Beneath every substation, there is a ground grid that provides proper grounding of all apparatus in substations (e.g. transformers, circuit breakers, capacitor banks, steel tower structures, etc.). The ground grid is usually made of copper-based cabling 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 purpose of this grid is to improve the safety of both personnel and apparatus in the substation. 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.
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). The test should differ from ground grid tests that require the use of AC power sources and inspect 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 / GGT500 devices with the GGT-M module and the cable connection diagram is illustrated in Figure 1.
Figure 1: Ground grid integrity testing with GGT device and GGT-M module
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 than needs to be inspected. An operator takes the GGT-M module along with the red color marked cable in order to control the testing remotely (away from the GGT main unit).
During the testing, GGT generates preselected DC current (e.g. up to 300 A for GGT500 model) between the reference and the tested grounding points. The current will be generated during the preselected test duration (e.g. 60 s) and the operator can stop the test after few seconds if the results are satisfactory. The typical time required for testing one grounding is approximately 2 minutes (including the cable connections).
The following parameters should be checked during ground integrity testing:
- Voltage drop between the reference and the tested grounding points
The voltage drop is measured by the GGT200/GGT500’s current outputs. The voltage drop on the current cables is automatically excluded from the result. In this way, the voltage drop between the reference and the tested grounding points is obtained. Criteria for determining the condition of the tested grounding depends on ground grid material. 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 or a rarely a failed test set. If this happens, check the clamp connections then select a different reference point and establish that it IS or is NOT a good reference point.
- Current flow inspection at the tested grounding point
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” (since it flows in a downward direction). For a single grounding connection, at least half of the total generated current should be the “DOWN” current (IDOWN ≥ ½ ITOTAL). In the case of multiple ground connections, it is acceptable that more than half of the groundings point complies with these criteria.