Coupled-Inductor Solid-State Circuit Breaker-Based Protection Scheme for MVDC Micro-Grids
Date of Award
Doctor of Philosophy
Electrical and Computer Engineering
Dr. Johnson Asumadu
Dr. Pablo Gomez
Dr. Richard Meyer
MVDC Power system Protection, Solid-State MVDC Circuit Breakers, Protection Scheme for MVDC Micro-Grids, Z-Source Impedance Circuit Breaker, Y-Source Impedance Circuit Breaker, Bi-directional Solid-State Protection Device
In this study, coupled-inductor solid-state Γ-Z and Y-Source impedance networks are utilized as automatic, fast, and reliable circuit breakers to protect the medium-voltage DC (MVDC) power systems including the MVDC micro-grids that integrate the renewable energy sources. These networks provide a natural zero-crossing point during the breaker action due to the occurrence of a fault. The absence of the natural zero-crossing point in DC systems is considered the main challenge in protecting the direct current power systems.
In the first part of this study, the Γ-Z-source DC breaker is analyzed under different conditions of coupling to confirm the operational validity and evaluate the performance of the breaker operation. The voltage, current, and impedance transfer functions are derived and plotted under different values of the coupling coefficient. Results verify that the inductor coupling coefficient of the breaker must be higher than a null value but need not be a perfect unity value to interrupt fault current. Also, as the turns ratio increases, the required coupling coefficient drops allowing designers more flexibility in selecting the coupling degree and the simulation results confirm that.
In the second part of this study, the Y-source impedance network topology is proposed as a novel idea to be an automatic bi-directional circuit breaker for MVDC systems. Under different conditions of coupling, the input impedance, voltage, and current transfer functions are presented, and the breaker is designed for two cases of the fault impedance slope rate. According to the mathematical results, this breaker has a unique feature that has a high gain-reverse current which is produced by two secondary inductors to turn the breaker SCRs OFF and cut the abnormal current. Simulation results show that the performance of the proposed breaker under fault and load change conditions verifies the mathematical analysis. In addition, the limitation of the coupling degree among the breaker inductors and the effect of the turns ratio is described. The simulation results confirm that effect with two different values of the turns ratio.
In the third part of this study, a protection scheme for a 12-bus MVDC micro-grid based on Γ -Z and Y-source impedance breakers is proposed. The grid is simulated and used to achieve this scheme, where the faults are applied at different locations of the grid to prove the operation of both breakers. The scheme shows that in some lines placing a unidirectional protection device is adequate. At the other lines, bi-directional devices are required to conduct the power in both directions and protect the grid in the original direction of the power flow and in case of a change in that direction. This is due to the changing in load distribution or disconnecting line(s) because a fault occurs. The results of these two tests that were applied to the grid proved that both topologies (Γ-Z and Y-source networks) of the breakers have the capability to clear the fault. Furthermore, the Y-source breaker allows the current to flow in the opposite direction if needed.
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Al-khafaf, Haider M., "Coupled-Inductor Solid-State Circuit Breaker-Based Protection Scheme for MVDC Micro-Grids" (2021). Dissertations. 3719.