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The focus of this paper is on analyzing the impact of conservation voltage reduction in the presence of active devices such as solar photovoltaic (PV) and developing controls that leverage these distributed energy resources. An event-driven predictive approach for real-time volt/volt-ampere reactive (VAR) optimization, along with a local two-level adaptive volt/VAR droop-based control algorithm for advanced distribution management systems, is introduced. The methodology covers aggregated and autonomous controls under different timescale operations, including the impact and effect of unpredicted events such as cloud transients on PV power production. Besides, the control schemes include the uncertainties in PV power generation and load power demand. The proposed methodology is validated in a real-time framework using the real-time digital simulator platform through co-simulation with models based on Python and OpenDSS (Open Distribution System Simulator). The developed methodology is tested on the modified IEEE 123-feeder test system. The results reveal that the proposed methodology works well in the presence of high penetrations of PV power, produces significant energy savings, and mitigates over-/undervoltage problems. [ABSTRACT FROM AUTHOR]

Conventionally, the converter of switched reluctance motor (SRM) is fed by the uncontrollable diode bridge rectifier (DBR), which leads to a low grid-side power factor (PF) and high current total harmonic distortion (THD). In this paper, an alternative solution for the grid-connected high-speed SRM drive system with improved PF is proposed. In the proposed drive system, the three-level active front end (AFE) is connected in cascade with the midpoint converter for SRM operation. A centralized strategy, which controls the AFE and SRM together, is proposed to govern the motor speed and grid-side PF by regulating the real power and reactive power of the system, respectively. Specifically, the real power, reactive power, and the voltage balancing of split capacitors are controlled by the model predictive directed power control (MP-DPC) algorithm, which significantly reduces the control complexity and guarantees the fast dynamic response. Consequently, satisfying speed regulation, high PF, low current THD, and bi-directional power-transfer capability are achieved. An idea-proofed testbench is constructed in laboratory, and the applicability of the proposed drive system is verified by a series of experimental results. [ABSTRACT FROM AUTHOR]

With increasing penetration of solar PV, some distribution feeders are experiencing highly variable net-load flows and even reverse flows. To optimize distribution systems under such conditions, the scheduling of mechanical devices, such as OLTCs and capacitor banks, needs to take into account forecasted solar PV and actual grid conditions. However, these legacy switching assets are operated on a daily or hourly timescale, due to the wear and tear associated with mechanical switching, which makes them unsuitable for real-time control. Therefore, there is a natural timescale-separation between these slower mechanical assets and the responsive nature of inverter-based resources. In this paper, we present a network admissible convex formulation for holistically scheduling controllable grid assets to position voltage optimally against solar PV. An optimal hourly schedule is presented that utilizes mechanical resources to position the predicted voltages close to nominal values, while minimizing the use of inverter-based resources (i.e., DERs), making them available for control at a faster time-scale (after the uncertainty reveals itself). A convex, inner approximation of the OPF problem is adapted to a mixed-integer linear program that minimizes voltage deviations from nominal (i.e., maximizes voltage margins). The resulting OPF solution respects all the network constraints and is, hence, robust against modeling simplifications.Simulation based analysis on IEEE distribution feeders validates the approach. [ABSTRACT FROM AUTHOR]

As the output power of wind farm is fluctuating, it is one of the important ways to improve the schedule ability of wind power generation to predict the output power of wind farm. The operation mode of tracking planned output takes the planned value issued by the grid dispatching as the control basis of wind power generation. This operation mode is easy to control, which not only meets the needs of power grid dispatching, but also improves the reliability and controllability of new energy generation. This paper introduces the traditional PI control strategy of SMES and combines the advantages of SMES and the characteristics of application scenarios, and introduces sliding mode variable structure control into SMES. Through simulation and comparison of two different control modes, it is verified that SMES adopting sliding mode variable structure control have the advantages of fast response speed and strong robustness, which can make the wind farm output stable power according to the planned value. [ABSTRACT FROM AUTHOR]

The penetration of photovoltaics (PVs) and electric vehicles (EVs) is increasing in active distribution networks (ADN), which may lead to severe voltage violation problems. This paper proposes a two-stage real-time Volt/Var control method to mitigate fast voltage violations. In the first stage, hourly on-load tap changer (OLTC) and capacitor banks (CBs) are scheduled based on the optimal power flow method. The optimization problem is formulated as a mixed-integer second-order cone programming (MISOCP) which can be effectively solved. In the second stage, a data-driven network partition method is proposed to select critical bus and assess the voltage violation risk of each control area, followed by the intra-day dispatch of EVs. Based on the partition results, reactive power of PVs and EVs is controlled in real-time to mitigate voltage violations. Considering the suddenly active power drop of PVs, a rule-based control strategy coordinating PVs and CBs is proposed to improve the reactive power reserve in ADN. The proposed approach is tested on the IEEE 33-bus and 123-bus distribution networks and simulation results verify the effectiveness both in network partition and real-time voltage control. [ABSTRACT FROM AUTHOR]

