Grid-connected Pulse Width Modulation Research Articles

Design and Implementation of Robust H∞ Control for Improving Disturbance Rejection of Grid-Connected Three-Phase PWM Rectifiers

In response to the high performance requirements of pulse width modulation (PWM) converters in grid-connected power systems, H-Infinity (H∞) control has attracted significant research interest due to its robustness against parameter variations and external disturbances. In this work, an advanced robust H∞ control is proposed for a grid-connected three-phase PWM rectifier. A two-level control strategy is adopted, where cascaded H∞ controllers are designed to simultaneously regulate the DC bus voltage and input currents even under load disturbances and non-ideal grid conditions. As a result, unit power factor, stable DC bus voltage, and sinusoidal input currents with lower harmonics can be accurately achieved. The design methodology and stability of the proposed controller are verified through a comprehensive analysis. Simulation tests and experimental implementation on a dSPACE 1103 board demonstrate that the proposed control scheme can effectively enhance disturbance rejection performance under various operating conditions.

Generalized Predictive dc-Link Voltage Control for Grid-Connected Converter

The main function of the grid-connected converter in many applications is to control the dc-link voltage with high performance, i.e., strong disturbance rejection capability and good dynamic response. Take the grid-connected pulsewidth modulation (PWM) rectifier of a motor drive system as an example, good disturbance rejection capability is essential for the dc-link voltage control to address the varying loads on the motor side, and the dynamic process of the dc-link voltage control is preferred to be fast and overshoot-free, so as to adaptively adjust the dc-link voltage according to the motor speed and reduce the switching losses. However, the performance of the conventional proportional-integral (PI)-based dc-link voltage control is not always satisfying and can be further improved. In this article, the generalized predictive control (GPC) method is applied to the dc-link voltage control of a grid-connected converter for the first time, which can provide both good disturbance rejection capability and satisfying dynamic performance. Moreover, stability analysis of the proposed GPC-based dc-link voltage control strategy is theoretically studied, and a parameter tuning guideline is provided. The effectiveness and advantages of the proposed method are validated with experimental results.

Analysis and Mitigation of Dead-Time Harmonics in the Single-Phase Full-Bridge PWM Converter With Repetitive Controllers

In order to prevent the power switching devices (e.g., an insulated-gate bipolar transistor, IGBT) from shoot-through in voltage-source converters during a switching period, the dead time is added either in the hardware driver circuits of the IGBTs or implemented in software in pulse width modulation (PWM) schemes. Both solutions will contribute to a degradation of the injected current quality. As a consequence, the harmonics induced by the dead time (referred to as “dead-time harmonics” hereafter) have to be compensated in order to achieve a satisfactory current quality, as required by standards. In this paper, the emission mechanism of dead-time harmonics in single-phase PWM inverters is, thus, presented considering the modulation schemes in detail. More importantly, a repetitive controller has been adopted to eliminate the dead-time effect in single-phase grid-connected PWM converters. The repetitive controller has been plugged into a proportional-resonant-based fundamental-frequency current controller so as to mitigate the dead-time harmonics and also to maintain the control of the fundamental-frequency grid current in terms of dynamics. Simulations and experiments are provided, which confirm that the repetitive controller can effectively compensate the dead-time harmonics and other low-order distortions, and also, it is a simple method without hardware modifications.

Parallel structure general repetitive controller for general grid‐connected PWM converters

This study investigates parallel structure general repetitive control (PSGRC) and its error convergence rate by using exponential function properties. PSGRC offers a general repetitive control solution for power converters to mitigate power harmonics distortions. PSGRC with appropriate settings will lead to various RCs with various error convergence rates at interested harmonic frequencies, e.g. conventional RC, dual-model RC, and odd harmonics RC. As application examples, PSGRC was applied into general grid-connected pulse-width-modulation (PWM) converter systems. Experimental results show the effectiveness and advantages of PSGRC: three/single-phase grid-connected PWM converters can achieve zero-error current tracking and very fast current error convergence rate upon demand.

