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Multilevel inverters (MLIs) are widely employed in high power and high voltage applications due to their advantages like better power quality and lower voltage stress across power switches compared to the traditional two-level inverter. However, the utilization of higher number of power semiconductor devices and capacitors results in less reliability. In this regard, a fault-tolerant MLI (FTMLI) with asymmetrical source configuration is developed which can tolerate the open-circuit and short-circuit switch failures (single and multiple switch faults). The proposed topology preserves the output power and balances the capacitor's voltage under pre and postfault conditions. Further, a new switching scheme is proposed to reduce the ripple content in the capacitor's voltage. Further, under postfault operation uses less number of conducting switches resulting in a better efficiency. The proposed FTMLI generates a higher number of voltage levels with less number of component counts when compared with recently proposed topologies. Moreover, this comparative study reveals the prevalence of the proposed converter in the recent literature. The conducted experimental test proves the effectiveness and fault-tolerant feature of the proposed topology. [ABSTRACT FROM AUTHOR]

Reports on the activities of the IES society for young professionals. [ABSTRACT FROM AUTHOR]

This paper presents the current control design procedure of a single-phase grid-tied five-level packed U-cell inverter (PUC5) with an LCL output filter. The PUC5 inverter is used as an interface of renewable energy sources, such as solar applications. The LCL filter is calculated according to the grid-tied operation and converter ratings. An optimal controller, based on a linear quadratic regulator with integral action, is designed to inject a sinusoidal current with low harmonic distortion at unity power factor. For that design, the PUC5 inverter is modelled in the D–Q frame. The sensorless voltage control is incorporated into the switching technique to balance the PUC5 capacitor voltage and generate a five-level waveform at the output. Experimental tests are performed on a laboratory benchmark to confirm the theoretical design. The results prove the efficiency and accuracy of the adopted control strategy in a steady state and under transients of grid current, grid inductance, ac and dc voltage amplitudes. [ABSTRACT FROM AUTHOR]

This paper proposes a novel sensor-less switching method based on logic gates for a five-level packed U-cell (PUC5) converter. It comprises only two level-shifted triangular carriers and logic gates. Hence, the number of triangular carriers is halved and the switching states table is eliminated, which cause remarkable reduction in complexity of the proposed modulation method. Moreover, employing the proposed sensor-less switching method leads to decrease in the PUC5 capacitor value by a factor of carrier ratio, fast self-balancing of the PUC5 capacitor voltage, and halving the output LC filter inductor and capacitor values in stand-alone mode, as well as the grid-link inductor value in grid-connected mode. In addition, the PUC5 converter start-up transient time is considerably decreased and the first switching harmonic cluster frequency is doubled, which lead to notable improvement of steady-state and dynamic performances of the PUC5 converter. The proposed sensor-less switching method has been implemented in both stand-alone and grid-connected operating modes of PUC5 converter. Provided simulation and experimental results verify the feasibility and effectiveness of the proposed sensor-less switching method as well as its improved dynamic and steady-state performances in both stand-alone and grid-connected modes. [ABSTRACT FROM AUTHOR]

This article studies the design and implementation of multi-objective predictive control (MO-PC) of a grid-connected seven-level packed U-cell (PUC7) inverter for minimising the line current total harmonic distortion (THD) and capacitor voltage error simultaneously. The weighting factor method is usually used as a simple method for solving the control problem in the literature. However, there are some difficulties and shortcomings in the calculation of weighting factors. Here, max–min selection strategy with together priority is adopted to reduce these deficiencies and improves the system performance. The switch model of the PUC inverter is derived and then applied in designing the MO-PC for grid-connected applications, where a controlled active or reactive power is injected into the utility. A comparative study among three strategies of weighting factor, fuzzy decision-making and max–min selection is performed to distinguish the proposed method superiority. Experimental results are given to validate the practicality of the applied controller in regulating the line current and capacitor voltage of the grid-connected PUC7 inverter. [ABSTRACT FROM AUTHOR]

