Organization of Data
The log files and line reports are helpful for organizing records, but post-processing is done on the waveform files. As shown in Figure 7-1, the waveform file consists of a header followed by 8000 records of time (ns), voltage rise, currents, and impedances.
Figure 7-1
Example of data file for Zed-Meter® instrument test of National Grid line 4VW_234 structure
It is recommended that post-processing be done in Excel, which offers a suitably rich set of options and is widely available in the electrical utility industry. Other programs, such as Matlab software from The MathWorks, Inc., can offer a richer set of resources in the signal processing toolboxes that are interesting to PhD-level engineers, but they are also susceptible to the common problem of “torturing data until it confesses,” whether it is innocent or guilty. Converting Waveform Files to Excel Format
The waveform files can be accessed with Excel (File, Open, All files). The Text Import Wizard will start automatically. The waveform files can be imported into the native Excel format by clicking the Finish button on the first screen of the Wizard. The description of the waveform files in Figure 7-1 can be used to identify the columns.
Producing Waveform Graphs in Excel
Generally, variables should be plotted against the vector of time values (ns) found in the first column (A).
Exporting Waveform Graphs from Excel to Portable Document Format
Generally, graphs exported from Excel have poor resolution and look bad when imported into technical papers and reports. One way to improve quality is to print the graph as an Adobe portable document format (.pdf) document, selecting high-resolution options. The conversion process produces a scalable document that satisfies most publication quality requirements. Establishing Confidence Level in Test Results
For identifying the sweet spot of the waveform, a plot of the lead and structure impedances, along with the local standard deviation of 32 values, 16 from each vector, has been used in many of the plots in this report. These graphs have also used a logarithmic axis for the impedance in order to see the three variables with good resolution on the same page.
Adaptive Filtering to Improve Confidence
The analyst has good confidence in the Zed-Meter instrument’s result if the standard deviation is a small fraction of the result. With an inherent instrument standard deviation of about 1 Ω, this means that measurements of towers with low impedance might need to be improved with post- processing. Offset subtraction and noise removal procedures have proved to be useful in some situations.
Offset Subtraction
The first set of post-processing duplicated the intended functions of the Labview software in the prototype instrument. The offset for each voltage and current signal was calculated from the average of the last 2000 points in the 8000-sample (80-μs) record. This value was then subtracted from every point. The sample-by-sample values of impedance were given by the measured potential, V, divided by I1 or by I2. The standard deviation of 15 samples using both Z1
and Z2 values was also computed.
This processing assumes that there is a quasi-static offset that does not change much over the 80- μs record length. The Zed-Meter instrument configuration used in this study does not provide a pretrigger record to conduct the offset process on the values before the trigger point. Preliminary work with oscilloscopes used about 25% pretrigger with a 10,000-sample record length [2, 3, 4]. When the offset is computed from the points near the front of wave, uncertainty about the change in noise signal over the 80-μs record length is reduced to uncertainty about the change over the 2-μs measurement interval. Now that the Zed-Meter instrument is automatically set up to use pretrigger values, these are the preferred data that are used to compute the necessary offset corrections for each averaged signal.
Tests at one tower in Manitoba showed a high noise level on the potential lead, with up to a 30-V offset in the tail of wave to be subtracted from every point. At this tower, the Zed-Meter
Table 7-1
Typical variations in Zed-Meter® instrument test results, S65R tower 67
Tower 67, Trial Number 1 2 3 4
Offset (V) -30 -11 +6 -18
Impedance (Ω), Offset Correction 3.79 3.78 5.17 5.73
Impedance (Ω), Denoising Process 4.17 4.31 5.43 5.90
There was less than one standard deviation difference among the results with offset correction. Table 7-1 also shows the effects of the digital artifact denoising process of about 3 Vp-p.
Denoising Digital Artifact
One Zed-Meter instrument prototype had internal digital noise problems that could have compromised a test campaign. The noise problem in this case was that, in the pretrigger record and again at the tail of the wave, the voltage should be constant and should average to zero. What was found was a repetitive pattern that was quantized into discrete levels, with an 8-sample period as shown in Figure 7-2.
Figure 7-2
Tail-of-wave noise pattern in Manitoba Hydro Zed-Meter® instrument trials
This pattern was fitted by adapting each value to give a minimum error on the tail of the wave, where the signal should be low. The repetitive pattern was extended along the entire vector of voltage values and subtracted. Figure 7-3 shows the degree of improvement that can be achieved with this relatively simple process.
Z(t) before denoising Z(t) after denoising
Figure 7-3
Improvement in measurement uncertainty with 16-sample denoising, Manitoba Hydro S65R_067
The facts that the repetitive pattern was synchronized to the sample rate, that it had discrete levels corresponding to quantization limits, and that it was not eliminated by averaging suggest some combination of internal software and hardware problems that were later resolved. For user- customized versions, the following suggestions to eliminate the noise should again be checked: • Are portions, rather than the entire vector of voltage values, being averaged?
• Is the dynamic range of the digitizer (expressed as number of effective bits versus sample rate) deficient at high sample rates?
• Is there internal coupling from the potential lead to the digitizer hardware?
The good experience with denoising can also be extended to cases in which, despite averaging, there is an interference signal in the pretrigger record. The same principles—identifying and fitting the noise record in Excel and subtracting the fitted noise signal from the original record— can achieve success in these cases.
