Search

Pole-zero cancellation technique has drawn much attention to tackle the filter resonance of the LCL-type grid-connected inverter for simplifying the stability-oriented design. However, its potential stability risk was rarely concerned. As one representative of this technique, the weighted average current (WAC) control is assessed. It is found that the LCL resonance in the state variable WAC is cancelled out by pole-zero cancellation, but the ones in some uncontrollable or unobservable state variables such as the grid current are still inherited, resulting in harmonic instability. From this perspective, this paper appeals that this technique should be cautiously used or even avoided. An extended WAC control which avoids pole-zero cancellation is proposed accordingly. The theoretical expectations are finally validated by simulation and experimental results.

Resonant pole cancelation is usually adopted in the current control or active damping to tackle the filter resonance in the LCL -type grid-connected inverter. However, its potential impact on the system stability has not been adequately assessed. This article provides a comprehensive investigation on three typical control schemes that utilize resonant pole cancelation. An important finding is drawn that the resonant pole cancelation violates the so-called internal stability, which can give rise to the undesired harmonic oscillation in grid current. From the physical insights, it is revealed that the resonant pole cancelation does not essentially eliminate the LCL resonance but merely “hide” it in the system, which can still be triggered and, thereby, turns into the culprit of the harmonic oscillation. The theoretical expectation is validated by the simulation and experimental results.

The feedforward schemes of the voltage at point of common coupling (PCC) have been widely used in grid-connected inverters to reject the current harmonics caused by the grid voltage distortion. However, in weak grid, the PCC-voltage feedforward tends to destabilize the grid-connected inverters due to the effect of time delay. In this article, this stability issue is explicitly elaborated by the impedance-based stability criterion and then addressed with an improved feedforward scheme, which uses the grid voltage instead of the PCC voltage as the feedforward variable. Considering that it is hard to sense the real grid voltage directly, a method to extract the grid voltage from the sensed PCC voltage is put forward to implement the proposed feedforward scheme. By carefully arranging the sampling instants according to the duty cycle, a dual sampling mode is adopted to ensure an accurate extraction of the grid voltage. Finally, simulations and experiments are performed on a 6-kW single-phase grid-connected inverter, which confirm that the proposed grid-voltage feedforward achieves superior harmonic rejection ability and strong stability under weak grid condition.

Both the capacitor-current-feedback (CCF) active damping and the point of common coupling (PCC) voltage feedforward can provide damping for the LCL -type grid-connected inverter. They are usually individually adopted, and negative damping will occur in a certain frequency range due to the digital control delay, leading to a nonminimum phase behavior. In this article, a hybrid active damping that combines the CCF and unit PCC voltage feedforward is studied. With properly designing the CCF gain, the positive damping range could sweep the entire frequency range with the variation of grid impedance. As a reward, the maximum profit of damping cooperation can be harvested, ensuring high robustness against both grid impedance variation and filter parameter fluctuation. The simulation and experimental results are provided to verify the effectiveness of the hybrid active damping.

Capacitor-current-feedback active damping has been widely used in LCL -type grid-connected inverters. However, the damping performance is deteriorated due to the negative equivalent resistance resulted by the digital control delays, and thus the grid-connected inverter is apt to be unstable under the grid impedance variation. To address this issue, this paper proposes the capacitor-current proportional-integral (PI) positive feedback active damping method that can ensure a positive equivalent resistance almost within the Nyquist frequency, i.e., the full controllable frequency range. In theory, the proposed damping method can be implemented by feeding back the capacitor current through a PI function. However, the integral term will continuously accumulate the noise and dc bias arising from the feedback signal. To overcome this drawback, a more practical implementation solution is drawn in this paper. Furthermore, a straightforward design procedure is presented for the convenience of selecting the proper controller parameters. With the proposed damping method and its practical implementation, high inverter robustness against the grid impedance variation can be achieved. Experiments are performed on a 6-kW prototype and the experimental results are in well agreement with the theoretical expectations.

When a grid-connected inverter is connected to a weak grid, the system may be unstable. An active damper can be connected to the point of common coupling (PCC), which simulates a virtual resistor to dampen the resonance and thus stabilize the system. In this paper, an adaptive tuning method of the virtual resistor is proposed, which can automatically regulate the virtual resistor to the critical value to stabilize the system, and thus reduce the power loss. Furthermore, the active damper is designed not to respond to the dominant low-frequency harmonic components in the PCC voltage introduced by the grid background harmonics, so that its power loss can be further reduced. In order to make the active damper more accurately simulate the virtual resistor in a wide frequency range, a harmonic-current-reference compensation method is proposed. The prototypes of a 6-kW grid-connected inverter and a 1-kVA active damper are built and tested to verify the effectiveness of the proposed control scheme of the active damper.

