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Local secondary control has been successfully used to regulate the frequency of inverter-based islanded microgrids without using communications. In this scenario, noticeable steady-state deviations have been observed in active power sharing caused by the inherent clock drift of the digital processors that implement the local control of inverters. This paper presents a control scheme that performs frequency regulation and improves the active power sharing under high-load conditions, thus alleviating the impact of clock drifts in this situation. The study introduces a theoretical analysis that quantifies the steady-state deviations in active power sharing. It also includes a design procedure for the control parameters based on static and dynamic specifications. Experimental tests validate the expected features of the proposed control. The experimental setup is based on a laboratory microgrid equipped with three independent digital signal processors with different clock drifts. [ABSTRACT FROM AUTHOR]

Estimation of critical control parameters is a desirable tool feature for stability analysis and impedance shaping of high voltage dc (HVdc) connected wind farms. Accurate estimation of such parameters would be enabled by access to detailed models, which is not always the case in real wind farms. Industrial secrecy is one of the main factors hindering the access to such models. This paper proposes a gray-box method that, with basic assumptions about the control structure of the wind energy conversion system (WECS), can estimate the parameters of its controllers. The method is based on the measurements of frequency domain equivalent impedance combined with nonparametric impedance identification used in the solution of an inverse problem. The method makes possible to specify which part of the equivalent WECS impedance has a major impact on the stability of the system and according to this, reshape the impedance to enforce stability. Once the critical controller bandwidth is identified with this method, an instability mitigation technique is proposed based on reshaping the impedance by retuning the critical controllers of the interconnected converters. In order to avoid interaction between the HVdc rectifier and the WECS inverter, the controllers of both converters need to be retuned in such a way that the q-axis impedance magnitude of the HVdc system is kept lower than the q-axis impedance magnitude of the wind farm at the frequency of the phase-locked loop bandwidth. The results show that the method ensures the stability of the system by retuning only the critical controller parameters. [ABSTRACT FROM AUTHOR]

Copyright of Technical Electrodynamics / Tekhnichna Elektrodynamika is the property of National Academy of Sciences of Ukraine, Institute of Electrodynamics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper proposes an effective model predictive control (MPC) scheme using a modulated optimal vector (MOV) and finite control options for a three-phase inverter with an LC filter. Unlike other MPC methods, the proposed MPC strategy exploits the unconstrained optimal vector (OV) of the continuous-control-set (CCS) MPC to limit the control options for the unconstrained mode. First, the analytical OV is derived based on a least-squares optimization. If the input constraints are not violated, the OV is applied with a space vector modulation (SVM) technique like the CCS-MPC. Otherwise, the OV is scaled into the MOV and only three control options are online evaluated to reselect the control input. Experiments are conducted on a three-phase inverter test bed with a TI TMS320F28335 digital signal processor to validate the improvements of the proposed method, especially the robust performances and fast responses. The comparative results with the FCS-MPC show the superior performances of the proposed scheme with smaller steady-state error and lower total harmonic distortion due to the analytical OV with SVM, more robustness to parameter uncertainties due to the disturbance observer, and faster dynamic response due to the online reselection of control inputs. [ABSTRACT FROM PUBLISHER]

It is mandatory to keep the total harmonic distortion in the output of most inverters below the limits prescribed by different international standards. The harmonic distortions caused due to nonlinear loads may not get corrected by typical magnitude controller, like PID controller. Repetitive controller is more effective in such situations. Often, repetitive controller is used as add-on block to the conventional magnitude controller. Design of repetitive controller can be easily implemented on digital platform. Rapid change in phase and magnitude of inverter-filter's transfer function near corner frequency causes instability problems in repetitive controller. Various techniques have been proposed by researchers to overcome this problem. Most of the reported techniques rely on digital compensation. However, it is hard to find a digital compensator, which simultaneously meets the phase and gain requirements at moderate computational effort. Here, a new output voltage feedback scheme has been proposed, which simplifies the requirement of digital compensation. The proposed scheme requires an extra hardware but at negligible added cost. In spite of this extra hardware, the proposed circuit turns out to be more energy efficient in comparison to few popular schemes, reported in literature, where some resistance is introduced in the inverter's filter for better stability of controller. In addition to the proposed modification the paper presents a simplified explanation of the various factors affecting stability of repetitive controller. The paper also discusses a novel experimental method for verifying the efficacy of repetitive controller in suppressing distortion of any particular harmonic order. [ABSTRACT FROM PUBLISHER]

