Model-based generation of low distortion currents in grid-coupled PWM-inverters using an LCL output filter

In this paper, a single phase inductance-capacitance-inductance (LCL) output stage for grid coupled inverters is designed and built. An accurate model and observer of the output filter and the distorted grid voltage are implemented. The paper deals with the construction of a 14-state model, and the feedback control loop to obtain adequate closed loop response. Simulations indicate a good performance of the controller, with a total harmonic current distortion (THD) below 1%. Experimental results confirm simulations, and illustrate the correct operation of the Kalman observer to estimate the distorted grid voltage (THD 3%). The observer only uses the inverter current measurement as input. The output filter effectively reduces the pulsewidth modulation harmonics in the grid current.     

A novel single-phase ZCS-PWM high-power-factor boost rectifier

This paper presents a novel single-phase high-power-factor (HPF) pulsewidth-modulated (PWM) boost rectifier featuring soft commutation of the active switches at zero current (ZC). It incorporates the most desirable properties of conventional PWM and soft-switching resonant techniques. The input current shaping is achieved with average current mode control and continuous inductor current mode. This new PWM power converter provides ZC turn on and turn off of the active switches, and it is suitable for high-power applications employing insulated gate bipolar transistors (IGBTs). The principle of operation, the theoretical analysis, a design example and experimental results from a laboratory prototype rated at 1600 W with 400 VDC output voltage are presented. The measured efficiency and the power factor were 96.2% and 0.99%, respectively, with an input current total harmonic distortion (THD) equal to 3.94%, for an input voltage with THD equal to 3.8%, at rated load     

基于SPWM逆变器控制系统的建模与仿真

在双环控制中,为了获得更好的控制效果,逆变器要实现状态反馈解耦。文章在状态反馈解耦的基础上,首先建立了SPWM数学模型,接着对提出的两种控制方案进行了比较,通过分析指令传函的动态跟踪性能和扰动传函的扰动抑制能力,选择了负载电流解耦的电感电流反馈,它是控制效果较好的一种方案,最后对所选的控制方案进行了系统仿真,结果表明输出电压波形质量高,动态响应好,扰动抑制能力强。     

基于dsPIC的PV逆变器的一种改进结构及其实现

针对目前PV光伏并网发电系统的核心逆变器的现状、结构和控制方法进行了详细的分析,从电网、PV系统及用户的需求出发,指出传统的单级全桥逆变器普遍具有不能处理较宽的输入PV电压,且需要重型工频升压变压器等缺点。在此基础上,本文创新设计并实现了一种基于单级全桥逆变器的并联耦合改进结构。实测证明这种并联耦合反激结构可以有效地减小通过大容量输入电解电容的纹波电流的RMS,从而延长电容的寿命;还可减小输出电流的纹波,从而降低输出电流的THD（谐波失真）;还可适应较宽的输入电压,减小交流纹波,减小磁芯,同时可以提供较高的额定输出电流等优点。     

Deadbeat controlled PWM inverter with parameter estimation usingonly voltage sensor

A technique based on deadbeat control theory is proposed to obtain a nearly sinusoidal PWM (pulsewidth-modulated) inverter output voltage using only a voltage sensor. The closed loop sampled-data feedback scheme inherently results in very fast response to load disturbance and nonlinear load, producing low total harmonic distortion. Parameter estimation of the plant provides a type of self-tuning of the proposed controller. A theoretical analysis, simulation, and experimental results are presented for a single-phase PWM inverter controlled by an Intel 8086 microprocessor     

A switching power amplifier with feedback for improving total harmonic distortion

A high efficiency and low distortion and switching power amplifier is proposed. We use feedback to reduce the total harmonic distortion (THD) and the DC output bias current. The experimental results show that the proposed circuit has 0.24% total harmonic distortion and 91% power efficiency while the DC output bias current is 12 μA at 1.5 V supply voltage.     

