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.

/pdf/dc-microgrid-protection-a-comprehensive-review-38dp561hfd.pdf

DC Microgrid Protection: A Comprehensive Review

Bidirectional DC-DC power converters are increasingly employed in diverse applications whereby power flow in both forward and reverse directions are required. These include but not limited to energy storage systems, uninterruptable power supplies, electric vehicles, and renewable energy systems, to name a few. This paper aims to review these converters from the point of view of topology as well as control schemes. From the point of view of topology, these converters are divided into two main categories, namely non-isolated and isolated configurations. Each category is divided into eight groups along with their respective schematics and a table of summary. Furthermore, the common control schemes and switching strategies for these converters are also reviewed. Some of the control schemes are typically applied to all DC-DC power converters such as PID, sliding mode, fuzzy, model predictive, digital control, etc. In this context, it should be noted that some switching strategies were designed specifically for isolated bidirectional DC-DC converters in order to improve their performance such as single phase shift, dual phase shift, triple phase shift, etc. The features of each topology and control scheme along with their typical applications are discussed, in order to provide a ground of comparison for realizing new configurations or finding the appropriate converter for the specific application.

/pdf/topologies-and-control-schemes-of-bidirectional-dc-dc-power-2o5z4fzkzm.pdf

Topologies and Control Schemes of Bidirectional DC–DC Power Converters: An Overview

For current-fed dual active bridge bidirectional dc–dc converters, all the possible switching patterns are summarized in view of the combinations of both side pulse-width modulation duty cycles and phase-shift angle. A control strategy is proposed for the current-fed dual active bridge converter to operate with the optimized patterns. The equivalent duty cycle of the secondary side is smaller than that of the primary side by a fixed value, which is optimized based on the soft-switching achievement and the circulating current minimization. The closed-loop control is easy to be implemented since there are only two independent variables, one-side duty cycle and the phase-shift angle. With the proposed control, zero-voltage switching can be achieved for all power switches throughout full range of load even at no-load condition. The typical working modes with the proposed control are given. The optimal design of the system parameters including the fixed time delta and dead time is illustrated. The converter has a very good performance, not only under steady states but also in transients. The conversion efficiency is high. The effectiveness of the proposed control is verified by the experimental results of a 1-kW laboratory prototype.

High-Efficiency Current-Fed Dual Active Bridge DC–DC Converter With ZVS Achievement Throughout Full Range of Load Using Optimized Switching Patterns

This paper presents a novel reactive-power minimization method under the three-level modulated phase-shift control to improve the efficiency for a wide operation condition. First, in order to unify all operation modes, a mathematic model of the dual-active-bridge converter with the three-level modulated phase-shift control by a harmonic analysis method is obtained. Then, the detailed analysis of the odd-order harmonic components of the active and reactive powers varied with the multiple control dimensions is presented. The soft-switching boundaries with respect to the voltage ratio and unified active power are specified. On this basis, a novel optimal three-level phase-shift (OPS) control strategy for minimizing reactive power is proposed. Experimental results of reactive power and operation efficiency among the proposed OPS method, and other methods are compared to verify the effectiveness of the OPS algorithm.

Minimum-Reactive-Power Scheme of Dual-Active-Bridge DC–DC Converter With Three-Level Modulated Phase-Shift Control

Ease of integration of the variable speed diesel generators resulting in substantial reduction of the fuel consumption is the key motivation for the development of the dc shipboard power system (SPS). One of the impediments to the widespread adoption of the dc SPS, however, has been the lack of comprehensive fault management strategies during the short-circuit faults. Such strategies comprise of fault detection, fault isolation, and reconfiguration of dc SPS. In the existing literature, all these aspects of fault management are dealt independently and mostly assuming ideal conditions. All the strategies are of utmost importance and it is needed to study them under a common framework which is the aim of this paper. This paper starts with a brief discussion on the characteristics of dc SPS along with recent modeling techniques. Subsequently, this paper describes the short-circuit fault studies, fault characteristics, and protection requirements. Finally, this paper outlines the working principle, advantages, and limitations of the fault detection, isolation, and reconfiguration strategies developed for the dc power system and analyzes their suitability to the dc SPS. This paper is concluded by identifying the future research trends needed for the development of the short-circuit fault management strategies of dc SPS for critical marine missions.