This paper proposes a capacitor voltage regulation method for the dual converter with a floating bridge for aerospace applications. This topology has previously been reported, but with a constrained voltage utilization factor due to the need for capacitor voltage regulations. In this paper, the effect of switching states on the voltage variation of capacitor is quantitatively modeled and an enhanced space vector modulation scheme with current feedback is proposed to achieve an active control of the floating capacitor voltages. This proposed method also allows further exploitation and utilization of converter voltage. The relationship between the allowed modulation index of dual converter and load power factor is obtained and expressed using a fitted polynomial equation. The advantages of the proposed method include boosted voltage utilization and superior performance in term of capacitor voltage balance. These advantages have been proven through simulation and experimental results on RL loads as well as with an open-end winding induction motor. The proposed modulation scheme can boost the converter voltage utilization by at least 10% while achieving full four-level operation. More importantly, the higher available voltage allows extending the constant torque region of the motor, the further beginning of field weakening operation could be postponed. [ABSTRACT FROM AUTHOR]

The instantaneous output power of the two-stage single-phase grid-connected photovoltaic (PV) inverter pulsates at twice the line frequency ($2f_{{\text{o}}}$), generating second harmonic current (SHC) in the front-end dc–dc converter and PV panel, which will affect the maximum power point tracking operation and deteriorate the overall conversion efficiency. The generating mechanism of the SHC is analyzed in this paper and it is pointed that in order to eliminate the SHC in the front-end dc–dc converter and PV panel, the voltage loop gain of the front-end dc–dc converter should be high enough at $2f_{{\text{o}}}$. Since there is a −180° phase abrupt at the resonant frequency of the input side filter capacitor and the inductor, the system may be unstable. To cope with this problem, the inductor current feedback active-damping scheme (ADS) is adopted. For further improving the loop gain at $2f_{{\text{o}}}$ , proportional–integral–resonant regulator with ADS (PIR + ADS) is proposed in this paper. Besides, a step-by-step closed-loop parameters design method is presented. Finally, a 3-kW two-stage single-phase grid-connected PV inverter has been fabricated and tested, and the experimental results verify the feasibility of the proposed control schemes. [ABSTRACT FROM AUTHOR]

A trinary multilevel converter uses a few heterogeneous H-bridge submodules (SMs) to synthesize an output voltage with multiple levels. Two major limitations of this converter are that it needs multiple dc sources and it cannot operate under pulsewidth modulation (PWM). This paper addresses these limitations by adding a low-voltage H-bridge SM to this converter and proposing a new switching algorithm. The resulting PWM-based asymmetric cascaded H-bridge converter uses only one dc source and synthesizes a high-quality output voltage. This improved converter can be a potential choice for real and reactive power support in power system applications. This paper also proposes methods for stability analysis and design of this converter. Experimental case studies evaluate the performance of the proposed converter for both standalone and grid-connected modes of operation. [ABSTRACT FROM AUTHOR]

This paper presents a method to enhance the dc-bus voltage-control loop of a single-phase grid-connected dc/ac converter, which improves its responses in terms of oscillation on its dc-bus voltage as well as its output ac current. Conventionally, the double-frequency (2-f) ripple is reduced by using a large electrolyte capacitor, which increases the cost and size of the system. A state-of-the-art approach is to use a notch filter (NF) to block the 2-f ripple in the voltage-control loop. This can significantly reduce the capacitor size. The existing presentations of this method, however, do not integrate the internal dynamics of the NF into consideration. This paper proposes a new way of implementing the NF, which allows integration of its internal variables into the control loop. The resulted system exhibits enhanced transient responses at both the dc-bus voltage and the output ac current. The proposed method is analyzed in detail and its effectiveness is verified through simulations and experimental results. [ABSTRACT FROM AUTHOR]

Light-emitting diodes (LEDs) are solid-state devices with specific $v$ – $i$ characteristics. In this paper, we study the basic requirement of the driving circuits and discuss the proper approach to drive LEDs in view of their characteristics. We compare voltage source driving and current source driving, and discuss their relative advantages and constraints. We specifically introduce the use of circuit duality principle for developing new current-source-mode (CSM) drivers that are less known but are theoretically more versatile compared to their conventional voltage-source-mode counterparts. The study highlights the effects of the choice of driving circuits in terms of the number and size of circuit components used, duty cycle variation, sensitivity of control, nonlinearity and control complexity of LED drivers. We propose a CSM single-inductor multiple-output (SIMO) converter, which demonstrates the advantage of having inductorless and easily controlled current-source drivers, and present a comparison of the CSM SIMO converter with the existing SIMO converters. We further illustrate that a high-voltage-step-down ratio can be naturally achieved by the CSM high-voltage-step-down converter without the use of transformers. This paper presents a systematic and comparative exposition of the circuit theory of driving LEDs, with experimental evidence supporting the major conclusions. [ABSTRACT FROM AUTHOR]

A modular multilevel converter (MMC) is regarded as a promising topology in high-voltage direct current systems. However, the MMC consists of numerous submodules (SMs) and switching devices, which lead to a considerable switching loss, and increased cost and size of the heat sink. To mitigate these issues, this paper presents a novel modulation method composed of fundamental frequency modulation (FFM) and multicarrier-based sinusoidal pulsewidth modulation schemes. The main purpose of this modulation method is the reduction of switching loss while maintaining good harmonic performance. However, the FFM scheme leads to the unbalanced capacitor voltage of each SM in the MMC. Accordingly, this paper additionally proposes the rotation method and selective voltage balancing control for SMs to ensure effective performance of the method. Simulation and experimental results verify the effectiveness and performance of the proposed modulation scheme through switching loss and spectral analyses. [ABSTRACT FROM AUTHOR]

