CYMGRD 7.0 – Training Exercise
CYMGRD 7.0 Training Exercise
Novemb er 18, 2015 2015 1
CYMGRD 7.0 – Training Exercise
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CYMGRD 6.5 – Training Exercise
TABLE OF CONTENTS
TRAINING EXERCISE .............................................................................. 3
1.1
SUBSTATION LAYOUT AND PARAMETERS ............................................. 3
1.2
METHODOLOGY.................................................................................... 4
1.3
STEP - 1 SOIL A NALYSIS ...................................................................... 4
1.4
STEP - 2 SAFETY CRITERIA TOLERABLE TOUCH & STEP V .................. 9
1.5
STEP - 3 GROUNDING I NSTALLATION DATA AND A NALYSIS ................ 9
1.5.1 Bus Data ........................................................................................ 9 1.5.2 Electrode Sizing ........................................................................... 10 1.5.3 Grid Installation Data ................................................................. 11 1.6
STEP - 5 DANGER POINT EVALUATION AND CONTOUR PLOTS .............. 14
1.7
STEP - 6 R EMEDIAL ACTION............................................................... 18
1.7.1 Installing Additional Conductors ................................................ 18 1.7.2 Surface Layer Resistivity ............................................................. 18 1.8
DISTINCT
AND
R ETURN ELECTRODES ................................................. 19
1.8.1 Distinct Electrodes ...................................................................... 19 1.8.2 Return Electrodes ........................................................................ 23
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CYMGRD 7.0 – Training Exercise
Training Exercise In this exercise a step-by-step sample studies are detailed: A primary electrode only model, below, and a Primary, Return and Distinct electrodes model. The descriptions of the two examples show how the most commonly used functions of the application are actually utilized.
1.1 Substation Layout and Parameters In this example, the grounding grid is square and symmetrical (meshes of equal area). It is 76.2 meters long and 76.2 meters wide. All conductors are buried at a depth of 0.5 meters. 9 conductors lie parallel to the X-axis and 9 parallel to the Y-axis. The diameter of all the conductors is 19.1 millimeters. Finally, 25 grounding rods are connected to the grounding grid in 5 Rows of 5 Rods each. The rods are 10.9 meters long with diameter 2.858 centimeters (1-1/8 inches). There are no auxiliary grounds in the vicinity and the fence of the station is to be disregarded, for now.
The grounding installation is in parallel with a resistance of 25 ohms, simulating the presence of overhead sky wire and counterpoise resistance. The total fault current is 6000 Amperes, but since the equivalent impedance of the sky wires is not infinite (9999 ohms), not all of that current will contribute to the station potential rise. The station surface is to be backfilled with crushed rock of 2500 Ohm-meter resistivity at a thickness of 0.2 meters. Safety design considerations focus concerns on an exposure duration of 500 ms and a weight of 70 Kilograms for the potential shock victim. 0
Ambient Temperature 20 C
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CYMGRD 6.5 – Training Exercise
1.2 Methodology The first step is to interpret the soil resistivity measurements and arrive at a soil model for the subsequent analysis. It is at this point that CYMGRD is used to calculate a two-layer soil model from the measurements. The second step is done automatically by CYMGRD as part of the soil model calculations. The maximum permissible touch and step voltages for the soil model is determined according to the IEEE Std 80-2000 and in accordance with the station surface treatment conditions and safety requirements. The third step is to enter the dimensions of the grounding assemblies and perform station potential rise analysis as well as to determine the station resistance. The fourth step is to carry out a danger point evaluation. The fifth step is applying any remedial actions necessary.
1.3 Step - 1 Soil Analysis Activate CYMGRD and define a new Study Title and Project Title.
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CYMGRD 7.0 – Training Exercise
Enter the soil measurements for Soil Type 2 Data in the appropriate ‘Data Entry’ view tab.
You will notice that as soon as the measurements are entered, they are reflected as dots on the ‘Soil Model’ chart in the ‘Workbook’ view.
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CYMGRD 6.5 – Training Exercise
Continue by opening the ‘Soil Parameters’ dialog box by selecting the ‘Parameters’ item in the ‘Soil’ menu, to define Soil Model and the safety analysis settings.
Click the lightning bolt symbol on the main toolbar to perform the analysis. The best fit curve will be generated and the reports will be shown in the ‘Soil Analysis’ tab of th e ‘Reports’ vi ew. This is the second step of the process.
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CYMGRD 7.0 – Training Exercise
The output report consists of the Input Data, Upper and Lower Layer resistivity, Thickness of the upper Layer, RMS Error of measurements and Tabulated Values of calculated and measured values and the % error of each.
`
Despite the fact that the results show a considerable RMS error of around 14 percent, the user can accept the soil model as is without discarding any measurements. But, should the user decide to reject some of the measurements in order to im prove (i.e. reduce) the RMS error.
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CYMGRD 6.5 – Training Exercise
The first measurement is a good choice due to its high percentage error and the rather small distance between electrodes which could affect the results. The RMS error is now 2.54 % which is acceptable and would be the soil m odel that will be used throughout this exercise.
