Abstract
This paper examines various aspects of the design process and subsequent field test measurements of a large and complex substation grounding system. The study and measurements show that soil layering and lead interference can have a significant impact on the appropriate test location that yields the exact substation ground impedance. Applying a specific percentage rule such as the 61.8% rule for uniform soils to obtain the true ground impedance may lead to unacceptable errors for large grounding systems. This poses significant problems when attempting to validate a design based on raw test data that are interpreted using approximate methods to evaluate substation ground impedance, and determine ground potential rise (GPR), touch and step voltages. Advanced measurement methodologies and modern software packages were used to obtain and effectively analyze fall of potential test data, compute fault current distribution, and evaluate touch and step voltages for this large substation. Fault current distribution between the grounding system and other metallic paths were computed to determine the portion of fault current discharged in the grounding system. The performance of the grounding system, including its GPR and touch and step voltages, has been accurately computed and measured, taking into account the impedance of the steel material used of the ground conductors and circulating currents within the substation grounding system.
Highlights
Appropriate power system grounding is important for maintaining reliable operation of electric power systems, protecting equipment, and insuring the safety of public and personnel
This paper examines various aspects of the design process and subsequent field test measurements of a large and complex substation grounding system
The complex and non-homogeneous nature of the soil, the intricate three-dimensional shape of the grounding system and topology of the entire power system network result in a very difficult task that requires appropriate specialized software packages and skilled professionals with adequate expertise in this field in order to account for the numerous factors that have to be considered during the design process and subsequent field measurement validation task
Summary
Appropriate power system grounding is important for maintaining reliable operation of electric power systems, protecting equipment, and insuring the safety of public and personnel. The exact location of the potential probe is well defined for some ideal cases, such as hemispherical or small grounding electrodes buried in uniform or layered soils [1,2,3,4,5,6] but must be evaluated adequately when the separation distances are not large enough In such cases, a value read at 61.8% may lead to significant errors on the measured ground impedance. To evaluate the grounding performance of the large system ground network, the following steps were carried out: 1) Soil resistivity and grid impedance measurements and interpretation; 2) Fault current split calculations; 3) Grounding system performance analysis. The analysis and the discussions can be used as a reference guide to study large grounding systems
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