Capacitor Voltage Feedback Research Articles

Capacitor Voltage Feedback Active Damping With Reduced Computation Delay for Improving Voltage Control Performance of LC‐Type Grid‐Forming Inverter

ABSTRACTIn low‐voltage applications using low‐Q LC filters, alias‐free capacitor voltage acquisition can be achieved, but synchronous capacitor voltage sampling under high‐Q filters used in high‐voltage applications in grid‐forming converters causes some degree of distortion and results in degradation of the power supply quality. And the inherent 1.5 sampling periods that delay in this approach limit the control bandwidth, reduce the stability region, and result in an inadequate time domain response. Despite these shortcomings, this approach is widely accepted in practice. The conclusion of this paper is that shifting the capacitor voltage sampling instant to 2 µs before the pulse width modulation (PWM) reference voltage update results in almost the same distortion as synchronous sampling. However, it can improve the dynamic performance of the system. A distortion‐acceptable univariate feedback voltage dual‐loop active damping control topology with much reduced computational delay is proposed, which is based on an internal active damping loop using a discrete lead compensator and a proportional resonance controller in the external voltage loop.

Analysis and Design of Voltage-controller Based on Single-state Feedback Active Damping for Grid-forming Converters

This paper investigates the analysis and design of digital single-loop voltage-controller for grid-forming voltage source converters with LC-filters. Inspired by the structure diagram of the passive damping method, the virtual resistance concept based on only capacitor voltage feedback is used to realize active damping without the need of any current sensor, reducing thus the system costs. It is revealed from the controller closed-loop analysis that the insertion of an integrator in series with a proportional and resonant controller can significantly improve stability margin and dynamic performance of the system, then only the proportional controller need to be determined for system stability and the resonant controller is merely used for zero steady-state tracking error, which facilitates the controller parameter tuning. A systematic design based on the root contours in the discrete z-domain is proposed to optimize the parameters of the controller. The largest distance between the poles of the closed-loop system and the unit circle is achieved, which indicates the fastest dynamic performance under desired stability margins. Meanwhile, a simple differentiator constructed by backward-Euler plus lead-compensator is used to replace the noise-sensitive derivative term. In order to evaluate the proposed control approach performance, the system is tested in laboratory setup under different load conditions. The obtained results verify the effectiveness of the proposed design method and validate the analysis.

An Expanded Lyapunov-Function Based Control Strategy for Cascaded H-Bridge Multilevel Active Front-End Converters

Cascaded H-bridge multilevel (CHBML) active front-end (AFE) converters exhibit some enticing benefits, comprising high adaptability for numerous applications, such as tractions, solid-state transformers, electric vehicle charging stations, and medium and high power electric drives. Yet, when the CHBML- AFE is operating under an unbalanced load condition, it is crucial to utilize an advanced control technique to maintain system stability. In this research, a Lyapunov Function (LF)-based control approach is utilized for regulating a single-phase CHBML- AFE with LCL filter to achieve global asymptotic stability. A capacitor voltage feedback is introduced and added to the traditional LF-based control strategy to reduce the resonance of the LCL filter. Furthermore, the proportional-resonant (PR) control procedure is utilized to derive the grid current reference, offering improvement to the robustness of the current control design. A balanced DC voltage control method is also employed to suppress the unbalanced DC voltage conditions among CHBML- AFE cells. In addition, the transfer function of the reference grid current and the actual grid current is evaluated for the CHBML- AFE with LCL filter parameters and their eventual variations in the employed control technique. The effectiveness of this control strategy is validated utilizing simulation and experimental studies.

A current sensorless control method for multi‐level active front‐end rectifiers with LCL filter

AbstractThis paper presents a current sensorless control strategy for a three‐phase multi‐level active front‐end rectifier with an LCL filter. The proposed control strategy is based on the mathematical model of the system. Opposed to the existing control methods that measure three‐phase currents as well as three‐phase voltages, the proposed control eliminates the current measurement requirement which leads to cost reduction. Moreover, unlike the existing current control methods that use two proportional‐integral (PI) controllers, the proposed current control does not use any PI controller in the current loop. The elimination of two PI controllers simplifies the controller design. The resonance damping requirement arising due to the LCL filter is tackled by active damping that is realized by using filter capacitor voltage feedback. The steady‐state and dynamic performances of the proposed control strategy are investigated experimentally under balanced, unbalanced and distorted grid conditions.

