A power distribution grid exhibits the characteristics of a weak grid owing to the existence of scattered high-power distributed power-generation devices. The grid impedance affects the robust stability of grid-connected inverters, leading to harmonic resonance, or even instability in the system. Therefore, a study of the stability of grid-connected inverters with high grid impedance based on impedance analysis is presented in this paper. The output impedance modeling of an LCL-type single-phase grid-connected inverter is derived, where the effects of the PLL loop and the digital control delays on the output impedance characteristics have been taken into account. To enhance the stability of grid-connected inverters with different grid impedance, a novel impedance-phased compensation control strategy is proposed by increasing the phase margin of the grid-connected inverters. Specifically, a detailed implementation and parameter design of the impedance-phased compensation control method is depicted. Finally, an impedance-phased dynamic control scheme combined with online grid impedance measurement is introduced and also verified by the experiment results.

An Interconnected Multi-microgrids (IMMGs) system takes advantage of various complementary power sources, and effectively coordinate the energy sharing/trading among the microgrids (MGs) and the main grid to improve the stability, reliability, and energy efficiency of the system. The core of this structure is to achieve the optimal distribution of energy sharing through proper strategies. However, the volatility and intermittent characteristics of renewable resources, time-varying loads in the MGs, their correlated power generations, the coupled energy among the MGs during energy trading, all bring about new challenges to achieving a stable operation and optimal scheduling in the power system. Many solutions have been proposed to solve these problems. In this paper, we provide an overview of the current energy management systems (EMS) in IMMGs, focusing on the IMMG structure, EMS objectives, timescales, and scheduling optimization structure. We then provide a review of distributed optimization algorithms in IMMGs. We conclude this survey with a discussion future directions.

As a special form of the Internet of Things, Smart Grid is an internet of both power and information, in which energy management is critical for making the best use of the power from renewable energy resources such as solar and wind, while efficient energy management is hinged upon precise forecasting of power generation from renewable energy resources. In this paper, we propose a novel least absolute shrinkage and selection operator (LASSO) and long short term memory (LSTM) integrated forecasting model for precise short-term prediction of solar intensity based on meteorological data. It is a fusion of a basic time series model, data clustering, a statistical model and machine learning. The proposed scheme first clusters data using k-means++. For each cluster, a distinctive forecasting model is then constructed by applying LSTM, which learns the non-linear relationships, and LASSO, which captures the linear relationship within the data. Simulation results with open-source datasets demonstrate the effectiveness and accuracy of the proposed model in short-term forecasting of solar intensity.

This paper presents sequence impedance modeling and analysis of modular multilevel converters (MMC) in a single-star configuration for Static VAR Compensation (STATCOM) application. Prior work on impedance modeling of MMC is limited to the double-star configuration for high-voltage dc (HVDC) transmission applications. Compared to that, single-star configuration uses only a single arm of switching modules in each phase, eliminating the second-harmonic circulating current and the need for the associated circulating current control. On the other hand, the lack of connection to a stiff dc bus makes it essential to include module capacitor voltage control in the model. A special form of the multi-harmonic linearization method is applied to take advantage of the simpler harmonic spectra and to reduce the complexity of the resulting models. The developed models are verified by simulation and experiment, respectively, and used to understand the similarities and differences in the impedance characteristics of the two configurations as well as the effects on stability. A representative wind power plant is presented as an example to demonstrate the application of STATCOM impedance models on the system stability analysis.

In certain regions and countries, the static synchronous compensator (STATCOM) is required in large-scale wind farms to provide reactive power support. Due to the limitations and complexity in distributed wind power systems, the STATCOM is considered as an effective and cheap alternative for impedance shaping to mitigate resonance. However, there is still a lack of a quantitative stability analysis for practical large-scale wind farms and an optimization approach for the STATCOM impedance to achieve so. Therefore, based on the sequence impedance matrix models of the Type-IV wind turbine generator (WTG) and STATCOM for the network modeling, this paper presents a sequence impedance networkbased stability analysis for practical wind farms to accurately reveal the stability of wind farms and quantitatively evaluate the damping provided by the STATCOM. A typical wind farm in Western China is exemplified to demonstrate the proposed stability analysis. Then, an intelligent parameter design-based optimization approach for the STATCOM impedance is proposed, which uses the heuristic intelligence algorithm to solve the STATCOM controller parameters to obtain the optimal stability margin of the wind farm under the constraints of desired performance. Finally, a comparison with the understanding-of-models-based optimization method confirms the efficacy of the proposed impedance optimization approach.

Sub and super-synchronous resonance problems caused by the increasing penetration of wind energy in the grid usually can be solved by reshaping the impedance of wind farms. Considering the feasibility and economics, the static synchronous compensator (STATCOM), which is already part of the wind farm in certain regions, becomes a preferred option for impedance shaping to improve the system stability. However, due to the difference between the application object and the application scenario, the control schemes for impedance shaping that widely used in the two-level voltage source converter are difficult to apply directly to the MMC-based STATCOM to solve the resonance problems in the practical wind farm. Therefore, in this article, a broadband impedance shaping control scheme for MMC-based STATCOM is proposed to generate an active parallel impedance, thus reshaping the impedance characteristics of the wind farm over the sub and super-synchronous frequencies for enhancing the system stability. Then, a passivity-based design method for this impedance is developed to ensure that the proposed control can provide sufficient damping at the desired frequency. With this broadband impedance shaping control scheme, the STATCOM can effectively suppress resonance in the wind farm under various operation conditions, which is confirmed by the hardware-in-the-loop experimental results.