A wind power short-term forecasting method based on discrete wavelet transform and long short-term memory networks (DWT_LSTM) is proposed. The LSTM network is designed to effectively exhibit the dynamic behavior of the wind power time series. The discrete wavelet transform is introduced to decompose the non-stationary wind power time series into several components which have more stationarity and are easier to predict. Each component is dug by an independent LSTM. The forecasting results of the wind power are obtained by synthesizing the prediction values of all components. The prediction accuracy has been improved by the proposed method, which is validated by the MAE (mean absolute error), MAPE (mean absolute percentage error), and RMSE (root mean square error) of experimental results of three wind farms as the benchmarks. Wind power forecasting based on the proposed method provides an alternative way to improve the security and stability of the electric power network with the high penetration of wind power.

Constant power loads may yield instability due to the well-known negative impedance characteristic. This paper analyzes the factors that cause instability of a dc microgrid with multiple dc-dc converters. Two stabilization methods are presented for two operation modes: 1) constant voltage source mode; and 2) droop mode, and sufficient conditions for the stability of the dc microgrid are obtained by identifying the eigenvalues of the Jacobian matrix. The key is to transform the eigenvalue problem to a quadratic eigenvalue problem. When applying the methods in practical engineering, the salient feature is that the stability parameter domains can be estimated by the available constraints, such as the values of capacities, inductances, maximum load power, and distances of the cables. Compared with some classical methods, the proposed methods have wider stability region. The simulation results based on MATLAB/simulink platform verify the feasibility of the methods.

Constant power loads (CPLs) are often the cause of instability and no equilibrium of DC microgrids. In this study, we analyze the existence and stability of equilibrium of DC microgirds with CPLs and the sufficient conditions for them are provided. To derive the existence of system equilibrium, we transform the problem of quadratic equation solvability into the existence of a fixed point for an increasing fractional mapping. Then, the sufficient condition based on the Tarski fixed-point theorem is derived. It is less conservative compared with the existing results. Moreover, we adopt the small-signal model to predict the system qualitative behavior around equilibrium. The analytic conditions of robust stability are determined by analyzing quadratic eigenvalue. Overall, the obtained conditions provide the references for building reliable DC microgrids. The simulation results verify the correctness of the proposed conditions.

Virtual synchronous generator (VSG) control introduces active power oscillation especially under the condition of multi-VSG. This article proposes a decentralized mutual damping control method that suppresses power oscillation in the multi-VSG grid. The idea is that as the local frequency is greater than the frequency of the VSG other than itself, the input energy of the local VSG is reduced, and thus dω_i/dt increases. On the contrary, when the local frequency is less than others, the input energy is increased, and thus dω_i/dt decreases. In this way, all frequencies get close to each other in the dynamic process with the method. The idea is verified by the stability analysis. Participation factor analysis and sensitivity analysis are presented to guide parameter design. Finally, the effectiveness of the decentralized mutual damping method is verified through the simulation results and experimental results.

When multiple virtual synchronous generators (VSGs) operate in parallel, severe power oscillation may be suffered. To deal with this issue, this article offers an adaptive virtual impedance method applied to a multi-VSG grid. The core idea is to change the output active power by adaptive virtual impedance. When the angular frequency of the local controller is greater than the angular frequency of the neighbors’ controllers, the output active power of local VSG falls, and thus dω_i/dt rises. Then, as the angular frequency is lower than neighbors’, the output active power rises, and dω_i/dt falls. As a result of this, the method facilitates the consistency of response curves of all angular frequencies. Besides, the stability of the offered algorithm is demonstrated. A final proof of the efficacy of the proposed control method is shown by simulation and hardware-in-loop results.