Capacitor Voltage Ripple Research Articles

Decoupling Capacitor Minimization of an MMC-Based Photovoltaic System With Three-Winding Power Channel

This article proposed a grid-connected photovoltaic (PV) converter based on modular multilevel converter (MMC), which can realize long-distance dc transmission while realizing the local consumption of PV. In addition to the advantage that there is no PV power mismatch between the upper and lower arms, the isolation transformer couples the two submodules of the upper and lower arms on a three-port transformer, and the differential-mode components of the capacitor voltage ripples in the upper and lower arms can be canceled between each other. Besides, through the double fundamental frequency circulating current control, the power exchange among MMC phase units can be realized, thereby eliminating the common-mode component of the capacitor voltage ripple. Through the combination of topology and control strategy, the low-frequency components of the voltage ripple can be completely canceled. Since the decoupling capacitance is reduced dozens of times, the volume and cost of the system are significantly reduced. Therefore, the life and reliability of the system are increased. Simulation and experimental results verify the effectiveness of the proposed topology and its control strategy.

Minimization of DC-Link Capacitance and Improved Operational Performance of a 5-Level Hybrid Multilevel DC-Link Inverter

This study analyzes a single-phase hybrid multilevel dc-link (MLDCL) inverter, in which a dc-link with two capacitors is integrated into a single dc source. When controlled under the conventional modulation scheme, the MLDCL inverter synthesizes a five-level output voltage. As the capacitor-voltage balancing depends on the fundamental period, the output voltage may contain unsolicited low-order harmonics. Moreover, in the conventional scheme, this inverter requires large dc-link capacitors. To avoid these problems, the present paper develops a simple phase-shifted pulse-width modulation scheme based on redundant switching states, which controls the MLDCL inverter and balances the capacitor voltage within each carrier period. The proposed modulation scheme also minimizes the dc-link capacitance by a factor of the frequency modulation ratio while reducing the total harmonic distortion of the output voltage and current. Unlike the conventional method, the proposed scheme considerably reduces the capacitor-voltage ripples and can be regulated by increasing the carrier frequency, thereby reducing the voltage stress on the switches and improving the reliability of the inverter. The feasibility of the proposed scheme over the conventional scheme is confirmed in a comparative study. Finally, the steady-state and dynamic performances of the proposed strategy are demonstrated in simulations and validated in experiments.

Optimization Scheme Based on High-Frequency Link Interconnection of Submodules

Submodule (SM) capacitor voltage ripple and imbalance are inherent problems with conventional modular multilevel converter (MMC). In addition, the large size capacitance in SM reduces the power density of MMC, and the life defect of electrolytic capacitor also decreases reliability of the converter. This article proposes a modified switched-capacitors based MMC (SC-MMC) topology with ripple-power decoupling channels. Without relying on any closed-loop control, the natural elimination of SMs ripple-power and automatic balance of SMs voltage can be realized simultaneously, which greatly increase the power density and simplify the control design of system. In this article, the ripple characteristic of the conventional MMC and the constraint on capacitance are studied, and an SMs ripple-power decoupling method is analyzed. The power characteristics and impedance constraint analysis of the switched-capacitors are described in detail and the configuration assessments including volume, cost, and efficiency of the SC-MMC are also analyzed. Finally, a down-scaled experimental platform is built, and the experimental results verify the correctness and effectiveness of the proposed scheme.

Advanced single-loop discrete-time control for T-type voltage source inverter with minimum capacitor voltage ripple modulation

This research focuses on improving the performance of the voltage source inverter (VSI), which has been widely used in practical applications such as uninterruptible power supply (UPS), photovoltaic (PV) systems in standalone mode. To reduce the total harmonic distortion (THD) of the output voltage without increasing the switching frequency, the T-type three levels inverter with enhanced modulation strategy, which minimizes the voltage ripple of the two input capacitors, is employed. In addition, a new single-loop fully digital control strategy in which various advanced control techniques such as proportional-integral observer, one-step ahead minimum prediction error, and model-based current command generator is also proposed to fulfill the strict requirements of the output voltage. The proposed control strategy is capable of achieving excellent output voltage regulation and high robustness against the parameters variation with low computational cost. Real-time simulations based on hardware in the loop (HIL) platform are conducted to confirm the validity of the method.

