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Abstract
Designs of saturated-cores fault current limiters (FCLs) usually implement conducting or superconducting DC coils serving to saturate the magnetic cores during nominal grid performance. The use of coils adds significantly to the operational cost of the system, consuming energy, and requiring maintenance. A derivative of the saturated-cores FCL is a design implementing permanent magnets as an alternative to the DC coils, eliminating practically all maintenance due to its entirely passive components. There are, however, various challenges such as the need to reach deep saturation with the currently available permanent magnets as well as the complications involved in the assembly process due to very powerful magnetic forces between the magnets and the cores. This paper presents several concepts, achieved by extensive magnetic simulations and verified experimentally, that help in maximizing the core saturation of the PMFCL (Permanent Magnet FCL), including optimization of the permanent magnet to core surface ratios and asymmetrical placement of the permanent magnets, both creating an increase in the cores’ magnetic flux at crucial points. In addition, we point to the importance of splitting the AC coils to leave the center core point exposed to best utilize their variable inductance parameters. This paper also describes the stages of design and assembly of a laboratory-scale single phase prototype model with the proposed PMFCL design recommendations, as well as an analysis of real-time results obtained while connecting this prototype to a 220 V grid during nominal and fault states.
Highlights
The continuous growth of energy generation and the worldwide efforts to integrate renewable energy sources in existing grids while maintaining a high level of power quality, result in an ever-increasing need to solve the problem of fault currents [1,2,3,4,5]
The concept of its operation consists of keeping the magnetic cores saturated during nominal grid performance so that the AC coils, mounted on the saturated cores and connected in series with the grid, exhibit a low impedance [9]
In order to control the process during assembly, to both obtain proper alignment and assure no ruptures to the magnets/cores, a DC current of 100 A was introduced into the AC coils to generate an opposing magnetic flux to the permanent magnets thereby saturating the core initially and minimizing the product B · ddxB
Summary
The continuous growth of energy generation and the worldwide efforts to integrate renewable energy sources in existing grids while maintaining a high level of power quality, result in an ever-increasing need to solve the problem of fault currents [1,2,3,4,5]. Being inductive on its own, the DC bias coil may interact with the AC grid coils through electromagnetic coupling and degrade the performances of the limiter or the power supply driving the DC coils, as well as degrading the power quality of the grid It was argued [11] that FCL designs based on permanent magnets (PMFCL), rather than electromagnets, may offer a better solution for the fault current limiters. This paper presents several concepts that contribute to optimizing the implementation of permanent magnets to assist in overcoming these challenges Adoption of these proposed concepts has been simulated and tested experimentally by building a low voltage prototype, and demonstrated improvements in performance as well as weight reductions of up to 20% in the permanent magnets and core materials when implemented. We present an analysis of real-time results obtained while connecting this prototype to a 220 V grid during nominal and fault states
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