Three-phase Grid-connected Photovoltaic Inverter Research Articles

Control approach of three‐phase grid connected PV inverters for voltage unbalance mitigation in low‐voltage distribution grids

Low-voltage distribution grids (LV) experience unbalanced voltage condition due to the high penetration levels of rooftop mounted single-phase photovoltaic (PV), the existence of single-phase transformers, unbalanced cable impedances and different load levels on each phase. The voltage unbalance (VU) has a harmful impact on the loads and equipment connected to grid that leads to overheating and fast thermal ageing. There are several solutions proposed to mitigate the VU, that requires the installation of extra hardware components or incorporate complex type of controllers that need special tuning procedures and special arrangements. Therefore, in this study a new and simple control approach of three-phase grid connected PV inverter is proposed to mitigate the unbalanced voltage. The new control approach is able to fully mitigate the unbalanced voltage without the need to extra hardware installation or negative sequence voltage magnitude and phase angle measurements, provided that the ampacity of the converter is high enough. The new system is tested in a LV grid with high PV penetration from Germany. The results show that the VU factor did not exceed 0.8% at the feeder connection point.

Real-Time Testing of a Fuzzy-Logic-Controller-Based Grid-Connected Photovoltaic Inverter System

This paper presents a novel fuzzy-logic-based high-performance control of a three-phase photovoltaic grid-connected inverter. With the aid of the inverter model and fuzzy-logic-based voltage and current-control schemes, a digital signal processor controller board DS1104 generates the sinusoidal pulsewidth modulated signals for the inverter operation in both stand-alone and grid-connected modes. An inverter prototype was built to verify the effectiveness of the control algorithm. The system demonstrates stable ac output voltage satisfactorily during both transient and steady state with grid and load disturbances. The control system generates 2.48% and 4.64% voltage and current total harmonic distortions, respectively. The output waveforms such as output voltage, injected current, and the system power flow are presented to validate the effectiveness of the control strategy.

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 Three-Phase Grid-Connected Photovoltaic Inverters Using Electrolytic or Polypropylene Film Capacitors

In grid-connected photovoltaic (PV) power stations, improving the life expectancy and long-term reliability of three-phase PV inverters is urgently needed to match the significantly higher lifetime of the PV modules. A key contribution toward such improvement is replacing the conventional electrolytic film capacitors by metallized polypropylene film ones. This paper presents a detailed evaluation of a conventional three-phase grid-connected PV inverter performance when replacing the electrolytic capacitor with a minimum value of metallized polypropylene film capacitor. Although the minimum dc bus capacitance leads to higher voltage ripples, such ripples were found to be within acceptable limits to operate the inverter satisfactorily. Simulation and experimental results are presented for a 5-kW grid-connected inverter prototype with a nominal dc voltage of 457 V to confirm the theoretical considerations.

Design of the Three-Phase Photovoltaic Grid-Connected Inverter

Photovoltaic grid-connected power generation is the inevitable development trend of solar photovoltaic systems,grid inverter is an essential part of photovoltaic grid-connected power generation. With the developed three phase current type PWM inverter device as the research object, and symmetry rules sampling method is applied to the inverter, through the method combined by the computer simulation and experiment research, the device was verified by simulation and experimental .The results show that: This method could not only effectively reduce the DC side inductance volume of the current-mode PWM inverter , and it could make output AC currents sinusoidal and in phase with grid voltages, but also improve the efficiency of the system, it will be more suitable for photovoltaic grid-connected.

Design and Control Scheme of Three-Phase Photovoltaic Grid-Connected Inverter

With the development of power electronics technology, a variety of unbalanced load are connected to the grid, causing the grid voltage imbalance and power quality decline, seriously affect the user's normal use. Traditional three-leg inverter can output three-phase symmetrical voltage in the case of three-phase symmetrical load. But it is difficult to output three-phase symmetrical voltage when the three-phase load is asymmetric. In order to solve the impact of unbalanced load on the power quality, the three-phase four-leg inverter which is obtained with the addition of a bridge arm on the basis of traditional three-leg inverter is adopted in this paper. Therefore, the three-phase symmetrical voltage can be acquired in the case of unbalanced load, at the same time, with the advantages of simple structure, small size, light weight, higher voltage utilization.

Application and Simulation of SVPWM Control Techniques in Three-Phase PV Grid-Connected Inverter

The space vector pulse width modulation (SVPWM), as a new kind of vector control technique, has the advantages of high voltage utilization, less distorted output, simple control method, etc. In this paper, the control technique is applied to three-phase photovoltaic (PV) grid-connected inverter system. It uses double loop control method combined with PI control technique. The simulation model of PV grid-connected inverter system is established in Matlab/Simulink. The simulation results show that the application of SVPWM technique effectively improves the dynamic performance of PV grid inverter system and the THD of grid-connected current is low.

Application of DE Algorithm over Control of Grid-Connected PV Inverter

A novel method for optimum vector PWM (OVPWM) control of three-phase grid-connected photovoltaic inverter was described. The control vector and its corresponding action duration composed of the PWM sequences generated by optimum vector PWM strategy. A Differential Evolution (DE) algorithm was applied to determine the action duration of the output voltage vector based on conventional vector PWM (CVPWM). The Matlab-based model of three-phase grid-connected photovoltaic generation system was established. Comparative analysis proving superiority of the OVPWM technique over the CVPWM, and the results of the simulation confirmed the feasibility and advantage of proposed OVPWM strategy.

Reduction of Current Harmonic Distortion in Three-Phase Grid-Connected Photovoltaic Inverters via Resonant Current Control

The resonant current control has been extensively employed to reduce the current harmonic distortion in a wide range of grid-connected distributed generation applications, including photovoltaic (PV) inverters, wind and water turbines, and fuel-cell inverters. However, the performance of these systems is deteriorated when the utility grid voltage experiences abnormal conditions such as voltage harmonics and imbalances. Several advanced control solutions have been recently introduced to cope with this problem but at the cost of a significant increase in the control computational load. This paper first analyzes the limitations of the standard resonant current control operating under abnormal grid conditions and then introduces a control scheme that improves the current harmonic distortion in such adverse conditions without increasing the computational load of the standard current control. This theoretical contribution is validated by means of selected experimental results from a three-phase PV inverter.

Research on Combined Control Strategy of Three-Phase Photovoltaic Grid-Connected Inverter Based on Fuzzy PI Controller

The control objective of three-phase photovoltaic (PV) grid-connected inverter is to generate high quality and stable AC sinusoidal output power with the same phase angle, frequency and amplitude compatible with the grid-connected voltage. To overcome the shortcoming of the normal current PWM control method and traditional PI algorithm, this paper researches a combined control strategy of three phase PV grid-connected inverter based on fuzzy PI controller. The combined control strategy switches different PWM current control method among the hysteretic PWM, SVPWM, hysteretic current SVPWM according to the error between command current and feedback current，and the d-axis command current is output of fuzzy PI controller of DC bus voltage. The simulation model of the combined control strategy for three phase PV grid-connected inverter based on normal PI controller is built with Matlab, simulation results verified the combination control strategy based on fuzzy PI controller has excellent performances.