A new level Increased NLM method of modular multi-level converters for reducing THD of output voltage

ABSTRACT This paper proposes a new modified level, increased nearest level modulation (NLM) method to improve the THD quality and enhance the quality of output voltage of modular multilevel converters (MMCs). Offshore wind farm links need connections from onshore national grids, and this paper is consistent with higher level voltage generation techniques by using MMC. By changing the on-number sub-modules (SM) at the middle position after using the round function, the generated gate logic by using the proposed method maintains a fixed magnitude value of phase voltage with a maximal number of (2 N + 1) levels, where N is the number of sub-modules per arm. The proposed method maintains the advantages of the conventional NLM such as simple implementation and low switching loss and avoids the disadvantages of the existing NLM method such as capacitor voltage ripple, peak value of arm inductor voltage, complexity in topology, etc. Both the advantages and disadvantages of the proposed and the conventional schemes are also studied, and an illustrated comparison shows the THD result differences for four methods. The system is simulated using MATLAB-Simulink to validate the feasibility of the proposed scheme and carried out by a hardware-in-the-loop simulation (HILS) system. By reviewing the simulation and experimental results, it was found that the proposed scheme improves the output voltage and THD quality.

Symmetrical Cascaded Switched-Capacitor Multilevel Inverter Based on Hybrid Pulse Width Modulation

Multilevel inverters have been widely used in various industrial applications such as renewable energy generation and electric vehicles. An improved circuit of symmetrical cascaded switched-capacitor multilevel inverter is proposed so that the reactive power is absorbed by its power supply instead of capacitors. Then, a special hybrid pulse width modulation strategy combing level-shifted pulse width modulation (LS-PWM) and phase-shifted pulse width modulation (PS-PWM) was developed for the inverter. With this modulation algorithm, the power between cascaded units is automatically balanced, and the voltage of the capacitor in each unit is also automatically balanced to the dc input voltage. In addition, the optimized capacitor voltage ripple makes it possible to use a smaller capacitor to produce a better output voltage waveform. Theoretical analysis, simulation and experimental results show that the equivalent switching frequency of the cascaded multilevel inverter is twice the original frequency so that the output voltage harmonics are only distributed near even multiples of the carrier frequency.

Modeling and Optimization of Low-Capacitance Half-Bridge Modular Multilevel Converters Operated With Average Submodule Capacitor Voltage Control

This article develops a new closed-loop analytical steady-state model of low-capacitance half-bridge Modular Multilevel Converters (MMCs) operated with the average arm energy control and submodule capacitor voltage (SMCV) ripple above the conventional value of 10%. In this control approach, the level of average SMCVs is regulated directly. As has been already shown in the literature, if the average SMCVs are not controlled, the circular interactions of the inherent low-frequency SMCV ripple and the modulation signal which affect the dc arm voltages will also lead to the variations in the average SMCVs. In order to compensate for these variations, a dc component has to be introduced in the modulation signal (in addition to the second harmonic which suppresses the second-order circulating currents). This is included in the proposed analytical modeling where the three key nonlinear equations are established and solved iteratively using Newton Raphson's method to calculate the steady-state modulation signal parameters. The effects of the introduced dc component in the modulation signal on the MMC electrical quantities are demonstrated and analyzed. The developed model is also compared with the existing analytical steady-state MMC models in the literature. It is further employed to optimize the parameters of the grid-connected MMC. The optimization is based on the improved utilization of the available modulation signal window and is performed with the aim to minimize the voltage stresses on the power devices. The analysis is supported by simulation and experimental results.

A Modified Modular Multilevel Converter for Motor Drives Capable of High-Torque Operation at Zero/Low Motor Speeds

Modular multilevel converter (MMC) is regarded as an emerging topology in medium-voltage (MV) variable-speed motor drive applications, particularly for driving fan-/pump-like loads. The main challenge is the large capacitor voltage ripple of MMC at low speed with high starting torque. In this brief, a simple and cost-effective modified MMC topology is introduced, which can provide high toque to motor during the zero/low frequency. The topology is particularly suitable for fan-/pump-like loads. Circuit structure and operation principle of this modified topology is explained, and the control schemes are also developed. Validity and effectiveness of this topology are verified by experimental results.

