Remaining stability and a fast dynamic response under weak grid conditions are vital but challenging to the phase locked loop (PLL)-based inverters due to the PLL's side effects. Moreover, the grid strength can vary significantly due to the high penetration of renewable energy generation (REG). It brings a new challenge for the PLL-based inverters, i.e., achieving robust stability and robust rapid response (RRR) with different short circuit ratios (SCRs). This purpose is hard to achieve by using the existing methods, which mainly focus on specific SCR conditions. Quantitative robust design is seldom considered, especially for achieving RRR. Based on a widely used PLL-based inverter, this paper presents a state-feedback compensator (SFC) that provides sufficient degrees of freedom (DOFs) for pole-placement. Furthermore, a time-domain forbidden region (TFR)-based parameter tuning method is proposed to utilize the DOFs provided by the SFC, by which quantitative robust stability and RRR design can be achieved with different SCR conditions. The effectiveness of the proposed method is verified through a simulation study and a 25kVA prototype experiment.

With the continuous growth of renewable energy capacity, power electronic inverters have been widely deployed in power grids, and their total capacity has surpassed that of conventional synchronous generators. To maintain the stability of the power system, some inverters are operated in the grid-forming mode (GFM) instead of the conventional grid-following mode (GFL) mainly through switching over the synchronization schemes. In the GFL mode, the synchronous angle is generated by the phase-locked loop (PLL) scheme, while in the GFM mode, it is achieved through the power synchronization control (PSC). Generally, the GFL mode exhibits better adaptability to strong grid, whereas the GFM mode is more stable to weak grid. However, the specific boundary between these two modes remains ambiguous and varies with control parameters, which poses challenges both to the selection of operation modes and to parameter design for specific grid conditions. Unifying these two operation modes is crucial to dig out of this dilemma. Against this background, the small-signal dynamic performance equivalence between the GFL and GFM modes is established just through adapting control parameters properly, without the need for switching between synchronous schemes. As a result, the characteristics of the inverters adapt to grid strength continuously.

Grid-forming (GFM) control enables doubly fed induction generators (DFIGs) to operate as voltage sources in islanded systems, but the stator voltage is highly sensitive to load transients, electromagnetic coupling, and parameter variations. This article proposes a disturbance-compensated single-loop variable-gain super-twisting control (DC-SLVGSTC) strategy for islanded GFM-DFIGs. The stator-voltage dynamics are first transformed into a single-loop matched-disturbance model, allowing the indirect rotor-side actuation, coupling effects, load variations, and model uncertainties to be handled in a unified compensation channel. Based on this model, a voltage-error-normalized variable-gain super-twisting controller is developed to improve transient recovery while suppressing rotor-voltage chattering. An adaptive disturbance observer (ADOB), which combines extended state observer (ESO) and generalized proportional-integral observer (GPIO) estimates through hysteresis-based fusion and smoothing, is further introduced for feedforward compensation. Lyapunov analysis establishes finite-time stability under bounded residual disturbance derivatives. Hardware experiments verify that the proposed method improves startup and load-step performance while maintaining robustness against parameter mismatch, DC-link voltage variation, and rotor-speed disturbance.

A doubly-fed induction generator (DFIG) based on grid-forming (GFM) control has the characteristics of a voltage source, providing grid stability and support capabilities, especially in weak grid conditions. During unbalanced fault ride-through (FRT), the DFIG is required to have the ability to support positive- and negative-sequence (PNS) voltage. First, a stator terminal voltage (STV)-based virtual impedance control in the PNS (PNVI) is proposed to provide stator voltage support capability. To reflect the unbalanced FRT capability and the voltage source characteristics of the GFM control, a stator internal electromotive force (SIE)-based PNVI is proposed. Leveraging this, a hybrid virtual impedance control with adaptive adjustment factors is proposed, combining operational domain and stability analysis, to achieve more flexible voltage support control. Finally, the effectiveness of the proposed method and theory is verified using a hardware-in-the-loop (HIL) platform