Model Predictive Control for Grid-Connected Current-Source Converter With Enhanced Robustness and Grid-Current Feedback Only

The current-source converter (CSC) features the second-order characteristic, and multivariable feedback is necessary to suppress the filter resonance. Model predictive control (MPC) has shown advantages in terms of the multiple-input–multiple-output (MIMO) system. This article contributes to the MPC scheme applied to CSC from two aspects. First, based on the typical one-step prediction, existing MPC with both grid-current and capacitor voltage feedback has been applied to mitigate the resonance issue but lacks the investigation and improvement of system robustness to parameter mismatch, especially in such a second-order system. Thus, the dual-disturbance observer is proposed, where both the additional capacitor voltage reference and the lumped input disturbance caused by parameter mismatch are estimated online. As a result, the grid-current reference tracking performance can be improved under mismatched parameters. Second, to reveal the resonance principle with different state feedback types, the tracking process of the grid current is intuitively analyzed in the time domain. The MPC scheme is further assessed by increasing the prediction horizon length, and the results show that the CSC can work well with only gird-current feedback. In this case, both the weighting factor and corresponding reference estimation for the capacitor voltage can be avoided, thus significantly simplifying the design stage and reducing the effect caused by the parameter mismatch. Experimental results verify the effectiveness of the proposed scheme and perspectives.

Design of Damping Strategies for LC Filter Applied in Medium Voltage Variable Speed Drive

In recent years, medium-voltage variable-speed drives have become popular in the industry. However, in some applications, the use of long cables can lead to overvoltages at the motor terminals, affecting the motor lifespan. Under such conditions, the use of passive filters is recommended. However, the use of inductive capacitive (LC) filters results in resonances, leading to control stability issues. This issue can be mitigated by introducing a resonance damping strategy. In this work, four damping strategies are implemented and designed: Passive damping, capacitor current feedback, capacitor voltage feedback, and notch filter-based damping. The paper performs a comparative study on the current control phase margin, current and voltage harmonic spectra, and overall changes in the control structure. Then, the effect of the damping strategy on the VSD control is evaluated, creating guidelines to support the selection of the appropriate damping strategy. The results indicate that capacitor voltage feedback stands out, since this strategy presents an interesting dynamic behavior while allowing the elimination of the passive damping losses at a relatively low cost.

Capacitor Voltage Balancing of a Grid-Tied, Cascaded Multilevel Converter with Binary Asymmetric Voltage Levels Using an Optimal One-Step-Ahead Switching-State Combination Approach

This paper presents a novel capacitor voltage balancing control approach for cascaded multilevel inverters with an arbitrary number of series-connected H-Bridge modules (floating capacitor modules) with asymmetric voltages, tiered by a factor of two (binary asymmetric). Using a nearest-level reference waveform, the balancing approach uses a one-step-ahead approach to find the optimal switching-state combination among all redundant switching-state combinations to balance the capacitor voltages as quickly as possible. Moreover, using a Lyapunov function candidate and considering LaSalle’s invariance principle, it is shown that an offline calculated trajectory of optimal switching-state combinations for each discrete output voltage level can be used to operate (asymptotically stable) the inverter without measuring any of the capacitor voltages, achieving a novel sensorless control as well. To verify the stability of the one-step-ahead balancing approach and its sensorless variant, a demonstrator inverter with 33 levels is operated in grid-tied mode. For the chosen 33-level converter, the NPC main-stage and the individual H-bridge modules are operated with an individual switching frequency of about 1 kHz and 2 kHz, respectively. The sensorless approach slightly reduced the dynamic system response and, furthermore, the current THD for the chosen operating point was increased from 3.28 to 4.58 in comparison with that of using the capacitor voltage feedback.