This paper develops a modulation based fault-tolerant (FT) strategy for restoring the operation of three-phase modular multilevel converters (MMCs) with faulty switches. This FT strategy is based on a proposed modified space vector modulation (SVM) technique that generates balanced line-to-line (line) voltages even in the case of a fault occurrence. In the postfault operation, the proposed strategy is able to restore the fundamental amplitude of the line to the neutral (load) voltages to that of the normal operation condition with a slightly increased voltage stress over the switches in the faulty phase. In this paper, first a brief background about MMCs and SVM technique is provided. Then, the proposed FT strategy and the modified SVM technique are presented. Finally, several simulation and experimental results are provided to validate operation of the proposed strategy. [ABSTRACT FROM AUTHOR]

This paper presents two capacitor voltage balancing (CVB) strategies for modular multilevel converter (MMC) applications. Both balancing schemes are based on model predictive control and are designed to efficiently solve a constrained optimal control problem, where the predicted capacitor voltage errors are included in the cost function with the demanded output voltage of a cluster being forced through an equality constraint. The first method proposed in this paper computes specific modulation indexes for each module using the explicit solution of a relaxed version of the original optimization problem. The second approach proposed in this paper reduces the complexity of the original problem by linearizing the objective function and using an optimal sorting network based on a greedy algorithm to solve this approximation. Considering the structures of both solution approaches, they are integrated into modulation schemes based on phase-shifted and level-shifted pulsewidth modulation algorithms, respectively. Experimental results obtained from a nine-cell single-phase MMC prototype demonstrate the good performance achieved with the proposed methodologies, as well as the implementation simplicity offered by the proposed CVB algorithms. [ABSTRACT FROM AUTHOR]

Although the probability of occurrence of station internal ac grounding faults in modular multilevel converter (MMC)-based high-voltage direct-current systems is low, they may lead to severe consequences that should be considered when designing protection systems. This paper analyzes the characteristics of valve-side single-phase-to-ground (SPG) faults in three configurations of MMC systems. Fault responses for symmetrical monopole MMCs are first studied. Upper arm overvoltages and ac-side nonzero-crossing currents arising from SPG faults in asymmetrical and bipolar configurations are then investigated. DC grounding using an LR parallel circuit is employed to create current zero-crossings, which will enable the operation of grid-side ac circuit breakers. The theoretical analysis is verified through simulations performed in PSCAD/EMTDC, with simulation results and the theoretical analysis showing a good agreement. The studies in this paper will be valuable for the design of protection systems for station internal ac grounding faults. [ABSTRACT FROM AUTHOR]

This paper presents a practical strategy for utilizing the submodule (SM) redundancy of a modular multilevel converter (MMC) for its fault tolerance. This strategy provides a systematic framework for balancing the tradeoff between two conventional methods for using the active redundancy and, thus, achieves operational flexibility. One of the existing methods improves SM reliability owing to less voltage stress on the SM components by employing all of the SMs to form the ac or dc voltages (voltage-sharing mode). The other avoids transients by keeping the average SM voltage constant at the cost of slightly increased stress on the SM components (fixed-level mode), which, however, can be controlled to provide the grid-adaptive operation by reserving the energy of the SMs not in service. We, thus, develop a new redundancy management scheme by integrating these two methods and exploiting their technical benefits to meet the PQ requirements and MMC control performance. This research provides a theoretical basis and a technical guide to determining the number of SMs, which can further increase the voltage steps as per the MMC and grid conditions. This paper also connects the remaining PQ capability of the MMC at a particular operating point with the SM redundancy concept by defining a potential redundancy, especially useful when the physical redundancy is exhausted. The theoretical findings and efficacy of the proposed strategy are validated through PSCAD/EMTDC time-domain simulations followed by experiments using a nine-level single-phase MMC system. [ABSTRACT FROM AUTHOR]

Nonregenerative pulsewidth-modulated (PWM) rectifiers are increasingly being considered for applications, where the power flow is unidirectional, such as power supplies for telecommunications, X-ray, the machine-side converter for wind energy conversion systems, etc. They use fewer active switches, which increase their power density and reduce cost. This paper proposes a novel reduced switch topology for a multilevel (five-level or higher) nonregenerative PWM rectifier. It uses only four controlled switches and eight diodes per phase for a five-level rectifier. Half of the diodes are naturally commutated (zero current switching) at the line frequency, which reduces switching losses. This topology has several other advantages compared to similar topologies reported in the literature, such as minimum voltage stress across the devices, elimination of transient voltage-balancing snubbers, no extra hardware for balancing the flying capacitors, the dc-link mid-point voltage, etc. In this paper, switching cycle average modeling and the carrier-based modulation strategy for this rectifier are also presented to maintain a balanced dc link and to regulate flying capacitor voltages, while achieving unity displacement factor at the rectifier input terminals. The overall performance of the rectifier is verified by experimental results. [ABSTRACT FROM AUTHOR]

