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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]

Conventional model predictive control (MPC) for three-level active neutral point clamped converter (ANPC) utilizes weighting factors to achieve the optimal control of multiple control objectives in a single cost function. However, the selection of these weighting factors has an impact on the performance of the controlled objectives significantly. Tuning of weighting factors is usually tedious and lacks theoretical background. Moreover, the predictive and evaluation process for multiple objectives optimization have to be carried out by enumerating all admissible switching states within one loop. To simplify the complexity of this process and eliminate the effect of weighting factors, this paper proposes a cascaded predictive control scheme for a 3L-ANPC inverter fed induction machine (IM) drive. With the proposed approach, three separate cost functions for three control objectives, stator flux vector, neutral point voltage and device loss distribution are evaluated in a cascaded way, so that the optimization of these control objectives is performed independently. A 3L-ANPC inverter driven IM drive is studied in this paper to verify the effectiveness of the proposed algorithm. Experimental results in different operating conditions confirm that the proposed method achieves satisfactory steady-state and transient performances comparable to the conventional predictive control approach. The weighting factors used in the conventional method are eliminated and the computational effort is reduced by 68% compared to the conventional method without any sacrifice in the drive performance. [ABSTRACT FROM AUTHOR]

Power sharing amongst sources can be improved in an islanded microgrid by utilizing the information of the network’s impedance. In this paper, an impedance based model reference adaptive system (Z-MRAS) scheme is proposed to estimate the impedance angle for converter based distributed generators (DGs) in the islanded mode. The focus is to enhance the proportional apparent power sharing between the DGs operated using generalized droop control (GDC). The performance of GDC depends on the effective tie-line impedance between the DGs and the proposed MRAS scheme provides the feedback of the estimated impedance angle. The accuracy of the impedance angle estimation is established for various tie-line $X/R$ ratios, and this results in enhanced power sharing performance for balanced, harmonic, unbalanced, dynamic and constant power load cases as well as parallel line and underground cable cases. The proposed method is validated through off-line simulations in MATLAB and controller hardware in loop (CHIL) using real time simulator (Opal-RT) including cases of meshed networks and DG plug and play for a modified CIGRE network. [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]

This paper presents a dual-hysteresis band control method for multi-drive systems based on a nine-switch inverter. The drive system contains two independent three-phase induction motors fed by a nine-switch inverter. Independent control of torque, speed, and stator current of two load stages is achieved. The proposed strategy offers a reliable control operation of both induction machines under individual load profiles. Instead of using independent sets of three-phase voltage source converters, the nine-switch inverter is used to drive separate machines. Thus, the volume and size of the power stage are noticeably reduced due to the use of fewer semiconductors. The theoretical concept and design steps of the proposed method are presented. The performance of the closed-loop independent motor control scheme is experimentally validated using a 3.2 kW lab-scaled nine-switch inverter prototype and Altera Cyclone IV Field-programmable gate array. [ABSTRACT FROM AUTHOR]

Due to the second order harmonic in the phase voltage, the voltage peak value is much higher than the amplitude of its fundamental component in the 6/4 variable flux reluctance machines (VFRMs). Conventionally, the amplitude of the fundamental component in the phase voltage is used to calculate the voltage constraint in flux-weakening control, which results in a large voltage margin and reduces the DC link voltage utilization. To deal with this problem in the VFRMs, the flux-weakening control considering the second order harmonic of the phase voltage is proposed in this paper. In this method, the voltage peak value is used to calculate the current operating points, by using which the maximum voltage utilization in flux-weakening region can be achieved. It shows by using the proposed method a wider the constant torque region owing to the lower voltage margin in the VFRMs, which exhibits higher power output in flux-weakening region. Finally, the method is verified by the test results in the VFRM. [ABSTRACT FROM AUTHOR]

Rotor fault is a condition that may occur in induction motors due to high forces and stresses on rotor bars or end rings. When this occurs, the machine may lead to a forced outage in production lines. Several methods have been proposed in the literature for the detection of rotor faults; however, they cannot work under variable frequency conditions, and they are very sensitive to mechanical load oscillations. This paper presents a novel frequency normalization methodology for reliable rotor defect diagnosis in variable-frequency drive-fed systems. The normalization is implemented by demodulating the stator current signal with a time-variant factor. As a result, the fundamental component of the stator current is normalized in a constant frequency value, whereas avoiding spectral leakage and frequency smearing. This enables the transient stator current signature analysis to accurately locate targeted fault signatures. The effectiveness of the proposed frequency normalization approach is demonstrated by analyzing the stator current of motors under non-stationary conditions. Experimental results show that the proposed methodology can detect and distinguish between load oscillations and rotor fault signatures for avoiding false rotor fault alarms. [ABSTRACT FROM AUTHOR]

