Capacitor Voltage Control Research Articles

Single-phase five level modified neutral point clamped grid connected inverter topology with front-end chopper control of DC-link capacitor voltages

Single-phase five level modified neutral point clamped grid connected inverter topology with front-end chopper control of DC-link capacitor voltages

A decoupling strategy for average value control and ripple suppression of capacitor voltages in modular multilevel converters based on notch filters

AbstractModular multilevel converters (MMC) have become an emerging hotspot in the field of medium‐voltage drives. A major challenge is the large voltage ripple of sub‐module capacitors at low rotor speeds with high torque. Therefore, it is necessary to achieve ripple suppression while maintaining the average values of the capacitor voltages at their reference. However, the average value control and ripple suppression of the capacitor voltages have a coupling effect during the circulating current control loop, which reduces the performance. This paper proposes a decoupling strategy based on notch filters (NF). First, the coupling problem existing in the traditional control strategy is analysed by theoretical analysis and simulation. Then, the principle and parameter design method of the improved decoupling strategy are introduced. Finally, the performance of the decoupling strategy is experimentally verified using a small prototype.

Power Flow Control with Capacitor Voltage Control of Hexagonal Modular Multilevel Converter as a Loop Power Controller

Power Flow Control with Capacitor Voltage Control of Hexagonal Modular Multilevel Converter as a Loop Power Controller

Inherently Decoupled Dc-Link Capacitor Voltage Control of Multilevel Neutral-Point-Clamped Converters

This letter derives and discusses the superiority of a simple dc-link capacitor voltage control configuration for multilevel neutral-point-clamped converters with any number of levels. The control involves n − 2 control loops regulating the difference between the voltage of neighbor capacitors. These control loops are inherently decoupled, i.e., they are independent and the control action of one loop does not affect the others. This method is proven to be equivalent to previously published approaches, with the added advantages of increased simplicity and scalability to a higher number of levels, all while imposing a lower computational burden. The good performance of such control is confirmed through simulations and experiments.

Experimental Investigation of Modified Level Shift PWM with Capacitor Voltage Balancing on Single Phase Modular Multilevel Converter

Modular Multilevel Converters (MMCs) are a prominent voltage source converter topology that is rapidly gaining popularity in medium/high power/voltage applications, including high-voltage DC transmission systems and electric vehicle systems. However, MMC has the critical issue of unbalanced submodule capacitor voltages and circulating current. The MMC distributes DC-link energy evenly among its submodule capacitors, rather than storing it in a large capacitor like conventional voltage source converters. In MMC, the submodule floating capacitors interface the DC input voltage and AC output voltage. Therefore, capacitor voltages must be balanced. The existing capacitor voltage control struggles to handle many gate pulses at medium and high voltages. An effective control scheme is needed to solve the issues mentioned earlier. This paper presents a performance analysis of a 1-Φ MMC using a modified level shift PWM technique based on Universal Control Modulation Scheme (UCMS) and a sorting-based capacitor voltage balancing algorithm. MATLAB/Simulink software implements the proposed control method for a 1-Φ , seven-level MMC. The outcomes of the simulation demonstration reveal that phase disposition PWM offers less harmonic distortion of output phase voltage than the other level shift PWM techniques, such as phase disposition PWM and alternate phase disposition PWM. The simulation results also demonstrate the effective use of the sorting-based capacitor balancing algorithm to regulate the voltages of the submodule capacitors. Finally, the real-time GUI tool validates the simulation results using an experimental prototype of 1-Φ MMC with the dSPACE MicroLabBox 1202.

An Active Virtual Space Vector Modulation Scheme for Three‐Level Neutral‐Point‐Clamped Converters

The performance of neutral‐point‐clamped (NPC) converters is subject to the regulation of neutral point balance. In the literature, researchers have addressed two main modulation methods: the active space vector PWM (SVPWM) technique and the virtual SVPWM (VSVPWM) method. However, those two approaches respectively suffer from reduced DC‐link voltage utilization and poor active regulation capability. To address this, an improved VSVPWM modulation scheme with actively strong voltage regulation capability is proposed in this paper. The fast and precise control of capacitor voltages is achieved by optimizing the synthesis scheme of the virtual vectors. When there is a voltage imbalance, the proposed technique can quickly restore and maintain the capacitor voltage balance till the full DC‐link voltage utilization without affecting the converter output quality. The experimental validation results on the NPC converter have demonstrated the benefits of the proposed active VSVPWM method. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

Model Predictive Control for Cascaded H-Bridge PV Inverter with Capacitor Voltage Balance

We designed an improved direct-current capacitor voltage balancing control model predictive control (MPC)for single-phase cascaded H-bridge multilevel photovoltaic (PV) inverters. Compared with conventional voltage balancing control methods, the method proposed could make the PV strings of each submodule operate at their maximum power point by independent capacitor voltage control. Besides, the predicted and reference value of the grid-connected current was obtained according to the maximum power output of the maximum power point tracking. A cost function was constructed to achieve the high-precision grid-connected control of the CHB inverter. Finally, the effectiveness of the proposed control method was verified through a semi-physical simulation platform with three submodules.

