Single-phase Grid-connected Inverter Research Articles

Review of Dual-Buck-Type Single-Phase Grid-Connected Inverters

Shoot-through issue of traditional bridge-type grid-connected inverters (GCIs) imposes a risk to GCIs' reliability and efficiency, so dual-buck-type single-phase GCIs are widely used in the applications requiring high reliability and high efficiency. These types of GCIs are reviewed in this article. As there is no shoot-through issue in the reviewed inverters, reliability of the inverters is improved. The dead time between switches are not required, so quality of the grid current is high. Freewheeling current flows through the independent freewheeling diode instead of the body diode of the switch, so efficiency of the system can be improved. Topologies of the dual-buck-type single-phase GCIs can be divided into two categories, i.e., buck and boost GCIs. Derivation of the reviewed topologies is given. Modulation strategies of the presented GCIs are illustrated. Different dual-buck-type topologies are compared. Finally, applications of the dual-buck-type single-phase GCIs are given.

An Inverter Model Simulating Accurate Harmonics With Low Computational Burden for Electromagnetic Transient Simulations

The electromagnetic transient (EMT) simulation of a power system involving power-electronics converters requires a fairly small time-step size to consider switching of the converters, thus leading to a heavy computational burden. To accelerate such simulations, this article generalizes the time average method (TAM), originally developed for real-time simulations, so that it becomes suitable to offline EMT simulations. For obtaining accurate current waveforms with a large time step, the TAM and the proposed method represent each leg of an inverter by voltage sources, and its output voltage is modified by interpolation at an instance of switching. The original TAM was intended for the primitive backward Euler method. This article contributes to generalize it for the trapezoidal integration method, which is widely used in offline simulation programs. In addition, the proposed method uses a simple formula to identify the switching instance for the implementation on off-the-shelf PCs, rather than a hardware counter in an field programmable gate array as used in the TAM. This article shows that the proposed method enables to extend the time step by a factor of five without deteriorating the accuracy. A case study demonstrates reduction of computational time by a factor of three for the offline simulation of a single-phase grid-connected inverter with reasonable reproduction of harmonics.

A finite control set model predictive control scheme for single-phase grid-connected inverters

The present article investigates a control scheme for single-phase grid-connected inverter based on the finite control set model predictive control (FCS-MPC) approach. The proposed grid integration scheme provides direct control of the active and reactive power (PQ) injected to the grid from distributed energy resources (DER) composed of a photovoltaic (PV) array as a renewable resource integrated with a battery bank as energy storage. The direct PQ control scheme includes also low voltage ride through (LVRT) capability of the inverter during voltage sag.The optimum inverter switching state is indicated via the employed FCS-MPC algorithm instantaneously such that the actual PQ, injected to the grid, will track the corresponding reference values.During normal operation, the active power, injected to the grid, is set to the maximum possible value, while the reactive power is nil. Under voltage sag, the PQ will be injected to the grid as a function of the percentage of voltage sag based on the existing grid codes and regulations.In single-phase systems, successful application of direct PQ control depends on accurately creating the fictitious orthogonal components of grid current and voltage required for instantaneous power computations. Therefore, the following three different orthogonal signal generation (OSG) methods are utilized in this study in order to create such virtual (quadrature) components: second-order generalized integrator (SOGI), all-pass filter (APF), and quarter cycle phase delay (QCPD). In addition, qualitative and quantitative analyses of the obtained results are involved in this study of the FCS-MPC PQ system in order to compare the three adopted OSG methods. The PSIM® software was utilized in this study for modeling and studying the overall FCS-MPC system and the DER grid integration system.According to the results, the investigated FCS-MPC approach is effective in providing quick response and flexible control of PQ injected to the grid through the use of the three OSG methods. Moreover, the investigated FCS-MPC scheme successfully involves the LVRT option in order to improve the dynamic grid voltage support during the permissible duration of voltage sag following the LVRT profiles and grid codes of many countries.

