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A double synchronous unified virtual oscillator controller (dsUVOC) is proposed for grid-forming voltage source converters to achieve synchronization to the fundamental frequency positive- and negative-sequence components of unbalanced or distorted grids. The proposed controller leverages a positive- and a negative-sequence virtual oscillator, a double-sequence current reference generator, and a double-sequence vector limiter. Under fault conditions, the controller enables to clamp the converter output current below the maximum value limited by the converter hardware while retaining synchronization without a phase-locked-loop (PLL) regardless of the balanced or unbalanced nature of grid faults. Consequently, balanced and unbalanced fault ride-through can be achieved without the need for switching to a back-up controller. The paper presents the systematic development of the double-synchronous structure along with detail design and implementation guidelines. Validation of the proposed controller is provided through extensive control-hardware-in-the-loop (CHIL) and laboratory hardware experiments.

There is a growing demand for multi-port power converters for managing multiple energy sources and loads. To that end, a dual-input single-output dc-dc converter is proposed in this article. The converter integrates two different and low-voltage dc sources into a dc bus. Pulse width modulation is used to control the transfer energy from the input ports to the output port. The discontinuous conduction mode (DCM) operation of the converter has enabled the soft-switching operation in the primary switches. Moreover, it helps avoid the high-frequency transformer (HFT) saturation when the two input voltage sources have different values. The detailed analysis of the proposed dc-dc converter is presented in this article along with the simulation and experimental results obtained from a 150 W hardware prototype to validate its operation.

Microgrid is a key to an efficient and resilient power grid operation. Thus in this paper, a robust control scheme is implemented namely modified frequency locked loop (MFLL), proficient in eliminating the voltage and current fluctuations because of phase/frequency variation. Therefore, enhancing the consistency of the supply across nonlinear load through a smooth transition from utility-interactive to islanded mode. It also achieves accelerated convergence and reduced steady-state error. MFLL is an easy control technique, with only one interfacing voltage source converter, which delivers a stable supply to critical loads during different operating modes. The implemented system has the main advantage of the capability of power-sharing between distributed generations (DGs) systems while regulating the voltage at the DC link. It also provides power quality improvement through harmonics suppression; voltage regulation during contingencies like unbalance in load and compensation in reactive power at the common coupling point following the prerequisite of the system. The performance through the implemented control algorithm is brought about in MATLAB 15b/Simulink platform. The performances of results are compared with already reported control strategies. Thus, simulated results comparison with existing work presents the competence of the implemented control strategy under dynamic conditions.

This paper introduces a new active-neutral-point clamped (ANPC) seven-level inverter with a switched-capacitor technique. The introduced inverter generates a seven-level ac output voltage with a maximum voltage-boosting gain of 1.5. Also, a simple modulation technique was proposed, where the capacitors voltages are naturally balanced by the series-parallel connection of the input voltage source. Compared with seven-level inverter topologies, the proposed topology is simpler and its control system does not requires sensors or balancing circuits to stabilize the dc-link voltages. Additionally, the dc-link voltage requirement is reduced by 33.33%, eliminating the need for an intermediate dc-boosting stage and making the introduced inverter a good candidate for applications such as photovoltaic panels and full-cells systems, both characterized by a low input voltage. The operation and performance of the proposed topology is validated through simulation and experimental results.

This paper introduces a new coupled inductor based dual-input ultra-high step-up DC-DC converter with a high voltage conversion ratio, low voltage stress across semiconductor components, and high efficiency features. The coupled inductor and voltage multiplier circuit (VMC) techniques have been utilized to enhance the voltage gain. The blocking voltages of power switches are clamped by VMC at low values and can be controlled via the turn ratio of the coupled inductors that decreases the voltage rate of power switches and converters cost. The presented dual-input structure can transfer power to the load from two independent voltage sources. Also, the inputs and output ports are common ground, which can reduce the electromagnetic interference (EMI) influences. Operation modes delineation, steady-state analysis, and comparison section are provided for describing the proposed structure performance. Finally, to confirm the effectiveness of the presented topology, experimental waveforms with a 200 W output power at 50 kHz operating frequency are presented.

This paper presents a new method for estimating the fault location in voltage source converter interfaced dc system. The proposed dc fault location estimation method evaluates the distance from the fault point to the dc line terminal by estimating the line inductance, hence it is designated as L location. Since the fault impedance usually only includes resistance, the proposed L location is less affected by the variation of fault resistance. In addition, the ripple caused by the distributed capacitive discharge is excluded from the dc line current when the proposed L location is applied in the dc system with the distributed-parameter transmission line. The cubic curve fitting method is used to reconstruct the current signal based on the least square method. The effectiveness of the proposed L location is verified with an OPAL-RT based multi-terminal dc system and experimental point-to-point and MTDC systems. The comparison with other methods, including the impedance estimation and the voltage travelling wave based location substantiates the comprehensive performance of the proposed method.