Single-loop current control is an attractive scheme for the LCL -type grid-connected inverter due to its simplicity and low cost. However, conventional single-loop control schemes, which command either the inverter current or the grid current, are subject to the specific resonance frequency regions. The weighted average current control, which splits the filter capacitor into two parts (in form of an LCCL filter) and commands the current flowing between these two parts, is independent of the resonance frequency, but on the other hand, it is limited by the poor sensitivity to the grid impedance variation and weak stability in the grid current. These limitations are comprehensively explained in this paper and then addressed by identifying that the single-loop weighted average current control is equivalent to the dual-loop grid current control with an inherent capacitor current active damping. By tuning the capacitor split proportion as a second degree of freedom, an optimal damping performance that is robust to the grid impedance variation can be naturally achieved using only the inherent damping. Thus, no extra damping is required, and the single-loop structure with only one current sensing turns to be adequate. Moreover, for convenience of practical implementation, an improved LCCL filter configuration is proposed to allow the use of two equal nominal capacitances for the split capacitors. Finally, experiments are performed to verify the effectiveness of the proposed method.

For the LCL -type grid-connected inverter, there are basically three current control schemes, namely the grid current control, the inverter-side inductor current control, and the weighted average current control. This paper builds a general mathematical model to describe the three current control schemes. In this model, the grid current is an equivalent target control variable, the capacitor current feedback serves as a damping solution, and the computation and pulse-width modulation delays are taken into account. Based on the general mathematical model, a comparative analysis of different control schemes is carried out in terms of the grid current stability. It reveals that when the inverter-side inductor current is controlled, the grid current shows the same stability as the inverter-side inductor current; but when the weighted average current is controlled, both the grid current and the inverter-side inductor current are critically stable even though the weighted average current can be easily stabilized. Moreover, the general mathematical model also provides a unified perspective to design different control schemes, which makes the controller parameter tuning more straightforward and effective. In this way, a set of controller parameters which yields high robustness against the grid-impedance variation can be selected for all the three current control schemes. Finally, a 6-kW prototype is built, and experiments are performed to verify the theoretical analysis.

Capacitor-current-feedback active damping is an effective method to suppress the $LCL$ -filter resonance in grid-connected inverters. However, due to the variation of grid impedance, the $LCL$ -filter resonance frequency will vary in a wide range, which challenges the design of the capacitor-current-feedback coefficient. Moreover, if the resonance frequency is equal to one-sixth of the sampling frequency $(f_{s}/6)$ , the digitally controlled $LCL$ -type grid-connected inverter can be hardly stable no matter how much the capacitor-current-feedback coefficient is. In this paper, the optimal design of the capacitor-current-feedback coefficient is presented to deal with the wide-range variation of grid impedance. First, the gain margin requirements for system stability are derived under various resonance frequencies. By evaluating the effect of grid impedance on gain margins, an optimal capacitor-current-feedback coefficient is obtained. With this feedback coefficient, stable operations will be retained for all resonance frequencies except $f_{s} /6$ . Second, in order to improve system stability for a resonance frequency of $f_{s} /6$ , a phase-lag compensation for the loop gain is proposed. Finally, a 6-kW prototype is tested to verify the proposed design procedure.

Two-stage single-phase inverters have been widely used as they can achieve voltage matching and galvanic isolation between the input and output. Due to the pulsating output power of the downstream inverter, an ac current at twice the output frequency, which is called second harmonic current (SHC), arises in the input side of the downstream inverter. This SHC will penetrate to the front-end dc-dc converter, leading to reduced conversion efficiency. This paper first analyzes the propagation mechanism of the SHC and load transient response of two-stage single-phase inverters from the viewpoint of output impedance. Then, based on the current mode control and load current feed forward, two control methods to achieve low SHC in the front-end dc-dc converter and fast dynamic performance during load transient are proposed in this paper. Finally, a 2-kW two-stage single-phase inverter prototype has been constructed and tested, and the experimental results are provided to verify the effectiveness of the proposed control methods.

Due to the effect of the computation and modulation delays on the capacitor-current-feedback active damping, the digitally controlled LCL-filtered inverter tends to be unstable as the LCL-filter resonance frequency approaching to one-sixth of the sampling frequency. Therefore, to guarantee sufficient stability margins, the guideline for choosing the LCL-filter resonance frequency is proposed in this paper. After the resonance frequency is selected, a systematic design method is proposed to facilitate the selection of the proper controller parameters. With this design method, a satisfactory region of the controller parameters for meeting the system specifications is obtained, from which the proper controller parameters can be easily determined. Moreover, it is convenient and explicit to optimize the system performance according to the satisfactory region. A 6-kW prototype is built and tested. The simulation and experimental results validate the theoretical analysis.

This paper investigates the capacitor-current-feedback active damping for the digitally controlled LCL-type grid-connected inverter. It turns out that proportional feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor due to the computation and pulse width modulation (PWM) delays. The LCL-filter resonance frequency is changed by this virtual impedance. If the actual resonance frequency is higher than one-sixth of the sampling frequency (fs/6), where the virtual impedance contains a negative resistor component, a pair of open-loop unstable poles will be generated. As a result, the LCL-type grid-connected inverter becomes much easier to be unstable if the resonance frequency is moved closer to fs/6 due to the variation of grid impedance. To address this issue, this paper proposes a capacitor-current-feedback active damping with reduced computation delay, which is achieved by shifting the capacitor current sampling instant towards the PWM reference update instant. With this method, the virtual impedance exhibits more like a resistor in a wider frequency range, and the open-loop unstable poles are removed; thus, high robustness against the grid-impedance variation is acquired. Experimental results from a 6-kW prototype confirm the theoretical expectations.