In this paper, a fixed switching frequency model predictive current controller is designed and implemented on a sensorless five-level packed U-cell (PUC5) inverter interfacing the utility. The predictive control methodology is based on a voltage vector generation technique for the cost function optimization by nullifying its derivative. The aim of the controller is to allow a fixed-frequency operation with no iterations while operating as a utility line current regulator. A pulsewidth modulator based on employing PUC5 redundant switching states fixes the PUC5 auxiliary dc bus capacitor voltage to its desired level with no necessity of a regulator; the overall technique reduces system complexity with less sensors and no loops in the predictive algorithm making it suitable for industrial applications. System discrete model is developed and experimental results verify the performance of the designed controller in making a fast response in regulating the grid current and balancing the capacitor voltage. [ABSTRACT FROM AUTHOR]

In this paper, a modified selective harmonic mitigation pulse amplitude modulation (SHM-PAM) is presented to be capable of canceling all triplen harmonic orders and suitable for single-phase application of five-level type of voltage source inverters. To this end, a new constraint is established for the two switching angles (α1, α2) to derive the new formula for the harmonics’ amplitude, which results in self-elimination of all triplen harmonics (e.g., 3rd, 9th, 15th, …). The fifth and seventh harmonic orders are mitigated through normal operation of the proposed SHM-PAM technique. It is also shown that the proposed technique is extendable to other multilevel voltage waveforms and a flowchart of self-elimination of all triplen harmonics has been presented. Mathematical analysis supported by experimental investigations show the desired performance of the proposed SHM-PAM algorithm on a two-cell single-phase cascaded H-bridge inverter as a typical five-level configuration in dealing with linear and nonlinear loads. Then, it is demonstrated that the maximum number of harmonic orders would be controlled with the minimum number of available angles in a low switching frequency voltage waveform. [ABSTRACT FROM AUTHOR]

This paper proposes new angles constraint for selective harmonic elimination-pulsewidth modulation (SHE)-PWM utilized in single-phase inverter applications. The proposed angles constraint naturally eliminates all triplen harmonics, whereas they conventionally appeared in all single-phase output voltages. The harmonic amplitude formula is developed based on the obtained angles constraint, and then modified single-phase SHE equations are defined to eliminate only specified low nontriplen harmonics. Therefore, more harmonics would be eliminated with the same switching frequency in modified SHE-PWM. Five-level voltage waveform has been opted to implement the introduced method, but it can be applied on all multilevel voltage waveforms such as seven-level with even number of angles. Unlike the conventional SHE technique, THD is analyzable in modified SHE to find optimum values for switching angles in order to control noneliminated harmonic amplitudes and minimize THD. Moreover, the cancellation of triplen harmonics empowers a single-phase inverter to supply nonlinear loads beside linear ones. Some experimental tests are accomplished using a single-phase five-level modified packed U-cell (MPUC) inverter. Results are discussed to prove the excellent performance of a single-phase MPUC inverter under both normal and harmonic loads and accurate elimination of determined triplen and nontriplen harmonics by applying the presented angles constraint. [ABSTRACT FROM AUTHOR]

In this paper, a novel controller based on the predictive technique has been introduced for a neutral-point-clamped grid-tied inverter. The aim of the developed method is to achieve a constant system switching frequency operation. The proposed method generates the voltage vector by nullifying the derivative of the cost function of a finite-set model predictive controller. The developed controller takes in action of injecting the desired grid current while a closed-loop space vector modulator regulates the capacitors voltages. A comparative study with a similar model predictive algorithm with constant switching frequency (MPC-CSF) is introduced and discussed in terms of computational complexity. Experimental results are provided to verify the performance and robustness of the new method over MPC-CSF with its ability of injecting a very low total harmonic distortion to the grid with almost unity power factor during normal and transient operations. [ABSTRACT FROM PUBLISHER]