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CYMGRD 7.0 – Training Exercise
1.4 Step - 2 Safety Criteria Tolerable Touch & Step V The second step is done automatically by CYMGRD as part of the soil model calculations. The maximum permissible touch and step voltages for the soil model is determined according to the IEEE Std 80-2000 and in accordance with the station surface treatment conditions and safety requirements.
1.5 Step - 3 Groundi ng Installation Data and Analysis 1.5.1
Bus Data
The Bus Data is the first set of information we need to enter. Bus Id Short Circuit Current Fault Duration Remote Contribution LG X/R Ratio Transmission and Distribution Tower Data
B1 6000 Amps 0.5 Seconds 100 % 15 Program Default
We can enter all pertinent general type data in the dialog box shown below, which is accessed from ‘Parameters’ item under the ‘Grid’ menu. Since there is no return electrode, the return current is 0. The Parallel impedance of the Sky Wires is considered to be 25Ω.
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CYMGRD 6.5 – Training Exercise
1.5.2
Electrode Sizing
Once the Short Circuit and Fault Duration are specified the program will suggest the minimum size of conductors that can withstand the fault current for the specified duration. This is automatically reported as soon as the desired conductors and rods are selected from the library.
Note that what is critical is the conductor Diameter. If a conductor is not listed in our library then specifying the diameter is sufficient to calculate the ground resistance of the conductor.
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CYMGRD 7.0 – Training Exercise
1.5.3
Grid Install ation Data
All conductors are buried at a depth of 0.5 meters. Nine conductors lie parallel to the X-axis and nine are parallel to the Y-axis. For analysis purposes, the conductors parallel to the Xaxis are subdivided into 8 elements and the conductors parallel to the Y-axis into 8 elements. In general we assume that an element is between connection joints of the conductors:
In Symmetrical Grids you can assume the f ollowing: •
The number of conductors parallel to X
N
•
Elements per Conductor would be
N-1
Note you can subdivide the conductor into any number of elements but the above guideline is a good way to start.
Conductors parallel to Y-axis.
Conductors parallel to X-axis.
The geometrical data can now be entered. Since the conductor assembly is a symmetrical arrangement (i.e. it can be defined as equally spaced and equidistant both X and Y directions), we will use the ‘Symmetrical Conductors’ tab in the ‘Data Entry’ view to supply the data. As soon as the data is entered the station layout will be displayed on screen.
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CYMGRD 6.5 – Training Exercise
The groundin g gri d is s quare and symm etrical (meshes of equal area). It is 76.2 meters long and 76.2 meters wide. All conducto rs are buried at a depth of 0.5 meters. 9 conductors lie parallel to the X-axis and 9 parallel to the Y-axis. The diameter of all the conduct ors is 19.1 milli meters.
We will now add the ground rods to the conductor assemblies. We can enter the grid and the rods in any order, but it is better to enter the grid layout first and then the ground rods. Similarly the symmetrical ground rod layout allows us to use the ‘Symmetrical Rod’ tab for entering the data. The rods are now shown superimposed on the grid conductors. The rod depth is defined as the distance from the surface of the earth to the top of the rods and is always entered as a positive value. 25 grounding rods are connected to the grounding grid in 5 Rows of 5 Rods each. The rods are 10.9 meters long with diameter 2.858 centimeters (1-1/8 inches). There are no auxiliary ground s in the vici nity and the fence of the station is to b e disregarded.
Complete Grid Layout wit h Conductor s and Rods
For illustration purposes the 3D representation of the Ground Grid and Electrodes is shown on the following page.
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CYMGRD 7.0 – Training Exercise
Perform a grid analysis by selecting the Grid Analysis engine and clicking on the lightning bolt icon.
The results are shown in the ‘Report’ view under the ‘Grid Analysis’ tab . The station Ground-resistance is found to be R g = 0.717249 Ω and a GPR of 4346.74 Volts.
This information alone is not sufficient to provide adequate information on the Safety of the Grid.
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CYMGRD 6.5 – Training Exercise
1.6 Step - 5 Danger point evaluation and contour plots To plot a contour of grid voltages (touch or surface), choose the corresponding engine as shown below, and then click on the lightning bolt icon.
The following dialog box is displayed. You can select the desired area to plot and the accuracy of the contour. The default coordinates are the extents of the Substation grid.
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CYMGRD 7.0 – Training Exercise
Touch voltages contours will be generated for the entire grid area. They can be shown in both 3-D and 2-D in the images that follow.
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CYMGRD 6.5 – Training Exercise
The Potential Contour Plot Report will also report the Touch Potential Threshold Color Coding and Maximum Touch Potential (X, Y) coordinates on the Grid. These Maximum Touch Potential points are also marked on the Substation Contour Plot with an (x).
