Answer:

1. The decrement factor has been corrected in the new guide. There is simply a typo in the old guide. However, in the derivation of the decrement factor in the new guide, Equation (77) is wrong and does not make sense. The final expression of the decrement factor in the new guide, Equation (79), is correct.
2. Your interpretation of the changes for the Cs factor in the new guide is correct. The Cs factor in the new guide is higher than that in the 86 guide. I did a test using MALT. For the case with K=-0.9, h=0.1 m, I modeled a spiral mat and the computed Cs is 0.727, which is close to that of the new guide (0.71 from Figure 11). I did not verify your statement about the 25% increase in allowable step and touch voltages in areas where surface materials such as asphalt (10,000 ohm-m) are applied. Obviously, the increase in the Cs factor will increase the threshold values for touch and step voltages.

Dr. Jinxi Ma, Manager of Analytical R&D, SES

Modeling Buried Cable, in FCDIST

Presently, FCDIST can not model buried conductors (although it is our plan to implement this option in the near future). Meanwhile, however, it is still possible to use FCDIST to carry out a fault study for buried cables using both the configuration and the Impedance method, in FCDIST. In order to use the Impedance method, the series and mutual impedances of the underground cables have to be computed. Once these impedances are obtained, a user can follow Chapter 5 in the "A Simple Substation Grounding Analysis" Engineering How To... Guide to prepare the input file. For the "tower" ground resistance, specify the ground resistance of any grounding along your cable sheath. Your span length should be the average distance between such grounds.

In both cases, the cable should be modeled aboveground (i.e., use the Configuration method), even if they are buried in reality. This is a good approximation as long as the cable is not buried at great depths. 

Dr. Winston Ruan, Senior Research Scientist, SES

Modeling Tap, in FCDIST

The scenario considered is as follows: a new tap is added which connects an existing substation (Substation A) with a new substation (Substation TAP). The distance between the two stations is short (few spans) in this case. The fault current distribution calculation is to be carried out at the new substation (Substation TAP), while the fault current data is only available at the existing substation. The question is: assuming that the fault current data at Substation TAP is the same as that at Substation A (since they are close), how to use FCDIST to compute the fault current distribution at Substation TAP when there is a fault.

This is a trivial task if TRALIN and SPLITS modules are used. This is because in SPLITS the tower impedances can be different along any terminals and the fault can be introduced at any locations. In a TRALIN/SPLITS model, the central site will be defined at Substation TAP and the existing substation is represented by a lower grounding impedance in one of the terminals.

The technique for using FCDIST to carry out this study is as follows. The central site is defined at the existing station (Substation A) with its grounding impedance properly entered. Introduce a terminal (Terminal TAP) corresponding to Substation TAP. The source current of Terminal TAP will be the sum of the total fault current at the central site (Substation A). The angle of the source current will be 180 degree out of phase with the angle of the total fault current at the central site. With this setup, the total fault current injected at the central site (Substation A) is forced to be zero and the fault is now moved to Substation TAP. At the same time, the lower grounding grid impedance at the existing substation (central site), as well as the influence of the overhead wires along the tap are also considered in the computation. The fault current injected at Substation TAP can be found from the Total Earth Current at Terminal TAP in the FCDIST Report.

Dr. Winston Ruan, Senior Research Scientist, SES

Did you know that the Preprocess and MonitorFault modules can be used independantly without the need to specify the right-of-way network?

In the Right-Of-Way software, the Preprocess module is a utility that retrieves data from, and makes modifications to, a SPLITS-compatible network definition input file initially produced by the ROW program or any of the CDEGS subpackages.

The MonitorFault module allows the user to automatically create faults along any transmission line at any combination of group of sections (spans) with the possibility to define arbitrary section increments between applied faults. The program generates summary output files containing pertinent information about the designated "victim" phase conductor (usually a pipeline or rail track) and a reference phase / line conductor (i.e., overhead ground wire) at every section, such as maximum phase (victim or reference) GPR, maximum earth injection current at each tower, maximum victim phase longitudinal current, maximum line-to-line voltage, etc.

In general, a user inputs the right-of-way configuration (network) and creates a circuit model (SPLITS file) corresponding to the right-of-way configuration first. Then the Preprocess and MonitorFault modules can be used for special file editing purposes. However, if one has created a SPLITS file using other software, for example, the SPLITS module of CDEGS, then one can directly use the Preprocess and MonitorFault modules without the need to specify the right-of-way original network data. The procedure is as follows:

1. Launch the ROW program.
2. Create a new project and scenario (assume that the scenario name is ScenarioName).
3. Exit the ROW program after saving the project.
4. Copy the SPLITS file which was created with another program, and rename it as SP_ScenarioName_0.f05.
5. Launch the ROW program and load the project that was now created.
6. The Preprocess and MonitorFault modules are available. One can start from here to make any operations in these two modules.

Mrs. Yexu Li, Research Scientist, SES

Question:

Where should I specify the observation points in order to get accurate values for the touch voltages over a large grid?

Answers:

By definition, the touch voltage is the difference between the earth potential at a given point and the potential of a nearby conductor connected to the grounding grid. Now, the earth potential will usually reach its maximum just above a buried conductor and can decrease quite fast (depending on the type of soil) as you move away from it. This is what explains why the touch voltage gets to be very large for points far away from any buried conductor.

Typically, the region of concern for touch voltages extends about 1 meter (3 feet) away from an energized, buried conductor. Moreover, the program can reliably interpolate the results to a distance of 1 m of a conductor only if the actual computation point is not much farther than 1 m, since the earth potential is not a "smooth function" ( i.e. it is full of spikes) and the interpolated results would be extremely sensitive to the location of the computation points.

The best solution is therefore to specify computation points in the MALZ input so that:

• they cover the region of interest
• they are no more than 1 m. apart in both the X and Y directions

• specifying a few observation surfaces in the corners of the grid and other "hot spots"
• specifying individual computation profiles running parallel to the grid conductors and about 1 m away from them (on both sides of the conductors)

Dr. Simon Fortin, Senior Research Scientist, SES

A CDEGS user asked:

I am studying a fast lightning transient signal using HIFREQ and FFTSES. In order to get a good resolution of the lightning signal in the time domain, I have to use a large number of time samples (2^11) and a short time window (300 microseconds). As a result, the program recommends computation frequencies up to 4 MHz. Is it necessary to run all these frequencies?

SES answers:

Generally, you do not have to run HIFREQ at all frequencies recommended by FFTSES. You only have to run HIFREQ at zero Hertz, as well as any other frequencies you believe are appropriate. For instance, you can choose to ignore frequencies recommended by FFTSES that are higher than a certain cut-off frequency if you realize that the input signal is too small past that frequency or if you expect the response function for the GPR or electromagnetic fields of interest to be negligible at those frequencies. The actual value of the cut-off frequency depends on the case being studied. It could be 1 MHz or even as low as 500 kHz. The criterion is that the modulated spectrum (product of the response and the input signal) should be negligeably small for frequencies larger than the cut-off.

Dr. Simon Fortin, Senior Research Scientist, SES