As the penetration of PV generation increases, there is a growing operational demand on PV systems to participate in microgrid frequency regulation. It is expected that future distribution systems will consist of multiple microgrid clusters. However, interconnecting PV microgrids may lead to system interactions and instability. To date, no research work has been done to analyze the dynamic behavior and enhance the stability of microgrid clusters considering the dynamics of the PV primary sources and dc links. To fill this gap, this paper presents comprehensive modeling, analysis, and stabilization of PV-based multiple microgrid clusters. A detailed small-signal model for PV-based microgrid clusters considering local adaptive dynamic droop control mechanism of the voltage-source PV system is developed. The complete dynamic model is then used to access and compare the dynamic characteristics of the single microgrid and interconnected microgrids. In order to enhance system stability of the PV microgrid clusters, a tie-line flow and stabilization strategy is proposed to suppress the introduced interarea and local oscillations. Robustly selecting of the key control parameters is transformed to a multiobjective optimization problem which is solved by genetic algorithm (GA). The proposed damping controller can effectively damp the power oscillations and provide robust control performance under variable operating conditions. Theoretical analysis, simulation results under various scenarios are presented to verify the effectiveness of the proposed scheme.

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Low-voltage ride-through (LVRT) requirements demand inverter-interfaced renewable energy power generation systems to remain connected in the presence of grid faults, by injecting required reactive current for voltage support. In this paper, a two-stage grid-connected photovoltaic inverter consists of a boost converter and a three-level T-type inverter is investigated. A stable decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL) is adopted appropriately for three-phase grid synchronization under faulty grids or abnormal conditions. Moreover, the DDSRF could extract and separate both positive- (PS) and negative-sequence (NS) components of grid voltage and current. During normal operation, the cascaded control structure of T-type inverter is composed of an outer proportional-integral-derivative (PID) dc-link voltage control loop and four inner multivariable-PI current control loops. Once grid voltage sags are detected by the separated d-axis PSvoltage component, the current references are calculated according to the sag depth so as to stay in maximum power point tracking (MPPT) mode or switch to Non-MPPT mode, meeting the latest LVRT requirements of China. The performance of the LVRT capability is evaluated by simulation and experimental results.

The high penetration level of inverter-based distributed generation (DG) power plants is challenging the low-voltage ride-through requirements, especially under unbalanced voltage sags. Recently, a flexible injection of both positive-(PS) and negative-sequence (NS) reactive currents has been suggested for the next generation of grid codes. This can enhance the ancillary services for voltage support at the point of common coupling (PCC). In light of this, considering distant grid faults that occur in a mainly inductive grid, this paper proposes a complete voltage support control scheme for the interface inverters of medium or high-rated DG power plants. The first contribution is the development of a reactive current reference generator combining PS and NS, with a feature to increase the PS voltage and simultaneously decrease the NS voltage, to mitigate voltage imbalance. The second contribution is the design of a voltage support control loop with two flexible PCC voltage set points, which can ensure continuous operation within the limits required in grid codes. In addition, a current saturation strategy is also considered for deep voltage sags to avoid overcurrent protection. Finally, simulation and experimental results are presented to validate the effectiveness of the proposed control scheme.

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