Search

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.

This paper proposes a design for a modified distribution static compensator (DSTATCOM) with a proposed current controller for microgrid (MG) applications. The modified DSTATCOM is incorporated in a three-phase four-wire network system to deal accurately at grid connected mode with balanced, unbalanced and excursion load for both nonlinear and inductive loads. Moreover, it handles the stability issues of the MG at fault caused islanding incidents. The design criteria of the modified DSTATCOM parameters is presented and discussed. The hysteresis current controller is introduced to drive the voltage source converter (VSC) and its reference voltage signal is generated based on the instantaneous symmetrical components theory (ISCT). Matlab/Simulink software is used to verify the performance of the modified DSTATCOM with the proposed controller under different load types and loading conditions for grid connected mode and islanded mode of MG operation. Based on the simulation results, the power factor is improved and the non-linear harmonics currents are compensated therefore the harmonic distortion of the supply currents and voltage are reduced, moreover the stability operation at fault cause islanding is maintained. The results show the superiority of the proposed modification of DSTATCOM in dealing with unbalanced, distorted and bad power factor system in comparing with conventional DSTATCOM.

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.