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.

This paper proposes a continuous-step-based model predictive control (MPC) method for permanent magnet synchronous motor (PMSM) drives that exhibits the satisfying steady-state performance and the controllable switching frequency. Different from finite-control-set MPC (FCS-MPC) and space-vector-modulation-based MPC (SVM-MPC) that apply either a single voltage vector or a complete voltage vector sequence of SVM, the proposed method determines an optimal voltage vector sequence over a longer and variable prediction horizon based on SVM, and then applies the voltage vectors falling within the first time step. First, the prediction horizon is adjusted online according to the reference switching frequency and the actual switching frequency. Second, the candidate voltage vector sequences during the prediction horizon are obtained by SVM, which not only include the traditional sequences but also some other sequences that are attractive in reducing current harmonic distortion. Third, taking nonzero harmonic current at sampling instants into account, the harmonic current rms values for different sequences are obtained and the optimal solution is selected. Finally, the voltage vectors falling within the first time step are applied. To validate the effectiveness of the proposed method, experiments are conducted on a 2.5 kW PMSM drive. Compared with SVM-MPC and FCS-MPC, the current total harmonic distortion of the proposed method is reduced by up to 36% and 64%, respectively.

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

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.

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.

A novel closed-loop torque ripple mitigation scheme is presented for master-slave winding motors. This strategy addresses the challenge of severe torque ripple in permanent magnet synchronous motors (PMSMs) that operate with a low carrier ratio to reduce inverter switching losses, and it overcomes the limitation of traditional open-loop control which fails when motor parameters are mismatched. The proposed method utilizes the amplitudes of two orthogonal components from harmonic IQ decomposition as feedback signals, effectively eliminating the influence of frequency. This approach actively suppresses torque ripple by regulating speed harmonics, requires no precise motor parameters, and demonstrates robust performance in maintaining effective ripple suppression even under conditions of parameter detuning. The practicality of this strategy is validated through simulation. The results show a significant reduction in total torque ripple compared to conventional methods.

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.

The development of electric vehicles and the need for scientific research and testing pose greater challenges to the flexibility, cost and speed of motor control testing. The development of low-cost, short-cycle motor control testing equipment has become increasingly important. In this paper, a permanent magnet synchronous motor emulator is proposed. By emulating the port characteristics of the target permanent magnet synchronous motor, the power electronic performance test of the motor control unit is realized without real motor and complex electromechanical load system. The emulator has the advantages of adjustable parameters and load, simple structure, small size and low cost, which can significantly save development time and cost. The simulation experiment is also carried out in MATLAB.

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.

Operation at low sampling-to-fundamental frequency ratios is challenging, but inevitable for high-speed electric drives due to the limited switching frequency. Synchronous modulation is commonly used to replace the asynchronous one to reduce harmonic distortion at these operation states. However, synchronous modulation faces the severe problems of poor dynamic response and intensive computation. To avoid falling into this dilemma, this article studies the improvement of asynchronous space vector pulsewidth modulation (SVPWM) at low sampling-to-fundamental frequency ratios, instead of following the traditional way of applying synchronous modulation. The conventional asynchronous SVPWM schemes are analyzed from the point of view of their stator flux trajectories, by which the reason for their poor harmonic characteristics at low sampling-to-fundamental frequency ratios is revealed. Improved asynchronous SVPWM schemes are proposed accordingly. The comparison results show that proposed modulation schemes have much less harmonic distortion than conventional schemes. Besides, the high dynamic response ability and simple implementation are preserved for the improved schemes. The effectiveness of the proposed modulation schemes is experimentally verified on an 18-kW interior permanent magnet synchronous machine (IPMSM) drive test rig.

A grid-connected inverter system based on a dual-winding induction Generators (DWIG) addresses issues such as broadband oscillations, lack of inertia, and weakened grid strength. To enhance the current performance of the DWIG under both periodic and aperiodic disturbances, an optimized active disturbance rejection control (ADRC) strategy is proposed. First, the model of DWIG and limitations of traditional linear ADRC are analyzed, revealing that the limited bandwidth of the extended state observer (ESO) leads to compromised disturbance rejection capability. To improve disturbance suppression, the ESO structure is optimized, resulting in a novel composite ESO (CESO). The CESO enhances disturbance estimation content, thereby improving estimation accuracy. Additionally, by embedding a quasi-resonant controller, the proposed CESO achieves precise estimation of periodic disturbances. Compared to existing ADRC techniques, this method significantly improves both dynamic response and steady-state accuracy. Simulations validate the effectiveness of the proposed scheme.

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.

During the recovery transient period after a zero-voltage fault in the power grid, grid-connected inverters need to quickly resynchronize to the power grid to avoid current surges or instability. The Synchronous Reference Frame Phase-Locked Loop (SRF-PLL) takes some time to re-lock phases when the power grid is restored. Due to phase deviation during re-phase-locking, grid-connected inverters are prone to cause overcurrent in the grid-connected current and pose an overcurrent risk. Therefore, this paper proposes a zero-voltage traversal phase-locked optimization scheme based on Extended State Observer (ESQ) with repetitive predictive control structure. The grid voltage information is observed by constructing a Proportional- Integral-Resonant( PIR)-ESO observer, and the phase of the grid voltage is obtained by using the arctangent function, effectively improving the rapidity and accuracy of phase locking. At the same time, Repetitive Predictive(RP) control is introduced to compensate for the inherent lag problem of the ESO. Simulation shows that this scheme can significantly reduce the phase difference between the power grid and the phase-locked loop during phase-locking, suppress grid-connected current overplay, and effectively improve the stability and reliability of the inverter during the period of grid voltage recovery

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.