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In photovoltaic systems, common-mode voltage (CMV) generates leakage current, which shortens the insulation lifespan and causes safety problems. As for neutral-point-clamped (NPC) inverters, zero CMV PWM (ZCMV PWM) can be implemented to eliminate leakage current, but unable to balance neutral-point (NP) voltage in dc side. This article presents a novel modulation technique for NPC PV inverter. The strategy contains ZCMV PWM and another two modulation modes which keeps CMV constant in one switching period, so as to eliminate leakage current. In order to balance NP voltage, NP current is adjusted by selection of optimal modulation mode and the leakage current is still well eliminated during the process of balancing NP voltage. Compared with conventional ZCMV PWM, the proposed scheme can remove both leakage current and NP voltage deviation. The output performance and limitations of the proposed method are also introduced in the paper. Experiments are provided to verify the validity of the proposed method. [ABSTRACT FROM AUTHOR]

The mechanical parameters, including the Coulomb and viscous friction coefficients together with the moment of inertia, are of great interest for the performance optimization and condition evaluation of surface-mounted permanent magnet synchronous motor (SPMSM) drives. This article contributes a speed-controller-independent mechanical parameter identification scheme using open-loop speed response due to signal injection, which has not been reported in existing works. It is designed based on the premise that the viscous friction torque is disproportional to the rotor speed, which is a new progress toward estimating the mechanical parameters with high precision. Moreover, as a practical full-participation-type solution, this scheme involves a new electrical parameter estimation technique based on differential algebra to assist in identifying the mechanical parameters. This auxiliary technique attains the current-controller-free estimation of the electrical parameters with the inverter nonlinearity eliminated, thereby addressing the current controller self-commissioning and serving to acquire the accurate electromagnetic torque. The experimental evaluations on two prototype SPMSM drive systems validate the feasibility of this suggested scheme. [ABSTRACT FROM AUTHOR]

A direct speed control strategy based on finite control set model predictive control (FCS–MPC) with a voltage smoother is presented herein. In the proposed concept, a finite set of smoothed voltage vectors characterized by an adjustable amplitude and a movable origin is introduced as voltage candidates in the FCS–MPC scheme. The controller predicts the future current and speed and outputs the optimal smoothed voltage using pulse-width modulation. Owing to this control scheme, an abrupt change in the output voltage, which causes a large current ripple, is avoided without additional computational costs. Simulation and experimental results obtained using a permanent magnet synchronous motor fed by a two-level three-phase inverter show that the proposed method effectively reduces the current ripple while achieving a high dynamic drive compared with the conventional FCS–MPC scheme. [ABSTRACT FROM AUTHOR]

In islanded ac microgrids, the mismatched impedances of the interfacing feeders between the inverters and the load bus cause poor reactive power sharing when the conventional frequency and voltage droop control technique is employed. Such operation endangers the whole microgrid reliability as it may lead to overloading certain inverters and, consequently, triggering protection relays and causing cascaded failure. Thus, this article proposes an accurate reactive power sharing strategy that considers the mismatched feeder impedances in islanded ac microgrids. It is based on the optimal tuning of the virtual complex impedance for each inverter. The proposed strategy has several advantages. First, it has a physical meaning as it establishes an explicit relationship between the mismatched values of the actual resistive–inductive feeders and the assigned values for the proposed virtual complex impedance to each inverter. Hence, further degradations in the microgrid voltages are prevented. Second, there is no need for prior knowledge about the actual feeder impedances in the design stage as they are estimated online from the available measurements. Finally, the proposed control is reliable and fault-tolerant as it copes with unexpected failures such as failure of (or sudden switching off) some inverters and communication disruptions/delays. It ensures accurate power sharing even under the primary controllers after losing communication links with the secondary controller. The simulation and experimental verification results are presented to validate the performance of the proposed technique. [ABSTRACT FROM AUTHOR]

Multilevel inverters (MLIs) featuring reactive power ability and common ground (CG) have become much popular in transformerless grid-tied photovoltaic applications. A switched-capacitor (SC) based five-level transformerless inverter configuration is thus proposed in this article, which requires only six switches, one diode, three capacitors, and one input dc voltage supply. Notably, only four power switches are operated in high frequency, thus, the switching losses are reduced. Moreover, the presented topology can effectively tackle the leakage current problem using a CG architecture. A multicarrier phase disposition pulsewidth modulation strategy is employed to achieve reactive power regulation and SC voltage self-balancing. To achieve the neutral point voltage balance of the presented five-level inverter, a simple closed-loop voltage-balancing control method is proposed. The operation principles with modulation strategy and voltage self-balancing of SC are discussed in depth. Comparisons between the proposed and the state-of-the-art MLIs are presented in detail. Finally, experimental tests on a single-phase 1.1-kW prototype validate the appropriate performance of the proposed inverter. [ABSTRACT FROM AUTHOR]

This article presents a nonlinear passivity-based control for a grid-connected cascaded H-bridge (CHB) photovoltaic (PV) system. The proposed control system is based on a port-controlled Hamiltonian model extracted from the synchronous reference frame system. Grid current dynamics is included in this model. Compared to existing solutions, the proposed control method has a robust performance and enhances the stability of the CHB-based PV system in the case of disturbances such as unequal irradiance over distinct PV arrays or a sudden sag or swell in the grid voltage. This controller also provides the independent control of each dc-link voltage. Therefore, the individual maximum power point tracking algorithms can be realized in each PV array, and the harvested energy can be maximized. Finally, simulation results and experimental investigations are conducted to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

The switched-capacitor of boost seven-level active neutral-point-clamped inverter has the large voltage ripple due to its inherent long discharging interval. Aiming at reducing the switched-capacitor voltage ripple and subsequently the capacitance, this article proposes a flexible third harmonic voltage modulation method. In specific, the long discharging interval can be divided into short intervals or even avoided by flexibly regulating the amplitude and phase-angle of third harmonic voltage based on various modulation indexes and power factors. An offline particle swarm optimization algorithm is employed to find the corresponding optimal amplitude and phase-angle of third harmonic voltage, which will further be put into the online look-up table to simplify the real time implementation. Experimental results verified the performance of the proposed modulation method. [ABSTRACT FROM AUTHOR]