A Wien bridge RC oscillator with fast amplitude control

The paper describes fast amplitude control in a Wien bridge RC oscillator. This fast control is achieved restoring ( within each period) a node voltage in the passive RC two-port of positive feedback. As a result the new value of the output voltage amplitude can be established with the delay which is not more than one oscillation period. The total harmonic distortion ( THD) is calculated and the condition of low distortion is established. The low-THD oscillator includes the circuit for damping-ratio control as well. The experimental data show that THD below 0-1% is achieved.     

Resonant-boost-input three-phase power factor corrector

This paper presents a novel three-phase power factor corrector (PFC) circuit which uses two power switches working in zero-voltage-switching (ZVS) condition. The two switches along with a high-frequency inductor constitute a high-frequency current source which is responsible for the energy transfer in the circuit. The input current is partly continuous and partly discontinuous. The total harmonic distortion (THD) in the input current has a low value of 4.5%, and the output DC voltage is very close to the peak line voltage. The operation of the converter is explained by identifying the different switching modes, and the simulation and experimental waveforms are presented     

Low order harmonic cancellation in a grid connected multiple inverter system via current control parameter randomization

In grid connected multiple inverter systems, it is normal to synchronize the output current of each inverter to the common network voltage. Any current controller deficiencies, which result in low order harmonics, are also synchronized to the common network voltage. As a result the harmonics produced by individual converters show a high degree of correlation and tend to be additive. Each controller can be tuned to achieve a different harmonic profile so that harmonic cancellation can take place in the overall system, thus reducing the net current total harmonic distortion level. However, inter-inverter communication is required. This paper presents experimental results demonstrating an alternative approach, which is to arrange for the tuning within each inverter to be adjusted automatically with a random component. This results in a harmonic output spectrum that varies with time, but is uncorrelated with the harmonic spectrum of any other inverter in the system. The net harmonics from all the inverters undergo a degree of cancellation and the overall system yields a net improvement in power quality.     

A centroid-based PWM switching technique for full-bridge inverterapplications

A novel centroid-based pulse-width-modulation (PWM) switching strategy is proposed which is suitable for full-bridge inverter applications. This method is evaluated, and its performance is compared with existing PWM switching strategies. The performance evaluation and comparison are based on the total harmonic distortion (THD) and number of pulses per cycle of the inverter output waveform. The objective of the new switching strategy is to minimize both the THD and low-order harmonics. Simulation results show that this technique yields a significant improvement in performance. In addition, a hybrid switching sequence is developed for the proposed scheme, which can lead to further reduction in switching losses     

Online grid measurement and ENS detection for PV inverter running on highly inductive grid

Photovoltaic (PV) and other sources of renewable energy are being used increasingly in grid-connected systems, for which stronger power quality requirements are being issued. Continuous grid monitoring should be considered so as to provide safe connections and disconnections from the grid. This letter gives an overview of the methods used for online grid measurement with PV inverters. Emphasis is placed on a method based on the injection of a noncharacteristic harmonic in the grid. Since this injection is regarded as a disturbance for the grid, different issues, i.e., the influence on total harmonic distortion (THD), the accuracy of line impedance measurement and the ENS (German abbreviation of Main Monitoring units with allocated Switching Devices) detection are studied. Laboratory results conducted on an existing PV inverter are presented to demonstrate the behavior of the PV inverter under different grid conditions. Some of the injection parameters are tuned in order to get an accurate measurement of line impedance.     

Design and analysis of power-CMOS-gate-based switched-capacitor boost DC-AC inverter