A profile along the main diagonal of the grid (i.e. from the lower left to the upper right corner) will be now be analyzed using a step value of 0.22 meters (the minimum step value). Select ‘Profile Plot’ from engine selection box on the main toolbar. Select the start and end coordinates of the profile using the mouse which will then open the ‘Profile Parameters’ dialog box. You can use this dialog box to refine the final coordinates for the analysis, and then click “Ok”
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CYMGRD 7.0 – Training Exercise
The surface touch and step potentials are shown once the analysis in complete. The results show that Touch Potentials exceed the threshold which is in line with the results of the contour plot in the previous section.
By analyzing the Potential Contour Color Coding on the Grid and hovering th e mouse above specific loc ations indic ate that there are danger Zones of the Grid towards th e edges of the grid.
The Grid is therefore deemed not safe and remedial action is required.
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CYMGRD 6.5 – Training Exercise
1.7 Step - 6 Remedial Acti on Try the following two Corrective Measures and Check if the Substation is clear from any Danger Zones.
1.7.1
Installing Addition al Conductors
Install the following four additional asymmetrical conductors
Re-Run the Grid analysis foll owed by t he Contour Plot and evaluate if th e safety cri teria is met and why?
Yes
No
Why 1.7.2
Surface Layer Resisti vity
•
Disable the four conductors installed previously by simply un-checking them.
•
In the Soil Parameters enter a surface layer of 5000 Ω-m instead of 2500 Ω-m.
Re-Run the Soil analysis fol lowed by the Contour Plot and evaluate if the safety criteria is met and why?
Yes
No
Why We will after evaluating both o ptions select the first option by adding the additional Ground Conductors. Note: Change the Surface Layer Resist ivi ty Back to 2500
-m
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CYMGRD 7.0 – Training Exercise
1.8 Distinct and Return Electrodes If a ground fault occurs within the substation, and that another electrode at a certain distance from the grid absorbs the current injected to the grid, this electrode becomes a Return electrode. Also, if adjacent building foundations are present, but are not energized, they will need to be modeled as a distinct electrode.
1.8.1
Disti nct Electro des
Let assume that there is an underground pipe that has some parts of it above ground near the vicinity of the substation. The objective is to determine if the has to be grounded by connecting it the Substation ground grid. Outer Radius
R2
12.5 cm
Inner Radius
R1
10.0 cm
This can be modeled as Hollow Conductors of inner radius R1 and outer radius R2. The Equivalent Radius and Diameter of the solid conductor would be as follows.
Req
=
2
R2
−
2
R1
Deq
=
2 Req
Coordinates of the Pipe
We can model this pipe as an Asy mm etr ic al Dis ti nc t Con du ct or having a Diameter of 150 mm.
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CYMGRD 6.5 – Training Exercise
Run the Grid analysis and notice that the Output Results will now report the distinct electrode potential which is at 1990.39 V.
This is much higher than the tolerable touch voltage of the substation of 918.5 V. Therefore the pipe has to be connected to the Grid by the follow ing Primary conduc tors.
Note: Remember to change the Distinct Electrode into a Primary Conductor since it is now con nected to the Grid.
Run the Grid analysis again and plot the resulting contour plot by modifying the plot settings with the right click of the mouse over the contour plot. •
Right Click Mouse Button → Settin gs
•
Chart → Title Change to “Contour Plot with Pipe Connected to Grid”
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CYMGRD 7.0 – Training Exercise
•
Chart → Graph → Background Change both “Color” and “Color 2” to white.
•
Chart → Graph → Contours Change Style to “Lines with Labels”.
•
Click on the Apply Button
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CYMGRD 6.5 – Training Exercise
The resulting Contour plot is shown below
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CYMGRD 7.0 – Training Exercise
1.8.2
Retur n Electro des
Let us now evaluate the effects of having a new energized installation next to the grid and assuming that the total fault current return path is through this installation. Therefore in the Grid Parameters you have to enter a Return Electrode Current of -6000 Amps to account for the effect of this new installation.
Return Symmetrical Square Grid
Return Asymmetrical Rod at the Lower Left Corner
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CYMGRD 6.5 – Training Exercise
Run the Grid analysis again and plot the resulting contour plot by modifying the plot settings with the right click of the mouse over the contour plot.
•
Right Click Mouse Button → Settin gs
•
Chart → Title Change to “Plot with Return Electrode Effects”
•
Chart → Graph → Background Change both “Color” and “Color 2” to white.
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CYMGRD 7.0 – Training Exercise
•
Chart → Graph → Contours Change Style to “Lines with Labels” and the number of “Levels” to 100.
•
Click on the Apply Button
The resulting Contour plot is shown below notice how the presence of the return electrode has altered the potential profile of the main substation and has created danger areas within the rectangular area show below.
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CYMGRD 6.5 – Training Exercise
Therefore the energized structure has to be connected to the Grid with the following two Primary conduct ors.
Note: Remember to change the Return Distinct Electrode Grid and Ground Rod into a Primary Conductor sinc e it is now connected to the Grid.
Run the Grid analysis again and plot the resulting contour plot by modifying the plot settings as per the previous sections.
The Analys is i s comp lete for thi s Trainin g Exercis e. However you need to noti ce that the new gri d requires fur ther investigation as there are some danger zones whi ch can easily be corrected by adding additi onal conductor s or rods as covered by this training session
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