance value.
If the tower has connections to OHGWs, the Zed-Meter instrument’s impedance profile will be subject to the same issues that affect low-frequency grounding. There will be a continuous
decline in the impedance as more and more adjacent towers take up their shares of test current. In Figure 6-9, the initial Zed-Meter reading of about 10 Ω falls to about 6 Ω at 10 μs from this effect.
Figure 6-9
Extended impedance profile (to 10 μs) obtained with grounded current reaction lead
It becomes increasingly complicated to compensate the tower base voltage for the influence voltages that arrive, delayed in time, from the adjacent towers.
Reasons for High Impedance Values
There are two main reasons why the Zed-Meter instrument’s result can be high. First, there can be a bad connection to diagnose if the right sort of current flow is not occurring in the test leads. Second, the impedance can be initially low and then rise to a value close to the impedance of the OHGWs. This is a characteristic of a poorly grounded tower situated in an area where the underlying resistivity of the rock or soil is high.
What Constitutes a High Reading?
The product of the lightning surge current with the Zed-Meter instrument’s reading gives a good estimate of the voltage that will appear on the transmission tower. The voltage across the
insulation will be a notable fraction of this potential, because the same coupling between test leads and nearby conductors also affects the OHGWs and phase conductors.
With a median lightning surge peak current of 31 kA, a 25-Ω reading will give a tower base voltage rise of 775 kV. Factoring in typical coupling, the peak voltage from this stroke can be withstood by an insulator with 0.65-m dry arc distance, typical of a string of four discs. If instead, the line is designed to withstand a severe current of 155 kA , which will be exceeded about 1.5% of the time, the potential rise and necessary insulation length to avoid backflashover
With typical transmission line reliability requirements that increase with voltage level, most utilities adopt limits for low-frequency resistance in the range of 25 Ω. For a compact electrode, such as the four foundations of a transmission tower, the corresponding Zed-Meter result is in the range of 11 to 16 Ω, depending on the time window selected for analysis. If the 25-Ω resistance has been achieved with the use of extensive buried counterpoise, then the Zed-Meter
instrument’s result will be higher and the lightning performance will be worse than expected.
Identifying Bad Connection to Current Leads
A current waveform with a lot of oscillation in the time range 0–200 ns followed by settling to a low value of a few mA indicates that there is a bad connection to the tower or current reaction lead at the Zed-Meter instrument.
With a 120-m lead length, a sudden drop in the current after 1000–1400 ns indicates that the current reaction lead is not connected to its terminating ground rod. This might have been deliberate—for example, in frozen soil—and is not a problem unless it results in excessive induced potential on the lead.
Indications of Local Soil Resistivity from Dipole Test Results
The surge impedance of the current reaction and remote potential leads in the dipole test is a function of two variables—the height of the leads over the soil and the resistivity of the soil. If the leads are laid on the earth surface, the following are some telltale signs that the underlying resistivity is high:
• A surge impedance of more than about 600 Ω.
• An early arrival of the reflection from the end of the current reaction lead. For a 120-m lead, the two-way speed at the speed of light, c, is 800 ns, so an early reflection arrival (0.8 c) for this lead length would be at 1000 ns.
• A reflection from the end of a grounded current reaction lead that drops the current almost to zero, behaving in nearly the same way as when the lead is ungrounded.
If some or all of these factors are noted in the dipole test, it is reasonable to expect a high
impedance reading from the Zed-Meter instrument when it is connected to the tower and to reset the automatic scaling to adjust to this possibility.
Reasons for Low or Negative Impedance Values
Some measurements with the Zed-Meter instruments have given results that were less than 1 Ω. As it turns out, most of these were obtained on isolated towers in areas of clay soil that had typical low-frequency resistivity of 50–80 Ωm. These towers push the limits of what can be achieved with the Zed-Meter instrument’s test method. For example, having a 1-m lead from the meter to the tower adds an inductance of about 1 μH. The L/Z time constant of this simple circuit