This paper deals with the short-circuit analysis of distribution systems dominated by Voltage Source Converters (VSCs). A novel methodology has been proposed in this paper for short-circuit calculation of power systems populated with VSCs where the control modes and potential current-saturated operation of power converters are considered. The studied distribution system has been modeled using an element-based steady-state formulation that includes the converter equations in both normal operation and fault scenarios. Then, equilibrium points of the studied distribution system are identified by solving the established systems of equations for different potential current-saturation states of the VSCs. The methodology identifies possible solutions involving different saturation states of different converters. The proposed methodology is applied to a specific case study of a distribution system penetrated with VSCs in both grid-forming and grid-following control. The short-circuit analysis results indicate that VSCs operation shows a significant impact on the short-circuit equilibrium points of the studied system. In addition, multiple equilibrium points may exist depending on whether the grid-forming converter is saturated or not. The existence of multiple equilibrium points is validated through dynamic simulations.

The microgrid (MG) which effectively utilizes distributed energy resources (DERs) is crucial to the modern power system. However, since the islanded MG with high penetration of DERs inherently lacks sufficient inertial support, the frequency regulation is challenging for the voltage source converter (VSC)-based MG. This paper presents a cascaded model predictive control (MPC) scheme for both the outer and inner loops of primary control, which improves frequency regulation capability with the advantage of satisfactory dynamic response and high tracking accuracy. In the outer loop, to enhance dynamic frequency characteristics, the MPC-based virtual synchronous generator (VSG) method with two control objectives is proposed. According to the movement direction of frequency under different load switching cases, the corresponding frequency response process speeded up or slowed down properly. In the inner loop, an improved finite-set double vector MPC (DV-MPV) is proposed to follow the output reference from the outer loop, which contributes to accurate frequency control by reducing tracking error, and faster dynamic response as well. The simulation and experimental results further demonstrated the effectiveness of the proposed method.

In single-phase voltage source inverters (VSI) under a weak grid, the frequency coupling, caused by the asymmetrical system structure, poses a challenge to system modeling and controller design. In this article, a frequency coupling suppression control strategy is presented, where all the frequency coupling components in the current reference are eliminated. As a result, the single-phase grid-tied inverter is directly modeled as a simple single-input single-output (SISO) admittance rather than a complicated multiple-input multiple-output (MIMO) admittance matrix. The SISO admittance model is simple, compact, and accurate, which facilitates design-oriented analysis. The admittance characteristic analysis shows that the Sym-PLL has a critical impact on the system stability under weak grid conditions. To improve the system stability, a prefilter-based impedance-shaping control strategy is proposed by mitigating the negative resistor behavior of the Sym-PLL. Finally, simulations and experimental results validate the effectiveness of the frequency-coupling suppression control strategy.

The real-time estimation of the AC power system equivalent parameters enables the grid-connected voltage source converter (VSC) to achieve more effective stability control during a short-circuit fault. In this paper, a novel Thevenin equivalent (TE) parameter estimation method based on local PMUs is proposed and applied to the phase-locked loop (PLL) synchronization stability control of VSC. First, the relationship between the variation of the measured voltage and the error of the estimated impedance is strictly derived based on the rules of norm inequality. Then, according to the analysis of the upper bound of the system voltage change, a voltage threshold is proposed to select the PMU measurements to ensure the accuracy of the estimation during the large disturbance. Finally, using the estimated TE parameters, the current control instruction of VSC during the fault is calculated in real-time to ensure a synchronous and stable operation. The simulations reveal that the proposed estimation method can accurately track the TE parameters after a fault. Furthermore, the associated current control actions during a fault can ensure the PLL synchronization stability of VSC.

Virtual oscillator control (VOC) is an emerging decentralized control technique for grid-forming inverter applications. In contrast to conventional phasor-based droop control or virtual synchronous machine control, VOC is a time-domain controller designed to emulate the dynamics of a nonlinear oscillator. VOC is a current controlled voltage source, lacking the ability to regulate the inverter terminal voltage, which will result in significant voltage deviations caused by dead-time effects, non-ideal semiconductor devices and output filter voltage drops. In this paper, a voltage and current dual-loop control structure augments the VOC to compensate these voltage deviations and regulate the inverter output variables directly. A complete small-signal model for a multiple inverter based microgrid with the proposed control structure is presented in order to assess system stability using eigenvalue and participation factor analysis. Analytical results show that the parameter related to the frequency regulation and the integral gain of the voltage controller affect the location of the system’s dominant modes significantly. The stability margin is determined by modifying these control parameters. Experimental results on a laboratory test microgrid verify the predication from the small-signal analysis and time-domain simulations.

This paper presents a combined control scheme that consists of two complementary control strategies, namely hierarchical control and autonomous control to handle contingencies in a multi-terminal high voltage direct current (MTDC) system. The proposed hierarchical control considers three optimal targets, i.e., equal or proportional power sharing, minimizations of transmission loss and total DC voltage variation. The primary control layer implements the droop control for voltage source converters (VSCs). While in the secondary control layer, voltage reference setpoints of the VSCs in droop control scheme are calculated by the proposed optimal power flow method with the equal or proportional power sharing as equality constraints. As the hierarchical control relies on communication system, the autonomous control for power sharing can be activated during communication system malfunctioning or rapid power variation caused by renewable energy sources. The combined control strategy can greatly increase the reliability of the MTDC system. The simulation results of a five-terminal MTDC network conducted in the MATLAB/Simulink software verify the performance of the proposed combined control strategy.

