Analysis of Grounding Resistance of Transmission Line Towers Measured by Three-pole Method and Clamp Meter Method
Grounding resistance measurement is a necessary means to check whether the grounding device meets the requirements of the regulations. The traditional grounding resistance measurement of transmission line poles and towers generally adopts the grounding meter method, which requires more than tens of meters of electrode leads to be arranged on site, which requires a lot of work. The clamp meter method is a new method that has appeared in recent years. It does not need current, voltage poles and external power supply, and does not need to disconnect the ground connection, as long as the clamp meter clamps the ground wire of the tower. The clamp meter method generally uses different frequency measurements. Since the loop resistance is measured by the clamp meter method, in addition to the grounding resistance of the grounding body, it can also be found that the entire grounding loop is caused by corrosion or poor contact of the weather, soil, or some grounding rods. The loop resistance of the circuit becomes large.
1. Measuring principle of three-pole method and clamp meter method
1.2 Three-pole method measurement principle
The three-pole method (also known as the current-voltage pole method) refers to the method of testing the grounding resistance of the tower with three electrodes consisting of the tower grounding grid, the current electrode and the potential electrode. Generally, the electrodes are arranged in a straight line method (more suitable for use on the transmission line site) ). The method and principle are shown in Figure 1.
Measuring principle: The test current is injected by the grounding device G (equivalent to a hemispherical electrode), and the potential drop curve between the grounding grid and the current electrode is shown in Figure 2. Theoretically, the resistance of the grounding device is the resistance relative to the zero potential point at infinity, then the potential difference of the grounding device relative to the zero potential plane at infinity is required, and the grounding resistance value is calculated by Ohm's law. The distribution law of the potential drop curve in Figure 2 is affected by the introduction of the current pole, causing electric field distortion, making the zero plane at infinity move closer to P, causing electric field distortion, and making the zero plane at infinity move closer to P middle
1.2 Measuring principle of clamp meter method
Measuring principle: the measurement of the clamp meter method (the overhead ground wire is directly connected to the tower), which is equivalent to the test circuit composed of the grounding resistance of the measured tower and the parallel impedance of adjacent multi-base towers. The equivalent schematic diagram is shown in Figure 4. Generally, each gear The resistance and inductance of the overhead ground wire can be ignored, then the final instrument measurement result R, ten, when there are enough towers in parallel (greater than ten bases or more), is much greater than △R, then the instrument measurement result is considered to be the grounding resistance of the measured tower.
The introduction of measurement error: The impact of the shunt of the tower's natural grounding body on the measurement results is not considered. When considering the impact of the tower’s natural grounding body on the test results, when the clamp meter method measures the local grounding resistance of the tower (the position of the downconductor of the tower under test), when the sum of RH and Ro (in parallel with Rj+ΔR) and Rj When the same order of magnitude or less, the artificial grounding electrode has a greater impact on the measurement results; when the anchor bolts are directly connected to the foundation reinforcement cage (Ro=0), the natural grounding body has a greater impact on the measurement results; there are tower foundation reinforcement cages and artificial In the case of direct welding of the ground electrode (RH=0), if the anchor bolts are also directly connected to the foundation reinforcement cage at the same time, the test result will be less than 1Ω, which is essentially the metal of the grounding down-lead, anchor bolts, and reinforcement cage. loop resistance.
2. Precautions when measuring with clamp meter method
(1) First check the electrical connection of the tower under test, confirm that the overhead ground wire is directly connected to the tower, and before removing the bolts of the ground down-conductor, confirm the electrical connection status of the down-conductor one by one (use the clamp meter to confirm the connection of the metal circuit); if If all the down-conductors are connected in good condition, keep any one of the down-conductors to measure the grounding resistance of the tower.
(2) If there is no obvious change compared with previous clamp meter measurement results, the current clamp meter measurement results are considered valid. If the measurement results of the clamp meter method are much greater than those of the previous clamp meter method, the three-pole method should be used for comparative measurement to determine the cause.
(3) When the condition of the line changes (such as replacement of the lightning protection line and grounding method, change of line direction, etc.) The grounding resistance of the tower is compared and measured.
(4) Under the condition of wet soil after rain or rainy season, the mutual resistance between the natural grounding body of the tower and the artificial grounding electrode decreases significantly, which will directly affect the correctness of the measurement results.