A multistage power CMOS-transmission-gate-based (CMOS-TG) quasi-switched-capacitor (QSC) boost DC-AC inverter is proposed and integrated with a boost DC-DC converter for a step-up application with AC or DC load. In this paper, using CMOS-TG as a bidirectional switch, the various topologies can be integrated in the same configuration for achieving two functions: boosting and alternating; boosting for getting a sinusoidal output in which the peak is the result of a many times step-up of the input; alternating to realize the positive/negative half sinusoidal of the output. The inverter does not require any inductive elements as inductor and transformer, so integrated circuit (IC) fabrication will be promising for realization. By using the state-space averaging technique, the large-signal state-space model of the inverter is proposed, and then both the static analysis and dynamic small-signal analysis are derived to form a unified formulation for inverter/converter. Based on this formulation, there are presented for theoretical analysis/control design, including steady-state power, conversion efficiency, voltage conversion ratio, output ripple percentage, capacitance selection, closed-loop control and stability, and total harmonic distortion (THD), etc. Finally, a six-stage QSC boost DC-AC inverter is simulated by PSPICE, and the simulations are discussed for some cases, including: 1) steady-state AC output, ripple percentage, and power efficiency; 2) transient response of the regulated inverter for load variation; 3) a practical capacitive load: electromagnetic luminescent (EL) lamp, and 4) efficiency, ripple percentage, and THD for different loads. The results are illustrated to show the efficacy of the proposed inverter.     

Design and Implementation of a Robust Current Controller for VSI Connected to the Grid Through an LCL Filter

This paper describes the design and implementation of a discrete controller for grid-connected voltage-source inverters with an LCL filter usually found in wind power generation systems. First, a theorem that relates the controllability of the discrete dynamic equation of the inverter with LCL filter and the sampling frequency is derived. Then, a condition to obtain a partial state feedback controller robust to grid impedance uncertainties and based on linear matrix inequalities is proposed. This controller guarantees the stability and damping of the LCL filter resonance for a large set of grid conditions without requiring self-tuning procedures. Finally, an internal model controller is added to ensure asymptotic reference tracking and disturbance rejection, significantly reducing the impact of grid background voltage distortion on the output currents. Experimental results are presented to support the theoretical analysis and to demonstrate the system performance.      

Two-level switching pattern deadbeat DSP controlled PWM inverter

A two-level switching algorithm of the deadbeat controlled PWM inverter is presented. Two levels, instead of three levels, are used in the pulse pattern. This scheme allows the use of higher switching frequency for a given computation time delay, which results in lower total harmonic distortion (THD) at the output. Control algorithms are derived. The proposed control scheme is implemented using a TI TMS320C14 DSP controlling an inverter to produce a very low THD sinusoidal output voltage. Simulation and experimental results are presented to verify the performance     

High-performance programmable AC power source with low harmonicdistortion using DSP-based repetitive control technique

This paper proposes a new control scheme based on a two-layer control structure to improve both the transient and steady-state responses of a closed-loop regulated pulse-width-modulated (PWM) inverter for high-quality sinusoidal AC voltage regulation. The proposed two-layer controller consists of a tracking controller and a repetitive controller. Pole assignment with state feedback has been employed in designing the tracking controller for transient response improvement, and a repetitive control scheme was developed in synthesizing the repetitive controller for steady-state response improvement. A design procedure is given for synthesizing the repetitive controller for PWM inverters to minimize periodic errors induced by rectifier-type nonlinear loads. The proposed control scheme has been realized using a single-chip digital signal processor (DSP) TMS320C14 from Texas Instruments. A 2-kVA PWM inverter has been constructed to verify the proposed control scheme. Total harmonic distortion (THD) below 1.4% for a 60-Hz output voltage under a bridge-rectifier RC load with a current crest factor of 3 has been obtained. Simulation and experimental results show that the DSP-based fully digital-controlled PWM inverter can achieve both good dynamic response and low harmonics distortion     

Current waveform quality from grid‐connected photovoltaic inverters and its dependence on operating conditions

The increasing installation of grid‐connected photovoltaics (PV) in the urban environment will lead to a significant penetration into the low voltage electricity supply network of small power electronic generators. Inevitably some disturbance to the electricity supply quality will result from these embedded generators. It is shown that the inverters used to grid connect PV arrays are susceptible to minor distortion of the network waveform and that this can result in higher levels of current waveform distortion, or harmonic disturbance, being sourced into the supply than would be expected from analyses which assume an ideal voltage waveform. The level of current distortion is shown to be very dependent on the type of inverter control used. Inverter operation is also a function of operating point; clearly a device at part load cannot be expected to deliver the same quality of current waveform as when operating under its rated design conditions. The impedance of the grid connection also has an impact on the inverter's operation. Copyright © 2000 John Wiley & Sons, Ltd.     