A triple-port solid state transformer (TP-SST) topology based on the combination of modular multilevel converter (MMC) and resonant push-pull converter (RP2C) is proposed in this paper. For an MMC-based SST, large size capacitor caused by fluctuating power and voltage imbalance of MMC sub-modules (SMs) are two important problems. Based on a simple open-loop control scheme, the RP2C based high frequency links (HFL) are characterized by a voltage source with fixed conversion ratio and low impedance, and then can form a power free-coupling channel between SMs and realize the automatic balance of capacitors voltage. Moreover, the fluctuating power can cancel each other based on their three-phase symmetry, which obviously reduces the capacitance size, and the 2nd-order harmonic circulating current of the arms could be eliminated at the same time, greatly simplifying the control of system. In this paper, the working principle of TP-SST are described, and the methods of HFL realizing SM voltage ripple elimination and self-balancing are analyzed in detail. In addition, a comparison with traditional SST in efficiency and cost is presented. Finally, simulation and experimental results verify the correctness and effectiveness of the proposed topology scheme.

Differing from a two-level voltage source converter (TL-VSC), the internal dynamics of a modular multilevel converter (MMC) are much more complicated. Since the fluctuation of submodule capacitor voltage is unavoidable when transmitting power, the MMCs output voltages cannot track the references perfectly. The interaction between the time-varying capacitor dynamics and controllers may lead to self-instability of the MMCs. In order to address this self-instability issue, this paper first establishes the small-signal closed loop transfer function matrices (TFMs) of the external and internal control loop by the harmonic linearization method. Further, a novel design principle of MMC controllers is proposed. Then, the critical impact factors on the stability of circulating current controller and AC voltage controller are investigated based on the generalized Nyquist criterion(GNC). The proposed models and analysis methods are validated by time-domain simulations in PSCAD/EMTDC.

This paper presents a general method to obtain the improved common ground quasi-Z-source series dc-dc converters. Utilizing the negative output characteristic of the quasi-Z-source converter (QZSC), the output voltage of the improved converter is the sum of the input voltage and the output voltage of original QZSC, which means that part of the energy is directly supplied by the input voltage source without passing through QZSC. Therefore, compared with the original QZSC, the proposed improved converter has advantages such as lower voltage stress, lower current stress, higher voltage gain and higher efficiency. More importantly, all these improvements are obtained with no obvious drawback. No additional component is required, the input and output remain common ground, and if the input current of original converter and the load current are continuous, the input current of improved converter is also continuous. In order to prove the feasibility of proposed method, firstly, the improved converters of three existing QZSCs are obtained. Then, the operation principle, parameter design, and comparison are introduced in sequence. Finally, the above analyses are verified by simulation and experiment results.

Voltage source converter (VSC) has been extensively used in power systems for interfacing renewable energies and other modern loads. Because of the nonlinear nature, it is important to establish a proper harmonic model of VSC devices when con-ducting harmonic analysis in power systems. This is especially true for low-order harmonics which are routinely encountered in present power systems. To this end, an analytical harmonic model that represents the low-order harmonic characteristics of the VSC is developed in this paper. The new model takes both the control loop and power loop (i.e., dc-link capacitor) of the VSC into account through rigorous mathematical derivations. The final result has shown that the VSC can be represented by a coupled impedance matrix at harmonic frequencies, which can be easily used for harmonic analysis in large-scale power systems. It also shows that the dc-link capacitor has a more noticeable impact on the harmonic impedance of VSC with a smaller dc-link capacitance and low-order harmonics. The correctness of the new model has been verified using EMT simulations and lab experiments. A complete procedure to construct the proposed VSC model is also established based on the research findings.

Capacitor-current active damping has been widely used in voltage source inverters with LC or LCL filters. However, the time delay poses an adverse impact on the damping performance, causing a negative-resistance behavior, and thus constraints the system stability. To address this issue, this letter proposes a compensation method, which eliminates the adverse impact by removing the time delay out of the capacitor-current loop. Consequently, the damping performance behaves as a constant positive resistance, and thereby the stable region of the damping gain is notably enlarged. Experimental results verify the effectiveness of the proposed method.

High-voltage direct-current (HVDC) systems using voltage source converters (VSCs) have been developed to suppress voltage fluctuations in offshore grids due to the intermittent power generation of offshore wind farms (OWFs). This paper proposes a new strategy for the optimal secondary voltage control of HVDC-linked OWFs. In the proposed strategy, optimal coordinated control of an HVDC system and OWFs is achieved to minimize real-time voltage fluctuations throughout an offshore grid. A full-order dynamic model of HVDC-linked OWFs is implemented, and its unobservable and uncontrollable state variables are then eliminated using a model reduction algorithm. Given the dynamic model, a linear quadratic Gaussian regulator is designed for optimal coordinated control, and an eigenvalue analysis is conducted focusing on the effects of communication time delays on the stability of the proposed strategy. Simulation case studies are also carried out to verify that the proposed strategy more effectively improves real-time regulation of offshore grid voltages, compared to conventional strategies, while ensuring voltage stability even for relatively large delays in communications systems.