This paper introduces an averaging control strategy of the phase-unit losses of modular multilevel converters (MMCs). The quantitative analysis of the MMC phase-unit loss distribution under unbalanced ac conditions is carried out by investigating the individual component of the MMC arm currents. It shows that the unbalanced arm currents will result in unbalanced phase-unit losses. To address this issue, this paper proposes a predictive closed-loop-control-based zero-sequence voltage injection method that results in much averaged loss distribution under unbalanced ac conditions. The essence of the new method is that it adjusts the three-phase sinusoidal references so that the unbalanced arm currents are fully utilized to average phase-unit losses. This paper shows the effectiveness of the new approach by electromagnetic transient simulations on PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper proposes a novel two-level single-stage direct ac–ac converter for realizing a 7.2-kV medium-voltage (MV) solid-state transformer (SST) based on 15-kV SiC mosfets. A new current-fed series resonant converter (CFSRC) topology is proposed to address major challenges in MV ac–ac converters such as achieving zero-voltage switching (ZVS) for the MV mosfets across wide voltage and load ranges and minimizing system capacitance. The topology is analyzed with both time-domain analysis and first harmonic approximation to provide useful equations for circuit design. Constant deadtime strategy is adopted, allowing partial ZVS to occur at low-voltage (LV) levels. ZVS behavior over wide voltage range is investigated, and calculation of the associated loss from partial ZVS is presented. System parameters are optimized based on the tradeoff between conduction loss and switching loss. The 15-kV mosfet has been tested continuously at a park voltage of 10 kV and 37 kHz, indicating stable device operation and an extremely high voltage × frequency figure of merit. Moreover, inherent cycle-by-cycle current limiting in the proposed CFSRC under output short-circuit circumstance is realized by paralleling diodes to the LV resonant capacitors. Without employing any additional current sensors, the input and circulating currents are limited to a safe range automatically when the short-circuit occurs. This paper presents detailed short-circuit protection operating principles and peak resonant current equation to aid the design of the resonant tank. A full-scale and compact SST that converts 7.2 kV ac to 240 V ac is developed to verify the theoretical analysis. This is the highest reported voltage rating for two-level-based power converters without device series connection. ZVS is verified and achieved over wide voltage and load ranges with a peak efficiency of 97.8%. A short-circuit experiment is conducted at 3-kV peak voltage to verify the analysis. Experimental results closely match the theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper proposes unconventional control techniques for a portable power supply consisting of electric double-layer capacitors (EDLCs), a dc–dc converter, and a low-impedance load such a plasma load. Between the EDLCs and the plasma load consuming high power, a dc–dc converter is required to control the output voltage from the rapidly decreasing voltage of EDLCs. In addition, the dc–dc converter should maintain a set point by responding to arcs occurred in plasma process. This paper proposed hybrid analog and digital controls for a nonideal boost converter which was setup by theoretical analysis. The proposed system is verified with experiment and simulation. As a result, the proposed control method can maintain the constant output voltage longer than a traditional analog pulsewidth modulation control. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) has been a promising candidate in high voltage direct current (HVdc) transmission systems. Due to the features of simple structure, low cost, and low power losses, half-bridge (HB) is usually adopted as the submodule circuit in the traditional MMC. However, an HB submodule circuit-based MMC does not have the capability of blocking dc short-circuit fault, which obstructs the development of low-cost HVdc systems with overhead lines. In order to solve this problem, an MMC with active clamped T-type submodule (ACTSM) to own dc fault blocking capability is proposed in this paper. Compared with other submodule circuits, the proposed ACTSM has many features, such as symmetrical blocking capability, low power losses, low voltage stress, and low amount of independent drive signals. The topology, operation principle, dc fault blocking capability, and distribution of power losses of the ACTSM are presented in this paper. The effectiveness of the ACTSM-based MMC is validated by both simulations and experiments. [ABSTRACT FROM AUTHOR]

In a sensorless vector control method, at low speeds and self-commissioning for AC motors, it is necessary to model dead time, rise/fall time, parasitic capacitors, and semiconductors’ voltage drop accurately. In this paper, voltage drop in each phase due to the nonlinear behavior of the inverter from the control system's viewpoint is precisely modeled by using two lookup tables. These lookup tables make the proposed model robust against the dc-link voltage and carrier frequency variations in the entire current range. In conventional methods at standstill, the measured voltage in the dc current test contains the nonlinearity of two phases. Therefore, to obtain the nonlinear voltage of each inverter phase, it is assumed that the values of inverter nonlinearity voltage at both high current and low current are equal. This simplification leads to distortion in measured voltage in the low currents. However, in this paper, the voltage drop equation of each phase is solved by numerical method that is realizable in conventional processors. The results obtained in actual voltage measurement show the superiority of the proposed method over traditional methods in the low currents. The proposed method is verified by using a voltage source inverter feeding a 2.2-kW three-phase induction motor. [ABSTRACT FROM AUTHOR]

