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This paper presents a different Finite Control Set – Model Predictive Control (FCS-MPC) for grid-connected three-phase bidirectional power inverters. These are typically used in dc or ac renewable-based microgrids (MGs), where bidirectional operation and fast dynamic response is required. The bidirectional grid-connected inverters are an essential part of MG, which inject energy into the ac grid or demand energy from it. The dynamic behavior of the system is a major concern since the current can suddenly change depending on the hierarchical controller. This paper proposes a different cost function using sliding mode theory, which offers a good dynamic response, reduced computational burden, and a parameter-free control model. The operation principle of the proposed controller is given and evaluated using a Hardware-In-the-Loop (HIL) system, but also experimentally with a 1kW laboratory prototype. The final results demonstrate the advantages of using this approach in grid-connected three-phase bidirectional power inverters in terms of dynamic response and reduced computational burden, making this solution technically attractive and viable. [ABSTRACT FROM AUTHOR]

The voltage rise problem due to the reverse power flow is one of the main obstacles to expanding the photovoltaic systems (PVSs) in distribution networks. In this paper, a decentralized control method for coordinated control of on-load tap changer (OLTC) transformers and PV inverters, is proposed for the voltage regulation of radial distribution networks. In this method, the allowable voltage range is divided into several zones, and appropriate corrective actions are adaptively taken according to each zone. To reduce the tension on the OLTC, first, the reactive power capability of the PV inverters is employed for the voltage regulation, and if it is insufficient, OLTC is used as the last solution. In this regard, the unused capacity of PV inverters is utilized for the reactive power compensation, considering the grid codes reactive power requirements. Furthermore, decentralized control based on a signaling method is used for coordination between OLTC and PVSs, which eliminates the need for communication links. Finally, the effectiveness of the proposed method is validated by the simulation results for the IEEE 33-bus distribution network, and the impact of different reactive power control methods on the number of OLTC operation is investigated. [ABSTRACT FROM AUTHOR]

The modern photovoltaic inverter has an active power-reactive power (Watt-VAr) control curve that is part of its management modes. The Watt-VAr curve, whose settings can be remotely modified, regulates the inverter reactive power output in function of its active power. This paper proposes the optimization of inverter segregated linear decision rules (LDRs), starting from the legacy Volt-VAr optimization in distribution networks. As opposed to classical LDRs that are interfaced with Volt-VAr optimization, the segregated LDRs are piecewise linear continuous curves that directly satisfy the reactive power capability of inverters, and their multiple segments can capture the correlation in solar power variability. The paper presents linear programming formulations for optimizing the segregated linear decision rules using stochastic optimization, robust optimization, and distributionally robust optimization. The optimization schemes are demonstrated on both meshed and radial networks, and their performance is validated via Monte-Carlo simulation. [ABSTRACT FROM AUTHOR]

With the rapid development of renewable energy, large amounts of power need to be transmitted to load centers, and series-capacitor compensation (SCC) plays an important role in renewable power transmission. However, it has been pointed out that SCC interacts with inverters and threatens system stability. This paper investigates the influence of SCC on inverter control, and proposes strategies for enhancing system stability based on the instability mechanism. First, the impacts of SCC on inverter current control and synchronization control are analyzed. A current control model is established by a system transfer function, and a synchronization control model focusing on transient stability is established based on the traditional synchronous reference frame phase-locked loop (SRF-PLL). Bode and nonlinear analysis methods are utilized in the stability analysis of both current control and synchronization control. It is found that SCC has little effect on inverter current control. However, it seriously affects synchronization control. SCC reduces the stability range of synchronization control, and causes system instability when there is a large frequency disturbance. In order to improve system stability, two approaches have been proposed. These approaches are optimizing a PI controller, and designing a band-pass filter (BPF) inside the PLL. Finally, simulations and experiments are presented to verify the correctness of theories. [ABSTRACT FROM AUTHOR]

The grid-connected inverter is a key device in the renewable energy power generation system and large-scale energy storage system, which the operational stability and reliability are the basis for the efficient and safe application of electrical energy. A real-time fault diagnosis method of a three-phase for grid-connected application combining a mixed logic dynamic (MLD) model and finitecontrol set model predictive control (FCS-MPC) is proposed. This paper not only realizes the open circuit fault diagnosis and location of the switching devices in the main power circuit, but also discusses the threshold issues and post-fault operations. The advantage of the proposed method is that it directly uses the control data and measurement signals of the controller without extra sensors and calculation, which will shorter the fault diagnosis time and occupy less calculation resource of the main processor. Simulation results illustrate the quickness of the fault identification and accurate position with robustness to the interference of the diagnosis method. Finally, the effectiveness of the diagnosis method was verified by a 1500 W experimental prototype in a laboratory. [ABSTRACT FROM AUTHOR]

One of the main challenges in the protection of the Inverter-Based Microgrids (IBM) is fault detection. Inverter-based resources have limited fault current; they also adopt different control schemes which make their fault current analysis and estimation more complex. These features make the traditional protection schemes inapplicable in IBMs. In this paper, two parameters are introduced and a protection scheme is proposed based on the introduced parameters to protect feeders and Distributed Generators (DGs) in IBMs. In the proposed scheme, in each sampling window, the last samples of the current waveform are predicted and the difference between the actual samples and the predicted samples are obtained; the voltage phase angle shift between the present and the past sampling windows is also computed; using these parameters and by introducing a new relay characteristic based on Mahalanobis distance the fault is detected in IBM. The effectiveness of the proposed protection scheme is verified through the simulation and experiment results. [ABSTRACT FROM AUTHOR]

This paper proposes a new control scheme to eliminate the 3rd harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical 3rd harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a 3rd harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup. [ABSTRACT FROM AUTHOR]