This paper presents a detailed power loss model for a microinverter with three different zero voltage switching boundary conduction mode (BCM) current modulation methods. The model is used to calculate the optimum peak current boundaries for each modulation method. Based on the power loss model, a dual-zone modulation method is proposed to further improve the microinverter efficiency. The proposed modulation method provides two main benefits: the addition of one more soft switching transition and low inductor peak current. The additional soft switching transition reduces switching losses by means of zero current switching. The lower peak current boundary reduces inductor rms current and conduction losses as well as allowing the output filter inductor to be smaller and more efficient. An improved BCM peak current control method was proposed and implemented on a microinverter prototype. The control circuit provides a highly accurate representation of the filter inductor current waveform and also provides galvanic isolation that simplifies control circuit design. The experimental results on a 400-W three-phase half-bridge microinverter validate the theoretical analysis of the power loss distribution and demonstrate that further improvement in efficiency can be achieved by using the proposed dual-zone modulation method. [ABSTRACT FROM AUTHOR]

The quality of injected current in multi-paralleled grid-connected inverters is a matter of concern. The current controlled grid-connected inverters with LCL filter are widely used in the distributed generation (DG) systems due to their fast dynamic response and better power features. However, designing a reliable control system for grid-connected inverters with LCL filter is complicated. Firstly, overcoming to system resonances due to LCL filters is a challenging task, intrinsically. This could become worse as number of paralleled grid-connected inverters increased. In order to deal with resonances in the system, damping methods such as passive or active damping is necessary. Secondly and perhaps more importantly, paralleled grid-connected inverters in a microgrid are coupled due to grid impedance. Generally, the coupling effect is not taken into account when designing the control systems. In consequence, depending on the grid impedance and the number of paralleled inverters, the inverters installed in a microgrid do not behave as expected. In other words, with a proper control system, a single inverter is stable in grid-connected system, but goes toward instability with parallel connection of other inverters. Therefore, consideration of coupling effect in the multi-paralleled grid-connected inverters is vital. Designing control systems for multi-paralleled grid-connected inverters becomes much more difficult when the inverters have different characteristics such as LCL filters and rated powers. In this paper, the inverters with different characteristics in a microgrid are modeled as a multivariable system. The comprehensive analysis is carried out and the coupling effect is described. Also, the control system design for multi-paralleled grid-connected inverters with LCL filter is clarified and a dual-loop active damping control with capacitor current feedback is designed. Finally, the proposed multivariable control system for a microgrid with three-paralleled grid-connected inverters with LCL filter is validated by simulation. [ABSTRACT FROM AUTHOR]

In recent years, new strategies of electric power generation are being developed and these are based on distributed generation systems ( DG ). With this new strategy of electric power generation based on renewable sources, there is a need to implement interfaces that allow the connection of these sources to the grid, as well as feed loads directly when operating in island mode or disconnected from the grid. Such interfaces are known as microgrids and the energy conversion systems that integrate them are very important. Therefore, the design of these units of energy conversion is a priority; therefore in this work we present the comparison of three methods of controllers design and the model of a single phase inverter for island mode operation in a microgrid. Particularly, the mathematical model is represented using transfer functions. This model describe the mathematical relationships that enable design in a simple and direct way the controllers of the different control loops that make up the energy conversion system. For the controller design in this operation mode, a conventional direct current method based on frequency method was first developed, from which a proportional-integral PI controller is designed. Second, an approximate method based on direct current to alternating current transformation ( dc-ac ), that allows the design of a proportional controller add generalized integrator ( P+IG ). This controller is obtained from the PI controller design in direct current. Third, an exact method is developed, which consists in applying a general methodology based on the frequency analysis. In particular, this work distinguishes the advantages of the second method, considering that its application allows the controllers design in a simple way for the different inverter control loops. [ABSTRACT FROM AUTHOR]