In this letter, the five-level packed U-cell (PUC5) inverter is reconfigured with two identical dc links operating as an active power filter (APF). Generally, the peak voltage of an APF should be greater than the ac voltage at the point-of-common coupling (PCC) to ensure the boost operation of the converter in order to inject harmonic current into the system effectively; therefore, full compensation can be obtained. The proposed modified PUC5 (MPUC5) converter has two equally regulated separated dc links, which can operate at no load condition useful for APF application. Those divided dc terminals amplitudes are added at the input of the MPUC5 converter to generate a boosted voltage that is higher than the PCC voltage. Consequently, the reduced dc-links voltages are achieved since they do not individually need to be higher than the PCC voltage due to the mentioned fact that their summation has to be higher than PCC voltage. The voltage balancing unit is integrated into the modulation technique to be decoupled from the APF controller. The proposed APF is practically tested to validate its good dynamic performance in harmonic elimination, ac-side power factor correction, reactive power compensation, and power quality improvement. [ABSTRACT FROM AUTHOR]

This paper presents a closed-loop space vector modulation (SVM)-based sliding mode controller (SMC) for a three-level grid-connected neutral point clamped (3L-NPC) inverter. The nonlinear SMC based on Gao's reaching law has been designed to control the grid current and inject desired amount of active and reactive power into the network. Due to using single dc source at the NPC inverter dc bus, neutral point voltage is controlled through redundant switching states and instantaneous dc voltage feedback integrated into SVM technique. Meanwhile, there is no external voltage controller involved, thus no associated fine tuning issues are existed. The performance of the proposed hybrid controller to inject a desired active/reactive power to the grid is investigated through external perturbations such as change in the line current amplitude/phase shift, ac voltage fluctuation, as well as dc voltage variation. Full converter state-space model was developed and simulated. Experimental results are provided to verify the fast dynamic performance, low content of line current THD%, and good voltage balancing of dc bus capacitors of the NPC inverter. [ABSTRACT FROM AUTHOR]

This paper presents a hybrid selective harmonic mitigation (SHM)-selective harmonic elimination (SHE) switching technique based on pulse-amplitude modulation (PAM) concept. It has been applied on a four-leg neutral-point-clamped (NPC) inverter to eliminate and mitigate more harmonic orders than recently proposed hybrid SHM-SHE-pulse width modulation (PWM) method while generating switching pulses at the same frequency. In conventional SHE and SHM techniques, equations are solved to attain the switching angles. Regarding the PAM, value of inverter dc voltage can be considered as an additional degree of freedom by which the flexibility of such techniques would be increased maintaining the switching frequency. In the proposed SHM-SHE-PAM, the conventional equations are reformulated to obtain constant switching angles for a vast range of modulation index (ma) applied on a four-leg NPC inverter. Switching pulses of the three-phase legs and the fourth leg are calculated to mitigate the nontriplen harmonics and eliminate the triplen ones, respectively. Due to the unique switching angles valid in the whole range for ma, the calculation time and volume (storage capacity) are significantly reduced leading to a simpler controller implementable on a low-risk and cheap AVR chip. Experimental tests’ results of a four-leg NPC inverter as UPS application prove the good dynamic performance and accuracy of the proposed implemented switching technique in producing associated pulses for the inverter switches at very low frequency to mitigate/eliminate undesired harmonic orders from the output phase/line voltage waveforms without using bulky filters. [ABSTRACT FROM AUTHOR]

