Single-phase Grid-connected Inverter Research Articles

The Output Current Controller of a Single-phase Grid-connected PV Inverter in Discrete Fourier Transform

The output current of grid-connected PV inverters should not excess harmonic contents. Also, because they are driven connected with grid, the reliability of electric power’s quality should be secured. But, the current controllers of the grid-connected PV inverter decrease the electric power’s quality due to not considering the harmonic contents of the output current. This paper proposed the current controllers of the grid-connected PV inverters to supplement the decrease of the electric power’s quality because of the harmonic contents of the electric power’s quality and the harmonic contents of the output current. The proposed current controllers controlled the harmonic contents of the output current by controlling gains of the current controllers in consideration of the harmonic contents of the output current. As a result, when total harmonic distributions of the grid voltage were 0.4% and 4.1%, the excellence of the proposed current controllers was shown through the experience, by obtaining excellent results of being controlled low as the total harmonic distributions of the output current were 0.756% and 0.78% respectively.

Suppression of zero-crossing distortion for single-phase grid-connected photovoltaic inverters with unipolar modulation

For single-phase grid-connected photovoltaic inverters, current-control with unipolar modulation can reduce the losses of power tubes and improve the efficiency compared with using the bipolar modulation. However, it suffers inherent zero-crossing distortion which is related to the grid current and voltage. In this paper, the reason for zero-crossing distortion is analyzed and simulated, also a novel current-control approach with commutating in advance is proposed to resolve this problem. This approach obtains a leading angle of the grid current by using the feedback of proportional-integral controller of the unipolar inverter, and then set the working mode of power tubes according to this angle through commutation.

Analysis and Solution of Current Zero-Crossing Distortion With Unipolar Hysteresis Current Control in Grid-Connected Inverter

In this paper, a kind of zero-crossing distortion (ZCD) generated in the grid-connected current of a single-phase grid-connected inverter with unipolar hysteresis current control is investigated. By establishing the mathematical model of the grid-connected inverter around the zero-crossing points (ZCPs), the reason for causing the current ZCD is presented. It is ascribed to the slope of the actual current less than that of the reference one around the ZCPs. The expressions of the critical phase angle (the starting point of the current ZCD) and the amplitude of the current distortion corresponding to the parameters of the inverter and connected utility grid are obtained, and the solution for the ZCD based on on-line leading phase angle compensation is proposed. Simulation and experimental results verify the accuracy and feasibility of the theoretical analysis and the proposed solution.

An effective passive islanding detection method for PV single-phase grid-connected inverter

This paper presented a low cost and effective passive islanding detection method for single-phase photovoltaic grid-connected inverters. An analog circuit for over/under voltage protection is developed to ensure fast detection and no delay to system. An under/over frequency circuit is also developed, for accurate and fast frequency detection with minimal external components. A new algorithm is developed in a low-cost PIC18F4550. An improved disconnection time in the proposed method compared with that in the previously developed method is an attractive solution for single phase grid connected inverters. The low cost, effective and minimal external component count are the advantages. A prototype is developed and tested to demonstrate the performance and feasibility of the proposed method. The experiment results verified that the proposed islanding detection method able to detect islanding operation effectively under various load types, inverter output powers and quality factors.

Grid-Connected of Photovoltaic Module Using Inverse Optimal Control

Switching converters and photovoltaic panels are nonlinear models, but existing PV inverter control methods are mostly based on the linearization control methods. The traditional linear methods are often difficult to eliminate static errors and the dynamic responses are not very satisfactory. In this paper, on the basis of nonlinear methods to control the switching converter, a mathematical model of discrete rotating coordinate system is given and an inverse optimal control method for single-phase grid-connected inverter control is proposed at the same time. This method achieved independent power control purposes due to implementing active and reactive power decomposition; and it is easier to use a microcontroller digital implementation for the discrete method. Simulation results show that this method can eliminate the static errors, and have a good dynamic response.

Singe-Phase Grid-Connected Inverter Based on Voltage-Oriented Control in Photovoltaic Generation Systems

The paper proposes a new control of a transformerless singe-stage single-phase grid-connected inverter in photovoltaic generation systems. The control scheme is mainly based on voltage-oriented control (VOC) with help of second order Generalized Integrator (SOGI). A cascaded control structure with an outer dc link voltage control loop and an inner current control loop is used. The currents are controlled in a synchronous dq reference frame using a decoupled feedback control. The simulated results have proven an excellent performance and verified the validity of proposed system.

