DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out.

/pdf/dc-microgrids-part-ii-a-review-of-power-architectures-69j8nn2rpd.pdf

DC Microgrids—Part II: A Review of Power Architectures, Applications, and Standardization Issues

Multilevel inverters have created a new wave of interest in industry and research. While the classical topologies have proved to be a viable alternative in a wide range of high-power medium-voltage applications, there has been an active interest in the evolution of newer topologies. Reduction in overall part count as compared to the classical topologies has been an important objective in the recently introduced topologies. In this paper, some of the recently proposed multilevel inverter topologies with reduced power switch count are reviewed and analyzed. The paper will serve as an introduction and an update to these topologies, both in terms of the qualitative and quantitative parameters. Also, it takes into account the challenges which arise when an attempt is made to reduce the device count. Based on a detailed comparison of these topologies as presented in this paper, appropriate multilevel solution can be arrived at for a given application.

Multilevel Inverter Topologies With Reduced Device Count: A Review

Review of multilevel voltage source inverter topologies and control schemes

This paper studies the relations of space-vector modulation (SVM) and carrier-based pulse-width modulation (PWM) for multilevel inverter. The PWMs' generation of SVM can be achieved by carrier-based PWM scheme, but the modulated wave of SVM is acquired by vectors' calculations and switching-states' selection. Based on different selection of redundant switching-states, there are many types of SVM modulated waves, some of which can function equivalently through proper selection of common-mode injections in the case of carrier-based PWM, the others have more freedoms in switching-states' selection than carrier-based PWM. Selection of more switching-states in SVM is propitious to optimize the output voltage, balance the dc power and so on. Then an improved PWM scheme is proposed based on the modulation waves of three-level SVM, which reserves the main advantages of SVM, and can be achieved easily. Finally, a five-level test circuit is built to verify this PWM scheme.

Comparisons of Space-Vector Modulation and Carrier-Based Modulation of Multilevel Inverter

This paper proposes a nonunit protection scheme for DC microgrids (DC MGs) utilizing only local measurements. This scheme is developed based on the natural characteristics of DC current and its first and second derivatives under fault transients. Since it is based on local measurements, the problems associated with the communication delay are avoided. The selected protection scheme detects and discriminates the faults within a few microseconds of its inception. In this paper, a method to calculate the thresholds, used for the protection scheme, is also discussed. The proposed scheme is validated on a ring-type DC MG architecture under different fault scenarios and tested through MATLAB/Simulink simulations.

A Non-unit Protection Scheme for DC Microgrid Based on Local Measurements

Neutral-point-clamped converters are increasingly applied in industrial drive systems as they allow the use of lower voltage devices in higher voltage applications, provide reduced output voltage total harmonic distortion (THD), and can develop low common mode voltage. Several distinct modulation strategies have been proposed in the past for eliminating the common mode voltage, providing low THD output voltage or reducing the neutral point current ripple. However, each of these strategies improves the performance of the converter on one metric while trading off performance in other metrics. Nearest three vector, radial state, and zero-common-mode types of space-vector modulation techniques are compared using metrics that clearly elucidate the performance tradeoffs. Analytical models, computer simulation results, and experimental verification are presented.

/pdf/comparative-evaluation-of-modulation-algorithms-for-neutral-3dfhioy3yw.pdf

Comparative evaluation of modulation algorithms for neutral point clamped converters

The AC power systems on navy ships use circuit breakers based on traditional commercial technology. New naval power systems employ higher voltage DC distribution and use solid state power converters that actively (and instantaneously) limit the available fault current. When conventional circuit breakers are used in these systems, they have a relatively long clearing time, causing the voltage to collapse for a significant time. The application of fast-acting solid state circuit breakers, based on IGBTs and IGCTs will eliminate such voltage dips and result in a superior power system. Solid state circuit breakers also offer advantages when used in traditional ship power systems by limiting fault currents to levels far below those dictated by the supplying generators. This leads to lower magnetic and thermal stresses on distribution components, lower energy delivered to faults, faster fault isolation and minimal voltage disturbance.

Circuit Breaker Technologies for Advanced Ship Power Systems

Future ship power systems require improved methods to achieve the objectives of flexible/reconfigurable power delivery, robust fault management and improved power quality for loads. A DC integrated fight through power (IFTP) system achieves this increased survivability goal through the use of in-line power conversion, where all of the ship service power is delivered to the zonal loads through staged power conversion. A new AC IFTP system is proposed in this paper, which results from the application of the industrial power-conditioning approach to shipboard power systems. The system architecture is presented and survivability modes are analyzed. Two power conditioning devices are vital to the effective operation of the AC IFTP and these are discussed and simulated in detail. After preliminary analysis, the power conditioning centric AC IFTP system promises reduced size and weight as the converter ratings are reduced. Further analysis is required to quantify these tradeoffs between survivability and weight for the AC IFTP system so that actual weight loss can be identified.

Improved ship power system - generation, distribution, and fault control for electric propulsion and ship service

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Comparative evaluation of modulation algorithms for neutral point clamped converters

Circuit Breaker Technologies for Advanced Ship Power Systems

Improved ship power system - generation, distribution, and fault control for electric propulsion and ship service