Novel Topology of Saturated-core Fault Current Limiter

Increasing levels of fault currents in utility grids are expected to result in mechanical and thermal overstresses of different power system components. To cope with these increased levels, expensive upgrades and replacements of currently employed Circuit Breakers might be required in the near future. Fault Current Limiters (FCLs) are expected to play an important role in the protection of future energy systems. They limit a fault current to a predetermined level and have no influence on the system during normal operation. A number of different FCL types have been proposed in literature. FCLs based on core-saturation effect have a number of crucial operation advantages over other FCL types: they have no fault reaction delay and no post-fault recovery delay; they have high reliability, low losses and can limit any number of consecutive faults. By introducing FCLs in power systems, above-mentioned expensive upgrades can be avoided. This thesis introduces a novel design of saturated-core FCLs which solves the problem of excessive weight of these devices. The new design utilizes design principle of insertion of an air gap in a core, typically used in design of inductors. Because of gap presence, the size of the core is approximately reduced by 70% in comparison to typical FCL design. The gap is inserted in such a way that it is a part of ac magnetic circuit but not a part of dc magnetic circuit. Thus, it is still possible to drive the core to saturation during normal operation. The new design uses one three-leg core per phase, where ac and dc windings are placed on outer legs. Based on the new core design, two FCL topologies are introduced: single-phase and three-phase FCL. Single-core FCL uses three cores for three phases whereas three-phase design utilizes only one core for all three phases. Therefore, the later topology reduces the amount of magnetic material further by more or less 66%. The drawback of three-phase FCL is that it limits only single-phase faults. The full voltage 10 kV/400 A prototype of three-phase FCL is built and tested. The measurements align very well with simulation results; they validate 3D FE transient model, built in Ansys, and prove the principle of operation