The grid-connection point of photovoltaic inverters may exhibit inductive characteristics (i.e., a weak grid) due to long transmission cables as well as multiple transformers. A large grid impedance can arouse impedance-based stability problems, sustained resonances and impose power-delivery limits. This paper discusses an impedance-compensated synchronous reference- frame-PLL-angle-correction method. The method presented in this paper compensates the PLL angle according to estimated or measured grid impedance in order to achieve improved performance in weak grids. Due to the lack of explicit small-signal impedance models around the aforementioned topic, this paper presents a complete small-signal transfer function model to analyze the impedance-based stability of the PLL-angle compensation method. The results are validated with real-time hardware-in-the-loop simulations and laboratory experiments. [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]

The main drawbacks of conventional direct torque control (DTC) drives are erratic switching frequency and high torque ripples. The multilevel inverter fed DTC drives are proved to be an alternative for eliminating these drawbacks. Apart from the commonly used three-level inverter fed drives, this paper presents an even-level inverter fed DTC drive. In this paper, a space vector modulated DTC algorithm for a four-level DTC drive is proposed and is implemented by feeding both ends of an open-end-winding induction motor with two-level inverters. The space vector modulation (SVM) algorithm uses a fractal based design for switching voltage vector generation and allied computations are performed in 60 degree co-ordinate system. Representation of vectors as integer values in 60 degree co-ordinate planes avoids fractional arithmetic, which simplifies the algorithmic computations. Detailed analyses of the drive performance under dissimilar operating modes are presented. The loading performance of the drive is experimentally verified on a 5-HP induction motor with a strain gauge type mechanical loading system. No-load experimental results were obtained from a laboratory model 2-HP induction motor drive operated in closed loop mode. [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]

Islanded microgrids have low inertia due to large penetration of non-inertial inverter based power sources. Such systems are prone to instability due to a higher rate of change of frequency (RoCoF) and large peak frequency deviation in case of a sudden load change or generation loss. This paper proposes a novel fast frequency control method for an islanded microgrid comprised of both inverter based distributed generators (DGs) and synchronous generators (SGs). The frequency controller estimates the power imbalance during the transient and compensates for it through a battery energy storage system (BESS). For the estimation of the power imbalance, an approximated linear model of the microgrid is developed considering multiple SGs operating in parallel. To transfer the compensated power from BESS to SGs, and also to maintain the state of charge (SoC) of the BESS, a SoC balancing control method is presented which is implemented by controlling the biased frequency of the SGs. The performance of the proposed frequency control method is verified using Typhoon real-time hardware-in-the-loop simulator. [ABSTRACT FROM AUTHOR]

To improve the torque accuracy for open-end winding permanent magnet synchronous motor (OEW-PMSM) with common dc bus, it is necessary to eliminate the zero-sequence torque (ZST) caused by zero-sequence current (ZSC) and the third back electromotive force. But the third flux linkage mismatch can enlarge ZSC which causes torque ripple. To deal with the above problem, firstly, a q-axis current injection method combined with deadbeat predictive current control (DPCC) is adopted to inject an extra current into the reference q-axis current. The extra torque generated by this injected current can compensate for ZST. Secondly, to ensure the accuracy of the q-axis injected current, a parameter identification method based on recursive least square (RLS) algorithm with forgetting factor is presented to identify the third flux linkage accurately. The identification value can reduce ZSC and torque ripple under different working conditions. And the accurate q-axis injected current can be obtained to counteract ZST and output accurate torque. Finally, three methods are comparatively analyzed in this paper, namely the traditional DPCC method, the DPCC method with q-axis current injection and the proposed DPCC method based on RLS with forgetting factor. The simulation and experimental results verify the effectiveness of the proposed DPCC method. [ABSTRACT FROM AUTHOR]

Grid connected inverter controllers require the magnitude and phase of the point of common coupling voltage for the generation of unit vectors from a phase locked loop. However, weak grid conditions or grid voltage sensor failures might substantially increase control difficulty in the inverter due to unknown grid impedance which causes instability in the phase locked loop. In this paper, an internal model based grid voltage estimation architecture has been presented. The proposed control architecture relies on the current measurements along with nominal values of filter parameters for the point of common coupling voltage estimation and unit vector generation. Detailed stiffness and robustness analyses are presented to show that the proposed control and estimation architecture is nearly insensitive to parameter variations. In comparison to conventional current control with both voltage and current sensors, the proposed current control scheme provides better transient response during both ideal and non-ideal grid voltage conditions. Experimental results on a reduced scale laboratory prototype validate the efficacy of the proposed architecture. [ABSTRACT FROM AUTHOR]