DC-DC converter with 50 kHz-500 kHz range of switching frequency for passive component volume reduction

This paper presents the relationship of switching frequency towards passive components volume of DC-DC boost converter. Principally, the inductor current ripple and capacitor voltage ripple must be considered in order to design the inductor and capacitor, respectively. By increasing the switching frequency, smaller size and volume of passive component can be designed. As the consequences, the switching loss increases during switching transition at turn-ON and turn-OFF conditions. This paper used soft-switching technique to reduce the switching loss at turn-ON condition. The soft-switching technique is realized by adding resonant circuit in DC-DC boost converter. The effectiveness of resonant circuit will be analysed, thus, the efficiency of the converter can be improved. The range of switching frequency considered in the experimental are 50 kHz to 500 kHz. A 100 W prototype has been developed and tested in order to verify the principle. The switching loss experimentally confirm reduced by implementing soft-switching technique with efficiency converter improved from 96.36% to 97.12% when 500 kHz of switching frequency is considered. The passive components volume reduction is achieved when high switching frequency is used where the total volume of passive component when 50 kHz and 500 kHz are 0.083 dm3 and 0.010 dm3, respectively.

Switched-Capacitor-Based Modular T-Type Inverter

Switched-capacitor-based multilevel inverters have been gaining increasing attention in recent years in view of their voltage-boosting capability and capacitor self-balancing properties. However, the existing topologies, which comprise frontend switched-capacitor cells and a backend H-bridge, result in longer discharging duration in some of their switched-capacitors. In addition, the adoption of backend H-bridge introduces common-mode voltage and necessitates multiple dc sources for voltage level extension. To resolve these problems, this article proposes a novel topology comprises a T-type inverter and n cascaded switched-capacitor cells. It requires only a single dc source and it manages to mitigate the common-mode voltage by connecting the neutral of ac output to the mid-point of dc link. All switched-capacitors in the proposed topology are ensured to be charged for at least half of the fundamental period to alleviate the capacitor voltage ripples and current spikes. The operation of the proposed topology is analyzed and experimental results are presented for validation.

Design and Simulation of Modular Multilevel Converter Fed Induction Motor Drive

Traditional modular multilevel converter (MMC) applications in medium voltage induction motor drive are difficult, particularly at low speeds because of the higher magnitude of the voltage ripple of the sub-module capacitor. This paper uses a hybrid MMC, particularly at low frequencies, to achieve a lower peak-to-peak voltage ripple of the sub-module capacitor. The vector control strategy with the closed-loop speed control indicates an accurate and wide-speed range. MATLAB / Simulink is used to simulate and obtain the simulation results of hybrid and traditional MMC with induction motor drive and compare from the standpoint of capacitor voltage ripple. The results are shown the reduction of peak-to-peak voltage ripple of the sub-module capacitor as the hybrid MMC is operated.

Design and Simulation of Modular Multilevel Converter Fed Induction Motor Drive

Traditional modular multilevel converter (MMC) applications in medium voltage induction motor drive are difficult, particularly at low speeds because of the higher magnitude of the voltage ripple of the sub-module capacitor. This paper uses a hybrid MMC, particularly at low frequencies, to achieve a lower peak-to-peak voltage ripple of the sub-module capacitor. The vector control strategy with the closed-loop speed control indicates an accurate and wide-speed range. MATLAB / Simulink is used to simulate and obtain the simulation results of hybrid and traditional MMC with induction motor drive and compare from the standpoint of capacitor voltage ripple. The results are shown the reduction of peak-to-peak voltage ripple of the sub-module capacitor as the hybrid MMC is operated.