This paper presents an enhanced current controller for improving the performance of a class of single-phase grid-connected inverters operating in weak and distorted grid conditions. An inverter designed to operate at normal (strong or stiff and clean) grid conditions may not perform satisfactorily during weak and distorted grid conditions. One major reason is the interfering dynamics of the synchronization or phase-locked loop (PLL). This paper proposes an enhanced control structure for a popular class of single-phase inverters to address this problem. The proposed idea is to include the PLL state variables into the main inverter controller thereby minimizing the undesirable interactions of the PLL with the other components. A method for optimally designing the controller gains is also proposed. Compared to the conventional one, the proposed controller is shown to have a more robust performance over a substantially wider range of weak and distorted grid conditions. Extensive simulation and experimental results are presented to validate the proposed controls. [ABSTRACT FROM AUTHOR]

Capacitor-current-feedback active damping has been widely used in LCL-type grid-connected inverters. However, the damping performance is deteriorated due to the negative equivalent resistance resulted by the digital control delays, and thus the grid-connected inverter is apt to be unstable under the grid impedance variation. To address this issue, this paper proposes the capacitor-current proportional-integral (PI) positive feedback active damping method that can ensure a positive equivalent resistance almost within the Nyquist frequency, i.e., the full controllable frequency range. In theory, the proposed damping method can be implemented by feeding back the capacitor current through a PI function. However, the integral term will continuously accumulate the noise and dc bias arising from the feedback signal. To overcome this drawback, a more practical implementation solution is drawn in this paper. Furthermore, a straightforward design procedure is presented for the convenience of selecting the proper controller parameters. With the proposed damping method and its practical implementation, high inverter robustness against the grid impedance variation can be achieved. Experiments are performed on a 6-kW prototype and the experimental results are in well agreement with the theoretical expectations. [ABSTRACT FROM AUTHOR]

The staircase modulation strategy is the best choice for asymmetric cascaded H-bridge multilevel inverter (ACMI) because it makes the output voltage to present a high number of levels and makes one of the ACMI modules to operate at low frequency. However, the staircase modulation is usually employed without giving its deserved attention. This paper presents a clear understanding of the staircase modulation strategy and its applications in both isolated and grid-connected ACMI.  The main novelty of this paper is the obtaining of the switching function for each module. The principle of operation is carefully described, also indicating how to command the ACMI transistors. The output voltage of the two superior modules is evaluated for different values of iterations. Special attention is given to the power distribution among the ACMI modules when the staircase modulation is applied. It is demonstrated on how the power is processed in each module considering an ACMI with three modules. The analysis is presented showing the staircase modulation within a closed-loop control strategy for isolated and grid-connected modes of operation, showing that the content of this paper is valid for both modes. Experimental results clarify and show the efficacy of the content presented in this paper. [ABSTRACT FROM AUTHOR]

Emulation of an electric machine allows the testing of a drive inverter and controller of an electric drive system, prior to the manufacture of a machine prototype. This paper presents the emulation of a variable flux machine (VFM). The VFM emulator system proposed in this paper uses a detailed look-up table-based machine model for the purpose of emulation. This allows the emulator to mimic all machine magnetic and geometric behaviors such as saturation and torque ripple. In addition, the machine emulator proposed in this paper uses high-performance high-bandwidth linear amplifiers as power amplifiers. This enables a high bandwidth; leading to a high accuracy machine emulation. A detailed control description for the proposed machine emulator system and a machine model look-up table data verification against a prototype machine is initially presented. Experimental results are then presented to validate the utility of the proposed VFM emulator system to emulate various machine transient behaviors at different magnetization levels. Experimental results obtained from the emulator are subsequently compared against the experimental results obtained from a prototype VFM drive to verify the emulation accuracy. [ABSTRACT FROM AUTHOR]

One promising solution for future large wind turbine power conversion is to use medium-voltage power conversion systems for high efficiency and high power-density, e.g., by using a cascaded structure presented in this paper. However, a medium-frequency (MF) oscillation issue was observed while testing the experimental prototype of the above system. Given the complex multivariable coupling models of the multiwinding transformer and multiple inverters, conventional single-variable theories are not suitable. Therefore, this paper presents a new decoupling analysis method. Using a proposed linear transformation matrix, the output currents of multiple inverters can be transformed to a summation (grid) current and difference currents, where the former is the summation of inverter currents and the latter are the differences between currents of any two inverters. This transformation has enabled the use of single-variable theories, e.g., Bode diagram to analyze the summation current and difference current separately. It was found that the MF oscillation issue is caused by the equivalent impedance of the summation current being much larger than that of the difference current. Specifically, using small controller parameters cannot ensure the grid current quality and using large parameters may cause oscillation in the difference current. The proposed method can provide guidelines to design the parameters of current regulators and filters in multiple-inverter systems. Experimental results of a downscaled wind energy conversion system are presented to verify the proposed method. [ABSTRACT FROM AUTHOR]

In islanded microgrids, voltage source inverters working in parallel are expected to provide regulation of the local frequency while granting active power sharing. This paper presents a local control approach at each inverter based on an event-driven operation of a parameter-varying filter. It ensures perfect active power sharing and controllable accuracy for frequency restoration without requiring the exchange of control data between inverters over the communication network. This paper includes stability analysis and design guidelines for the control parameters using a modeling approach that considers the interaction between inverters. Selected experimental results on a three-inverter laboratory microgrid corroborate the effectiveness of the proposed control scheme, and outlines its advantages with respect to previous similar schemes and the performance cost that implies not using communications. [ABSTRACT FROM AUTHOR]

Three-phase grid-connected inverter modeling depends on the equivalent resistance and inductance between the inverter and the grid. However, these parameters are not fixed during the operation of the inverter and vary with the operating conditions. In this paper, a new globally stable adaptive controller is proposed to estimate these values online and update the controller during its operation. A model reference adaptive system (MRAS) based on two fictitious quantities with no physical significance is utilized, namely $M = v \cdot i^*$ and $N = v^* \times i$ , where $v$ and $i$ are the steady-state values of voltage and current in synchronously rotating frame of reference. Detailed stability analysis of the proposed estimation technique has been presented using a Lyapunov energy function-based approach proving the global stability of the technique. Small-signal modeling and analysis of the proposed scheme has also been presented so as to understand the local dynamics of the system. The guidelines to select the parameter of the proposed MRAS system along with updating the controller are elaborated in this paper. Detailed simulation based on MATLAB/Simulink and PLECS, along with experimental studies through a Texas Instruments TMS320F28377S microcontroller, validates the feasibility of the proposed control architecture. [ABSTRACT FROM AUTHOR]