Removing Noise Using Pretrigger Record
The use of a pretrigger period of 1500 ns can show currents and voltages that are nonzero, even after averaging many test impulses. One example of interference with a period of about 500 ns (frequency of 2 MHz) was recorded during tower impedance tests at the National Grid in the United Kingdom. Figure 7-4 shows that peak-to-peak currents of about 4 mA, with
Line ZK, tower 1011 (ZK_1011) Line 4VW, tower 2341 (4VW_2341)
Figure 7-4
Pretrigger records for Zed-Meter® instrument impedance tests at National Grid
The complex nature of the noise in the ZK_1011 record will be difficult to fit with a simple set of sine and cosine wave functions. However, the 1100-kHz pretrigger noise in the 4VW_2341 record has a relatively simple form that could be fitted by a sine term and a linear term, using the Excel Solver add-in to minimize an error sum by adjusting the frequency, phase, and amplitude of the correction function.
Validating a Novel Test Lead Arrangement
It has proved to be helpful to conduct numerical analyses of the frequency and time domain response of Zed-Meter instrument’s test lead configurations. In particular, the calculations have given increased confidence that the rising impedance profiles in high-resistivity soil have a sound physical basis. Other features of the test waves, such as the effect of lead direction and orientation, have also been tested. The use of additional lead length to optimize the sweet spot of the Z(t) profile was optimized with the calculation results. Also, the need to have a short and direct connection to the tower leg was an output of numerical analysis.
When field test results are returned with a note or sketch showing what was done to obtain a result, it might be necessary to determine a correction factor for the result. This would normally be done by a specialist in time-domain electromagnetics, using one of the standard modeling programs, such as a version of the Numerical Electromagnetics Code (NEC).
Validating Against What?
The standard in-line test lead configuration uses current reaction and remote potential leads that run along the right of way, parallel to the OHGWs and phase conductors. The leads are oriented at 180°. Changes to the lead orientation and geometry have generally been evaluated against the benchmark result.
It is reasonable to ask how well the Zed-Meter instrument actually simulates the response of the tower to real lightning. This breaks into two separate questions: what happens for a stroke to midspan, and what happens for a stroke to the top of the tower.
Modeling the lightning channel above the tower is feasible, but it calls for the adoption of an engineering model of the channel that accurately reproduces the nearby electric and magnetic fields. This is a rather specialized domain, and the results can depend to some extent on the choice of return stroke model. An interim step is to evaluate the response of the tower and OHGW to a current injected at midspan as shown in Figure 7-5.
Figure 7-5
Reference result: impulse injection into midspan
Qualitatively, after noting that there will be a time delay of about 400 ns for the pulse to reach the tower base, there is reasonable agreement between results. For a soil resistivity of 50 Ωm, the voltages in both cases stabilize at about 2 V in the time range from about 200 ns to 700 ns after the initial peak. The result is about 18 V for the case of 1000-Ωm soil. With 20,000-Ωm soil, the voltage no longer reaches a constant value. Instead, it rises continuously, to a level of 100 V at 900 ns. Effectively, the tower is acting as a lumped capacitance in this case, and the ground electrode has no effect on the resulting voltage. (See Figure 7-6.)
Pulse injection at midspan Zed-Meter instrument injection at tower base
50 Ωm
1000 Ωm
20,000 Ωm
Figure 7-6
Tower base potential to remote ground for midspan and Zed-Meter® instrument current injection
0 0.5 1 1.5 2 2.5 -60 -40 -20 0 20 40 60 80 100 120 Time [microseconds] V o lt ag e [ V ] V tower-grd.ref Seg.No.110 0 0.5 1 1.5 2 2.5 -50 0 50 100 150 Time [microseconds] Vol tag e [ V ] V tower-grd.ref Seg.No.1 0 0.5 1 1.5 2 2.5 -15 -10 -5 0 5 10 15 20 25 30 Time [microseconds] V o lt age [ V ] Vtower-grd.ref Seg.No.110 0 0.5 1 1.5 2 2.5 -5 0 5 10 15 20 25 Time [microseconds] V ol tage [ V ] Vtower-grd.ref Seg.No.1 0 0.5 1 1.5 2 2.5 -2 -1 0 1 2 3 4 5 6
Sim# Pulse Injection GW-50 BIS
Time [microseconds] V ol tage [ V ] V tower-grd.ref Seg.No.1 0 0.5 1 1.5 2 2.5 -2 0 2 4 6 8 10 12 14 Time [microseconds] V o lt age [ V ] Vtower-grd.ref Seg.No.110
NEC-2 for Simple Coupling Estimates
The NEC-2 computer program has some notable advantages for analyzing coupling. It is freely available on the Internet, and it is supported by the amateur (ham) radio community. Typically, it is used to calculate the impedance and radiation pattern of antennas at one or a narrow band of frequencies. The antenna geometry is described by a file with a list of start and end points, wire radius, and other factors. Lossy soil can be modeled, using the Sommerfeld-Norton model, but one limitation in calculations is that the antenna segments cannot penetrate into the lossy ground. For time-domain calculations, computations must be performed for large numbers of points covering the entire frequency spectrum of interest. The results must then be transformed into the time domain, using an inverse Fourier transform function in Excel, Matlab, or another program. The NEC-2 program comes with a specific case study for Beverage antennas. This case can be modified easily to match the characteristics of the Zed-Meter instrument’s current reaction and remote potential leads. For evaluating coupling in cases with buried wires, the NEC-4 computer program must be used as described in Appendix F. The Zed-Meter system computations in this document have been carried out in the frequency range of 195.3 kHz to 100 MHz with the increment step of 195.3 kHz. This corresponds to the time range of 0–5.12µs with a 5-ns sampling interval.
8
REFERENCES
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