Beneath every substation, there is a ground grid that provides proper grounding of all objects in substations (e.g., transformers, circuit breakers, steel tower structures, fences, etc.). The ground grid is usually made of copper-based connections arranged as a square mesh of varying sizes (e.g., from 1 m x 1 m to larger mesh sizes). Each crossing is joined by welds or by clamps. The grounding grid has a dual purpose: to carry faulty currents into the earth without affecting the operation of any protective equipment and to provide safety for personnel assuring that they are not exposed to an electric shock that could result from the excessive step or touch potential. Over time, a grid can deteriorate due to corrosion, ground movements, grid fatigue, high energy conductance (lightning), and construction damage. All this can cause various safety problems.
There are several available test methods for inspection and condition assessment of substation ground grid. Different methods can be combined and used together to provide more reliable information about ground grid conditions. Commonly used test methods for grounding grid condition assessment are:
- Ground impedance measurement methods (e.g., the two-point, three-point, and four-point methods)
- Continuity/integrity testing
- Touch and step voltage measurements
- Soil resistivity measurement
The maintenance strategy usually involves test methods for condition assessment of the ground grid. Soil resistivity measurement is performed during the substation design stage. The ground grid impedance measurement (with step and touch voltages) is performed regularly during periodical maintenance. However, those tests are not effective in detecting the grid connections issues that affect its continuity.
The ground grid continuity/integrity (a non-destructive test method) is the most relevant test method/technique for measuring the electrical characteristics of the substation grounding system. The test is described in international standards – IEEE Guide for Safety in AC Substation Grounding IEEE Std. 80-2000 (Revision of IEEE Std. 80-1986). The test should have differed from the ground impedance measurement that requires the use of AC power sources and inspection of the soil resistivity.
Ground Grid Integrity Testing Using GGT
This application note gives a detailed description of the ground grid integrity testing with GGT200 & GGT500 devices and the GGT-M module. The test is controlled remotely by the battery-operated GGT-M module that has wireless communication with GGT main unit.
The test setup of GGT200 / GGT300 devices with the GGT-M module and the cable connection diagram is illustrated in Figure 1.
GGT is a powerful DC current source that provides DC test currents up to 300 A. High output voltage enables testing with long cables (e.g., 60 m) and measurement of a wide range of resistance values. Long test cables are very important for this application because they simplify the test procedure. The test device can be placed in one place during the measurement of all grounding points in the substation. Since substation contains dozens of grounding points, features that save time and simplify work are crucial.
To simplify the test procedure additional GGT-M module has been developed. It provides remote control of the test device and enables the remote start/stop of the test and monitoring of measured results. It also increases the safety of personnel because the starting and stopping of the test are controlled directly from the measurement point.
Accessories and Test Setup
The integrity test verifies the continuity between two different ground points of the grid. In this way, connections with the ground grid are verified confirming that the grounding line can carry operating and fault currents.
The GGT test set is provided with a set of current-carrying cables and sense leads, black color marked cable (e.g., 10 m / 33 ft.), and the red color marked cable (10 m + 20 m + 20 m extension, totally 50 m / 165 ft.). The black color-marked cable should be connected to a good reference ground point (e.g., usually near the center of the substation and at a major piece of apparatus like a transformer or breaker that has multiple ground connections). The red color marked cable is sequentially connected to exposed ground leads in the substation that needs to be inspected.
An operator takes the GGT-M module along with the red color-marked cable to control the testing remotely (away from the GGT main unit).
The test procedure consists of the following steps:
- Connect the current and heavy-duty sense cables 2 x 10 m (figure 3) to the GGT device
- Connect 20 m extensions (figure 4) (if purchased) to the red 10 m current cable
- Connect the current and sense 2 x 35 cm cables at the other ends of the already connected current cables
- Make a proper grounding of the GGT (e.g., connect the grounding screw of the GGT to a PE using only the manufacturer provided grounding cable)
- Connect the current clamp (figure 5) to the GGT-M module
- Turn ON the GGT device
- Connect the “black” current cable to the reference ground point
- Turn ON the GGT-M module and the current clamp
- Take the GGT-M module (with connected current clamp) and the “red” current cable to the grounding point under inspection
- Connect the “red” current cable to the inspected ground point
- Open the jaws of the current clamps and connect them to the tested grounding (between the “red” clamps and the ground surface)
- Set the test current and test duration on the GGT-M module and start the test by pressing the Ω button
- The measured test parameters will be displayed on the GGT-M and GGT200/GGT300 devices and automatically saved in the device’s internal memory
- Continue the testing of the next grounding points by repeating steps 9 – 12.
During the test, GGT generates preselected DC current (e.g., up to 300 A for the GGT300 model) between the reference and the tested grounding points. The current will be generated during a preselected test duration (e.g., 60 s) and the operator can stop the test after a few seconds if the results are satisfactory.
The typical time required for testing one grounding is approximately 2 minutes (including the cable connections). This time refers to steps 9 – 12.
If the distances are too large for full substation testing, the reference lead (“black” current cable) can be connected to another allegedly good reference point. The first test in this case should be done with the “red” current cable connected back to the original reference point to verify a pass.
Interpretation of the Results
The following parameters should be checked during ground integrity testing:
The Voltage Drop Between the Reference and the Tested Grounding Points
According to the IEEE Std, 80-2000, when 300A is passing through the ground grid between a reference point and the ground point under test, the acceptable voltage drop should rise approximately 1.5 V for each 50 ft. (15,24 m) of straight distance from the reference point. A voltage drop higher than 1,5 V indicates a potential bad ground connection.
Criteria for determining the condition of the tested grounding depend on ground grid material. It should be taken into consideration that sometimes the ground grid can be made by using Fe-Zn strips which might have higher resistance values, compared to the copper grid.
As a reference we can use values from the table below, for copper-based groundings and use of 300 A test current:
Note: If the voltage drop is too high for several initial measurement points, or the test set fails to deliver the 300 A test current, this means a bad path OR it can be a bad reference point. If this happens, check the clamp connections then select a different reference point and execute the test again.
Current Flow Inspection at the Tested Grounding Points
The current probe connected to the GGT-M module is used for the measurement of the current below the “red clamps” connection point. This current flows directly to the ground grid and it is called “DOWN current”.
a) For a single grounding connection, the “DOWN” current can be considered satisfactory if the voltage drop is in line as explained above and at least 200A flows to the ground conductor (300A is a total generated current).
b) For the equipment in a substation with multiple grounds, the ground can be considered satisfactory if the voltage drop is in line as explained above and at least 150A flows to the ground conductor (300A is a total generated current).
Inspection of the Resistance Values
In such cases when the instrument cannot generate the selected current (e.g.,300A), because of the high burden, then the calculated resistance values can also be used for the analysis.
The GGT-M module besides the current and voltage values is also displaying the resistance values calculated by using the total generated current and current recorded by current clamps.
These values can be compared with the obtained resistance values from the previous testing or with the expected values, or to compare the results between the test risers and search for those with abnormally high resistance values.
The objective is to determine whether the equipment, frames, structures, etc. are connected properly to the ground grid with very low resistance (less than a couple of mOhm).
Note: These are only guidelines, and each ground should be inspected subjectively on its regulations and standards.
May 17, 2022