This paper proposes a robust optimal centralized controller to mitigate the small signal instability caused by the interaction between the control loops of rectifier interfaced active loads (RIALs) and the inverter interfaced distributed generations. The proposed controller is based on a robust linear quadratic regulator with prescribed degree-of-stability (LQRPDS). In order to reduce communication system burden and improve the flexibility and reliability of the proposed controller, a kalman state estimator has been designed. The optimal values of the diagonal weighting matrices of LQRPDS and kalman estimator are obtained by formulating a bi-objective optimization problem, which is solved using a fast and elitist multi-objective non-dominated sorting genetic algorithm (NSGA-II). A full dynamical linearized state space model of an islanded MG represented in synchronous (DQ) reference frame has been considered for analyzing the stability. However, schur balanced model order reduction technique has been used to reduce the complexity in the design of the controller. Eigenvalue analysis and time-domain simulations have been presented to demonstrate the robust performance of the controller under various load configurations as well as under step load disturbances. The efficacy of the proposed centralized controller is verified by comparing its performance with that of LQR based centralized controller. [ABSTRACT FROM AUTHOR]

In the context of distributed generation and renewable energy penetration toward smart grid, grid-connected inverter with LCL filter has drawn many attentions, whose current control conventionally requires several sensors to realize active damping and grid synchronization. In this paper, a novel full status observation strategy based on extended state observer (ESO) is proposed using inverter current feedback only and without grid voltage sensor. The proposed observation strategy contains observation and transformation process. Unlike conventional Luenberger observer, by using ESO, the system parameters do not appear in observation process, thus the observer dynamics and parameter mismatch error can be separately handled, providing more robust observation dynamics during parameter variation. Parameter mismatch study was carried out, and it is found that purposely choosing the observer parameters smaller than the real value can achieve relatively low estimation error and a large adaption range for parameter variation. The proposed observation strategy is simple to implement, without the need of expert knowledge-based parameter tuning such as pole placement. Experimental tests validated that the proposed observation-based control is able to give satisfactory performance in both dynamic and steady states, as well as adaption for system parameter variation. [ABSTRACT FROM PUBLISHER]

A current-limiting droop controller is proposed for single-phase grid-connected inverters with an LCL filter that can operate under both normal and faulty grid conditions. The controller introduces bounded nonlinear dynamics and, by using nonlinear input-to-state stability theory, the current-limiting property of the inverter is analytically proven. The proposed controller can be operated in the set mode to accurately send the desired power to the grid or in the droop mode to take part in the grid regulation, while maintaining the inverter current below a given value at all times. Opposed to the existing current-limiting approaches, the current limitation is achieved without external limiters, additional switches, or monitoring devices and the controller remains a continuous-time system guaranteeing system stability. Furthermore, this is achieved independently from grid voltage and frequency variations, maintaining the desired control performance under grid faults as well. Extensive experimental results are presented to verify the droop function of the proposed controller and its current-limiting capability under normal and faulty grid conditions. [ABSTRACT FROM PUBLISHER]

Droop control strategy is widely applied in ac microgrid to realize the seamless mode transition and achieve average power sharing among different inverters. However, there is a lack of analysis about the operation performance of droop control strategy in grid-connected mode. In this paper, grid fluctuation and control accuracy of reactive power are illustrated to be the main influence factors for power flow control according to the small-signal model. The feedforward of grid frequency and voltage magnitude is proposed to mitigate the impacts of grid fluctuation on power flow control. Since the feedforward items are achieved through the synchronous reference frame phase-locked loop with the input of filter capacitor voltage, extra sensors for grid voltage are avoided. Moreover, voltage magnitude control is introduced to enhance the control accuracy of reactive power flow. In addition, modified droop control loops are also proposed to improve the system stability. With the implementation of the proposed control scheme, improved power flow control and enhanced system stability are achieved with the feature of seamless mode transition maintained. Finally, the major contribution of this paper is verified by a laboratory prototype. [ABSTRACT FROM PUBLISHER]

Multiphase electric machines are known to have problems with stator current harmonics. The harmonics can be eliminated by various current harmonic compensation methods. However, using the active harmonic compensation can increase the required output voltage of the inverter supplying the machine. Because the maximum voltage is limited, complete elimination of the current harmonics may not always be possible. For such cases, this paper introduces a strategy for a partial compensation of the current harmonics. The aim of the proposed strategy is to provide a maximum reduction in the magnitude of the harmonics when the available voltage is limited. The strategy is verified by experimental results. [ABSTRACT FROM AUTHOR]

