Thais Gama de Siqueira

MODELING AND CONTROL OF A THREE-PHASE ISOLATED GRID-CONNECTED CONVERTER FOR PHOTOVOLTAIC APPLICATIONS

This paper describes the modeling and control of a three-phase grid-connected converter fed by a photovoltaic array. The converter is composed of an isolated DC-DC converter and a three-phase DC-AC voltage source inverter The converters are modeled in order to obtain small-signal transfer functions that are used in the design of three closed-loop controllers: for the output voltage of the PV array, the DC link voltage and the output currents. Simulated and experimental results are presented.

Micro-inverter for integrated grid-tie PV module using resonant controller

Two-stage isolated converters for photovoltaic (PV) applications commonly employ a high-frequency transformer on the DC-DC side, submitting the DC-AC inverter switches to high voltages and forcing the use of IGBTs instead of low-voltage and low-loss MOSFETs. This paper shows the modeling, control and simulation of a single-phase full-bridge inverter with high-frequency transformer (HFT) that can be used as part of a two-stage converter with transformerless DC-DC side or as a single-stage converter (simple DC-AC inverter) for grid-connected PV applications. The inverter is modeled in order to obtain a small-signal transfer function used to design the P+Resonant current control regulator. A high-frequency step-up transformer results in reduced voltage switches and better efficiency compared with converters in which the transformer is used on the DC-DC side. Simulations and experimental results with a 200 W prototype are shown.

Resonant (P+RES) controller applied to voltage source inverter with minimum DC link capacitor

Voltage source inverters use large electrolytic capacitors in order to decouple the energy between the utility and the load, keeping the DC link voltage constant. Decreasing the capacitance reduces the distortion in the inverter input current but this also affects the load with low-order harmonics and generate disturbances at the input voltage. This paper applies the P+RES controller to solve the challenge of regulating the output current by means of controlling the magnitude of the current space vector, keeping it constant thus rejecting harmonic disturbances that would otherwise propagate to the load. This work presents a discussion of the switching and control strategy.

Modified droop control for low voltage single phase isolated microgrids

This paper develops a control strategy for converters in low voltage single phase isolated microgrids. Droop control technique for low voltage is used to control the active power as function of the voltage amplitude, and reactive power as function of frequency deviation. This control suppresses the use of physical communication between the power converters by simply utilizing local measurements, allowing its connection to the system without adjustment. Some concepts as microgrids and types of inverters connected, such as grid-forming, providing and supporting inverters are introduced. The individualized droop control for low-voltage systems is demonstrated, where it is more convenient to use active power as function of the magnitude of the voltage. A novel modified droop control equation is proposed to compensate the voltage drop at line resistance. The line impedance influence in power-sharing is also reduced. The validity of this new approach is proven by simulations that compare both droop control types and presents the effective improvement in active power-sharing and voltage regulation at microgrids common bus. A reduction of the power-sharing difference from 1041W to 341W is achieved with the new equation, and a voltage regulation from 204V to 210V. The reactive power-sharing is also verified.

Approximate method for modeling and control of a two-stage power converter

In this paper, it is presented a procedure to simplify the well-known average model as well as to approximate the controller design for a Pulse-Width-Modulation (PWM) based power converter that transforms energy from some DC source (such as batteries or fuel cells) into an AC voltage source. The power converter studied is formed by two stages: a DC-DC bidirectional converter (boost) and a DC-AC H-bridge converter (inverter). It is demonstrated that second-order average models of those power converters can be simplified to obtain first-order approximate models, which in turn are used with frequency analysis to design approximate controllers that perform well compared to the complete design procedure, as it is observed in the simulation tests. As a result, parameters of the controllers for the power converter are calculated directly from the specifications of the power stages and control loop constraints. Hence, this method can be used to implement adaptive controllers as well.

Multi-converters droop control in single-phase microgrid

This paper develops the droop control strategy to connect multiple different types of converters in parallel in an isolated single-phase low voltage microgrid. First, the traditional droop control for low voltage microgrids is explained. Then three different types of converters in a microgrid are described, and different droop equations are proposed for each converter to overcome the line impedance drawback when sharing active power. The new equations also include the priority feature that prioritizes the renewable sources instead of the batteries. Simulations of a modeled microgrid with four converters and two resistive loads are performed. Accurate active power sharing is achieved even with different values of line impedance, the voltage at load bus is regulated, and the functionality of the equations for selecting the priority of generation and the correct power balance to avoid damages in the microgrid elements is verified.

Modeling and simulation of photovoltaic systems under non-uniform conditions

The efficiency of a photovoltaic (PV) system depends on very intermittent environmental variables such as irradiance and temperature. Understanding of the behavior of PV systems in function of these variables depends on mathematical models for PV cells, modules and arrays. This becomes more complex when partial shading occurs. This paper presents three different MATLAB-based modeling strategies for estimating the I–V and P-V curves of PV systems considering different levels of irradiance and temperature under partial shading conditions. This work is useful for power electronics designers and researchers who work with PV systems under partially shaded conditions.

Autonomous microgrid with energy storage system

This paper shows the development of a strategy to connect multiple different types of converters in parallel in an isolated single-phase microgrid harmoniously. First, the droop control strategy for microgrids and the particularities of the microgrid operation are explained, such as controlling the voltage and current of the battery to maintain its integrity. Then three different types of converters in a microgrid are described, and different droop equations are proposed for each converter to perform the correct control. These equations also include the priority feature that prioritizes the renewable sources instead of the batteries, when feeding loads in autonomous operation. Simulations of a modeled microgrid with four converters and two loads are performed. The functionality of the equations are verified in terms of priority selection of generation, the correct power balance and the control to avoid damages in microgrid elements, especially to the batteries.

Analysis of ensemble models in the medium term hydropower scheduling

The medium term hydropower scheduling (MTHS) problem involves an attempt to determine, for each time stage of the planning period, the amount of generation at each hydro plant which will maximize the expected future benefits throughout the planning period, while respecting plant operational constraints. Besides, it is important to emphasize that this decision-making has been done based mainly on inflow earliness knowledge. To perform the forecast of a determinate basin, it is possible to use some intelligent computational approaches. In this paper one considers the Dynamic Programming (DP) with the inflows given by their average values, thus turning the problem into a deterministic one which the solution can be obtained by deterministic DP (DDP). The performance of the DDP technique in the MTHS problem was assessed by simulation using the ensemble prediction models. Features and sensitivities of these models are discussed.