The continuity and integrity test is one of the relevant techniques for assessing the condition of substation grounding systems. This article explains how to detect inconsistencies in the grounding grids, including poor connection of HV equipment to the ground grid and deterioration of the grid itself, and suggests suitable field tests and appropriate equipment for detecting those inconsistencies.
The test’s objective is to determine whether all the connections are established with low resistance. For that purpose, a high DC source will be used to detect any open circuit or isolated structure or equipment in a substation. A power source that can provide up to 300 A is recommended for this application. Case studies will be presented to show a simple way of detecting bad connections in the grounding system, which can be very dangerous for personnel.
A fundamental part of the electrical substation is a ground grid that provides proper grounding of high-voltage apparatus, switchgear equipment, buildings, steel tower structures, etc. The grounding grid is placed underneath the entire electrical substation, which has a dual purpose:
- Establishing a connection to carry fault currents to and from the earth.
- Safety for personnel in the vicinity assuring they are not exposed to electric shock that could result from the excessive step or touch potentials. This is accomplished by connecting all groundings and metal sections using an equipotential ground mesh.
The ground grid is usually made of copper-based (Cu) or galvanized-steel tape (Fe-Zn) arranged as a square mesh of varying sizes, depending on the substation size (e.g., from 1 m x 1 m to larger mesh sizes). Each crossing is joined by welds or 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 grids are buried in the ground, it is challenging to inspect and verify if corrosion and connection issues are present. For this reason, it is beneficial to have a non-destructive testing method capable of verifying the integrity of the grounding grid.
TEST METHODS
Several available test methods exist for inspection and condition assessment of substation ground grids. The various methods can be combined or used together to evaluate the condition of the grounding grid. Commonly used test methods for grounding grid condition assessment include:
- Ground impedance (resistance) measurement (e.g., the two-point or three-point methods)
- Soil resistivity measurement (four-point method)
- Touch and step voltage measurements
- Continuity/integrity testing
Soil resistivity measurement is usually performed during the substation design stage. The ground resistance measurement (with step and touch potentials) is performed regularly during periodical maintenance. It is the most common test to measure the overall resistance to remote earth. The objective is to check whether the measured resistance value is like the one that was obtained during the installation stage.
However, these tests are not effective in detecting the ground grid connection issues that affect its continuity. This paper will present a non-destructive test method to verify the integrity of the ground grid. The test procedure, advantages, and case studies are also presented.
The ground grid test can be done by using an AC or DC source, but this paper focuses on the DC high-current method. The use of high DC provides reliable and accurate results by means of measuring the stable value of voltage, as well as the current magnitude and direction to verify the continuity of the ground connections.
CONTINUITY AND INTEGRITY TESTING
The ground grid continuity/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 Std. 80-2013, IEEE Guide for Safety in AC Substation Grounding (Revision of IEEE Std. 80-1986)
- IEEE Std. 81-2012, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System (Revision of IEEE Std. 81-1983).
This test confirms the continuity between two different ground points of interest verifying that the grounding line can carry operating and fault currents (Figure 2).
Test Procedure
According to IEEE Std. 80, a typical test set should contain the current source up to 300 A, voltage and current measurement channels, and two test current leads. One of the two test leads is connected to a reference ground point; the other is connected to a ground point to be tested. A transformer neutral should be used as a reference point whenever possible. For large substations, several reference grounding points may be necessary, where each new reference point must be verified first. Any ground riser that satisfactorily passes the test criterion may be verified as a reference.
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 sequentially connected to exposed ground risers in the substation that need to be evaluated.
To improve this kind of measurement, voltage sense leads are also included, which significantly improves the accuracy of the results.
Using only current cables requires measuring their voltage drop each time after the test is performed and then subtracting that value from the measurements. This certainly helps but they are made of copper and conduct 300 amps, so they will heat, and the resistance will change during the test. The condition of the clamps and the presence of oxide, foreign matter, etc., at the point of connection may also create an error. The unknown and variable contact resistance is being measured in a way that can be significant, given that the resistance to the ground grid and the resistance of the ground grid itself is normally very low.
Test Procedure Steps (Figure 3)
- Connect the black current cable to the reference ground point (below any bonding connections or clamps) — transformer neutral in this case.
- Take the control module (with connected current clamp) and the red current cable to the evaluated grounding point.
- Start the test and record the voltage drop at each current injection point.
- Use the DC clamps to measure the amount of current in the ground riser below the red clamps.
- Record the total current generated by the instrument.
- Continue testing the next grounding points.
INTERPRETING THE RESULTS
The following parameters should be checked during ground grid integrity testing:
- Voltage drop between the reference and the tested grounding points. According to IEEE Std. 80, 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. The voltage drop of each riser in the vicinity should be similar. It should be noted that sometimes the ground grid can be made by using Fe-Zn strips, which might have higher resistance values compared to the copper grid.