Neutral-to-ground overvoltage may occur in noneffectively grounded power systems because of the distributed parameters asymmetry and resonance between Petersen coil and distributed capacitances. Thus, the constraint of neutral-to-ground voltage is critical for the safety of distribution networks. In this paper, an active grounding system based on a single-phase inverter and its control parameter design method is proposed to achieve this objective. Relationship between its output current and neutral-to-ground voltage is derived to explain the principle of neutral-to-ground voltage compensation. Then, a practical current detection method is proposed to specify the reference of compensated current. A current control method consisting of proportional resonant and proportional integral (PI) with capacitive current feedback is then proposed to guarantee sufficient output current accuracy and stability margin subjecting to large range of load change. The PI method is taken as the comparative method and the performances of both control methods are presented in detail. Experimental results prove the effectiveness and novelty of the proposed grounding system and control method. [ABSTRACT FROM AUTHOR]

In the distributed generation environment, parallel operated inverters play a key role in interfacing renewable energy sources with the grid or forming a grid. This can be achieved by operating the inverters in parallel with an effective control strategy. Hence, qualitative output in terms of voltage and power in a parallel inverter demands a control strategy for abating the certain performance parameters. The technical constraints like load sharing capability, stability, total harmonic distortion, steady state and transient response determines the performance indices of the control strategy in connection with micro grid. The paper presents a comprehensive review of control strategies of parallel inverter based on the communication and control loops. The classification and transfiguration of control strategy from centralized approach to distributed approach and from distributed approach to droop approach were reviewed in the paper. The paper also focuses on the ancillary services that improve the performance indices of the system with multi loop approach of droop control in connection to micro grid. This paper provides an insight of the researcher's perception in emphasizing the power quality issues to achieve regulated and qualitative output with different control strategies. The paper also cites the progress of the research on parallel inverter control by inoculating the issue and the resolution in the literature. This could help the future researchers in addressing the power quality issues associated with parallel inverter control in connection to micro grid. Based on the reviewed state of art, the paper finally reports the suitability and future scope of the control strategy as its concluding remark. [ABSTRACT FROM AUTHOR]

In this article, an optimal virtual flux predictive direct power control (VF-PDPC) is proposed for three-phase grid connected voltage source inverter (VSI) operating under unbalanced and distorted grid supply voltage. The concept of virtual flux and extension pq theory is introduced in the conventional PDPC, where the former enhances its resilience toward distorted supply voltage while the latter enhances its resilience toward unbalanced supply voltage. The inherent advantages of the conventional PDPC scheme, such as simple implementation, fast dynamic responses, and no demand of modulators are retained. Getting rid of the complicated positive and negative sequence extraction of grid components, power compensation, or any tuning work, the proposed optimal VF-PDPC achieves excellent power tracking capability, and sinusoidal grid currents with low harmonics under various grid voltage conditions. Simulation results are presented to verify the effectiveness of the proposed control scheme when compared to the conventional PDPC, conventional VF-PDPC, and optimal PDPC. [ABSTRACT FROM AUTHOR]

This paper presents the results of comprehensive testing and subsequent detailed analysis of the obtained test results, evaluating harmonic and interharmonic performances of photovoltaic inverters (PVInvs) for a range of different operating conditions. The presented results indicate significant power-dependent changes in harmonic and interharmonic emissions of tested PVInvs for different supply voltage conditions (presence of voltage waveform distortions and various source impedance values). To correctly quantify and describe these changes in PVInv performance, this paper discusses and applies measurement procedures and metrics for evaluating harmonic and interharmonic emission recommended in existing standards, as well as some additional metrics and indicators. For some operating conditions, tested PVInvs significantly increase both harmonic and interharmonic emissions, and this paper also discusses the impact of PVInv control (e.g., maximum power point tracking control) as a possible origin of the interharmonic distortion. [ABSTRACT FROM PUBLISHER]

This paper considers torque regulation of a variable-speed synchronous machine fed by a line-commutated rectifier and a load-commutated inverter. The proposed control approach is model-predictive control where both the rectifier and inverter firing angles are considered as control inputs. Conventional controllers assign different tasks to the rectifier and inverter firing angle. In contrast, the model-predictive controller coordinates the firing angles and this improves the dynamic performance and disturbance rejection. In particular, the proposed controller handles line side under voltage conditions better than a conventional PI controller. The nonlinear model-predictive torque controller has been implemented on an embedded system and applied in an experimental test bed. The experiments confirm that the controller is able to successfully ride through line side under voltage conditions. [ABSTRACT FROM AUTHOR]