First-Order and High-Order Repetitive Control for Single-Phase Grid-Connected Inverter

With the increasing demand of users for power sources and quality, how to provide high-quality renewable clean energy has become a key issue of power electronics. The main idea of this paper is to develop a composite control including a PI control and repetitive control for a single-phase grid-connected inverter to eliminate the effects of harmonics, which can obtain better steady-state and dynamic responses of the single-phase inverter system and reduce the net current harmonics. The modelling of a single-phase inverter is first introduced; then a first-order repetitive control is developed for the proposed grid-connected inverter. Moreover, a high-order repetitive controller is adopted to further improve the robustness against the uncertainties in the period of signals. The stability and performance analysis are given for the first-order repetitive control and high-order repetitive control. Finally, comparative simulations are conducted in a circuit-level inverter model, which show the effectiveness of the proposed method.

Capacitor Voltage Full Feedback Scheme for LCL-Type Grid-Connected Inverter to Suppress Current Distortion Due to Grid Voltage Harmonics

For the LCL -type grid-connected inverter, grid voltage full feedforward scheme is an effective method to improve the quality of the injected grid current. However, in the applications where a step-up transformer is adopted and its leakage inductance serves as the grid-side inductor, it is difficult to directly measure the grid voltage. For this case, the capacitor voltage full feedback scheme is proposed in this article to suppress the injected grid current distortion due to the grid voltage harmonics, and the full feedback function includes proportional, derivative, and second-derivative components. It is found that the derivative component counteracts the capacitor current feedback active damping, and both of them can be removed. Thus, the current sensor for sensing the capacitor current is saved. Instead, the LCL filter resonance is damped by the proportional and second-derivative feedback components, and a low-pass filter is added to the second-derivative component for ensuring a positive equivalent resistance within the full controllable frequency range. Experimental results from a 6-kW single-phase grid-connected inverter are provided to verify the effectiveness of the proposed method.

A Robust Grid-Voltage Feedforward Scheme to Improve Adaptability of Grid-Connected Inverter to Weak Grid Condition

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.

Improvement of the power quality of single‐phase grid‐connected inverter by filter‐based control scheme

In this study, a filter-based control scheme is developed for a single-phase grid-connected inverter system with the local load. Improving the quality of the local load voltage in the grid-connected mode and injecting clean current to the grid at the same time is the main objective of the proposed scheme. Moreover, unity power factor at the grid side for different types of the local load is ensured by the proposed controller. Furthermore, this control approach ensures the seamless transfer between grid-connected and stand-alone operation modes without adjusting the controller structure and without any resynchronisation scheme. The proposed scheme is validated by the Lyapunov stability analysis. Moreover, an experimental testbed has been implemented to further validate the controller in the real-time environment, as well as, the performance is compared to a conventional approach.

A Novel Selective Harmonic Compensation Method for Single-Phase Grid-Connected Inverters

Conventional harmonic compensation techniques have several drawbacks, such as: 1) additional hardware needs; 2) complex control structure; 3) difficulties in parameter tuning; 4) issues in applicability with the reference frame of the fundamental current controller. In addition, most of them are not good enough in terms of compensating harmonics under the light load condition, which can be mainly attributed to the inaccurate detection of harmonics. In order to cope with the abovementioned problems, a novel digital lock-in amplifier (DLA)-based harmonic compensation method is proposed in this article. Since the proposed method can detect harmonics very accurately without additional hardware, it is simple, robust, and easy to implement. The detected amplitude and phase information of a certain harmonic are used to reconstruct the harmonic and it is used to compensate that harmonic through a negative feedback loop. The theoretical background of the DLA, mathematical analysis, simulation, and experimental results are presented to corroborate the validity and feasibility of the proposed harmonic compensation method. A 5-kW single-phase grid connected inverter is built to verify the performance of the proposed harmonic compensation method by a comparison with those of conventional harmonic compensation methods.