Direct Predictive Current Control of a New Five-Level Voltage Source Inverter

This article proposes a direct predictive current control approach to regulate the flying capacitor voltages along with the output currents in a new five-level voltage source inverter under a wide range of output frequencies and load power factors. Also, the proposed approach maintains the flying capacitor voltage ripples well below the desired limits (10%-15%) throughout the operating range. The sampled data model of a new five-level voltage source inverter is developed to predict the future behavior of the control variables such as output current, and flying capacitor voltage and its ripples. A three-objective cost function with reference and predicted control variables is formulated. The cost function is evaluated for all possible switching states, then an optimum switching state which gives the low cost value is selected. The optimum switching state is applied to the converter in one sampling interval. The feasibility of the proposed control method is verified on a new five-level voltage source inverter by using MATLAB/Simulink simulations and experimental studies on a scaled-down laboratory prototype with dSPACE Microlab Box. Furthermore, the performance of the proposed approach is analyzed in terms of voltage and current harmonic distortion, and flying capacitor voltage ripples at different current magnitudes, output frequencies, and load power factors.

A Capacitor Current and Capacitor Voltage Ripple Controlled SIDO CCM Buck Converter With Wide Load Range and Reduced Cross Regulation

It is well-known that the ripple control technique benefits from fast load transient response and small cross regulation for single-inductor dual-output (SIDO) dc–dc converter. However, existing capacitor current ripple (CCR) controlled SIDO buck converter suffers from incomplete operation state and single switching path. Thus, it has a limited stable load range. In order to extend the stable load range and suppress cross regulation of SIDO buck converter in continuous conduction mode (CCM), a novel ripple control technique, called as capacitor current and capacitor voltage ripple (CCVR) control, is proposed in this article. The operation principle of the proposed CCVR controlled SIDO CCM buck converter is presented, and its discrete iterative map model is established. Furthermore, the stability and stable load range have been discussed by using bifurcation diagrams. Based on the established small signal model, Bode plots are given to show the performance in suppressing cross regulation. Finally, simulation and experiment results are provided to verify the theoretical analysis. It shows that the proposed CCVR control technique can be applied to SIDO CCM buck converter to extend its stable load range and suppress its cross regulation.

Self-Balanced Switched-Capacitor Thirteen-Level Inverters With Reduced Capacitors Count

This letter describes two improved 13-level inverters based on switched-capacitor. Compared with their original structure, which is published recently, one less high-voltage capacitor is required in the proposed inverters and the blocking voltage of their inverting half-bridge is reduced by half. In addition, the new inverters inherit various advantages of the original structure, such as a high boost factor of 6, self-balanced capacitor voltages, and reduced voltage ripples. Circuit description, operation principle, hybrid PWM modulation, and capacitor voltage ripples are analyzed and the feasibility of the proposed inverters is finally verified by experimental results.

Pareto Frontier of the Arm Energy Ripple and the Conduction Losses of a Modular Multilevel Converter

Modular multilevel converters can achieve high power quality and voltage ratings, becoming a good alternative for high-voltage direct-current applications. However, the circulating current and capacitor voltage ripple remain significant drawbacks, mainly due to the impact on efficiency and power density. In this work, a stationary operation of a modular multilevel converter is optimized by calculating the Pareto frontier of the arm energy ripple and the conduction losses for different operating conditions. From the set of optimal solutions, the best trade-off between energy ripple and power losses can be chosen depending on the application requirements. Therefore, an optimal operating point regarding capacitor voltage ripple and circulating current was found, optimizing efficiency and allowing a reduction of capacitance, hence size and cost. The theoretical analysis and results are validated experimentally on a prototype test-bench.

Capacitor voltage ripple reduction in MMC-HVDC system using flat bottom current method

Modular-multilevel converters (MMCs) are considered a useful technology in high-voltage direct current (hvdc) transmission systems. Since it offers many advantages like modular configuration with scalable output voltage. Voltage ripples on sub-module (SM) capacitors in MMC and circulating currents are two major challenges, which can affect the converter stability and performance. Conventionally, direct modulation method is used to insert or bypass SMs in MMC producing circulating current and SMs capacitor voltage oscillations. The closed loop and open loop control methods are used to overcome drawbacks in direct modulation method. However, these methods rely on the accurate measurements and estimation of SM capacitor voltages. A flat bottom current method is proposed in this paper to minimize the amplitude of ripple voltages on SM capacitors in MMC-based HVDC systems. Limited capacitor energy changes leading to reduce capacitor voltage ripples are observed by the proposed methodology using direct-modulation method without using circulating current suppression controller. The proposed method is tested on a two-terminal MMC-based HVDC system implemented by vector control approach in PSCAD/EMTDC software. Analytical results are verified by simulation results in reducing capacitor voltage ripples based on the direct modulation approach.