An improved capacitor voltage full feedforward control strategy for LCL‐type grid‐connected inverter based on control delay compensation

For the LCL-type grid-connected inverter, when the capacitor voltage feedforward is applied, the delay in the digital control system could change the phase characteristics of capacitor voltage feedback and affect the stability of the system. To solve the problem, a delay compensation method based on lead compensator is proposed in this paper, which can reduce the impacts of control delay on the capacitor voltage full feedforward control system. In this study, a lead compensator is introduced into the feedforward channel to compensate the phase lag, and the compensation function is designed to compensate for the phase margin but not change the system amplitude gain. The proposed strategy can keep the phase margin of the system above 25° when the grid impedance changes from 0 to 2.6 mH (0.1 p.u.), which enhances the robustness of the grid-connected inverter under weak condition. The grid current distortion caused by grid background harmonics is greatly reduced and is kept within 4%. Moreover, the proposed strategy has fast dynamics while ensuring the system stability. Finally, the simulation and experimental results are presented to verify the effectiveness and robustness of the proposed control strategy.

A novel active damping control based on grid-side current feedback for LCL-filter active power filter

LCL filter is widely used in grid-connected converters for good harmonic mitigation performance. However, resonance poles of LCL filter challenge system stability, especially in a digital control system. This paper studies the suppression method of the active power filter output LCL filter resonance. Because of the defect that the traditional active damping based on the filter capacitor current or capacitor voltage feedback needs to add additional sensors, a novel active damping with grid-side current component feedback is proposed. And analysis and design of the feedback function, the resonance suppression characteristics, and the impact on harmonic compensation performance are analyzed in detail respectively and proposes a novel active damping control method based on a quasi-resonant controller grid-side current feedback. On this basis, a mathematical model of the SAPF system with the proposed novel active damping was established, and the resonance characteristics were analyzed. Finally, the proposed method was verified through relevant experiments.

Stability Analysis and Key Parameters Design for Grid-Connected Current-Source Inverter With Capacitor-Voltage Feedback Active Damping

In a three-phase grid-connected current-source inverter system with the capacitor-voltage feedback (CVF)-based active damping method, a high-pass filter is usually employed in the CVF loop to mitigate its impact on system dynamics. Meanwhile, the inherent delays in a digital control system brought by pulsewidth modulation (PWM) and the digital control will make stability analysis and parameters design of the control system more complicated. In this article, the impacts of the high-pass filter and the inherent delays are considered for stability analysis and parameters design. The impacts of the cutoff frequency of the high-pass filter and the CVF damping coefficient on the system stability are analyzed in the discrete domain, and an optimal CVF damping coefficient is obtained. Moreover, a key parameters design method for the control system is proposed. The method simplifies the procedure of key parameters designing, which can mitigate the resonance of the system caused by the CL filter effectively with better dynamics and steady state performance. Finally, simulation and experimental results are provided to verify the effectiveness of the proposed method in this article.

A Robust Controller for a Grid-Tied Inverter Connected Through an LCL Filter

This article presents a control strategy for three-phase inverters connected to grids through an LCL output filter. For this topology, it is known that when output current and capacitor current are fed back through certain optimal gains, the system is unfailingly unstable for a grid inductance value called “critical.” For the critical inductance value, the system's resonance frequency is equal to one-sixth of the sampling frequency. In this article, it is first checked that capacitor voltage positive feedback can stabilize this system. In addition, it is found that there exists a proportional relationship between the common coupling point voltage and the capacitor voltage. Based on this relationship, it is proposed to add grid voltage feedforward instead of capacitor voltage feedback for obtaining a stable system for every grid inductance value, including the critical value. In order to validate our assertion, this control law is tested via simulations and also, experiments on a prototype are included. Simulation and experimental results show the good performance of the proposed scheme.

