Capacitor Voltage Fluctuation Research Articles

Capacitor Voltage Fluctuation Suppression Method Based on Improved MMC Topology for Variable-Frequency Drive Application

Modular multilevel converter (MMC) has been widely applied in HVDC systems. When MMC is applied in variable-frequency drives, the sub-module (SM) capacitor voltage fluctuation becomes seriously at low output frequency. To suppress the fluctuation, a useful method is high-frequency injection. However, the high-value common-mode voltage would cause the insulation problem to the load. To address this issue, a hybrid MMC with a series of full-bridge SMs (FMs) inserted at the dc side is given. The high-value common-mode voltage can be eliminated by the additional arms. Meanwhile, the SM capacitor voltage fluctuation of the upper and lower arms of MMC system can be suppressed by an optimal high-frequency injection method. Simulation and experiment results confirm the validity of the given topology.

A voltage fluctuation compensation control strategy at DC side for integrated power quality control device

The power quality control process in the integrated power quality control device, especially for three-phase unbalance control, usually leads to the capacitor voltage fluctuations on DC side. In rectifier control strategy, the output of the voltage loop is the reference value of the inner current loop, and the DC-side voltage is the reference value of the voltage loop. Therefore, the DC-side voltage instability will overlay a fluctuating component on the reference value, which affects the accuracy of the inner current loop control, causing a considerable impact on the transient performance of the integrated power quality control system. Hence, the stability of the DC-side voltage becomes the main factor of the current loop control strategy. Unfortunately, in practical applications, such fluctuations on DC side are inevitable. In this paper, the DC-side capacitor voltage fluctuation mechanism is studied, and a comprehensive power quality control strategy based on feed-forward compensation is proposed to mitigate the DC capacitor voltage fluctuation influence. The proposed compensation control strategy prevents the impact of DC-side voltage fluctuations on the control system. The simulation conducted on the developed experimental platform verified the feasibility and capability of the feed-forward control strategy of the integrated power quality control system.

Operation and control schemes of a novel direct AC-AC modular multilevel converter

AC-DC-AC modular multilevel converter (MMC) and direct AC-AC MMC are two kinds of interfaces to integrating AC motors into power grid, and they are easily damaged from large capacitor voltage fluctuation with varying motor speed. Up to now, none of the methods could balance the reliability and power density of MMC-based motor drives system. To find a better solution to this problem, a novel direct AC-AC MMC topology is proposed in this paper. Operating principle of this topology is introduced and relative multi-loop feedback control method is designed for it. Then, simulation and experiments are conducted for verification. Compared with the traditional MMC with high-frequency injection method, the proposed topology has no common-mode voltage problem and the current stress is reduced by 38.5%. Compared with the half-bridge-based AC-DC-AC MMC, the number of capacitors and inductors of the proposed topology is reduced by 20.8% and 25%. In comparison with the direct AC-AC MMC (modular multilevel matrix converter), the number of power devices of the proposed topology is reduced by 28%. The proposed topology has higher reliability and power density.

A Novel Modular Multilevel Converter With Ripple Current Elimination Channels Based on Isolated CLLC Resonant Circuits

This paper proposes a novel multilevel topology “Modular-Isolated-Multilevel-Converter” which achieves almost zero low frequency capacitor voltage fluctuation. It inherits the structure of MMC but replaces the half bridge module by the newly proposed Isolated Half-Bridge (IHB). The fundamental and 2^nd order harmonic frequency current originally in the MMC module capacitor have been eliminated through connecting the secondary sides of the IHB at the same level of the three phases together. The elimination is due to that the 1^st and 2^nd order components in the arm current are 120° phase shifted in three phases. Therefore, the module capacitance is reduced by more than 10 times since it only carries switching frequency ripple. Moreover, the arm inductance can also be significantly reduced since the 2^nd order harmonic current disappears. The topology is specifically suitable for variable frequency drive application, because its capacitance and inductance are not affected by the output frequency. The challenges of zero frequency start-up when MMC is adopted for VSD can be addressed here. In the paper, the operation principle of the proposed MIMC is fully analyzed and the mathematical model is built. Moreover, a methodology of capacitor sizing and arm inductor design for general MMC topology is proposed. The detailed design considerations for MIMC are also discussed and presented. The plant modeling and control strategy have been proposed for MIMC. A 55-kW simulation is carried out to verify the theoretical analysis. And a 6-kW downscaled hardware prototype is also developed to demonstrate the benefits of the new topology over the traditional MMC.