Standalone MicroGrid (MG) suffers from high voltage unbalancing due to both single phase and unbalanced loads. This paper presents a comprehensive survey about the available techniques and controllers for MG power quality improvement. Recent trends and schemes for the standalone MG power quality improvement are reported in detail. The techniques which concern with the MG voltage unbalancing issue are highlighted. In addition, this paper develops a simple scheme for balancing the MG phase voltages. The proposed scheme employs a new interfacing inverter topology and control. The three phase compact unit interfacing inverter of the Distributed Generators (DGs) has been rebuilt in form of three single phase inverter units. Each single phase inverter unit is controlled independently to achieve MG phase voltage balancing. At different load operating conditions, the three single phase inverter units run at different values and types (leading or lagging) power factors (PF). Employing the proposed inverter control achieves perfect phase voltages balancing at the most MG buses. Before employing the proposed inverter control, the voltage unbalancing ratio at some buses exceeds than 6%. This ratio approximately shrinks to zero after including the proposed inverter control. The performance of the proposed inverter control has been tested under different load conditions. In addition, the performance of the proposed inverter control is compared with both the old inverter control (unity power factor inverter) and the conventional inverter control (constant power factor inverter) performance. Only, a simple modification to the old inverter control is sufficient to implement the new inverter control. The proposed inverter control is very simple and highly economical attractive. [ABSTRACT FROM AUTHOR]

In this paper, a new method for intelligent robust control design is presented that achieves the best possible convergence rate of the system, utilizing the knowledge on the range of uncertain parameter. Thus resulting in enhanced stability and performance. The proposed method is applied to the grid-connected voltage source inverter (VSI) system with uncertainties in grid-impedance. Simulation and experimental results illustrate the efficacy of the proposed scheme. Comparison with existing methods shows that the proposed scheme can provide better reference tracking, stability for a wider uncertainty range, and improved transient and steady-state performance with low implementation cost. [ABSTRACT FROM AUTHOR]

Attributing to the advantages of high efficiency, low electromagnetic interference (EMI) noise and closest to the pulse-width-modulation (PWM) converter counterpart, zero-voltage-transition (ZVT) PWM soft-switching inverters are very suitable for high-performance applications. However, the conventional control algorithms intended for high efficiency generally results in voltage distortion. Thus, this paper, for the first time, proposes a high-precision control method to eliminate the dead-time effect through controlling the auxiliary current in the auxiliary resonant snubber inverter (ARSI), which is a typical ZVT PWM inverter. The dead-time effect of ARSI is analyzed, which is distinguished from hard-switching inverters. The proposed high-precision control is introduced based on the investigation of dead-time effect. A prototype was developed to verify the effectiveness of the proposed control. The experimental results shows that the total harmonic distortion (THD) of the output current of the ARSI can be reduced compared with that of the hard-switching inverter, because the blanking delay error is eliminated. The quality of the output current and voltage can be further improved by utilizing the proposed control method. [ABSTRACT FROM AUTHOR]

In some special occasions with strict size requirements, such as mine hoists, improving the design accuracy of the forced-air cooling systems of NPC three-level inverters is a key technology for improving the power density and decreasing the volume. First, a fast power-loss calculation method was brought. Its calculation principle introduced in detail, and the computation formulas were deduced. Secondly, the average and dynamic power losses of a 1MW mine hoist acting as the research target were analyzed, and a forced-air cooling system model based on a series of theoretical analyses was designed with the average power loss as a heat source. The simulation analyses proves the accuracy and effectiveness of this cooling system during the unit lifting period. Finally, according to an analysis of the periodic working condition, the maximum power-loss range of a NPC three-level inverter under multi cycle operation was obtained and its dynamic power loss was taken into the optimized cooling system model as a heat source to solve the power device damage caused by instantaneous heat accumulation. The effectiveness and feasibility of the optimization design based on the dynamic power loss calculation of the maximum power-loss range was proved by simulation and experimental results. [ABSTRACT FROM AUTHOR]