Power management integrated circuits (PMICs) with on-chip compensation are widely used to power multiple loads in mobile devices with increased power density. However, there are two issues for on-chip compensated PMICs. First, on-chip compensation reconfigurability is required to obtain the appropriate response in various passive components. Second, a converter for a processor requires adaptive voltage positioning (AVP) to reduce the output capacitor size. In this paper, a novel reconfigurable on-chip compensated current-mode buck converter with AVP is proposed to solve the aforementioned issues without requiring high-speed and high-resolution analog-to-digital converter (ADC). A reconfigurable accurate load line control scheme and on-chip compensation are proposed to achieve an accurate load line and AVP in various load lines and passive components. The control scheme, small-signal model, and circuit implementation are illustrated in this paper. The proposed buck converter was implemented into an integrated circuit to verify the analysis. [ABSTRACT FROM AUTHOR]

Power converters and electric motor drives when tightly regulated behave as constant power loads. These loads are different from resistive loads and have destabilizing negative impedance characteristics, which impact a system's stability. A boost converter is intrinsically nonlinear and is a nonminimum phase system at the output voltage with respect to the control input. The linear approximation of this boost converter loaded with a constant power load has a zero and poles in the right half of the $s$ -plane, making the system unstable and very difficult to control. Control techniques that employ some form of system inversion cannot be implemented for a nonminimum phase system. This paper describes a technique to modify the nonminimum phase boost converter to a minimum phase for a constant power load, further implementing the input–output linearization technique to stabilize the system. This paper also provides a methodological analysis of the problem followed by the proposed solution. Furthermore, it verifies the analysis of the proposed solution through simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a split-capacitor four-wire current source inverter, which is the dual of the split-capacitor four-wire voltage source inverter. Since the midpoint of the dc link is tied to the neutral point of ac filter capacitors, the common-mode voltage (CMV) is reduced significantly. Consequently, the leakage current issue is effectively addressed. The proposed circuit is cost-effective as no extra switch is added. This paper, first, establishes the equivalent common-mode circuit of the proposed inverter. The impact of the neutral line inductance on CMV is also analyzed. Then, a specific modulation is introduced to balance the dc-link voltages/currents. To achieve good input/output performance, a nonlinear control method is developed based on time-domain models. Finally, all the proposed methods and related theoretical analysis are verified by simulations and experimental results. [ABSTRACT FROM AUTHOR]

Voltage-controlled voltage-source converters (VSCs) are widely applied in islanded microgrids, dynamic voltage restorers, and uninterruptible power supplies, etc. Typically, an LC filter with a large filter capacitance is employed to improve the quality of output voltage and system robustness against external disturbances. Nevertheless, a large capacitance may lead to a low LC-resonant-frequency and cause unstable voltage control of VSCs when a single-loop proportional-resonant (PR) or resonant controller is used. This paper proposes a novel design of single-loop voltage control, where a negative proportional gain is adopted in the PR voltage controller, and it is revealed that the voltage control can be stable even with a low LC -resonant-frequency, e.g., lower than one-third of the system sampling frequency. To tackle the poor dynamic issue introduced by the negative proportional gain, this paper further proposes a closed-loop zeros configuration-based control method, which can flexibly tune the system dynamic response without affecting its stability. Finally, simulation and experimental results are provided to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes a new power decoupling control method for a direct ac-to-ac converter. The ac-to-ac converter consists of two half-bridge converters, two input filter capacitors, and a series-resonant circuit, which enables it to convert the single-phase line-frequency ac input to the high-frequency ac output directly. The proposed power decoupling control method stores input power pulsation at double the line frequency in the input filter capacitors. Thus, the proposed control method realizes a unity power factor in the line-frequency ac input and a constant amplitude current in the high-frequency ac output without any additional switching device or energy storage element. This paper theoretically discusses the principle and operating performance of the proposed control method and confirms the effectiveness of the proposed control method in experiments using an isolated ac-to-dc converter based on the direct ac-to-ac converter. As a result, the proposed power decoupling control method effectively improved the displacement power factor of the line-frequency ac input current to more than 0.99 and reduced the voltage ripple in the dc load to 4%. [ABSTRACT FROM AUTHOR]

This paper discusses the control of a modular multilevel converter (MMC) used as a grid interface for the klystron modulators in the compact linear collider (CLIC). The converter has a DC side load which takes short-duration power pulses, causing high DC side power fluctuations that are not tolerable if seen by the AC grid. The DC–AC power decoupling capability of the MMC enables mitigation of the power ripple on the AC side, guaranteeing compliance with power quality requirements. However, the pulse repetition rate of the CLIC modulators is synchronized the 50-Hz AC grid and this induces permanent power imbalance in the arms of the MMC, causing voltage deviation and overmodulation unless appropriate balancing strategies are implemented. Unlike existing arm balancing methods that control 50-Hz circulating currents to balance the arm powers, the method proposed in this paper introduces an augmented modulation strategy where modulation signals are redistributed among arms based on the demand from a balancing controller. The resulting controller has lower complexity and its simple structure enables an easier design of the balancing loop, which guarantees predictable dynamics in operation. The effectiveness of the method has been demonstrated in simulation for the full-scale CLIC converter ratings and experimentally on a 7-kW MMC prototype operating with a 3.3-kA pulsed DC load. [ABSTRACT FROM AUTHOR]