Optimal Decoupled Discontinuous Modulation for StatComs Based on the Cascaded H-Bridge Converter

Conventional discontinuous pulse width modulation (DPWM) strategies for cascaded H-bridge static compensators suffer from poor inter-phase capacitor voltage balance control during unbalanced grid voltage conditions. Specifically, the logic-based algorithm that calculates the zero-sequence voltage (ZSV) for discontinuous operation interacts with the closed-loop capacitor voltage control. This paper proposes an optimal DPWM strategy based on a finite-set optimization that chooses the best ZSV candidate that minimizes the control interactions while clamping a converter voltage. Furthermore, the effect of the different ZSV candidates on the switching loss is also considered within the optimization. Extensive experimental study is provided to demonstrate the effectiveness of the proposed approach.

Stand-Alone Wind Power System Operating with a Specific Storage Structure

In this paper we analyze a low power wind system operating in autonomous mode. We present a control structure that ensures the maximization of wind energy conversion and the balance between required and produced power. We have studied two stand alone wind system structures based on the capacitor voltage control and the buffer battery operation. The simulation results have been obtained in Matlab/Simulink environment, using PowerSim toolbox.

Analytical Investigation and Optimization of DC Capacitor Voltage Control in the Enhanced Multi-Level STATCOM System

In this paper introduces a novel method for controlling the balance of dc capacitor voltage in a STATCOM, along with a comprehensive logical approach for the balance management system. A balancing control approach has been presented to address the abnormality of the dc capacitor voltage, which is generated by the anomaly of dynamic power absorbed and spent by the chain. This method takes into account the superposition of dynamic voltage vectors. In this approach, a voltage component is added to the output voltage of the append operation in order to modify its inherent power. Furthermore, a thorough scientific approach grounded on vector analysis is introduced. This technique facilitates the deconstruction of the equilibrium control mechanism, including both its stability and boundary constraints. The STATCOM uses a fuzzy logic controller to enhance its aesthetic appeal. A vector analysis-based evaluation is administered to ascertain the optimal approach for equilibrium control. This analysis contrasts the proposed methodology with the other two commonly used methodologies. The findings of this comparison indicate that the suggested strategy for equilibrium control has the benefit of notable stability and a well-defined threshold limit. Reenactments are conducted to illustrate this particular component of the system.

A Dual-Ring Control Structure for Enhancing the Fault-Tolerance Ability Under Communication Interruption in MMC

The control structure of a modular multilevel converter (MMC) consists of a central controller and numerous local controllers with complex communication links. However, the communication interruption may lead to MMC operation failure or frequent bypassing of submodules (SMs). Although the fault-tolerance can be improved by adding extra communication link for each local controller in centralized star control structure, it will greatly increase the cost considering the large number of SMs. In this article, a novel dual-ring control structure based on consensus algorithm is proposed to enhance the fault-tolerance ability of MMC to the communication interruption. Compared with the star control structure, control tasks in central controller, such as capacitor voltage control and circulating current control, can be reasonably allocated to all local controllers. Each local controller only needs to communicate with adjacent local controllers to obtain the average value of specified information. Two types of communication interruptions, occurring between central controller and local controller or among local controllers, are analyzed respectively with the corresponding redundancy design implemented. Simulated and experimental results show that proposed control structure greatly enhance the fault-tolerance ability to communication interruption as well as effectively reduces the computational burden of central controller compared with star control structure.

Circulating Current Control for a Three-Phase Modified Modular Multilevel Converter With a Reduced Number of Current Sensors

The high capacitance requirement becomes a significant problem when modular multilevel converters (MMCs) are applied to medium or low-voltage systems. This paper proposes a capacitor voltage control scheme for a three-phase modular multilevel converter (MMC) with optimized capacitor sizing. Unlike the existing circulating current control, the proposed controller uses the middle capacitor voltage to control the second-order harmonic component of the circulating current. Therefore, it is unnecessary to measure the two arm currents; only one voltage sensor is required, which reduces the number of current sensors. The proposed controller can effectively control the circulating current to be constant or inject second-order harmonics to reduce the capacitor sizing. The detailed analysis, controller design, and performance of the proposed scheme are presented in this paper. The effectiveness of this study is confirmed by simulation and experimental results.