In this study, a real-time controller implementing closed-loop space vectormodulation (CLSVM) technique combined with DC bus voltage control is applied on a three-phase four-wire three-level neutral point clamped (NPC) inverter to be used in sustainable energy conversion systems with higher efficiency and lower switching frequency than conventional two-level inverters. The redundant switching states of CLSVM is dictated based on the voltage feedback obtained from DC link in order to constantly balance the DC bus capacitors voltages under various load conditions. The main feature of this technique is the use of only voltage sensors for balancing the DC capacitors voltages in four-wire system which makes the algorithm act fast, while permitting reduction of DC bus capacitors voltage ripple significantly and therefore smaller capacitors are used. The NPC inverter has been modelled and effects of redundant switching states have been analysed mathematically on DC capacitors voltages. Comprehensive experimental tests are conducted on a four-wire three-leg NPC operating as uninterruptible power supply application for sustainable energy conversion to validate the designed approach and the dynamic response under various load conditions including linear, non-linear, balanced, unbalanced and motor loads. [ABSTRACT FROM AUTHOR]

This paper proposes a sliding-mode control (SMC)-based scheme for the variable-speed direct-driven wind energy conversion systems (WECS) equipped with a permanent magnet synchronous generator connected to the grid. In this paper, diode rectifier, boost converter, neutral point clamped inverter, and L filter are used as the interface between the wind turbine and grid. This topology has abundant features such as simplicity for low- and medium-power wind turbine applications. It is also less costly than back-to-back two-level converters in medium-power applications. The SMC approach demonstrates great performance in complicated nonlinear systems control such as WECS. The proposed control strategy modifies reaching law (RL) of the sliding mode technique to reduce chattering issue and to improve total harmonic distortion property compared to conventional RL SMC. The effectiveness of the proposed control strategy is explored by simulation study on a 4-kW wind turbine, and then verified by experimental tests for a 2-kW setup. [ABSTRACT FROM PUBLISHER]

A multilevel boost power factor correction (PFC) rectifier is presented in this paper controlled by cascaded controller and multicarrier pulse width modulation technique. The presented topology has less active semiconductor switches compared to similar ones reducing the number of required gate drives that would shrink the manufactured box significantly. A simple controller has been implemented on the studied converter to generate a constant voltage at the output while generating a five-level voltage waveform at the input without connecting the load to the neutral point of the dc bus capacitors. Multicarrier pulse-width modulation technique has been used to produce switching pulses from control signal at a fixed switching frequency. Multilevel voltage waveform harmonics has been analyzed comprehensively which affects the harmonic contents of input current and the size of required filters directly. Full experimental results confirm the good dynamic performance of the proposed five-level PFC boost rectifier in delivering power from ac grid to the dc loads while correcting the power factor at the ac side as well as reducing the current harmonics remarkably. [ABSTRACT FROM PUBLISHER]

This paper presents a new family of buck type power factor corrector rectifiers that operates in continuous-conduction mode (CCM) and generates a multilevel voltage waveform at the input. Due to CCM operation, commonly used ac-side capacitive filter and dc-side inductive filter are removed from the proposed modified packed U-cell rectifier structure. Dual dc output terminals are provided to have a five-level voltage waveform at the input points of the rectifier where it is supplied by a grid via a line inductor. Producing different voltage levels reduces the voltage harmonics which affects the grid current harmonic contents directly. Low switching frequency of the proposed rectifier is a distinguished characteristic among other buck type rectifiers that reduces switching losses and any high switching frequency related issues, significantly. The proposed transformerless, reduced filter, and multilevel rectifier topology has been investigated experimentally to validate the good dynamic performance in generating and regulating dual 125-V dc outputs terminals as telecommunication boards feeders or industrial battery chargers under various situation including change in the loads and change in the main grid voltage amplitude. [ABSTRACT FROM PUBLISHER]

