Submodule Capacitor Voltage Ripple Research Articles

Submodule capacitance requirement reduction with capacitor voltage ripple suppression in MMC

In modular multilevel converter (MMC)-based high-voltage direct current transmission system, hundreds of submodules (SMs) with large SM capacitance in each arm result in expensive cost and bulky volume. In full-bridge submodules (FBSMs)-based MMC (FB-MMC), a novel capacitor voltage ripple suppression method based on three available variables manipulation is proposed to reduce SM capacitance requirement. In the interaction of these available variables, the dominant fundamental-frequency and second-order harmonic fluctuations of SM capacitor voltage can be eliminated under different power factors. Under the proposed method, three-to-five times frequency fluctuation components will be reduced as much as possible. By establishing capacitor voltage fluctuation model, the concrete mathematical expressions of the three desired available variables under different power factors can be derived, which are easily implemented to suppress capacitor voltage ripple. Compared with normal operation, SM capacitor voltage ripple can be reduced by 80% with the proposed method when the power factor of 0.9–1 is considered. In other words, SM capacitance requirement can be reduced by 80% under the same constraint of capacitor voltage ripple. Therefore, the cost and volume of SM capacitors can be significantly reduced. Simulation results confirm the feasibility and validity of the proposed method.

A Modular Multilevel Converter With Active Power Filter for Submodule Capacitor Voltage Ripples and Power Losses Reduction

Modular multilevel converter (MMC) topology has been widely used in the medium-/high-voltage applications. However, large capacitance is needed to suppress submodule (SM) capacitor voltage ripples. This article presents a modified MMC with the active power filter (APF-MMC) paralleled between the N th SM of the upper arm and the first SM of the lower arm in each phase without changing the original topology. It is intended to decrease the capacitor voltage fluctuations and power losses of SMs by diverting the phase low-frequency ripple powers into the storage capacitor of the additional active power filter circuit. Both the detailed SM capacitor voltage fluctuations, which can be reduced up to 50% of the original value in the traditional MMC topology, and the SM average currents, which are proportional to SM switching losses and can be reduced up to 87.83% of the original value, are analyzed. The arm rms currents are also derived, which are related to SM conduction losses. Moreover, the operating principle, modulation method, and control strategy are presented in detail. Finally, the APF-MMC and the traditional MMC topology simulation and experimental platforms were built. The simulation and experimental results demonstrate the validity and performance of the APF-MMC topology and control strategy by comparing with the traditional MMC topology.

Capacitor Voltage Ripple Suppression in a Modular Multilevel Converter Using Frequency-Adaptive Spatial Repetitive-Based Circulating Current Controller

In this article, a new circulating current control architecture is proposed to suppress the submodule capacitor voltage ripple in a modular multilevel converter (MMC). The proposed cascaded control architecture uses the spatial repetitive controller (SRC) in both the outer voltage loop and the inner current loop, and can effectively suppress the second harmonic component of the submodule capacitor voltage ripple even under variable line frequency conditions. The dual SRC-based circulating current controller injects a second harmonic component into the circulating current to suppress the submodule capacitor voltage ripple. The SRC, which is a frequency-adaptive periodic digital controller, incorporates a phase sampling technique to achieve dynamic change in the sampling frequency, can adaptively tune its gain characteristics according to operating line frequency of the MMC. Unlike the existing techniques, the proposed controller uses the submodule capacitor voltage information for circulating current harmonic reference generation and, hence, does not require additional sensing of the output current. Detailed mathematical analysis and architecture of the proposed controller are presented in this article. A 400-V, 1-kW MMC laboratory prototype is developed and experimental results demonstrating the performance of the proposed circulating current control architecture for variable frequency and load conditions are also presented.