Analysis and design of L + LCL‐filtered dual‐frequency single‐phase grid‐connected inverter

To increase the efficiency of the grid-connected inverter, this study proposes an L + LCL-filtered dual-frequency single-phase grid-connected inverter. The proposed inverter consists of the low-frequency unit and high-frequency unit. The low-frequency unit transmits power to the grid at the low switching frequency. The high-frequency unit uses the feed-forward method to eliminate grid switching current ripples without current harmonics detection. The passive damping LCL filter is used to attenuate high-frequency unit switching harmonics. The operation principle of the proposed inverter is analysed, and the parameter design method of the inverter is introduced. An experimental prototype is built to test the performances of the inverter. Simulation and experimental results show that the grid current harmonics are effectively suppressed, and the efficiency of the proposed inverter can be higher than that of the L-type inverter and LCL-type inverter.

A Stationary Reference Frame Current Control Algorithm for Improvement of Transient Dynamics of a Single Phase Grid Connected Inverter

This paper proposes a stationary reference frame current control algorithm for a single-phase grid-connected inverter (GCI) for improvement of transient dynamic performance. Disturbance, i.e., grid voltage in a target system, is estimated using a stator current observer, and the estimated disturbance is applied to a current controller for implementation of disturbance rejection control (DRC). In the proposed current control algorithm, the disturbance rejection control algorithm is applied to reduce the overcurrent occurring in the single-phase grid-connected inverter when grid faults happen. In this paper, the AC phase current of a single-phase inverter is controlled, instead of the current vector, which is a DC signal. To compensate for the drawbacks of controlling the AC phase current, such as phase lag and steady-state error, command feedforward control is also applied in the proposed control system. The proposed control algorithm is mathematically derived and represented in transfer functions and implemented via simulation and experiment.

A Fast and Smooth Single-Phase DSC-Based Frequency-Locked Loop Under Adverse Grid Conditions

This article proposes a frequency-locked loop (FLL) with fast and smooth dynamic performance for single-phase grid-connected inverters under adverse grid conditions. Different from modeling with the orthogonal signals of the same amplitudes or modeling with the ideal single-phase signal, the proposed linear frequency estimation model is developed with the difference of signal amplitudes that are generated by cascaded delayed signal cancellation operators. Based on this model, an enhanced variable-step-size least mean square algorithm is proposed to construct the FLL, where a smooth factor and a damping factor are integrated. In addition, the convergence analysis and the parameter tuning are also given. Compared with some well-known methods, the proposed FLL can provide smoother dynamic performance with relatively fast response speed under grid conditions with small frequency deviation or different levels of phase/amplitude jump, which are verified by experiments.

Leakage Current Reduction in Single-Phase Grid-Connected Inverters—A Review

The rise in renewable energy has increased the use of DC/AC converters, which transform the direct current to alternating current. These devices, generally called inverters, are mainly used as an interface between clean energy and the grid. It is estimated that 21% of the global electricity generation capacity from renewable sources is supplied by photovoltaic systems. In these systems, a transformer to ensure grid isolation is used. Nevertheless, the transformer makes the system expensive, heavy, bulky and reduces its efficiency. Therefore, transformerless schemes are used to eliminate the mentioned disadvantages. One of the main drawbacks of transformerless topologies is the presence of a leakage current between the physical earth of the grid and the parasitic capacitances of the photovoltaic module terminals. The leakage current depends on the value of the parasitic capacitances of the panel and the common-mode voltage. At the same time, the common-mode voltage depends on the modulation strategy used. Therefore, by the manipulation of the modulation technique, is accomplished a decrease in the leakage current. However, the connection standards for photovoltaic inverters establish a maximum total harmonic distortion of 5%. In this paper an analysis of the common-mode voltage and its influence on the value of the leakage current is described. The main topologies and strategies used to reduce the leakage current in transformerless schemes are summarized, highlighting advantages and disadvantages and establishing points of comparison with similar topologies. A comparative table with the most important aspects of each converter is shown based on number of components, modes of operation, type of modulation strategy used, and the leakage current value obtained. It is important to mention that analyzed topologies present a variation of the leakage current between 0 to 180 mA. Finally, the trends, problems, and researches on transformerless grid-connected PV systems are discussed.