Series and parallel resonance active damping of three‐phase buck‐type dynamic capacitor for reactive compensation and harmonic suppression

Series and parallel resonance tend to occur and cause harmonic distortion when the distribution system contains a shunt power capacitor to compensate inductive load and dynamic capacitor (D-CAP) to suppress harmonics. This study focuses on the series and parallel resonance active damping of three-phase buck-type D-CAP, so as to achieve better power quality control. Two virtual resistor damping methods for series resonance, D-CAP power capacitor voltage or D-CAP front-end buffer inductor current feedback, are introduced and compared. Then, parallel resonance is damped by generating certain harmonic currents in the phase with the selective harmonic voltage at the point of common coupling (PCC), equivalent to controlling the D-CAP as a virtual harmonic resistor. In addition, the power balance of D-CAP is interpreted to ensure no additional energy control loop is needed for the active damping. To further improve the parallel resonance damping, the self-adjusted damping gain is proposed by the closed-loop regulation of harmonics in PCC voltage. Finally, coordinated control between reactive and multiple harmonic currents is introduced for the whole combined control to avoid over-modulation. A wide variety of experimental results from a 33 kVar/220 V laboratory prototype are provided to demonstrate the validity of the combined control.

Wide frequency range active damping of LCL‐filtered grid‐connected converters

It can be challenging to guarantee the stability of grids with many converters with LCL filters connected due to the presence of multiple resonances within the system. This paper presents an active damping technique to mitigate multiple resonance effects and harmonics in power converters connected to weak grids. The proposed technique employs grid current and capacitor voltage feedback to achieve active damping for a wide range of multiple resonance frequencies. The effectiveness of the proposed wide frequency active damping and improved controller stability are demonstrated through frequency domain analysis and experimental results for single and parallel grid connected converters.

Imbalanced Load Regulation Based on Virtual Resistance of A Three-Phase Four-Wire Inverter for EV Vehicle-to-Home Applications

As a new application area of electric vehicles, vehicle to home (V2H) recently becomes one of the most attractive research areas. In this paper, a three-phase four-wire inverter using four half-bridge legs is adopted to realize the V2H functionality. Such three-phase inverter acts as the grid-side ac/dc part of the battery charger. In order to deal with the imbalanced load, an independently controlled neutral module has been adopted to form the neutral line and provide the path for the zero-sequence current, together with split dc-bus capacitors. Each phase employs the independent sinusoidal pulse width modulation control with the virtual resistor paralleled with the load, namely, notch-filter-incorporated capacitor voltage feedback control, which damps harmonics around the resonant frequency at various load conditions. In addition, this paper quantifies the load imbalance versus the dc-link voltage oscillation and provides the guidance for the selection of the dc-link capacitance. Finally, a 10-kW three-phase four-wire inverter working at the V2H mode is built and tested, with the three-phase imbalanced load imposed to validate the proposed design and control strategy.

Wide Bandwidth Control for Multi-Parallel Grid-Connected Inverters with Harmonic Compensation

This paper proposes a virtual impedance-based bandwidth control method for multi-parallel harmonic-compensation grid-connected inverters (HCGIs). Firstly, the influence of the resonance points caused by the interaction of multiple HCGIs on the control bandwidth is analyzed, and the analysis result shows that the control bandwidth becomes narrow due to the appearance of a new resonance point. Then, to increase the control bandwidth of multi-parallel HCGIs, six different types of virtual impedance circuits are constructed and compared, and the bandwidth control method based on virtual impedance by capacitor voltage feedback is proposed. Following that, the relationship between feedback coefficient and bandwidth is established, the design approach of parameters for the proposed method is presented. Finally, the proposed method is confirmed by the simulation and experimental tests. The simulation and experimental results show that the proposed control method can effectively shift resonance frequencies right to solve the issue of control bandwidth reduction in multi-parallel HCGIs systems, while without affecting the low-frequency harmonic current compensation.

