Search

This paper introduces a hybrid modular multilevel converter (MMC) with the full-bridge submodules (FBSMs) inserted in the ac side. It is intended for redistributing arm powers of the MMC system to reduce the submodule (SM) capacitor voltage fluctuation by injecting high-frequency powers in upper and lower arms without disturbing the output voltage. New power distribution of the hybrid MMC system can be realized by controlling the SMs and FBSMs working under the high-frequency powers. The proposed FBSMs in the hybrid MMC system also perform as the harmonic power generator to eliminate the common-mode voltages in the ac side. Since the FBSMs locate in the ac side of the hybrid MMC system, the traditional MMC control method can be directly applied in arm control of the hybrid MMC system. Meanwhile, relationship between the arm powers and SM capacitor voltage has been analyzed in detail to reduce the SM capacitor voltage ripples by redistributing the frequency spectrum of arm powers. Simulation and experimental results have also verified the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents an alternative approach to address the control and modulation problem of single-phase three-level converters applied in the high-speed railway electrical traction drive system. Following the principle of deadbeat predictive direct torque control of ac motors, this paper discusses an improved direct power control (DPC) method based on a deadbeat active and reactive power prediction technique. Comparing with the conventional PI-based DPC scheme, the proposed deadbeat predictive DPC scheme can provide these advantageous features: lower current harmonics and THD index, lower active and reactive power ripples, and fewer adjusted parameters. Moreover, compared with PI-based DPC with the PI parameters optimization, this approach can also easily obtain fast dynamic response but without the main voltage orientation. A single-phase three-level space vector pulse width modulation (SVPWM) with inherent neutral-point voltage balancing capability is adopted, which can be combined with DPC scheme as an overall control and modulation system. A series of simulation and experimental tests have been conducted to demonstrate an excellent performance of the deadbeat predictive DPC. In addition, the neutral-point-voltage balancing ability of the adopted SVPWM method has been verified. [ABSTRACT FROM PUBLISHER]

Conventional photovoltaic (PV) single/double stage grid – tie centralized and string inverters suffers from the problem of lower efficiency, high filter size, and limited depth of operation for remaining in connection with the grid, particularly under the lower insolation/partial shading condition. This paper proposes a staggered PV connection through cascaded multilevel converter (CMC) for PV-grid tie application utilizing independent maximum power point tracking controller, providing the larger depth of operation under partial shading condition with smaller filter size and electromagnetic interference (EMI). For dynamic and steady-state performance evaluation, a d-q frame-based control algorithm is investigated for the single-phase PV-CMC system. Furthermore, to establish the stability of the proposed controller, a detailed plant model is also investigated along with the detailed comparative analysis for operation of PV under partial shading condition for a conventional PV based centralized and string inverter vis-à-vis proposed PV-CMC approach. The improved performance analysis is demonstrated both through simulation and experimentation. [ABSTRACT FROM PUBLISHER]

This paper studies the techniques for modeling modular multilevel converter (MMC) based multi-terminal HVDC (MTDC) systems in the electromechanical transient mode. Firstly, the mathematical model of the MMC and its corresponding equivalent circuit are established, which are similar to those of the two level converters. Then, a power flow calculation method for AC/DC systems containing MMC-MTDC systems is developed. Two dynamic models for MMC-MTDC systems are developed in the paper. One is the detailed model, taking into account of the AC side circuit, the inner controllers, the modulation strategies, the outer controllers and the MTDC circuit. The other is the simplified model, which only reserves the outer controllers and partial dynamics of the MTDC circuit based on a quantitative analysis of the detailed model's dynamic processes, and it can be used in electromechanical transient simulation with a larger step size. Both the detailed and the simplified models are implemented on PSS/E and compared with the accurate electromagnetic transient models on PSCAD in a four terminal MMC-MTDC system; the result proves the validity of the developed models. Lastly, a stability study of a modified New England 39-bus system is executed, and the result shows that the AC fault can be isolated well in an MMC-MTDC asynchronously connected AC grid. [ABSTRACT FROM PUBLISHER]

