This article proposes an analytical methodology to evaluate the performance of the main partial power processing (PPP) architectures in terms of the improvements in the system's conversion efficiency. This analysis considers the influence of the system's voltage gain, the auxiliary dc/dc converter's efficiency, and the possibility of bidirectional power flow. Herein, the key PPP architectures are, thus, modeled and benchmarked. The presented results attest to the series configuration as the most efficient PPP circuit solution, with no limits on the system voltage gain, contrary to the generalized results found in today's literature. To assess these results and the significance of the proposed analysis, a well-known, simple, and cost-effective flyback topology has been designed and tested for a series PPP circuit solution able to effectively interface a 5-kW battery energy storage system (BESS) to a 700-V dc grid. A relatively high power conversion efficiency and compact hardware are achieved due to the reduced size requirements on the input and output filtering stages. Above all, while explaining the PPP concept, this study shows that even converter circuits known for their low power efficiency can be used to derive highly efficient systems. A design approach is, thus, provided to facilitate the design of the presented PPP circuit, and measurements are, finally, carried out to compare the obtained results with the expected ones derived from the developed analytical models.

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This paper presents a protection framework for low voltage dc grids, which segments these grids into zones and tiers according to their fault current potential and provided protection. Furthermore, the technology and applications of different protection devices are examined. It is demonstrated that the utilization of fast fault interruption and fault limiting inductors are vital for the protection of low voltage dc grids. Moreover, a design of a solid-state circuit breaker is presented, and this devices is experimentally verified. The experimental results showed that the total time for the detection and interruption of faults can be lower than $1 \mu \mathrm{s}$ with solid-state protection devices.

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Low temperature electrolysis brings the possibility of achieving the production of fuels and chemical feedstocks without any carbon footprint. Power electronic converters are vital components of future electrolyser systems in terms of overall cost and efficiency. This paper presents the current state of art in power electronics for low temperature electrolysis and all the major steps of designing an electrolysis system are discussed from the modeling of electrolysers to the system architecture. The most promising routes are pointed out and backed up with results from both experimental and theoretical studies found in the literature.@en

Solid-state circuit breakers (SSCB) show great promise to become the key element in the protection of low-voltage direct current microgrids. SSCBs operate in the microsecond range and employ semi-conductor devices that have strict safe operation area limits. Therefore, the design of the SSCB needs to consider the effects of fault detection delays and semi-conductor safe operation area limitations. This paper derives SSCB design criteria that consider the effect of different detection methods with different detection delays under varying system constraints. The design space is investigated in a sensitivity analysis, which provides insights into the operation boundaries of SSCB and explains how a combination of fault detection methods can reduce the SSCB size. The insights from the theoretical and sensitivity analysis are used to propose an SSCB design flowchart. SSCB prototype is developed and tested in different scenarios under nominal grid voltage and current. The derived design constraints can be used for efficient SSCB design and also to evaluate the effects of different protection schemes on the required SSCB size.

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Electrification of ships is one of the hot topics in the marine industry. This is due to the stringent guidelines by the International Maritime Organisation (IMO) for curbing the green house gas emissions from the marine sector. In this paper, the state-space modeling approach is used to model bipolar dc grids on ships. A ferry is used as a test case. The modeling is done for the radial and zonal architecture with similar components. The dynamic simulation and stability analysis of the two architectures reveal that zonal architecture is potentially more stable than the radial architecture.

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Due to the distributed nature of future electrical power systems, decentralized control is essential for these grids. This paper shows that converters that have identical voltage thresholds switch off simultaneously even if some could have remained operational. Therefore, inadequate system and energy utilization can occur when decentralized demand or supply response is utilized. The Grid Sense Multiple Acces (GSMA) algorithm proposed in this paper ensures that, after a change occurs in the system, a subset of the converters remains connected to the grid, without the need of utilizing any form of communication. This is achieved by introducing an exponential backoff time between failed connection attempts. Furthermore, several simulations and experiments are conducted to illustrate and validate the behavior of the GSMA algorithm, showing that it can be applied to dc grids in order to improve system and energy utilization.

