Capacitor Voltage Fluctuation Research Articles

A DC Capacitor Voltage Control Method for Active Power Filters Using Modified Reference Including the Theoretically Derived Voltage Ripple

This paper proposes a new dc capacitor voltage control method suitable for active power filters equipped with a small dc capacitor. The proposed control method calculates the theoretical dc capacitor voltage ripple and provides it to the dc capacitor voltage feedback controller as the reference. As a result, the proposed method has the capability to regulate the mean value of the dc capacitor voltage under a large voltage ripple condition. The proposed method also allows us to use a high feedback gain for suppressing the capacitor voltage fluctuations without any performance deterioration in the harmonic compensation. Experimental results confirm a good harmonic compensation and an effective voltage regulation performance under a small dc capacitor condition. The results also demonstrate a significant improvement in the capacitor voltage regulation even when a sudden load change occurs.

An Improved Phase-Shifted Carrier Modulation for Modular Multilevel Converter to Suppress the Influence of Fluctuation of Capacitor Voltage

Modular multilevel converter (MMC) is considered to be one of the most promising multilevel topologies from the medium- to high-voltage power applications. However, by reason of energy exchange between the dc side and ac side, the fluctuation of sub-module (SM) capacitor voltage for the MMC system is inevitable. Moreover, this fluctuation would deteriorate the total harmonic distortion (THD) of the output voltage. The influence of the fluctuation of the SM capacitor voltage on the output voltage is analyzed mathematically in detail. In this paper, an improved phase-shifted carrier (PSC) pulse width modulation (PWM) for the MMC is presented to suppress the influence of the fluctuation of SM capacitor voltage. This improved modulation can be regarded as a nonlinear compensation applied in the traditional PSC-PWM method. The THD of the output voltage is improved with this modulation method noticeably. Simulation and experiment results verify the effectiveness of the proposed modulation.

Independent Control of Input Current, Output Voltage, and Capacitor Voltage Balancing for a Modular Matrix Converter

This paper presents a novel control scheme for the modular matrix converter (MMxC), in terms of balancing capacitor voltages in addition to controlling output voltage and input current along with a comprehensive mathematical model. The proposed technique has been designed based on a fully decoupled current control of the positive-, negative-, and zero-sequence components of each MMxC subconverter in addition to the feedforward instantaneous power control to suppress the capacitor voltages fluctuation. The frequency of the zero-sequence is the output frequency for the load. Therefore, the positive-sequence current component is used to control the input current to achieve unity input power factor, the negative-sequence current component is used to balance the capacitor voltages among all MMxC arms, and the zero-sequence current component is used to control the output voltage. The effectiveness of the proposed control scheme for controlling the MMxC has been verified theoretically, using simulation software, and experimentally, using a laboratory prototype based 3 kVA, 200 V.

A Modified Modular Multilevel Converter With Reduced Capacitor Voltage Fluctuation

The requirement of a large number of sizeable capacitors in traditional modular multilevel converter (MMC) submodules is seen as a major barrier hampering its widespread use in medium-voltage applications. To reduce the value of capacitance, this paper presents a modified MMC (M-MMC) topology, which offers inherent alleviated capacitor voltage fluctuations compared with traditional MMC. Its operating principle, modulation, and capacitor voltage balancing strategies are described in detail. Moreover, a novel capacitor voltage fluctuation suppression scheme is also proposed to further reduce the second-order capacitor voltage ripples. Finally, both a three-phase prototype of M-MMC and traditional MMC have been built in the laboratory. The effectiveness of the proposed M-MMC topology and control methods is experimentally verified by comparing with the traditional MMC.

Asymmetrical Eleven-Level Reduced Switch Cascaded Topology and it’s Switching Schemes

The emphasis of this paper is on selection scheme and its switching pattern of cascaded multilevel inverter. This has been designed as an application to feed induction motor and industrial drives. The main reason to work on this multilevel inverter is due to – low EM interferences, low dv/dt burden, lower rating switches used, and enhanced output of better efficiency. These advantages are when compared to conventional inverters. The level of the inverter can be increased to nth level by increasing the H-bridge blocks in cascade. Here the design is of eleven level multilevel inverter. The controlling schemes used is sinusoidal pulse width modulation (SPWM). These controlling has been done for higher performance as load voltage, lower THD, and lower capacitor voltage fluctuations. The performance can be understood and discussed through the simulation result as done below.

