Yiwen Geng

Wide Damping Region for LCL -Type Grid-Connected Inverter With an Improved Capacitor-Current-Feedback Method

This paper has presented a stability analysis of a LCL-type grid-connected inverter in the discrete-time domain. It has been found that even though the system is stable when the resonance frequency f,. is higher than one-sixth of the sampling frequency (f8/6), an effective damping scheme is still required due to the potential influence of the grid impedance. With a conventional proportional capacitor-current-feedback active damping (AD), the valid damping region is only up to f8/6. This however is not sufficient in the design process for obtaining a high quality output current and the system can easily become unstable due to the resonance frequency shifting. Considering the resonance frequency design rules of the LCL filter, this paper proposes an improved capacitor-current-feedback AD method. With a detailed analysis and proper parameter design, the upper limit of the damping region is extended to f8/4, which can cover all the possible resonance frequencies. Then, the damping performance of the proposed AD method is studied. It shows that the optimal damping is obtained when the actual resonance frequency is (fr + f8/4)/2. Moreover, an approximate calculation for the optimal damping coefficient R is given. Finally, the experimental results have validated the effectiveness of the proposed AD method.

Grid Harmonics Suppression Scheme for LCL -Type Grid-Connected Inverters Based on Output Admittance Revision

In this paper, the influence of grid harmonics on the output current of grid-connected inverters with an LCL filter is investigated by means of the output admittance. With the complex transfer model of the output admittance, the full-feedforward scheme of grid voltage is derived, which, however, is difficult to be implemented due to the derivative terms in the feedforward link. A detailed theoretical analysis is also presented to explain the compensation error introduced by the active damping and delays when the grid voltage proportional feedforward is adopted. Then a feedforward scheme based on the band-pass filter (BPF) is proposed to compensate the grid harmonics at the selected frequencies, and the parameters of the BPF are derived to revise the output admittance. It has also been found that with the commonly used phase-locked loop (PLL), an additional admittance matrix is introduced. The compensation effect will be degraded when the PLL with a high bandwidth is used for tracking grid phase accurately. Therefore, a modified PLL is proposed to revise the output admittance again, for suppressing the output current distortion arising from the grid harmonics, which propagate to control system through the PLL. Finally, the experimental results verify the effectiveness of the proposed scheme, where the current harmonics are effectively suppressed.

An Improved Grid-Voltage Feedforward Strategy for High-Power Three-Phase Grid-Connected Inverters Based on the Simplified Repetitive Predictor

When faced with distorted grid voltage, more harmonics will appear in the output currents of the grid-connected inverters. The grid-voltage feedforward strategy, as the most direct solution to compensate the harmonics, however, is seriously affected by the errors in the grid-voltage feedforward loop, such as delays. This issue is more significant for high-power inverters, where the switching frequency is relatively low (<; 5 kHz), and the grid-interface inductance is small (<; 0.5 mH). The errors mainly include the signal distortion caused by the conditioning circuits, the control delay of the digital controller, and the zero-order hold (ZOH) characteristic of pulse width modulation (PWM). In this paper, several improvements have been made to reduce the signal distortion and compensate the delays. A second-order Butterworth low-pass filter in the conditioning circuit is carefully designed with the maximum flat magnitude response and the almost linear phase response to avoid distorting the measured grid voltage. Furthermore, based on the conventional repetitive predictor, an open-loop simplified repetitive predictor is proposed to compensate the delays in the grid-voltage feedforward loop. Three predictive steps are achieved by the open-loop simplified repetitive predictor to compensate the delays: one step for the delay caused by the conditioning circuit, the second step for the control delay of the digital controller, and the third step for the ZOH characteristic of PWM. The effectiveness of the improved grid-voltage feedforward strategy are experimentally validated on a 250-kVA solar power generation system, where the current harmonics are effectively attenuated. In addition, the inverter starting current is suppressed.

Stability Analysis of Grid-Connected Inverters with an LCL Filter Considering Grid Impedance

Under high grid impedance conditions, it is difficult to guarantee the stability of grid-connected inverters with an LCL filter designed based on ideal grid conditions. In this paper, the theoretical basis for output impedance calculation is introduced. Based on the small-signal model, the d-d channel closed-loop output impedance models adopting the converter-side current control method and the grid-side current control method are derived, respectively. Specifically, this paper shows how to simplify the stability analysis which is usually complemented based on the generalized Nyquist stability criterion (GNC). The stability of each current-controlled grid-connected system is analyzed via the proposed simplified method. Moreover, the influence of the LCL parameters on the stability margin of grid-connected inverter controlled with converter-side current is studied. It is shown that the stability of grid-connected systems is fully determined by the d-d channel output admittance of the grid-connected inverter and the inductive component of the grid impedance. Experimental results validate the proposed theoretical stability analysis.