This paper discusses the extension of electromechanical stability models of voltage source converter high voltage direct current (VSC HVDC) to multi-terminal (MTDC) systems. The paper introduces a control model with a cascaded DC voltage control at every converter that allows a two-terminal VSC HVDC system to cope with converter outages. When extended to an MTDC system, the model naturally evolves into a master-slave set-up with converters taking over the DC voltage control in case the DC voltage controlling converter fails. It is shown that the model can be used to include a voltage droop control to share the power imbalance after a contingency in the DC system amongst the converters in the system. Finally, the paper discusses two possible model reductions, in line with the assumptions made in transient stability modeling. The control algorithms and VSC HVDC systems have been implemented using both MatDyn, an open source MATLAB transient stability program, as well as the commercial power system simulation package EUROSTAG. [ABSTRACT FROM PUBLISHER]

This paper proposes a startup sequence for a modular multilevel converter (MMC) HVDC-connected offshore wind farm (OWF) and demonstrates how it can be simulated using an electromagnetic transient-type tool. The objective is to analyze inrush currents of transformers and cables (HVAC and HVDC). Modeling accuracies and computational performances are examined using three existing modeling methods for the MMCs. [ABSTRACT FROM PUBLISHER]

This paper is the continuation of Part I in which a quasi-Z-source modular cascaded converter (qZS-MCC), comprising front-end isolated qZS half-bridge dc–dc converter submodules (SMs), for dc integration of high-power photovoltaic (PV) systems is proposed. The qZS-MCC-based PV system features modular structure, high-voltage dc collection of PV power, simple control with a unified and constant duty cycle for the front-end isolation converter of all SMs, and low qZS impedance due to no double-line-frequency pulsating power. Here, control scheme of the qZS-MCC PV system integrated into the dc collection grid is investigated. Dynamic model of the system is established for controllers design and time-domain transient simulation. Experimental tests are carried out on the downscaled prototype as a proof-of-concept of the proposed control and modeling, demonstrating the validity of the proposed approaches. [ABSTRACT FROM PUBLISHER]

The increased penetration of nonlinear loads and power-electronics-based distributed generation (DG) systems may introduce power quality issues to the distribution power system. However, if controlled and regulated properly, the DG–grid interfacing converters are able to improve the distribution system efficiency and power quality. This paper focuses on the distribution system harmonic control through the DG–grid interfacing converters. Two alternative DG systems, namely, current-controlled DG and voltage-controlled DG, are considered. While most of the previous works on harmonic compensation are based on the current-controlled method, a novel harmonic control scheme using a voltage-controlled method is developed in this paper. The voltage-controlled method is more flexible and has similar compensation performance compared to the conventional current-controlled method. In addition, by avoiding the implementation of a harmonic current tracking loop, the proposed voltage-based method can be seamlessly incorporated into a voltage-controlled DG unit, which is important to provide direct voltage and frequency support in a microgrid. Moreover, the possible harmonic circulating current among multiple DG systems is also investigated in this paper. Simulations and experimental results from a three-phase 5-kVA laboratory DG prototype are provided to validate the discussion. [ABSTRACT FROM AUTHOR]

In photovoltaic power systems, both photovoltaic modules and switching-mode converters present nonlinear and time-variant characteristics, which result in a difficult control problem. This paper presents an in-depth analysis and modeling to discover the inherent features of a photovoltaic power system. The method of successive linearization simplifies the nonlinear problem back to the linear case. This paper also presents the use of Youla parameterization to design a stable control system for regulating the photovoltaic voltage. The experimental and simulation results demonstrate the effectiveness of the presented analysis, design, and implementation. [ABSTRACT FROM AUTHOR]

This paper presents two single-phase to three-phase conversion systems for a three-phase load application. The load is connected to a single-phase grid through an ac–dc–ac single-phase to three-phase converter. The single-phase rectifier is composed of two parallel single-phase half-bridge rectifiers. The first proposed topology is composed of a full-bridge three-phase inverter, i.e., three-leg inverter, while the other topology is composed of a two-leg inverter. Suitable modeling, including the circulation current, and control strategy are presented. A pulsewidth modulation (PWM) technique using a single or double carriers PWM implementation is presented. Proposed topologies permit to improve the harmonic distortion. In addition, the P5L converter can reduce the converter power losses. Finally, simulation and experimental results are presented for validation purposes. [ABSTRACT FROM AUTHOR]