This paper proposes a new modulated model predictive control method for the control of modular multilevel converters (MMCs) in voltage source converter-high voltage dc systems. The proposed method retains the advantages of the conventional finite control set-model predictive control methods by programing the nonlinear properties of the MMC into the design calculations while minimizing the ac line and circulating current ripples and steady-state error by generating modulated switching signals with a fixed switching frequency. In this paper, first, the predictive modeling of the MMC is provided. Next, the proposed control method is described. Then, the application of the proposed method to an MMC system is detailed. Finally, experimental results from an MMC system connected to a three-phase grid are provided to validate the theoretical outcomes. [ABSTRACT FROM AUTHOR]

This paper presents theoretical and experimental discussions on low-voltage-ride-through operation of a modular multilevel single-delta bridge-cell (SDBC) inverter intended for utility-scale photovoltaic (PV) systems. Modern grid codes require grid-tied inverters to provide dynamic grid support during grid-fault events by injecting reactive current. This paper discusses decoupled positive- and negative-sequence reactive-current control, focusing on asymmetric voltage sags with imbalanced magnitude and phase relationships. The main objective is to present a feedforward control method based on calculation of the zero-sequence current required for achieving power balance during normal and grid-fault conditions. Moreover, this paper demonstrates a practical method that minimizes overcurrent stress in the three inverter clusters by adjusting active power drawn from PV arrays. Experimental results on a three-phase 12.6-kVA system prove that the SDBC inverter is capable of seamlessly operating through asymmetric voltage sags. [ABSTRACT FROM AUTHOR]

In this paper, a dc–ac buck converter is proposed as a grid-side converter in high voltage dc (HVdc) transmission systems for high-power renewable energy source grid integration. The proposed architecture consists of multimodules of half-bridge voltage source converters (HB-VSCs). The dc terminals of the HB-VSCs are connected in series across the entire dc link (i.e., capacitor tapped), whereas their ac outputs are connected to multiwinding transformers to provide the three-phase terminals for ac grid integration. The proposed grid-connected capacitor-tapped multimodule converter (CT-MC) architecture, inspired from the HVdc shunt tap proposed by ABB, provides a dc–ac conversion with a relatively moderate voltage rating of semiconductor devices. It also provides operation with a lower number of semiconductor devices, gate driver circuits, voltage sensors, and lower total MVA rating of semiconductor devices (67%) compared with the conventional three-phase HB modular multilevel converter, which positively affects the system cost and reduces the computational burden of the employed controller. In this paper, the operational concept and control of the CT-MC are presented along with a capacitor voltage balancing approach. Simulation results are presented during normal and abnormal conditions to show the viability of the proposed architecture. Finally, a scaled down prototype for the CT-MC is employed to validate the converter operation. [ABSTRACT FROM AUTHOR]

This paper presents a quantitative analysis of the losses of the hybrid cascaded modular multilevel converter (HC-MMC). Regulation of full-bridge (FB) submodule-capacitor voltages in an HC-MMC requires dedicated strategies that affect the voltage harmonics generated by the converter's main power stage and hence its switching patterns and losses. This paper uses an electromagnetic transient simulator (PSCAD/EMTDC) to estimate the semiconductor losses using the switches’ turn-on and turn-off characteristics. The paper examines the losses of a 45-MW HC-MMC for two recently introduced voltage-regulation methods based upon the modified sinusoidal pulse width modulation (PWM) and an existing method based upon the trapezoidal PWM. Losses of a conventional FB-MMC that offers dc-fault blocking capability are also investigated and compared with those of HC-MMC. The loss comparison results indicate that: first, the two voltage-regulation methods for HC-MMC based upon the modified sinusoidal PWM result in markedly lower semiconductor losses than the trapezoidal PWM method; second, compared with the original HC-MMC, a mixed submodule HC-MMC may lower the semiconductor losses at certain operating points with proper addition of mixed submodules; and finally, HC-MMC may have up to 9.0% lower semiconductor losses (relative) than the conventional FB-MMC. [ABSTRACT FROM AUTHOR]

This paper proposes generation of a 15-level (14 concentric) dodecagonal voltage space vector structure (DVSVS) for a star connected induction motor drive. The proposed multilevel DVSVS is obtained by cascading two inverters, namely a primary and secondary inverter. The primary inverter is a five-level (5L) structure formed by stacking two three-level flying capacitors with individual reduced dc sources and the secondary inverter is also a 5L structure formed by cascading two capacitor-fed cascaded H-bridges (CHB). The active power is supplied by the primary inverter, while the secondary inverter acts as switched capacitor harmonics filter, and capacitors in the secondary inverter are balanced naturally irrespective of load power factor for entire modulation index. The high-voltage dc supply fed primary inverter is operated in quasi-square wave mode, while the high frequency switching is applied to low voltage CHBs, thus, reducing the overall switching loss. The proposed scheme gives the advantages of both DVSVS and multilevel structure, thus, making it one of the solutions for battery or stacked dc-fed applications. The paper also presents the experimental results as well as comparison study with the existing topologies to support the advantages of proposed scheme. [ABSTRACT FROM AUTHOR]

