Low-voltage dc (LVdc) microgrids emerge as a viable alternative to ac microgrids. A large research interest is noted toward fast and selective protection of dc grids, typically focusing on hybrid or full solid-state solutions. In this article, the use of fuses as short-circuit protection in low-voltage dc microgrids is evaluated. The main advantage of fuses is that they are simple, cheap, standardized, and have low steady-state losses. A theoretical basis is formed to model dc short-circuit currents in grids with a limited short-circuit availability. The outcomes are applied to evaluate the possibilities of fuse protection in LVdc grids. It was found that fuses are an effective means of protection, although the required amount of capacitance at the output of the voltage balancing converter can be high, which impacts the total system cost. A fuse-based protection strategy is presented that highlights the need for additional capacitance to clear faults compared to the necessary capacitance for system stability. An experimental setup was built to validate the claims.

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Fuse-Based Short-Circuit Protection of Converter Controlled Low-Voltage DC Grids

Since building-integrated photovoltaic (BIPV) modules are typically installed during, not after, the construction phase, BIPVs have a profound impact compared to conventional building-applied photovoltaics on the electrical installation and construction planning of a building. As the cost of BIPV modules decreases over time, the impact of electrical system architecture and converters will become more prevalent in the overall cost of the system. This manuscript provides an overview of potential BIPV electrical architectures. System-level criteria for BIPV installations are established, thus providing a reference framework to compare electrical architectures. To achieve modularity and to minimize engineering costs, module-level DC/DC converters preinstalled in the BIPV module turned out to be the best solution. The second part of this paper establishes converter-level requirements, derived and related to the BIPV system. These include measures to increase the converter fault tolerance for extended availability and to ensure essential safety features.

https://www.mdpi.com/1996-1073/12/8/1532/pdf

Review on Building-Integrated Photovoltaics Electrical System Requirements and Module-Integrated Converter Recommendations

Embedded BIPV module-level DC/DC converters: Classification of optimal ratings

DC-DC Buck-Boost power converters are extensively used in several applications, from renewable energy sources to most industrial equipment. Nevertheless, to achieve proper integration of certain devices it is often necessary to provide converters with extended output voltage. This paper presents a new DC-DC power converter topology with Buck- Boost operation and extended output voltage using a single power semiconductor. Additionally, this converter operates with reduced voltage stress in the power semiconductor. The performance of this power converter will be proven trough simulations and experimental tests.

A Single-Switch DC/DC Buck-Boost Converter with Extended Output Voltage

A mission profile-based reliability analysis framework for photovoltaic DC-DC converters

In this paper, a two-stage power-electronics topology is presented for use in a BIPV module-level converter, connected to a low-voltage DC microgrid. The converter is built up as a cascade of two topologies, being an interleaved boost converter and a phase-shifted full bridge. The interleaved boost converter was chosen to lower the current stresses for the components and to reduce the required input and output capacitance due to the ripple current cancellation in the in- and output. The full bridge is chosen for its galvanic isolation and to perform the high step-up via the transformer turns ratio. It is shown that the maximum intermediate voltage level in between both converters can be derived from the continuous conduction mode of operation. Simulations are provided that show the correct operation of the overall structure. An experimental prototype has been built to demonstrate the overall performance of the converter.

Study on a cascaded DC-DC converter for use in building-integrated photovoltaics

An increasing number of electrical loads and sources in the distribution grid require a conversion step from DC, or AC at variable frequency, to be connected to the traditional 50/60Hz AC grid. This sparks an increasing interest for Low Voltage DC microgrids as an alternative. LVDC grids enable a reduction of the losses and an increase of the transfer capacity compared to their AC equivalent. However, fast and selective fault protection still remains a major obstacle for the breakthrough of LVDC grids. The challenge is twofold: On the one hand, a protection strategy for fast fault identification is required, on the other hand there is a need for protection devices capable of fast fault clearance in an LVDC grid. This paper first gives an overview of the current state-of-the-art of fault indicators and methods for fault identification in DC grids, addressing the first part of the challenge. Subsequently, an overview is given of the interruption devices that are currently available for fault clearance in DC grids and their (dis)advantages, addressing the second challenge.

Fault Identification and Interruption Methods in Low Voltage DC Grids — A Review

This paper presents an experimental comparison between two series resonant full bridge converters for use in BIPV applications. The first converter employs Si MOSFETS and the second converter uses GaN HEMTs for the switches of the full bridge. The practical implementation and design of the boards and the trade-offs between components are highlighted. The efficiency, power density and thermal performance of the converters are discussed in detail. Practical issues, such as secondary oscillatory behaviour, in the development of a phase-shifted full bridge are highlighted. The used transformer is modeled, including all parasitic elements. Several solutions to avoid transient oscillations in the secondary are set forward. To mitigate these high-frequency voltage transient overshoots, the best solution was found to equip the primary of the converter with a series resonant tank.

Experimental Comparison of the Efficiency, Power Density and Thermal Performance of Two BIPV Converter Prototypes

The strive to increase the penetration of renewables in urban areas has attracted the use of facade Building Integrated Photovoltaics (BIPV). From a system efficiency and cost perspective, an LVDC microgrid is preferred over a traditional AC grid to interconnect the BIPV modules inside the building. To address the problem of partial shading and further reduce the engineering effort and subsequent installation costs, Module-Level Converters (MLCs) are preferred. The frame-integration, however, rises questions regarding protection and safety of the installation. Assuming the converters cannot be serviced after installation, a high reliability and hence fault-tolerance is required. This paper will focus on LVDC pole-to-earth faults in the context of BIPV MLCs. The current state-of-the-art in commercially available systems is given and recommendations concerning the used grounding configuration and protection methodologies are formulated. Focus will be put on the integration and cost aspect of the solutions.

Earth Fault Analysis and Safety Recommendations for BIPV Module-Level Converters in Low-voltage DC Microgrids

Low-Voltage DC grids are gaining more attention nowadays due to an increased compatibility with DC loads and generators, higher power transfer capability and active power flow control within the network. Renewable energy sources such as PV, and storage systems such as batteries and fuel cells require power electronics converters to couple them to the grid. To this end, boost converters are commonly used. This paper will present a novel feedforward boundary conduction mode algorithm that is specifically suited for boost converters in Low-Voltage DC applications. PLECS simulations and experimental results are presented to demonstrate the working principle. The major advancement of this algorithm compared to the state of the art in boundary or discontinuous conduction mode control algorithms is the avoidance of additional zero-current detection circuits.

M. Dalla Vecchia

Papers

Study on a cascaded DC-DC converter for use in building-integrated photovoltaics

Fault Identification and Interruption Methods in Low Voltage DC Grids — A Review

Experimental Comparison of the Efficiency, Power Density and Thermal Performance of Two BIPV Converter Prototypes

Earth Fault Analysis and Safety Recommendations for BIPV Module-Level Converters in Low-voltage DC Microgrids

A Boundary Conduction Mode Feedforward Control Algorithm for Boost Converters in Low-Voltage DC Applications