Wake steering is widely regarded as an effective strategy for mitigating wake losses and enhancing wind farm energy production through active yaw misalignment. However, most existing wake steering optimization methods implicitly assume ideal yaw execution without accounting for turbine yaw control logic and execution dynamics, leading to systematic performance deviations when deployed in real wind farms. This study develops a wake steering optimization framework that explicitly accounts for realistic turbine yaw behaviour. By integrating data-driven identification of yaw control characteristics with a probabilistic yaw response representation and a two-level optimization scheme, the proposed method captures uncertainty-induced, multi-modal, and directionally coupling effects in yaw execution. Gaussian-Hermite Sparse Grid Quadrature is employed to efficiently propagate these effects within the optimization process. Simulation results based on a commercial wind farm demonstrate that the proposed framework achieves higher overall energy production than conventional wake steering optimization methods. Overall, the proposed framework provides a systematic and customizable approach for wake steering optimization under realistic yaw execution, supporting more reliable long-term operational improvements in real-world wind farms.

The open-end winding (OEW) motor drive is a promising topology for electric vehicles due to its merits in voltage superposition and fault tolerance. While the dual three level (dual-3L) voltage source inverter (VSI) offers superior output quality compared to the dual two-level (dual-2L) VSI by generating five-level output voltage, it requires twice the number of switching devices, leading to increased cost and size. As a compromise, a hybrid 2L/3L VSI topology with a common dc bus has been introduced, achieving five-level output with 25% fewer switching devices than the dual-3L VSI. However, this hybrid topology inherently suffers from high switching loss due to doubled switching actions per cycle. To address this issue, this paper proposes a novel modulation strategy designed to achieve ZSC suppression and NPV control while minimizing switching actions. By clamping the 2L VSI and redistributing the zero sequence compensation voltage solely to the 3L VSI, the total number of switching actions per cycle is reduced to that of a single-VSI. Furthermore, NPV control is achieved without increasing switching actions by shifting the modulation voltages of only the most effective phase. Experimental results validate the effectiveness of the proposed strategy in simultaneously achieving ZSC suppression, NPV balance, and switching actions reduction.

In new energy vehicles, the 800 V high-voltage platform is increasingly adopted to enhance power level and charging speed. However, increasing bus voltage presents significant challenges to drive system, such as cost of the higher-voltage-rated switching devices, switching surge, electromagnetic noise, motor insulation, switching loss, and battery management. Dual-inverter-based open-end winding (OEW) drive is considered to be a highly promising drive configuration, due to its merits in output voltage, fault tolerance, system efficiency, and others. Compared with dual two-level voltage source inverter (dual-2L VSI) configuration, dual three-level (dual-3L) VSI configuration is more applicable to today's increasing maximum speed requirement of the motor, because it can generate more phase voltage levels. However, the increased number of switching devices results in increased cost and volume, which also limits its use in practical engineering. To overcome this dilemma, a novel hybrid 2L/3L VSI topology with a common dc bus is proposed in this article. This hybrid topology can also generate five-phase voltage levels, which are the same as dual-3L VSI topology, but it reduces the number of switching devices by 25%. To operate this new hybrid drive system, a carrier-based pulsewidth modulation (CBPWM) strategy is employed to simultaneously achieve zero-sequence current (ZSC) suppression and active neutral-point voltage control (ANPVC) while ensuring the effectiveness of the former remains entirely unaffected by the latter. Furthermore, its capability to balance the NPV under ZSC suppression condition is analyzed for the first time. Finally, the feasibility and the effectiveness of the hybrid drive topology and the control strategy are validated through simulation and experiment.

