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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]

In this paper, we demonstrate how latency insertion method (LIM) can be used for real-time simulation of high switching frequency power electronics systems and how this approach can be implemented for scalable Field Programmable Gate Array (FPGA) execution. We first present a summary of the LIM and how this method can be developed into a solver for high-performance FPGA execution. We then present how common power electronics topologies—buck and boost—can be modeled using the LIM approach, followed by how the LIM model of a three-phase dc/ac converter is created. Afterward, we demonstrate the accuracy of the developed FPGA solver through a set of power electronic system examples, with each example compared with near ideal results of same example provided by a traditional electromagnetic transient type solver. We complete the paper analyzing the scalability of the proposed approach on FPGA devices, both in terms of achievable time step as well as of resource usage. [ABSTRACT FROM AUTHOR]

In several electrical dc–ac power conversions, the ac output voltage is higher than the input voltage. If a voltage-source inverter (VSI) is used, then an additional dc–dc boosting stage is required to overcome the step-down VSI limitations. Recently, several impedance source converters are gaining higher attentions [1], [2], as they are able to provide buck-boost capability in a single conversion stage. This paper proposes the merging of the boost stage and the VSI stage in a single stage dc–ac power conversion, denoted as split-source inverter (SSI). The proposed topology requires the same number of active switches of the VSI, three additional diodes, and the same eight states of a conventional space-vector modulation. It also shows some merits compared to Z-source inverters, especially in terms of reduced switch voltage stress for voltage gains higher than 1.15. This paper presents the analysis of the SSI and compares different modulation schemes. Moreover, it presents a modified modulation scheme to eliminate the low frequency ripple in the input current and the voltage across the inverter bridge. The proposed analysis has been verified by simulation and experimental results on a 2.0-kW prototype. [ABSTRACT FROM PUBLISHER]

Superjunction MOSFETs in cascode configuration with low-voltage silicon MOSFETs are evaluated in this paper. The proposed structure combines the good switching performance provided by the cascode configuration with the benefits of the silicon technology such as its robustness, maturity, and low cost. This paper aims to explain and to demonstrate the reduction of switching losses of superjunction MOSFETs in cascode configuration with respect to their standalone counterparts (directly driven). A detailed simulation analysis of power loss contributions is carried out under hard-switching operation. Moreover, experimental evidence is provided using a boost converter (100–400 V) operating in continuous conduction mode for different switching frequencies (100–400 kHz) and output power levels (180–500 W). [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]

A recently reported modeling methods for a switched capacitor converter either assume output as “firm” voltage or a resistor to facilitate calculation. When the size of output filter capacitor is moderate, accompanied by noticeable output ripple, previous modeling results may not provide accurate prediction and design guidelines. In this paper, a new modeling technique taking into account the output capacitor effect is proposed for simple dual-phase switched capacitor converters without complex coupling loops. The proposed model employs transient calculation, takes charge redistribution phase into account, and includes the duty cycle, switching frequency as well as output capacitor simultaneously in the final output impedance formula, which enhances accuracy of the model. It is suitable for interleaved and noninterleaved switched capacitor converters. The simulation and experimental results for the unity gain SC converter are provided to verify the proposed theory. [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]

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]

Multilevel cascaded H-bridge converters are promising candidates for large-scale photovoltaic power plants. They allow direct connection to medium-voltage distribution networks without the presence of bulky line frequency power transformers. Owing to the stochastically variable nature of irradiance level, ambient temperature, and other factors, power levels in the three phases are expected to be unequal. The power imbalance condition creates unexpected problems with this topology, which was initially designed to operate under balanced power conditions. To deal with this issue, the paper proposes three novel zero-sequence injection methods as an expansion to the conventional zero-sequence injection method. Results obtained from simulations and a 430-V 8-kW three-phase seven-level cascaded H-bridge prototype are presented to verify the effectiveness and feasibility of the proposed methods. [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]

The main objective of this work is to provide an overview and evaluation of discrete model-predictive control (MPC)-based maximum power point tracking (MPPT) for photovoltaic systems. A large number of MPC-based MPPT methods have been recently introduced in the literature with very promising performance; however, an in-depth investigation and comparison of these methods has not been carried out yet. Therefore, this paper has set out to provide an in-depth analysis and evaluation of MPC-based MPPT methods applied to various common power converter topologies. The performance of MPC-based MPPT is directly linked with the converter topology, and it is also affected by the accurate determination of the converter parameters; sensitivity to converter parameter variations is also investigated. The static and dynamic performance of the trackers is assessed according to the EN 50530 standard, using detailed simulation models, and validated by experimental tests. The analysis in this work aims to present useful insight for practicing engineers and academic researchers when selecting the maximum power point tracker for their application. [ABSTRACT FROM AUTHOR]