A Sliding-Mode Controller With Multiresonant Sliding Surface for Single-Phase Grid-Connected VSI With an LCL Filter

This paper presents a sliding-mode control (SMC) scheme via multiresonant sliding surface for single-phase grid-connected voltage source inverter with an LCL filter to eliminate the grid current tracking error as well as suppress its total harmonic distortion (THD). In general, the design of SMC leads to a sliding surface that is a linear combination of the system state variables and the generated references. The sliding surface would drift while the system parameters change or external disturbance exists, which affects the tracking error and THD of system output seriously. Moreover, for ac tracking system, integral SMC can reduce, but not fully alleviate the sinusoidal tracking error and has limited ability to suppress the grid current harmonics, especially for high-order harmonics. In order to fully eliminate the grid current tracking error and suppress its THD effectively, multiple resonant terms of the grid current error are added to the sliding function. It is used for the first time in the SMC. This method can be used for an arbitrary ac tracking system. Simulation and experimental results on a 5-kVA single-phase grid-connected inverter prototype show the effectiveness of the proposed control strategy. The tracking precision of the grid current is about 0.91% and the THD of the grid current is 0.76%.

A 2ndOrder Harmonic Compensation Method for Wind Power System Using a PR Controller

This paper proposes a compensation method for the <TEX>$2^{nd}$</TEX>-order harmonic of single-phase grid-connected wind power generation systems. Theoretically, a single-phase grid-connected inverter system has no choice but to cause the <TEX>$2^{nd}$</TEX>-order harmonic to DC-link voltage. The reference active current is affected by the DC-link voltage. The output current from the reference active current is distorted by the <TEX>$1^{st}$</TEX> and <TEX>$3^{rd}$</TEX>-order harmonic. The proposed method can compensate, conveniently, the reference active current with the <TEX>$2^{nd}$</TEX>-order harmonic. To reduce the <TEX>$2^{nd}$</TEX>-order ripple in the reference active current, proposed method takes a PR controller as a feed-forward compensator. PR controllers can implement selective harmonic compensation without excessive computational requirements; the use of these controllers simplifies the method. Both the simulation and experimental results agree well with the theoretical analysis.

Control of Single-Phase Grid-Connected Inverters With Nonlinear Loads

This paper presents a control of single-phase grid-interactive inverters with nonlinear loads. The utility-connected inverter should operate at both stand-alone and grid-connected modes. However, the waveform qualities of the grid current in grid-connected mode and the output voltage in stand-alone mode are poor under the nonlinear critical load with the conventional control. The impact of the nonlinear load on the grid current is analyzed. The proposed control is illustrated in detail. By adding the load current into the filter inductor current loop, the influence of the nonlinear load on the grid current can be eliminated, and the waveform quality of the output voltage in stand-alone mode can be improved. The control method is simple and easy to be achieved. The grid-connected inverter is stable at both modes. The filter inductor is chosen. Simulation and experimental results from a 1-kW dual-buck full-bridge grid-interactive inverter with a diode rectifier load verify the theoretical analysis.

Modularized Bidirectional Grid-Connected Inverter With Constant-Frequency Asynchronous Sigma–Delta Modulation

The objective of this paper is to propose a modularized bidirectional single-phase grid-connected inverter (BGI) with constant-frequency asynchronous sigma-delta modulation (CF-ASDM) control for the renewable energy (RE)-supplied dc power system. The proposed BGI can be operated either in the inverter mode (IM) or rectified mode (RM); both are necessary features for the RE-supplied dc power system. By comparing to the conventional ASDM with variable switching frequency, the proposed CF-ASDM can simplify the filter design and can reduce the computational burden of the digital signal processor while maintaining the merit of quick dynamic response. Moreover, the proposed BGI has a redundant central controller which can increase the system reliability. The major contribution of this paper includes developing the modularized BGI with CF-ASDM control scheme for IM and RM operations thoroughly and applying it to the RE-supplied dc power system successfully. In this paper, the mathematical equations to achieve the constant switching frequency operation for the CF-ASDM will be derived. Computer simulations and hardware experiments are shown to verify the performance of the proposed modularized BGI with CF-ASDM control.

