Oswaldo Lopez-Santos

Robust Sliding-Mode Control Design for a Voltage Regulated Quadratic Boost Converter

A robust controller design to obtain output voltage regulation in a quadratic boost converter with high dc gain is discussed in this paper. The proposed controller has an inner loop based on sliding-mode control whose sliding surface is defined for the input inductor current. The current reference value of the sliding surface is modified by a proportional-integral compensator in an outer loop that operates over the output voltage error. The stability of the two-loop controller is proved by using the Routh-Hurwitz criterion, which determines a region in the Kp-Ki plane, where the closed-loop system is always stable. The analysis of the sliding-mode-based control loop is performed by means of the equivalent control method, while the outer loop compensator is derived by means of the Nyquist-based Robust Loop Shaping approach with the M-constrained Integral Gain Maximization technique. Robustness is analyzed in depth taking into account the parameter variation related with the operation of the converter in different equilibrium points. Simulations and experimental results are presented to validate the approach for a 20-100-W quadratic boost converter stepping-up a low dc voltage (15-25-V dc) to a 400-V dc level.

A simple digital sinusoidal reference generator for grid-synchronized power electronics applications

This paper presents a simple method to generate low distorted sinusoidal references improving the power quality or the accuracy of power measurement in grid-synchronized applications. The proposed control strategy uses a half cycle detector, combining a square-wave discrete PLL with some functions embedded in a low-cost digital device. Using the proposed method it is possible to dedicate the digital device to perform the more difficult and complex numerical functions. The discrete PLL generates a low frequency (LF) signal with the same frequency and phase of the grid and a signal whose frequency is an integer number of times higher than the grid frequency (HF). Then, despite of the possible frequency variations, a constant number of samples-per-period is imposed in the control algorithms reducing deviations and errors. A descriptive study of the control loops provides the understanding to define the required controller parameters. The method is experimentally evaluated in a Sine-wave Pulse Width Modulated (SPWM) inverter, a grid-tied inverter and a monitoring system for a two-stage photovoltaic microinverter showing its simplicity and high applicability.

Sliding-mode control of a transformer-less dual-stage grid-connected photovoltaic micro-inverter

A transformer-less topology of a grid-connected photovoltaic micro-inverter applying sliding-mode control is discussed in this paper. The proposed structure is a dual-stage topology with a quadratic boost converter in the DC-DC stage and a full-bridge inverter in the DC-AC stage. The quadratic boost uses a multi-loop scheme with a sliding-mode current controller and a proportional-integral (PI) compensator regulating the output voltage. The full-bridge inverter injects real power to the grid by means of a sinusoidal reference sliding-mode tracking-loop whose amplitude is defined by a maximum power point tracker (MPPT). This preliminary work involves an analytical discussion and several simulation results of each converter and of the overall system.

Non-linear control of the output stage of a solar microinverter

ABSTRACT This paper presents a proposal to control the output stage of a two-stage solar microinverter to inject real power into the grid. The input stage of the microinverter is used to extract the maximum available power of a photovoltaic module enforcing a power source behavior in the DC-link to feed the output stage. The work here reported is devoted to control a grid-connected power source inverter with a high power quality level at the grid side ensuring the power balance of the microinverter regulating the voltage of the DC-link. The proposed control is composed of a sinusoidal current reference generator and a cascade type controller composed by a current tracking loop and a voltage regulation loop. The current reference is obtained using a synchronized generator based on phase locked loop (PLL) which gives the shape, the frequency and phase of the current signal. The amplitude of the reference is obtained from a simple controller regulating the DC-link voltage. The tracking of the current reference is accomplished by means of a first-order sliding mode control law. The solution takes advantage of the rapidity and inherent robustness of the sliding mode current controller allowing a robust behavior in the regulation of the DC-link using a simple linear controller. The analytical expression to determine the power quality indicators of the micro-inverters output is theoretically solved giving expressions relating the converter parameters. The theoretical approach is validated using simulation and experimental results.