A low-frequency fluctuation suppressing method for a seven-level hybrid T-type nested neutral point clamped converter

This paper proposed a low-frequency fluctuation suppressing method for a seven-level hybrid T-type nested neutral point clamped (7L-HTNNPC) converter that can be applied in medium-voltage and high-power powertrain systems. The 7L-HTNNPC converter is a combination of a 3L-ANPC unit and a four-level T-type neutral point clamped (4L-TNNPC) unit. The 3L-ANPC unit is modulated to switch at a low fundamental frequency while the remained 4L-TNNPC unit is operated at a high switching frequency. Compared to the existing modulation method, the proposed method has the advantage of resuppressing flying capacitor voltage fluctuations and reducing current harmonics with a low flying capacitance. Simulation results have been presented to verify the proposed low-frequency fluctuation suppressing method for the 7L-HTNNPC converter.

A Modular Multilevel Converter Topology With Novel Middle Submodules to Reduce Capacitor Voltage Fluctuations

This letter proposes a modified topology of modular multilevel converter for medium-voltage applications, which can reduce the ac-voltage fluctuation in the submodule (SM) capacitor at low-frequency operation. A novel structure of middle SM is inserted between the upper and lower arms, which has lesser count of components compared with the existing ones, leading to the reduction in cost and power losses. At the low operating frequency, furthermore, the middle SM can attenuate the power fluctuation in arms by distributing the high-frequency power components. Therefore, the SM capacitor voltage fluctuation is significantly alleviated without imposing the common-mode voltage on the ac side. The effectiveness of the proposed topology has been verified by experimental results.

Small-Signal Stability Modeling for MMC-Based DC Grids With Voltage Slope Control and Influence Analysis of Parameters

In the flexible DC grids, selections of slope coefficients in system level controls and designs of smoothing reactors have a crucial influence on the system stability. In order to obtain their optimal values, a comprehensive small-signal stability model with the DC voltage slope control and smoothing reactors is proposed in this paper. Firstly, the linear state-space model of the entire system based on the small-signal method is deduced according to the coupling equations between the inputs, the outputs, and state variables related to converter stations and the AC sides, controllers, and the DC grid with smoothing reactors. Meanwhile, the influences of the MMC bridge arm circulating current and the capacitor voltage fluctuation of the sub-module are considered in this model. Secondly, the impacts of control parameters, smoothing reactor parameters, and bridge arm inductances are analyzed on the system stability through the Lyapunov stability theory. Finally, the simulation of Zhangbei four-terminal DC grid project in China is established to testify the correctness of the presented small-signal model and the conclusions about the impact of parameters on the system stability. The proposed method will provide some theoretical guidance for the system design and the parameter selection in DC grid applications.

Suppression control strategy of MMC sub-module capacitor voltage in variable-frequency situation

Modular multilevel converter (MMC) topology has received widespread attention in flexible DC transmission, power quality control, and other fields because of its high modularity and malleability characteristics, and has become one of the most promising topologies. However, owing to the characteristics of the MMC itself, the fluctuation of the capacitor voltage of the MMC sub-module is inversely proportional to the MMC working frequency. One of the main technical challenges for MMC is effectively suppressing the fluctuation of the capacitor voltage of the MMC sub-module when working at low frequencies. This study first analyzes the reasons for the fluctuation of the MMC sub-module capacitor voltage at low frequency. Then, based on the results of this analysis, we propose a suppression control strategy for the capacitor voltage fluctuation of the MMC sub-module in variable-frequency working situations. Finally, an MMC module was built using the MATLAB/Simulink environment for simulation verification, and a five-level MMC physical experimental platform was built for experimental verification. The simulation and experimental results demonstrate that the proposed suppression control strategy for the MMC sub-module capacitor voltage effectively suppresses the fluctuation of the capacitor voltage under variable-frequency working situations.