Multi-Objective Genetic Algorithm Based Coordinated Second- and Third-Order Harmonic Voltage Injection in Modular Multilevel Converter

The submodule (SM) capacitor voltage ripple and the root mean square (RMS) of the arm current are related to the costs and losses of the modular multilevel converter (MMC). The optimization issues have received great attention. The second-order circulating current control and third-order harmonic voltage injection are two conventional optimization methods, but the existing research neglects their coupling effects. In this paper, these two methods are comprehensively investigated and coordinately combined. First, the various performances of MMC under multiple injections are characterized. Then, a multi-objective optimization model considering the output capability of arm voltage and the energy balance of capacitors, is proposed to reduce the SM capacitor voltage ripple and the RMS of the arm current. The amplitudes and phases of the injected second- and third-order harmonic voltages are designed coordinately by genetic algorithm. The simulation and experimental results have validated the effectiveness and feasibility of the proposed method.

Deadbeat Predictive Current Control for Modular Multilevel Converters With Enhanced Steady-State Performance and Stability

Model predictive control (MPC) methods are popularly employed in modular multilevel converters (MMCs) due to their fast dynamic response and multiobjective control capability. However, they present some inherent problems, such as computational complexity, variable switching frequency, poor steady-state performance, and tedious weighting factor selection. This article develops a deadbeat predictive current control method for MMCs. This method can realize the reference tracking of ac current and circulating current in one sampling period without error, and thus provide a fast dynamic response as conventional MPC methods. Besides, switching state or voltage level evaluation, cost function calculation as well as weighting factor selection are not required. Therefore, it has a very low calculation burden, which is independent of the number of submodules (SMs). Since a modulation stage is utilized, a fixed switching frequency and consequently a satisfactory steady-state performance are obtained. The effects of time delay, parameter mismatch, and SM capacitor voltage ripple on the control algorithm are discussed. Also, the corresponding improvement measures are provided to further enhance the steady-state performance and system stability of MMCs. The effectiveness and performance of the developed control algorithm are verified by experimental results.

Equivalent Model of Modular Multilevel Converter Considering Capacitor Voltage Ripples

Modular multilevel converter (MMC) is regarded as a topology with high potential for high-voltage direct current transmission system. However, the distributed capacitors lead to significant submodule capacitor voltage ripples, which have great influence on the terminal characteristics of MMC. In this paper, an equivalent model considering submodule capacitor voltage ripples and circulating current suppression control is established to explore the terminal characteristics of MMC. In the model, MMC is equivalent to the cascade of buck/boost converter and two-level voltage source converter (VSC) from a terminal perspective. The influence of capacitor voltage ripples is represented by the equivalent series impedances, whose characteristics are investigated in detail with harmonic linearization method. Then, the simplified equivalent series impedances are obtained. Based on the equivalent model, the study of terminal characteristics is greatly simplified by directly applying conclusions of two-level VSC and buck/boost converter to MMC. Furthermore, the control strategy design and impedance modeling of MMC have been carried out to show how the equivalent model can be used in practical applications. Finally, the effectiveness of the proposed model is verified by detailed simulation results.

Device Loading and Reliability Analysis of Modular Multilevel Converters With Circulating Current Control and Common-Mode Voltage Injection

The submodule (SM) capacitor voltage ripple and circulating current (CC) are major technical challenges in modular multilevel converters (MMCs). The generation of CC in the MMC results in an increase of SM capacitor voltage ripple, voltage/current stress on power switching devices, and power losses. Therefore, minimizing the CC and SM capacitor voltage ripple is essential for the smooth operation of an MMC. Prior studies and research have focused on different control strategies and modulation schemes to minimize the CC and SM capacitor voltage ripple. This paper presents an impact of active CC control and common-mode voltage (CMV) injection on thermal loading of power switching devices. To evaluate the performance comparison between different control approaches, a generalized control framework for MMC is established. An electrothermal simulation is performed to study the thermal loading on power switching devices. The efficacy of the generalized control scheme is evaluated through PLECS simulations and hardware-in-the-loop experiments. Finally, the thermal loading and reliability of MMCs are evaluated for different modulation schemes with the active CC control and CMV injection.

