Numerical Analysis of Transient Performance of Grounding Grid with Lightning Rod Installed on Multi-Grounded Frame

Abstract

In large substations, many lightning rods are installed on multi-grounded frames. The lightning rods, the frame, the grounding grid and the soil form a whole body, and the lightning current will be discharged from many grounding points. In this paper, based on the partial element equivalent circuit method, a numerical model, in the time domain, is developed to simulate the lightning-caused electromagnetic transients on the frame and the grounding grid. The model is verified by field testing and by comparison with commercial software. The model has several features: (1) it has a simple time domain form; (2) it is stable due to a staggered arrangement of space and time variables and an implicit difference scheme used, and (3) the dimension of the equations is relatively small because the unknown variables are divided into several groups, which are calculated one by one. With this method, the transient characteristics of the grounding grid with lightning rods on the frame are calculated, and the factors affecting the results are analyzed. It can be seen that although the frame causes the ground potential rise in an evenly distributed manner, compared with the situation in which the lightning strikes an independent lightning rod, the ground potential decrease rate near the main grounding point is almost the same because most of the current still enters the soil from the grounding electrode closest to the lightning strike. Therefore, even if there is a frame, the nearby facilities should take the same protective measures as in the case of an independent lightning rod. The ground conductors near the grounding points of the frame should be dense enough to reduce the potential gradient. The equipment should be kept at least 10 m away from the grounding point for lightning.