This paper presents a new single-source three-phase seven-level inverter with an inherent three-time voltage boosting capability for medium-voltage applications. The proposed topology is comprised of series connection of two switched-capacitor (SC) networks with an added half-bridge module per phase. Each of such integrated SC networks requires a single capacitor associated with three active power switches. The three-time voltage boosting feature of the proposed topology can reduce the dc-link voltage requirements by 50% in comparison to the traditional neutral point clamped (NPC), flying capacitors, active NPC (ANPC), hybrid clamped ANPC, and cascaded H-bridge topologies, and 75% to advanced ANPC topologies. It can also reduce the number of required switches and capacitors as well as their voltage stresses compared to those state-of-the-art topologies reported in the literature. By integrating “n” number of added SC networks in the proposed seven-level basic topology, the number of output voltage levels can be extended to 2n+3 per phase. A finite control set model predictive control algorithm is also derived to control the converter in both the active and reactive power exchanges modes without distorting the generated grid current quality. The capacitor voltage balancing is inherent in the proposed topology, and thus, there is no need for any additional voltage balancing circuit, which further reduces the control complexity. The performance of the proposed topology and its control algorithm are validated by several measurement results. [ABSTRACT FROM AUTHOR]

Interactions of identical inverters in weak grids have received wide attentions. However, in a practical wind farm, parameters of grid-side inverters and lengths of collecting lines are not completely the same. In this case, a multi-inverter system is not symmetric, and thus the interaction mechanism could be more complicated than that of identical inverters. To investigate the interaction mechanism of differently parameterized inverters, this paper proposes a clustering-based modeling method, in which inverters with identical parameters are classified as one group. The clustering-based model allows for efficient stability assessment using three low-order transfer function matrices, instead of a full-order dynamic model. The root locus analysis is applied in the clustering-based model for stability analysis. Results show that the interconnection of distinct groups of inverters deteriorates common current stability, but has no impact on interactive current stability. The simulation and rtlab-based experimental results validate the effectiveness of the proposed model and the interaction mechanism of differently parameterized inverters. [ABSTRACT FROM AUTHOR]

The open-end winding permanent magnet synchronous motor (OW-PMSM) with common DC bus configuration provides a path for zero-sequence current (ZSC). Apart from the zero-sequence voltage (ZSV) generated by inverters modulation, ZSC also comes from the third harmonic back-EMF. However, the third harmonic back-EMF contains the position information of the rotor, which can be extracted by an appropriate strategy to achieve position sensorless drive. In order to achieve sensorless drive and ZSC suppression simultaneously, this paper proposes a novel general-integral extended state observer (GI-ESO) based method. The third harmonic back-EMF of the OW-PMSM is estimated and calculated in three-phase stationary coordinate system by GI-ESO. And then, a single-phase phase lock loop observer (SPLL) is exploited to extract the position and speed information of the rotor. Meanwhile, the estimated third harmonic back-EMF is directly fed forward to SPWM as the ZSV reference, then the output voltages of the inverters contains the ZSV that can exactly counteract the third harmonic back-EMF, so as to realize the suppression of ZSC. Compared with the method of combining the fundamental back-EMF detection and proportional resonance (PR) regulator, this method can realize sensorless drive and ZSC suppression at the same time with only one scheme. Benefiting from the use of GI-ESO, the observer bandwidth can be relatively low, which helps suppress noise and extend the speed range of sensorless operation. Moreover, this method has strong robustness against motor parameters. The proposed strategy is validated by experiments. [ABSTRACT FROM AUTHOR]

–This paper proposes an efficient and optimal reduced control set model predictive flux control (RCS-MPFC) for a three-level neutral-point-clamped voltage source inverter (3L-NPC VSI) fed induction motor. The proposed algorithm reduces the computational time in the prediction stage without causing any suboptimality. The optimal voltage vector selected by the proposed method produces the same cost function value as that of the conventional FCS-MPFC which requires enumerating all 27 voltage vectors. Therefore, the proposed algorithm achieves the same performance as the conventional method in the entire range of operation of IM drives while the computational effort is significantly reduced. Experimental results verify the effectiveness of the proposed algorithm and its superior performance compared to the existing RCS-MPFC scheme. [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]

In this paper, we present two methods to identify the admittance of a 2.3-MVA commercial grid-following inverter through frequency-domain data and time-domain data. In addition to the well-known harmonic injection method to obtain the admittance at frequency points followed by vector fitting, we adopt a step response-based method where the step response data of the converter are collected and their $s$ -domain expressions are obtained. In turn, the $s$ -domain admittance model of the converter is found. The two methods can be used for cross validation. Step responses and frequency-domain responses obtained from the two methods are compared. Results show that the time-domain data-based method leads to an admittance comparable with that from the frequency-domain measurement in the range less than 60 Hz. In addition, two insights are obtained from the frequency-domain admittance measurements. First, when the $dq$ -frame is aligned to the point of common coupling (PCC) voltage, the per unit DQ-domain admittance directly reflects operating conditions. Second, existence of negative-sequence control can be detected via sequence-domain admittances. [ABSTRACT FROM AUTHOR]