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Due to the sharp growth in the adaptation of electric vehicles (EV) in the Netherlands and the objectives of the Dutch Climate Accord is to encourage electric mobility, in the coming decades a substantial number of new EV charging facilities needs to be provided. Efficient planning of EV charging infrastructure is coupled with the notion of range anxiety, which is likely to be severely high in case of soon-to-be EV drivers. This study aims to estimate the cost of developing a new charging infrastructure under five scenarios of range anxiety in Amsterdam East. Employing a Linear Integer Programming optimization model, on the basis of geographic data on car registration, existing EV chargers, and electricity substations, it is obtained that if drivers use 90% of their battery before using a charging facility, the existing charging infrastructure needs to be expanded by only 31% to accommodate almost seven times larger number of EVs–the threshold set by the European Union (EU) legislation on the deployment of alternative fuel infrastructure. If drivers use only 30% of the batteries; however, an increase of 167% in infrastructure is inevitable (accounting for almost five million euro of cost). Second, at any point along the range anxiety spectrum, if the interval between charging session increases for 1 day, the overall cost decreases by more than 30%. These findings are discussed, and two policy approaches are proposed: (1) information technology approach; (2) demand-response approach, on the basis of EU legislation on energy efficiency and deployment of alternative fuel infrastructure.

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Historically speaking, alternating current (ac) has been the standard for commercial electrical energy distribution. This is mainly because, in ac systems, electrical energy was easily transformed to diffierent voltages levels, increasing the efficiency of transmitting power over long distances. However, technological advances in, for example, power electronics, and societal concerns such as global warming indicate that a re-evaluation of the current distribution systems is timely. Direct current (dc) distribution systems are foreseen to have advantages over their ac counterparts in terms of efficiency, distribution lines, power conversion and control. Moreover, most renewable energy sources and modern loads produce or utilize dc, or have a dc link in their conversion steps. However, the stability, control, protection and standardization of these systems, and the market inertia of ac systems are major challenges for the broad adoption of dc distribution systems. Steady-State, Dynamic and Transient Modeling Adequate models of dc distribution grids are required for the analysis, design and optimization of these systems. In this thesis new and improved methods are proposed for steady-state and dynamic modeling. Two novel steady-state methods are presented, which are shown to be better than the methods in existing literature with respect to convergence, computational effort and accuracy. Furthermore, a dynamic state-space model is proposed that can be efficiently applied to any system topology, and can be used for the stability analysis of these systems. Moreover, an improved symmetrical component decomposition method is presented, which enables simplied (fault) analysis. Transient models for dc distribution systems are briefly discussed, but the development of transient models is outside of the scope of this thesis. Algebraic and Plug-and-Play Stability As a result of the decreasing conventional generation, the inertia of electrical grids is signicantly decreased. Furthermore, more and more tightly regulated load converters that have a destabilizing effect on the system's voltage (and frequency) are proliferated throughout the grid. Consequently, the stability of systems with substantial renewable generation is more challenging. In this thesis a method to algebraically derive the stability of any dc distribution system is presented. Moreover, utilizing a Brayton- Moser representation of these systems, two simple requirements are derived for plug-and-play stability (i.e., stability requirements that can be applied to any system, even systems that are subjected to uncertainty or change). Decentralized Control Strategy and Algorithm Decentralized control is essential to deal with the trend to decentralize generation and segment the distribution grid, and to manage the potential absence of a communication infrastructure. In this thesis a decentralized control scheme is proposed that ensures global stability and voltage propriety for dc distribution grids. The control scheme divides the acceptable voltage range into demand response, emission, absorption and supply response regions, and species the behavior of converters in these regions. Furthermore, it is shown that inadequate energy utilization can occur, when voltage dependent demand response is utilized. Therefore, the Grid Sense Multiple Access (GSMA) is proposed, which improves the system and energy utilization by employing an exponential backoff routine. Decentralized Protection Framework and Scheme Because of the absence of a natural zero crossing, low inertia, meshed topologies and bi-directional power ow, the protection of low voltage dc grids is more challenging than conventional ac grids. In this thesis a decentralized protection framework is presented, which partitions the grid into zones and tiers according to their short-circuit potential and provided level of protection respectively. Furthermore, a decentralized protection scheme is proposed, which consists of a modied solid-state circuit breaker topology and a specied time-current characteristic. It is experimentally shown that this protection scheme ensures security and selectivity for radial and meshed low voltage dc grids.@en