The intelligent substations should be developed to perform multifunction operations, including power flow control, voltage regulation, and cost-effective power quality (PQ) improvement. In this article, an intelligent substation by a distribution transformer integrated with a capacitive-coupling winding tap injection static synchronous compensator (CWTI-DSTATCOM) is proposed for balance and unbalance operations. Compared with the state-of-the-art approaches, CWTI-DSTATCOM can operate at a low dc voltage with a small active inverter rating, which also can provide distributed voltage, reactive, and unbalanced current compensation. The power rating reduction mechanism is mathematically revealed via balance and unbalance compensation analysis, which is further used to determine the optimal inverter power rating of unbalance operation via an optimal LC filter design method. Furthermore, a comprehensive compensation control strategy is developed to coordinate the transformer taps and the active inverter under balanced and unbalanced operations. Simulation and real-time digital simulator hardware-in-the-loop experiments are performed to prove the validity and effectiveness of the proposed topology over inverter rating reduction and PQ enhancement. [ABSTRACT FROM AUTHOR]

This article investigates a decoupled vector current control strategy for grid-tied single-phase nine-level Packed E-Cell (PEC) inverters for vehicle-to-grid (V2G) technologies. The control design is based on a loop shaping approach, where performance specifications such as desired closed-loop bandwidth, closed-loop stability, robustness to pure time-delays in control loops/process, and decoupling between direct ($d$) and quadrature ($q$) axis of control signals are specified in the frequency domain. The control design problem is formulated as a convex optimization problem whose solution results in a multivariable controller in the $dq$ framework. The effectiveness of the proposed control strategy in terms of robust stability, current tracking, and improved total harmonic distortion is evaluated by experimental and simulation results under several case studies. The results also show that the designed controller provides the desired transient response as well as decoupling between the $dq$ output currents. [ABSTRACT FROM AUTHOR]

Linear modulation techniques (LMTs) of an asymmetrical six-phase machine (ASPM) with two isolated neutral points synthesize the desired voltage vectors by applying at least five switching states. Different choices of applied voltage vectors, sequences in which they are used, distribution of dwell-times among the redundant switching states give rise to a large number of possible LMTs. It is desirable that these LMTs avoid more than two transitions of a particular inverter leg within a carrier period. Only a subset of existing LMTs of ASPM follows this rule. Through an innovative approach, this article finds a way to account for all possible infinitely many LMTs that follow the rule of at most two transitions per leg. Another important criterion for the selection of an LMT is its current-ripple performance. Therefore, through numerical optimization, this article finds optimal LMTs among the above infinite possible LMTs for all reference voltage vectors in the linear range and for the whole feasible range of a machine parameter. This parameter is related to the leakage inductance of the machine and impacts the current ripple performance of ASPM. An optimal hybrid strategy is proposed with these optimal techniques, which outperforms all existing techniques in terms of the current ripple. The theoretical analysis is validated through simulation in MATLAB and experiments performed up to 3.5 kW on a hardware prototype. [ABSTRACT FROM AUTHOR]

Cascaded split-source inverter (CSSI) is a single-stage modular multilevel structure. This article proposes the multidimensional pulsewidth modulation (MD-PWM) for this topology. All multilevel modulations are a particular version of this approach. The principles of this geometrical method are based on selecting the optimal points and determining the switching sequences and time intervals. By considering the operational principles of CSSI, this method is modified to accurately control the voltage of dc links, charge inductors with constant duty cycles, control dc and ac sides of cells independently, and reduce the reverse recovery effects of input diodes. MD-PWM, with a low computational burden, can be implemented for CSSI with any number of voltage levels. The output power can be distributed equally or unequally among cells, and both buck and boost operation modes are realized for each cell. Also, this method is extended for the operation of the converter with unequal input sources. For an asymmetric topology with N cells, harmonics are located at multiples of N times switching frequency. Thus, this strategy eliminates low-frequency distortions from the output voltage of asymmetric CSSI. The simulation and experimental results verify the performance and accuracy of the proposed technique and mathematical relations. [ABSTRACT FROM AUTHOR]

This article explores a single-source UXE-type asymmetrical multilevel inverter for its operation on 13-, 11-, and 9-levels with boosted output voltage. The 13- and 11-level operations require a single voltage sensor to maintain the auxiliary dc link at $V_{\text{dc}}/2$ and produce 1.5 and 1.25 boosting using redundant states and exhibit lower capacitor inrush currents. For the 9-level operation, the topology exhibits self-balancing of the dc-link at $V_{\text{dc}}$ and a boosting of 2. With a conventional pulsewidth modulation (PWM) for the 13-level operation, the inverter can drive a predominantly inductive load of 0.35 lagging at unity modulation index. For the 11-level operation, the levels cease to maintain beyond the modulation of 0.95. Thus, a hybrid PWM is proposed to fully utilize the redundant states for the charging of the switched-capacitors and enhance the inverter active load capability in the 13-level mode by 21% at unity modulation index ($m_a$) and address a 0.8 power factor load at the $m_a$ of 0.9. For the 11-level operation, the levels are maintained at any $m_a$ with the hybrid PWM. The operation of the inverter is also verified for nonlinear load. Further, the 9-level operation with twice boosting and self-balancing is presented. The results are verified on MATLAB/Simulink and validated experimentally. [ABSTRACT FROM AUTHOR]