This paper deals with a cooperative control of multiple distributed generation (DG) systems with power quality (PQ) enhancing capabilities. This control algorithm is realized using an adaptive harmonic signals extraction method based on variable weight mixed norm least mean fourth (LMF)-least mean square (LMS) adaptive detection method. This adaptive technique extracts all the necessary information required for the development of the proposed cooperative control algorithm. With this control algorithm, the voltage source inverters (VSIs) of the DG systems contribute to PQ improvement by mitigating harmonics and reactive currents in the distribution system. This algorithm operates the VSIs in maximum power generation and total harmonic distortion (THD) improvement modes. In maximum power generation mode, the algorithm utilizes the remaining capacity of the VSIs for compensation of nonactive currents drawn by the dedicated loads. Upon finishing the dedicated loads' compensation, the remaining capacity is spent for compensation of the loads dedicated to other DG systems that are not in the position to render full compensation services. In THD improvement mode, the algorithm prioritizes compensation of harmonic currents generated by the loads. The efficacy of the proposed cooperative control algorithm for multiple DG systems is demonstrated via simulation and hardware-in-the-loop (HIL)-based experimental results.

In this paper, a novel modular multilevel dc-dc converter with high power-density potential is proposed based on a cascaded string of voltage source submodules (VSM) merged with a current source submodule (CSM) that is configured to rapidly invert the polarity of the current in the VSM string. The proposed hybrid converter combines the i) modularity advantages of the modular multilevel converter (MMC) and ii) the high effective frequency advantages of the flying capacitor converter (FCC). Unlike other hybrid non-isolated MMC structures the proposed converter features a novel CSM stage enabling i) a near 50% reduction in string current, and ii) the input source and output load to be commonly grounded. In addition, the proposed operational approach enables self-balancing of the VSMs and a simple overall control structure. This article introduces the main operating principles of this novel topology termed the Current Shaping Modular Multilevel Forward Converter (CS-M2FC) along with a proposed control design. A peak efficiency of 97.1% is measured for a laboratory-scale prototype at 950-137V and 1.45 kW.

Fast frequency response (FFR) is an option in maintaining system security under low inertia conditions. Power electronics interfaced energy resources such as wind and photovoltaics have the capability to deliver FFR. At present, almost all installed power electronics interfaced energy resources are connected to the grid by the phase locked loop (PLL)-synchronized voltage source converter (VSC) and they usually implement inertial response by measuring the rate of change of frequency (RoCoF). However, RoCoF measurements are highly susceptible to disturbances in the grid. In this paper, a novel inertia controller is developed for PLL-synchronized VSC interfaced energy resources to implement natural inertial response without measuring RoCoF. A thorough stability analysis is performed to study the impact of proposed inertia controller on the VSC in weak grid. The proposed inertia controller makes the deviation of DC voltage proportional to the deviation of system frequency. Further, a method of primary frequency control is proposed by measuring DC voltage rather than grid frequency. The proposed FFR scheme of inertia and primary frequency control is applied to a direct-drive windfarm connected to a grid, and its correctness and advantages are verified by simulation.

Low-voltage multi-terminal DC system is one of the important forms in distribution network. Its stability issue has drawn worldwide attention for the characteristics of low inertia and weak damping. This paper aims to analyze the interactive behavior between the voltage source converter (VSC) and the oscillation mode of the DC network. First, this paper proposed a modeling method for DC voltage control loop considering the time-lag effect of inductor energy storage behavior on the AC side of the VSC. Based on this model, the interactive behavior between VSC and DC network including the influence of droop coefficients and load power under DC voltage-AC power (Vdc-Pac) droop control mode and under DC voltage-DC power (Vdc-Pdc) droop control mode is studied. Then, the overall state-space model of the system is established. Furthermore, the stability of a low-voltage three-terminal DC system under different control modes and parameters is compared and detailed analyzed. Finally, the corresponding simulation and experimental verification validate the theoretical analysis. The research results show that under the same capacity and control parameters, Vdc-Pac mode can better improve the capability of power regulation and system stability. This research can provide an effective method for system control modes design.

Wound Rotor Induction Machine (WRIM) connected to stator and rotor Voltage Source Converters (VSCs), known as the Double-Inverter-fed WRIM drive, can be viewed as two virtual Squirrel Cage Induction Machine (SQIM) drives, when seen from air-gap. In this article, this feature of WRIM is explored to develop an improved Dual Direct Torque Control (DDTC) technique such that, two individual and independent conventional DTC controllers can be directly employed to control the stator and rotor VSCs. The improved technique is inculcated with all the benefits of conventional DTC controller, especially the benefit of zero dependency on rotor position. Furthermore, the increased torque ripple with the conventional DTC controllers is mitigated by modifying the Switching Tables (STs) such that, the Voltage Vectors (VVs) emulated in the air-gap are equivalent to a 3-Level Neutral Point Clamped (3L-NPC) VSC. The STs are developed by analyzing the WRIM as an Open-End Winding Induction Machine (OEWIM). To demonstrate the performance of the proposed DDTC technique, the experimental results obtained from 3 kW WRIM are presented, under various transients and steady state conditions. The reduction in the torque ripple is demonstrated by estimated torque ripple waveforms, at different speeds.

