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Research

Fault Current Limiter surge protection concept based on a permanent magnet biased core by Jeremy Hall and Faris Al-Naemi

25 May 2021

Introduction

A new magnetic structure is described, based on interacting AC and DC fluxes in a device comprising permanent magnets and soft magnetic alloy cores in a novel, highly efficient arrangement to render the composite core electrically invisible under normal conditions, but able to respond quickly with a high impedance to limit damaging fault currents.

Aims & Objectives

A small, bench-top prototype was built to demonstrate the concept for a distributed magnet, orthogonally biased FCL. The test was designed to compare the current limiting capability of the FCL with an air-cored reactor coil of similar dimensions. The prototype was designed to operate with a normal current of around 3 Amps and a prospective fault current of around 30 Amps.

Materials & Methods

Fig. 1: Schematic showing toroidal iron cores (i/d 90mm, o/d 110mm, width each 10 mm width) with biasing magnets (Sm2Co17 magnets, arc length and height 30 mm) and AC coil windings (800 turns, evenly disrtibuted). An 800-turn “air-cored” coil with similar over dimensions (not shown) was fabricated using a non-magnetic, non-conducting material core.

FEM Modelling

The magnetic circuit was optimised using a non-linear magnetostatic solver. A time-step non-linear transient solver was used to simulate the dynamic behaviour of the device. The output of the transient solver was coupled to an electrical circuit model to calculate the instantaneous current profile.

Fig. 2: Flux density distributions for coil currents 0A (above) and 30A (below). The FEM shows the magnetic field set up by the winding is sufficient to de-saturate the toroidal cores at high current leading to an increase in core permeability, with the FCL acting to limit the short-circuit current relative to the air-cored coil due to the increase in inductance and hence the current limiting impedance.

Prototype testing & modelling results

The FCL core was connected in series with the power supply and the load resistor. The current through the FCL was recorded for a range of supply voltage when the load resistor was briefly shorted. The test was repeated with the FCL replaced by the air-cored coil.

Fig. 3: Comparison of instantaneous fault current calculated by FEM transient solver for FCL and air-cored coil
Fig. 4: Short-circuit test results for the FCL and air-cored coil. The linear data trend of the air-cored coil is extrapolated beyond the range which could be achieved with the laboratory power supply and maximum current rating of the winding insulation
  • For a ‘normal’ current of 3 Amps there is only small difference between the FCL and air-cored coil.
  • For a current of 28 Amps, the impedance of the FCL is around 3 times that of the air-cored coil. In the FEM simulation at 30 Amps a change in current limitation was found to be approximately 4.5 times. This is reasonable given unavoidable differences between the models (such as assumption of isotropic permeability for grain oriented steel and reluctances introduced by small air gaps between the wound toroidal cores and the pole faces of the magnets).
  • The performance of the core was found to be repeatable, showing no degradation of the permanent magnets following a “fault event”, as predicted by the FEM simulation.

Conclusions

Repeatable, current limiting capability has been demonstrated in a toroidal core, distributed magnet, orthogonally biased permanent magnet biased Fault Current Limiter.


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