A method for the identification of mechanical parameters based on integral calculation is presented. Both the moment of inertia and the friction constant are identified by the method developed here, which is based on well-known mechanical differential equations. The mechanical system under test is excited according to a pre-determined low-frequency sinusoidal motion, minimizing the distortion, and increasing the accuracy of the results. The parameters are identified using integral calculation, increasing the robustness of the results against measurement noise. Experimental data are supported by simulation, confirming the effectiveness of the proposed technique. The performance improvements shown here are of use in the design of speed and position controllers and observers. Owing to its simplicity, this method can be readily applied to commercial inverter products. [ABSTRACT FROM AUTHOR]

Copyright of Automatika: Journal for Control, Measurement, Electronics, Computing & Communications is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper presents a new control strategy based on Lyapunov-function and proportional-resonant (PR) controller for single-phase grid-connected LCL-filtered voltage-source inverters (VSIs). While Lyapunov-function-based control guarantees the global stability of the system, the PR controller is employed to process the grid current error and determine the inverter current reference. However, it is shown that the conventional Lyapunov-function-based control (CLFBC) together with the PR control cannot damp the inherent resonance of the LCL filter. Therefore, this control approach is modified by adding a capacitor voltage loop so as to achieve the desired resonance damping. In addition, a transfer function from the reference grid current to actual grid current is formulated in terms of the LCL-filter parameters and their possible variations in the proposed control strategy. An important consequence of using the PR controller is that the need for performing first and second derivative operations in the generation of inverter current reference is eliminated. Also, a zero steady-state error in the grid current is guaranteed in the case of variations in the LCL-filter parameters. The computer simulations and experimental results obtained from a 3.3-kW system show that the proposed control strategy exhibits a good performance in achieving the required control objectives such as fast dynamic response, zero steady-state error, global stability, and sinusoidal grid current with low total harmonic distortion (THD). [ABSTRACT FROM PUBLISHER]

Parabolic current control is an attractive current control method with fast transient response and constant switching frequency. Due to the good dynamics of the parabolic current control, it can be employed in voltage source inverters to improve the system performance such as minimizing the distortion of current waveforms or voltage waveforms. To implement the parabolic current control, a current sensor is required, associated with the current conditioning circuit and parabolic carrier generators. Since the parabolic current control is based on the real-time information of the inductor current, any phase delay or propagation delay of the sensor itself and the conditioning circuitry, or limited resolution of parabolic carrier generators, could impact the current control performance. Since the parabolic current control compares analog signals to generate the required control signals, noise from the control board impacts the control precision as well. This paper will explore solutions to these problems. First, the inductor current of the voltage source inverter is analyzed and the parabolic current control strategy is studied, then a sensorless parabolic current control method is proposed. The new sensorless parabolic control method utilizes a current emulator to rebuild the inductor current on a microcontroller. To avoid a dc offset on the ac-side output voltage caused by the current emulator, an additional control loop in the current emulator is added. The effectiveness of the proposed methods is experimentally verified by the use of an H-bridge voltage source inverter. [ABSTRACT FROM AUTHOR]

This paper proposes a novel approach for testing dynamics and control aspects of a large scale photovoltaic (PV) system in real time along with resolving design hindrances of controller parameters using Real Time Digital Simulator (RTDS). In general, the harmonic profile of a fast controller has wide distribution due to the large bandwidth of the controller. The major contribution of this paper is that the proposed control strategy gives an improved voltage harmonic profile and distribute it more around the switching frequency along with fast transient response; filter design, thus, becomes easier. The implementation of a control strategy with high bandwidth in small time steps of Real Time Digital Simulator (RTDS) is not straight forward. This paper shows a good methodology for the practitioners to implement such control scheme in RTDS. As a part of the industrial process, the controller parameters are optimized using particle swarm optimization (PSO) technique to improve the low voltage ride through (LVRT) performance under network disturbance. The response surface methodology (RSM) is well adapted to build analytical models for recovery time ( R t ), maximum percentage overshoot (MPOS), settling time ( T s ), and steady state error ( E ss ) of the voltage profile immediate after inverter under disturbance. A systematic approach of controller parameter optimization is detailed. The transient performance of the PSO based optimization method applied to the proposed sliding mode controlled PV inverter is compared with the results from genetic algorithm (GA) based optimization technique. The reported real time implementation challenges and controller optimization procedure are applicable to other control applications in the field of renewable and distributed generation systems. [ABSTRACT FROM AUTHOR]