A new transformer-less three-level grid-tied inverter is proposed in this article. By employing the switched-capacitor (SC) modules, a new topology is derived. The proposed topology has some features which are high efficiency, the capability of voltage boosting without utilization of any additional boost stage, and suppressing the leakage current by using common grounded strategy. In this article, a peak current control is employed to generate the gate pulses of power semiconductor switches and adjustment of both active and reactive powers. By doing so, the grid injected current is controlled and adjusted to have a proper quality. The operation modes, design consideration, and a comparison with some recently proposed grid-connected topologies are done through this paper. Finally, the performance and feasibility of the proposed inverter are confirmed with a 500 W experimental prototype. [ABSTRACT FROM AUTHOR]

In this paper, a novel and non-invasive stator inter-turn short circuit (ITSC) online detection method is presented for an induction machine (IM), driven by a two-level voltage source inverter (2L-VSI) via finite control set model predictive control (FCS-MPC). The main idea of the proposed method is to utilize the controller itself as an observer: under the presence of a fault, the distribution of inverter switching states significantly deviates from the original balanced case. Therefore, by inspecting the inverter switching vectors, which are the outcomes of the FCS-MPC routine's online optimization procedure, a stator fault can be detected efficiently. It is observed that both the zero-vector allocation over the complex plane and the allocation among the active vectors are influenced by the presence of a stator short-circuit fault. The proposed fault detection strategy introduces little to no extra burden for processor and memory. Experimental results verify the proposed method, and inter-turn short circuits of two and three turns are confidently detected and located for a 500 W, two-pole IM. [ABSTRACT FROM AUTHOR]

This article presents a potential wind energy conversion system (WECS) using the recently proposed brushless induction excited synchronous generator (BINSYG), which can operate as a self-excited synchronous-machine. In BINSYG, an induction-machine (IM) and a synchronous-machine (SM) are wound for different number of poles and are embedded in the same machine structure. By operating the SM in self-excited mode, dc-link voltage of grid-side inverter can be controlled smoothly from the stator side of the IM. A set of conditions has to be maintained for stable operation in self-excited mode. This is particularly critical when the machine needs to be operated in wide variation in speed. So far, no stability-study is available to explore the stable-operating-zone, when the machine operates in self-excited mode. Therefore, in this paper stability of self-excited BINSYG is investigated to check suitability of the proposed system for wind energy applications. The study clearly indicates the presence of a critical speed (i.e. a minimum speed below which self-excitation is not possible). The movement of eigen values are presented to show the influence of several parameters on stable behavior of the system. Simulations using PLECS and supporting experimental-results using a laboratory-prototype (configured for 4/6 poles, 5.5 kVA) have been presented. [ABSTRACT FROM AUTHOR]

This article proposes a Virtual State Damping (VSD) plugin to output current controllers of distributed generation (DG) inverters. The VSD is designed to suppress the resonance usually present in LCL-type grid-connected DG systems, providing robustness to the control system against grid impedance uncertainties or variations. Typical active damping techniques apply an equivalence impedance model to design their structures. The proposed VSD has the advantage of not needing to take such analysis into account for its design, i.e., it is not dependent on the LCL filter's physical model. The VSD structure applies the model reference theory, and it does not require additional sensors for its implementation. The paper also presents the VSD design methodology and stability analysis. Real-time domain emulation (RTDE) results and 5kW lab-prototype experimental results demonstrate the effectiveness of the proposed technique. [ABSTRACT FROM AUTHOR]

This article investigates discharge bearing currents in inverter-fed electric motors. Industrial reports indicate that among prevalent bearing failures, bearing currents are one of the influent drivers leading to a variety of tribological issues and premature wear in bearings. While the root-causes of bearing currents have been extensively reported over the years, the response of bearing under voltage stress and the factors that influence bearing endangerment through electric discharges have been far less studied. An online measurement of the bearing voltage is proposed on a motor test-bench to estimate both the discharge activity and the energy of the discharges. The designed test bed allows reproduction of representative bearing currents leading to electrical micropitting. This paper presents extensive measurements results that highlight the critical operating conditions and parameters that worsen the discharge phenomenon inside bearings. Testing protocols are performed to quantify the influence of each parameter separately. While the bearing voltage magnitude play a significant role in the discharge phenomenon, other parameters such as successive startups, bearing temperature, and type of grease deserve appropriate investigations to avoid or mitigate the well-known shortened bearing life under inverter operation. [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]