Voltage source back-to-back power converters are widely used in grid-tied applications. This paper presents a quasi-centralized direct model predictive control (QC-DMPC) scheme for back-to-back converter control without a dc-link outer-loop controller. Furthermore, the QC-DMPC is experimentally compared with a conventional proportional-integration (PI) dc-link controller-based DMPC (PI-DMPC) scheme. For the QC-DMPC scheme, the dc-link voltage is directly controlled by a grid-side predictive controller using a dynamic reference generation concept and load-side power estimation. For the PI-DMPC scheme, the dc-link voltage is controlled by an external PI controller. Both schemes are implemented on a field programmable gate array (FPGA)-based platform. Effectiveness of the proposed QC-DMPC is verified by both simulation and experimental data. Moreover, FPGA implementation issues (resource usage and timing information), dc-link control performance, and robustness to parameter variation of the two DMPC schemes are compared in detail. The results emphasize that the QC-DMPC may outperform the PI-DMPC scheme in normal operation but with a slightly higher usage of FPGA resources. However, PI-DMPC scheme is more robust when parameter variations are considered. [ABSTRACT FROM PUBLISHER]

The feasibility of future multiterminal dc (MTDC) systems depends largely on the capability to withstand dc-side faults. Simulation models of MTDC systems play a very important role in investigating these faults. For such studies, the test system needs to be accurate and computationally efficient. This paper proposes a detailed equivalent model of the modular multilevel converter (MMC), which is used to develop the MTDC test system. The proposed model is capable of representing the blocked-mode operation of the MMC, and can be used to study the balancing control of the capacitor voltages. In addition, the operation of the MMC when redundant submodules are included in the arms can also be studied. A simplified model of a hybrid high-voltage dc breaker is also developed. Hence, the developed test system is capable of accurately describing the behavior of the MMC-based MTDC system employing hybrid HVDC breakers, during fault conditions. Using time-domain simulations, permanent dc-side faults are studied in the MTDC system. In addition, a scheme to control the fault current through the MMC using thyristors on the ac side of the converter is proposed. [ABSTRACT FROM AUTHOR]

This paper studies a regenerative cascaded multilevel converter that substitutes active front ends for the diode-based front ends only in part of cells, and proposes a power tracking control strategy that does not need any motor parameters. During the induction motor's deceleration, by controlling the two kinds of cells' output voltages according to the motor's power factor angle, the converter makes the motor-generated energy wholly collected by regenerative cells; thus, other cells' dc-link voltages remain stable. Regeneration limitation is analyzed. The simulation results of a 6-kV six-cell cascaded converter and experiments on a laboratory cascaded converter verify that the converter can transfer the motor power back to the grid and allows the motor to decelerate more quickly. Moreover, the control strategy is suitable for the drive systems whose regenerated power cannot be estimated or is varying. [ABSTRACT FROM PUBLISHER]

In this paper, a generalized switching technique for cascaded H -bridge (CHB) converters is proposed. To take advantage of both low- and high-frequency modulation techniques, a hybrid modulation method is utilized. The proposed technique employs the low-frequency switching for voltage balancing of the dc-link capacitors, and the high-frequency sinusoidal pulse-width modulation switching for shaping the input ac current. Using the proposed technique, the stable operating region of CHB converters is extended. The proposed technique is capable of maintaining the dc voltages balanced even under critical operating conditions in which the load of one H-bridge cell is removed. The validity and effectiveness of the proposed method is confirmed by several simulation and experimental tests on a seven-level CHB rectifier. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is a newly introduced switch-mode converter topology with the potential for high-voltage and high-power applications. This paper focuses on the design and control methods for fault-tolerant operation using redundant submodules (SMs), which is one of the most important features in the MMC topology. By comparing three design schemes of redundant SMs, the most economic and reliable scheme is identified. In addition, a mathematical model of the MMC with arms containing different numbers of SMs is developed. Based on the identified scheme and mathematical model, an energy-balancing control strategy is proposed to keep the MMC operating normally under SM fault conditions. Finally, time-domain simulations of a 61-level MMC system are performed, using the PSCAD/EMTDC software, while experiments are carried out on a 41-level MMC prototype. The simulation and experimental results validate the design scheme, mathematical model, and proposed energy-balancing control strategy. [ABSTRACT FROM AUTHOR]