In this paper, a new cascaded nonlinear controller has been designed and implemented on the packed U-cell (PUC) seven-level inverter. The proposed controller has been designed based on a simplified model of PUC inverter and consists of a voltage controller as an outer loop and a current controller as an inner loop. The outer loop regulates the PUC inverter capacitor voltage as the second dc bus. The inner loop is in charge of controlling the flowing current, which is also used to charge and discharge that capacitor. The main goal of the whole system is to keep the dc capacitor voltage at a certain level results in generating a smooth and quasi-sine-wave seven-level voltage waveform at the output of the inverter with low switching frequency. The proposed controller performance is verified through experimental tests. Practical results prove the good dynamic performance of the controller in fixing the PUC capacitor voltage for various and variable load conditions and yet generating low-harmonic seven-level voltage waveform to deliver power to the loads. Operation as an uninterruptible power supply (UPS) or ac loads interface for photovoltaic energy conversion applications is targeted. [ABSTRACT FROM AUTHOR]

In this paper, a model-predictive control (MPC) has been designed and implemented on the packed U-cell (PUC) inverter, which has one isolated dc source and one capacitor as an auxiliary dc link. The MPC is designed to regulate the capacitor voltage at the desired magnitude to have seven voltage levels at the output of the inverter. Since grid-connected application is targeted by this application, the inverter should be capable of supplying requested amount of active and reactive power at the point of common coupling (PCC) as well. Therefore, MPC should also consider the line-current control to monitor the exchange of reactive power with the grid while injecting appropriate active power at low total harmonic distortion (THD). Various experimental tests including change in dc-source voltage, active power variation, and operation at different power factor (PF) have been performed on a laboratory prototype to validate the good performance obtained by the proposed MPC. The dynamic performance of the controller during sudden changes in dc capacitor voltage, supply current, and PF demonstrates the fast and accurate response and the superior operation of the proposed controller. [ABSTRACT FROM PUBLISHER]

Uninterruptible power supply (UPS) is the promising candidate to ensure high quality of delivering power from renewable energy resources to the AC loads especially sensitive and critical ones. In this study, optimised harmonic elimination modulation technique is extended to four-leg neutral-point-clamped (NPC) with neutral point voltage balancing and it has been experimentally validated. It is demonstrated that by controlling the fourth leg to eliminate more harmonics while operating at low switching frequency makes the four-wire NPC an ideal candidate for an UPS application in energy conversion systems. Non-triplen, 5th to 23rd harmonics orders from the output voltage waveforms are eliminated by optimally computing the switching angles for phase legs. Moreover, switching angles computed for the fourth leg are also considered to help removing triplen harmonics containing 3rd, 9th, 15th, and 21st orders from phase voltages. It is mathematically proved that the designed selective harmonic elimination technique ensures the neutral point voltage balancing. The obtained experimental results confirm the proper performance of the proposed technique in supplying symmetrical/asymmetrical loads from renewable energy resources while keeping the inverter neutral point voltage balanced. [ABSTRACT FROM AUTHOR]

In this paper, a new mode of operation has been introduced for packed U-cell (PUC) inverter. A sensor-less voltage control based on redundant switching states is designed for the five-level packed U-cell (PUC5) inverter, which is integrated into switching process. The sensor-less voltage control is in charge of fixing the dc capacitor voltage at half of the dc source value results in generating symmetric five-level voltage waveform at the output with low harmonic distortion. The sensor-less voltage regulator reduces the complexity of the control system, which makes the proposed converter appealing for industrial applications. An external current controller has been applied for grid-connected application of the introduced sensor-less PUC5 to inject active and reactive power from inverter to the grid with arbitrary power factor, while the PUC auxiliary dc bus is regulated only by sensor-less controller combined with new switching pattern. Experimental results obtained in stand-alone and grid-connected operating modes of proposed PUC5 inverter prove the fast response and good dynamic performance of the designed sensor-less voltage control in balancing the dc capacitor voltage at desired level. [ABSTRACT FROM PUBLISHER]