Adaptive control strategy of solid state transformer with fast dynamic response and enhanced balance performance

Solid state transformer (SST) is designed as a power transmission device between AC grids and DC grids. However, the power fluctuation causes unbalanced power sharing among dual active bridge (DAB) converters and large voltage fluctuations on dc buses. Here, an adaptive virtual impedance control (AVIC) strategy is proposed to improve the balance performance of SST. Adaptive virtual impedance controller gets the same dynamic response for DAB converters, which matches dynamic impedance of DAB converters. Thereby the power balance can be achieved even the load changes. Meanwhile, an adaptive feedforward control scheme is proposed to reduce the influence of load fluctuation on DC bus voltage. The adaptive feedforward scheme also considers the parameter mismatch, thereby the balance of SST will not be destroyed. Moreover, the balanced power sharing achieved by adaptive controller eliminates the difference between actual transmission power and compensation power in each rectifier, which further suppresses the intermediate capacitor voltage fluctuations. Finally, simulation and StarSim experimental results verified the correctness and effectiveness of the proposed control strategy.

Power Decoupling Control of MMC and Small-Signal Stability Analysis of AC/DC Distribution Network With Renewable Energy

AC/DC hybrid distribution network can realize the high penetration utilization of renewable energy and modular multilevel converter (MMC) is the key equipment to connect AC grid and DC grid. In this paper, the direct modulation based on the virtual resistor and the reference value of dc-bus voltage is adopted, and instantaneous-value model of three-phase half-bridge MMC is derived by introducing differential-mode and common-mode component representation. Then, the decoupling control of AC active power and reactive power is designed based on the linear active disturbance rejection control theory (LADRC). The total disturbances including the coupling term and capacitor voltage fluctuation can be estimated by extended state observer (ESO), and then cancelled exactly. Based on the simplified average-value model of MMC, the small signal stability analysis of three-terminal AC/DC distribution network with 26-level MMCs corresponding to the Tangjiawan-Jishan1-Jishan2 demonstration project is carried out by root locus and bode diagram method, which gives the guidance for the choice of the main circuit component and controller parameters, and shows that the virtual resistor greatly reduces the resonant peak of AC/DC distribution network. Simulation and experiment results verify the effectiveness of the power decoupling control strategy of MMC and the AC/DC distribution network can realize complex multi-directional power flow.

Reduction of Torque Ripples in PMSM using Fuzzy Controlled based Driving Converter

In this paper we have introduce a four switch PWM (Pulse Width Modulation) inverter for controlling the operation of PMSM (Permanent Magnet Synchronous Motor). The inverter is controlled with 49 rule base fuzzy controller controlling the current component in the feedback loop. The torque of the machine is reduced with the help of second order filter stabilizing the output from the fuzzy controller. The second order torque harmonics are produced by DC capacitor voltage fluctuations are demonstrated first, and a very simple method for compensation is presented by introducing a novel non-orthogonal coordinate transformation. The design of the three phase four switch PWM inverter is modeled in MATLAB software with all graphical representations and reports generated with dynamic characteristics.

Research on fault-tolerant operation strategy of rectifier of square wave motor in wind power system

In wind power system, the randomness of wind energy increases the probability of components fault. once the switch fault of the PWM rectifier is detected, it will lead to the distortion of the motor voltage and current waveform, the increase of torque ripple, the fluctuation of filter capacitor voltage, and the increase of harmonic content, which will affect the control performance and operation safety of the system. A common fault in a three-phase PWM rectifier is the open-circuit fault of the switch. This paper analyzes the current waveform distortion of the square wave motor when the rectifier switch fault, and proposes different fault-tolerant control strategies for different situations of switch fault, include single, double and triple switches fault. This method is based on several special driving modes under power generation situation of square wave motor. The strategy is easy to implement and does not need to change the circuit topology. It can realize fault-tolerant control of switch faults in the same side half bridge or the same bridge arm of the three-phase PWM rectifier. The effectiveness of the proposed fault-tolerant strategy is verified by simulation and experiment at the end of the article.