The supply-side current of an inverter-fed induction motor (IM) is easily accessible and less affected by noises than the motor stator current. This paper proposes a novel sensorless speed estimation method for an IM using the spectrum of the supply-side current. However, due to the influence of fundamental supply frequency, it is difficult to carry out the speed detection straight from the supply-side current signal. In this paper, an analytic model for characterizing the supply-side current of an inverter-fed IM using the modulation theory and the switching function concept is developed. Then, the Hilbert transform is applied to extract the speed harmonic component from the supply-side current. Finally, an interpolated Goertzel algorithm is put forward to enhance the estimation precision of IM speed. The performance of the proposed sensorless speed estimation method is proved by the speed estimation results obtained from hardware experiments of an IM fed by an inverter under different load conditions. [ABSTRACT FROM AUTHOR]

In modern power systems, the increasing penetration of renewables and power electronics, particularly inverter-based wind and solar power generation, is altering power system dynamics and bringing new stability concerns. One challenging issue that is attracting considerable attention is the wide range of power oscillations associated with multiple parallel grid-connected inverters. In such systems, the characteristics in terms of resonance and oscillation are significantly different from single-inverter systems. This paper investigates the mutual interaction and stability issues of multiple grid-interfacing inverters with LCL-filters in power-electronics-based power systems under various grid conditions. The investigation reveals that such interactions between power inverters and the grid may excite multiple resonances at various frequencies under certain grid conditions. The nodal admittance matrix concept, which was originally from power systems engineering, is adopted here. Moreover, this paper further develops an Interaction-Admittance model that can effectively describe these mutual interactions in terms of a physical network admittance. We apply our model to various scenarios such as stiff grid conditions and inductive grids with/without power factor correction capacitors. The results with the proposed framework demonstrate an intuitive interpretation of multi-inverter system resonance and instabilities. Finally, simulations and experiments on a lab-scale system are provided to verify the theoretical analysis. [ABSTRACT FROM AUTHOR]

Inverter transformers are one of the key components in a PV plant and their operating patterns are considerably different from that of transformers in a substation. Moreover, these transformers experience higher level of harmonics generated by the inverters. Therefore, investigating the ageing profile of the inverter transformer is a timely requirement. In this paper, the ageing of inverter transformers in a medium-scale solar PV plant is studied by utilizing various real-time measurements including load current, top-oil temperature (TOT), ambient temperature, moisture in top oil, solar irradiance and individual current harmonics. An updated moisture-dependent thermal model (MDTMH), with consideration of current harmonics is developed to estimate the hot-spot temperature (HST). The influence of weather conditions, which directly determine the load current of inverter transformers on the loss of life (LOL) of an inverter transformer is also investigated. Furthermore, the effect of current harmonics levels on the inverter transformer's HST and LOL is explored. Finally, comparisons of HST and LOL of inverter transformers connected to PV arrays with three different tracking technologies, i.e., fixed-tilt, single-axis and Dual-axis PV arrays are performed. [ABSTRACT FROM AUTHOR]

The power quality of virtual synchronous generator (VSG) inevitably deteriorates in the presence of local nonlinear loads and distorted grid. In this paper, the conflict involved in the simultaneous elimination of distortion for both the inverter local load voltage and the grid exchanged current is first described. A unified control structure is presented that enables a tunable tradeoff between the two constrained harmonic sources. Then, a hybrid harmonic suppression scheme is proposed to enable the further improvement of the adaptability of VSG, which mainly consists of a local voltage harmonic control loop and an adaptive grid current-controlled loop. The local voltage harmonic control loop aims to scale down the inverter output impedance via a negative feedback loop, while the grid current-controlled compensator is intended to counteract the adverse effects from a weak grid via an additional voltage, which leads to substantially lower total harmonic distortion for both the local load voltage and the grid current at the same time. Small-signal modelling is performed to investigate the system stability and its robustness to parameter perturbations. The effectiveness of the proposed methodology is verified using hardware-in-the-loop simulations. [ABSTRACT FROM AUTHOR]

The instantaneous output power of the two-stage single-phase grid-connected photovoltaic (PV) inverter pulsates at twice the line frequency ($2f_{{\text{o}}}$), generating second harmonic current (SHC) in the front-end dc–dc converter and PV panel, which will affect the maximum power point tracking operation and deteriorate the overall conversion efficiency. The generating mechanism of the SHC is analyzed in this paper and it is pointed that in order to eliminate the SHC in the front-end dc–dc converter and PV panel, the voltage loop gain of the front-end dc–dc converter should be high enough at $2f_{{\text{o}}}$. Since there is a −180° phase abrupt at the resonant frequency of the input side filter capacitor and the inductor, the system may be unstable. To cope with this problem, the inductor current feedback active-damping scheme (ADS) is adopted. For further improving the loop gain at $2f_{{\text{o}}}$ , proportional–integral–resonant regulator with ADS (PIR + ADS) is proposed in this paper. Besides, a step-by-step closed-loop parameters design method is presented. Finally, a 3-kW two-stage single-phase grid-connected PV inverter has been fabricated and tested, and the experimental results verify the feasibility of the proposed control schemes. [ABSTRACT FROM AUTHOR]

Motors and inverters are increasingly being used in vehicles. As more vehicle components are being adopted, requirement for safety and installation space are increasing. Growing attention is being paid to the use of six-phase permanent magnet synchronous motors (PMSMs) and their parallel control by two inverters, because of their reduced size and enhanced safety. This paper proposes a pulsewidth modulation (PWM) control algorithm for a six-phase PMSM. In low-voltage vehicle components such as electric power steering, the current is usually acquired with resistors placed under the low-side switching devices. In the proposed control algorithm, the neutral voltage of the two inverters is shifted to either a lower or higher voltage within a certain range to allow for the signal conditioning time delay after the switching device turns on or off, and the time taken for the phase current to rise or settle. The paper proposes a novel PWM control algorithm that ensures that the on-time of the low-side switching devices satisfies the restriction on current sensing with resistors and reduces the current in the capacitor at the same time. The work contributes to reducing the current in a practical configuration involving low-voltage vehicle components. [ABSTRACT FROM AUTHOR]