Offset error compensation algorithm for grid voltage measurement of grid-connected single-phase inverters based on SRF-PLL

This paper proposes a method for compensating the offset error in the grid voltage measurement process of grid-connected single-phase inverters. In general, the offset error in the grid voltage sampling process results in a fundamental frequency component in the synchronous reference frame phase locked loop (SRF-PLL). As a result, the dq-axis currents and phase current based on the synchronous reference frame PI current regulator include the unwanted DC component, as well as first- and second-order harmonic ripples when compared with the grid frequency due to the distorted grid angle. Therefore, in this paper, the influences of the offset error are mathematically analyzed based on the SRF-PLL. In particular, the d-axis integrator output of the SRF-PLL with a PI controller is selected to detect the offset error. Then it is compensated using a simple proportional-integral controller. Moreover, the root mean square function is easily applied to obtain the offset error. The usefulness of the proposed algorithm is verified through simulation and experimental results.

Analysis and Improved Design of Phase Compensated Proportional Resonant Controllers for Grid-Connected Inverters in Weak Grid

Phase compensated proportional resonant (PR) controllers with phase leading angles embedded into resonant controllers, are used to improve the system stability, when the grid-connected inverter is connected to a weak grid with large grid impedance. However, an inappropriate phase leading angle will cause obvious anti-peak in the magnitude response of phase compensated PR controller. Thus, the phase leading angle should be selected carefully. The conventional phase compensation (CPC) method for selecting the phase leading angle can maximize the system phase margin, but at the expense of large peaks in the magnitude response of sensitivity transfer function. Meanwhile, the bandwidths of resonant controllers will be reduced. In the paper, an improved phase compensation (IPC) method is proposed, which does not cause peaks in the magnitude response of sensitivity transfer function while providing sufficient phase margin. Experimental tests were carried out on a single phase grid-connected inverter. The effectiveness of the proposed method is proved in terms of the steady-state and transient performances, as well as the performance under the grid frequency deviation.

Analysis and Implementation of Improved Software Phase-Locked Loop for Single-phase Grid-connected Inverter

For single-phase grid connected inverter, based on the traditional closed-loop structure of three-phase phase-locked loop (PLL), an improved software PLL was proposed. The basic principle of the phase-locked loop was analyzed and its mathematical model was deduced. In addition, the software implementation method of the phase-locked loop was presented. Then, the influence resulted from the change of amplitude, frequency and phase on the grid voltage was simulated with PSIM software. The results show that the software phase-locked loop can realize the phase tracking at any time in the cycle effectively and quickly, without waiting for the zero crossing of the grid. Finally, the software phase-locked loop proposed in this paper was implemented respectively by using TI’s DSP TMS320F28035 and TMS320F2808 and applied to the 500W dual-channel single-phase grid-connected micro-inverter and 5kW single-phase grid-connected inverter. The simulation and experiment results verify the proposed software phase-locked loop.

A novel photovoltaic inverter topology based on common mode current suppression

ABSTRACT Aiming at the problems of large leakage current and low efficiency of existing transformerless single-phase grid-connected photovoltaic inverters, a novel transformerless single-phase grid-connected photovoltaic inverter topology is proposed. In this topology, two freewheeling diodes are added to the single-phase full-bridge inverter, and the filter inductors are moved between the upper and lower bridge arms, so that the common mode voltage remains constant during the switching period and the common mode current is effectively suppressed. Finally, through simulation and the experimental platform of 3kW single-phase grid-connected photovoltaic inverter, it is verified that compared with the traditional full-bridge topology, the novel topology can effectively suppress the common mode current, while compared with the H5 and H6 topology, the novel topology reduces the system loss and improves the system efficiency.

