Medium Voltage DC Distribution Network Modeling,Control,and Stability Analysis

The interests in multi-terminal medium voltage DC(MVDC)distribution network as the future alternative for AC distribution system in commercial and industrial applications have been steadily growing.The MVDC distribution finds extensive applications in;de-risking high voltage DC(HVDC)transmission networks,cost-effective AC distribution network reinforcement,renewable energy integration,rail transport application,urban electrification,etc.Thus,this network presents great prospects for technology development,new markets,and advanced modern power networks with numerous benefits.However,the MVDC distribution network concept is relatively new with no current commercial terrestrial installation in the world.As such,many challenges need to be addressed such as MVDC distribution network model development,DC voltage control,and system stability studies to foster MVDC research and development(R&D)and market deployment.Therefore,the main purpose of this study is to develop and ascertain the multi-terminal MVDC distribution network as a controllable and stable architecture for terrestrial power systems applications.Thus,the study pursued to:establish the multi-terminal MVDC distribution network dynamic model;identify its suitable primary control scheme;develop its hierarchical control scheme and evaluate the influence of CPL power rating,PI control parameters,and damping coefficients on the large-signal stability of the multi-terminal MVDC distribution system.Systematic mathematical modeling,basic primary control implementation,and proportionate integral(PI)controller parameter optimization for the multi-terminal MVDC distribution network realized a credible combined dynamic model for control design and stability analysis.The key primary level control features in the multi-terminal MVDC distribution network for the master-slave,voltage margin,conventional droop,and DC voltage droop with dead-band schemes were evaluated and the droop with dead-band control identified as the best with the lowest steady-state error and superior transient response.At the primary level,the droop with dead-band control was further enhanced to an adaptive droop as well as integrated with VSG in PSCAD/EMTDC whereas at the secondary level the DC optimal power flow(OPF)algorithm was implemented using fmincon nonlinear optimization in MATLAB within a co-simulation environment.The proposed hierarchical control scheme guaranteed desirable operational control,stability,security as well as minimal losses in the MVDC distribution network for RE integration.The large-signal stability criterion for a scaled-down droop-controlled multi-terminal MVDC distribution network with constant power load(CPL)using Brayton-Moser's mixed potential theory was derived and the impacts of the CPL power rating,PI control parameters,and damping coefficients verified through MATLAB/Simulink simulations.The results show that the large-signal stability of the MVDC system reduces with increase in CPL rating and source converter droop coefficients as well asincreases with increase in proportionate coefficients and optimized damping coefficients.Thus the droop-controlled MVDC distribution network with optimized PI control parameters and damping coefficients have enhanced dynamic response and large-signal stability margin.For future research work,collaborations of simulations and experimental validations on a similar or extended multi-terminal MVDC distribution network are recommended to provide more practical insights on control design and stability analysis.