High penetration of the inverter-based renewable energy generation is diminishing the grid strength due to its limited overload capacity. Under certain operation conditions, particularly during short-circuit ground faults, substantial reactive power currents are required from generating units. Increasing the current rating of the power electronic devices can meet this demand; however, the cost is also increased due to the low utilization rate. To address this issue, this article proposed a flexible full-scale converter topology for a wind turbine (WT). Typically, these turbines have high generator ratings, leading to the construction of converters with more than one back-to-back converter operating in parallel. To accommodate the varying requirements of the power grid under both normal and fault conditions, the generator-side inverter units included in the whole converter are designed with flexible functionality. Specifically, they can be assigned, as conventionally done, to the generator side under normal grid condition; however, one or more of these units can be switched to operate in parallel with the grid-side inverters during grid fault conditions, thereby enhancing the full-scale converter's current capacity to support reactive current to the grid. Consequently, grid strength is increased without increasing the extra current capacity of the converter, simply by introducing flexibility to the generator-side inverters. This approach also enhances the hardware utilization rate. The proposed concept is validated through MATLAB simulations, and the feasibility of switching the generator-side inverter to the grid side is further confirmed through experiments made on a small-scale laboratory platform.

This paper explores the application of dissipativity-based iterative learning control (ILC) to discrete-time singular systems (SSs). Firstly, the P-type learning algorithm is adopted to demonstrate that the dissipative discrete-time SSs have the accurate trajectory tracking along the iteration axis. Then, a novel linear matrix inequality (LMI)-based sufficient condition is presented to ensure the dissipativity of such SSs, and a criterion for solving the LMI is established. Furthermore, the ILC via dissipativity analysis is investigated for discrete-time irregular SSs. In addition, the robust ILC and dissipativity analysis are addressed for discrete-time SSs with uncertain parameters. Finally, the effectiveness of the developed method is validated through two examples.

With the rapid development of renewable energy, power system characteristics have been significantly changed by including numerous power electronic inverters, which has led to a series of problems, such as wideband oscillations, inertia deficiency, weakened grid strength, etc. To solve these problems, the authors proposed a dual winding induction machine (DWIM)-based generating system. The DWIM is utilized instead of the conventional inverter as an interface for renewable energy integration. One set of the DWIM stator windings is connected to the grid directly, while the other is connected to an inverter. The DC side of the inverter is powered by renewable energy resources or energy storage units. With the help of the proposed system, the power system is expected to regain the characteristics of the synchronous generator-dominated system such as strong stability, rotational inertia, and large short-circuit currents. In order to make the proposed system applicable, this paper focuses on the control methods. By analyzing the double-dq mathematical model and the vector space decomposition (VSD) mathematical model of DWIM, as well as the relationship between them, two equivalent control methods are proposed. These control methods enable independent control of the active and the reactive power exchanged with the grid. Some experiment results verified the effectiveness of these control methods and then the feasibility of the DWIM-based generating system.

In an open-end winding (OEW) drive fed by dual three-level voltage source inverters (dual-3L VSIs) with a common dc bus, the zero-sequence current (ZSC), the common-mode voltage (CMV) as well as the neutral-point voltage (NPV) are three critical factors needing to be considered simultaneously. ZSC can cause torque ripples and additional losses. CMV can produce shaft current, contributing to bearing failure. NPV adversely affects the quality of the output waveform and voltage safety of the power electronic tubes. However, how to achieve these three objectives simultaneously is still an open question. To solve it, a multiobjective pulsewidth modulation (MOPWM) strategy is proposed. In this strategy, the vectors with zero CMV amplitude are selected at first to eliminate CMV variation. Following that, some redundant vector pairs consisting of two of these vectors are built according to their spatial positions and their effects on the ZSC and the NPV, which provides the degree to control ZSC or NPV independently. As a result, the conflicts among these objectives are resolved. Moreover, a compensation scheme is proposed to address the negative effect of the dead time on the CMV variation. Finally, the feasibility and the effectiveness of the MOPWM strategy are validated through experiments.