A Control Scheme for a Quasi-Z Source Three-Phase Inverter

This paper presents a novel control scheme for a three-phase quasi-z source inverter (qZSI) using a capacitor voltage and input current-based sinusoidal pulse width modulation (SPWM) technique. The proposed scheme combines the advantages of both qZSI and SPWM techniques to achieve improved performance. The SPWM technique utilizes a sinusoidal modulation signal, which is compared with a high-frequency triangular carrier wave and two shoot-through (ST) references to determine the switching states for the three-phase qZSI. The positive and negative ST references are obtained from the capacitor voltages and input current, allowing for control of the DC bus voltage and ST states of the inverter. Additionally, the proposed control scheme generates the three-phase modulation signal through decoupling control in the dq reference frame. The detailed analysis of the control scheme includes its operating principle, transient state, steady-state responses, and the effects of parameter variations. Simulation studies are conducted using MATLAB/Simulink to assess the performance of the three-phase qZSI under the proposed control scheme. The simulation results demonstrate the effectiveness of the control scheme in terms of output voltage quality, DC bus voltage control, and robustness against reference variations. Overall, the proposed capacitor voltage and input current-based SPWM control scheme for the three-phase qZSI shows promising performance improvements and robustness, as confirmed through comprehensive simulation studies.

Refine control methodology and implementation of capacitor voltage control for improved switched inductor Z‐source inverter

SummaryNowadays, Z‐source inverter (ZSI) has received several attentions compared to other power converters owing to their simplicity and reliability. In this paper, a refine control methodology of capacitor voltage control is presented based on third‐order small signal analysis. The proposed control is applied to the improved switched inductor ZSI (ISL‐ZSI). Proportional, integral, and derivative controller (PID) is adopted due to their capability to operate with large operation levels, reliable operation, and cheaper implementation. In addition, the function of this control can be achieved even with incomplete system data or parameters variations. ISL‐ZSI has been presented because of its advantages such as the high gain of direct current voltage, low voltage stress for both the Z‐network and the inverter bridge switches, and ensuring good soft starting. In addition, the inrush current has been removed based on its configuration. The proposed control is validated by both experimental and simulation results under input voltage change, load change, and steady‐state operations using DSP F28335 and MATLAB SIMULINK Real‐Time Workspace.

Detailed Wideband Impedance Modeling and Resonance Analysis of Grid-Connected Modular Multilevel Converter

Modular multilevel converter (MMC) tends to cause resonance instability in interconnected systems. Current studies on the stability of MMC mainly focus on high-voltage and large-capacity power systems, while the impedance modeling process of MMC ignores the harmonics of the submodule (SM) capacitor voltages and the influence of voltage feedforward control. The applicability of the MMC impedance model with fewer submodules is doubtful. According to the circuit structure and control mode, the AC-side sequence impedance model of the MMC is established, which takes into account the capacitor voltage balance control of submodules and voltage feedforward control. Based on the RT-Lab control hardware-in-loop (CHIL) test platform, the MMC impedance frequency scanning was carried out to verify the accuracy of the impedance modeling method. The influence of control parameters in different frequency bands on the impedance characteristics of MMC was studied. The AC terminal voltage feedforward causes phase lag in the mid-/high-frequency range and increases the risk of resonance. In the low-frequency range, the dynamics and control of capacitor voltages reduce the impedance magnitude of MMC. Using the resonance phenomenon of an MMC connected to a weak grid as an example, the high-frequency resonance mechanism caused by control parameters is analyzed from the perspective of the negative damping effect. The simulation results show that the detailed wideband impedance model can improve the accuracy of the stability analysis results.

