In high-voltage power systems, grounding devices play a critical role in fault current dissipation, lightning surge protection, and ensuring safe equipment operation. Whether it is the large grounding grid at power plants or substations, or the grounding electrodes of transmission line towers, their performance is closely related to the electrical conductivity characteristics of the surrounding soil. Soil resistivity, as the key parameter characterizing soil conductivity, serves as the primary basis for determining electrode dimensions, layout configurations, and burial depth during grounding design, and also provides an essential reference for assessing the condition of grounding systems during operation.
The power industry has strict standard requirements for soil resistivity testing. DL/T 475 “Guidelines for Measurement of Grounding System Parameters” clearly defines the general principles, methods, criteria, and testing cycles for soil resistivity measurement, applicable to commissioning acceptance tests and preventive tests of grounding systems in power plants, substations, converter stations, wind farms, photovoltaic power stations, energy storage stations, and transmission line towers. This article systematically elaborates on the technical principles, operational procedures, and industrial applications of soil resistivity testers based on this standard framework.
Measurement Principles and Standard Basis
Basic Principle of the Four-Electrode Method
Currently, the four-electrode method (Wenner method) is widely used in the power industry for measuring soil resistivity. The basic principle involves arranging four electrodes in a straight line at equal intervals within the soil, with electrode spacing denoted as a. An alternating current I is injected into the ground through the two outer current electrodes (C1, C2), while the potential difference V between the two inner potential electrodes (P1, P2) is measured. The apparent resistance R is then calculated as R = V/I.
Soil resistivity ρ is computed using the following formula:
ρ = 2πaR
where a is the electrode spacing (m), and R is the measured resistance value (Ω).
When operating in the “soil resistivity” measurement mode, users can directly read the ρ value after inputting the electrode spacing.
Regulatory Requirements of the Standard System
The core standard for soil resistivity testing in the power industry is DL/T 475 “Guidelines for Measurement of Grounding System Parameters.” This standard has undergone multiple revisions, with the 2017 edition removing single-electrode method testing and further standardizing the application of the four-electrode method. It applies to grounding system testing for various power facilities, including power plants, substations, converter stations, step-up substations and wind turbines in wind farms, photovoltaic power stations, energy storage stations, traction substations for electrified railways, and transmission line towers.
Additionally, for power frequency grounding impedance testing of large grounding grids, JJG 1180-2021 “Verification Regulation for Power Frequency Grounding Impedance Testers for Large Grounding Grids” specifies clear requirements for initial verification, subsequent verification, and in-service inspection of test instruments. This regulation applies to testing devices operating at frequencies between 45 Hz and 55 Hz with a working current of no less than 1 A.
Technical Features of Testing Instruments
Modern digital soil resistivity testers typically employ microprocessor control and fast filtering technology, offering multiple measurement modes such as two-wire, three-wire, and four-wire configurations. Typical instruments have a soil resistivity measurement range from 0.00 Ω·m to 9999 kΩ·m, with a measurement accuracy of ±2% reading ± several digits. The test frequency is usually around 128 Hz, and these instruments generally include data storage and transmission capabilities.
For testing large grounding grids, some instruments support advanced functions such as dual-frequency measurement (45 Hz/55 Hz), step voltage, and touch voltage measurements, with short-circuit currents reaching up to 5 A.
Field Measurement Operational Procedures
Electrode Arrangement Requirements
The accuracy of the four-electrode method depends heavily on proper electrode placement. Standards require that the four electrodes be arranged in a straight line at equal distances, with the electrode spacing a significantly greater than the electrode burial depth. Typically, the burial depth should not exceed 1/20 of the spacing—i.e., satisfying the condition a > 20h, where h represents the electrode burial depth.
Control of Detection Depth
The four-electrode method measures the average soil resistivity within approximately 0.75 times the electrode spacing depth. By adjusting the electrode spacing, soil resistivity data at different depths can be obtained. To characterize layered soil properties in engineering applications, multi-point measurements using non-uniformly spaced electrodes should be conducted to plot the variation of soil resistivity with depth. The DL/T 475-2006 standard already incorporates the non-uniform four-electrode method, providing a standardized basis for testing layered soil resistivity.
Interference Mitigation Measures
High-voltage power sites are subject to complex interference sources; therefore, proper interference avoidance is essential during testing:
Underground metallic structures: Electrodes should be arranged perpendicularly to underground metal pipelines and main grounding grid conductors, and kept away from cable trenches, metal water pipes, and other conductive components to minimize measurement errors caused by current diversion.
Electromagnetic interference control: Substations and converter stations have strong power-frequency electromagnetic fields; instruments must possess sufficient anti-interference capability. Testing should be halted if ambient interference voltage exceeds 20 V or interference current exceeds 2 A, and the source of interference must be identified and eliminated.
Environmental conditions: Soil moisture significantly affects resistivity; testing should not be performed immediately after rainfall. According to DL/T 475 requirements, preventive tests should preferably be carried out during periods when soil conditions are relatively stable.
Key Applications in High-Voltage Power Industry
Transmission Line Tower Grounding
Soil resistivity testing for tower grounding is a critical foundation for lightning protection design. For new lines, soil resistivity determines the configuration and dimensions of grounding electrodes; for existing lines, periodic test results guide grounding system upgrades. During measurement, it is necessary to verify the connection status between the tower grounding electrode and the tower body, and disconnect them if needed to eliminate measurement errors due to current diversion.
Substation and Power Plant Grounding Grids
Large-scale grounding grids typically cover extensive areas, face high on-site interference, and require high accuracy. As specified in DL/T 475, soil resistivity testing should ideally be conducted simultaneously with grounding impedance testing. Soil resistivity data are used for verifying grounding grid designs and assessing equipment condition. Instruments selected must comply with JJG 1180 regulations regarding operating current and frequency.
Renewable Energy Station Grounding
Grounding systems in wind and photovoltaic power plants often feature dispersed locations and complex terrain. DL/T 475-2017 includes substations, wind turbines, photovoltaic plants, and energy storage stations within its scope. These facilities require attention to variations in soil conditions across different turbine positions and array zones, and site-specific testing may be required in distinct zones.
Soil resistivity testing forms the foundational link in the grounding technology framework of high-voltage power engineering. Centered around DL/T 475, the standard system provides comprehensive technical guidance for testing methods, instrument selection, and data application. Engineers must thoroughly understand the principles and inherent limitations of the four-electrode method, strictly follow operational requirements such as electrode placement, interference control, and testing cycles, and ensure accurate and comparable test data. With the continuous development of renewable energy and smart grids, soil resistivity testing technologies must further evolve toward layered measurement and online monitoring to support the safe and stable operation of power systems.
Post time: Sep-17-2026