The application of photovoltaic (PV) as a source of electrical energy in the distributed generation (DG) systems are gaining more attention with the advances in power electronics technology. The one of the key technologies in the PV based DG systems is grid connected inverter that is utilized to interface PV power systems into the utility grid. Multilevel multifunctional grid connected inverters (ML-MFGCIs) are new breed of power converter used in large scale PV applications and have superior advantages such as lower switching power dissipation, lower harmonic distortion and lower electromagnetic interference (EMI) outputs. ML-MFGCIs perform the high quality power from PV systems and provide flexible functionality with improved power quality (PQ), voltage and reactive power support and increased capability of the auxiliary service for the utility grid. This paper presents a detailed analysis of various ML-MFGCI configurations for 1-phase and 3-phase systems and control strategies to compensate the different PQ problems. Almost 100 papers including the practical applications and recent research studies on ML-MFGCIs are reviewed and analysed.

Investigation of multilevel multifunctional grid connected inverter topologies and control strategies used in photovoltaic systems

This paper proposes a novel robust and adaptive sliding-mode (SM) control for a cascaded two-level inverter (CTLI)-based grid-connected photovoltaic (PV) system. The modeling and design of the control scheme for the CTLI-based grid-connected PV system is developed to supply active power and reactive power with variable solar irradiance. A vector controller is developed, keeping the maximum power delivery of the PV in consideration. Two different switching schemes have been considered to design SM controllers and studied under similar operating situations. Instead of the referred space vector pulsewidth modulation (PWM) technique, a simple PWM modulation technique is used for the operation of the proposed SM controller. The performance of the SM controller is improved by using an adaptive hysteresis band calculation. The controller performance is found to be satisfactory for both the schemes at considered load and solar irradiance level variations in simulation environment. The laboratory prototype, operated with the proposed controller, is found to be capable of implementing the control algorithm successfully in the considered situation.

Sliding-Mode Control of PWM Dual Inverter-Based Grid-Connected PV System: Modeling and Performance Analysis

In this paper, a disturbance observer-based fuzzy sliding mode control (DOBFSMC) strategy is proposed for a single-phase PV grid-connected inverter. In the fact that the uncertainties caused by the inverter component parameter variations and the changes of climatic conditions may seriously affect the control performance of the inverter, a disturbance observer is designed to estimate these disturbances in real time and a sliding mode controller designed with the output information of the disturbance observer is employed to control the output voltage of the DC–AC inverter, and a fuzzy system is used to approximate the upper bound of the observation error between the actual disturbance and its observation value in order to improve the performance of the control system. The inverter has strong robustness since the disturbances can be adaptively compensated, and the chattering is greatly reduced since the switching gain can be very small as the upper bound of observation error is estimated by the fuzzy system. Finally, simulation results verify the effectiveness of the proposed method and demonstrate it can work reliably under different conditions.

Disturbance Observer Based Fuzzy Sliding Mode Control of PV Grid Connected Inverter

This paper presents a dynamic droop load sharing scheme based on the available generation capacity of the distributed generation (DG) units. Since conventional droop schemes share loads proportional to units’ ratings, they suffer from the inability to maintain an efficient operating point when their input renewable power varies, without imposing their new operating point on other connected DGs in the microgrid. This problem is mainly due to the insensitivity of the droop scheme to the varying nature of the renewable resources used, including wind and solar photovoltaic (PV). A control method is proposed for PV systems; however, it is applicable for all types of droop-controlled renewable DG. A stability analysis of the proposed scheme on DG units is also presented to identify theoretical and practical limits. The proposed scheme identifies the dc operating zone of the inverter-based source as irradiance level changes and conditions the droop parameters appropriately for an efficient load sharing based on available generation, while the rating of each unit is also taken into account. The proposed scheme provides energy saving, since energy demand from a local auxiliary generator is reduced. The proposed method is validated using MATLAB/SIMULINK simulations.

Implementation and Stability Study of Dynamic Droop in Islanded Microgrids

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A solar PV water pumping solution using a three-level cascaded inverter connected induction motor drive

Dual-inverter for grid-connected photovoltaic system: Modeling and sliding mode control

In this paper a novel power conversion structure for grid-connected photovoltaic applications is presented. This structure is based on a multilevel inverter and on a LeBlanc transformer. The proposed multilevel power converter uses two single-phase voltage source inverters and a four wire voltage source inverter. The Le-Blanc transformer is connected at the output of the multilevel inverter. The configuration of the PV system is based on the multi-string technology. The structural design of this new power converter allows a seven level shaped output voltage wave at the output of the multilevel inverter.  To control this power converter a sliding mode controller with a vectorial modulator is used. This control system allows obtaining a fast a robust response for the multilevel inverter. Several experimental results are presented, confirming the expected performance of the proposed power conversion structure.

https://www.ijrer.org/ijrer/index.php/ijrer/article/viewFile/131/pdf

A Grid Connected Photovoltaic System with a Multilevel Inverter and a Le-Blanc Transformer

This paper presents a quasi-Z-source (qZS) converter that allows to supply multilevel voltages for the switched reluctance machine (SRM) drive. The use of qZS converter in SRM drives allows obtaining better high speed operation characteristics since higher voltages can be achieved. The proposed drive uses a qZS converter but designed in order to be possible to apply, to the SRM, different voltage levels on the magnetizing and demagnetizing stages. The presented converter also allows to regulate the output voltages and the difference between the voltage levels. Thus, it is possible to improve the efficiency of the drive, by reducing the switching frequency, as well as having better characteristics for high-speed operation. Different configurations of the converter are presented and the modulator associated to the SRM drive is described. In order to verify the features of the proposed system several results are also presented and discussed.

A Quasi-Z-Source Converter to Feed a Switched Reluctance Drive with Multilevel Voltages

In this paper a novel power conversion structure for grid-connected photovoltaic applications is presented. The proposed structure uses a four-wire voltage source inverter, two single-phase voltage source inverters and a scott transformer. PV modules arrangements can be divided in several groups. This power conversion structure is suitable for multi-string technology. The proposed structure allows multilevel output voltage. A sinusoidal pulse with modulator particularly adapted to this power converter structure is also proposed. Several simulation results are presented in order to verify the effectiveness of the proposed system.

A modular multilevel power converter system for photovoltaic applications

In this paper it is presented a system controller for a power converter connoted to a photovoltaic system and in a smart grid. This system controller has been developed in order to transmit all the produced active power of the photovoltaic system to the grid. However, it was also developed to use the power converter as reactive power compensator. Most of the time the photovoltaic system it is not producing its rated value. In this way, the power converter can also be used as a reactive power compensator in order to exploit the apparent power of the power converter. A single-stage three-phase power converter is used. To control this converter a sliding mode approach is used. This allows obtaining a fast and robust controller. A maximum power point tracking algorithm associated to the sliding mode controller is used. Several numerical simulations are presented in order to confirm the effectiveness of the proposed system.

Chen Hao

Papers

Dual-inverter for grid-connected photovoltaic system: Modeling and sliding mode control

A Grid Connected Photovoltaic System with a Multilevel Inverter and a Le-Blanc Transformer

A Quasi-Z-Source Converter to Feed a Switched Reluctance Drive with Multilevel Voltages

A modular multilevel power converter system for photovoltaic applications

Controlling a power converter as a photovoltaic power source and compensator in smart-grids