Minimization of the DC Component in Transformerless Three-Phase Grid-Connected Photovoltaic Inverters

The dc component is a special issue in transformerless grid-connected photovoltaic (PV) inverter systems and may cause problems regarding system operation and safety. IEEE standard 1547-2003 has defined the limit for dc component in the grid-side ac currents, e.g., below 0.5% of the rated current. The dc component can cause line-frequency power ripple, dc-link voltage ripple, and a further second-order harmonic in the ac current. This paper has proposed an effective solution to minimize the dc component in three-phase ac currents and developed a software-based approach to mimic the blocking capacitors used for the dc component minimization, the so-called virtual capacitor. The “virtual capacitor” is achieved by adding an integral of the dc component in the current feedback path. A method for accurate extraction of the dc component based on double time integral, as a key to achieve the control, has been devised and approved effective even under grid-frequency variation and harmonic conditions. A proportional-integral-resonant controller is further designed to regulate the dc and line-frequency component in the current loop to provide precise control of the dc current. The proposed method has been validated on a 10-kVA experimental prototype, where the dc current has been effectively attenuated to be within 0.5% of the rated current. The total harmonic distortion and the second-order harmonic have also been reduced as well as the dc-link voltage ripple.

Performance Evaluation of Split Output Converters With SiC MOSFETs and SiC Schottky Diodes

The adoption of silicon carbide (SiC) MOSFETs and SiC Schottky diodes in power converters promises a further improvement of the attainable power density and system efficiency, while it is restricted by several issues caused by the ultrafast switching, such as phase-leg shoot-through (“crosstalk” effect), high turn-on losses, electromagnetic interference (EMI), etc. This paper presents a split output converter, which can overcome the limitations of the standard two-level voltage source converters when employing the fast-switching SiC devices. A mathematical model of the split output converter has been proposed to reveal how the split inductors can mitigate the crosstalk effect caused by the high switching speed. The improved switching performance (e.g., lower turn-on losses) and EMI benefit have been demonstrated experimentally. The current freewheeling problem, the current pulses and voltage spikes of the split inductors, and the disappeared synchronous rectification are explained in detail both experimentally and analytically. The results show that the split output converter can have lower power device losses compared with the standard two-level converter at high switching frequencies. However, the extra losses in the split inductors may impair the efficiency of the split output converter, which is verified by experiments in the continuous operating mode. A 95.91% efficiency has been achieved by the split output converter at the switching frequency of 100 kHz with suppressed crosstalk, lower turn-on losses, and reduced EMI.

Capacitor-Voltage Feedforward With Full Delay Compensation to Improve Weak Grids Adaptability of LCL-Filtered Grid-Connected Converters for Distributed Generation Systems

LCL-filtered grid-connected converters are widely used for distributed generation systems. However, the current regulation of such converters is susceptible to weak grid conditions, e.g., grid impedance variation and background harmonics. Paralleling multiple harmonic compensators (HCs) is a commonly used method to suppress the current distortion caused by grid background harmonics, but the control bandwidth should be wide enough to ensure system stability. In order to enhance the adaptability of LCL-filtered grid-connected converters under weak grid operation, this paper proposes an improved capacitor-voltage-feedforward control with full delay compensation. When used with converter-side current feedback, the proposed control can keep system low-frequency characteristic independent of grid impedance and provide a high-harmonic rejection capability without using additional HCs. Moreover, it completely avoids the design constraints of an LCL filter, i.e.,

An Overlap-Time Compensation Method for Current-Source Space-Vector PWM Inverters

\omega _{r}< \omega _{s}/ 6

Parameters Design and Optimization for LC-Type Off-Grid Inverters With Inductor-Current Feedback Active Damping

is required for single-loop converter-side current control. Therefore, a higher resonant frequency can be designed to achieve a wider control bandwidth and to lower the current distortion caused by the paralleled filter capacitor branch. Experimental results are finally presented to verify the proposed control, which are also in good agreement with theoretical analysis.

Improving weak grids adaptability of ZCZ-filtered grid-connected converters with delay-compensated capacitor-voltage feedforward control

In voltage-source inverters, it is necessary to add the dead time between gating signals of the upper and lower switches to prevent short circuit in a phase leg. But in current-source inverters (CSI), due to the large series inductor in dc link, it is necessary to add the overlap time in gating signals of switches during the commutation process for avoiding voltage spikes generated by the intermittent dc current. However, the overlap time will cause negative influence in the output current waveform. In this paper, the current commutation process of CSI within a carrier period is investigated, which shows that the overlap time can cause output-current-vector errors. The output-current-vector errors are then analyzed, which are equivalent to a superposition and introducing additional low-order harmonics in the output current waveform. The relationship between harmonic distortion and the overlap time is derived. An overlap-time compensation method is then proposed, which compensates the dwell time of vectors according to the relations of voltages at the ac side of the inverter. The proposed overlap-time compensation method can effectively decrease the current harmonics introduced by the overlap time and improve the output current of the inverter. Simulation and experimental results are finally provided to verify the feasibility and validity of the proposed method.