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]

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]

An asymmetric three-level neutral point diode clamped converter for switched reluctance motor drives is proposed in this paper. The modulation method, the dc-link voltage balancing algorithm, and the current control scheme of this converter are presented. The proposed three-level converter is compared with the conventional two-level asymmetric half-bridge converter in terms of cost, current ripple, noise, and power losses. Compared to the conventional two-level converter, the proposed three-level converter has much lower current ripple, lower noise, and higher efficiency. The effectiveness of the proposed converter is verified by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

High-power dc–dc converter is the essential component for connecting the renewable energy sources to medium-voltage dc (MVDC) grids. In this paper, a novel zero-voltage zero-current switching converter with two full-bridge cells sharing a bridge leg and connecting the secondary windings of two transformers in series is proposed for MVDC applications. One big feature of the proposed converter is that the required inductance used for energy transmission is reduced remarkably and even can be embedded in the transformer in some cases. Special but simple control strategy adopted by the converter makes it work in discontinuous current mode, which can realize zero-current switching for the main switches and rectifier diodes in the whole load range. Meanwhile, the auxiliary switches with small current rating can realize zero-voltage switching naturally. Hence, the switching loss is reduced, which is very important for high-power applications. The effects of the turns ratio of auxiliary transformer on the total loss and needed inductance are comprehensively analyzed and proved. With a detailed parameters design procedure, a simulation model is established in the software PLECS and the operation principle of the converter is verified. A 120 V–1200 V/1 kW prototype was built to validate the operation principle of the proposed converter. [ABSTRACT FROM AUTHOR]

The stacked polyphase bridges (SPB) converter consists of several submodules that all are connected in series to a voltage source. The total dc-link voltage should split in a balanced way among the submodules. This does not always occur inherently. This paper presents an analysis of the capacitor voltage stability for the SPB converter. From the analysis, criteria for stability are derived and three alternatives of a suitable balancing controller are designed. The proposed controller alternatives and their associated stability properties are verified on an experimental setup and by simulation. [ABSTRACT FROM PUBLISHER]

This paper describes a novel modeling approach for self-oscillating resonant power converters operating under a control method based on the variable structure of the system (VSS). Using a fundamental harmonic approximation, the link between control and switching frequency in steady state is found to be accurately described by an affine function that depends on the load. Besides, this link has been extended in order to characterize the control-to-switching frequency dynamics, resulting in a novel small-signal continuous-time model. The resulting control-to-switching frequency transfer function can be appended to the well-known models for frequency modulation, in order to obtain a complete control-to-output system. The new model exposes that the dynamics of the converter under the VSS-based control approach presents advantages with respect to the conventional methods as frequency modulation: 1) reduced dc gain variation for uncertain loads and 2) improved phase margin at high frequencies. The dynamic modeling is complemented with the design of a controller for regulating the output voltage of a parallel resonant converter, whose performance and robustness are compared with the standard frequency modulation. Numerical simulations and experimental results confirm and verify the analytical derivations. [ABSTRACT FROM AUTHOR]

This paper proposes an individual phase current control (IPCC) method based on optimal zero-sequence current separation for star-connected cascade STATCOMs under unbalanced grid voltage. The IPCC is adopted to get better adaptation to unbalanced conditions. Each cluster of the cascade STATCOM has its own phase current control loop and dc-voltage feedback loop, and it can generate the necessary negative-sequence current automatically to rebalance power. However, the zero-sequence current which is unnecessary and harmful in star configuration is introduced in the process. An optimal zero-sequence current separation is presented to remove the zero-sequence current by reconstructing the reactive current command. This improved IPCC enables star-connected STATCOMs adapt to unbalanced conditions without the need for any power balancing algorithms. Also, the operation range of the proposed control method is analyzed. Simulation research studies are performed to verify the operation range analysis, and the experimental operation is tested on a 25-level ±10 Mvar/10-kV STATCOM product installed at a wind farm. [ABSTRACT FROM PUBLISHER]