A Control Strategy for Suppressing Submodule Capacitor Voltage Fluctuation of MMC Based on Circulating Current Voltage Drop Balance

The submodule capacitor voltage fluctuation of the modular multilevel converter (MMC) leads to the increase of the size of the submodule capacitor, the increase of system cost, and the decrease of system reliability and stability. In order to solve the problem, the mathematical model of circulating current voltage drop is derived, and the mechanism of circulating current on the submodule capacitor voltage fluctuation is studied through the submodule switching function. A control strategy for suppressing the submodule capacitor voltage fluctuation is proposed, which is based on circulating current voltage drop balance (CCVDB). The simulation model is built to compare three control strategies for suppressing submodule capacitor voltage fluctuation under the step changes of DC bus voltage and active power, namely, the circulating current is suppressed to zero, the second-order component of the submodule capacitor voltage is suppressed to zero, and the CCVDB strategy. The research results show that the CCVDB strategy can effectively suppress the submodule capacitor voltage fluctuation, reduce the capacitance demand, and reduce the total harmonic distortion (THD) of the submodule capacitor voltage, which verifies the feasibility and effectiveness of the CCVDB strategy.

Capacitor Voltage Ripple Estimation and Optimal Sizing of Modular Multi-Level Converters for Variable-Speed Drives

This work deals with the sizing of submodule capacitors for modular multilevel converters (MMCs) used for variable speed drives for medium voltage and high voltage electric machines. Variable speed drives are required to run over a wide frequency range from zero to a few hundred Hz or even over 1 kHz. Under low frequency operation (<30 Hz), a large current ripple flowing through the MMC submodule capacitors will cause significant capacitor voltage fluctuation. This article proposes an optimal submodule capacitor selection method based on the common-mode voltage injection. The proposed sizing model includes two functions: 1) accurately estimating the capacitor voltage ripple based on the drive system parameters; and 2) identifying the minimum capacitance of the submodule capacitors based on the allowed maximum capacitor voltage fluctuations. Simulation and experimental results show that the error between the actual and estimated values of the voltage fluctuation is within 3% using the proposed model.

Capacitor voltage feedforward decoupling control based on third harmonic injection modulation for five-level active neutral point clamped converter

For the five-level active neutral point clamped (5L-ANPC) converter, the coupling problem between the DC-link capacitor voltages and flying-capacitor (FC) voltages will increase the capacitor voltage fluctuations. The capacitor voltage fluctuations and current harmonics of the space vector pulse width modulation (SVPWM) are less than those of the sinusoidal pulse width modulation (SPWM), because the maximum linear modulation index of the SVPWM is higher than that of the SPWM. However, SVPWM has difficulty in realizing the decoupling control of the capacitor voltages, and the computation complexity of SVPWM is much higher than that of SPWM. This paper proposes a third harmonic injection modulation algorithm for the 5L-ANPC converter. The proposed algorithm injects the third harmonic into the modulation waves of SPWM, which makes the maximum linear modulation index of SPWM the same as that of SVPWM, to reduce the capacitor voltage fluctuations and current harmonics. The calculation of the third harmonic only requires a standard sorting algorithm and a simple linear operation; thus, it is easy to implement. Based on the third harmonic injection modulation algorithm, two variables are introduced to control the neutral point (NP) current and the FC current. And the two variables are used to feedforward compensate the modulation waves, realizing the decoupling control of the capacitor voltages. The proposed decoupling control method can further reduce the capacitor voltage fluctuations. Experimental results are presented to verify the validity of the proposed algorithm.

Cumulative Fatigue Damage Balancing for Modular Multilevel Converter

With many advantages, modular multilevel convert (MMC) has been extensively used in high and medium voltage power transmission projects. The thermal performance and reliability of components in MMC are key issues in system operation. However, in the current research on the reliability of MMC components, there are few methods to improve service lifetime expectancy of the components. This paper proposes a balance control algorithm, based on the cumulative fatigue damage of components, feedback to the control terminal for allocating the sub-module (SM) operating state and generating trigger pulses. Finally, the effectiveness of the proposed algorithm is verified and discussed in case studies. It is found that this algorithm is able to improve the aging degree of components in the meanwhile, the improvement in MMC reliability comes at the cost of slightly increasing capacitor voltage fluctuations and total harmonic distortion (THD). Due to the limitation of capacitor thermal performance, SM capacitor banks become a weak link in MMC reliability.