Existing dc and ac bypass-assisted two-level three-φ inverter topologies can only reduce the peak-to-peak (P-to-P) common mode voltage (CMV) value by 66.6%. However, the dv/dt of CMV remains unchanged. This issue can be easily addressed by using a multilevel inverter but it increases the size, cost, and complexity of the system. As an alternative, this article proposes a novel three-phase inverter topology where one leg (out of three) of the inverter is configured to produce three-level pole voltage while the other two legs produce two-level pole voltages. This results in a unique space vector (SV) diagram with 12 SVs, whereas a 2-level inverter has only 8 SVs. A new pulsewidth modulation scheme is proposed that can utilize the SV diagram in such a way that dv/dt of CMV is reduced by 50% along with a 66.6% reduction in CMV P-to-P value in each switching cycle. Another advantage of the proposed topology is the reduced conduction losses due to fewer switches conducting at any given time. The proposed topology is compared with the existing solutions to prove its advantages. Simulation and experimental results are presented for a three-phase induction motor load to validate the various claims. [ABSTRACT FROM AUTHOR]

In this article, a nine-level multilevel topology is proposed by stacking of low voltage basic inverter cells and then cascading with H-bridges. The dc-link is split into four equal halves by series-connected capacitors from a common dc-link. Three common coupling points (CCPs) are formed due to the stacking of four capacitors, which are connected across the dc-link. The CCP voltage balancing is ensured by drawing or injecting zero instantaneous currents from each of these points at any instant. A six-phase symmetrical IM is used to generate equal and opposite instantaneous phase currents by applying exactly equal and opposite phase voltages for two opposite phases from the six-phase inverter. Simultaneously, the capacitor voltage balancing of H-bridge inverter cells is also achieved by proper selection of redundancies from the available pole voltage redundancies. The concept is verified with a nine-level inverter laboratory prototype, which consists of a five-level stacked inverter, and followed by two low voltage cascaded H-bridge inverters in each phase. Detailed experimental results of steady-state and transient conditions are included to validate the proposed scheme. [ABSTRACT FROM AUTHOR]

Driven by the demands of carbon footprint reduction, the conventional fossil-based generators are gradually being replaced by grid-forming inverter. One crucial aspect of grid-forming inverter is synchronization stability, which is not only related to the inverter power-frequency (P-f) dynamic but also influenced by power grid characteristics as well as grid impedances. From the perspective of a local inverter, it is difficult to know the detailed information. As a result, severe oscillations and even the loss of synchronization may occur and pose a great threat to the power grid. To overcome this problem, a black-box approach is presented in this article. Initially, a frequency-domain identification approach is proposed to acquire the power grid characteristic by the inverter itself. Based on the measurement results, the P-f dynamic of grid-forming inverter is explicitly reshaped to ensure the synchronization stability. The proposed methodology does not require any prior power system knowledge or the trivial modeling process. Finally, the effectiveness of the proposed method is verified by hardware experimental results. [ABSTRACT FROM AUTHOR]

Three-phase four-wire grid-forming (GFM) inverters are promising to interface distributed energy resources into low-voltage networks. However, these inverters are prone to overcurrent under grid faults. Physically increasing the inverter current capacity is not cost-effective to cope with complicated fault conditions. In this article, an adaptive fault ride-through (FRT) scheme based on instantaneous saturators and virtual negative- and zero-sequence resistances is proposed. It features not only overcurrent limitation by modifying voltage references, but also seamless transition between normal and grid fault conditions. The proposed FRT scheme is first analyzed from different aspects, including the virtual sequence resistances, grid short-circuit ratio, fault types, and fault levels. The virtual sequence resistances are then designed to be adaptive to ensure high voltage quality at the healthy phase. The proposed FRT scheme is verified by MATLAB/Simulink simulations under asymmetrical faults. A laboratory platform with a grid-connected 3kW GFM inverter is further constructed to demonstrate its effectiveness (a video of the experimental results under three asymmetrical faults is attached). [ABSTRACT FROM AUTHOR]

In this work, a ten-level inverter scheme is presented to extend the linear modulation range (LMR) by using a unique space vector pulsewidth modulation (SVPWM) technique, such that no lower order harmonics (such as 5th, 7th, 11th, and 13th, etc.) are present in the phase voltage as compared to six-step mode operation. The proposed scheme can increase the peak phase fundamental voltage of the inverter from 0.577 to $0.637 V_{dc}$ (maximum possible peak phase fundamental voltage of a conventional hexagonal space vector structure (SVS) voltage source inverters while operating in six-step mode), irrespective of the load power factor (p.f), where $V_{dc}$ is the dc link voltage of the inverter. The ten-level inverter structure is formed using a two-level inverter and an H-bridge (HB) in cascade from one end and a floating capacitor-based two-level inverter cascaded with an HB from the other end to drive an open-end winding induction motor (OEWIM). All the HB capacitor voltages are balanced by using space vector redundancy. The claim of balancing the capacitor voltages throughout the whole modulation range is verified experimentally in this article. Experimental results at different steady-state and transient conditions are shown to validate the proposed inverter scheme’s efficacy in increasing the LMR. [ABSTRACT FROM AUTHOR]

In order to improve the control performance of conventional finite-control-set model predictive control (FCS-MPC) method and keep the balance between the steady-state performance and switching frequency of the system, this article proposes an improved MPC method based on the conventional MPC method. First, the reference voltage vector of the next control instant is predicted to determine the candidate voltage vectors of the next two control instants. Then, an optimal vector selection way is proposed, in which candidate voltage vectors selected by the reference voltage vector and designed cost function structure in series. Moreover, the selection process of optimal voltage vector from the candidate voltage vectors is analyzed. Finally, the comparison experiments between the conventional FCS-MPC method and the proposed MPC method are conducted. And experimental results show the validity of the proposed control strategy. [ABSTRACT FROM AUTHOR]

This article declares two control strategies, named “average-model based” (AMB) and “energy-based” (EB) control methods, to control a monoinverter dual parallel surface-mounted permanent magnet synchronous motor (MIDP-PMSM) with different parameters. Up to now, it has been assumed that two parallel PMSMs have the same parameters, so the controller has been designed based on this simplifier assumption. To extend the control subject to MIDP-PMSMs with different parameters, two new methods are proposed and compared in this article. The average value of both motors’ parameters is used in the AMB control design, while in the proposed EB control method, each motor’s parameters separately contribute to designing the control laws. The proposed methods are based on the “mean-difference” technique. These methods have been compared by mathematical analysis regarding the torque per total ampere ratio, the copper losses and the dynamic response of motors’ speed. The MATLAB simulation and experimental implementation have been carried out to verify the proposed methods. The results have shown that the EB control method has better performance than the other. [ABSTRACT FROM AUTHOR]