Solar-PV inverter for the overall stability of power systems with intelligent MPPT control of DC-link capacitor voltage

This paper demonstrates the controlling abilities of a large PV-farm as a Solar-PV inverter for mitigating the chaotic electrical, electromechanical, and torsional oscillations including Subsynchronous resonance in a turbogenerator-based power system. The oscillations include deviations in the machine speed, rotor angle, voltage fluctuations (leading to voltage collapse), and torsional modes. During the night with no solar power generation, the PV-plant switches to PV-STATCOM mode and works as a Solar-PV inverter at its full capacity to attenuate the oscillations. During full sun in the daytime, on any fault detection, the PV-plant responds instantly and stops generating power to work as a Solar-PV inverter. The PV-farm operates in the same mode until the oscillations are fully alleviated. This paper manifests the control of the DC-link capacitor voltage of the Solar-PV inverter with a bacterial foraging optimization-based intelligent maximum power point tracking controller for the optimal control of active and reactive power. Kundur’s multi-machine model aggregated with PV-plant is modeled in the Matlab/Simulink environment to examine the rotor swing deviations with associated shaft segments. The results for different test cases of interest demonstrate the positive outcomes of deploying large PV-farms as a smart PV-STATCOM for controlling power system oscillations.

A Closed-Loop High-Frequency Current Shaping Method to Achieve Trapezoidal Transformer Current in a Current-Fed Dual Active Half-Bridge Converter for Minimum RMS Current and Wide-Range ZVS

This article proposes the use of primary and secondary side duty ratio control along with the phase shift control in a current-fed dual active half-bridge (DAHB) converter to achieve trapezoidal transformer current, which results in zero voltage switching (ZVS) of all the switches and minimum rms current even at light loads, for a wide range of input voltage. The high-frequency transformer current is directly sensed at specific transition instants by a delay-adjusted sampling strategy to be used as control variables in a current control structure. This is better because the voltage control of capacitor voltages would have required more expensive sensors. A simple and accurate small-signal model of the converter is developed, and the relevant transfer functions are derived. The controller design details are provided. Finally, experimental results on a 1-kW laboratory prototype demonstrate the effectiveness of the closed-loop duty ratio control in achieving trapezoidal transformer current resulting in improved efficiency and ZVS of the switches, compared to a simple phase shift control.

Operation Characteristics of Discontinuous Current Mode for a Dual-Active-Bridge AC-DC Converter with an Active Energy Buffer

This paper discusses operation characteristics of discontinuous current mode (DCM) for an ac-dc isolated converter for on-board battery chargers of electric vehicles. The converter discussed in this paper consists of a front-end diode-bridge rectifier, an active energy buffer, and a dual-active-bridge dc-dc converter. The energy buffer circuit absorbs the power pulsation form the single-phase ac line into a small-rated capacitor, and thus, makes it possible to reduce a required capacitance of the dc-link capacitor. This paper explains details of the DCM control method including a technique of the ripple cancel of the inductor current, to realize the power factor correction of the line current and the voltage control of the buffer capacitor at the same time without complex calculations. Then, a reference value of the inductor current and an operation range of the dc-output voltage are discussed. Experimental results obtained by a 220-V, 6.6-kW experimental setup demonstrate a sinusoidal-line current and a controlled buffer capacitor voltage as well as a smoothed dc-output current.

A New Modulation for Leakage Current Reduction of Modular Multilevel Converter

Leakage current is an important problem in non-isolated inverters, especially implemented in a medium- and high-voltage system. First, several analyses had carried out to reduce the high-frequency component of common-mode voltage (CMV) and the leakage current of a three-phase modular multilevel converter (MMC). Then, a new modulation method is proposed to keep CMV as the half of dc voltage and suppress the leakage current effectively. The peak value of leakage current can be suppressed under 5 mA, which complies with the VDE-0126-1-1 standard. Meanwhile, the parasitic capacitor voltage can be balanced automatically without the additional capacitor voltage control. Finally, the proposed scheme is verified in MATLAB/Simulink and tested on the hardware-in-loop platform. The effectiveness of the proposed scheme is verified by the experiment results.

Impedance Shaping Effects and Stability Assessment of Circulating Current Control Schemes in Modular Multilevel Converters

Control of circulating current can help modular multilevel converters (MMCs) improve the internal performance, but only the conventional circulating current suppression control has been considered in impedance models to investigate MMC-grid interactions. In this paper, the sequence impedance responses of circulating current injection methods based on instantaneous information are modeled for stability analysis. Compared with circulating current control schemes based on predefined references, the circulating current injection methods based on instantaneous information introduce couplings among circulating current, output current and capacitor voltage control loops, which influence the MMC impedance in the low frequency range. This paper reveals that injection of second harmonics in the circulating current reduces the capacitor voltage ripples, but the system stability is compromised due to the couplings. The MMC stability boundaries for different circulating current controllers are defined with the help of sequence impedance model and Bode-plot-based analysis. The impedance model and stability assessment are validated by frequency scan results and case studies.