Trapezoidal Current Modulation for Bidirectional High-Step-Ratio Modular DC–DC Converters

A modular dc–dc converter (MDCC) has been proposed for high-step-ratio interconnection in dc grid applications. To further optimize the performance of the MDCC, this article presents a trapezoidal current modulation with bidirectional power flow ability. By giving all the submodule (SM) capacitors an equal duty to withstand the stack dc voltage, their voltages are balanced without additional feedback control. Moreover, based on soft-switching performance and circulating current analysis, three-level and two-level operation modes featured with high efficiency conversion and large power transmission, respectively, are introduced. The control schemes of both modes are designed to minimize the conduction losses. Besides, the SM capacitor voltage ripples with different switching patterns are compared, and the option for ripple minimization is presented. A full-scale case study is provided to introduce the design process and device selection of the MDCC. The experimental tests based on a downscaled prototype are finally presented to validate the theoretical analysis.

Modeling and Analysis of Modular Multilevel Converter in DC Voltage Control Timescale

This paper presents a small-signal model of modular multilevel converter (MMC) based on the motion equation concept in the dc voltage control (DVC) timescale (around 10 Hz). The relations between the active/reactive powers and the phase/magnitude dynamics of the internal voltage vector are developed with considering the submodules (SMs) capacitor voltage ripple. With the proposed model, the stability mechanism of the DVC timescale in an MMC-HVdc system can be well interpreted with the equivalent inertia and damping coefficients. It is found that the fundamental frequency ripple in the capacitor voltage of SMs will result in new oscillation modes in DVC timescale when compared with the conventional two-level voltage source converter. Furthermore, the coupled relationship between the reactive power and phase dynamics of internal voltage is revealed, which has great effect on the system stability property. Then, comparative studies via eigenvalues analysis show the proposed model can hold the main behaviors of concern, and the correctness of the proposed model is verified by comparisons with detailed time-domain simulations and experimental tests. Finally a single MMC connected to infinite-bus grid is taken as an example to validate the feasibility of the proposed model for dynamics analysis.

Failure Prediction of Submodule Capacitors in Modular Multilevel Converter by Monitoring the Intrinsic Capacitor Voltage Fluctuations

Modular multilevel converters (MMCs) are emerging as a promising topology for medium- and high-power applications. Aluminum electrolytic capacitors (AECs) are usually employed in MMCs as floating capacitors due to their high volumetric efficiency and low price. AECs gradually deteriorate over time due to electrolyte vaporization, and for this reason, they have been recognized as one of the most fragile components in the converter. As they continue to age, the AEC's capacitance decreases and its equivalent series resistance increases, which can result in an increase of submodule (SM) capacitor voltage ripple, power loss and could damage the operation of the MMC with prolonged use of aged capacitors. To prevent such damage, monitoring the health of capacitors is an important step to enhance the reliability of the MMC by predictive maintenance. This paper presents a failure prediction scheme for SM capacitors in the MMC by monitoring the SM capacitor voltage oscillations. Detailed simulation studies are carried out for a five-level MMC in the PLECS® platform and verified experimentally. The estimated capacitance through simulations and experiments is in close agreement to that value measured using the LCR meter.

Modular multilevel converter with DHB energy‐balancing channels for medium‐voltage adjustable‐speed drives

This paper presents a modular multilevel converter (MMC) configuration that utilises energy exchange between submodules (SMs) of upper and lower arms, for energy rebalancing. The configuration is applicable to mediumvoltage high-power variable-speed drives with any number of motor phases, where the traditional MMC topology experiences challenging shortcomings. With the out-of-phase alternation of the fundamental ripple power in upper and lower arms, the proposed MMC configuration decouples this ripple power by employing dual half-bridge modules linking opposite SMs in upper and lower arms of the same MMC-leg. This counter-balances arm ripple-power through bidirectional power transfer between opposite SMs, resulting in a reduction in the SM capacitance and the MMC system stored energy. The proposed MMC configuration solves the problem of wide SM capacitor voltage fluctuation, especially at low operating frequencies, where the SM capacitor voltage ripple profile is almost constant, independent of the operating frequency. Therefore, the configuration is able to drive multi-megawatt machines from stand-still to the rated speed, at rated torque. The operation of the proposed converter topology is elucidated in detail, and its effectiveness is verified through simulation and experimentation.