Short-Circuit Fault Management in DC Electric Ship Propulsion System: Protection Requirements, Review of Existing Technologies and Future Research Trends

Several papers have been published recently on TPS control of dual active bridge (DAB) converter, however, no complete study of the converter operation behaviour exists, that takes into account all switching modes in both charging and discharging (bidirectional) power transfer. In this paper, six switching modes and their complements with opposite power transfer direction are defined with their operational constraints. Exact expressions for power transferred are derived with no fundamental frequency assumptions and range of power transfer for each mode is also defined to characterize mode limitations. Detailed constraints for zero voltage switching (ZVS) are also obtained. A new definition for converter reactive power consumption is introduced. This is based on calculation of inductor apparent power which avoids fundamental frequency approximations as well as the vague negative (back flowing) power definitions in recent papers. All known DAB phase shift modulation techniques including conventional, dual and extended phase shift, represent special cases from triple phase shift, therefore the presented analysis provides a generalised theory for all phase shift based modulation techniques.

/pdf/comprehensive-steady-state-analysis-of-bidirectional-dual-4i4pqxd96i.pdf

Comprehensive steady state analysis of bidirectional dual active bridge DC/DC converter using triple phase shift control

Reactive power flow increases dual active bridge (DAB) converter RMS current leading to an increase in conduction losses especially in high power applications. This paper proposes a new optimized triple phase shift (TPS) switching algorithm that minimizes the total reactive power of the converter. The algorithm iteratively searches for TPS control variables that satisfy the desired active power flow while selecting the operating mode with minimum reactive power consumption. This is valid for the whole range of converter operation. The iterative algorithm is run offline for the entire active power range (−1pu to 1pu) and the resulting data is used to train an open loop artificial neural network controller to reduce computational time and memory allocation necessary to store the data generated. To validate the accuracy of the proposed controller, a 500-MW 300kV/100kV DAB model is simulated in Matlab/Simulink, as a potential application for DAB in DC grids.

/pdf/reactive-power-minimization-of-dual-active-bridge-dc-dc-1udfch48yr.pdf

Reactive power minimization of dual active bridge DC/DC converter with triple phase shift control using neural network

Fast isolation and detection of DC faults is currently a limiting factor in high power DC transmission grid development. Recent research has shown that the role of DC/DC converters is becoming increasingly important in solving various DC grid challenges such as voltage stepping, galvanic isolation and power regulation. This paper focuses on an additional important feature of bidirectional dual active bridge (DAB) DC-DC converters which make it attractive for future DC grids; it's inherent fault isolation capability which does not need control intervention to limit fault current in case of the most severe DC faults. Detailed analytical, simulation and experimental study are performed by subjecting the converter to DC short circuit faults at its DC voltage terminals. The results obtained have shown significant advantage of DAB where fault current is less than rated current during the fault duration. Thus no control action is necessary from the non-faulted bridge to limit fault current and no external DC circuit breakers are required. This advantage makes DAB converter feasible for DC grid integration.

/pdf/dc-fault-isolation-study-of-bidirectional-dual-active-bridge-3fwgv62sk5.pdf

DC fault isolation study of bidirectional dual active bridge DC/DC converter for DC transmission grid application

This paper presents a new structure of a power sharing controller for a modular based dual active bridge (DAB) DC/DC converter. The modular topology is based on input-series output-parallel (ISOP) connection of DAB modules suitable for a voltage step down application at medium voltage level. Since DAB has bi-directional power flow capability, this connection would act as its dual input-parallel output-series (IPOS) counterpart in the case of reverse power flow. The proposed power sharing controller ensures uniform power sharing among the modules even in the case of parameter mismatches as well as manages converter operation during pole-to-pole and individual module DC faults. Results for the proposed controller are justified using a detailed Matlab simulation model.

/pdf/power-sharing-controller-for-modular-dual-active-bridge-dc-42cmrupij5.pdf

Yasen A. Harrye

Papers

Comprehensive steady state analysis of bidirectional dual active bridge DC/DC converter using triple phase shift control

Reactive power minimization of dual active bridge DC/DC converter with triple phase shift control using neural network

DC fault isolation study of bidirectional dual active bridge DC/DC converter for DC transmission grid application

Power sharing controller for modular dual active bridge DC/DC converter in medium voltage DC applications