This paper proposes a novel method to determine and control active and reactive power output of the virtual ocillator control-based inverter in grid-connected mode. An analytical expression is introduced to describe the quasi-steady-state relationship between the parameters of the inverter including its controllers, and the power injection and voltage at the inverter terminals. We introduce a new, complex valued parameter (K) to the conventional virtual oscillator control (VOC) scheme in order to control the inverter output power. We compare analytic and numeric solutions of the above analytical expression with time-domain simulation results, using different K values and line impedances to validate the analytical expression. In the second part of the paper, we design a controller: we use the above expression to derive K for given reference powers and use adaptive integrators to compensate for the remaining errors. We compare this control scheme to a method that uses PI controllers, and also to a simple decoupling method for independent control of active and reactive power. The controller based on the analytical method exhibits very good dynamic and decoupling performance. A single VOC-controlled inverter connected to a nonstiff grid is simulated in MATLAB/Simulink for demonstration, and is also validated using real-time simulations. [ABSTRACT FROM AUTHOR]

In this paper, a generalized droop control (GDC) is proposed for a grid-supporting inverter based on a comparison between traditional droop control and virtual synchronous generator (VSG) control. Both the traditional droop control and VSG control have their own advantages, but neither traditional droop control nor VSG control can meet the demand for different dynamic characteristics in grid-connected (GC) and stand-alone (SA) modes at the same time. Rather than using a proportional controller with a low-pass filter, as in a traditional droop control, or fully mimicking the conventional synchronous generator parameters in a VSG control, the active power control loop of the GDC can be designed flexibly to adapt to different requirements. With a well-designed controller, the GDC can achieve satisfactory control performance; unlike a traditional droop control, it can provide virtual inertia and damping properties in SA mode; unlike a VSG control, the output active power of an inverter with GDC can follow changing references quickly and accurately, without large overshoot or oscillation in the GC mode. Moreover, given specific controller parameters, the GDC can function as both a traditional droop control and a VSG control. The GDC's controller parameter design is more intuitive and flexible, and this paper provides a distinct design process. Finally, the effectiveness of the proposed control method is validated by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

In grounded photovoltaic (PV) systems, dc leakage to earth is formed by the contribution of the PV modules, inverters, and dc circuitry. Ground faults in such systems occur when there is an unintentional connection between any dc-carrying conductor with a grounded surface or earth. On the occurrence of ground faults, dc leakage will be intensified and will flow through the earth or other conductive paths, before returning back to the source. Thus, it is highly probable that some portion of the dc leakage will attempt to return to the source through any low-resistance paths that exist in the nearby vicinity, for example, through nearby buried pipelines. Therefore, unattended dc leakage may induce stray current corrosion concerns on these pipelines. To this extent, this paper presents a detailed dc interference modeling for assessing the dc leakage activity in PV systems—particularly under undetected fault conditions. First, the modeling carried out embraces the topologically accurate consideration of all sources of dc leakage that are associated with the grounded PV systems. A second feature presented in the paper is the modeling of blind-spot dc faults and their impact on third-party infrastructure. [ABSTRACT FROM AUTHOR]

A new controller for a grid-connected inverter with an LCL filter is proposed in this paper. The system is described by its complex representation, and the controller is designed using the complex root locus method. The complex representation allows a considerable reduction in the order of the system, simplifying the design task and making it possible to use advanced techniques, such as the complex root locus. The new complex controller adds an extra degree of freedom that makes it possible to move the poles of the systems and to improve the stability and speed of response compared with the conventional controls. This paper includes a detailed discussion of the effect of the gains of the controller on the root locus. The proposal is validated with simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper studies the significant impact of distributed-energy-resource (DER) inverters (which are connected to the same node) on anti-islanding protection of DER-installations. As it concerns the harmonic injection-based schemes, the induced harmonic voltage component that is provoked by each inverter is compromised, as synchronous injection (injection at the same time intervals) among the inverters cannot be reassured. Therefore, harmonic injection-based anti-islanding schemes will fail if the harmonic voltage component falls below a certain level. In this framework, a novel metric (i.e., the upgrade factor, UF) is introduced, indicating the supplementary power (of DERs) that can be installed in the same point of common coupling without compromising the effectiveness of the harmonic injection scheme. As this metric enables the evaluation of harmonic injection schemes prior to the future integration of DERs, it could be a substantial addendum into the relevant standards that evaluates the anti-islanding features of grid-tied DER-inverters. Finally, this paper proposes a technique (implemented in the software level), which maximizes UF and reassures the effectiveness of the harmonic injection schemes under the power variations of installed inverters. Extensive simulations and experimental tests have been conducted that verify the effectiveness of the proposed strategy under various penetration-levels and islanding-network conditions. [ABSTRACT FROM AUTHOR]

This paper analyzes the influence of phase-locked loop (PLL) on the stability of LCL-type single-phase grid-connected asymmetrical cascaded H-bridge multilevel inverter (ACHMI) with synchronous reference frame proportional-integral (SRF-PI) grid current control under weak grid scenarios. The ACHMI system is composed of power stage circuit and control system, where the control system contains the dual-loop current control strategy established in the hybrid reference frame, the SRF-PLL, and the hybrid modulation method employed to synthesize the multilevel output voltage. The small-signal model of the whole ACHMI system is first established by using a simple step-by-step derivation method, and then, the small-signal analysis method is adopted to linearize the ACHMI, which is then utilized to derive the impedance model of the ACHMI system. Furthermore, an improved impedance stability criterion is derived, which is then employed to analyze the system stability. By using this criterion, the stability of the ACHMI can be evaluated with the variation of the bandwidth of PLL, the output power factor angle of the ACHMI, and the amplitude of the grid current reference signal under weak grid conditions. In this paper, a systematic design procedure for the optimal selection of the PI controller of the PLL is presented, which guarantees the steady-state performance and dynamic response of the ACHMI system. With this design method, the dual-loop current control and PLL can be taken into account simultaneously when analyzing the stability margin of the ACHMI. Finally, the simulation and experimental results from a down-scaled grid-connected ACHMI prototype system are provided to confirm the validity of theoretical analysis. [ABSTRACT FROM AUTHOR]