Fluctuation voltage control and fault-tolerant operation of modular multilevel converters with zero-sequence injection

SUMMARY This paper focuses on voltage fluctuation mitigation and fault-tolerant control methods for modular multilevel converters (MMC). The reason for MMC output fluctuation caused by the floating capacitor voltage fluctuation at low frequency is carefully analyzed. To reduce voltage fluctuation, two control freedom degrees of high-frequency zero-sequence voltage and phase-to-phase circulating current are proposed. In fully implementing MMC functionalities, we comprehensively investigated its fault-tolerant operation without redundant submodules for fail-safe energy transfer. A fundamental-frequency zero-sequence-based neutral-shift method is adopted to enable continuous operation and avoid complete shutdown of the MMC system. These methods enhance both the reliability and availability of MMC while avoiding the undesired unbalanced stresses among the healthy submodules. Rigorous simulation studies are performed to demonstrate the validity and effectiveness of the proposed control methods. Copyright © 2013 John Wiley & Sons, Ltd.

Multiobjective daily Volt/VAr control in distribution systems with distributed generation using binary ant colony optimization

This paper presents a multiobjective daily voltage and reactive power control (Volt/VAr) in radial distribution systems, including distributed generation units. The main purpose is to determine optimum dispatch schedules for on-load tap changer (OLTC) settings at substations, substation-switched capacitors, and feeder-switched capacitors based on the day-ahead load forecast. The objectives are selected to minimize the voltage deviation on the secondary bus of the main transformer, total electrical energy losses, the number of OLTCs, and capacitor operation and voltage fluctuations in distribution systems for the next day. Since this model is the weighted sum of individual objective functions, an analytic hierarchy process is adopted to determine the weights. In order to simplify the control actions for OLTC at substations, a time interval-based control strategy is used for decomposition of a daily load forecast into several sequential load levels. A binary ant colony optimization (BACO) method is used to solve the daily voltage and reactive control, which is a nonlinear mixed-integer problem. To illustrate the effectiveness of the proposed method, the Volt/VAr control is performed in IEEE 33-bus and 69-bus distribution systems and its performance is compared with the genetic, hybrid binary genetic, and particle swarm optimization algorithms. The simulation results verify that the BACO algorithm gives better performances than other algorithms.

A Comprehensive Investigation of Four-Switch Three-Phase Voltage Source Inverter Based on Double Fourier Integral Analysis

This paper presents a comprehensive analytical investigation of the four-switch three-phase (B4) voltage source inverter. The double Fourier integral analysis is used as an analytical approach to determine the phase-leg switched voltage spectrum under condition of natural sampling. The performance of two proposed switching sequences are subsequently evaluated when the harmonic copper losses are chosen as performance criterion. For a clear identification of dc-link voltage, the spectrum of dc-link current is characterized by convolution of switching function spectrum and corresponding phase current spectrum, in which fundamental component appears causing offset and fluctuation in dc-link two-capacitor voltages. The offset can be suppressed by certain switching states, whereas the effect of fluctuation must be neutralized through analytical compensation for modulating waveforms. Otherwise, symmetry of three-phase output currents and reliable operation of the system will not be retained. Finally, the analytically calculated spectrum of output voltage and dc-link current are demonstrated by comparing with those obtained by fast Fourier transform (FFT) analysis of simulated waveforms in MATLAB/SIMULINK. The capacitor voltage offset suppression and fluctuation effect neutralization are carried out by simulations and experiments, in which the results confirm the validity of the analytical investigation.

Simulation study of a modular multilevel high‐voltage direct current based on voltage source converter system for an offshore wind farm

SUMMARYSome new technologies of modular multilevel converter (MMC) for high‐voltage direct current based on voltage source converter transmission applications were presented in this article. It focused on the triggering method of MMC and the strategy of capacitor voltage balancing. First, MMC topology and its characteristics were introduced. The method of comparing multiple triangular carriers with the reference wave for generating the trigger pulse was proposed. Then, the submodules in each arm were sorted according to their capacitor voltages, and the trigger pulses were assigned to them according to ranking result. This method ensured that the instantaneous values of capacitor voltages within the same arm were equal to each other. On the basis of the equivalent circuit of the converter, it was proved that the circulating current can make capacitor voltages unbalance between arms. Some component must be added to control the signals of the converter for reducing the capacitor voltages fluctuation and for improving the quality of output waveforms. Finally, the simulation study of the proposed trigger method and capacitor voltages balancing strategy for MMC‐based high‐voltage direct current based on voltage source converter system was implemented using “MATLAB/Simulink” software. The results show that the designed method is correct and effective. Copyright © 2011 John Wiley & Sons, Ltd.