More than 170 countries have already established renewable energy targets to meet their national increasing energy demand and also to keep their environment sustainable. Due to a number of features, the use of the multi-megawatt solar photovoltaic (PV) power plants is becoming the preferred choice for escalating and updating the power systems all over the world. Moreover, the solar PV power plant is also the first choice for meeting rapidly growing demands; as it can be installed relatively quickly, say in 6–12 months, compared to that of the fossil-fuel-based plants that may require more than 4–5 years. The traditional low-voltage (288–690 V) converter-based system requires a step-up transformer and a line filter to interconnect a solar PV power plant with medium-voltage grids. Recently, the use of medium-voltage converters without a step-up transformer and a line filter has become more attractive for direct medium-voltage grid integration of solar PV power plants. This paper aims to review the necessity and the technical challenges in developing medium-voltage power electronic converters, including the converter circuit topologies and control techniques used in the development of medium-voltage converters to interconnect solar PV power plants to medium-voltage grids directly. In this paper, a comprehensive review of the current research activities and the possible future directions of research to develop medium-voltage converter technologies to provide for a cost-effective grid integration of solar PV power plants are presented. [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]

The dual-loop control strategy in a hybrid reference frame (HRF) for single-phase voltage-source inverters in islanded operation mode is studied, which applies a capacitor voltage shaping loop in the synchronous reference frame (SRF) and a capacitor current shaping loop in the stationary reference frame (HRF-based v+ ic control strategy). This strategy is able to achieve the purpose of active damping, fast dynamic response, and zero reference tracking error. However, due to the inherent characteristics of SRF-based voltage loop and the digital control delay, the performance of the system is degraded and the control parameter design of the HRF-based v+ ic control strategy shows great difficulties. To overcome these shortcomings, in this paper, a systematic parameter design guideline for the HRF-based v+ ic control strategy is proposed to ensure the system stability and optimize the performance of the system under control delay condition. The mathematic model of the HRF-based v+ ic control strategy is established with the consideration of control delay in this paper. Based on this model, a satisfactory region of the system stability indexes can be obtained by stability specifications of the system, and the optimal control parameters can be calculated according to the stability indexes selected from the satisfactory region. By using this method, the system stability and robustness can be guaranteed. Finally, the experimental results are presented to validate the effectiveness of the presented optimal control parameter design methodologies. [ABSTRACT FROM AUTHOR]

Phase compensated proportional resonant (PR) controllers with phase leading angles embedded into resonant controllers, are used to improve the system stability, when the grid-connected inverter is connected to a weak grid with large grid impedance. However, an inappropriate phase leading angle will cause obvious anti-peak in the magnitude response of phase compensated PR controller. Thus, the phase leading angle should be selected carefully. The conventional phase compensation (CPC) method for selecting the phase leading angle can maximize the system phase margin, but at the expense of large peaks in the magnitude response of sensitivity transfer function. Meanwhile, the bandwidths of resonant controllers will be reduced. In the paper, an improved phase compensation (IPC) method is proposed, which does not cause peaks in the magnitude response of sensitivity transfer function while providing sufficient phase margin. Experimental tests were carried out on a single phase grid-connected inverter. The effectiveness of the proposed method is proved in terms of the steady-state and transient performances, as well as the performance under the grid frequency deviation. [ABSTRACT FROM AUTHOR]

Recent technical advances in control, protection and interconnection of distributed power generation units imply that it is practically viable and economically profitable to keep them as backup generators in isolated operating modes. Therefore, along with the development of islanding detection techniques, seamless operation in transition between islanded and grid connected modes is required and more sophisticated control strategies are needed to recognize the existing working condition and adjust the performance to meet the strict standards of grid interconnection. This paper presents a new adaptive control structure, based on internal model control (IMC), which uses multiple models and an inherent islanding detection method through an optimized switching mechanism to tune the operation of a three-phase inverter under transitions between islanded and grid-tied conditions. By applying a power synchronization method, the system emulates the operation of a synchronous machine which is needless to rely on a phase-locked loop to synchronize during the transitions. Hardware co-simulation environment in Simulink/PLECS and Xilinx System Generator have been utilized to evaluate the transient behavior of the controller in discretized domain and verify its robustness during parameter variations and load switching conditions. Various switching rules have been applied and a comparison of their effect in transient response is demonstrated. The results, taken from several case studies, confirm the significant robustness of the proposed control methodology. [ABSTRACT FROM AUTHOR]

This paper presents a modification of the Vienna rectifier (VR) to obtain more than three voltage levels at the DC line connection, while preserving the unity power factor at the AC grid side. The proposed unidirectional multilevel there-phase rectifier, called the “Negev rectifier”, has a reduced device count and utilizes a unique novel Time Division Multiplexed based control scheme, which makes the challenging N-level VR expansion possible. The Negev rectifier can serve as a grid interface for supplying dc voltage to multilevel inverters with 4 up to 7 voltage levels, eliminating the burden of inverter-side voltage balancing control. The Negev hardware can be constructed by two complementary topologies, making it suitable for either high current or high voltage applications. [ABSTRACT FROM AUTHOR]