Capacitor Voltage Balance Control of Hybrid Modular Multilevel Converters With Second- Order Circulating Current Injection

The capacitor voltage imbalance between the full-bridge sub-modules (FBSMs) and half-bridge submodules (HBSMs) may occur in the hybrid modular multilevel converters under the overmodulation operation. In this paper, the sub-module output voltage model under overmodulation is established first to explore the mechanism of capacitor voltage imbalance. With the established sub-module output voltage model, the complementary second-order voltage component in FBSMs and HBSMs is disclosed, which makes the second-order circulating current a novel control freedom to balance the capacitor voltages. With proper design of the magnitude and the phase angle of the injected second-order circulating current, the energy between the FBSMs and HBSMs is transferred and the capacitor voltage imbalance under overmodulation is eliminated. Moreover, the relationship between the sub-module capacitor voltage ripples and injected circulating current is also investigated to optimize the capacitor voltage ripple. Finally, the simulation and experiment results show that the proposed second-order circulating current injection method can not only balance the capacitor voltage but also reduce the voltage ripples of capacitors and current stress of semiconductor devices.

Analysis and Injection Control of Circulating Current for Modular Multilevel Converters

The circulating current of modular multilevel converter (MMC) has a great impact on the power losses, the submodule (SM) capacitor voltage ripples, and the rating of power devices. Existing analysis of circulating current has considered the capacitor voltage and the modulation signal. In this paper, the effect of semiconductor devices on the circulating current is further presented. In order to realize the different circulating current injection targets under different conditions, a unified online calculation scheme by using the instantaneous information of MMC is proposed in this paper to generate the circulating current injection references. It avoids detecting the amplitude and phase of the output current or using an extensive lookup table. Besides, a mathematical analysis model is built and it reveals the impact on the capacitor voltage ripple and the rms value of arm current with different injection references, thus guide the determination of the injection ratio. Experimental results based on a three-phase MMC prototype with six SMs per arm are presented to demonstrate the analysis of circulating current and the proposed circulating current injection references generating method is verified by different circulating current control targets with different loads.

Modified Modular Multilevel Converter to Reduce Submodule Capacitor Voltage Ripples Without Common-Mode Voltage Injected

This paper presents a modified modular multilevel converter with the three-terminal middle-cells inserted between arms and ac-side for medium voltage applications. It is intended to reduce the submodule capacitor voltage ripples by controlling the high-frequency powers flowing among arm branches for low-frequency output operations without common-mode voltage injected in ac-side. Due to the physical connection among the upper arm, the lower arm and ac-side, the middle-cell can be regarded as the excess degree of freedom to modify the dc-side and ac-side characteristics. Therefore, the common-mode voltage in ac-side can be eliminated by the inverse common-mode voltage produced by the middle-cell in ac-side. Moreover, owing to the unique structure of the middle-cell, the middle-cell capacitor voltage also results in a small ripple due to the high-frequency current flowing into it by proper control. The operation principle and control strategy of the middle-cell has been proposed in detail. Simulation and experimental results have verified the effectiveness of the proposed topology.

Review on low‐frequency ripple suppression methods for MMCs for medium‐voltage drive applications

Advantages of the modular multilevel converter (MMC) such as modularity, scalability, fault-tolerant ability, low device switching frequency, transformerless grid connection etc. have made it the most promising converter topology for medium-to-high-voltage, high-power applications. However, in MMC-based medium-voltage drives, the excessive fundamental-frequency submodule capacitor voltage ripple at low speeds or start-up poses a major technological challenge. A lot of low-frequency ripple suppression methods have been proposed to address this problem. This study summarises the state of the art in this area and classifies these methods into three categories. The pros and cons of them are discussed. An in-depth analysis of these methods from a practical viewpoint is given. Some future research trends are also discussed.

