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Lightning overvoltages in power stations

The complex installations of power stations must be protected against direct lightning strokes, but also against surge waves coming from power lines. While the protection against direct lightning strokes is ensured through a protective system with vertical rods or ground wire, the protection against the overvoltage waves coming from power lines is achieved with variable resistance surge arresters.

High voltage generation, overvoltages in power grids and insulation coordination
High voltage generation, overvoltages in power grids and insulation coordination (on photo: SiNTEF-Energy-Lab; credit: Sintef)

Lightning overvoltage analysis in power stations aims selection and proper installation of surge arresters. It is also necessary to verify if the power stations’ devices are in the protective area of the existing surge arresters.

The level of lightning overvoltages in power stations can be determined in two manners:

  1. A comprehensive analysis of the surge waves’ propagation process, considering its multiple reflections and refractions in different points of the station;
  2. The transient regime analysis, generated by the lightning surge waves, which excites the power station’s complex circuits, using the laws of electrical circuits.

The second approach is practically useless without some adequate software applications support. First method is also very complex, if an un-simplified scheme of the power station is used.

Even so, after many studies on physical models it seems that it is possible to simplify the power stations’ equivalent scheme, redistributing the equipment’s surge capacities.

In this sense, the so called moment rule is applied, redistributing the equipments’ capacities in the main points of the scheme, depending on the distance between the concentration node and the point where capacity was in the first scheme.

Usually the surge capacities of the equipments are concentrated in the points were transformers and surge arresters are installed in the power station.

Simplified equivalent diagrams used for lightning overvoltages determination in power stations
Figure 1 – Simplified equivalent diagrams used for lightning overvoltages determination in power stations: a) diagram with upstream capacity; b) diagram with upstream surge arrester, related to wave propagation direction.

The oversimplification of the power stations’ scheme involves inherently larger calculation errors, as a single propagating path is considered, with only one surge arrester connected, and a single concentrated capacity. In this situation all capacities are concentrated in transformers’ point.

Depending on the relative position of those two circuit elements, and the propagating path of the incident wave, the following situation can be considered:

Title:High voltage generation, overvoltages in power grids and insulation coordination – Marcel Estrate, Mircea Guşă and DragoÅŸ Machidon
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High voltage generation, overvoltages in power grids and insulation coordination
High voltage generation, overvoltages in power grids and insulation coordination

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