This paper presents a steady-state model and associated power flow equations that can be applied to any dc grid. State-of-art power flow methods and two newly proposed methods are discussed and applied to the proposed steady-state model. A standardized IEEE test feeder is used to benchmark the power flow methods with respect to accuracy, convergence and computational efficiency. It is shown that the two new methods have a superior performance compared to the existing techniques for the steady-state analysis of most common dc grids, providing up to a 93 % increase in computational efficiency for the system that was analyzed in this paper. Therefore, it is demonstrated in this paper that these power flow techniques can be used for the operation, planning, optimization, market simulation, and security assessment of practical dc grids.

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Since the voltages and currents in dc grids do not have a natural zero-crossing, the protection of these grids is more challenging than the protection of conventional ac grids. Literature presents several unit and non-unit protection schemes that rely on communication, or knowledge about the system's topology and parameters in order to achieve selective protection in these grids. However, communication complicates fast fault detection and interruption, and a system's parameters are subject to uncertainty and change. This paper demonstrates that, in low voltage dc grids, faults propagate fast through the grid and interrupted inductive currents commutate to non-faulted sections of the grid, which both can cause circuit breakers in non-faulted sections to trip. A decentralized plug-and-play protection scheme is proposed that ensures selectivity via an augmented solid-state circuit breaker topology and by utilizing the proposed time-current characteristic. It is experimentally shown that the proposed scheme provides secure and selective fault interruption for radial and meshed low voltage dc grids under various conditions.

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Constant power loads combined with low inertia form a major challenge for future distribution grids. This paper presents a state-space representation to model dc distribution systems. Two methods are discussed to analyze the (small-signal) stability of these dc distribution systems; an algebraic method and a Brayton-Moser method. The system models and the methods for stability analysis were verified using an experimental dc microgrid set-up. Furthermore, it was found that the instability of dc distribution systems can be classified into two categories: equilibrium instability and oscillatory instability.

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Scalable and robust low-voltage direct current (LVdc) distribution networks require solutions, allowing flexible power flow control and reliable short-circuit protection. In this paper, the continuous full-order large-and small-signal models of a partially rated power flow control converter (PFCC) are derived utilizing the generalized averaging method. The large-signal model of the PFCC is coupled with a model of the LVdc grid. Due to the state-space representation, the combined model of the PFCC and the LVdc grid is suitable for easy algorithmization, and efficient simulation. These advantages make them essential tools for studying and optimizing of scalable LVdc systems with decentralized power flow control based on the PFCC. The PFCC models provide insights into controller design and stability analysis. The models are experimentally validated, and the functionality of the PFCC is demonstrated in a laboratory-scale microgrid.

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To tackle the challenges of future distribution systems, dc is being reconsidered. However, broad adoption of dc distribution systems requires additional research into the modeling, stability, protection and control of these systems. Previous research presents modeling methods that only consider monopolar configurations and do not take mutual couplings into account. Therefore, this paper presents a state-space method that can be applied to any dc distribution system, regardless of configuration and mutual couplings. Moreover, it shows how the state-space matrices can be derived in a programmatic manner. Furthermore, the models are validated using an experimental dc microgrid set-up. Due to the mathematical nature, the presented modeling method can be applied easily, and the stability and control can be analyzed algebraically.

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The number of electric vehicles in the Netherlands has sharply increased over the past decade. This has caused a need for the allocation of a substantial amount of new electric vehicle chargers around the country, which in turn has been acknowledged by a variety of legislative bodies. However, the approach of how these new charging infrastructures need to be spatially distributed has yet to be decided, including the distance that an electric vehicle charger could be allocated from the final destination of its driver. The hypothesis of this study is that if residents walk a longer distance to/from these charging stations, the chargers could be shared by a greater number of electric vehicle owners, and the total cost of the new charging infrastructure could be reduced. By using linear integer programming, the minimum cost of allocating new fast-charging stations in a central, densely populated area of Amsterdam, accounting for 7% of the city’s population, is calculated. The results show that if residents were to walk for five minutes (roughly 400 metres) instead of two and half minutes (roughly 200 metres), the overall cost of new electric vehicle chargers could be reduced by more than 1 million euros. The study also found that both the cost of new charging stations and their efficiency of use are vastly affected by the portion of the charging infrastructure that is saved for people visiting the area. The findings of this study are discussed in detail, including the proposal of potential further studies.