In this article, various converters integrated with renewable sources to the utility grid along with its control strategies are proposed. The proposed system uses a buck–boost converter to extract maximum power from photovoltaic (PV) and maintain power balance between the system and utility grid through a voltage source converter (VSC) employing bidirectional power flow. The main feature of the buck–boost converter presented here is to boost the dc voltage of the load by integrating the PV and dc output of the VSC in series. Moreover, the boost converter is utilized to extract the maximum power from a wind source. Additionally, a voltage management scheme is proposed to achieve a regulated constant voltage magnitude higher than that of a conventional voltage source converter integrated with the utility grid. Besides, it can be realized using a conventional voltage source converter along with the existing renewable sources in the system. Finally, the proposed system is validated through simulation and experimental results under various operating conditions.

In this paper, using Lyapunov’s stability theorem, the transient stability conditions for a grid-following Voltage Source Converter (VSC) are found. These conditions take into account both the grid specifications and the VSC’s dynamics. The derived conditions are based on a well-known nonlinear model of the VSC’s Phase-Locked Loop. To evaluate the stability of the nonlinear system, Lyapunov’s direct method is employed. To this end, a new Lyapunov function is proposed, and its characteristics are analysed. Using this Lyapunov function, the domain of attraction of the system’s equilibrium point is calculated. Additionally, a novel system strength index based on the domain of attraction of the system is proposed. The privilege of this index over the conventional indices are absoluteness, VSC’s dynamics consideration, and comparability of different VSCs with each other from a stability point of view. In the end, the correctness of the proposed stability analysis is validated via simulation in Matlab/PLECS and experiment.

Large scale renewable energy systems interfaced with voltage source converters (VSCs) experience several challenging issues when integrated to weak grids. One of the specific issue i.e. control loop interactions of vector-controlled VSC under high grid impedance leads to several instability mechanism ultimately resulting in unwanted tripping of the VSC unit. As a countermeasure, a unique nonlinear adaptive stabilizing control (NASC) technique has been proposed in this paper to enhance the dynamic stability of the VSC during such scenarios. A small-signal model is developed first to examine the instability mechanism of VSC connected to weak grids under abnormal operating conditions and varying set points of the power controller. The dominant modes are identified from the analysis and a stabilizing controller is developed based on adaptive dynamic surface control philosophy for enhancing its stability. The proposed controller has been proved to be uniformly ultimately bounded using Lyapunov analysis. Besides, the stabilizing mechanism of VSC is analyzed with the proposed controller by performing small-signal stability analysis of the entire system. The efficacy of the proposed control scheme is demonstrated by performing numerical simulations in MATLAB 2017a and conducting experimental validations in controller-hardware-in-the Loop (CHIL) platform.

This paper develops a new control strategy for voltage source converter (VSC) based multi-terminal high voltage dc (MT-HVDC) grids for enhancing the mutual frequency support between MT-HVDC interconnected asynchronous ac systems. The existing droop control structure of VSCs is augmented with an additional feedback voltage-based loop that generates a voltage shifting term to be added to the original dc voltage reference. The addition of this integrator-based controller nullifies the difference between voltage errors seen at adjacent converter stations. This action helps to overcome inaccuracies in the mutual frequency support caused by the uneven voltages across a dc system. The proposed strategy uses only local variables for the continuous-time control; thus, it does not rely on a high-bandwidth communication network, and its performance is not affected by the change of ac systems parameters. Modal analysis is carried out to investigate the stability of the proposed controller. Finally, comprehensive simulation studies are used to verify the proposed strategys performance and compare it with recently reported alternative controllers.

A step-up single-phase seven-level inverter is proposed in this paper. The proposed multilevel inverter (MLI) employs one voltage source and two symmetrical capacitors. Two capacitors are provided with self-voltage balancing ability, eliminating the requirement for sensors or additional control loop. The proposed topology generates bipolar levels inherently without the use of inverting H-bridge, so that the total standing voltage of switches is greatly reduced. The blocking voltages of all switches are limited to the input voltage, thus the proposed topology has low-voltage stress. Furthermore, the proposed structure has the advantages of voltage-boosting capability and high efficiency. The maximum output voltage is 1.5 times the input voltage. The prominent merits make it suitable for distributed generation applications. In this paper, the circuit configuration of the proposed topology, operation principle, and the phase disposition pulse width modulation (PD PWM) strategy are presented. Furthermore, the mathematical calculations of power loss and parameter comparisons are performed to demonstrate the superiority. Finally, experimental results are presented to verify the feasibility of the proposed topology.

This paper presents a control of the parallel operation of diode rectifier based high-voltage direct current (DR-HVDC) and voltage source converter based HVDC (VSC-HVDC) links. The two links are connected to offshore wind farms (OWFs). The VSC-HVDC controls the voltage and frequency of the offshore AC-grid needed for the OWFs start-up. The proposed grid forming control is based on controlling the OWFs voltage by controlling the active power balance at the VSC-HVDC AC-bus. When the OWFs inject more power than a certain active power reference established for the VSC-HVDC, the OWFs voltage increases and the DR-HVDC starts conducting. Then, the offshore voltage automatically accommodates the required value for transmitting the OWFs power through the DR-HVDC link while the system frequency is controlled by the VSC-HVDC through satisfying the reactive power balance. The power transmitted through the VSC-HVDC can be set by adjusting the corresponding power command while the DR-HVDC will automatically close the power balance. The main advantage of the proposed control system is that the VSC-HVDC can be sized just to support the system energization; while once the OWFs are in operation, the DR-HVDC will be loaded automatically. Simulation results verify the proposed control system.