Enhancement of dc bus voltage utilization for a five-level inverter with single dc source and capacitor-fed H-bridge (CHB) units is investigated in this paper. A carrier-based modulation technique is used for boosting the dc bus utilization, which is established by providing detailed mathematical analysis. The five-level inverter used here is realized by cascading a CHB unit to each phase of a three-level neutral point clamped inverter. The increase in dc bus voltage utilization owes to the pole voltage redundancies offered by CHB units. The floating capacitors of H-bridge units are balanced within a quarter fundamental cycle using the switching state redundancies of pole voltage levels. The aforementioned modulation technique allows the inverter to enhance the dc bus utilization from 0.577 $\text{V}_{\text{dc}}$ to 0.63 $\text{V}_{\text{dc}}$ under unity power factor. This enhancement is obtained in the linear modulation range without increasing the dc bus voltage, and thus, the inverter can operate without the presence of low-order harmonics in its phase voltages. The strength of this paper lies in its detailed mathematical analysis for finding out the limiting modulation index and power factor condition in the light of floating capacitor voltage balancing issue. Simulation as well as experimental verification of the modulation scheme is carried out on an induction motor drive under various operating conditions. It is shown that this carrier-based modulation technique is suitable for any single source inverter topology with one CHB unit per phase. [ABSTRACT FROM AUTHOR]

This paper proposes a novel method for instantaneous balancing of neutral-point (NP) voltages with stacked multilevel inverters (MLIs) for variable-speed drives. The stacked MLI uses series-connected dc sources and NPs (connecting points of dc sources) to obtain the desired levels. The balancing of NP voltages are obtained by using a low-voltage-capacitor-fed cascaded H-bridge (CHB) per phase of a symmetrical six-phase induction machine (IM), which ensures zero current drawn from NPs (at any given instant). Since no current is drawn from NPs, the single dc-link operation with stacked capacitors is also possible. The scheme is suitable for applications, where low-voltage dc sources and batteries are stacked to form a dc link. A variable-speed operation is done using a seven-level inverter scheme for a symmetrical six-phase IM, which is formed by three dc-link stacked capacitors cascaded with two low-voltage-capacitor-fed CHBs per phase. Furthermore, the method is extended for an open-end IM to obtain a seven-level common-mode eliminated space vector structure using a single dc link. The generalization of this method for any stacked $n$ -level inverter without NP voltage deviation is also presented in this paper. The experimental results and analysis are included to validate the proposed method. [ABSTRACT FROM AUTHOR]

This paper addresses the resonance problem in a parallel-inverter-based grid-connected system. Harmonic interactions between inverters and the grid exhibit various number of complicated characteristics that threatens the system stability and power quality. In this paper, a method of harmonic resonance analysis is proposed to investigate the resonance problem in a multi-inverter grid-connected system based on resonance modal analysis (RMA) and participation factor (PF) calculations. Initially, an impedance-based analytical approach is employed and expanded to an electricity distribution network that is dominated by multiple inverters with LCL filters and proportional resonant controllers. Then, the RMA is introduced to investigate the resonant interactions among inverters and the grid. Specifically, harmonic resonance characteristics are addressed through transfer functions and the proposed method considering variation of inverter number, different inverter combinations, and critical system components. Finally, time domain simulations on the PSCAD/EMTDC platform are presented. The case study results validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Soft magnetic powder cores with their high saturation flux density and low core loss are excellent alternatives for filter inductors in inverter-based applications. However, their nonlinear current-dependent inductance characteristics pose a challenge for control of grid-connected inverter with LCL filter. In this paper, the variable inductance conundrum is discussed and a modified direct digital control method based on a variable-structure inductance estimation model that takes into consideration wide nonlinear variation in both inverter- and grid-side inductances is proposed. The proposed method is shown to have better grid-voltage harmonic rejection and improved stability margins. However, investigation of stability which is conventionally based on nominal values of filter inductors cannot predict instabilities over the entire range of inductance variation. Hence, a parametric approach to conventional stability methods with a parameter space defined by variation in actual and estimated inductance is explored in this paper. The effect of line impedance on stability is also investigated with impedance-based stability criterion by considering line inductance as an additional dimension in the parameter space. A pattern in stability margins is observed due to inductance variation, with inductance at its minimum being most vulnerable to instability. Experimental results measured from a 5 kW single-phase grid-connected inverter with various LCL filters have verified the feasibility of the proposed control method. The experimental results also match the analytical results with reasonable accuracy. [ABSTRACT FROM AUTHOR]

We derive analytically expressions for the probability distribution of the propagation delay time of logic signals through a CMOS inverter when the inverter power supply voltage $V_{{\text{dd}}}$ (more precisely, gate voltage $V_{{\text{GS}}}$) is itself probabilistic for some reason (e.g., system noise or a random external electromagnetic disturbance). Since the propagation delay time is a function of the value of $V_{{\text{dd}}}$ , then some values of $V_{{\text{dd}}}$ may produce values of the inverter propagation delay time that cause the circuit in which the inverter resides to malfunction (e.g., by resulting in an inverter delay time value not conforming with the delay time margins for that circuit). Based upon the derived inverter propagation delay time probability distribution, we then derive a probability for the malfunction of a circuit containing a single inverter subject to such a stochastic $V_{{\text{dd}}}$. In this paper, we present no discussion of the application of our formalism to example probability density functions for $V_{{\text{dd}}}$ ; we rectify this situation by presenting three detailed such examples in a companion paper in this journal. [ABSTRACT FROM AUTHOR]

