DC microgrids have attracted significant attention over the last decade in both academia and industry. DC microgrids have demonstrated superiority over AC microgrids with respect to reliability, efficiency, control simplicity, integration of renewable energy sources, and connection of dc loads. Despite these numerous advantages, designing and implementing an appropriate protection system for dc microgrids remains a significant challenge. The challenge stems from the rapid rise of dc fault current which must be extinguished in the absence of naturally occurring zero crossings, potentially leading to sustained arcs. In this paper, the challenges of DC microgrid protection are investigated from various aspects including, dc fault current characteristics, ground systems, fault detection methods, protective devices, and fault location methods. In each part, a comprehensive review has been carried out. Finally, future trends in the protection of DC microgrids are briefly discussed.

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DC Microgrid Protection: A Comprehensive Review

A modified Z-source breaker topology is introduced to minimize the reflected fault current drawn from a source while retaining a common return ground path. Conventional Z-source breaker topologies do not provide steady-state overload protection and can only guard against extremely large transient faults. The Z-source breaker can be designed for considerations affecting both rate of fault current rise and absolute fault current level, analogous in some respects to a thermal-magnetic breaker. Detailed analysis and design equations are presented to provide a framework for sizing components in the Z-source breaker topology. In addition, the proposed manual tripping mechanism enables protection against both instantaneous current surges and longer-term overcurrent conditions. The fault operation intervals of the proposed Z-source breaker topologies are both demonstrated in SPICE simulation and validated in experimental characterization.

Analysis and Design of DC System Protection Using Z-Source Circuit Breaker

This paper proposes a multilevel high-frequency-link dc transformer (MDCT) based on dual active phase-shift principle for medium-voltage dc (MVDC) power distribution application. The proposed MDCT employs multilevel and multiplex conversion principle, which brings many advantages and makes the operation of MDCT quite different with the traditional dc transformer (TDCT) and modular multilevel converter. Compared to the TDCT scheme, the proposed MDCT has smaller circulating current and higher power factor; it can operate as a dc breaker to cutoff the connection with the MVDC distribution grid absolutely when a short fault occurs in the distribution grid; the redundant design can be achieved when some submodules failure to improve the reliability. In the paper, the topology, operation principle, modulation method, switching characterization, voltage, and power characterization, and control strategy of MDCT are presented and analyzed comprehensively. At last, a MDCT prototype is built and the experimental results verify the correctness and effectively of the proposed solution.

Modular Multilevel High-Frequency-Link DC Transformer Based on Dual Active Phase-Shift Principle for Medium-Voltage DC Power Distribution Application

This paper proposes a new $\boldsymbol {n}$ -port hybrid dc circuit breaker for offshore multiterminal HVDC grid application. The $\boldsymbol {n}$ -port dc circuit breaker can substitute $\boldsymbol {n-1}$ hybrid dc circuit breakers at a dc bus with $\boldsymbol {n-1}$ adjacent dc transmission lines. The system-level behavior of the proposed multiport hybrid dc circuit breaker is similar to other hybrid dc circuit breakers. The operation principles of the proposed multiport dc circuit breaker are introduced, analyzed, and compared to the existing solution in this work. The components ratings are compared to the existing solution, and the functionality of the proposed device is verified by simulation.

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Multiport Hybrid HVDC Circuit Breaker

DC microgrids have some attractive characteristics, which have being investigated to utilize in future electrical power systems. However, they have several essential challenges, such as limiting and interrupting fault current, which should be rectified to have high-performance dc microgrids. In this paper, an efficient dc bidirectional fault current limiter and interrupter (FCLI) based on Z-source topology is proposed to overcome some of the challenges in dc microgrids. The FCLI is a multifunction device, by which power flow direction control, circuit breaking, fault current limiting, and interrupting in a dc system can be handled. Analytical analysis of the FCLI is discussed in detail for different operation modes. Performance of the proposed FCLI in the different operation modes is evaluated using some simulations and experiments. The results confirm high capability of the device to realize appropriate operation in dc systems. Fast response time of fault current limitation and interruption, simple and reliable structure, and negligible conduction loss are the main features of the proposed FCLI.

A Z-Source-Based Bidirectional DC Circuit Breaker With Fault Current Limitation and Interruption Capabilities

The Z-source dc circuit breaker has been introduced as a new circuit for quickly and automatically switching off in response to faults. A modified Z-source breaker design is introduced for the operation at medium-voltage dc with future applications in naval ship power systems. Compared to existing designs, the respective design will allow for greater control of step changes in load. This new design also limits capacitor current in the circuit and can be easily modified for fault detection. Analysis of the breaker operation is presented during both the fault and step changes in load. Low-voltage laboratory validation of the breaker was carried out on two different versions of the proposed circuit.

Modified Z-Source DC Circuit Breaker Topologies

The z-source breaker has been introduced as a new circuit for quickly and automatically switching off in response to faults. A modified z-source breaker design is introduced for operation at medium-voltage dc with future applications in naval ship power systems. Compared to existing designs the respective design will allow for step changes in load. This new design also limits capacitor current in the circuit and can be easily modified for fault detection. Analysis of the breaker operation is presented during both fault and step changes in load. Low voltage laboratory validation of the breaker was carried out on two different versions of the proposed circuit.

Modified z-source DC circuit breaker topologies

The development of medium-voltage DC power distribution for future Naval ships brings questions about the protection methods to be used in various places within the architecture. This paper starts with overlaying the current-time curve of a solid-state DC breaker with a traditional DC breaker. Using both methods can address slowly encroaching faults and rapidly occurring faults. A medium-voltage DC system demonstrator is described which includes solid-state breaker protection. The system is tested in the laboratory where it is demonstrated that a hard fault can be isolated from a controlled rectifier by the solid-state breaker within the ultra-fast time period (sub-millisecond). With this system, faults of various time scales (slow, fast, and ultra-fast) can be addressed using traditional breakers, power electronic controls, and solid-state breakers.

Allan Overstreet

Papers

Modified Z-Source DC Circuit Breaker Topologies

Modified z-source DC circuit breaker topologies

Solid-state breaker protection in MVDC systems