The inverter's impedance is influenced by its control algorithm and control parameters simultaneously. It is important to obtain a proper impedance shape for an inverter to achieve the design purpose, e.g., sufficient stability margin and strong filtering ability. Different control schemes and corresponding parameter tuning have been proposed to realize these design purposes against different adverse grid conditions. However, these control schemes may have to be combined in practice because different grid conditions may be coupled in the real world. Under this situation, all the control parameters have to be returned to coordinate the control schemes. Impedance-based analysis provides a general vision to test the stability margin and harmonic suppression ability of the grid-inverter system. However, the existing impedance-based analyses cannot realize the quantitative parameter tuning for further improvement. In this article, three common design purposes are formulated as impedance shaping problems, including stability margin improvement, filtering ability enhancement, and phase-angle shaping. Then, a general parameter tuning method based on optimization theory is proposed for impedance shaping, which can provide a quantitative tuning result directly after an iteration.

Rotor position information of interior permanent magnet synchronous motors (IPMSMs) is crucial when it comes to control performance. Position sensor increases the cost of the control system as well as its hardware complexity. Thus, position estimation has attracted a great deal of attention. High frequency injection-based estimation methods are often used at standstill and low speeds. Estimation methods that work by injecting the pulsating high frequency square wave voltage signal into the stationary reference frame are attractive due to their merits of simplicity and stability. However, the estimation accuracy is limited in practice, due to the negative effects of the voltage errors caused by inverter nonlinearity as well as those of the cross saturation of the machine itself. To enhance accuracy, a method that suppresses these effects is proposed in this paper. With the proposed method, the negative effects of inverter nonlinearity and cross saturation are overcome. Consequently, the harmonics in the estimation are removed. Besides, the method used for compensating the position deviation due to the cross-saturation effects is a general method. Thus, calibration processes dedicated to a specific machine are not required in this method. Experiments on an 18-kW test bench verify the effectiveness of the proposed method.

With the rapid development of renewable energy, the power system is characterized by both a high percentage of renewable energy sources and a high percentage of power electronic equipment, which has caused problems such as wideband oscillations, inertia deficiency, weakened grid strength and so on. In order to solve these problems, a dual winding induction machine (DWIM) based grid-tied inverting system is proposed in this paper. The DWIM is used instead of the conventional inverter, interfacing renewable energy sources or energy storage units to the grid. In this system, one set of the stator windings of the DWIM is connected to the grid directly, while the other is controlled with an inverter. Energy transfer is accomplished through electromagnetic induction effects. Although there is no extra mechanical load on the shaft, the mechanical inertia is available to actively damp the grid oscillations and frequency deviations. A control strategy, achieving independent control of the active and reactive power exchanged with the grid, is also proposed. The effectiveness of the inverting system is verified through simulations and experiments. Comparing with the synchronous condenser (SC) shows that this new system possesses all of the merits of the SC in addition to the inverting function. Therefore, it inherits the advantages of both SC and grid-tied inverters.

Equipped with energy storage has become the trend in the development of wind power, at the same time, with more new energy sources being integrated into the power grid, the short-circuit ratio (SCR) of the grid-connected point of the wind storage power station is decreasing, which has an impact on the power exchange between the wind storage power station and the power grid. In order to study the influence of the wind storage power station connected to the weak grid on the active power transmission capacity of the wind storage power system, a line impedance model was established based on the doubly-fed induction generator (DFIG) and battery energy storage systems(BESS), and the influence of different short-circuit ratios on the voltage and power of the wind storage system was analyzed and studied. Finally, the effectiveness of the proposed strategy is verified through simulation.

Offshore wind farm is the main trend of wind power development in the future. Diode rectifier unit (DRU) shows great potential in offshore wind farm because of its economic and reliability advantages. In this paper, firstly, the mathematical model of system based on DRU is established and the basic principle of grid-forming control is analyzed. Then the stability under standard operating conditions is proved. Secondly, a startup strategy based on improved active power loop is proposed in order to avoid the AC voltage fluctuation in the starting process. Then, the synchronization strategy of multiple wind turbines (WTs) and current decouple strategy are proposed. Finally, the simulation results show that the strategies have good effects during the startup process.