Asymmetric Duty Control of a Dual-Half-Bridge DC/DC Converter for Single-Phase Distributed Generators

A dual-half-bridge (DHB) converter is integrated with a half-bridge 60-Hz inverter as a converter/inverter system for a small distributed generator. This topology provides an isolation between the power source and the load with a 100-kHz transformer, and therefore, the system volume is small. On the other hand, it reduces the number of switching devices greatly. However, the half-bridge inverter causes severe capacitor-voltage fluctuations. The unbalanced voltage problem can be solved by controlling the converter switching: In addition to the conventional phase-shift control method, an asymmetric charging method is used that charges the upper and lower secondary capacitors differently. This means that the voltage imbalance is corrected by adjusting the switching time of the secondary switches. Furthermore, a decoupling control algorithm can be derived from this approach. The usefulness of this method is then validated by simulation and experimental results.

High Performance of Sinusoidal Pulse Width Modulation Based Flying Capacitor Multilevel Inverter fed induction Motor Drive

This paper focused on the development of capacitor voltage balancing methods in a flying capacitor multilevel inverter (FCMLI) fed induction motor drive. For improving the performance of flying capacitor multilevel inverter, a switching pattern selection scheme is implemented. The proposed method has been designed a seven-level flying capacitor multilevel inverter by using sinusoidal pulse width modulation technique. The selected pattern has been exposed to give superior performance in load voltage, total harmonics distortion and capacitor voltage fluctuation. The performance of proposed strategies is confirmed through simulation investigations.

ON CONTROL OF INTERMEDIATE CAPACITOR VOLTAGE IN SINGLE-STAGE AC/DC PFC CONVERTER

To reduce the intermediate storage capacitor voltage stress in single-stage power-factor-corrected (S2PFC) converters due to the change of load, some effective approaches such as variable frequency control and negative bus voltage feedback have been proposed. However, this intermediate capacitor voltage still varies largely against the change of line voltage. This paper proposes a S2PFC converter which is constructed in a way such that the amount of energy to and from the intermediate storage capacitor can be varied when the line voltage is changed, hence narrowed the range of intermediate capacitor voltage fluctuation. This is achieved by an auxiliary bidirectional switch, which operates at zero-voltage-switching (ZVS) condition. A dual-loop voltage mode control is proposed for the converter to achieve power factor correction, output voltage regulation, and intermediate capacitor voltage control simultaneously. Simulation and experimental results show that the proposed approach is effective.

Capacitor voltage balancing in multilevel flying capacitor inverters by rule-based switching pattern selection

A rule-based scheme is investigated for capacitor voltage balancing in a multilevel flying capacitor inverter (MFCI). Without using voltage feedback, the scheme determines the best switching pattern for maintaining nil mean current in all capacitors, hence minimising the capacitor voltage fluctuation. The method is developed to work with the selective harmonic elimination (SHE) technique for sinusoidal voltage generation applied to control static VAR compensators. The method has been designed using a four-cell five-level MFCI as an example. The selected pattern has been shown to give superior performance in load-voltage total-harmonies distortion level and mean capacitor voltage fluctuation. The method is validated experimentally for this form of MFCI.

Sliding mode control of the energy recovery chopper in a C-dump switched reluctance motor drive

A control strategy for the energy recovery chopper in a capacitor-dump (C-dump) switched-reluctance motor (SRM) drive is proposed. The strategy is based on sliding-mode control theory and allows continuous-output-current operation of the chopper to be attained with low semiconductor current peaks and low capacitor voltage fluctuations. The dynamic equations of the chopper, which are formulated from the point of view of the capacitor voltage control, show that the chopper operates as a boost DC-to-DC converter in a regenerative mode. The sliding-mode control is then designed, and the effects of the converter and control parameters on chopper performance are pointed out. Experimental results are presented and discussed. >