Many research works report the use of LCL filters for grid-interfaced PV systems. Indiscreet selection of passive LCL filter parameters may result in poor filtering and increased power losses. LCL filter parameter selection is critical to get a good performance and at the same time ensure that losses and costs stay low. This paper proposes a novel approach to design the LCL filter for a grid-interfaced PV system. A cost-based minimization approach on a cost per joule basis for each of the filter elements is explored to obtain the optimal filter design. Simulation results for a 10-kVA grid-connected inverter with an LCL filter are presented in order to verify the efficacy of the proposed design. The impact of designed filter parameters on the ripple current is also discussed in this paper. The total inductance of the LCL filter is varied and filter parameter values which give lowest cost is identified while meeting harmonic standard requirements and the fundamental voltage drop across the filter. Finally, the proposed design is also experimentally validated via a hardware prototype. Results validate the design method in terms of the harmonic attenuation and total harmonic distortion. [ABSTRACT FROM AUTHOR]

In this paper, the theoretical analysis of the position estimation error in position-sensorless operation using pulsating square wave signal injection is presented. The purpose of this paper is to analyze the phenomenon that the position estimation performance varies depending on the injection frequency. Mathematical derivation supported by simulation and experimental results shows that, in no-load condition, the voltage distortion induces position estimation error with 3rd-order harmonics if the frequency of the injected square wave is the same to the switching frequency. While, if the frequency is a half of the switching frequency or heavier load condition in either injection frequency, the effect of nonideal characteristics of inverter disappears and there is no 3rd harmonics in the position estimation error. Therefore, only the effect of cross-coupling inductance is reflected in the estimation error. As the result, the position estimation error appears in the form of 6th-order harmonics due to the spatial harmonics of inductances, and the error is relatively small compared with the switching frequency injection case. The theoretical analysis was verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a new dc–ac converter with a reduced number of semiconductor switches and gate drive and control circuit components. A salient feature of this inverter is its pure sinusoidal line-to-line voltages with no need for output filter. Also, the proposed inverter improves the voltage utilization factor of the input dc supply compared to the four-switch three-phase inverter (first best topology in the literature from the number of switches point of view) and standard six-switch inverter with sine pulsewidth modulation. The proposed inverter is capable of operating with a wide range of output voltages from zero to the full value of the dc input voltage by appropriately altering the instantaneous duty cycle. A three-phase universal inverter with only three power switch has been proposed in this paper for the first time ever, to our knowledge. For the purpose of providing a good compromise between fast transient response and stability, a model predictive controller is proposed. All the design expressions have been derived. A comparison with other dc–ac converters is given to show the merits of the proposed converter. Finally, satisfactory circuit operation is confirmed by experimental results from a laboratory prototype. [ABSTRACT FROM AUTHOR]

In a sensorless vector control method, at low speeds and self-commissioning for AC motors, it is necessary to model dead time, rise/fall time, parasitic capacitors, and semiconductors’ voltage drop accurately. In this paper, voltage drop in each phase due to the nonlinear behavior of the inverter from the control system's viewpoint is precisely modeled by using two lookup tables. These lookup tables make the proposed model robust against the dc-link voltage and carrier frequency variations in the entire current range. In conventional methods at standstill, the measured voltage in the dc current test contains the nonlinearity of two phases. Therefore, to obtain the nonlinear voltage of each inverter phase, it is assumed that the values of inverter nonlinearity voltage at both high current and low current are equal. This simplification leads to distortion in measured voltage in the low currents. However, in this paper, the voltage drop equation of each phase is solved by numerical method that is realizable in conventional processors. The results obtained in actual voltage measurement show the superiority of the proposed method over traditional methods in the low currents. The proposed method is verified by using a voltage source inverter feeding a 2.2-kW three-phase induction motor. [ABSTRACT FROM AUTHOR]

This paper proposes a high-efficiency single-phase T-type boundary conduction mode (BCM) microinverter. The conventional full-bridge BCM microinverter has achieved zero voltage switching (ZVS) and thereby improved the efficiency, but it suffers from high switching losses under light load conditions. The proposed T-type BCM microinverter reserves ZVS and uses a multilevel technique to further decrease the switching losses. The BCM operation with multilevel technique will have too low switching frequency when the grid voltage approaches half of the dc link voltage. To solve this problem, this paper adopts a third operation mode for the T-type switching leg to maintain the switching frequency above a minimum value. The corresponding mode transitions are also detailed to ensure a smooth operation. Because of the turn-offdelay of the freewheeling transistor, the actual lower current boundary deviates from the programmed one, which will distort output current. To address this issue, this paper also proposes a boundary compensation method. A prototype has been built for performance verification, which can test both full-bridge and T-type topology. Compared with the full-bridge BCM microinverter, the proposed T-type BCM microinverter has a higher efficiency over the whole load range. [ABSTRACT FROM AUTHOR]

This paper presents the digital implementation of a boundary controller with unipolar switching characteristic for single-phase voltage source full-bridge inverters. This paper expands the application of a second-order switching surface-based control method to unipolar switching of single-phase voltage source inverters (VSIs) using a finite-state machine. The finite-state machine has been formulated considering four different states of the inverter; positive,zero1, negative, andzero2. The second-order boundary control governs the current state of the system and provides proper switching action to keep the system within the desired reference. The control law is implemented digitally in F28m35x digital control card. A full-bridge inverter topology is used to achieve the three-level voltage switching. Various simulations and experiments were performed in a 550 VA, 120 V, 60 Hz VSI with a digitally implemented controller to verify the theoretical predictions. A high-quality voltage output was obtained for various loading conditions. The transient performance of the controller was investigated using a reference and load changes. A comparison of the implementation was made with the existing classical controllers to verify the fast-dynamic response of the system. [ABSTRACT FROM AUTHOR]