For AC microgrids with three-phase unbalanced loading conditions, the four-wire voltage source inverters (FWVSIs) have become an advisable interfaced converter between the source and loads. Predictive control has been applied to FWVSIs in recent years, but model errors are not solved well, which is caused through parameter mismatch, sampling error and time delay. Thus, the accurate following between reference value and predicted value is difficult to achieve. To this end, a dual-predictive control based on adaptive error correction (DPCEC) for the FW-VSIs is presented. Firstly, the state-space function of FW-VSIs under reference frame is built. Then, the DPCEC strategy is proposed based on the above issue. Therein, an adaptive error correction strategy is severally embedded into both the outer and inner prediction loop. Noting that the impacts of different negative factors can be simultaneously processed and corrected through adaptive error correction strategy. Not only does the proposed control strategy achieve better performance in steady state, but also retains fast dynamic response in transient state. Finally, simulation and experimental results which verify the high performance of the proposed control techniques are provided.

This paper presents a new power electronic topology for a STATCOM allowing the generation of multilevel AC voltages. This topology is characterized by a modular configuration since it uses two well known four-leg two-level voltage source inverters. A Scott transformer with multiple secondary windings connects the two four-leg inverters to the grid. One of the important features of the proposed power topology is the capability to generate AC voltages with high number of levels, in spite of using only two four-leg two-level voltage source inverters. To maximize the number of voltage levels a geometric progression ratio of 3 between the windings of the transformers connected in cascade is proposed. However, to reduce the turns-ratio span, alternatively a fractional geometric ratio of 1.5 is also proposed and analyzed. A control strategy adapted to the proposed power converter topology is also presented. Besides the control of the reactive power the regulation of the four-leg inverter DC voltage will also be considered. To support the theoretical studies and assumptions, simulations and experiments are presented. Experimental results were obtained using a low power laboratory prototype. The results shown give evidence to the characteristics and features of the proposed power converter topology and control system

The voltage-source grid-supporting inverters (GSIs) are important to establish and support the frequency and voltage of a microgrid (MG) in both islanded and grid-connected modes. Traditionally, a cascaded control strategy that includes an external droop-based control loop and an inner proportional-integral (PI) based voltage and current control loop is adopted to control a GSI, which shows barely inertia and limited control performance. In this paper, a novel cascaded control strategy is proposed for a GSI. In the external loop, a VSG-based control strategy is adopted to increase the frequency stability of the MG, and a novel synchronization control method that does not use a complex PLL unit is proposed to satisfy the plug-and-play requirements of a GSI. In the inner voltage and current control loop, the paper proposes an SMC-based vector control strategy which uses the fixed switching frequency and inherits the robustness and fast response speed of the SMC control. Both simulation and experiments demonstrate the effectiveness and improved performance of the proposed control strategy.

The main objective of this study is to focus on dynamical analysis of full-scale direct-driven permanent magnet synchronous generator (PMSG)-based wind energy conversion systems (WECSs) configured with back to back (BTB) voltage source converters (VSCs). The mathematical methods have employed to discuss the properties WECSs in a theoretical manner, which are proven to be cost-effective. It can be seen that the considered model contains the nonlinearities in terms of currents and rotation speed of shaft. The Takagi-Sugeno (T-S) fuzzy approach is considered to be an effective tool to approximate those nonlinearities into local linear form without compromising the model characteristics. Technically, WECSs are networked to each other, hence event-triggered-based control (ETC) scheme is appropriate in terms of reducing of packet loss and enhancing the stable performance of the WECSs in an effective manner. Further, ETC scheme provides the user-designed condition to activate the controllers that helps to restrict the unnecessary network transmissions and minimize the leakages. Theoretically, by the virtue of fuzzy Lyapunov function, intensive attention is focused on deriving the theoretical-based sufficient conditions in terms of solvable linear matrix inequalities (LMIs), which ensure the global stability of the closed-loop model based on Lyapunov stability theory. Detailed numerical simulations are performed with an experimental range of model parameters that evidence the effectiveness of the proposed ETC scheme in WECS.

In three-phase three-wire (3P3W) voltage-source converter (VSC) systems, utilization of filter inductors with deep saturation characteristics is often advantageous due to the improved size, cost, and efficiency. However, with the use of conventional synchronous frame current control methods, the inductor saturation results in significant dynamic performance loss and poor steady-state current waveform quality. This article proposes an inverse dynamic model-based compensation (IDMBC) method to overcome these performance issues. For this purpose, two-phase exact modeling of the 3P3W VSC control system is obtained. Based on the modeling, the inverse system dynamic model of the nonlinear system is obtained and employed such that the nonlinear plant is converted to a virtual linear inductor system for linear current regulators to perform satisfactorily. Further, to control phase currents in the synchronous frame, a two-phase coordinate transformation is proposed. The IDMBC method is tested via dynamic command response and waveform quality simulations and experiments that employ saturable inductors reaching down from full inductance at zero current to 1/9th inductance at full current. The results obtained demonstrate the suitability of the method for 3P3W VSCs employing saturable inductors. Publisher's Version

This paper proposes an analytical method for the selection of the machine parameters and inverters power ratings of a synchronous motor operated in Open-End Winding (OEW) configuration with one of the two Voltage Source Inverters (VSIs) connected to a floating capacitor (FC) in order to meet a specific torque-speed profile. With the derived normalized expressions and curves all the suitable synchronous motors operated with an OEW-FC and a standard 2L-VSI drive can be easily compared according to the power rating and desired speed-torque characteristic.