The current research indicates that the implementation of grid-forming (GFM) control in doubly-fed induction generator (DFIG) wind turbines (WTs) has the potential to enhance the stability of electrical systems in weak grids. However, there is a scarcity of studies on the electromechanical stability of DFIG under GFM control. The investigation begins with formulating a small-signal model for the electromechanical system of a GFM DFIG connected to a weak grid. The theoretical analysis reveals that, as the grid strength diminishes, the system becomes susceptible to electromechanical oscillations near the natural frequency (NF) of the drivetrain. Subsequently, the paper scrutinizes the drivetrain damping branch within the control system, establishes an average energy dissipation model during the drivetrain NF oscillation period, and examines the mechanisms causing drivetrain oscillations (DOs). Additionally, optimization recommendations for oscillations are presented from the perspective of GFM control parameters. Nevertheless, achieving a balance between drivetrain stability and electrical stability remains challenging, especially in extremely weak grids. Therefore, the paper proposes a hybrid d-q axis voltage reference drivetrain damping control (HVRDC) tailored for GFM control. Theoretical analysis demonstrates that, even in the presence of an extremely weak grid, the proposed strategy ensures the stability of the GFM DFIG-based WT. Finally, the accuracy of the theoretical analysis is substantiated through a control-hardware-in-loop (CHIL) experiment.

With the rapid increase in the single machine capacity of new energy distributed generation systems, in order to ensure transmission efficiency, the transmission voltage also needs to be correspondingly increased. In medium voltage highpower topology structures, cascaded H-bridges are widely used due to their unique advantages. Meanwhile, as the proportion of distributed generation increases, the power system gradually presents a “dual high” characteristic. In order to improve the stability of the “dual high” system, it is required that the distributed generation system has a certain frequency support characteristic. Therefore, this paper proposes a cascaded H-bridge structure grid-connected inverter based on virtual synchronous machine control to meet the development requirements of medium voltage high-power grid-connected systems. Moreover, the optimization of virtual synchronous machine control during transient processes, namely the adaptive control of J and D parameters, is carried out based on this control method. A Simulink simulation model is built based on the cascaded H-bridge structure to verify the effectiveness of the analysis method.

With the increasing growth of clean energy, the inertia of the power grid continues to decrease, and energy storage devices continue to play a role. In order to solve this problem, this paper proposes a virtual impedance based energy storage VSG control method. By adding virtual impedance in the control link, it resists the power coupling phenomenon caused by grid impedance in weak grids, effectively improving the grid stability of energy storage devices. Finally, a simulation model was established in MATLAB, and the Bode plot was used to demonstrate the inductive characteristics of the energy storage model output with the addition of virtual impedance.

Inverters in a renewable-energy-generation based power station (PS) may be produced by different manufacturers whose control schemes cannot be exposed to each other. Hence, the system-level stability prediction and stability-margin-improvement-oriented parameter tuning should be completed in a black-box manner. Based on the Nyquist theorem and the Routh-Hurwitz criterion, a gradient-based black-box (GBBB) modeling method is proposed for the inverters in the PS. The GBBB model is a black-box encrypted function with tunable control parameters as inputs. Its outputs contain impedance values, open-loop-stability factors, and parameter-participation-factors of the impedance for Nyquist-based stability judgment and margin-improvement-oriented parameter tuning of the PS. Based on the GBBB models of the inverters, the PS's stability margin is described by black-box cost functions. Then, a gradient-descent based parameter tuning method is proposed where the gradients are calculated using the outputs of the GBBB models according to the chain rule. All the inverters' parameters can be optimized in theory iteratively to improve the PS's stability margin. Under the given workflow, only the GBBB models and the PS topology are needed, which means the control details of the inverters are unexposed, i.e., the intellectual property would not be infringed.