This paper proposes a simplified deadbeat based predictive torque control (DB-PTC) for interior permanent magnet synchronous machine (IPMSM) drives using space vector modulation (SVM). The proposed DB-PTC is implemented in stationary reference frame, which simplifies the design. It contributes to greatly diminish the torque and flux ripples by using SVM and predictions. Low computational complexity is achieved since iterative optimization is not needed in the proposed method. Moreover, one cycle delay is compensated through predictions. In addition, to further reduce the torque and flux ripples, three-level simplified neutral point clamped inverter (3L-SNPCI) is employed. It features fewer switching devices and less DC-link capacitor voltage imbalance problem compared to the other types of three-level inverters. A novel SVM scheme is proposed to fully exploit the advantages of this inverter while ensuring smooth voltage vector transition. Experimental results are provided to verify the validity of the proposed method. [ABSTRACT FROM AUTHOR]

Hybrid cascaded-parallel microgrid is becoming a new emerging structure to integrate multiple low-voltage power sources. This paper presents a unified distributed control strategy to implement power sharing control in hybrid cascaded-parallel microgrid under both resistive-inductive (RL) and resistive-capacitive (RC) load, where a sign function is introduced to automatically match load characteristic. Active power and reactive power regulators without frequency drop are developed, and low bandwidth communication network is employed to support power management and improve system redundancy. Furthermore, small signal model of hybrid cascaded-parallel microgrid with RL load and RC load is established. Also, small signal stability and dynamic performance of the proposed distributed control strategy is investigated. Simulation results show that the unified distributed control strategy is able to implement desirable power sharing under different load types with superior control performance. Also, the proposed control strategy is able to improve system redundancy and support plug-and-play operation of microgrid. [ABSTRACT FROM AUTHOR]

This paper compares transient responses of virtual oscillator control (VOC) to the conventional droop control method, under various inverter terminal voltage amplitude and frequency regulations. The system of interest is an islanded microgrid of three three-phase inverters sharing a resistive load. By letting the two controllers have similar steady-state droop characteristics, the virtual oscillator and droop controlled systems are compared in a small-signal framework with the aid of eigenvalue analyses. The analytical results are verified by simulating the two systems in MATLAB/Simulink. In the simulations, the third inverter in each system is connected to share the load when the other two inverters are already synchronized. Transients are compared by examining synchronization time after the third inverter is connected. The key finding is VOC outperforms droop control in terms of time responses when the terminal voltage frequency regulation range is allowed to be wide; otherwise, the droop control responds faster than the VOC technique. The same conclusions are found for two other load types: constant power load and nonlinear load. In addition, transient energy losses are investigated, which shows positive correlation with the time response. [ABSTRACT FROM AUTHOR]

Reliability of power systems can be improved by the use of voltage source inverters (VSIs) with fast dynamic control, which can handle uncertainties induced by renewable energy resources and nonlinear loads. To ensure fast-dynamic response and simplified controller design, various model predictive control (MPC) methods are investigated in this paper. In the conventional MPC design for three-level VSIs, the cost function requires an additional term to ensure the balance of the neutral-point voltage (NP-V), which however can considerably impact the main goal of the MPC. To address this issue, an enhanced MPC using virtual space vectors is proposed, which allows the use of a significantly reduced weighting factor for the term in the cost function for NP-V balancing, while retaining fast-dynamic response. Additionally, the proposed MPC leads to reduced harmonic distortion in the VSI output voltage and current. Both simulation and controller hardware-in-the-loop studies are performed to demonstrate the effectiveness of the proposed enhanced MPC in an operating microgrid under both steady-state and fast-transient conditions. [ABSTRACT FROM AUTHOR]

This paper investigates an offshore wind farm based on modular dual-active-bridge (DAB) isolated DC-DC converter. The DAB converter is adopted due to its compact design and the galvanic isolation between its input and output terminals. The synchronous generator in each wind turbine is operated individually with the generator-side rectifier to maximize the power production. The output terminals of multiple DAB converters are connected in series so that the high voltage for DC transmission is achieved and the expensive offshore step-up substation is eliminated. The DC transmission voltage is excluded from the primary side of the DAB transformer by its galvanic isolation. To balance the power sharing among the DAB converters, their input terminals are connected in parallel. In addition, the droop voltage balancing control is presented for the series offshore wind farm. As a result, the input and output voltages of each DAB converter are governed by the same droop characteristic, and the DAB converters’ output voltages are kept balanced. The time-domain simulation results show that the DC voltages of each DAB converter and the high voltage direct current (HVDC) transmission system remain stable during the temporal and spatial variations in the wind power generation. [ABSTRACT FROM AUTHOR]