In order to achieve an efficient pulsed power compensation for the ac-sourced pulsed power system, a multifunctional compensator (MFC) of modified T-type semi-two-stage converter is proposed in this article. The power decoupling capacitor is in parallel with one of the dividing capacitors of the three-level ac/dc converter, so that the voltage of decoupling capacitor is able to vary in a very wide range without increasing the voltage stress of the converter. The semi-two-stage compensator achieves better compensation performance and less decoupling capacitor than the single-stage circuit while having higher efficiency and lower converter power rating than the two-stage solution. Operating principles of the semi-two stage MFC are analyzed in detail, and a modulation strategy is presented to achieve two-path power flow between the decoupling capacitor and ac power system. In order to improve the dynamic response of the MFC during the transition between different pulse frequency loads, a power-difference feed-forward control and a dual low-pass filter control are proposed for the compensation. A prototype is built and tested to verify the effectiveness and feasibility of the proposed MFC and control methods. [ABSTRACT FROM AUTHOR]

High-performance comprehensive optimization of line current ripple, zero-sequence circulating current (ZSCC), and switching losses is a challenge for parallel interleaved inverters. This article proposes a modified discontinuous pulse width modulation (PWM) method for two parallel interleaved two-level voltage-source inverters (2L-VSIs), which are regarded as a single 3L-VSI to optimize its vector sequence. The line current ripple is the priority optimization target to guarantee the principle of the nearest three vectors is applied in each subsector. To reduce the ZSCC and switching losses, the vector combinations with smaller changing rate of ZSCC are selected, and the phase-leg carrying the highest current is clamped. The detailed design procedure of 3L-vector sequence and how to distribute to 2L-VSIs are revealed. The quantitative performance comparison between the proposed PWM method and existing ones in terms of line current ripple, ZSCC, and switching losses are made. Experimental results confirm the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In this work, a multilevel 42-sided polygonal space vector structure for suppression of lower order harmonics for open-end induction motor (OEIM) drive applications is proposed. The proposed power circuit topology consists of two inverters feeding an OEIM from either side. The main inverter fed with a single dc-link providing active power for motoring operation is switched at low switching frequency. The secondary inverter fed with a capacitive supply is switched at high frequency to suppress lower order harmonics upto 39th order, up to the base speed of operation allowing maximum utilization of the dc-link. The advantages of lower order harmonic suppression in motor phase voltage, for polygonal space vector structures, are combined with multilevel inverter topology. This offers various advantages like multilevel operation with low inverter switching for the main power delivery inverter and harmonic suppression throughout the modulation range. The low switching frequency primary inverter generates harmonics of the order $6*k \pm 1$ , where $k = 1, 2, 3,{\ldots }$. The harmonics are suppressed by the capacitor-fed secondary inverter, which supplies only reactive power for suppression of lower order harmonics in motor phase voltage. Use of a single dc-link facilitates four-quadrant operation of the inverter. The proposed scheme is validated for steady-state and dynamic performance by experimental results. [ABSTRACT FROM AUTHOR]

The article proposes a new vector analysis based model predictive control (VAMPC) method for multilevel converters with four different switch configurations. It is based on track of an error vector using fundamental state vectors, and can achieve deadbeat control of four switching states multilevel converters with multiple control objectives. Compared with conventional finite control set model predictive control (FCS-MPC) method, VAMPC can achieve excellent spectrum characteristics in the case of a slight increase in computational burden. The VAMPC method is experimentally validated on a 100-kHz GaN-based three-level flying capacitor (FC) converter. Compared with the FCS-MPC, VAMPC leads to significant reduction in steady-state control errors. For example, the inductor current total harmonic distortion of the FC converter is now reduced by more than 60%. [ABSTRACT FROM AUTHOR]

For the grid-connected inverter, grid voltage sag can be a typical phenomenon. From the prospective of safe and robust operation, the inverter should be stable when the grid voltage sag (or large disturbance) occurs. Although the inverter stability in this case has attracted considerable attentions, few studies have given suggestions on the system stability boundary and parameter design. To fill this gap, this article deeply discusses the large-signal stability of the system under different situations and reveals the quantified impact of the phase-locked loop (PLL) on the large-signal stability of the system based on the fundamental current-source model. The large-signal stability criterion and boundaries for grid voltage sags are established, which are instructive to the design of PLL parameters. Besides, a novel design guideline for the PLL parameters is proposed to ensure the robust operation of the system both under the steady-state and grid voltage sags, as well as to achieve good dynamic performance after large disturbance occurs. The theoretical, simulation, and experimental results show that the proposed stability boundary has certain accuracy and the PLL parameters design guideline can ensure the system stability both under steady-state and grid voltage sag conditions. [ABSTRACT FROM AUTHOR]

The focus is on the maximum power point tracking implementation for the buck-boost voltage mode control of a single-phase multilevel inverter based on a three-level neutral point clamped quasi-Z-source topology. To utilize shoot-through states only when boost function is needed and avoid it in the buck mode, two different control approaches are required. This article proposes merged control system which provides switching between different control algorithms in the buck and boost modes. In particular, a hysteresis band between the buck and boost modes is proposed which provides smooth and stable performance. It also includes features of the implementation of the global maximum power point tracking, which can scan the entire input voltage range in up to 10 s. Experimental results showed that proposed control system provides high and stable maximum power point tracking efficiency in different operation points. Finally, the key features are verified, the advantages, drawbacks, and an eventual potential for other applications are presented in conclusions. [ABSTRACT FROM AUTHOR]