This letter investigates the magnetic integration of the LCL filter in grid-connected inverters. By sharing an ungapped core and arranging the windings properly, the fundamental fluxes generated by the two inductors of an LCL filter cancel out mostly in the common core. Thus, the common core with low flux level can be dramatically reduced. Although the reluctance of the common core can hardly be zero, which implies an inevitable coupling between the integrated inductors, the proposed magnetic integration scheme is still attractive if the cross-section area and magnetic material of the common core are made reasonable. Experimental results from both single-phase and three-phase grid-connected inverters verify the effectiveness of the proposed method.

Due to the significance of extracting the grid voltage information, the grid synchronization system plays an important role in the control of grid-connected power converters, and various grid voltage synchronization schemes have been proposed. This paper adopts the complex-vector-filter method (CVFM) to analyze the grid synchronization systems. With this method, the pairs of scalar signals, for example, the α- and β-axis components in the stationary α-β frame, are combined into one complex vector. As a consequence, the grid synchronization systems can be described with the complex transfer functions, which is very convenient to evaluate the steady-state performance, for example, the fundamental and harmonic sequence decoupling/cancellation, and dynamic performance of these systems. Moreover, the CVFM also provides a more generalized perspective to understand and develop the grid synchronization systems. Therefore, some of the representative systems are reanalyzed with the CVFM in this paper. A generalized second-order complex-vector filter and a third-order complex-vector filter are proposed with the CVFM to achieve better dynamic performance or higher harmonic attenuation. Moreover, a brief comparison of the complex-vector filters analyzed in this paper is presented. The effectiveness of the CVFM and the proposed two complex-vector filters are verified by the simulation and experimental results.

The injected grid current regulator and active damping of the LCL filter are essential to the control of LCL-type grid-connected inverters. Generally speaking, the current regulator guarantees the quality of the injected grid current, and the active damping suppresses the resonance peak caused by the LCL filter and makes it easier to stabilize the whole system. Based on the proportional-integral (PI) and proportional-resonant (PR) compensator together with capacitor-current-feedback active-damping which are widely used for their effectiveness and simple implementations, this paper proposes a simple step-by-step controller design method for the LCL-type grid-connected inverter. By carefully dealing with the interaction between the current regulator and active damping, the complete satisfactory regions of the controller parameters for meeting the system specifications are obtained, and from which the controller parameters can be easily picked out. Based on these satisfactory regions, it is more convenient and explicit to optimize the system performance. Besides, the insight of tuning the controller parameters from these satisfactory regions is also discussed. Simulation and experimental results verify the proposed step-by-step design method.

A family of isolated buck-boost dc/dc converter for wide input-voltage range is proposed in this paper, and the full-bridge (FB) boost converter, being one of the typical topologies, is analyzed. Due to the existence of the resonant inductor (including the leakage inductor), the FB-boost converter can only adopt the two-edge-modulation (TEM) scheme with the FB cell being leading-edge modulated and the boost cell being trailing-edge modulated to minimize the inductor current ripple over the input-voltage range, and a phase-shift-control-scheme-based TEM with the use of the market available controller IC such as UC3895 is proposed, which realizes phase-shifted control for the FB cell to achieve zero-voltage switching. In order to improve the reliability and efficiency of the FB-boost converter, a three-mode dual-frequency control scheme is proposed, in which the FB-boost converter operates in boost, FB-boost and FB modes in low, medium and high input voltage regions, respectively, and for which the expression of the inductor current ripple is derived in this paper. As the input voltage in the FB-boost mode is close to the output voltage, the inductor current ripple in this mode is much smaller than that in the other modes, and the switching frequency of the boost cell in this mode can be lowered to one-(2N+1)th of the preset switching frequency to reduce the switching loss, and hence, to improve the efficiency. A 250-500 V input, 360 V output, and 6 kW rated power prototype is fabricated to verify the effectiveness of the design and control method. The average efficiency over the input-voltage range is 96.5%, and the highest efficiency attained is 97.2%.

The grid-connected inverter with an LCL filter has the ability of attenuating the high-frequency current harmonics. However, the current distortion caused by harmonics in the grid voltage is difficult to be eliminated. Increasing the loop gain can reduce the current distortion, but this approach is compromised by the system stability requirement. Without increasing the loop gain, applying feedforward of the grid voltage can suppress the effect of grid voltage harmonics. This paper proposes the feedforward function of the grid voltage for the grid-connected inverter with an LCL filter. Specifically, the proposed feedforward function involves proportional, derivative, and second derivative of the grid voltage, and can be simplified according to the dominant harmonics in the grid voltage. The proposed feedforward scheme can effectively suppress the current distortion arising from the grid voltage harmonics, and the steady-state error of the injected current can be substantially reduced even if a conventional proportional and integral regulator is applied. A 6-kW experimental prototype has been tested to verify the effectiveness of the proposed feedforward scheme.