In this paper, we propose an idea to integrate more renewable energy into the existing China power grid and to provide remote islands with a reliable power supply. The idea is to expand the existing point-to-point line-commutated converter (LCC) HVDC transmission link into a hybrid multiterminal HVDC (hybrid MTDC) system, which contains LCC stations and voltage-source converter (VSC) stations. Accordingly, this paper proposes a novel control strategy for this system called active voltage feedback control, which does not need any high-speed communication system under various disturbances, such as wind speed variations, load fluctuations, faults, etc. This paper also adds secondary frequency regulation to the synchronverter, which is one way of combining the VSC converter with synchronous machine behavior. Then, this paper applies an approach to limit its current under fault conditions. Finally, the entire hybrid MTDC system is modeled in PSCAD/EMTDC. Simulation results show that the improved synchronverter is able to achieve secondary frequency control, and the presented active voltage feedback control works very well when exposed to various disturbances. [ABSTRACT FROM AUTHOR]

In this paper, a single current sensor technique (SCST) is proposed for single-phase full-bridge inverters. The proposed SCST measures the currents of multiple branches at the same time, and reconstructs the average inductor, capacitor, and load current in a single switching cycle. Since all of the branches' current in the LC filter and the load are obtained using the SCST, both the inductor and the capacitor current feedback schemes can be selectively applied while taking advantages of each other. This paper also analyzes both of the current feedback schemes from the view point of the closed-loop output impedance. The proposed SCST and the analysis in this paper are verified through experiments on a 3kVA single-phase uninterruptible power supply (UPS). [ABSTRACT FROM AUTHOR]

This paper proposes a single-phase grid-connected wound rotor machine that has the single-phase stator and three-phase rotor windings. The machine has no auxiliary winding or capacitor unlike conventional single-phase machines. Nevertheless, it can operate in all four quadrants of the torque–speed plane with a three-phase inverter. For adjustable speed drives, an isolated three-phase inverter is applied to the rotor windings while the stator winding is directly connected to a single-phase source. The grid filter and rectifier of the conventional system are eliminated and the rotor-side slip rings can be also removed by the inverter integration. So the overall structure of the proposed drive system is simple and cost effective. In this paper, the proposed machine is modeled into a modified d–q model considering the absence of q-axis stator coil. Its characteristics are analyzed and vector control methods of the grid power factor, dc-link voltage, and speed are proposed. For more efficient control, the optimal rotor current set is calculated from the minimum copper loss condition. The system has wider operating areas than other single-phase drive systems. The feasibility of the proposed system is verified by experiments. [ABSTRACT FROM AUTHOR]

This paper presents an asymmetrical cascaded multilevel inverter based on a five-level transistor-clamped H-bridge power cell. A two-cell configuration with a dc link voltage ratio of 4 : 1 is presented. The modulation method is based on the fundamental frequency switching. An analysis of the inverter output voltage and harmonics is presented. Injection of the third harmonic voltage as to maximize the output voltage and the issue of the inverse power flow or regeneration in the low-voltage cell are presented. The prototype of the proposed inverter was built, and its functionality was verified through experimental results. [ABSTRACT FROM AUTHOR]

Highlights: [•] A secondary control architecture for a low-voltage AC microgrid is proposed. [•] This secondary layer relies on a primary layer analyzed in a previous article. [•] Each inverter feeds the reactive power demand of its associated loads. [•] A centralized controller transmits compensating values for voltage and frequency. [•] A model predictive controller is applied for frequency restoration. [ABSTRACT FROM AUTHOR]

Highlights: [•] A control layer for a low-voltage AC microgrid with distributed storage is proposed. [•] The inverters interact with the microgrid through RL impedance. [•] Decoupled droop control techniques are proposed and analyzed. [•] The dynamic response can be adjusted without modifying the droop coefficients. [•] The resulting primary control layer can be easily actuated for power dispatching. [ABSTRACT FROM AUTHOR]