Study of DQ Rotating Frame Control for Single-Phase Photovoltaic Grid-Connected System

Nowadays solar photovoltaic energy has become an important alternative energy sources. This paper focuses on single-phase PV grid-connected system, a reference frame which used for three-phase inverter has been used in this single-phase grid-connected inverter. The in-quadrature signal is generated used T/4 transport delay technique. This single-phase grid-connected inverter uses space vector pulse width modulation(SVPWM) method. Finally, the 3KW small power single-phase photovoltaic inverter system model is built to verify the presented control strategy by using the Matlab/Simulink.

Energy-Balance Control of PV Cascaded Multilevel Grid-Connected Inverters Under Level-Shifted and Phase-Shifted PWMs

This paper presents an energy-balance control strategy for a cascaded single-phase grid-connected H-bridge multilevel inverter linking <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> independent photovoltaic (PV) arrays to the grid. The control scheme is based on an energy-sampled data model of the PV system and enables the design of a voltage loop linear discrete controller for each array, ensuring the stability of the system for the whole range of PV array operating conditions. The control design is adapted to phase-shifted and level-shifted carrier pulsewidth modulations to share the control action among the cascade-connected bridges in order to concurrently synthesize a multilevel waveform and to keep each of the PV arrays at its maximum power operating point. Experimental results carried out on a seven-level inverter are included to validate the proposed approach.

Derivation, Analysis, and Implementation of a Boost–Buck Converter-Based High-Efficiency PV Inverter

In this paper, a single-phase grid-connected transformerless photovoltaic inverter for residential application is presented. The inverter is derived from a boost cascaded with a buck converter along with a line frequency unfolding circuit. Due to its novel operating modes, high efficiency can be achieved because there is only one switch operating at high frequency at a time, and the converter allows the use of power MOSFET and ultrafast reverse recovery diode. It also features a robust structure because the phase leg does not have a shoot-through issue. This paper begins with theoretical analysis and modeling of this boost-buck converter-based inverter. And the model indicates that small boost inductance will lead to an increase in the resonant pole frequency and a decrease in the peak of Q, which results in easier control and greater stability. Thus, interleaved multiple phases structure is proposed to have small equivalent inductance; meanwhile, the ripple can be decreased, and the inductor size can be reduced as well. A two-phase interleaved inverter is then designed accordingly. Finally, the simulation and experiment results are shown to verify the concept and the tested efficiency under 1-kW power condition is up to 98.5%.

단상 계통연계 인버터의 SRF 전력제어 방법

화석에너지의 고갈과 환경오염 문제를 해결하기 위한 대안인 신재생 에너지를 이용하는 분산발전 시스템은 핵심 구성 요소인 PWM 인버터의 전력제어가 요구된다. 본 논문에서는 단상 계통연계 인버터 시스템의 전력제어를 위해 계통 임피던스를 고려한 SRF(synchronous-reference-frame) 전력제어 방법을 제안한다. 제안한 SRF 전력제어 방법은 복잡한 계통 임피던스의 추정 없이 계통 임피던스의 공칭값(nominal value)에 기반해 단상 인버터에 전압 기준값(reference)을 제공하여 독립운전 모드와 계통연계 모드에서 모두 운전이 가능함을 나타내었다. 또한, 유효전력과 무효전력의 독립적인 제어가 가능한 장점을 가진다. 계통을 포함하는 시뮬레이션을 통해 제안한 SRF 전력제어 방법의 타당성을 검증한다. 시뮬레이션 결과는 제안한 SRF 전력제어 방법을 통해 단상 계통연계 인버터 시스템의 전력 흐름을 적절히 제어할 수 있음을 보여주고 있다.

A modified H-bridge multilevel inverter for photovoltaic system

This paper presents a modified H-bridge seven-level single-phase grid-connected inverter for photovoltaic system with a novel pulse-width-modulated (PWM) control scheme. The inverter is capable of producing seven output-voltage levels (Vdc, 2Vdc/3, Vdc/3, 0, -Vdc, -2Vdc/3, -Vdc/3) from the dc supply voltage. Perturb and Observe (P&O) algorithm is used to extract maximum power from the PV module. MPPT and current control algorithm were implemented in an eZdsp board, TMS320F2812. The proposed system was implemented in a prototype.