Steady-State Analysis of Inductor Conduction Modes in the Quadratic Boost Converter

The conduction modes of the quadratic boost converter are explored in the context of high-dc-gain applications. A complete analytical description of the steady-state behavior of the converter operating in four possible inductor current discontinuous-conduction modes is presented. Boundaries between modes are determined together with the corresponding transitions. The study covers the operation of the converter using a pulse width modulator for constant switching frequency, and employing a hysteresis comparator for variable switching frequency. Both cases are analyzed for the same set of converter parameters and different constant resistive loads, while the converter is fed by a low dc voltage ranging from 20- to 30-V dc. Several simulations are used to verify the waveforms and average values predicted by the theoretical analysis. Experimental results in a 60-W prototype are in good agreement with the theoretical predictions, and efficiency measurements reveal that one of the discontinuous-conduction modes can compete with the continuous-conduction mode in high-dc-gain applications.

On-line Visualization and Long-Term Monitoring of a Single-Phase Photovoltaic Generator Using SCADA

This paper describes the design and implementation of a Supervisory, Control & Data Acquisition (SCADA) system dedicated to realize on-line visualization and long-term monitoring of the performance of a single-phase single-module photovoltaic grid-connected installation. The main objective of this development is to ensure continuous monitoring of the efficiency and relevant power quality indicators of a two-stage microinverter providing a comprehensive treatment of the electrical variables. The system consists of a software component developed in LabVIEW and a hardware component including specialized sensors and analogue electronics. Both components communicate synchronously through an acquisition card, which enhances visualization and accuracy of computations. The mathematical expressions employed to obtain variables and indicators are listed, explained and verified by means of simulated results.

Discrete Time Nested-Loop Controller for the Output Stage of a Photovoltaic Microinverter

This paper presents a comprehensive study of the digital implementation of the control requirements of the output stage of a two-stage solar microinverter. This approach uses a synchronized nested-loop controller which ensures the tracking of an internally generated high-quality current reference, the estimation and cancelation of the effect of the DC-link voltage ripple in the control loops and the regulation of the average value of the DC-link voltage. The proposed control architecture is validated by means of simulation results comparing operation of the inverter using continuous time, quasi-discrete time and discrete time implementations.

Derivation of a global model of a two-stage photovoltaic microinverter using sliding-mode control

This paper develops a global model of a two-stage grid-connected microinverter which uses sliding-mode control laws in both stages. In the study, it is considered that the microinverter is fed by a current source representing the photovoltaic module with a capacitor connected in parallel. The model of the DC-DC stage is obtained from the linearization of the ideal sliding dynamics of the converter considering that it is controlled through the input current. The DC-AC stage tied to the grid is considered as the load of the DC-DC stage. This representation allows including the voltage regulation loop used to stabilize the DC-link voltage through the value of the output current. The model is used to evaluate the local stability of the DC-link of the microinverter which represents the power balance of the system. The results are validated by means of simulations and experimental results.

Event-Triggered Digital Implementation of MPPT for Integration of PV Generators in DC Buses of Microgrids

This paper presents an event-triggered approach to optimally implement a Maximum Power Point Tracking (MPPT) algorithm into a Digital Signal Processor (DSP). The proposed method allows improving the amount and distribution of time required for executing control tasks. The used nested loop control architecture has an outer loop of MPPT generating the conductance reference used by an inner loop which regulates the input conductance of a DC-DC converter. This last loop enforces a sliding-mode loss-free-resistor behavior for the power converter by means of a simple hysteresis comparator. Computations required by the MPPT algorithm are synchronously executed by the two possible commutation events produced by the inner loop during a switching period. Then, the acquisition of signals must be activated only at an instant before each one of the switching events, releasing the most of the time to implement other tasks. This last characteristic and the use of a nested loop control architecture facilitate the integration of the other essential control functions for photovoltaic (PV) generators in microgrids. Simulation and experimental results confirm the high potetialities of this implementation approach.

Adaptive Sampling Frequency Synchronized Reference Generator for Grid Connected Power Converters

This paper introduces a simplified method for digital generation of high-quality references required for control of single-phase grid-connected (GC) power converters, which can generate synchronized sinusoidal waveforms at the same frequency of the input signal or its harmonics. Therefore, its application can be useful for active power filtering, high power factor rectification, and grid integration of renewable energy sources. A hybrid analog-digital implementation is proposed integrating an Adaptive Sampling Frequency Moving Average Filter (ASF-MAF) and a discrete-time Proportional-Integral (PI) controller into a Digital Signal Processor (DSP) operating with a sampling frequency defined by an external hardware-based Voltage Controlled Oscillator (VCO). The main advantages attributed to the method are immunity to harmonic content, accuracy in computations despite of frequency changes, flexibility to produce phase displacements and reduced computational cost. Performance of the proposal was verified by means of simulation and experimental results.