Constant on-time V^2 control for point-of-load buck converters has instability issues in the cases that the output capacitors' RC time constants are small. This paper intensively studies the proposed solution using capacitor current ramp compensation, which is a superior solution featuring fast response and universality. A frequency-domain small-signal model based on describing function method is proposed in this paper. The time-domain large-signal response to the load step change is analyzed. The analysis illustrates the unique transient response behaviors of the capacitor current ramp compensated V^2 control. The design optimization methodology based on frequency-domain and time-domain analysis is presented. The proposed model and the design guidelines are verified by the experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a generalized power-decoupling control scheme using a multiport isolated bidirectional converter for a multilevel inverter, which has multiple dc links inside. In the proposed method, a single power-decoupling capacitor is needed for all the dc links in the multilevel inverter cell. First, a prototype of the power-decoupling concept of individual H-bridge cells in the multilevel inverter is proposed, using a separate power-decoupling circuit. Then, a more advanced one-step power-decoupling method is proposed. The lifetime and reliability of the multilevel inverter is improved as film capacitors replace the large capacitance electrolyte capacitors. A multi-input ports/single output voltage-fed dual half-bridge converter (MDHB) is used for the power-decoupling circuit. A steady-state analysis for the peak and root mean square of the MDHB current is carried out for the loss breakdown. The currents are functions of the switching frequency, phase shift, leakage inductance, turn ratio, and output voltage, which make the multiport transformer design complex. A design methodology is proposed, which takes into account the design of the copper and core losses as functions of the switching frequency and number of turns. Furthermore, a special winding method for the input port is illustrated to obtain identical leakage inductances for the uniform current distribution in the multiport transformer. The proposed MDHB employs a current-sensorless power-decoupling control that contributes to the spontaneous ripple rejection of all the dc links without individual link current information, as well as to the cost and size reduction. Hence, the ripple-rejection controller is independent of the control configuration of the multilevel inverter, and also available for universal applications of various inverter topologies. Since the primary-input ports of MDHB share a single magnetic core for interfacing the ripple power to the unified secondary ripple capacitor, the controller design becomes difficult in considering the dynamic interaction among the ports, along with the average-voltage control loop design. In this paper, the dynamic analysis and controller design procedure of the circuit is also presented. The power decoupling is achieved even when the ripple frequency is other than the double frequency of the inverter output, since the single-pole transfer function of the small-signal model of the MDHB allows sufficient phase margin, along with high bandwidth. The proposed power-decoupling method for the multilevel inverter is validated with the help of simulation and 1.2-kW hardware prototype experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive fault current limiting control (AFCLC) for modular multilevel converters (MMC). Without introducing extra current limiting devices, this control scheme enables fast fault current suppression during DC faults. The AFCLC will be triggered automatically once DC faults occur. By adaptively reducing the output DC voltages of MMCs, the fault current can be suppressed. Compared with the existing current limiting methods, the proposed AFCLC has a better performance on fault current limiting, since it only depends on the real-time operating condition and no fault detection delay is imposed. Firstly, the principle of the proposed AFCLC together with the mathematical analysis is disclosed. Then, the sensitivity analysis of the impact of key control parameters on the current limiting effect is investigated. Finally, the effectiveness of AFCLC is demonstrated in a four-terminal HVDC grid test model. The simulation results show that the proposed AFCLC can reduce the interrupted current and energy absorption of a DCCB from 10.39 kA and 38.24 MJ to 4.62 kA and 8.32 MJ, respectively. The simulation results also prove that the AFCLC will not affect the accuracy of the DC fault detection algorithms under DC faults. [ABSTRACT FROM AUTHOR]

This paper proposes a dynamic sliding mode control (SMC) approach to the robust voltage regulation of dc–dc boost converters by using interval type-2 fuzzy neural networks (IT2FNNs). First, uncertainties caused by the perturbation of the input inductor and the output capacitor are represented with some bounded approximation errors, by the utilization of a Takagi–Sugeno (T–S) fuzzy modeling approach. Based on the represented model of the boost converter, a new type of sliding surface is designed depending on the duty cycle and reference inputs of the converter. Then, a dynamic SMC law is designed, by considering that the perturbation of the uncertain parameters, including input inductor, output capacitor, load resistor, and input voltage, is bounded. Meanwhile, we adopt an exponential plus power approaching law in the sliding mode controller for fast reachability of the sliding surface and a small chattering in the duty cycle input. Moreover, in terms of the considered uncertainties, a novel IT2FNN-based dynamic SMC law is derived, by applying simplified ellipsoidal-type membership functions in the type-2 fuzzy neural network. To improve the capacity to manage the uncertainties, some online learning algorithms for the updating of the IT2FNN are designed by a gradient descent method (GDM), without the requirement of the boundedness of the uncertainties. The resulting tracking error system is synthesized to be bounded stable based on the designed IT2FNN-based dynamic SMC. Finally, the effectiveness of the proposed adaptive IT2FNN-based dynamic SMC method is verified by some comparative simulation results. [ABSTRACT FROM AUTHOR]