This paper presents an approach for systematically modeling a class of microgrid (MG) systems. The derived model accommodates grid-connected and islanded operation of the MG simultaneously, and allows modeling of converter-based as well as directly interfaced resources. Full modeling of virtual synchronous machines and actual synchronous generators is addressed. The originally nonlinear model is then converted to a linear model whose eigenvalues determine local stability of the MG. The model is used to analyze impacts of cross-coupling droop terms on the MG stability. The conclusions of this analysis are as follows. Addition of cross-coupling droop terms reduces the losses, compromises the stability in the grid-connected mode, and improves the stability in standalone mode. Therefore, for an MG that is intended to continuously operate at grid-connected and standalone mode, only a modest amount of cross-coupling terms is recommended to increase the efficiency without compromising the grid-connected stability. Simulation and experimental results are presented to verify the derivations. [ABSTRACT FROM AUTHOR]

Low-order harmonic currents in high-speed permanent magnet brushless dc (PM BLDC) motors cause large rotor eddy-current loss, resulting in high temperature rise and even overheating in the rotors, thus their application in the field of high-speed drive is limited. The rotor eddy-current loss can be reduced by using a split-phase winding method. In this paper, various time–space orders of harmonic eddy-current losses in the rotors of the PM motors with and without split-phase winding are quantified. The calculated results indicate that the rotor harmonic eddy-current losses generated by the low-order harmonic currents, which inherently exist in PM BLDC motors, are greatly reduced. Furthermore, associated with the characteristics of harmonic currents of PM BLDC motors, the mechanism of drastically reducing the rotor harmonic eddy-current loss of high-speed PM motors by using the split-phase winding method is revealed and experimentally validated. [ABSTRACT FROM AUTHOR]

This paper proposes a new direct torque control (DTC) method for a three-level neutral point clamped (3L-NPC) inverter-fed interior permanent magnet synchronous motor (IPMSM) drive. Theoretically, in comparison to a two-level inverter, a 3L-NPC inverter is advantageous for reducing the torque and flux ripples in DTC drives. Nonetheless, the ripples can still be excessive and detrimental if low switching frequencies are desired. To reduce the torque and flux ripples while maintaining a low average switching frequency, a switching table-based DTC scheme employing two voltage vectors in each sampling period is proposed in this paper. A duty ratio calculation method with minimal parameter dependence is developed, taking into account the dynamic torque and flux characteristics of an IPMSM. In addition, the problems of neutral point voltage fluctuations and smooth voltage vector switching, which are inherited from the 3L-NPC inverter, are addressed. Simulation and experimental results are presented to prove the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

The capacity to provide ancillary services has become more and more important for generators in the distribution grid. New grid codes have been implemented which regulate inverter behavior regarding different aspects of system safety and stability. This mainly includes the frequency-dependent power reduction or the allocation of reactive power. These standards affect mains monitoring because the power control directly changes penetration after the generator has islanded. In this paper, equations for active and reactive power balance are set up and developed further. Non-Detection Zones for different control methods are shown. These are then described and interpreted in detail to show how different effects that result from grid codes interfere with one another. Frequency-dependent power reduction was found to change the size and shape of the Non-Detection Zones. Reactive power control results in a shift of the Non-Detection Zones. In fact, even the initial operating point before loss of mains has an impact on inverter behavior after islanding. Islanding operating points are sometimes undefined which leads to oscillation. An interpretation of the results and advice for future revisions of grid codes conclude the paper. [ABSTRACT FROM PUBLISHER]

This paper outlines a reduced-order aggregate dynamical model for parallel-connected single-phase grid-connected inverters. For each inverter, we place no restrictions on the converter topology and merely assume that the ac-side switch-averaged voltage can be controlled via pulsewidth modulation. The ac output of each inverter interfaces through an $LCL$ filter to the grid. The closed-loop system contains a phase locked loop for grid synchronization, and real- and reactive-power control are realized with inner and outer PI current- and power-control loops. We derive a necessary and sufficient set of parametric relationships to ensure that a reduced-order aggregated state-space model for an arbitrary number of such paralleled inverters has the same model order and structure as any single inverter. We also present reduced-order models for the settings where the real- and reactive-power setpoints are different and where the inverters have different power ratings. We anticipate the proposed model being useful in analyzing the dynamics of large collections of parallel-connected inverters with minimal computational complexity. The aggregate model is validated against measurements obtained from a multi-inverter experimental setup consisting of three ${\text{750-VA}}$ paralleled grid-connected inverters, hence establishing robustness of the analytical result to parametric variations seen in practice. [ABSTRACT FROM AUTHOR]