Control Design Guidelines for Single-Phase Grid-Connected Photovoltaic Inverters With Damped Resonant Harmonic Compensators

The injection of low-harmonic current to the electrical grid is nowadays regulated by international standards. In order to accomplish these standards, selective harmonic compensation is carried out in grid-connected photovoltaic inverters by means of resonant harmonic compensators. This paper gives a systematic design procedure for selecting the gains and parameters of these harmonic compensators. Other factors considered in the design process include frequency deviation, grid synchronization, and transient response. Both a design example meeting the requirements of grid interconnection and selected experimental results from a digital-signal-processor-based laboratory prototype are also reported.

Development of a MATLAB/Simulink Model of a Single-Phase Grid-Connected Photovoltaic System

Because of their deployment in dispersed locations on the lowest voltage portions of the grid, photovoltaic (PV) systems pose unique challenges to power system engineers. Computer models that accurately simulate the relevant behavior of PV systems would thus be of high value. However, most of today's models either do not accurately model the dynamics of the maximum power point trackers (MPPTs) or anti-islanding algorithms, or they involve excessive computational overhead for this application. To address this need, a Matlab/Simulink model of a single-phase grid-connected PV inverter has been developed and experimentally tested. The development of the PV array model, the integration of the MPPT with an averaged model of the power electronics, and the Simulink implementation are described. It is experimentally demonstrated that the model works well in predicting the general behaviors of single-phase grid-connected PV systems. This paper concludes with a discussion of the need for a full gradient-based MPPT model, as opposed to a commonly used simplified MPPT model.

MPPT Scheme for a PV-Fed Single-Phase Single-Stage Grid-Connected Inverter Operating in CCM With Only One Current Sensor

The cost and efficiency of a photovoltaic (PV)-based grid-connected system depends upon the number of components and stages involved in the power conversion. This has led to the development of several single-stage configurations that can perform voltage transformation, maximum power point tracking (MPPT), inversion, and current shaping-all in one stage. Such configurations would usually require at least a couple of current and voltage sensors and a relatively complex control strategy. With a view to minimize the overall cost and control complexity, this paper presents a novel MPPT scheme with reduced number of sensors. The proposed scheme is applicable to any single-stage, single-phase grid-connected inverter operating in continuous conduction mode (CCM). The operation in CCM is desirable as it drastically reduces the stress on the components. Unlike other MPPT methods, which sense both PV array's output current and voltage, only PV array's output voltage is required to be sensed to implement MPPT. Only one current sensor is used for shaping the buck-boost inductor current as well as for MPPT. The information about power output of the array is obtained indirectly from array's voltage and the inductor current amplitude. Detailed analysis and the flowchart of the algorithm for the proposed scheme are included. Simulation and experimental results are also presented to highlight the usefulness of the scheme.

Linear Current Control Scheme With Series Resonant Harmonic Compensator for Single-Phase Grid-Connected Photovoltaic Inverters

This paper deals with an important aspect in the operation of single-phase grid-connected photovoltaic inverters, i.e., injecting low harmonic current into the electrical grid. Specifically, we propose a linear current control scheme with a resonant harmonic compensator connected in series with a tracking regulator (the standard harmonic compensator location is in parallel with the tracking regulator). The series connection provides an efficient attenuation of the grid voltage background distortion, an accurate synchronization with the grid voltage, and a low computational time in relation to the standard control scheme. Experimental results from a digital signal processor-based laboratory prototype validate the features of the proposed control scheme.

Energy-Balance Modeling and Discrete Control for Single-Phase Grid-Connected PV Central Inverters

This paper presents a two-control loop design considering the nonlinear time-varying characteristics of a single-phase grid-connected photovoltaic (PV) full-bridge central inverter. The control scheme design is based on the energy-balance modeling of the PV system and enables the design of a voltage loop linear discrete controller ensuring the stability of the system for the whole range of PV array operating conditions. A set of experimental results carried out on a laboratory prototype is provided to validate the proposed approach.