Active Power Decoupling for a Modified Modular Multilevel Converter to Decrease Submodule Capacitor Voltage Ripples and Power Losses

The usage of too many sizeable capacitors significantly increases the volume and cost of the modular multilevel converter (MMC), and limits its spread in medium- and high-voltage applications. By adding a buck-type active power filter (APF) circuit into a particular MMC topology [of which each phase multiplexes one submodule (SM)], a modified MMC (APF-M3C) topology, which inherently has the ability to suppress capacitor voltage fluctuations and decrease the power losses compared with conventional MMC topology, is proposed in this article. Both the SM capacitor voltage ripples, which can be reduced up to 50% of original value and are related to the SM capacitance, and the arm currents, which are related to SM power losses, are calculated in detail. Its topology, and operating principle is presented in detail, and a novel modulation method for the APF-M3C topology are proposed. In addition, an improved control method for the voltage balancing is also recommended to further reduce system cost and power losses. Finally, the APF-M3C topology and traditional MMC simulation and experimental platforms were built. Simulations and experimental results verify the feasibility and effectiveness of the APF-M3C topology and control strategy by comparing with the conventional MMC.

DC‐link capacitor voltage balance strategy of independent double three‐phase common bus five‐level NPC/H‐bridge inverter based on finite control set

This study aims at the DC-link capacitor voltage imbalance problem of independent double three-phase common bus five-level neutral point-clamped/H-bridge inverter, which is applied to an electromagnetic launch system under instant high-power and high-current conditions. First, the traditional method of voltage balance strategy based on redundant vectors is described, and the key factors leading to the DC-link capacitor voltage fluctuation is analysed in detail. Second, the objective function is constructed to predict the variation trend of DC-link capacitance voltage difference. Third, an optimised voltage balance strategy is proposed. At first, the optimised strategy calculates the variation trends of the voltage difference for all three-phase switch vector combinations. Then, the results make up a voltage balance finite control set. At last, compare the results in the set to select the vector combination corresponding to the minimum variation for output. Finally, tested by the semi-physical real-time simulation platform, and the proposed optimised voltage balance strategy is verified effective for improving DC-link capacitor voltage imbalance and inhibiting the voltage difference fluctuation.

Carrier phase shift modulation and voltage equalization control strategy in modular multilevel converter

In order to solve the problem of voltage fluctuation of sub-module capacitors in Modular Multilevel Converter, based on carrier phase-shifted sinusoidal pulse width modulations, this paper analyzes the effect of different carrier phase shift modulation and the number of sub-modules on a single bridge arm on the number of output levels and uses proportional control to make capacitor voltage balance and PI control to suppress bridge arm circulation. The method has a good stability effect on the sub-module capacitor voltage.A 31-level model is built in MATLAB/simulink to analyze the capacitor voltage fluctuation, output current and voltage distortion rate, and bridge arm current of the sub-module. The voltage and circulation fluctuation range is compared when there is no circulation suppression. It is verified that the method is simple and effective.

Tracking Control of Modular Multilevel Converter Based on Linear Matrix Inequality without Coordinate Transformation

Modular multilevel converters (MMCs) play an important role in the power electronics industry due to their many advantages such as modularity and reliability. However, one of the challenges is to suppress fluctuations of circulating current and capacitor voltage and to ensure the quality of the output current. In this paper, the upper and lower arm voltages are employed as control inputs to control the output current and circulating current which are fed back to track the desired value. Based on linear matrix inequality (LMI), the control law of the MMC with multi-input system is designed to optimize the control value. The optimum arm voltage is divided by the SM nominal capacitor voltage to determine the number of SMs inserted into the upper/lower arm. The Voltage Sorting Algorithm (VSA) is then used to suppress the capacitor voltage fluctuation. The proposed tracking control strategy is implemented in MATLAB/Simulink. The results show that even under a small number of SMs (4 per arm), the output current can track the desired values and have better harmonic performance (current THD: 5.42%,voltage THD: 5.67%), and the fluctuations of the circulating current can be suppressed. Furthermore, it has better robustness and three-phase variable load fault tolerance.