This paper proposes a distributed control strategy for coordination of distributed energy resources (DERs) in low-voltage resistive microgrids. The proposed framework consists of two level structures; primary and secondary control. Unlike the existing distributed control solutions, the proposed method is based upon the practical assumption of network impedance being resistive. The primary control level consists of a V-I droop mechanism, where GPS timing is used to synchronize the control agents. A consensus-based distributed secondary control method is introduced to improve the voltage regulation and load sharing accuracy of the V-I droop method. In the proposed approach, the d-axis component of the voltage is altered so as to regulate the average microgrid voltage to the rated value while guarantying proper sharing of active power among the DERs. Additionally, the q-axis component of voltage is adjusted to perform proper current and, accordingly reactive power sharing. The proposed control methodology accounts for the distribution line impedances. It features a plug-and-play environment; prior system knowledge is not required, and an arbitrary DER can enter the microgrid without any need for additional synchronization. An ac microgrid is prototyped to experimentally demonstrate the efficacy of the proposed method. [ABSTRACT FROM AUTHOR]

In AC microgrids, voltage source inverters (VSIs) have become widely employed to interface DC energy sources to the AC network. Apart from power generation, VSI-based energy sources can also perform ancillary services such as demand side management using conservation voltage reduction (CVR). In conventional distribution systems, CVR involves lowering the voltage levels of the system within permissible limits to reduce the amount of power consumed by certain loads. However, in autonomous microgrids, the application of CVR for demand side management has not been fully explored yet. Thus, in this paper, a CVR strategy for a droop-controlled VSI-based distributed generation (DG) operating in an autonomous microgrid is proposed. This strategy employs the deviation in DG output voltage magnitude and frequency, caused by an increase in power demand during peak and overloading conditions, to determine the amount of voltage compensation required to reduce the microgrid bus voltages. In addition, a controller to coordinate between the proposed CVR strategy and load shedding is also developed to operate the DGs during overloading conditions. The proposed strategy is verified with a microgrid test system, and its performance is validated on MATLAB/Simulink software environment. [ABSTRACT FROM AUTHOR]

To reduce the current ripples of finite control set model predictive control, this paper proposes a continuous voltage vector model-free predictive current control method for the surface mounted permanent magnet synchronous motor (SMPMSM). In the proposed method, the continuous phase and the amplitude of the voltage vector are optimized in a sequence based on their uncoupling feature, and then the steady-state current fluctuation can be reduced. Only six active voltage vectors are enumerated, and the optimal phase is obtained by establishing the Lagrange interpolation polynomial between the cost function and the phase of voltage vectors. Then, the amplitude of the voltage vector with the optimal phase is optimized based on the principle of minimizing the cost function, and the optimal voltage vector is synthesized by a three-vector method. In addition, the ultra-local model of the SMPMSM drive system from the previous study is used to improve the robustness of the proposed method. The current dynamic and steady-state responses are demonstrated through the simulation and experimental results. [ABSTRACT FROM AUTHOR]

Output filters of the grid inverter usually have a large footprint in the total system. In order to increase the power density of the grid inverter, a passive integrated structure for the output filter with flexible multilayer foils is presented. Compared with a discrete output filter, the volume of the integrated filter is reduced by at least 20%. In this paper, the design of the integrated output filter is discussed. Then, a distributed electromagnetic component model of the integrated filter is used to verify the proposed design. Finally, a prototype of the integrated output filter is built and used in a grid inverter. Experimental results are included and compared with the results derived from the discrete output filter to prove the validity of this integrated filter design. [ABSTRACT FROM AUTHOR]

The increasing penetration of photovoltaic (PV) systems in distribution networks often causes overvoltage problems. One solution to address this issue is the provision of reactive power (RP) by the PV converters. This can cause increased power losses on the PV converters leading to additional operational costs. However, the manufacturers of commercially available PV converters provide data regarding the converter losses only under unity power factor (PF). The data are also limited regarding the technical details of the PV converters. This paper presents a methodology to estimate analytically the power losses in two-stage PV converters under an RP provision based on the efficiency curves at PF = 1 for different PV voltages and the limited information given in the PV converter datasheet. The losses are separately estimated for the dc–dc converter and the dc–ac inverter, because the losses on the former are not affected by the RP, while the losses on the latter are. The method is validated with field measurements of PV converter losses under RP provision and with detailed simulations. [ABSTRACT FROM AUTHOR]

An enhanced DC–DC converter is proposed in this paper, based on the combination of the Cuk and SEPIC converters, which is well-suited for solar photovoltaic (PV) applications. The converter uses only one switch (which is ground-referenced, so simple gate drive circuitry may be used), yet provides dual outputs in the form of a bipolar DC bus. The bipolar output from the DC–DC converter is able to send power to the grid via any inverter with a unipolar or bipolar DC input, and leakage currents can be eliminated if the latter type is used without using lossy DC capacitors in the load current loop. The proposed converter uses integrated magnetics cores to couple the input and output inductors, which significantly reduces the input current ripple and hence greatly improves the power extracted from the solar PV system. The design methodology along with simulation, experimental waveforms, and efficiency measurements of a 4-kW DC–DC converter are presented to prove the concept of the proposed converter. Furthermore, a 1-kW inverter is also developed to demonstrate the converter's grid-connection potential. [ABSTRACT FROM AUTHOR]