Switched-capacitor (SC) converters are gaining popularity due to their high power density and suitability for on-chip integration. Soft-charging and resonant techniques can be used to eliminate the current transient during the switching instances, and improve the power density and efficiency of SC converters. In this paper, we propose a split-phase control scheme that enables the Dickson converter to achieve complete soft-charging (or resonant) operation, which is not possible using the conventional two-phase control. An analytical method is extended to help in the analysis and design of split-phase controlled Dickson converters. The proposed technique and analysis are verified by both simulation and experimental results. An 8-to-1 step-down Dickson converter with an input voltage of 150 V and rated power of 36 W is built using GaN FETs. The converter prototype demonstrated a five fold reduction in the output impedance (which corresponds to conduction power loss) compared to a conventional Dickson converter, as a result of the split-phase controlled soft-charging operation. [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]

This paper investigates the different control objectives, such as loss reduction, neutral point (NP) balance and noise reduction of the space vector modulation (SVM), for the three-level neutral point clamped (NPC) converter. A detailed loss model and simulation model is built for quantitative loss and NP voltage ripple analysis. An improved SVM method is proposed to reduce NP imbalance and switching loss/noise simultaneously. The coordinately selected small vectors consider both the NP charge and the pulse sequence so that the minimized NP ripple and switching events are guaranteed in one switching cycle. In addition, the switching events between switching cycles are also considered to reduce the total switching loss. This method ensures an evenly distributed device loss in each phase leg and also a constant system efficiency under different power factors. The control result for NP balance and loss reduction is verified by using both a simulation model and an experimental prototype on a 200-kVA three-level NPC converter hardware. [ABSTRACT FROM AUTHOR]

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]

Due to the high integration of renewable energy, the power systems may become more susceptible to unbalanced grid voltages, grid frequency disturbances, harmonic resonances and others. Therefore, several works have pointed out that the ancillary services provided by the energy storage systems can minimize these issues. The combination of energy storage and static synchronous compensators (ES-STATCOM) has been proposed to reduce the cost. Modular multilevel converters (MMC) are featured as a good solution for ES-STATCOM realization, due to their high efficiency, modularity and inherent fault-tolerant structure. However, the study of the integration of the MMC based ES-STATCOM in a complex power system composed of power electronic converters, transmission system, renewable plants, transformers, etc, leads to a high computational burden. Therefore, this work presents a detailed design and implementation of three efficient simulation models of the MMC ES-STATCOM. The performance of these models compared during a grid frequency support case study by a 100 MVA/33 kV ES-STATCOM. In addition, the performance of these models is evaluated during a battery charging process and a grid symmetrical fault. The models are compared through the following aspects: processing time, normalized integral of absolute error, state-of-charge estimation and active and reactive power dynamic response. [ABSTRACT FROM AUTHOR]

This letter presents an approach for modular multilevel converters that enables a stable operation with failed nonredundant modules. This is achieved by using a neutral shift with a dedicated power feed-forward control. The feasibility of the approach is verified by simulation. [ABSTRACT FROM AUTHOR]

Power electronic converters having capability of driving hybrid (ac and dc) loads are becoming popular in specific applications such as standalone nanogrid. The proposed hybrid switched inductor impedance source converter (HLZSC) is derived from the switched inductor impedance source inverter (LZSI). It can independently regulate both ac and dc voltages in continuous conduction mode (CCM). However, discontinuous conduction mode (DCM) injects significant dc ripple voltage that restricts HLZSC operation for wide load variations and standalone ac or dc loads. Modification in the form of forced continuous conduction mode to replace DCM is, therefore, proposed for modified HLZSC (or MHLZSC), which can decouple the voltage regulation of ac and dc load without getting affected by the modes of operation. MHLZSC also permit the standalone operation of ac/dc load. The control scheme for HLZSC/MHLZSC, which is similar to the established simple boost control of ZSI, is discussed briefly to utilize the salient features of the proposed converter. Theoretical analysis of the proposed converters is supported with the simulation and experimental results. Interestingly, the proposed converters—HLZSC and MHLZSC—avoid the use of lossy snubber resistance. [ABSTRACT FROM PUBLISHER]

This study focuses on the control of instantaneous complex power of a grid-connected voltage-source inverter (VSI). In this study, a new space-vector-modulation-based direct power control approach is proposed: the robust predictive direct power control (RP-DPC). The proposed predictive control algorithm ensures that both, instantaneous real and imaginary powers, track the reference with high speed and accuracy reducing steady-state errors. In order to reduce the total harmonic distortion (THD) in the output currents, a fundamental frequency positive sequence detector is used in conjunction with a prediction grid voltage observer. Comparative simulations and experimental results of a 10-kW three-phase grid-connected VSI showing the steady-state and transient performance of the proposed RP-DPC are given. A low THD and balanced output currents are maintained even under severe voltage unbalance conditions. [ABSTRACT FROM AUTHOR]