Pole-zero cancellation technique has drawn much attention to tackle the filter resonance of the LCL-type grid-connected inverter for simplifying the stability-oriented design. However, its potential stability risk was rarely concerned. As one representative of this technique, the weighted average current (WAC) control is assessed. It is found that the LCL resonance in the state variable WAC is cancelled out by pole-zero cancellation, but the ones in some uncontrollable or unobservable state variables such as the grid current are still inherited, resulting in harmonic instability. From this perspective, this article appeals that this technique should be cautiously used or even avoided. An extended WAC control which avoids pole-zero cancellation is proposed accordingly. The theoretical expectations are finally validated by simulation and experimental results. [ABSTRACT FROM AUTHOR]

In this article, the stability problem is investigated for a class of time-invariant current controlled LCL-type weak-grid-tied inverter (WGTI) system. Considering that the existing stability analysis methods rarely effectively handle the adverse effects of both sinusoidal pulsewidth modulation (SPWM) saturation and plant uncertainty, and the corresponding theoretical results are poor in accuracy and completeness, this article proposes an intuitive stability analysis method in Laplace-domain to ensure the system robustness against SPWM saturation and plant uncertainty. Specifically, the describing function (DF) is employed to characterize the averaged SPWM saturation nonlinear model in Laplace domain. By designing equivalent functions without the poles inside the right-half s-plane, the Nyquist criterion required by the DF is equivalently converted into the Hurwitz criterion of closed-loop characteristic polynomial with interval coefficients containing uncertain circuit parameters. Subsequently, by means of the Kharitonov’s theorem, the interval characteristic polynomial is transformed into four Kharitonov polynomials with fixed parameters that are easy to analyze. The state-and-disturbance-observer (SDO)-based current controlled WGTI system is adopted as a case of application study. In order to reduce the conservatism of the Kharitonov’s theorem applied here, a “pointwise” stability analysis on the grid impedance is implemented. Finally, the controller parameters are determined by stability region analysis with different grid impedance points, and the effectiveness of the proposed method is further verified by a series of steady-state and transient experiments. [ABSTRACT FROM AUTHOR]

Accurate rotor position estimation is important to ensure the high-performance operation of position sensorless permanent magnet synchronous motor (PMSM) drives. To suppress the estimated rotor position harmonic error (ERPHE) caused by inverter nonlinearities, flux spatial harmonics, and the current measurement error in high-frequency signal injection-based schemes, an adaptive iterative learning control-based online suppression method is proposed in this article. This method constructs a P-type iterative learning rotor position observer with the forgetting factor to obtain the tracking error and generate the compensation value in real time. The stability, the convergence, and the parameter sensitivity of this observer are analyzed in detail. To improve the suppression effect of the ERPHE for different loads and speeds, an iterative learning gain self-tuning strategy is investigated. The proposed method does not require the offline detection and the discrimination of harmonic frequencies, which has strong suppression ability for different harmonic error components. The effectiveness is verified by experiments on a 2.2-kW interior PMSM drive platform. [ABSTRACT FROM AUTHOR]

This article proposes a low-profile coplanar magnetic coupling resonance wireless power transfer system, which can be expanded segment by segment to power more receivers over a long distance with load-independent constant current output. Due to the low structural profile, it can easily conform to the environment such as desktops, floors, and walls, occupying a small space. By placing low-profile ferrite plates on or under the junctions of adjacent coplanar coils, the main coupling coefficients of the adjacent coils are significantly enhanced, and therefore, the power transmission efficiency (PTE) of the system is greatly improved. With the fixed operating frequency, load-independent current characteristics, simple structure, coplanarity with the environment, good scalability, and high PTE, the system can be easily utilized in various medium and long-range multiload WPT applications. The article describes its principle in detail and clarifies the influence of losses on system performance such as output current characteristics, PTE, and the maximum number of the charging segments. The experimental platform with two and three receivers was established to verify the theory, and the measurement results are in good agreement with the theoretical analysis. The maximum measured PTE reached 93.8% and 93.6% for two and three receivers, respectively. [ABSTRACT FROM AUTHOR]

This article designs parameter adaptation algorithms for online simultaneous identification of a two-parameter sigmoid inverter model for compensating inverter nonlinearity to reduce the voltage error in flux estimation for a position sensorless motor drive. The inverter model has two parameters, $a_2$ and $a_3$ , where $a_2$ is “plateau voltage” and $a_3$ is a shape parameter that mainly accounts for the stray capacitor effect. Parameter $a_3$ is identified by the $(6k\pm 1)$ th order harmonics in measured current. Parameter $a_2$ is identified by the amplitude mismatch of the estimated active flux. It is found that the classic linear flux estimator, i.e., the hybrid of voltage model and current model, cannot be used for $a_2$ identification. This article proposes to use a saturation function based nonlinear flux estimator to build an effective indicator for $a_2$ error. The coupled identifiability of the two parameters is revealed and analyzed, which was not seen in literature. The concept of the low current region where the two-way coupling between $a_2$ and $a_3$ occurs is established. In theory, it is suggested to stop the inverter identification in the low current region. However, the experimental results in which dc bus voltage variation and load change are imposed have shown the effectiveness of the proposed online inverter identification and compensation method, even in low current region. [ABSTRACT FROM AUTHOR]

This article proposes a straightforward and effective control strategy to reduce the periodic speed ripple of the permanent-magnet synchronous motor. The control strategy can be described as injecting current harmonics whose amplitude and phase can be adjusted automatically to compensate for periodic speed ripple. Compared with the resonant controller with the Tustin transform, the method proposed in the article can track speed ripple frequency without frequency displacement, realizing no steady-state error. Besides, the system stability is analyzed, and the phase compensation scheme is proposed to increase the phase stability. Moreover, the equivalence between the harmonic injection module and the resonant controller has been analyzed, which provides a simple way to understand the phase compensation of the resonant controller. The theoretical analysis and experimental results are presented to verify the feasibility of the proposed harmonic injection method. [ABSTRACT FROM AUTHOR]

