What is the purpose of a ground grid in a substation, and how is its effectiveness measured?

A ground grid in a substation is a network of interconnected conductors buried beneath the substation surface. Its primary purpose is to provide a low-impedance path for fault currents to return to the source, ensuring rapid operation of protective devices and minimizing the risk of electrical shock. It achieves this by limiting ground potential rise (GPR) during fault conditions and by reducing step and touch potentials within the substation. Ground potential rise is the increase in the electrical potential of the ground grid relative to remote earth during a fault. Step potential is the voltage difference between the feet of a person standing on the ground near a grounded object. Touch potential is the voltage difference between a grounded object and the feet of a person touching that object. A well-designed ground grid minimizes these potentials, protecting personnel and equipment. The effectiveness of a ground grid is measured primarily by its ground resistance. Ground resistance is the resistance between the ground grid and remote earth. Lower ground resistance indicates a more effective ground grid. Ground resistance is typically measured using the fall-of-potential method, which involves injecting a test current into the ground grid and measuring the resulting voltage drop at various distances. Other measurements include measuring step and touch potentials at various locations within the substation to verify they are within safe limits as defined by standards such as IEEE Std 80. Regular inspections of the ground grid connections and conductors are also performed to ensure their integrity and prevent corrosion. Soil resistivity measurements are also taken to assess the conductivity of the soil and determine if any improvements are needed to the ground grid design.