Design and Analysis of Robust Active Damping for LCL Filters Using Digital Notch Filters

Resonant poles of LCL filters may challenge the entire system stability especially in digital-controlled pulse width modulation (PWM) inverters. In order to tackle the resonance issues, many active damping solutions have been reported. For instance, a notch filter can be employed to damp the resonance, where the notch frequency should be aligned exactly to the resonant frequency of the LCL filter. However, parameter variations of the LCL filter as well as the time delay appearing in digital control systems will induce resonance drifting, and thus break this alignment, possibly deteriorating the original damping. In this paper, the effectiveness of the notch filter-based active damping is first explored, considering the drifts of the resonant frequency. It is revealed that when the resonant frequency drifts away from its nominal value, the phase lead or lag introduced by the notch filter may make itself fail to damp the resonance. Specifically, the phase lag can make the current control stable despite of the resonant frequency drifting, when the grid current is fed back. In contrast, in the case of an inverter current feedback control, the influence of the phase lead or lag on the active damping is dependent on the actual resonant frequency. Accordingly, in this paper, the notch frequency is designed away from the nominal resonant frequency to tolerate the resonance drifting, being the proposed robust active damping. Simulations and experiments performed on a 2.2-kW three-phase grid-connected PWM inverter verify the effectiveness of the proposed design for robust active damping using digital notch filters.

The Comprehensive Design and Optimization of the Post-Fault Grid-Connected Three-Phase PWM Rectifier

The comprehensive design and optimization, including the optimized modulation approach, detailed modeling, and reliable control algorithm, is presented in this paper to guarantee the stable operation of the postfault three-phase pulsewidth modulation (PWM) rectifier. The effects of the space vector modulation approaches on the capacitor current are investigated. Based on the analytical results, the method introducing the smaller capacitor current is selected as the optimized modulation approach, which is of paramount importance to avoid the permanent failure of the dc-link capacitors. Then, with the optimized modulation approach, a control-oriented model for the post-fault PWM rectifiers is derived based on the α - β stationary frame, where the capacitor voltage oscillation and deviation are taken into account. The proposed model-based controllers do not require the phase-locked loop (PLL), and the elimination of the PLL means that the post-fault rectifiers can never lose synchronization with the grid. Furthermore, an equation of the required dc voltage for linear modulation is proposed, and using the proposed equation, a correct dc voltage is selected to reject the overmodulation. The experimental results validate the effectiveness of the proposed comprehensive design and optimization. Finally, a conclusion can be drawn from the experimental results-that the dc-link capacitor current and the linear modulation should be taken into account when selecting the dc voltage in the post-fault operation.

An <italic>LCL</italic>-<italic>LC</italic> Filter for Grid-Connected Converter: Topology, Parameter, and Analysis

In order to further cut down the cost of filter for grid-connected pulsewidth modulation (PWM) converter under the more and more stringent grid code, a new kind of high-order filter, named LCL - LC filter, is presented in this paper. The resonant frequency characteristics of the filter are analyzed, and a parameter design method on the base of the characteristics is also proposed in the paper. The proposed parameter design method can easily make full use of the existing research results about the traditional LCL filter parameter design. And then a parameter robustness analysis method based on four-dimensional graphics is proposed to analyze parameter robustness of the presented filter. Compared with the traditional one, the proposed analysis method can analyze the filter performance under variations of several parameters at a time without any iteration. The comparative analysis and discussion considering the LCL filter, the trap filter, and the LCL - LC filter, are presented and verified through the experiments on a 5 kW grid-connected converter prototype. Experiment results demonstrate the accuracy of theoretical analysis and prove that the presented filter has a better performance than two others.

Voltage Distortion Approach for Output Filter Design for Off-Grid and Grid-Connected PWM Inverters

This paper proposes a novel voltage distortion approach for output filter design based on the voltage transfer function for both off-grid and grid-connected Pulse Width Modulation (PWM) Inverters. The method explained in detail is compared to conventional methods. A comparative analysis is performed on an example of L and LCL-filter design. Simulation and experimental results for the off-grid and the grid-connected single phase inverter prove our theoretical predictions. It was found that conventional methods define redundant values of the output filter elements. Assumptions and limitations of the proposed approach are also discussed.