Dual Role CDSC-Based Dual Vector Control for Effective Operation of DVR With Harmonic Mitigation

For the effective operation of a dynamic voltage restorer (DVR), a control strategy plays a significant role. This paper presents an enhanced control strategy for DVR using dual role cascaded delay signal cancellation (CDSC)-based dual vector control (DVC) under unbalanced and distorted grid conditions. Based on the numerical analysis, it is found that the CDSC prefilter is a promising solution when grid voltage is distorted by symmetric, asymmetric harmonics, and unbalanced sag. Mainly, the CDSC prefilter extracts instantaneous symmetrical components of the grid voltage required for voltage sags detection and generation of fundamental component of reference voltage for the DVR. A CDSC-based DVC algorithm with inductor current and capacitor voltage feedback is implemented in a synchronous reference frame, which tracks the fundamental DVR reference voltages. An extractor based on the modified CDSC strategy is designed to extract harmonics from load voltage during distorted grid conditions. These extracted harmonic components (nonfundamental) are added in phase opposition with fundamental component and fed to pulse width modulation block to generate reference voltages. Experimental studies are conducted on scaled down (100 V, 0.5 kVA) laboratory prototype DVR to verify the effectiveness of the proposed control algorithm under unbalanced and distorted grid conditions.

Comparative study on Lyapunov‐function‐based control schemes for single‐phase grid‐connected voltage‐source inverter with LCL filter

This study deals with various Lyapunov-function-based control (LFBC) schemes proposed for a single-phase grid-connected voltage source inverter with LCL filter. Use of LCL filter causes resonance which may adversely affect the controller's stability. The conventional (CLFBC) scheme employing the inverter-side current guarantees the asymptotic global stability, but it is not able to damp the resonance. As a remedy to the poor resonance damping, the adoption of grid-current and capacitor-voltage feedback schemes have been investigated. Although the former offers a globally asymptotically stable system, it cannot improve the poor resonance damping. However, the CLFBC with capacitor-voltage feedback scheme not only preserves the global stability, but also improves the resonance damping substantially. The analytical equations of the closed-loop poles for each control scheme are provided which can be used to compute the pole positions and the damping ratio needed for a desired response. Furthermore, the effect of changing controller gains on the loci of closed-loop poles is also studied. Simulation and experimental results obtained from 3.3 kW system demonstrate that the CLFBC with capacitor-voltage feedback scheme not only offers a global stability, but also leads to good quality sinusoidal grid current with reasonable total harmonic distortion and fast dynamic response.

Impact of phase-locked loop on stability of active damped LCL-filter-based grid-connected inverters with capacitor voltage feedback

Active damped LCL-filter-based inverters have been widely used for grid-connected distributed generation (DG) systems. In weak grids, however, the phase-locked loop (PLL) dynamics may detrimentally affect the stability of grid-connected inverters due to interaction between the PLL and the controller. In order to solve the problem, the impact of PLL dynamics on small-signal stability is investigated for the active damped LCL-filtered grid-connected inverters with capacitor voltage feedback. The system closed-loop transfer function is established based on the Norton equivalent model by taking the PLL dynamics into account. Using an established model, the system stability boundary is identified from the viewpoint of PLL bandwidth and current regulator gain. The accuracy of the ranges of stability for the PLL bandwidth and current regulator gain is verified by both simulation and experimental results.

A continuous-time delta-sigma ADC for portable ultrasound scanners

A fully differential fourth-order 1-bit continuous-time delta-sigma ADC designed in a 65 nm process for portable ultrasound scanners is presented in this paper. The circuit design, implementation and measurements on the fabricated die are shown. The loop filter consists of RC-integrators, programmable capacitor arrays, resistors and voltage feedback DACs. The quantizer contains a pulse generator, a high-speed clocked comparator and a pull-down clocked latch to ensure constant delay in the feedback loop. Using this implementation, a small and low-power solution required for portable ultrasound scanner applications is achieved. The converter has a supply voltage of 1.2 V, a bandwidth of 10 MHz and an oversampling ratio of 16 leading to an operating frequency of 320 MHz. The design occupies a die area of $$0.0175\hbox { mm}^2$$ . Simulations with extracted parasitics show a SNR of 45.2 dB and a current consumption of $$489 \,\upmu \hbox {A}$$ . However, by adding a model of the measurement setup used, the performance degrades to 42.1 dB. The measured SNR and current consumption are 41.6 dB and $$495\,\upmu \hbox {A}$$ , which closely fit with the expected simulations. Several dies have been measured, and an estimation of the die spread distribution is given.

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.