The Value of Continuity in a 2 5mm Cable
Introduction
One of the fundamental parts of the electrical substation is a ground grid which provides proper grounding of all apparatus in substations (including transformers, circuit breakers, capacitor banks, steel tower structures, etc.). The grounding grid is placed underneath the entire electrical substation which has a dual purpose: operating grounding, carrying faulty currents into the earth without affecting the operation of any protective equipment, and safety for personnel in the vicinity assuring that they are not exposed to an electric shock which could result from the excessive step or touch potentials.
The ground grid is usually made of copper-based (Cu) or galvanized steel tape (Fe-Zn) arranged as a square mesh of varying size, depending on the substation size (e.g. from 1 m x 1 m to larger mesh sizes). Each crossing is joined by welds or by clamps.
Over time, this grid can deteriorate due to corrosion, ground movements, grid fatigue, high energy conductance (lightning), and construction damage. All this can cause various safety problems. Since the grounding grid is buried underground, it is difficult to inspect and verify if there are corrosion and connection issues present. For this reason, it is beneficial to have a non-destructive testing method capable of verifying the integrity of the grounding grid.
There are several available test methods for inspection and condition assessment of substation ground grid. It is important to mention that 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 usually 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.
This article will present a non-destructive test method to verify the integrity of the grounding grid. The test procedure, advantages, and a case study will be presented. The article also gives a detailed description of the ground grid integrity testing with the DV Power GGT test device and its GGT-M module that has wireless communication with the GGT main unit and provides remote control.
The ground grid test can be done by using either AC or DC current, but this article will be focused on DC current method. In the presented case study the results obtained by using both AC and DC methods are compared and the conclusion is presented.
Continuity / Integrity Testing
The ground grid integrity test 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) and IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System IEEE Std 81-2012 (Revision of IEEE Std 81-1983). 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 is capable of carrying operating and fault currents.
The DV Power GGT device is specially designed for this test. GGT is a powerful DC current source that provides DC test currents up to 300 A. High output voltage (9 V DC) enables testing with long cables (e.g. 70 m) and measurement of a wide range of resistance values [3]. 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 hundreds 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.
Test Procedure
The integrity test is used to detect any bad connection, open circuit, or isolated structure or equipment in a substation grounding system. According to the standard IEEE Std 80-2000, a typical test set should contain the current source up to 300 A, voltage and current measurement channels, and two test leads. One of the two test leads is connected to a reference ground point and the other test lead is connected to a ground point to be tested. The test device generates the current between the connection points and measures the voltage drop across the ground circuit. The current that flows down to the ground grid should be measured with a proper current clamp. Keeping the reference point connected, the second test lead is moved to grounding points of other equipment and structures until the entire substation ground grid is tested.
The test setup of the GGT device with the GGT-M module and the cable connection diagram is illustrated in Figure 3. The GGT test set is provided with a set of current-carrying cables, black color marked cable 50 ft. (15 m) long and the red color marked cable 150 ft. (45 m) long. 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.
The operator takes the GGT-M module along with the red color-marked cable to control the test remotely (away from the GGT main unit).
The test procedure consists of the following steps:
- Connect the current cables to the GGT device
- Connect the current clamp to the GGT-M module
- Connect the "black" current cable to the reference ground point
- Take and connect the GGT-M module (with connected current clamp) and the "red" current cable to the inspected grounding point
- Open the current clamp jaws and connect them to the tested grounding (between the "red" clamps and the ground surface)
- Continue the testing of the next grounding by repeating the steps
During the testing, GGT generates preselected DC current (up to 300 A) between the reference and the tested grounding points. The current will be generated during preselected test duration (e.g. 60 s) and the operator can stop the test after a few seconds if the results are stable.
Interpretation of Results
The following parameters should be checked during ground grid integrity testing:
- The voltage drop between the reference and the tested grounding points
According to the IEEE Std, 80-2000 standard condition assessment of a ground grid can be done by comparing the voltage drop with a known reference value, or previous test results. When the test current of 300 A is used, the expected value for the copper grid is approximately 1.5 V/50 ft (15.24 m) between test points. 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. Additionally, the ground grid can be evaluated by comparing the resistance values with each other and determining the grounding points, which have abnormally high values.
- Inspection of the resistance values
Conclusions about the substation grounding can also be made by comparing the obtained resistances with the results from the previous testing. In case the previous results do not exist then they should be compared with other relative resistances within the same substation. The resistance values can be taken directly and easily from the GGT-M module.
Case Study
In this case study, the integrity of a complete ground grid in a 110/35/10 kV substation was tested, where the interruptions of the several connections to the mutual grounding were detected (metal reflector towers and substation gantry towers), exposing the personnel to the possible dangerous touch potentials (electrical shock).
The purpose of this test is to verify that a new part of a ground grid in the mentioned substation has been properly installed and at the same time to check the integrity of the existing part of a ground grid. Whenever safety is a concern, particularly in older substations, the ground integrity test for verifying the continuity of the grid at any point should be performed before any other tests.
To verify that there is a low-resistance path for ground currents, all accessible ground leads need to be inspected, as well as those that are buried under the earth's surface.
The testing procedure was performed according to the most relevant method described in the international standard – IEEE Guide for Safety in AC Substation Grounding IEEE Std. 80-2000 (Revision of IEEE Std. 80-1986).
