This paper focuses on the stability analysis and damping performance improvement of dc microgrids. The small-signal model of a dc microgrid has been derived. Eigenvalue analysis results reveal the relationship between the system stability and different factors of dc microgrids, including types of dc load, the droop coefficient, line parameters, etc. It shows that the poorly damped LC circuits in dc microgrids reduce the system damping and bring in high frequency oscillations. To improve the damping performance, a frequency-dependent virtual impedance approach is proposed, which can effectively shape the high frequency impedance of the dc bus voltage control units, improve the system stability, and mitigate the oscillations. Detailed design guidelines on virtual impedance to achieve good stability and transient performance are also provided. Simulation and experimental results are obtained to confirm the validity of the proposed approach.

Multiple distributed dc micorgrids (MGs) in a close vicinity can be connected to each other to form a dc MG cluster (especially with isolated bi-directional dc-dc converters (IBDCs) as the active interconnected devices). A novel coordinated power control framework for such a cluster has been proposed, in which the dc MGs adopt the standard droop control, and a unified control is proposed for the IBDCs. In normal operation, autonomous flexible interlinking power control among the dc MGs can be achieved through the unified control of the IBDCs, making the demand of the power terminals to be proportionally shared among the slack terminals in the cluster. Furthermore, with the unified control, the interlinked IBDCs can transfer to dc MG support control mode automatically without control scheme switching and operation mode detection under emergency situations. In addition, when there is a dc bus fault in one of the subsystems, the interconnected IBDC can lockout its control signals and isolate the dc fault, to ensure stable operation of the rest of the dc MGs within the cluster. Finally, PSCAD/EMTDC based simulation verifications and experimental results obtained from a hardware prototype have been provided.

The high-precision parameters in distribution networks are difficult to obtain, which brings difficulties to the model-based methods and analysis. With the widespread deployment of high-precision measurement units, data-driven methods have greater advantages in practice. In addition, with massive integration of distributed renewable generation and fast electric vehicle chargers, the fluctuations of net load increase significantly. The data-driven power flow calculation method based on the linear model becomes difficult to obtain accurate results for the low nonlinear adaptability. To improve the data-driven power flow calculation accuracy under high penetration of renewable distribution generation, this paper proposed an approach with high adaptability to the nonlinearity of power flow. Based on the thought of Koopman operator theory, the nonlinear relationship in power flow calculation is converted into a linear mapping in a higher dimension state space, which can significantly improve the calculation accuracy. Case studies on different IEEE cases have demonstrated that the proposed method can realize higher accuracy in power flow calculation with large-scale power fluctuations, compared to the existing data-driven method, in both mesh and radial networks. Finally, measurement data of a practical 10kV distribution network has been further used to verify the effectiveness of the proposed method in practical applications.

In this paper, a hierarchical control framework is proposed for multi-terminal dc (MTDC) grids with large-scale renewable power generations. In the primary control, dc voltage droop control is adopted for the multiple voltage source convert- ers (VSCs) connected to ac main grids. A novel optimal control strategy with enhanced flexibility and compatibility has been developed in the secondary control, which can simultaneously address issues including dc grid loss minimization, power sharing accuracy among droop based VSCs, and influence of possible power disturbances during the optimization intervals on dc voltages security. After thorough theoretical analysis and study on an tested five-terminal dc grid, a co-simulation platform based on PSCAD/EMTDC and Matlab has been setup to verify the proposed hierarchical control, with the dynamics of main circuits of MTDC dc grid and primary controllers simulated in PSCAD, and the novel secondary optimal control strategy implemented in Matlab.

This paper proposes a novel variable-inertia emulation control (VIEC) scheme that enables voltage-source-converter based high-voltage DC (VSC-HVDC) transmission systems to flexibly support AC grid frequency stability like synchronous generators. The VIEC scheme allows us to extract the energy from the augmented DC capacitance for inertia emulation by controlling the DC voltage without affecting the stability of the AC system connected on the remote side. In particular, with the proposed VIEC scheme, the inertia time constant can be flexibly emulated and its value can be automatically adjusted according to the rate of change of grid frequency as well as grid frequency deviations. This leads to more effective inertial support across different timescales. Modal analysis is carried out to investigate the impacts of inertia and DC capacitance, and to obtain the optimal control parameters. The effectiveness and advantages of the proposed VIEC scheme are demonstrated on an IEEE benchmark system in the presence of faults and load changes.