- Current flow inspection. The current measurement is done with the current clamps connected to the GGT-M module (as seen in Figure 3).
a) For a single grounding connection, a DOWN current can be considered acceptable if the voltage drop is in line as explained above and at least 200 A flows to the ground conductor (300 A is a total generated current). DOWN current is the current we measure with the red current clamp mentioned in Figure 3. For example, when the main unit injects 300 A, most of that current should go DOWN through the ground riser/conductor under test — if the connection to the ground grid is in good condition — and one part of the injected current will flow through the other parallel paths, as explained in Figure 4.
b) For equipment in a substation with multiple grounds, the ground can be considered acceptable if the voltage drop is in line as explained above and at least 150 A flows to the ground conductor (300 A is a total generated current).
- Inspection of Resistance Values. Conclusions about substation grounding can also be made by comparing the obtained resistance values with the results from the previous testing. If 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 instrument or GGT-M module.
Two resistance values can be observed while using this method. The equivalent resistance, which includes all connections connected in parallel (Figure 4), is calculated by the total generated current from the instrument and the measured voltage drop with the sense leads.
The resistance of single connection RX is calculated by the current recorded with the current clamp and measured voltage drop with the sense leads.
CASE STUDY
In this case study, the integrity of a complete ground grid was tested in a substation where the interruptions of several connections to the ground grid were detected, exposing personnel to possible dangerous touch potentials (electrical shock).
The purpose of this test is to verify that a new part of a ground grid in the 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 points (Figure 5) must be evaluated.
The principle of measurement is based on the U-I method (Ohms law) where a high DC current is injected into a test object and the voltage drop is measured across the test object. Down current was measured at the same time for a more detailed inspection.
Measurement Results
In this case, a test current of 300 A DC was selected and was sufficient to perform all the tests and obtain stable results.
The continuity test to the mutual grounding of all metal parts in the substation — including metal supporting structures of HV apparatus, power transformers housing, metal lighting towers, gantry towers, MV switchgear, protection and control panels, lightning rod connections, and the substation fence — were performed. The results can be seen in Table 1.
Remarks
Test Point 14 should be evaluated because increased voltage drop and resistance indicated a possible bad connection.
Test Point 15 should be evaluated since only 85 A went down and the rest of the current went up through the parallel path, indicating a bad connection.
Test Point 18 has an open connection because all the injected current went up through the parallel path.
CONCLUSION
Buried underground, the grounding grid cannot be visually evaluated during most electrical maintenance procedures. It may seem static/inert, but during lightning impacts or stress during a fault, a grid can be severely damaged, interrupting its continuity, introducing increased resistance across the connections, etc. Even though the grid was previously in known good condition and cleared the fault as designed, there are no obvious indications about its condition after it is subjected to these conditions. The high-current method of testing ground grid continuity provides an efficient, reliable, and accurate method of detecting faulty connections in a short time.
REFERENCES
IEEE Std. 80-2000, Guide for Safety in AC Substation Grounding (Revision of IEEE Std. 80-1986).
IEEE Std. 81-2012, Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System (Revision of IEEE Std. 81-1983).
DV Power. M-GGTM00-307-EN. GGT Series User Manual. Available at https://www.dv-power.com/download/ggt-series-user-manual/.
DV Power. A-GGT000-303-EN. Application Note: Ground Grid Integrity Testing. Available at: https://www.dv-power.com/download/ground-grid-integrity-testing/.
DV Power. Test Report 1 Special Measurement in Transformer Station 110/35/10 kV. Available on request.
DV Power. Test Report 2 Special Measurement in Transformer Station 110/35 kV. Available on request.
Evelin Sokolović, Goran Skelo, Alija Smaka. CIGRE R.B3.02, “The Stake-Less Measurement Method in the Continuity Test of the Lightning Protection System,” 8th CIGRE Session, Neum, Bosnia and Herzegovina, 2007. Not available online.
A.S. Gill, PE. “High-Current Method of Testing Ground Grid Integrity,” NETA World, Vol. 10, No. 2 Winter 1988–1989. Not available online.
Vedran Mulic is a Regional Sales Manager and Application Engineer at DV Power, Sweden, with over 8 years of technical experience. He is in charge of developing equipment for transformer and substation grounding systems testing and works as a Technical Support Engineer for several territories. Since graduating with an MSc in power electrical engineering, Mulic worked as a Commissioning Engineer for power plants and electrical substations. He gained his experience mostly by testing protection and control IEDs, doing factory acceptance (FA) and site acceptance (SA) testing, and testing instrument and power transformers in Europe, the Middle East, and North Africa.