MVDC technology is a promising solution to avoid installation of new AC networks. MVDC can provide optimum integration of large-scale renewable energy sources, the interconnection of different voltage levels of DC and AC grids with the ancillary services. The development in MVDC depends significantly on the DC-DC converters. Such converters support the modern trends of utilising medium-frequency transformers in power networks. Research on isolated converters technology is in its infancy and limited by the conversion ratio and component ratings. Besides, there is no standards exist covering specific aspects of isolated converter product. Thus, a review of such converters is needed. This work presents, for the first time, a review of the DC-DC power converter families in MVDC grids including the leading families which are isolated and non-isolated converters, as well as other subfamilies comparing the specifications and characteristics. Also, the applications of these converters are provided by focusing on the essential requirements for each application.

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Isolated and Non-Isolated DC-to-DC Converters for Medium Voltage DC Networks: A Review

Analysing the power system with a goal of determining its stability is based on models of system components encompassing adequate assumptions to formulate an appropriate mathematical model in the time scale that properly describes the phenomenon under study.

Small signal stability

Advances in voltage-source converters (VSCs), as well as their successful application in VSC-HVDC systems, have motivated growing interests and research in medium-voltage direct current (MVDC) traction power systems (TPSs) for high-speed rail (HSR) applications. As an emerging power-converter-based infrastructure, this study reviewed developments that shape two key evolving pieces of equipment—namely, high-power traction substation (TSS) converters, and power electronic transformers (PETs)—for MVDC TPS as well as prospects for smart grid (SG) applications in the future. It can be deduced that cost-effective and robust high-power TSS converters are available from hybrid modular multilevel converters (MMCs) for enhanced performance and fault-tolerance capability. In addition, silicon carbide (SiC) MMC-based PETs with input-series-output-parallel (ISOP) configuration are present for greater weight/size reduction and efficiency for MVDC rolling stock design. Finally, the implementation of a smart MVDC TPS incorporating a sophisticated railway energy management system (REM-S) based on the smart grid principles is feasible in the future, with numerous benefits. However, there are related challenges, like knowledge gaps on these technologies, the high costs involved, and lack of standardization to overcome to realize widespread future commercial deployment.

MVDC railway traction power systems, state-of-the art, opportunities, and challenges

In this research study, a multiterminal voltage source converter (VSC) medium voltage DC (MVDC) distribution network hierarchical control scheme is proposed for renewable energy (RE) integration in a co-simulation environment of MATLAB and PSCAD/EMTDC. A DC optimal power flow (DC OPF) secondary controller is created in MATLAB. In PSCAD/EMTDC, the main circuit containing the adaptive DC voltage droop with a dead band and virtual synchronous generator (VSG) based primary controller for the VSCs is implemented. The simulation of the MVDC network under the proposed hierarchical control scheme is investigated considering variations in wind and solar photovoltaic (PV) power. The network is also connected to the standard IEEE-39 bus system and the hierarchical scheme tested by assessing the effect of tripping as well as restoration of the REs. The results show that during random variations in active power such as increasing wind and PV power generation, a sudden reduction or tripping of wind and PV power, the primary controller ensures accurate active power sharing amongst the droop-based VSCs as well as regulates DC voltage deviations within the set range of 0.98–1.02 pu with an enhanced dynamic response. The DC OPF secondary control optimizes the system’s losses by 38% regularly giving optimal droop settings to the primary controllers to ensure proper active power balance and DC voltage stability. This study demonstrates that the hierarchical control strategy is effective for RE integration in the MVDC distribution network.

Multiterminal Medium Voltage DC Distribution Network Hierarchical Control

Control and protection of MMC-based HVDC systems: A review

The multi-terminal voltage sourced converter medium-voltage DC (VSC-MVDC) distribution network has high prospects for future development, hence the proposed alternative for the AC distribution in commercial and industrial applications. However, a number of obstacles stand on its way in realising its immense potential in the modern power system. This study presents a review and focuses on the complexity of controlling DC voltage/power flow in the multi-terminal DC (MTDC) grids and the possible DC voltage coordinated control approaches aimed at identifying the suitable control strategies for the proposed MVDC distribution grids. Several centralised and distributed DC voltage control strategies for MTDC have been considered. In conclusion, the droop-based control strategies have been identified as reliable, flexible, and expandable with high dynamic performance and minimal communication needs in MTDC grids. These strategies are suitable and adaptable for stable and secure operation of the proposed multi-terminal MVDC distribution network. The study recommends a comparative study of the key DC voltage coordination control features of the droop-based control strategies using the multi-terminal VSC-MVDC distribution network dynamic model for future studies.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/joe.2018.8841

