Risk determination in respect to sufficient protection against lightning overvoltages

Proper rating and placing of surge arresters in HV substations is vital for safe protection of the equipment and high availability of the entire power transmission system. However, a substation is a complex system for steep surges, and for many configurations it cannot be said easily whether all parts of the substation are protected sufficiently by the surge arresters installed or not. Computer simulation of lightning surges and their magnitudes inside a substation can help determining the risk of damages of equipment by lightning overvoltages and supports optimized placing and rating of surge arresters. This paper describes typical problems out of the daily practice. It shows the complexity of insulation co-ordination problems with the need for an individual consideration of each substation. Computer based simulations can show the maximum overvoltage stresses at different locations of a substation for different switching conditions and surge arresters provided already in the planning stage. Representative modeling of transient overvoltages will be shown as well as the general way how to carry out such studies. Main input parameters and influencing factors will be discussed showing possible variations to design the transmission system which would reduce overvoltage stresses. Considering the statistical nature of lightning surges helps to define the failure risk at reasonable amount and optimization of the protection devices necessary.