In conventional SR driving schemes, the drain to source voltage of SR is detected to generate its driving signal, However, the driving signal by this method will lose a portion of duty cycle due to the parasitic inductance in SR package and on-board parasitic inductances. This phenomenon is severe under heavy load and deteriorates the efficiency of the converter. In this paper, a novel LLC synchronous rectifier (SR) driving scheme based on secondary rectification current emulation is proposed. An auxiliary winding of the transformer and an auxiliary winding of the resonant inductor located on the secondary side are in series to generate a superposed voltage signal. The secondary rectification current is emulated out according to the superposed voltage signal by an integration circuit. After that, the SR driving signal, which is not affected by parasitic inductances, is generated with a logic circuit. Detailed theoretical analysis and Circuit implementation have been presented. Finally, a 400V input and 12V/20A output half bridge LLC resonant converter prototype has been built up to verify the feasibility of the proposed SR driving scheme.

The full-bridge can further improve the output power through parallel inverters. The inverter output voltage of single-switch LC resonant circuit is a combination of trapezoidal wave and half sine wave, there is no relevant research on parallel connection of single-switch LC resonant circuit. Therefore, this paper presents a novel input-parallel single-switch LC resonant circuit, this topology adopts switchable secondary networks for constant-current (CC) and constant-voltage (CV) output, and prevents shoot-through of power switches. The decoupling between transmitters simplifies the analysis and calculation. The input-parallel structure avoids the unbalanced input voltage of each inverter and improves the stability of the WPT system. CC and CV output modes can be achieved by controlling one relay without adding DC-DC converters or changing switching frequency. This paper includes decoupling equivalent analysis of coils, topologies analysis of CC mode and CV mode, calculation of equivalent input AC voltage source, design of magnetic coupler and circuit parameters. Finally, a 1-kW experimental prototype is built to verify the theoretical analysis, the maximum efficiency can reach 92.5%.

Model-based predictive power control (MPPC) is a well-known and useful technique for control of electric drives and renewable energy generation systems. However, this strategy relies on the knowledge of accurate system models and parameter values and would otherwise lead to tracking errors in applications because of inevitable parameter uncertainties. Also, step delays in MPPC implementations have to be compensated to eliminate errors. This paper proposes a predictive control application to control active and reactive powers exchanged between the grid and the grid side converter (GSC) interfacing renewable energy sources such as wind farms or photovoltaic. The proposed MPPC minimizes the power tracking error based on a proposed cost function and the control system output is the voltage reference for the electronic converter that is converted into switching pulses by the modulation stage. The proposed control strategy is designed to eliminate tracking errors and has fixed switching frequency while featuring a fast-dynamic response, low current THD, and low computational burden. The method has been evaluated by using Matlab/Simulink environment and an experimental 5 kW grid-connected voltage source converter. Finally, the proposed MPPC method has been critically compared to the previous MPPC approaches.

In this article, a novel 13-level inverter Single source, Switched-Capacitor Multi-Level Inverter (SSC-MLI), is proposed. This topology is suitable for renewable energy applications using less input voltage source magnitude. This structure is capable of boosting the input voltage six times with the help of switched capacitors. The capacitors are automatically balanced without any control algorithms, complex circuits, or closed-loop controllers. The advantages of the proposed structure are high power density and high efficiency by use of only switches. Since, there is no diodes there is no forward conduction loss, and no reverse recovery delay. The maximum blocking voltage across individual switch is three times the input voltage. The functionality of the SSC-MLI is described in detail. The capacitance calculation and optimum value of capacitors are discussed. A suitable comparison is presented for the proposed structure with the existing literature to check the inverter performance. The power loss for exiting a 13-level inverter is presented. The simulation is carried out for both pure resistive and inductive load. Later, the experimental results are presented for variation in frequency, dynamic change in load, variation in modulation index, and step-change in input voltage to validate the proposed topology performance and feasibility.

This paper proposes a novel three-phase hybrid rectifier topology and control method for high power electrolytic hydrogen production application. It is composed of a primary rectifier, i.e., a 6-pulse silicon controlled rectifier (SCR) and an auxiliary converter, including a pulse width modulation (PWM) voltage source converter (VSC) and a phase-shift-full-bridge (PSFB) converter. SCR undertakes most of the power consumed by electrolysis cell (EC), and the auxiliary converter can compensate dc current ripples and absorb ac current harmonics of primary rectifier simultaneously. Compared with high power PWM rectifier, the proposed hybrid rectifier can keep the main benefits of PWM rectifier, such as the low dc current ripple to realize improved efficiency of electrolytic hydrogen production and high grid current quality at ac side, while significantly reducing the cost. The effectiveness of proposed hybrid rectifier is verified by the simulation as well as experimental results.

