Integration of Sodium Metal Halide Energy Storage Systems in Telecommunication Microgrids: Performance Analysis of DC-DC Converter Topologies

The present paper proposes an integrated method for modelling and designing Energy Storage Systems (ESSs) based on Sodium Metal Halide Batteries (SMHBs). The implementation of the proposed methodology for designing an SMHB-ESS used for supporting telecommunication DC microgrids is presented. The motivation concerning this specific case study is the role assumed by battery technology in improving the reliability and robustness of telecommunication DC microgrids. In this context, the SMHBs, due to their operative temperature, dynamic power response and robustness against cell breakdown, represent one of the most suitable technologies, mainly when challenging environmental conditions occur. The motivation for implementing an integrated design approach is the non-linear behaviour of SMHBs, which requires a high accuracy in battery modelling and in managing DC-DC interfacing for full SMHB capacity exploitation. To highlight the advantages of this novel approach, a comparison between the SMHB- ESS designs considering, as the DC-DC converter, a buck–boost topology actually implemented in the commercial systems and a Dual-Active-Bridge (DAB) converter, specifically developed for this kind of battery, was investigated. Considering different operating conditions in a specific DC telecommunication microgrid, the designed configurations of SMHB ESSs were simulated. Finally, a comparison of simulation results is presented and discussed, highlighting that DABs, despite their greater complexity compared to buck–boost converters, present advantages in terms of flexibility, dynamic performances and efficiency, increasing the available SMHB capacity by 10%

Soft-start procedure for a three-stage smart transformer based on dual-active bridge and cascaded H-bridge converters

Power electronics based three-stage smart transformers (STs) can be seriously damaged by inrush currents and overvoltages during the start-up phase if the control of the stages is not correctly coordinated. Hence, it is crucial to design properly the start-up procedure, especially in case of modular architectures with distributed dc-links. The design of the start-up procedure depends on the ST power stages topologies, their control systems, and the operation modes. This article proposes a soft-shift start modulation technique that allows to limit the inrush current in the dc/dc isolation stage during the dc-link capacitors precharging. A fast voltage-balancing control, performed by the dc/dc isolation stage, is introduced to avoid overvoltages and unbalanced voltage conditions among the different power cells. Under the proposed method, fast control dynamics is guaranteed thanks to the high frequency bandwidth of the dc/dc isolation stage converters. Theoretical analysis, based on a detailed small signal model of the ST, and simulations are used to demonstrate the principle of the operation. Experimental results, carried out in an ST prototype, confirm the performances of proposed solution in realizing a smooth start-up without voltage/current overshoots

Battery models for battery powered applications: A comparative study

Battery models have gained great importance in recent years, thanks to the increasingly massive penetration of electric vehicles in the transport market. Accurate battery models are needed to evaluate battery performances and design an efficient battery management system. Different modeling approaches are available in literature, each one with its own advantages and disadvantages. In general, more complex models give accurate results, at the cost of higher computational efforts and time-consuming and costly laboratory testing for parametrization. For these reasons, for early stage evaluation and design of battery management systems, models with simple parameter identification procedures are the most appropriate and feasible solutions. In this article, three different battery modeling approaches are considered, and their parameters' identification are described. Two of the chosen models require no laboratory tests for parametrization, and most of the information are derived from the manufacturer's datasheet, while the last battery model requires some laboratory assessments. The models are then validated at steady state, comparing the simulation results with the datasheet discharge curves, and in transient operation, comparing the simulation results with experimental results. The three modeling and parametrization approaches are systematically applied to the LG 18650HG2 lithium-ion cell, and results are presented, compared and discussed

AC-DC interface converters for MW-scale MVDC distribution systems: A survey

Focusing on the relevant pros provided by power conversion onboard introduction (e.g. reduction of power system weight/volume), a widespread use of power converters is foreseeable in future MVDC shipboard power systems. For conveniently exploiting the power converters capability in the marine environment, a preliminary investigation about the attainable topologies must be carried out. In this paper the focus is limited to the AC-DC interface power conversion stage. Starting from the analysis of the power devices fully available on the market, four AC-DC interface converters topologies are proposed: a 12-pulse thyristor rectifier, an IGCT based rectifier, a modular multilevel IGBT rectifier and finally an IGBT Dual Active Bridge (DAB) based rectifier. With references to the present standards issues, the survey highlights advantages and disadvantages of the presented topologies providing a preliminary comparison in terms of rough estimation of weights, filtering elements, scalability (both in power and in voltage), power quality performance, galvanic isolation, complexity, etc

Power-Electronics-Based Power Distribution System of a MVDC Ship: AC/DC Interface Converters and Control System

Next generation of MVDC ships will be characterized by a power-electronics-based power distribution system. Since onboard power generation is in AC, special attention is pointed at the AC/DC interface converters forming the MVDC bus of the shipboard power distribution system. In this paper preliminary design of two AC/DC power conversion stages and their voltage control is provided. Besides the respective local control systems, a coordination strategy is required between the two AC/DC power conversion stages in order to achieve loads power sharing. In the proposed case study, a MVDC bus control based on the droop control theory is adopted. Simulation results support the proposed coordination strategy

Phase-Controlled Thyristor Sub-Synchronous Damper Converter for a Liquefied Natural Gas Plant

In electrified liquefied natural gas (LNG) plants variable frequency drives (VFDs) interact with turbine-generator (TG) units creating torsional vibrations known as sub-synchronous torsional interactions (SSTIs). Torsional vibrations can be dangerous for an LNG plant when they involve torsional instability. The stability of an LNG plant depends on the plant configuration and on the number of TG units and of VFDs. In such peculiar configurations stability cannot be achieved acting of the VFDs control system. Alternatively, dedicated equipment is needed to damp the torsional vibrations. In this paper, a sub-synchronous damper (SSD) converter is used to mitigate the SSTI phenomena. The SSD converter consists of a thyristor H-bridge regulating the phase of the additional torque provided at the TG unit air-gap. A phase control system is proposed and is based on the torsional torque oscillations measurement. An adaptive reference signal is employed, also guaranteeing high performance in island-mode operation. The proposed solution increases the damping of the LNG plant in all the considered configurations. The LNG plant successful operation is validated by comprehensive results