Lucas S. Xavier

Adaptive saturation scheme for a multifunctional single-phase photovoltaic inverter

Power inverters are essential components of photovoltaic systems and are designed to process the rated power generated by solar plant. Due to variations in solar irradiance, inverters have a current margin, which can be used in reactive power and harmonics compensation. This possibility, known as multifunctional operation, can be interesting in terms of power factor improvement and voltage control at point of common couple (PCC). Many works propose control strategies for multifunctional inverters. However, during multifunctional operation current limit of the inverter cannot be exceeded. Therefore, inverter needs to compensate partially reactive power and harmonics of the load. This fact is few related in literature. This work proposes a scheme of current dynamic saturation in order to compensate partially reactive power and harmonics of the load. This scheme is based on peak detection algorithm and prioritizes reactive power compensation. Conservative Power Theory (CPT) is used to separate the load current components. Simulation results show performance of the proposed dynamic saturation technique, providing adequate operation of the inverter and partial compensation.

Current control strategy for reactive and harmonic compensation with dynamic saturation

The main objective of a photovoltaic (PV) inverter is inject the PV power into the grid. However, due to variations in solar irradiance, inverters have a current margin, which can be used in reactive power and harmonics compensation. This possibility, known as multifunctional operation, can be interesting in terms of power factor improvement and reduction of nonlinear loads impact at point of common couple (PCC). This work applies the instantaneous power theory in order to obtain the harmonic and reactive components of the load current. These components are used as references in the inverter control strategy. Simulation results show reduction in the grid distortion, with improvements in the power quality indexes. However, during multifunctional operation, inverters present a current limit that cannot be exceeded. Thus, this work introduces a dynamic saturation scheme for a multifunctional three-phase inverter. This strategy possibilities partial or total reactive and harmonic compensation without injection of low order harmonics.

A novel adaptive current harmonic control strategy applied in multifunctional single-phase solar inverters

The inverter multifunctional operation is based on the harmonic current compensation, generated by nonlinear loads. The traditional harmonic detection methods tracks all harmonic contents of the load current and the control tuning tends to be complex with low flexibility. In many works, the proportional resonant (PR) controllers are used to control the inverter current reference. However, one PR controller needs to be designed for each harmonic frequency and this fact increases the control algorithm complexity. Therefore, this work proposes a novel adaptive current harmonic control strategy applied in multifunctional single-phase solar inverters. The strategy is based on a novel detection method of the harmonic load current. The harmonic current detection method is frequency adaptive and able to detect the load harmonic current with higher amplitude. This method consists in a cascade association of two phase-locked loop based on second order generalized integrator (SOGI-PLL). The detected frequency is used as feedback by the proportional resonant controller. Therefore, only two PR controller are required: one to track the fundamental component and another to track the harmonic component with higher magnitude. Simulation and experimental results show the performance of the proposed control strategy, improving significantly the grid current quality.

Adaptive saturation for a multifunctional three-phase photovoltaic inverter

This paper presents an inverter control scheme, based on proportional-resonant controllers, where the inverter works in multifunctional operation. In this case, the reactive power and harmonic compensation of the load are ancillary services. A resonant control is implemented in stationary reference frame (αβ), where is not necessary the use of a phase-locked loop (PLL). The instantaneous power theory (IPT) is used to detect the harmonic current and reactive power of the load. These components are used as references in the inverter control strategy. Most important fact and less related in literature is the inverter current limitation strategy. Thus, this work introduces a dynamic saturation scheme for a multifunctional three-phase inverter. This strategy possibilities partial or total reactive power and harmonic current compensation without injection of low order harmonics, ensuring that the inverter current does not exceeds the rated current.

