Voltage sags in power distribution networks

Voltage sags have become increasingly important when considering the various power quality issues that cause inconvenience to customers. For some customers voltage sags cause especially high costs. Thus, power distribution companies should understand the voltage sags experienced in their networks and their options in terms of decreasing the influence of voltage sags. In this thesis, voltage sag mitigation includes the use of conventional means and regular power system components. This thesis introduces an extended method of fault positions to calculate the voltage sag distribution experienced in power distribution networks. The method uses only input data that is typically available and the basic fault calculations already in use in power distribution companies. The thesis also presents calculated and measured voltage sag distributions from Finnish power distribution networks. In addition, the thesis shows how various, specific power system characteristics affect the sag distribution. Sag-sensitive customers should be supplied by their own main transformers and the local distribution network kept as limited as possible. For overhead line networks, power distribution transformers should be protected against overvoltages using surge arresters instead of spark gaps. Underground cables are superior to overhead line networks, but mixed networks having both underground cables and overhead lines in the same distribution network should be avoided. Surprisingly, the increased level of distribution automation often gives rise to the most severe voltage sags. Since basic decisions in the development of power distribution networks may have a positive as well as negative influence on voltage sag characteristics, more awareness of these effects is needed. In this thesis, the main focus has been voltage sags. However, voltage sags represent only one power quality issue to be taken into account in the planning and operation of power distribution networks. Thus, voltage sag analysis should not be a separate part of power distribution planning but should be included as one, important element in a comprehensive power system analysis. The economic calculations presented in this thesis showing the especially high costs caused by voltage sags strengthen this claim.reviewe