Studies On The Arrangement & Coordination Of SPDs Protecting The Secondary Equipments In Substation

With the wide use of power system automation in electrical power substations, it's more and more important to protect the secondary equipments against the lightning surge. These equipments would be in malfunctioning or even be broken when they were disturbed by lightning surge. It'll affect the automation systems severely. One approach is to install surge protective devices (SPDs) to protect the secondary equipments from the lightning surge. The characteristic of the protected equipments and the performance of SPDs involve their locations. Up to now, SPDs made from the same manufacturer will be located in the same circuit optionally or by manufacturer's advice because there is no promulgated uniform standards about surge protection for secondary systems in substations. There were many disagreements within the existing international or domestic standards, especially to such problems as the cascade coordination, the effective protection distance of SPDs and backup over-current protection. This dissertation involves some principle reseaches for reasonable location of SPDs.The main electromagnetic interferences (EMI) on substation secondary equipments were summarized in the dissertation at first, then the problems with SPD's location were described, and the facts that affect the protection distance of SPDs and the cascade coordination were studied via experimentations and simulations. Some useful principles were concluded: the protection distance of SPDs is shortest when the input impedance of the protected equipment is infinite; the input impedance, the withstand voltage of the protected equipment, and the parameters of cable that connects the protected equipment and SPD will affect the effective protection distance; the scheme with same clamping voltage will meet the cascade coordination generally; for the scheme in which the clamping voltage of front stage SPD's is higher than that of the latter stage SPD's, the diverting current would be inversion; cascade coordination could be implemented more easily with the first stage being combination SPD (MOV parallelized with spark gap), whereas the coordination effect may be affected by the disperse discharge voltage of the gap.Researches on the layout of low-voltage system in substations, which affect the location of SPD, were also presented in the dissertation. No SPD is needed at the front point of the line when the appropriate SPD is located at each bus of low-voltage system, and less SPD or no SPD may be needed at the end of the short line when there are enough lines.The dissertation deals with a method of optimization design for the location scheme of SPD. Based on the risk of failure, fuzzy logic was used to determine the SPD's position by using MATLAB. The approach allows the designer to select as less SPD as possible at the appropriate position based on the acceptable risk of failure of the protected equipments.