This article introduces a fault-tolerant reconfiguration topology and control strategy for symmetric open-winding multiphase machines. Bidirectional switching devices are proposed to connect the inverter bridge legs on different ends when the open-circuit fault occurs, and the reconfigured circuit can regenerate the missing voltage phasors due to the faults. Furthermore, to prevent overcurrent problems on the reconfigured bridge legs and maximize the torque output capability of the reconstructed system, winding currents are optimized with zero-sequence component injection during fault-tolerant operation. The experiments implemented on the five-phase system validate that the proposed fault-tolerant scheme is effective and has higher torque output capability compared with the traditional fault-tolerant solutions. [ABSTRACT FROM AUTHOR]

Beyond the impacting presence of nonlinear loads, low-voltage microgrids also experience low energy efficiency and resonance phenomena when operating interconnected to a distribution grid that suffers from distorted voltages. This article proposes a model-free control strategy capable of coordinating inverters existing within a dispatchable microgrid, allowing to operate it as a single-controllable entity that behaves like a resistor at selected harmonic frequencies. Such resistive shaping uses a centralized control architecture to steer inverters to distributively compensate reactive and harmonic currents, supporting active current sharing. Consequently, the microgrid point-of-common-coupling operates with a high power factor when the grid imposes distorted voltages. Additionally, if resonant components exist, the strategy supports harmonic resonance damping, which minimizes deterioration of voltage quality. For instance, comparative results show that, for the considered scenario, the proposed resistive shaping damps resonances up to 50% better than a previous approach that compensates harmonics using sinusoidal current synthesis. Simulation results carried out on a three-phase low-voltage microgrid testbench, considering three inverters, demonstrate the abovementioned capabilities of the proposed approach. Experimental results based on a single-phase microgrid prototype comprising two inverters with two linear loads and one nonlinear load validate the applicability of the method to real-life implementations. [ABSTRACT FROM AUTHOR]

The system stability and injection of maximum active power to the grid are among the most important technical issues in cascaded H-bridge (CHB) based photovoltaic (PV) systems, especially when the irradiation of PV arrays connected to CHB cells are not uniform. In other words, the unequal irradiation of PV arrays leads to the unbalanced output power of H-bridge cells, which causes overmodulation and increased grid current total harmonic distortion in the CHB inverter. Conventional control methods cannot maintain the stability of a grid-connected CHB inverter under asymmetric operating conditions. So, this article proposes a new CHB-based structure with reserve batteries and a novel modulation technique to stabilize the system in heavy mismatching conditions and avoid overmodulation. Furthermore, the number of switches and series batteries is reduced, and the number of output voltage levels is increased compared to the conventional PV-CHB system with energy storage. The validity of the proposed solution is verified by simulation in MATLAB/Simulink, and then the experimental results are presented to confirm the theoretical and simulation results. [ABSTRACT FROM AUTHOR]

Cascaded split-source inverter (CSSI) is a new single-stage modular multilevel topology. Each cell of this converter converts dc to ac power in the buck or boost operation mode without any additional power switch. This article develops a design method based on a detailed model for CSSI. This model shows that energy storage elements experience double-fundamental frequency ripples besides high-frequency ones. It accurately calculates voltage gains and capacitor voltage and inductor current ripples. On the other hand, common carrier-based multilevel modulations have been modified in terms of both reference and carrier signals to control the inverter under symmetric and asymmetric conditions. With proposed simple modulations, the cell inductor is charged with the constant duty cycle. Also, the reverse recovery of diodes occurs in the minimum level of input current once per switching cycle and output power is distributed equally between cells. These characteristics reduce power losses, the input inductance of each cell, and the complexity of the system model. By phase-shifted pulsewidth modulation (PWM), the output voltage harmonics of three-cell CSSI are centralized around multiples of the triple switching frequency. In this method, high-frequency ripples on the network impedance are one-third of those in level-shifted PWM. Also, CSSI has been compared with cascaded multilevel inverters with voltage boosting ability and other SSI-based multilevel topologies. Validity of operation principles, mathematical relations, and modulation schemes are verified through simulation and experimental results. [ABSTRACT FROM AUTHOR]

Rotary assembly stations are commonly used in automated manufacturing processes. These production platforms require the speed control of the plate and need power for the actuators and sensors located on the rotating frame. These requirements are usually met through a gearmotor and sliding electrical contacts. Among the solutions that avoid the slip-rings, this article focuses on an integrated electric drive consisting of a gearless five-phase wound-rotor induction machine fed by two inverters, one placed on the stator reference frame and one placed on the rotating one. The main contribution is a novel control scheme that maximizes the power transferred to the rotating loads in compliance with the thermal constraint of the rotor and the voltage constraint of the rotor inverter. The theoretical analysis allows deriving an analytical solution to the control problem, and the experimental results confirm the effectiveness of the proposed algorithm. [ABSTRACT FROM AUTHOR]

In this article, the slow-scale instability occurring in the three-level T-type inverter with a passive memristive (3LT2IPM) load is investigated. The average model of the 3LT2IPM, whose coefficient matrix is nonlinear periodic time-varying, is constructed, both harmonic balance method used to calculate the approximate solution of the average model and Floquet theory used to identify the circuit dynamic states are applied to explore the mechanism of the slow-scale instability emerging in the 3LT2IPM. Theoretical results indicate that the slow-scale instability of the 3LT2IPM is caused by Hopf bifurcation emerging in a region where the frequency is higher than line frequency but much lower than switching frequency. Also, the conditions of three theoretical parameters that make the theoretical analysis results as accurate as possible are presented. Different parameters impact on the stability boundary of the 3LT2IPM in various design parameter spaces are discussed, and the Floquet multiplier sensitivity is analyzed to identify key parameters for the stability of the 3LT2IPM, which are helpful to guide parameter adjustment of the 3LT2IPM to ensure its stable operation in practice. Finally, hardware experiment is established and experimental verification is provided. Physical experiments agree well with simulations, which together demonstrate the correctness of theoretical analysis. [ABSTRACT FROM AUTHOR]

