Grounding grid design for high voltage substations : an assessment of effectiveness for lightning currents

• Main Author: Hanaffi, Farhan bin
• Published: University of Strathclyde 2016
• Subjects: 621.319
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.698541

An electrical grounding system is an important element to ascertain a safe environment for both humans and equipment during fault or transient conditions. The performance of grounding systems under lightning current is quite different from the conventional frequency based power. In order to understand the grounding grid behaviour under lightning current, researchers typically carry out experiments on actual grounding systems or on laboratory scaled models. Although experiments can provide insights of the actual grounding operation, the shortcoming is that a large area of lab space is required which reflects into high costs. As an alternative, computer simulation has been introduced, and can be categorised into three different approaches, namely circuit approach, transmission line approach or electromagnetic approach. In this work, the simulations are performed based on the electromagnetic approach under three dimensions (3D) mode due to its accurate results. For further understanding, a comparison between circuit and electromagnetic approaches is also carried out, where the resulting outcome shows that the circuit approach underestimates the impulse impedance at injection point compared with simulations by the electromagnetic approach. When the electromagnetic approach is applied, a finite element method is used to solve the partial differential electromagnetic equations in the time domain. Thereafter, the simulations results are validated with the existing published results covering the electromagnetic simulations by using the method of moment (MOM), and as well as actual field experiments. In addition, simulations are performed to understand the effect of different parameters, including lightning current, soil parameters, grounding design, and location of injection point of lightning current. Moreover, a comparison study is carried out for potential rise between power frequency and impulse current at different grid sizes. The study shows the potential generated at injection point for both current and saturation point when the grid size reaches a certain point. It’s important to consider both types of current to get better grounding grid design. Besides that, empirical equations are used out to calculate the effective area under lightning conditions, where the effect of the down-conductor is taken into consideration as part of the grounding model. The effective area is an important parameter for the optimization of the grounding grid design when increasing grounding size does not improve the impulse impedance. Transient ground potential rise (TGPR) above the ground is another interesting parameter to analyse. In this work, a good correlation is shown between the effective area and the impulse impedance at the injection point with rising transient ground potential. It is found that the TGPR is larger when it is closer to the injection point, but only lasts for a few microseconds. Step voltage evaluations are performed for different standing positions of the human above the grid, including the distance of the step voltage location from the injection point, and the effect of grid size to step voltage value.