This paper presents a robust hierarchical control system of distributed generation converters for robust microgrid operation and seamless transfer between grid-connected and isolated modes. The proposed control scheme employs 1) an internal model voltage controller with a variable structure control element to provide means for mitigating fast and dynamic voltage disturbances, such as dynamic voltage disturbance during mode transition and normal operation, and 2) a droop-based power-sharing controller with an active damping feature to mitigate large power-angle swings and oscillations associated with large-signal disturbances (e.g., mode transition and heavy loading conditions). The proposed voltage and power sharing controllers provide high disturbance rejection performance against voltage disturbances and power angle swings, respectively. Accordingly, robust microgrid operation with seamless transfer in the transition mode has been obtained. Comparative experimental results are presented to validate the effectiveness of the proposed control scheme.

DC microgrids are gaining high momentum under the smart grid environment. DC microgrid stability can be an issue under high penetration of tightly regulated power converters used to interface distributed resources and loads. This paper addresses dc microgrid stability under high penetration of tightly regulated power electronic converters; and proposes three simple and computationally efficient active damping solutions that can be implemented to stabilize a controlled voltage-source converter (VSC) interfacing a dc- microgrid to an ac system. The proposed active damping methods depend on reshaping the VSC impedance by injecting internal-model-based active damping signal at the outer, intermediate and inner control loops of the voltage-oriented VSC interface. Small signal analysis is conducted to assess the system stability under different compensation schemes. Moreover, the reshaped source impedance of the VSC interface and the modified voltage-tracking dynamics are derived under different compensation schemes. Sensitivity and robustness analyses are provided to assess the dynamic coupling among active damping and voltage tracking controllers. Evaluation results, based on a detailed model of a dc microgrid with multiple tightly regulated converter-interfaced loads, are provided to validate the developed models and demonstrate the effectiveness and robustness of proposed techniques.

Hybrid ac/dc power networks are recently emerged in distribution generation (DG) systems with widespread acceptance under the smart grids environment. However, system-level dynamic interactions might be yielded due to the active control nature and tight regulation of power converters to meet load/generation requirements. This paper presents an assessment and mitigation strategies of such interactions in hybrid networks. A typical and comprehensive hybrid network composed of a DG power park, dc microgrid, islanded ac microgrid interfaced by voltage-source converter (VSC), and a grid-connected VSC is considered. Mathematical modeling and analysis of the input/output admittances of these entities are provided to evaluate the overall system stability based on the Nyquist admittance ratio criterion. It can be shown that the tight regulation of VSCs introduces incremental negative admittances reflected to the common dc link which significantly degrades the system stability. Therefore, active compensators are proposed to actively reshape the input dc-side admittance of the VSCs so that the Nyquist criterion is satisfied. Time- domain -large signal model of a typical hybrid network is implemented to validate the analytical results.

Reference-current-based active compensation techniques are widely used to actively damp the negative admittance effect associated with constant power loads in power electronic systems. These methods, however, yield strong coupling among the active compensator, speed, and current control dynamics when applied to motor drives. To overcome these difficulties, a new reference-voltage-based active compensator (RVC) is proposed to mitigate the negative admittance effect in permanent-magnet synchronous motor drives with tight-speed regulation. The proposed RVC suppresses the interaction dynamics between the compensator and motor drive dynamics in the low- and medium-frequency regions and maintains stability and effective damping performance along the whole loading trajectory of the motor drive. In addition, the proposed compensator has a very simple structure, and it can easily be augmented to existing motor drives with minimal interactions with existing drive control strategy and parameters. A theoretical comparative analysis and test results are provided to demonstrate the validity and effectiveness of the proposed technique.

This paper introduces a new topology, yet simple and efficient, for a grid-connected wind-solar cogeneration system. A permanent magnet synchronous generator-based full-scale wind turbine (FSWT) is interconnected to the utility-grid via back-to-back (BtB) voltage-source converters (VSCs). The dc-link capacitor has been utilized to directly intertie a photovoltaic (PV) solar generator. No dc/dc conversion stages are required, and hence the hybrid system is simple and efficient. Moreover, the proposed topology features an independent maximum power point tracking for both the wind and the solar generators to maximize the harvesting of the renewable energy. The regulation of the VSCs is achieved via the vector control in the rotating reference frame. The detailed small-signal models for the system components are developed to characterize the overall stability. The influence of the utility-grid faults on the performance of the proposed system is also investigated. Nonlinear time-domain simulation results under different operating conditions are presented to validate the effectiveness of the proposed topology.

This paper addresses ac microgrid stability under high penetration of converter-interfaced loads (CILs). According to the Nyquist admittance ratio criterion, destabilizing interaction dynamics are yielded among the interfacing voltage-source inverter (VSI) of the ac microgrid and CILs. Active stabilization techniques are proposed to maintain the overall system stability from the VSI side using admittance-based small signal analysis. Sensitivity and robustness analyses are presented to assess the dynamic coupling among active damping and voltage tracking controllers of the interfacing VSI. A complete Matlab/Simulink model of an ac microgrid with multiple CILs is used to demonstrate the effectiveness of the proposed active compensation techniques.