Detecting and locating the open-circuit (OC) fault of an insulated-gate bipolar transistor (IGBT) rapidly and accurately is very important to improve the security and reliability of the drive system. However, due to the symmetry of the dual-inverter topology, it is difficult to achieve fault detection for open-end winding permanent magnet synchronous motor (PMSM). Thus, this article proposes a fault diagnosis technology for open-end winding PMSM fed by a dual inverter with a common dc bus. In this method, the faulty phase and the faulty case are identified by the average value of the output of the closed-loop controller, which is used to eliminate the differential-mode current in the system. Then, the faulty phase is isolated by removing the gate signals. Finally, the faulty IGBT is located by restructuring the faulty phase and analyzing the current of the faulty phase. In this method, the OC fault can be detected and the faulty IGBT can be located accurately. Experimental results are shown to validate the proposed detection method. [ABSTRACT FROM AUTHOR]

This article presents a ground breaking traction drive for fuel cell-powered light rail vehicles based on a multilevel cascade converter with H-bridge cells. The converter provides dc–ac power conversion with unique buck–boost capability to compensate for the variation of fuel cell terminal voltage with the load power. In comparison to a conventional boost inverter, based on the cascade connection of a boost dc–dc converter and a voltage source inverter, the proposed converter has the significant benefits of much smaller inductor size, lower dc side current ripple, and multilevel voltage waveforms at the ac output. This article presents the converter modeling and design of the boost inductor and submodule capacitors, taking into account the full speed range of the motor. The concept has been fully validated on a bespoke experimental prototype driving an induction motor, showing the suitability of the proposed converter as a traction drive for hydrogen-powered rail vehicles. [ABSTRACT FROM AUTHOR]

This article details and solves a small-signal stability-constrained optimal power flow (SSSC-OPF) for inverter-based ac microgrids. To ensure a sufficient stability margin during optimal generation, a small-signal stability constraint is embedded into the conventional OPF formulation. This condition is enforced using a Lyapunov stability equation. A reduced-order model of the microgrid is adopted to alleviate the computational burden involved in solving the resulting SSSC-OPF. Even then, the resulting stability conditions are highly nonlinear and cannot be handled using the existing methods. To tackle the nonconvexity in the SSSC-OPF due to the presence of the nonlinear stability constraint, two distinct convex relaxation approaches, namely semidefinite programming and parabolic relaxations, are developed. A heuristic penalty function is added to the objective function of the relaxed SSSC-OPF, which is solved sequentially to obtain a feasible point. While off-the-shelf tools fail to produce any feasible point within hours, the proposed approach enables us to solve the SSSC-OPF in near real time. The efficacy of the proposed SSSC-OPF is evaluated by performing numerical studies on multiple benchmarks as well as real-time studies on a microgrid system built in a controller/hardware-in-the-loop setup. [ABSTRACT FROM AUTHOR]

Conventional predictive control (PC) has been employed for machine drives due to good dynamic response and easy implementation. However, the disturbance caused by machine parameter mismatch is one of main barriers to its widespread application. In order to deal with this issue, this work proposes a novel model-free predictive current control (MFPCC) for surfaced permanent magnet synchronous machines (SPMSM). The contribution of this work is that a novel extended state observer (ESO) is proposed and the proposed MFPCC only utilizes the input and output knowledge of the plant without using any machine parameter in controller, which can effectively suppress the disturbances. In addition, the effect of the initial machine inductance parameter mismatch on the conventional ESO of MFPCC is analyzed in detail. The proposed MFPCC is validated and compared against three methods, namely PC without ESO, PC using ESO, and MFPCC using ESO. The experimental results have been carried out to verify the effectiveness of the proposed MFPCC. [ABSTRACT FROM AUTHOR]

In the electrical engineering field, there are two types of system which can potentially generate resonance under excitation, one is the electrical system with inductor and capacitor, the other is the mechanical system with spring-mass characteristic. A lot of research on active damping control algorithms for grid-connected inverters with LCL filter and inverter-driven machine with multirotating masses have been demonstrated. However, research works for these two systems were carried out independently and there is a lack of systematic comparison for modelling and control between these two systems. This article will unify the mathematical models and active damping control algorithms for these two systems. It is found that the mathematical models and control structures are fundamentally the same. The existing or future potential active damping control algorithms used in electrical system can be applied in mechanical system and vice versa to avoid reinventing the wheel. Parameter sensitivity analysis for controller and feedback gains was performed for electrical systems in the discrete z-domain. For mechanical systems, it is found that a substantial electromagnetic torque overshoot was introduced when applying the active damping control and it was analyzed quantitatively with various damping coefficients to guide the inverter design. Finally, experimental tests were done to verify the findings. [ABSTRACT FROM AUTHOR]

This article presents energy-function based model predictive control (EF-MPC) for single-phase split source inverters. Unlike conventional MPC methods, the proposed EF-MPC is designed by considering the stability of inverter. According to the Lyapunov's direct method, the stability of inverter can be assured if the derivative of energy-function is always negative. Motivated from this fact, the design of cost function in EF-MPC is based on the energy function which involves dc- and ac-side inductor current errors. It is shown that the coefficient used in the formulation of energy function is redundant which eliminates the weighting factor required in the multiobjective cost function. Elimination of the weighting factor not only simplifies the controller design in the practical implementation, but also removes the weighting factor tuning process needed in the conventional predictive methods. Another advantage of the proposed EF-MPC is that the amplitude of low-frequency component in the dc-side inductor current is suppressed. The steady-state and dynamic performances of the proposed EF-MPC are investigated experimentally under linear and nonlinear loads. In both cases, the dc-side variables are regulated at the desired values while the sinusoidal output voltage with low total harmonic distortion is obtained in the ac-side. [ABSTRACT FROM AUTHOR]