Single-phase multifunctional inverter with dynamic saturation scheme for partial compensation of reactive power and harmonics

Single and three-phase photovoltaic inverters are essential components of the photovoltaic (PV) systems to extracting the PV power and injecting it into the grid. Thus, in order to extract the maximum power of the solar array for various solar irradiation tracks, it is used a maximum power point tracker (MPPT) algorithm. Due to variations in solar irradiance, inverters have a current margin, which is not explored during the day. Thereby, many works have proposed the multifunctional operation. This concept consists in aggregate to the inverter control strategy other functions, such as harmonics and reactive power compensation. However, most important fact and less related in literature is the necessity of techniques to compensate partially reactive power and harmonics of the load, ensuring that the inverter works below the rated current. Hence, the present work proposes a current dynamic saturation scheme in order to compensate partially reactive power and harmonics of the load during the multifunctional operation. Simulations show that the dynamic saturation prevents the inverter to inject low-order harmonics, while ensuring the operation below the system rated current. Furthermore, control performance is evaluated for five grid-connected PV system in parallel association, in order to show the effectiveness of proposed control strategy for various dispersed PV systems in the grid. To ensure that the proposed method is applied with the maximum efficiency of the PV system, this work compares, during inverter multifunctional operation, the instantaneous and dynamic efficiency between three MPPT algorithms proposed in literature: perturb and observe; dP - perturb and observe; modified perturb and observe.

Saturation scheme for single-phase photovoltaic inverters in multifunctional operation

Single and three-phase photovoltaic inverters are responsible to extract the photovoltaic array power and inject it into the grid. Due to variations in solar irradiance, inverters have a current margin, which is not explored during the day. Thereby, many works have proposed multifunctional operation. This concept consists in aggregate to the inverter control strategy other functions, such as harmonics and reactive power compensation. However, most important fact and less related in literature is the necessity of techniques to compensate partially reactive power and harmonics of the load, ensuring the inverter work below the rated current. Hence, the present work proposes a current dynamic saturation scheme in order to compensate partially reactive power and harmonics of the load during the multifunctional operation. Simulations show that the dynamic saturation prevents the inverter of inject low-order harmonics while ensuring the operation below the system rated current. By applying the multifunctional operation, the grid current THD is reduced from 104.64% to 1.54% and reactive power is fully compensated, resulting in a considerable grid improvement.

Design of a current harmonic detector method applied in photovoltaic inverters with ancillary service capability

The multifunctional operation of photovoltaic (PV) inverters consists in using the PV inverter for ancillary services as reactive power injection and harmonic current compensation. The multifunctional operation can improve the ac-grid power quality. Thus, this paper presents a dynamic method based on the Second Order Generalized Integrator coupled with a Phase Locked Loop (SOGI-PLL) structure to detect the most predominant harmonic current components in the distribution system. This work intends to show the improvement of the detection method by the use of negative feedback on the SOGI. It is used an extension of this detector to detect multiple harmonic currents according to its amplitude. Besides, the influence of the discretization method is presented and the error in this conversion is analyzed. At last, it can be seen a performance improvement in the detection of the harmonic current amplitudes.

LCL filter losses due to harmonic compensation in a photovoltaic system

Nowadays, non-linear loads are one of the major issues regarding the system power quality. Additionally, the growing presence of distributed generation (DG) systems, which employs converters to connect power sources to the grid may also affect power quality due to its switching. To better utilize such converters, multifunctional operation is usually employed. In order to reduce the harmonics generated by the switching, passive filters interface converter connection to the grid and any loads as well. This work performs an analysis of how the multifunctional operation of the photovoltaic inverter, specifically during harmonic current compensation, affects the efficiency of the system. Therefore, the power losses of the LCL filter are analyzed.

Comparison of control strategies for grid-connected photovoltaic systems during unbalanced voltage dips

Unbalanced voltage dips are common disturbances in power systems. Therefore, grid-connected photovoltaic systems need to be able to work during these disturbances. Conventional control strategies do not consider the negative sequence current control, which is an important limitation. In this context, this paper compares through simulations four strategies of negative sequence current control: Notch filter based strategy, measured signals decoupling based strategy, reference and error signals decoupling based strategy and resonant controllers based strategy. All simulations are performed in Matlab/Simulink environment. Representation of point of common coupling voltage (PCC) uses experimental data of an unbalanced voltage sag. Comparisons during unbalanced voltage sags and the impact of voltage harmonics on techniques are analyzed. Obtained results shows resonant controllers based strategy has a superior behavior, in terms of robustness and simplicity.