Harmonic current filtering and resonance damping have become important concerns in the operation and control of the islanded microgrids. To address these challenges, this paper proposes a control method for the inverter-interfaced distributed generation (DG) units to autonomously share the harmonic currents and resonance damping burdens. The approach employs a load compensator based on the decomposition of output current, in addition to the outer droop-based power controller and the inner voltage and current controllers. The load compensator consists of a virtual-fundamental-impedance loop for the enhanced reactive power sharing and a variable-harmonic-impedance loop which allows to counteract the harmonic voltage drops across the grid-side inductance of the DG inverter and also to dampen out harmonic resonance propagation in the microgrid. Finally, the laboratory tests on a three-phase islanded microgrid setup are carried out to validate the performance of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Microgrid is an effective way to integrate the distributed energy resources into the utility networks. One of the most important issues is the power flow control of grid-connected voltage-source inverter in microgrid. In this paper, the small-signal model of the power flow control for the grid-connected inverter is established, from which it can be observed that the conventional power flow control may suffer from the poor damping and slow transient response. While the new power flow control can mitigate these problems without affecting the steady-state power flow regulation. Results of continuous-domain simulations in MATLAB and digital control experiments based on a 32-bit fixed-point TMS320F2812 DSP are in good agreement, which verify the small signal model analysis and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Abstract: An application of a novel steady-state simulation of a monovalent inverter-driven water-to-water heat pump with a desuperheater is presented. The simulation is capable of modeling and managing the different operation modes of the system such as combined space heating and domestic hot water, in order to provide realistic results, from which four control schemes based on two electricity tariff plans were developed. The schemes were tested on a subject low energy house for Tokyo and Sapporo, in Japan, for the coldest day of the year and for the heating season. Emphasis was given to the short-term domestic hot water storage. The results show that the application of the schemes with a flexible electricity tariff had lower cost over the scheme that used the flat rate pricing plan. Moreover, it was shown that no unique scheme was able to minimize the cost and electrical consumption for both locations. Additionally, a tradeoff between lowest cost and lowest energy consumption was identified for Sapporo. The results of this research could be applied to develop a more robust control on monovalent heat pump systems. [Copyright &y& Elsevier]

Distributed generation and distributed energy storage systems can be controlled to improve power system stability. This article proposes control strategies for adopting distributed generation units to autonomous microgrids. The proposed distributed generation interface is utilized not only to control the active power flow, but also to provide essential ancillary services, such as voltage regulation and frequency control, to the autonomous microgrid. A coordinated voltage and frequency control of the inverter-based distributed generation and distributed energy storage system is presented. Moreover, a frequency estimation algorithm based on orthogonal filters is adopted for the frequency control loop. One advantage of the proposed control system is its unified structure for the different operating modes. Extensive simulations of the proposed coordinated control of the inverter-based distributed generation and distributed energy storage system are conducted to evaluate the dynamic performance of the proposed microgrid. [ABSTRACT FROM AUTHOR]

We propose a concept of microgrid for electric power distribution with lattice topology in which every node is interconnected with a number of closest neighbors. Such a toppology directly reflects special arrangement of households throughout subdivisions in cities. Each node of the microgrid is assumed to be an electric load and possibly a power-delivering inverter simultaneously. We introduce a method of local control for the inverters to collectively regulate the voltage and frequency in the microgrid. Our control method does not require data communication between the nodes nor is it necessary to define a dominant node to act as a central utility. Therefore all inverters remain autonomous at all times. We provide numerical simulation results demonstrating stability and controllability of voltage and frequency in the microgrid. [ABSTRACT FROM AUTHOR]

The theory, design, implementation and experimental validation of a hardware-circuit-based diagnosis method for open-switch faults in voltage source inverters are discussed. Operating states of the inverter under both healthy and faulty conditions are analysed. The estimated voltages can be achieved as pre-fault voltages by means of the switching function model of the inverter under a normal condition. Then, the error voltages between the estimated ones and measured ones can be used to detect faults. To achieve reliable diagnosis, the design employs both the amplitude limit and the duration limit to filter the error voltages so as to eliminate effects of the deadtimes and the switching processes of power devices. The proposed method is validated by experiments. [ABSTRACT FROM AUTHOR]

Specializing in the design and production of electric motors, mechanical gearboxes and inverters, Italian company Motive has recently widened its product range with a solution dedicated to automatic pressure control in hydraulic systems and with an island or hybrid drive for solar pumps and motors. [ABSTRACT FROM AUTHOR]