Finite control set model predictive control (FCS-MPC) is a control strategy with fast response and a simple and flexible structure. However, when the control plant is complicated such as a multiphase electric machine, the application of FCS-MPC faces clear challenges. This article for the first time investigates the FCS-MPC for a nine-phase open-end winding (OW) permanent magnet synchronous machine, which is powered by nine H-bridge inverters with a common dc bus. First, in order to solve the challenge of substantial iterations in the conventional FCS-MPC, the number of control sets is simplified by reconfiguring the high level in switching states. Then, to eliminate the zero-sequence current caused by the common dc bus, the zero common-mode voltage (CMV) vector is selected. Subsequently, duty-ratio optimization is used to further reduce the available vectors. By the abovementioned measures, the number of iterations is reduced from 19 171 to 18. In order to suppress the harmonic current, the virtual voltage vectors (VVs) are designed. Each VV is synthesized by two zero CMV vectors, which can eliminate all the third and fifth harmonics in the output voltage. In addition, to achieve symmetrical pulsewidth modulation pulse sequences, a general pulse generation method for OW drive systems is proposed. Finally, the control performance of different control sets and harmonic weighting factors are evaluated and compared, and the experimental results have verified the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

The low-frequency current ripple, which is inherent in the two-stage single-phase grid-tied fuel cell power system, gives harmful effects on fuel cell stack itself and causes a reduction in the lifetime, efficiency, and operating performance of system. This article first presents a direct current control scheme to improve the dc and ac power quality with a dc-link voltage notch filer and a proposed improved model predictive control (MPC) method. However, since the number of the switching states is low, the fuel cell dc current and grid current may suffer contaminated and distorted, when the conventional MPC algorithm implementation is straightforward. With the aid of feedforward duty ratio and the rotation angle of reference grid current, the optimization space can be reduced in real time, avoiding evaluating all the feasible virtual-vectors, and then the improved MPC has been proposed. Finally, the correctness and effectiveness is verified by the simulation and experiment results. [ABSTRACT FROM AUTHOR]

Digital active gate driving has been shown to effectively manage the switching performance for power devices with the adjustable driving waveforms. However, most of the studies are based on a dedicated test circuit rather than a practical inverter with the sinusoidal output current. In fact, it is the dependence on the load current that makes the design of the gate driving profiles a critical issue. To investigate the digital active gate driver in an inverter application, this article has applied optimal patterns adapting to time-varying output load current. Prior to the search of optimal patterns, the proper design of time slot is discussed with three different time resolutions. With the proper resolution determined for patterns, multiple optimizations are carried out for different current conditions. In this way, the search for optimal patterns can be completed in an efficient time. Next, a lookup table of optimal switching pattern in correspondence with each certain load current condition was built in advance. According to the output load current, the optimal pattern is selected based on the lookup table. Compared to conventional constant driving waveform, the power loss has been reduced by 7% with full optimal lookup table applied. [ABSTRACT FROM AUTHOR]

Finite control set model predictive control (FCS-MPC) has been widely recognized in the field of electrical drive control during the past decades, due to its merits of quick dynamic response and low switching frequency. However, it is inherently penalized by high tracking deviations in the steady-state as well as exhaustive search among the switching sequences. To cope with this issue, a low-complexity gradient descent-based finite control set predictive current control (GD-FCSPCC) combined with a backtracking iteration approach is proposed in this article, aiming to improve the control performance by effectively tracking the reference value. First, FCS-PCC is reformulated as a quadratic programming (QP) problem from a geometric perspective. Consequently, the convexity of QP problem is proved to underlying the gradient descent, which minimizes the tracking deviations in an effective manner. Thus, the optimal solutions are selected by optimizing the reformulated objective functions. To reduce the number of the searched control inputs, a two-layer generalized decision-tree is employed. The procedures are repeated in several iteration periods optimized via a backtracking method, until the stopping criterion is satisfied. The effectiveness of the proposed GD-FCSPCC is experimentally validated on a 2.2 kW induction machine testbench. [ABSTRACT FROM AUTHOR]

A multitransmitter (multi-TX) inductive power transfer system would not only offer better spatial freedom but also improve the overall output capability. Under heavy-load condition, it is desirable to fully utilize the output power capability, and the overall input power needs to be balanced among the multiple TXs. This article proposes a simple and low-cost current sharing method to balance the input power. The current distribution is analytically studied for an example two-TX system, and the sharing path is proposed to balance the power under coupling variation and coil inductance mismatch. It also explores the influence of sharing path on the efficiency and zero-phase operation. Besides, the proposed method is valid for other compensations and can be extended for N-TX (N $>$ 2) systems. Finally, a two-TX system is implemented to verify the sharing effect. The current imbalance factor is effectively reduced from 37% to 2.7% under the worst coupling cases. [ABSTRACT FROM AUTHOR]

In this article, a series–series wireless power transfer (WPT) system combined with an inverse coupled current doubler rectifier (ICCDR) is proposed, as a very appropriate WPT topology for high current applications. The ICCDR uses an autotransformer to reduce the losses on the rectifier stage, which is suitable for high-current low-voltage output applications. Furthermore, a comprehensive comparison between the proposed WPT system with ICCDR and the traditional WPT system with full-bridge rectifier is presented. In the proposed WPT system, the primary parameter configuration and circuit behavior keep the same, while the secondary side altered, contributing to a volume reduction on the resonant capacitor. The system overall efficiency and power density are improved significantly. Accordingly, a 10 kW 400 V/48 V WPT prototype for battery charging is constructed and tested to validate this proposal. The output voltage is in the range between 38 and 55 Vdc, and the rated output current is 200 A. The overall efficiency of the proposed system reaches 94%, improving 2% efficiency with 220 W energy saving with respect to the traditional diode-rectifier system, when 9.5-kW output power is delivered. Theoretical analysis and experimental results have verified that the proposed system shows great advantages in high-power WPT applications. [ABSTRACT FROM AUTHOR]

This article proposes an analytical method for the selection of machine parameters and inverter power ratings of a synchronous motor operated in an open-end winding (OEW) configuration with one of the two voltage-source inverters (VSIs) connected to a floating capacitor (FC) in order to meet a specific torque–speed profile. With the derived normalized expressions and curves, all the suitable synchronous motors operated with an OEW-FC and a standard two-level VSI drive can be easily compared according to the power rating and desired speed–torque characteristic. [ABSTRACT FROM AUTHOR]