This paper presents a novel configuration for a high-Q tunable waveguide (WG) filter with a constant absolute bandwidth (BW). The key feature of this filter is that it is tuned by a single tuning element. This paper investigates the theory of coupling behavior of single septum and double septa to achieve constant absolute BW. The theory is presented for both the coupling matrix-based and inverter-based filter topologies. The filter design methodology is lucidly presented covering the design strategy adopted for interresonator coupling, input–output coupling and tuning element. The tuning mechanism of the proposed filter is explained with measurement results presented for a $Ku$ -band tunable WG filter designed at 15 GHz with a 2% BW to achieve 15% tuning range. The filter promises to be useful in emerging 5G millimeter-wave applications, where the filter size is very small to accommodate many mechanical tuning elements. Furthermore, the proposed design methodology is scalable, i.e., the tuning mechanism is independent of the filter order. [ABSTRACT FROM AUTHOR]

This paper proposes a split-capacitor four-wire current source inverter, which is the dual of the split-capacitor four-wire voltage source inverter. Since the midpoint of the dc link is tied to the neutral point of ac filter capacitors, the common-mode voltage (CMV) is reduced significantly. Consequently, the leakage current issue is effectively addressed. The proposed circuit is cost-effective as no extra switch is added. This paper, first, establishes the equivalent common-mode circuit of the proposed inverter. The impact of the neutral line inductance on CMV is also analyzed. Then, a specific modulation is introduced to balance the dc-link voltages/currents. To achieve good input/output performance, a nonlinear control method is developed based on time-domain models. Finally, all the proposed methods and related theoretical analysis are verified by simulations and experimental results. [ABSTRACT FROM AUTHOR]

A multifunctional grid-connected inverter (MFGCI) has been a promising solution to mitigate power quality problems in distributed generation system and microgrid applications. However, the capability of a conventional MFGCI is limited when compensating a voltage-based power quality problem. In this paper, a new configuration of MFGCI is proposed to improve both voltage-based and current-based power quality issues and is named shunt-series-switched MFGCI (SSS-MFGCI). The SSS-MFGCI can be connected in series or parallel to the grid by implementing bidirectional switches to provide different modes of operation depending on various grid disturbances. The required compensating current and voltage are analytically derived considering the grid and load conditions for voltage regulation. This paper also presents the compensation strategies, control scheme, and transition scheme between different modes of the proposed SSS-MFGCI. Simulation results to prove the concept are also included for unbalanced sag and swell cases. [ABSTRACT FROM AUTHOR]

Grid-tied inverters and associated control techniques have gained importance in the domain of distributed generation. Different methods of control have been compared in the literature depending upon the ability to meet total harmonic distortion limits, damping offered to resonant oscillations, and stable operation. Multiple-loop methods compared to their single-loop counterparts have proved to be highly effective for meeting the grid regulations. In the literature, while single-loop and two-loop control systems for LCL filter have been assessed, not much study on three-loop control (TLC) has been done, possibly due to the hardware complexity and cost imparted by the increase in the number of sensors. In this paper, a TLC structure for grid connected inverters is analyzed for stability and parametric variations. Robustness of TLC is compared with single-loop grid current feedback control and a two-loop control structure. An approach to estimate the state variables is proposed in this paper, which cuts down the number of sensors and cost of implementation. The performance of the system is analyzed through simulations and results are validated in a hardware setup, under dynamic and steady-state conditions, thereby showing that this is a promising approach. [ABSTRACT FROM AUTHOR]

This paper introduces a new bidirectional single-phase inverter topology. The proposed topology has three ports: a dc port, an ac port, and a ripple port. The ac and dc ports are bidirectional to support rectifier or inverter operation. A small inductor, which is of alternating current, exchanges power between different ports of the single-phase system. The proposed topology is capable of accomplishing voltage step-up or step-down, suppressing the ripple power, and performing inversion or rectification operation in one stage of power conversion. The proposed configuration offers an active decoupling function, which not only eliminates the double-frequency ripple power at the dc port but also achieves minimum capacitance requirements to minimize the size of the decoupling capacitor. This facilitates the use of a very small thin-film capacitor, which offers a much longer life-cycle and higher reliability compared with a bulky electrolytic capacitor. A control approach is also developed to regulate the dc-port and ac-port currents and manage the ripple power through proper distribution of power between each port. Moreover, a very small capacitor can be placed in parallel with the inductor to reinforce the proposed configuration with soft-switching operation, which enhances the overall efficiency and minimizes the voltage stress over the semiconductor devices. This converter is capable of accommodating an arbitrary number of dc or single-phase ac sources and/or loads configuring a multiple-input multiple-output inverter without introducing any additional passive elements or sacrificing the performance of the inverter. The primary focus of this paper is dc-to-ac conversion mode of operation, in spite of the fact that it can easily be configured to serve as a rectifier. Experimental and simulation results are presented to validate the operation of the proposed topology and its control strategy. [ABSTRACT FROM AUTHOR]

This paper proposes a novel discontinuous pulse-width modulation (DPWM) method to reduce the current harmonics and improve the system efficiency for a single-phase three-level neutral-point clamped inverter. In single-phase inverters, the unipolar pulse-width modulation (UP-PWM) method is commonly used. However, this method has the disadvantage of power losses due to numerous switching operations. Conventional DPWM methods reduce the power losses and improve efficiency but increase the current total harmonic distortion (THD). To overcome these weaknesses, this paper proposes a hybrid DPWM switching method combining two PWM methods: the UP-PWM method and the conventional DPWM method called one-pole clamped PWM method. Since the proposed DPWM method offers all the advantages of both PWM methods, the optimal performance—with regard to the power losses and current THD—is obtained. The combination of two PWM methods is investigated by analyzing the power losses and current THD. Based on the analysis, the process determining the optimal operating condition is introduced. The effectiveness of the proposed DPWM method is demonstrated through simulations and experiments. [ABSTRACT FROM AUTHOR]