This paper introduces a new nonlinear voltage-mode controller for a PWM dc-dc buck converter operating in continuous conduction mode. The sliding-mode control technique is used to derive a simplified equivalent control law based on the averaged dc-dc buck converter dynamics. As opposed to the reported design approaches, the proposed control scheme only requires the output voltage in the feedback loop without the need to the capacitor current. Thus, the analogue implementation cost is reduced and the drawback of adding a capacitor current sensor to the power converter is eliminated. The control equation is realized in a simple analogue circuit using few op-amps and resistors, which is suitable for industrial applications. The system modeling and control design procedure along with the derivation of the existence and stability conditions are presented. The control system performance is analyzed using MATLAB and SaberRD simulations. Furthermore, a simple analogue prototype of the simplified nonlinear voltage-mode controlled dc-dc buck converter is developed on a PCB. The simulation and experimental results show that the controller maintains a constant switching frequency, exhibits fast transient response, and provides good tracking performance under large disturbances. [ABSTRACT FROM AUTHOR]

The steady-state performance of modular multilevel converter (MMC) can be optimized by the following methods: circulating current injection, third-harmonic signal injection, and average capacitor voltage control. However, most of the papers, due to the deficiencies of their MMC models, are limited to the study of adopting just one method and only optimizing a single-aspect of the performance. Thus, this article proposes a general MMC model for the comprehensive study of the steady-state performance optimization. The general model is built based on an equilibrium equation between the internal quantities and the external quantities of the MMC. This general model is capable of analyzing the optimization where the optimizing methods are combined and used to improve the multiaspect performance of the MMC. In addition, more influence factors are taken into account; hence, the calculation error can be smaller than 1% and even the minor effect of each optimizing method can be reflected. The proposed model is compared with traditional models; and it is found that both dc and fundamental components in modulation signals will be affected by the injected circulating current, rather than the constant values in the traditional models. Finally, the proposed MMC model is verified by both simulation and experiment. [ABSTRACT FROM AUTHOR]

With the development of the MMC-HVDC, the lightweight requirements of the converter valves have become increasingly urgent. The reduction of submodule (SM) capacitance is difficult because the ripple of capacitor voltage cannot be too large, which is one of the key factors that affect the weight and the volume of the MMC. On the basis of the fundamental frequency and double fundamental frequency components are the critical factors for voltage ripples, this paper proposes an arm inductor switching control (AISC) strategy to reduce the voltage ripples of SM capacitor by controlling the inductance value of the bridge arm. The proposed strategy and the existing circulating current suppression controller (CCSC) are researched in terms of mathematical principles. A two terminal DC transmission model is built in PSCAD/EMTDC simulation environment, then the capacitor voltage fluctuation curves of non-capacitance-reducing control, CCSC, and proposed strategy with the same parameters are simulated. Afterwards, an experiment prototype is built to verify the simulation results. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

This paper presents voltage regulation and current tracking control strategies for three phase two-level grid-connected power converters. By using power-invariant Park’s transformation, an averaged mathematical model of power converters is obtained in dq synchronous reference frame. Then a novel control strategy using adaptive control and ${H}_{\infty }$ technique is proposed to regulate the dc-link output voltage as well as track a desired current reference for three-phase power rectifiers. More specifically, an efficient adaptive controller is established in the external loop for regulating dc-link output voltage in the presence of external disturbances. A set of ${H}_{\infty }$ controllers are designed in the internal loop to force the input currents track their desired values. Finally, simulation results obtained from the proposed control method are presented, analyzed, and compared with that of sliding mode control, and the superiority of the proposed control laws is verified. [ABSTRACT FROM AUTHOR]

The second harmonic current compensator (SHCC) can be added into the single-phase converters for compensating the second harmonic current (SHC) and thus removing the undesired electrolytic capacitor. In this paper, a virtual impedance, which is in the form of a capacitor and a resistor connected in parallel, is introduced to be in parallel at the bus-side port of the SHCC for effectively compensating the SHC while guaranteeing the system stability. This virtual parallel impedance is realized by feeding forward the bus-side port voltage of the SHCC, and thus the SHCC can be modular designed. The closed-loop parameter design of the SHCC is also presented. A 3.3-kW prototype of a two-stage single-phase power factor correction converter is built and tested in the laboratory, and the experimental results verify the effectiveness and feasibility of the proposed control method. [ABSTRACT FROM AUTHOR]

This paper presents an augmented design method for the dc/dc modular multilevel converter considering the control (e.g., the phase difference between arms ac voltages) and hardware aspects (e.g., type and number of submodules (SMs)) of the converter simultaneously. The proposed augmented design determines the number of SMs and their types (e.g., half- or full-bridge) in each arm and phase difference between arms ac voltages that minimize the total converter losses. Computationally-efficient analytical and semi-analytical methods are proposed to estimate the conduction and switching losses. To verify the effectiveness of the proposed method, the losses obtained from the proposed analytical and semi-analytical methods are compared with the results of the detailed converter switching model implemented in the Simulink environment. The performance of the converter designed by the augmented approach is compared with the conventional topology in terms of total losses and the number of SMs. The comparative study showed that the proposed design method yields a converter with lower total losses and number of SMs. Moreover, the proposed design method extends the operation regime of the converter, especially when the dc links voltage levels are close; this extension is not possible in the conventional modular dc/dc converter. [ABSTRACT FROM AUTHOR]