Control Scheme and Performance Analysis of Dual-Frequency Single-Phase Grid-Connected Inverter Interfaced With Weak and Distorted Grids

Grid impedance variation and voltage harmonics may cause instability of the grid-connected inverter system and reduce the grid current quality under weak grid. Switching losses and electromagnetic interference of inverter can be reduced by using lower switching frequency. However, it is a challenging task to achieve stable operation of LCL-type grid-connected inverter using the active damping method at the low switching frequency. In this paper, an improved control strategy is proposed for the dual-frequency inverter, which contains a power inverter and an auxiliary converter. For the proposed control strategy, the ability to suppress the influence of the grid voltage harmonics and the robustness against grid impedance variation are analyzed. In order to reduce the switching losses, the electrical energy is delivered to the power grid through the power inverter at a low switching frequency. By using the switching ripple compensation method and the grid voltage feedforward scheme, the switching current ripple caused by the power inverter and the current harmonics caused by the grid voltage harmonics can be suppressed. Since the filters used in the proposed inverter are all L type filters, the system stability is guaranteed under weak grid. The simulation and experimental results are provided to verify the effectiveness of the proposed grid-connected inverter system under weak grid.

Fuzzy Logic Control for Low-Voltage Ride-Through Single-Phase Grid-Connected PV Inverter

This paper presents a control scheme for a photovoltaic (PV) system that uses a single-phase grid-connected inverter with low-voltage ride-through (LVRT) capability. In this scheme, two PI regulators are used to adjust the power angle and voltage modulation index of the inverter; therefore, controlling the inverter’s active and reactive output power, respectively. A fuzzy logic controller (FLC) is also implemented to manage the inverter’s operation during the LVRT operation. The FLC adjusts (or de-rates) the inverter’s reference active and reactive power commands based on the grid voltage sag and the power available from the PV system. Therefore, the inverter operation has been divided into two modes: (i) Maximum power point tracking (MPPT) during the normal operating conditions of the grid, and (ii) LVRT support when the grid is operating under faulty conditions. In the LVRT mode, the de-rating of the inverter active output power allows for injection of some reactive power, hence providing voltage support to the grid and enhancing the utilization factor of the inverter’s capacity. The proposed system was modelled and simulated using MATLAB Simulink. The simulation results showed good system performance in response to changes in reference power command, and in adjusting the amount of active and reactive power injected into the grid.

Integrated Common and Differential Mode Filter With Capacitor-Voltage Feedforward Active Damping for Single-Phase Transformerless PV Inverters

Recently, several publications have discussed the design of LCL filters and current controllers for grid-connected converters. However, the focus was the LCL filter used as a differential mode (DM) filter. Besides its conventional application as DM filter, a modified LCL filter configuration can be used as an integrated common mode (CM) and DM filter, which is particularly interesting for transformerless photovoltaic (PV) inverters, whereby the CM leakage current must be limited to comply with standards. This article proposes the joint design procedure for an integrated CM and DM filter plus the current controller used in single-phase grid-connected transformerless PV inverters. The increased number of components and resonance frequencies, compared to the conventional LCL filter, requires a careful analysis of the location of resonance and anti-resonance frequencies, taking into account parameters variation. The design procedure considers the extended region of stability for the DM resonance frequency allocation provided by the filter capacitor-voltage feedforward active damping with converter-side current feedback. It also considers continuous and discontinuous unipolar pulsewidth modulation. Design examples and simulation results for 1.5 and 10 kVA are presented. The effectiveness of the proposed design procedure is verified by experimental results obtained from a 1.5 kVA inverter.

An Enhanced Control System for Single-Phase Inverters Interfaced With Weak and Distorted Grids

This paper presents an enhanced current controller for improving the performance of a class of single-phase grid-connected inverters operating in weak and distorted grid conditions. An inverter designed to operate at normal (strong or stiff and clean) grid conditions may not perform satisfactorily during weak and distorted grid conditions. One major reason is the interfering dynamics of the synchronization or phase-locked loop (PLL). This paper proposes an enhanced control structure for a popular class of single-phase inverters to address this problem. The proposed idea is to include the PLL state variables into the main inverter controller thereby minimizing the undesirable interactions of the PLL with the other components. A method for optimally designing the controller gains is also proposed. Compared to the conventional one, the proposed controller is shown to have a more robust performance over a substantially wider range of weak and distorted grid conditions. Extensive simulation and experimental results are presented to validate the proposed controls.