In this paper, a buck based active-clamp circuit for current-fed isolated DC-DC converters is proposed. The proposed active-clamp circuit can not only suppress the voltage spikes caused by the current difference between the current-fed inductor and leakage inductor of the transformer, but also transfer the energy of the above process to the input voltage source. Different from the traditional active-clamp circuit, the proposed active-clamp circuit needs only one switch regardless of the number of input phases in the main circuit. Moreover, the control of the proposed active-clamp circuit is decoupled with the main circuit, which makes the design and implementation of the system controller easier. A 400W experimental prototype has been built to verify the effectiveness of the proposed circuit.

The Current Shaping Modular Multilevel DC-DC Converter (CS-MMC) is a recently proposed class of modular multilevel dc-dc converters for dc distribution grid applications consisting of a single string of cascaded voltage source submodule cells (VSMs), a current source submodule (CSM) and notably no series inductor. In the CS-MMC, since it is the CSM that shapes the string current and there is no series inductor, this frequency can readily be in the medium-frequency range, enabling VSM cells to be realized with low cell capacitance. In the previous work, only continuous conduction mode (CCM) operation of the CS-MMC had been considered which led to several limitations including elevated switching losses and low utilization of the semiconductor devices in low output voltage applications. In this work, discontinuous conduction mode (DCM) operation is proposed for the CS-MMC which addresses these limitations. In traditional dc-dc converters, DCM is determined by the inductor current ripple. However, unique to the CS-MMC, DCM is determined by the VSM capacitor voltage ripple. In this work, the proposed DCM operational approach is presented along with its analysis and control. Both simulation and experimental results from a laboratory scale converter system are provided for validation.

This work aims to develop a solar- battery energy storage (BES) based system, which ensures an uninterruptable supply to loads irrespective of the availability of the grid. This system comprises of a solar photovoltaic (PV) array, a BES, the grid and local residential loads. A new control is implemented such that the active power demand of residential loads, is fed from the PV array, a BES unit and the utility grid. In this system, the power control operates in different power modes, which delivers the benefits to the end-users with the integration of BES and an excess of PV array power, which is sold back to the grid. For this, effective control logic is developed for the grid-tied voltage source converter (VSC). Moreover, this system deals with the issue of an integrating power quality enhancement along with the power generation from the solar PV source. The cascaded delayed signal cancellation (CDSC) based phase-locked loop (PLL) is implemented for grid synchronization during the grid voltage distortion. The developed control is easily implemented in a real-time controller (dSPACE-1202).

High field intensity electromagnetic radiation effect test is difficult to simulate in large-scale space, and the existing bulk current injection (BCI) technology has a limited application of the nonlinear system test. Starting from the coupling mechanism of shielded wire, an analysis model about the response of the equipment under test (EUT) is built-in radiation and injection conditions. In addition, the equivalent corresponding relationship originates between the injected excitation source voltage and the radiation field intensity. The basis for obtaining the source of the injection excitation and performing extrapolation tests is found, and the test method of BCI replacing high field intensity electromagnetic radiation is developed. Subsequently, the passing-through load and communication station tests are performed to verify the theoretical derivation and the test method. As indicated from the research results, the BCI is capable of accurately replacing the high field electromagnetic radiation effect test of the EUT, and the maximum test error reaches 2.7 dB, capable of satisfying the actual needs of the project.

Challenges in long distance power transmission and large-scale integration of renewable energy systems drive the development of high-voltage direct current (HVDC) systems. HVDC systems can be configured as combinations of line-commutated converters (LCCs), voltage source converters (VSCs) and hybrid systems. However, each hybrid configuration poses its own challenges and requires unique control. Thus, it is necessary to explore other possible configurations, especially different configurations in more complex hybrid multiterminal DC (MTDC) systems and future dc grids. This paper develops the steps for integration of alternate arm converters (AACs) in hybrid MTDCs and demonstrates the operation of AACs in a hybrid MTDC system. The detailed system structure and control hierarchy are developed. The performance of the hybrid MTDC system under multiple operating scenarios including ac and dc faults are demonstrated, verifying the feasibility of an LCC and AAC-based hybrid MTDC system.

The modular multilevel converter (MMC) has emerged as the preferred choice for voltage source converter (VSC)-based high voltage direct current (HVDC) systems due to its low losses, low harmonic distortion, modularity, and redundancy. These advantages come at the expense of a complex control system with the strong coupling among its control variables, which complicates the design procedure of both its individual control loops and the DC link controllers. In fact, the performance improvement of a control loop could lead to degraded performance of other loops. Hence, to deal with such a complex dynamic system, this article suggests adopting multivariable H optimal control techniques in order to ensure stability and optimized performance of a VSC-HVDC link. First, a full-order, centralized multi-input-multi-output (MIMO) controller is derived based on H optimization and used as a benchmark for the system performance level. Then, a fixed-structure, decentralized MIMO controller is synthesized to make a compromise between the optimal performance and feasibility of practical implementation. Finally, simulations are conducted to evaluate and compare the performance and robustness degradation due to the migration from a high-order, centralized controller towards a low-order, decentralized controller.