Review of the DC voltage coordinated control strategies for multi-terminal VSC-MVDC distribution network

Modelling and control of a VSC medium‐voltage DC distribution network with DFIG wind farm

The Brayton-Moser’s mixed potential theory is a popular approach for large-signal stability analysis of DC-based systems with constant power load (CPL). The criterion from the conventional mixed potential theory contains multiple time-varying parameters which are complicated for convenient stability analysis hence the need for simplification. On the other hand, stability analysis of droop-controlled multiple source converter-based DC systems are scarce. Thus, the large-signal stability criterion of a scaled-down droop-controlled multi-terminal MVDC distribution network with CPL using Brayton-Moser’s mixed potential theory was simplified and derived to show that CPL power rating, PI control parameters, droop and damping coefficients have profound effects on the system’s large-signal stability. By way of MATLAB/Simulink simulations, the large-signal stability of the MVDC distribution system was observed to reduce with increase in CPL power rating and source converter droop coefficients as well as increase with increase in proportionate coefficient values and use of optimized unlike non-optimized damping coefficients. Therefore, the droop-controlled MVDC distribution network with optimized PI control parameters, droop and damping coefficients have enhanced dynamic response and large-signal stability margin proving the accuracy and validity of the simplified large-signal stability criterion.

Multi-terminal Medium Voltage DC Distribution Network Large-signal Stability Analysis

The stable operation and control of the proposed terrestrial multi-terminal Voltage Source Converter (VSC) medium voltage DC (MVDC) distribution network is a remarkable motivation towards developing the future universal DC grid interconnecting the low voltage DC systems on one side and the high voltage DC systems on the other. The small-signal stability analysis are important in designing these power electronic based systems (PEBS). In this study, small-signal stability approaches for PEBS are reviewed with the aim of identifying suitable methods for the proposed MVDC distribution network. It is established that the impedance-based methods are suitable for local stability analysis complementing very well with the state-space modelling-eigenvalue analysis method best for global stability analysis. The Lyapunov linearisation method offers simple, adaptable, and accurate small-signal and large-signal stability study. On the other hand, the bifurcation analysis is an emerging non-linear approach with promising efficiency and precision. In conclusion, there is no single small-signal stability criterion that encompass an acceptable level of accuracy, adaptability, and efficiency. Thus, the impedance-based methods combined with state-space modelling-eigenvalue analysis, Lyapunov linearisation, and bifurcation theory can be suitable small-signal stability approaches for the multi-terminal MVDC distribution network.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/joe.2018.8842

Small-signal stability analysis for the multi-terminal VSC MVDC distribution network; a review

The design and function verification of aircraft electromechanical management system mainly based on test have the disadvantages of long developing cycle and high cost. Therefore, combined with the idea of Model-based Systems Engineering (MBSE), it is of great engineering significance to establish a simulation platform of integrated control management system to replace the test and form an integrated simulation support environment for the research of integrated control management. A modeling and simulation method of electrical interface function and performance is developed based on Saber, which aims at aircraft electrical interface characteristics of electromechanical management system. With analog modeling, digital modeling, analog-digital modeling and embedded modeling, mechanical and electrical management electrical interface components and circuits are modelled. The electromechanical management electrical interface integrated simulation system, which is in high precision, fine granularity, and high degree of confidence, is built to promote the process of aviation simulation instead of test. 

Wang Kunyu

Papers

Review of the DC voltage coordinated control strategies for multi-terminal VSC-MVDC distribution network

Modelling and control of a VSC medium‐voltage DC distribution network with DFIG wind farm

Multi-terminal Medium Voltage DC Distribution Network Large-signal Stability Analysis

Small-signal stability analysis for the multi-terminal VSC MVDC distribution network; a review

Modeling of electrical interface of electromechanical management system based on Saber