Adaptive Current Control for Grid-Connected Converters With LCL Filter

This paper presents a discrete-time adaptive current controller for grid-connected pulse width modulation voltage source converters with LCL filter. The main attribute of the proposed current controller is that, in steady state, the damping of the LCL resonance does not depend on the grid characteristic since the adaptive feedback gains ensure a predefined behavior for the closed-loop current control. An overview of model reference adaptive state feedback theory is presented aiming to give the reader the required background for the adaptive current control design. The digital implementation delay is included in the model, and the stability concerning the variation of the grid parameters is analyzed in detail. Furthermore, current distortions due to the grid background voltage are rejected without using the conventional stationary resonant controllers or the synchronous proportional-plus-integral controllers. Simulation and experimental results are presented to validate the analysis and to demonstrate the good performance of the proposed controller for grid-connected converters subjected to large grid impedance variation and grid voltage disturbances.

PI State Space Current Control of Grid-Connected PWM Converters With LCL Filters

Design and analysis of PI state space control for grid-connected pulsewidth modulation (PWM) converters with LCL filters based on pole placement approach is addressed. State space control offers almost full controllability of system dynamic. However, pole placement design is difficult and usually requires much experience. In this paper, a suitable pole placement strategy is proposed, which ensures fulfilling the requirements, which are commonly specified with respect to rise time, overshoot, and proper resonance damping. Controller parameter expressions are derived in terms of system parameters and specified poles and zeros. Hence, straightforward controller tuning for a particular system setting is possible. Performance is analyzed by means of transfer function-based calculations, simulations with MATLAB, and experimental tests. Dynamic performance and robustness against grid impedance variations are addressed as well as harmonic rejection capability and other practical issues.

Investigation of Active Damping Approaches for PI-Based Current Control of Grid-Connected Pulse Width Modulation Converters With LCL Filters

This paper deals with various active damping approaches for PI-based current control of grid-connected pulsewidth-modulation (PWM) converters with LCL filters, which are based on one additional feedback. Filter capacitor current, as well as voltage feedback for the purpose of resonance damping, are analyzed and compared. Basic studies in the continuous Laplace domain show that either proportional current feedback or derivative voltage feedback yields resonance damping. Detailed investigations of these two approaches in the discrete <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$z$</tex></formula> -domain, taking into account the discrete nature of control implementation, sampling, and PWM, are carried out. Several ratios of LCL resonance frequency and control frequency are considered. At high resonance frequencies, only current feedback stabilizes the system. At medium resonance frequencies, both approaches have good performance. At low resonance frequencies, stability gets worse, even though voltage feedback offers slightly better damping properties. Measurements validate the theoretical results.

A novel pulse width modulation for grid-connected multilevel inverter

This paper presents a single-phase five-level grid-connected photovoltaic inverter with a novel dual reference modulation technique. Two reference signals identical to each other with an offset equivalent to the amplitude of the triangular carrier signal were used to generate pulse width modulation (PWM) signals. The inverter consists of a full-bridge inverter and an auxiliary circuit comprising of four diodes and a switch. The inverter produces output voltage in five levels: 0, +1/2Vdc, Vdc, −1/2Vdc, and −Vdc. A digital proportional-integral (PI) current control algorithm is implemented in DSP TMS320F2812 to keep the current injected into the grid sinusoidal and to have high dynamic performance with low total harmonic distortion (THD). The validity of the proposed inverter is verified through simulation and is implemented in a prototype. The experimental results are compared to conventional single-phase three-level grid-connected pulse width modulation (PWM) inverter in terms of THD.

Limitations of Voltage-Oriented PI Current Control of Grid-Connected PWM Rectifiers With $LCL$ Filters

Voltage-oriented PI control of three-phase grid-connected pulsewidth-modulation rectifiers with LCL filters is addressed. LCL filters require resonance damping. Active resonance damping is state of the art to face the problem, but it is still under investigation because of the manifold solutions. It is often realized using many sensors and/or complex control algorithms. In contrast, pure PI control requires only one set of current sensors, and its implementation and design are rather simple and well known from the L filter control. PI control has already been shown to be a suitable solution also for LCL filters, but there are limitations. These are investigated in this paper. System stability is analyzed with respect to different ratios of LCL filter resonance and control frequencies. The latter are important parameters for system design and control. Both line and converter current control are analyzed. For a certain range of frequency ratios, the voltage-oriented PI control gives stable performance without additional feedback, but for ratios outside this range, stable operation is impossible. Experimental tests validate the theoretical results. In addition, an experimentally determined LCL filter transfer function is shown in this paper, which shows a lower resonance peak as expected from commonly used filter models.