A novel suppressed-link rectifier-inverter topology with near unity power factor

This work describes a novel method in improving the input current total harmonic distortion (THD) as well as the power factor of a three-phase suppressed-link rectifier-inverter circuit. This proposed method makes use of only three bi-directional low power static switches with a relatively simple gating circuit. This paper illustrates how the proposed method is superior in reducing the input current THD of a rectifier-inverter set to about 5%, which is in line with the requirements of IEEE standard 519-1992. This is accomplished without the use of any filter or complex wave shaping techniques. A delta-modulated (DM) voltage source inverter (VSI) with proportional integrator forms the output stage of the converter. It helps to provide constant volts per hertz operation without the need for additional feedback circuitry and complexity. Moreover, this novel DM technique also helps to provide a smooth transition from the pulse width modulation (PWM) to square wave, hence allowing full utilization of the DC bus voltage.     

Optimal Predictive Control of Three-Phase NPC Multilevel Converter for Power Quality Applications

This paper presents the optimal control of the ac currents, the dc voltage regulation, and the dc capacitor voltage balancing in a three-level three-phase neutral point clamped multilevel converter for use in power quality applications as an active power filter. The ac output currents and the dc capacitor voltages are sampled and predicted for the next sampling time using linearized models and considering all the 27 output voltage vectors. A suitable quadratic weighed cost function is used to choose the voltage vector that minimizes the ac current tracking errors, the dc voltage steady-state error, and the input dc capacitor voltage unbalancing. The obtained experimental results show that the output ac currents track their references showing small ripple, a total harmonic distortion (THD) of less than 1%, harmonic contents that are 46 dB below the fundamental, and almost no steady-state error (0.3%). The capacitor voltages are balanced within 0.05%, and the balancing is assured even when redundant vectors are not chosen. Near-perfect capacitor dc voltage balancing is obtained while reducing current harmonic distortion. Some experimental evidence of robustness concerning a parameter variation was also found, with the optimum controller withstanding parameter deviations from $+$100% to $-$50%. Compared to a robust sliding mode controller, the optimal controller can reduce the THD of the ac currents or reduce the switching frequency at the same THD, being a suitable controller for power quality in medium-voltage applications.      

A current mode feed-forward gain control system for a 0.8 V CMOS hearing aid

A current mode feed-forward gain control（CMFGC）technique is presented,which is applied in the front-end system of a hearing aid chip.Compared with conventional automatic gain control（AGC）,CMFGC significantly improves the total harmonic distortion（THD）by digital gain control.To attain the digital gain control codes according to the extremely weak output signal from the microphone,a rectifier and a state controller implemented in current mode are proposed.A prototype chip has been designed based on a 0.13μm standard CMOS process.The measurement results show that the supply voltage can be as low as 0.6 V.And with the 0.8 V supply voltage,the THD is improved and below 0.06%（-64 dB）at the output level of 500 mV_p-p,yet the power consumption is limited to 40μW.In addition,the input referred noise is only 4μV_rmsand the maximum gain is maintained at 33 dB.     

Harmonic distortion in single-channel MOS integrated circuits

Expressions assuming a simple square-law MOSFET model are presented for the low-frequency harmonic distortion of an enhancement-mode source follower. These theoretical results are compared to measurements of several integrated versions of the three circuit types. For a given fabrication process, the main factors determining the amount of distortion for all three circuits are the quiescent output voltage and the output swing; to a first order, the distortion does not depend on bias current or device geometries. The distortion of an enhancement-mode source follower has a similar behavior to that of an enhancement-load inverter with the same output quiescent voltage and output swing; both distortions are nearly proportional to the body-effect coefficient. For the same output quiescent voltage and output swing, the distortion of the depletion-load inverter is the highest among the three circuits, but is practically independent of process parameters.