The principle of measurement is based on the U-I method (Ohms law) where the high DC current is injected to a test object and the voltage drop is measured across the test object. An example of the test setup during the ground grid integrity test is shown in the following figure:
The test set consists of the high current source (0–300 A DC) and two current test leads. One test lead is connected to a reference point, which is usually a transformer grounding, and the other test lead is connected to the ground riser to be tested (in this case holding structure of current transformers). Keeping the reference lead connected, the second test lead is moved around on the other equipment and structures until the entire substation ground grid is tested.
The current probe is used for the measurement of the current below the "red clamps" connection point which provides additional data to evaluate the ground path. This current flows directly to the ground grid and is used for the resistance calculation. The voltage drop caused by the current cables is automatically excluded from the results. In this way, the voltage drop between the reference and the tested grounding points is obtained.
Characteristics of the substation ground grid
A grounding grid of the substation is constructed as a mutual grounding of the operational grounding, protective grounding, and lightning protection system. The same grid consists of two different materials, Fe-Zn strip 25 x 4 mm and 40 x 3 mm (under the two 110 kV line bays and one transformer bay), and of the copper grounding rod 50 mm2 (under one 110 kV line bay, transformer bay and 35 kV outdoor switchyard). These two different grounding grids are galvanically connected during the substation upgrade, and all the apparatus and metal parts are connected to the same.
The substation fence is connected to the separated grounding system using the Fe-Zn strip and will be examined as well.
Measurement results
The tests were performed according to the diagram in Figure 5. In this case, the test current of 100 A DC was selected and was sufficient to perform all the tests and obtain stable results.
The tests of the correct connection to the mutual grounding of all metal parts in substation like metal supporting structures of HV apparatus, power transformers housing, metal lighting towers, gantry towers, MV switchgear, protection and control panels, lightning rode connections, as well as the substation fence, etc., were performed and the results can be seen in the following table:
Notes: Based on the experience, acceptable values can go up to 100 mΩ. The increase of resistance is approximately 1 mΩ per meter (distance between reference and measure point). A longitudinal resistance of the Fe-Zn strip (25 mm x 4 mm) is in this case three times higher than the resistance of used copper rope (50 mm2).
Id – the current measured with current clamps (I down) and used for the resistance calculation
Rd – resistance calculated using Id
Rt – resistance calculated using total current (I test)
I test = 100 A
Remarks:
- Fe-Zn strips (3 pcs.) on the old gantry towers at the 110 kV line bays have galvanic interruptions with the mutual grounding system.
- Fe-Zn strip of the metal lighting tower has the galvanic interruption with the mutual grounding system.
Conclusion
Conclusions about the integrity of the grounding grid are made by comparing the obtained results with the results from the previous testing. In case the previous results do not exist then they should be compared with other relative resistances within the same substation. One way to evaluate a ground grid is to compare the values with each other and determine the test risers, which have abnormally high values.
If the measured values significantly deviate from the average values (or it is not possible to inject any current from the instrument), then the bad connection (or no connection) with the grounding grid is suspected and should be further investigated.
One can also evaluate a ground grid by comparing the voltage drop with a known reference value (typically 1.5 V/50 ft between test risers) and determining the weak ties between the risers [2]. During the condition assessment, it is important to take some factors into account which can affect the results like ambient temperature, humidity, change in the specific ground resistance, etc.
The following table shows guidelines about the interpretation of the results which are based on the experience. The recommendations are in the case of the Fe-Zn strips.
| Test results | Rt < 0,1 Ω | 0,1 Ω < Rt < 1 Ω | Rt > 1 Ω | The instrument shows an open connection |
| Conclusion | Good connection | Questionable connection | Bad connection | Open connection |
| Further investigation | Not required | Only if necessary | Required | Required |
In the case of the resistances in the range 0,1 Ω < Rt < 1 Ω, the results should be supplemented by additional testing to find where the bad connection is (if exist). Further investigation is recommended in case test results show a significantly higher resistance value compared with other test points. Test points with questionable results should be revealed to carry out a visual inspection.
Sometimes, in the case of the longer length of a test object, the resistance higher than hundreds of mΩ could be expected but it is always required to perform some additional tests or to compare the results with adjacent or similar cases.
Even resistance of less than 0,1 Ω can be bad in case of the short testing distances which means that the longitudinal resistance of the used grounding material should be respected during the investigation.
In this particular case where the instrument reported open connections, from the aspect of the galvanic connection (continuity), it is recommended to repair the damage.
Although the integrity test is very practical and convenient to perform, its results can only be analyzed subjectively. Because of the importance of a reliable ground grid, it is useful and valuable to involve this test in the routine/periodical test procedure. This test method is especially important in the case of older substations with the grid buried for a long time where a visual check has not been possible for a long time. The ground grid and its connections can corrode over time. Low resistance soil also influences corrosion of the ground grid.
Ground grid integrity testing method is accepted and described in the IEEE standards and guides (IEEE Std 80-2000 and IEEE Std 81.2012). Also, this study aimed to compare the described method with the AC testing methods which are generally accepted within the national transmission companies by following their state regulations given in [5], and [6] ("Technical standards for earthing the electrical substation and plants with the nominal voltage higher than 1 000 V"). During this research, a satisfactory match of the results was obtained.
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November 3, 2021
Source: https://www.dv-power.com/?p=28929
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