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Many modelling methods for the analysis of dc distribution grids only consider monopolar configurations and do not allow for mutual couplings to be taken into account. The modelling method presented in this paper aims to deal with both of these issues. A state-space approach was chosen for its flexibility and computational speed. The derived approach can be applied to any dc distribution system regardless of its configuration and takes into account mutual couplings between phase conductors. Moreover, the state-space matrices can be derived in a programmatic manner. The derived model was verified empirically and by a reference model created in Simulink using PowerLib blocks. Subsequently, an illustrative system was analyzed, which showed the utility of the presented method in analyzing the dynamics of dc distribution systems. The presented method could especially be useful for the analysis, design and optimization of, for example, the stability and control systems of dc distribution systems.@en

Changes in distribution grids pose significant challenges with respect to the control and management of these grids. Stability and decentralized control are vital to ensure the availability and accessibility of plug-and-play dc distribution grids that are (temporarily) without communication. Therefore, this paper presents guidelines for these grids that ensure global stability and a decentralized control strategy that implements these stability guidelines. The stability guidelines are derived using a Brayton-Moser representation of the system to arrive at a Lyapunov candidate function. Furthermore, the decentralized control strategy implements these stability guidelines and ensures that the voltages in the system remain within a specified range. Additionally, several simulations are performed to illustrate the stability of the system and the behavior of the control strategy under different scenarios.

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Employing bipolar dc distribution systems introduces the possibility of imbalance in the system. To analyze these systems it is crucial to create novel modelling techniques. This paper presents a method to decompose dc distribution systems into symmetrical components. The presented method simplifies the analysis of balanced, unbalanced, and fault conditions of bipolar dc distribution systems. Equivalent circuits for several network components in the symmetrical domain are derived and are shown to be independent under symmetrical conditions. Furthermore, a dynamic analysis is performed in the symmetrical domain showing that the method simplifies the analysis of dc distribution systems. Additionally, the symmetrical domain equivalent circuits of several fault conditions are derived.@en

AbstractDue to an increasing number of power generation units and load devices operating with direct current (DC) at distribution level, there is a potential benefit of leading efforts toward building a DC distribution system. However, the implementation of DC distribution systems faces important challenges, including the market inertia of AC systems and standardization. Many of the benefits that are attributed to DC can only be realized if a complete DC system is developed, and not if only a few components are replaced. This paper presents the concept of a universal DC distribution system, as envisioned by the authors. The universal DC distribution system could be implemented in various use cases, but could also completely replace AC distribution grids. The paper covers the possibilities of having DC nanogrids inside buildings, DC microgrids in neighborhoods, and the connection to AC and DC medium voltage grids. Furthermore, considerations regarding flexibility, electricity market design, control, and protection are presented.@en

Due to low inertia and constant power loads, stability is a major issue for dc distribution grids. Previous research focuses mostly on the (mathematical) analysis of the stability, but does not provide any guidelines to ensure stability. This paper presents four guidelines that ensure the existence of an appropriate equilibrium and stability around that equilibrium. From a linearized model of the dc distribution grid the location of the equilibrium and the stability of the system are derived. Subsequently, guidelines for the existence and propriety of the equilibrium, and the static and dynamic stability around that equilibrium are developed. Furthermore, the accuracy of these guidelines is determined. Additionally, possible applications of the guidelines are discussed.@en

Instability caused by low inertia and constant power loads is a major challenge of DC distribution grids. Previous research uses oversimplified models or does not provide general rules for stability. Therefore, a comprehensive approach to analyze the stability of DC distribution systems is desired. This paper presents a method to algebraically analyze the stability of any DC distribution system through the eigenvalues of its state-space matrices. Furthermore, using this method, requirements are found for the stability of three example systems. Additionally, a sensitivity analysis is performed, which considers the effect of several system parameters on the stability and disputes some overgeneralized conclusions of previous research. Moreover, various simulations are performed to illustrate the dynamic behavior of a stable and an unstable DC distribution system.@en