Permanent Magnet Fault Current Limiters for Electrical Power Protection Systems

The main aim of this work is to design and test a permanent magnet fault current limiter (PMFCL) to limit the fault current in electrical power system network. The (fit and forget) device presented in this thesis is based on two important aspects; the best selection of the state of the art soft and hard magnetic materials and the design topology of PMFCL. Rare earth material is used as a permanent magnet, which is the main source of excitation to keep the non-oriented silicon steel iron core in magnetic saturation state. During the normal operation of the device the saturated core offers low impedance to the grid and during the fault state the core inherently rushed to high impedance state that limits the high short circuit current. A commercial Finite Element software (FEM) was used in the device modelling techniques from the commencement till the end of the final design. The work commenced with the verifications and investigations of a recently reported model in 2D (FEM). Then, a prototype of small scale, in two design configurations of the same PMFCL specifications, was built and tested at the electrical laboratory of Sheffield Hallam University to verify the simulation results and to access the design of the PMFCL device. After that, the 11-kV PMFCL for substation distribution transformer was designed. Finally, the low voltage toroidal core PMFCL was proposed for the existing renewable energy and future wind-photovoltaic (wind-PV) step-up transformer. The dry type PMFCL current-inductance profiles were obtained by 3D (FEM) magneto static solver to predict the behaviour of the devices in the abnormal condition. The calculated RMS current, using 3D (FEM) time saving inductance-current approach, agreed with the peak transient currents obtained by the lengthy computation process time-step solver. The current limitation capability has been calculated in comparison with the air-cored of similar specifications as the PMFCL device and a useful reduction in the fault current has been achieved. The simulation results proved that the proposed PMFCL topologies (toroidal and square-shaped) can protect the renewable energy generator-transformer and real power grids from the fault current. Both the toroidal and full scale PMFCL devices initial and energy cost over an expected service of life have been evaluated. The merit of the PMFCL device is that it reduces downtime during power system’s outages by mitigating the severe fault current in the first half cycle.