Fran Gonzalez-Espin

An Adaptive Control System for Three-Phase Photovoltaic Inverters Working in a Polluted and Variable Frequency Electric Grid

The proportional + resonant (PR) controller has been proposed in the past as a suitable method to control the current generated by the grid-connected photovoltaic voltage source inverters. Due to the fact that information regarding the frequency of the grid is needed to use this control technique, the synchronous reference frame phase-locked loop (SRF-PLL) is commonly used. To assure that the total harmonic distortion of the injected current (THDi) meets the appropriate standards, even if the grid voltage is polluted and its frequency varies, an adaptive control strategy is presented in this paper. This control strategy can improve the behavior of both, the conventional SRF-PLL and the conventional PR controller, when they are used in a polluted grid with a time varying frequency. The experimental results obtained by means of a digitally controlled 10-kVA inverter, show up that the THDi of the injected current is improved when the proposed adaptive control strategy replaces the conventional one.

Robust Model-Following Control of Parallel UPS Single-Phase Inverters

This paper presents a robust control technique applied to modular UPS inverters operating in parallel. When compared to conventional PI control, the proposed technique improves the response of the output voltage to load steps and to high distorted output currents. Furthermore, an excellent distribution of currents between modules is achieved, resulting in a fine power equalization between the inverters on stream. The crossover frequency of the different loop gains involved is moderate, so that robustness to variations of the operation point and to tolerances of the output LC filter components values is achieved. A comparative study with a two-loop conventional P1 control scheme is presented. Experimental results on a 1 kVA modular UPS system confirm the viability of the proposed scheme.

Dynamic Analysis of Three-phase Photovoltaic Inverters with a High Order Grid Filter

The aim of this paper is to analyze the control loops stability of three-phase photovoltaic inverters for distributed power generation. The main issues regarding the design of the control stage are outlined, including a small signal model of the photovoltaic cells for control purposes and the influence of high order grid filters on the system stability. Experimental results carried out on a 100 kW inverter are presented to validate the analytical results. The experimental inverter generates a balanced set of currents with a distortion lower than 3%.

Application of rapid prototyping tools for a hierarchical microgrid control implementation

This paper presents the application of rapid prototyping tools for the implementation of hierarchical microgrid automation and control algorithms in Real-Time/FPGA platforms based on a Triphase 3PExpress and a National Instruments CompactRIO platforms (NI cRIO). A three layer hierarchical control structure has been used, where the primary layer implements the power electronics controls, the secondary layer implements the microgrid controls and automation strategy, and the tertiary layer implements the energy management algorithms. The rapid prototyping tools offer an environment that allows the implementation of concepts related to the coordination, control and management of a Commercial and Industrial (C&I) building microgrid to provide high quality power to both the local loads (e.g. harmonic distortion) and the grid (e.g. ancillary services). The microgrid is composed of a three-phase diesel generator, a Li-ion battery, a motor-generator test rig for wind turbine emulation, a load bank with resistive, inductive and capacitive passive load emulation capabilities, a resistive load bank, a fault emulator and a back to back converter to emulate loads or distributed resources.

Systematic small signal modeling and stability analysis of a microgrid

This paper presents a systematic, scalable, small-signal modelling approach for microgrids. Each subsystem is modelled separately and then all are interfaced using the Simulink Control-Design Toolbox. The main advantage is practicality, as analytical models of whole microgrids may be derived systematically and easily. The application to a subsystem-level (control design) and system-level analysis (microgrid stability) is shown, as well as experimental results that validate the correct operation of the microgrid.

Configuration, modelling and control of a microgrid for commercial buildings application

Distributed generation with a high penetration of renewable energies has been presented as an alternative for traditional centralized power plants. This distributed generation can be managed by means of microgrids. The microgrid concept can be extended to commercial and residential buildings, where there exist growing opportunities in energy saving and market profit for stakeholders. The aim of this paper is to present an overview of a microgrid for commercial buildings, with integrated control of thermal and electrical aspects, as well as to describe each of the electrical power sources, including a small-signal state-space representation and the control structure of each of the generators. The generators described in this paper are a small permanent magnet wind turbine, a storage unit based on a Li-Ion battery and a combined heat and power (CHP) based on a gas reciprocating engine connected to a synchronous generator.

Low-frequency reduced-order modeling approach and implementation of grid emulation in hardware-in-the-loop platforms

A simple and effective modelling approach of the electrical grid and implementation in Hardware in the Loop (HIL) device is presented in this paper. A low-frequency reduced-order modelling approach has been followed, which reduces dramatically the computational requirements and implementation complexity in comparison to an equivalent full grid model, and offers an intuitive way to tune the grid dynamic behaviour.

Operating modes of a commercial and industrial building microgrid with electrical generation and storage

This paper describes the several operating modes of a Commercial and Industrial (C&I) building microgrid that allows for seamless transition between the grid connected and the islanded modes. The microgrid described in this paper is composed of a diesel generator, a Li-Ion battery system and electrical loads. The Li-Ion battery system is interfaced with the grid by using a power electronics converter, whilst the synchronous machine of the diesel generator is directly connected to the building electrical grid. Depending whether the diesel generator is running or not during the transition and its operating mode, the battery system will remain in grid connected mode or will adapt its control strategy to grid forming or droop operation.

Modular d-q frame small-signal modelling and analysis of a microgrid

In this paper, a systematic, scalable small-signal modelling approach for microgrids is described. First, each subsystem is modelled separately, and then all are interfaced using Simulink Control Design Toolbox. The main advantage of this method is its practicality, as analytical models of whole microgrids are derived systematically and easily. A real microgrid is modelled and the presented method is applied to perform analysis both at subsystem-level (control design) and at system-level (microgrid stability). Also, experimental operation of the microgrid is illustrated.

A Variable Multi-Rate Plug-in Repetitive Controller for single-phase inverters operating in the islanding mode

The aim of this paper is to present a Repetitive Controller which is able to adapt its frequency response according to the frequency of a reference signal. The proposed controller is based on the Multi-Rate feature, and is intended to control the output voltage of an inverter operating in the islanding mode, when the Droop Control Method is implemented. The Variable Multi-Rate Plug-in Repetitive Controller is easy to design, and it allows obtaining a much better output voltage total harmonic distortion than that obtained by means of the conventional Repetitive Controller or the Proportional+Integral controller.