Hybrid Modular Multilevel Converter With Redistributed Power to Reduce Submodule Capacitor Voltage Fluctuation

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.

A Star-Channel Modular Multilevel Converter for Zero/Low-Fundamental-Frequency Operation Without Injecting Common-Mode Voltage

This paper proposes a star-channel modular multilevel converter (MMC). It overcomes the zero/low-fundamental-frequency operation issues of conventional MMC while not injecting the common-mode voltage (CMV). The proposed converter is characterized by the cross connections of upper three arms and lower three arms through two sets of star-channel branches. These star-channel branches rebalance the power among three phase legs, resulting in minimized submodule capacitor voltage ripples even at zero/low-fundamental-frequency operation without CMV injection. This feature enables the proposed converter to interface medium/high-voltage DC and variable-frequency AC systems. A 4160-V 1-MW simulation model is performed in MATLAB/Simulink, and a 208-V 5-kW experimental setup is constructed in the lab. Both of the simulation and experimental results verify the feasibility of the proposal.

Decoupled Current Control With Synchronous Frequency Damping for MMC Considering Sub-module Capacitor Voltage Ripple

Capacitor voltage ripple in the fundamental frequency is distinctive in the performance of modular multilevel converters (MMCs), unlike other voltage-source converters. First, in order to reveal the influence of capacitor voltage ripple, the coupling effect of conventional MMC double-loop control strategy is investigated to explain the unsatisfactory response. Then, decoupled current control with synchronous frequency damping is proposed, eliminating the coupling terms in the presented average value MMC model, which can improve the system dynamic behavior. Moreover, the damping index is defined and designed in the proposed control strategy to compromise between current decoupling and oscillation damping. Furthermore, the dynamic MMC characteristics with the decoupled current control and appropriate damping index are explored with detailed theoretical analysis. Finally, the simulation results validate the effectiveness of the proposed control strategy.

Extended Controllability of HVDC Converters in the Vector Space

This paper investigates the controllability of modular multilevel converters for high voltage direct current applications. The paper provides an in-depth analysis of a computationally efficient method to calculate the required amount of triplen harmonic injection (THI) and to devise an overmodulation strategy based on a three-dimensional representation of the converter control voltages in the vector space. The proposed method allows simultaneous converter operation and power quality control in an extended ac voltage range without degrading the submodules (SM's) capacitor voltage ripple or spectra of generated waveforms with respect to well-known THI methods. The advantages of the presented technique with respect to traditional THI techniques are: first, the converter controllability is extended during ac-side voltage disturbances; second, the SM's capacitor voltage ripple is upper bounded; and third, the insulation stress due to the common-mode voltage at the transformer's secondary windings is minimized. The performance of the claimed features is supported by simulation results for both steady-state and transient operating conditions.

Average-Value Model of Modular Multilevel Converters Considering Capacitor Voltage Ripple

The modular multilevel converters (MMCs) are attractive topologies for the high-voltage direct current (HVDC) transmission system. Since the complicated structure of MMC with hundreds sub-modules per arm, building an accurate model to design the appropriate control strategy is one of the main technical challenges. In this paper, the average-value model (AVM) of the MMCs considering the sub-module capacitor voltage ripple is established to explore the control dynamic performance of MMCs. The effect of the sub-module capacitor voltage ripple on the MMC dynamic behavior is explored and the equivalent impedances of the DC and AC sides in the MMC converters are extracted and evaluated. Based on the equivalent impedances, a particular AVM of MMCs, which comprises the equivalent capacitors reflecting the capacitor voltage ripple, is presented. Furthermore, the detailed interaction of the AC side voltages, AC side currents and arm voltages and the coupling relationship of AC output current i <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> and i <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</sub> can be also precisely described by the introduced AVM. Finally, the effectiveness of the presented model is verified by the PSCAD/EMTDC simulations.