This paper proposes a new high-efficiency photovoltaic (PV) converter for grid connection through a high-leg delta transformer. The converter is composed of a symmetrically connected boost converter and three half-bridge inverters. One of the three half-bridge inverters is connected to the boost converter, and the others are directly connected to the PV terminals. As a result, this circuit configuration enables to reduce the power losses in both boost converter and inverters. This paper also proposes a new cooperative control method between the symmetrically connected boost converter and inverter. The control method can reduce the average switching frequency to 75% of that in a conventional one, resulting in a great reduction in the switching power loss. Experimental results confirm that the proposed circuit configuration makes it possible to improve its European efficiency from 91.6% to 94.5%. Moreover, system performance is evaluated on the assumption of maximum power point tracking operation. [ABSTRACT FROM AUTHOR]

This paper comprehensively analyzes the stability of a grid-interfacing inverter with the LCL -filter in the discrete domain, where the LCL-filter, along with the controller, is modeled in a polar coordinate. System open-loop and closed-loop poles are analytically studied and expressed in the z -domain. Through the poles movement and distribution analysis, the relationship between system stability and the ratio of resonance frequency over sampling frequency is mathematically revealed and calculated as well as the system control gain limit. Moreover, this paper demonstrates that a grid-voltage feedforward regulator would significantly alter the inverter stability in a weak power system. By means of Jury stability criterion, the stability status under different filter resonance frequency is given. The selection of resonance frequency and filter parameters makes a considerable difference on system behavior. Finally, to improve the robustness against grid inductance variation, a conservative design recommendation of filter parameters and control gain is given. Through the tests on a laboratory-scale prototype, the theoretical analysis is validated by experimental results. [ABSTRACT FROM AUTHOR]

This paper presents theoretical and experimental discussions on low-voltage-ride-through operation of a modular multilevel single-delta bridge-cell (SDBC) inverter intended for utility-scale photovoltaic (PV) systems. Modern grid codes require grid-tied inverters to provide dynamic grid support during grid-fault events by injecting reactive current. This paper discusses decoupled positive- and negative-sequence reactive-current control, focusing on asymmetric voltage sags with imbalanced magnitude and phase relationships. The main objective is to present a feedforward control method based on calculation of the zero-sequence current required for achieving power balance during normal and grid-fault conditions. Moreover, this paper demonstrates a practical method that minimizes overcurrent stress in the three inverter clusters by adjusting active power drawn from PV arrays. Experimental results on a three-phase 12.6-kVA system prove that the SDBC inverter is capable of seamlessly operating through asymmetric voltage sags. [ABSTRACT FROM AUTHOR]

This paper presents the development of methods and mathematical models to determine the DC/AC average efficiency of inverters that allow to optimize estimates of electricity generation of photovoltaic systems. The adaptive hybrid mathematical model of DC/AC average efficiency of inverters of photovoltaic systems proposed in this paper may be composed of three function settings–Linear, Lognormal, and Polynomial–considering the influence of the relative power, which varies with the sizing factor inverter and the DC input voltage. Determining of the average efficiency with a high degree of accuracy guarantees greater reliability in estimating electricity generation of photovoltaic systems. Depending on the specific behavior of each model and topology of inverter, the adaptive hybrid mathematical model determines the fit functions with highest coefficient R 2 , and estimates with accuracy the DC/AC average efficiency that represents the operation and dynamic behavior of the inverters. [ABSTRACT FROM AUTHOR]

This paper presents an intelligent-based fault-tolerant system for a solar photovoltaic (PV) inverter. Artificial neural network based controller is used to monitor, detect, and diagnose the faults in solar PV panels, battery, semiconductor switches, and inverters. The cascaded multilevel inverter is connected across the combination of solar PV panel and battery for dc–ac conversion. The major advantage of the proposed topology is that it can deliver power from source to the load even under fault and partial shaded conditions. The paper also reviews on various faults in the solar PV energy conversion process and provides suitable solutions for each circumstance. Simulations are undertaken in MATLAB 2016a/Simulink and experimental investigation is carried out for a 3 kWp solar PV system. The results have proven that the system is capable to deliver power in spite of faulty environments. The comparison and discussion of the results were made to show the effectiveness of the proposed system. [ABSTRACT FROM AUTHOR]

This paper presents an inverter topology named the multi-source inverter that aims to connect several independent DC sources to the same AC output using a single stage of conversion. This power converter has been developed for applications, such as electrified powertrains. Compared to the conventional hybrid powertrains that use a DC/DC converter to provide an adaptable voltage to the load, the multi-source inverter can drive a traction motor from variable DC voltages without the use of an additional power converter. Hence, the high power DC/DC converter can be dissociated from the traction mode and its use will be restricted to the engine $^{\prime }$ s start-up and regenerative braking, allowing a significant reduction of its power rating from tens of kWatts to a few kWatts. The overall efficiency of the electrified traction drive can also be improved due to the single conversion stage between the battery and the motor. In this paper, the multi-source inverter topology is introduced and its different operating modes are determined through the study of the inverter circuit. Closed-loop control simulations and experiments with a scaled-down prototype and an induction motor were performed to validate the effectiveness of the proposed topology and concept. [ABSTRACT FROM AUTHOR]

This paper proposes generation of a 15-level (14 concentric) dodecagonal voltage space vector structure (DVSVS) for a star connected induction motor drive. The proposed multilevel DVSVS is obtained by cascading two inverters, namely a primary and secondary inverter. The primary inverter is a five-level (5L) structure formed by stacking two three-level flying capacitors with individual reduced dc sources and the secondary inverter is also a 5L structure formed by cascading two capacitor-fed cascaded H-bridges (CHB). The active power is supplied by the primary inverter, while the secondary inverter acts as switched capacitor harmonics filter, and capacitors in the secondary inverter are balanced naturally irrespective of load power factor for entire modulation index. The high-voltage dc supply fed primary inverter is operated in quasi-square wave mode, while the high frequency switching is applied to low voltage CHBs, thus, reducing the overall switching loss. The proposed scheme gives the advantages of both DVSVS and multilevel structure, thus, making it one of the solutions for battery or stacked dc-fed applications. The paper also presents the experimental results as well as comparison study with the existing topologies to support the advantages of proposed scheme. [ABSTRACT FROM AUTHOR]