This paper focuses on the common-mode voltage reduction and the power quality of matrix converters for a low-voltage transfer ratio of less than 0.5 by the direct space vector modulation (DSVM) method. The principles of selecting suitable switching configurations based on new space vector sectors are described, and an entire switching table for the new control strategy is developed. The matrix converter with the proposed method reduces peak-to-peak common-mode voltage remarkably. The new DSVM method has advantages over the conventional DSVM method, such as lower total harmonic distortion of line-to-line output voltage and a reduction in switching loss by a minimum number of switching commutations. The proposed DSVM method can be easily implemented in software. Simulation and experimental results are shown to verify the effectiveness of the proposed DSVM method. [ABSTRACT FROM AUTHOR]

Low system voltage in traction networks, mainly caused by active power absorption of locomotives, adversely affects the performance of the locomotives and also the power transmission capability of the catenary line. This paper introduces a voltage support scheme to compensate for the adverse effects of low system voltage. The proposed method is based on the injection of reactive power through the current-controlled line-side converter of locomotives. Comparing the catenary voltage with its reference value, the error is fed to a high-order controller. The controller generates the quadrature (q)-axis reference value of a current control strategy, which is responsible for the reactive power injection. To design the high-order controller, adopting the nonparametric models of the system at various locations, an optimization-based loop-shaping approach is used. The loop shaping guarantees the stability and the acceptable performance of the closed-loop system for various locomotive positions in the network. The performance of the proposed control strategy is evaluated based on simulation results in MATLAB/PLECS environment. Moreover, implementing a scaled-down laboratory setup, the performance of the proposed scheme is experimentally evaluated. [ABSTRACT FROM AUTHOR]

Proper operation of a multilevel flying capacitor converter (FCC) necessitates a closed-loop voltage balancing strategy. This paper proposes a space vector modulation (SVM)-based approach that benefits from the switching state redundancy of an n-level FCC to implicitly carry out the voltage balancing task within the switching strategy. Based on the voltage deviations of the capacitor voltages, a cost function is defined and minimized to choose the proper redundant switching states among the available switching states. The performance of a grid-connected four-level FCC under the proposed SVM strategy for various operating conditions is studied and evaluated based on time-domain simulations in the PSCAD/EMTDC environment, and experimentally verified. The studies demonstrate the capability of the proposed SVM strategy to regulate the capacitor voltages at their nominal reference values. [ABSTRACT FROM AUTHOR]

Performance of a carrier-based pulsewidth modulation (CB-PWM) strategy can be improved by the inclusion of a zero-sequence voltage in the modulation-reference signal. This paper proposes a new CB-PWM strategy for a three-level neutral-point-clamped (NPC) converter, which is based on a zero-sequence voltage injection. By inclusion of the zero-sequence voltage, the sinusoidal-modulation reference is modified to 1) carry out the voltage-balancing task of the dc-link capacitors, with no additional control effort, 2) reduce the switching losses, and 3) reduce the low-frequency voltage oscillations of the neutral point. The proposed strategy is an alternative approach to the nearest three-vector (NTV) space-vector modulation (SVM) strategy and is obtained by the analysis of the NTV-SVM strategy and establishing a correlation between the NTV-SVM and the CB-PWM strategies. The salient features of the proposed scheme, as compared with the NTV-SVM strategy, are: 1) its reduced computational processing time which is attractive for digital implementation and 2) its reduced switching losses. Compared with the existing CB-PWM strategies, the proposed strategy offers 1) capability to balance the capacitor voltages and reduce the NP voltage oscillations and 2) reduced switching losses. Performance of the proposed CB-PWM strategy for a three-level NPC converter based on time-domain simulation studies in the MATLAB/SIMULINK environment is evaluated and also experimentally verified. [ABSTRACT FROM AUTHOR]