M-Type and CD-Type Carrier Based PWM Methods and Bat Algorithm-Based SHE and SHM for Compact Nine-Level Switched Capacitor Inverter

This paper presents new and efficient modulation techniques applied on the recently introduced compact nine-level switched-capacitor inverter (C9LSCI). The paper also discusses the performance of the inverter under selective harmonic elimination (SHE) and mitigation (SHM) based on the heuristic Bat-Algorithm (BA) technique. Two new modulation techniques employing CD-type carrier waves and M-type carrier waves are proposed, showing a rise in efficiency, an increase in RMS voltage, reduction in capacitor voltage ripple, and a reduction in harmonic distortion (THD). Four recently introduced modulating signals, namely the third harmonic injection, thirty-degree bus clamped PWM (THTDBCPWM), third harmonic injection sixty-degree bus clamped PWM (THSDBCPWM), one pole clamped PWM (OPCPWM). Loss balancing two-pole clamped PWM (LBTPCPWM) are applied to the inverter with the proposed carriers, and a comparative analysis is carried out on different performance parameters. The proposed carriers' efficacy in Alternate Phase Opposition Disposition (APOD), Phase Opposition Disposition (POD), and Phase Disposition (PD) is validated. Further, Bat algorithm (BA) is applied as a heuristic approach to determine the optimum angles for the application of SHE and SHM. The efficacy of all the above techniques on the C9LSCI is verified in MATLAB/Simulink environment and further validated on an experimental setup.

A generalized switched-capacitor step-up multilevel inverter employing single DC source

In this paper, a new generalized step-up multilevel DC-AC converter is proposed, which is suitable for applications with low-voltage input sources such as photovoltaic power generation and electric vehicles. This inverter can achieve a high voltage gain by controlling the series-parallel conversion of DC power supply and capacitors. Only one DC voltage source and a few power devices are employed. The maximum output voltage and the number of output levels can be further increased through the switched-capacitor unit's extension and the submodule cascaded extension. Moreover, the capacitor voltages are self-balanced without complicated voltage control circuits. The complementary operating mechanism between each pair of switches simplifies the modulation algorithm. The inductive-load ability is fully taken into account in the proposed inverter. Additionally, a remarkable characteristic of the inverter is that the charging and discharging states among different capacitors are synchronous, which reduces the voltage ripple of the frontend capacitors. The circuit structure, the working principle, the modulation strategy, the capacitors and losses analysis are presented in detail. Afterwards, the advantages of the proposed inverter are analyzed by comparing with other recently proposed inverters. Finally, the steady-state and dynamic performance of the proposed inverter is verified and validated by simulation and experiment. 1

Dynamic Phasor Modeling and Analysis of Three-Phase DC/DC/AC MMCs for Hybrid AC/DC Grids

The first comprehensive three-phase state-space dynamic phasor model (DPM) for the emerging class of dc/dc/ac modular multilevel converters (MMCs) is presented that accounts for both dc/dc and dc/ac power transfer mechanisms. The phasor model is derived in the α β 0-frame and includes frequency components from dc to third harmonic, which is shown to yield a high degree of matching for both steady-state and transients when compared against switched and time-averaged models, the latter of which is also derived in this work. An analytical solution procedure is then proposed that takes advantage of the phasor model time-invariance to find the full state solution at any operating point including the requisite open-loop modulating signals. Several analyses are carried out for two different dc/dc/ac MMC case study designs to explore the utility and versatility of the proposed DPM and solution procedure. Results show the patterns of internal current stresses and capacitor voltage ripples for the arms can vary significantly and in a nonsymmetrical manner, which is in stark contrast to the traditional dc/ac MMC.