In this paper, selective harmonic mitigation (SHM) and selective harmonic elimination (SHE) modulation techniques have been combined to produce appropriate pulses for a four-leg three-level neutral point clamped (NPC) inverter. In the proposed hybrid method, the SHM technique is applied to phase legs in order to mitigate non-triplen harmonics (fifth, seventh, $\ldots$) , and the fourth leg is controlled by the SHE principle to eliminate important low-order triplen harmonics (third, ninth, $\ldots$) completely. In this context, it is demonstrated that using SHM instead of an SHE modulator for phase legs leads to a reduced vast range of harmonic orders in the NPC output voltage, significantly. Moreover, it is proved that the presented hybrid switching technique has the ability of dc-bus capacitor voltage balancing, inherently. Furthermore, analyses are performed to calculate the dc-bus capacitances accurately to show 5% voltage ripple acceptably. Simulation and experimental results validate a good dynamic performance of the proposed hybrid modulation technique in firing the NPC inverter switches to generate low-harmonic voltage waveforms supplying balanced and unbalanced loads while balancing the dc capacitor voltages in a four-wire network with small dc-link capacitors. [ABSTRACT FROM PUBLISHER]

Power converters produce a vast range of harmonics, subharmonics and interharmonics. Harmonics analyzing tools based on the Fast Fourier Transform (FFT) assume that only harmonics are present and the periodicity intervals are fixed, while these periodicity intervals are variable and long in the presence of interharmonics. Using FFT may lead to invalid and undesired results due to the above mentioned issues. They can also lead to problems such as frequency blending, spectral leakage and the picket-fence effect. In this paper, the group-harmonic weighting (GHW) approach has been presented to identify the interharmonics in a power system. Afterwards, a modified GHW has been introduced to calculate the proper bandwidth for analyzing the various values of interharmonics. Modifying this method leads to more precise results in the FFT of a waveform containing inter harmonics especially in power systems with a fundamental frequency drift or frequency interference. Numerical simulations have been performed to prove the efficiency of the presented algorithm in interharmonics detection and to increase the accuracy of the FFT and the GWH methods. [ABSTRACT FROM AUTHOR]

Excitation system of synchronous generator can help to an effective voltage control and power system stability support. In fact, for the stability improvement, this excitation system should be able to apply proper input voltage to the generator field winding. In this paper limiters and protective circuits of generator excitation system are explained. Two important types of these limiters are over and under excitation ones. The over-excitation limiter protects the generator against over-temperature caused by high field current for long time. Under-excitation limiter prevents the decrease of generator field current to the values lower than the stability margin or the thermal limit of end side of the stator core. Various models of the over and under excitation limiters have been presented in the literature. Tuning these limiters can be performed in different ways by considering the generator,s capability curves. The limiter models and their parameters tuning affect voltage stability straightly. In this research, after implementation the proper model for our case study, the tuning of limiters is obtained by analyzing well known voltage stability methods. Some simulations have been done in Matlab/Simulink to analyze the effect of the selected model and tuned limiters on voltage stability both in single and multi-machines system. Also the dynamic parameters of a real system (Bistun power plant) are used as the case study. The results issued from this study are also discussed and validate the simulations. [ABSTRACT FROM AUTHOR]

Hysteresis Current Control (HCC) is widely used due to its simplicity in implementation, fast and accurate response. However, the main issue is its variable switching frequency which leads to extraswitching losses and injecting high-frequency harmonics into the system current. To solve this problem, adaptive hysteresis current control (AHCC) has been introduced which produces hysteresis bandwidth which instantaneously results in smoother and constant switching frequency. In this paper the instantaneous power theory is used to extract the harmonic components of system current. Then fixed-band hysteresis current control is explained. Because of fixed-band variable frequency disadvantages, the adaptive hysteresis current control is explained that leads to fixing the switching frequency and reducing the high-frequency components in source current waveform. Due to these advantages of AHCC, the switching frequency and switching losses will be diminished appropriately. Some simulations are done in MATLAB/Simulink. The Fourier Transform and THD results of source and load currents and the instantaneous switching frequency diagram are discussed to prove the efficiency of this method. The Fourier Transform and THD results of source and load currents are discussed to prove the validity of this method. [ABSTRACT FROM AUTHOR]