By utilizing multiline voltage-source (VSC)-based flexible ac transmission system (FACTS) controllers, independent controllability over each compensated line of a multiline system can be achieved. While VSC-based multiline FACTS controllers emerged as a new opportunity to control two independent ac systems, the main constraints and limitations that are presented to the conventional transmission-line protection systems need to be investigated. In this paper, the impacts of VSC-based FACTS controllers on distance relays while controlling the power flow of compensated lines are evaluated analytically and by detailed simulations for different fault types and locations. [ABSTRACT FROM PUBLISHER]

The matrix converter is a single-stage ac–ac power conversion device without dc-link energy storage elements. Any disturbance in the input voltages will be immediately reflected to the output voltages. In this paper, a modified space vector modulation strategy for matrix converter has been presented under the abnormal input-voltage conditions, in terms of unbalance, nonsinusoid, and surge (sudden rising or sudden dropping). By using the instantaneous magnitude and phase of input-voltage vector to calculate the voltage modulation index and input-current phase angle, this modified modulation strategy can eliminate the influence of the abnormal input voltages on output side without an additional control circuit, and three-phase sinusoidal symmetrical voltages or currents can be obtained under normal and abnormal input-voltage conditions. The performance of the input currents is analyzed when the matrix converter uses different modulation strategies. Some numerical simulations are presented to confirm the analytical results. Tests are carried out on a 5.5-kW matrix converter prototype. Experimental results verify the validity of the proposed strategy. [ABSTRACT FROM AUTHOR]

Common-mode voltage (CMV) is responsible for overvoltage stress to the winding insulation and bearing damage of an ac motor. High dv/dt of CMV raises leakage currents, which can cause serious problems such as motor damage and electromagnetic noise to the equipment installed near the converter. This paper proposes two modulation strategies for indirect matrix converters that substantially eliminate CMV. The first method is developed by using the suitable couple of nonzero space vectors instead of zero space vectors in the inverter stage. The maximum voltage transfer ratio of this method is unaffected and remains 0.866. The second one, which is useful for low-voltage operation, is approached by choosing the medium and the lowest positive line-to-line input voltages in the rectifier stage to generate the dc-link voltage and by placing suitable zero space vectors in the inverter stage. In the second method, the maximum voltage transfer ratio is 0.5. Both methods can mitigate the peak value to 42%. In addition, while the first method provides the same performance to the conventional method, the second method can reduce significantly the harmonic components of line-to-line output voltage. Simulation and experimental results are shown to verify the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

This paper proposes a new topology for bidirectional multiinput direct dc–ac converter for clean-power-generation system, therefore, at the input port, a boost converter is used to meet the requirement of many distributed generation systems, such as photovoltaic, and fuel cell systems. The boost converter can increase the dc input voltage. This point results in a turn ratio reduction of high-frequency isolating link transformer. Furthermore, the boost inductor reduces the input current ripple, therefore, the saturation of the transformer can be avoided for higher output currents. The count of used devices has been reduced in the proposed converter. As a result, the cost, size, and the volume of the converter can be reduced. The reduced conversion steps increase the efficiency of the converter. The converter is analyzed step by step to show the principles of the operation in providing energy management based on duty-cycle variations. The system capability in different operation conditions has been simulated, and the simulation results have been compared with measurement results. Dynamic analysis and control design of the bidirectional dc–ac converter is presented based on nonlinear control by input–output feedback linearization. [ABSTRACT FROM AUTHOR]

In this paper, a cascade controller is designed and analyzed for a boost converter. The fast inner current loop uses sliding-mode control. The slow outer voltage loop uses the proportional–integral (PI) control. Stability analysis and selection of PI gains are based on the nonlinear closed-loop error dynamics. It is proven that the closed-loop system has a nonminimum phase behavior. The voltage transients and reference voltage are predictable. The current ripple and system sensitivity are studied. The controller is validated by a simulation circuit with nonideal circuit parameters, different circuit parameters, and various maximum switching frequencies. The simulation results show that the reference output voltage is well tracked under parametric changes, system uncertainties, or external disturbances with fast dynamic transients, confirming the validity of the proposed controller. [ABSTRACT FROM AUTHOR]