Jose G. de Matos

Power Control in AC Isolated Microgrids With Renewable Energy Sources and Energy Storage Systems

This paper presents a new strategy to control the generated power from energy sources existing in autonomous and isolated microgrids. In this particular study, the power system consists of a power electronic converter supplied by a battery bank, which is used to form the ac grid (grid former converter), an energy source based on a wind turbine with its respective power electronic converter (grid supplier converter), and the power consumers (loads). The main objective of this proposed strategy is to control the state of charge of the battery bank limiting the voltage on its terminals by controlling the power generated by the energy sources. This is done without using dump loads or any physical communication among the power electronic converters or the individual energy source controllers. The electrical frequency of the microgrid is used to inform the power sources and their respective converters about the amount of power that they need to generate in order to maintain the battery-bank charging voltage below or equal its maximum allowable limit. Experimental results are presented to show the feasibility of the proposed control strategy.

Power control in AC autonomous and isolated microgrids with renewable energy sources and energy storage systems

This paper presents a new strategy to control the generated power that comes from the energy sources existing in autonomous and isolated Microgrids. In this particular study, the power system consists of a power electronic converter supplied by a battery bank, which is used to form the AC grid (grid former converter), an energy source based on a wind turbine with its respective power electronic converter (grid supplier converter), and the power consumers (loads). The main objective of this proposed strategy is to control the state of charge of the battery bank limiting the voltage on its terminals by controlling the power generated by the energy sources. This is done without using dump loads or any physical communication among the power electronic converters or the individual energy source controllers. The electrical frequency of the microgrid is used to inform to the power sources and their respective converters the amount of power they need to generate in order to maintain the battery-bank state of charge below or equal its maximum allowable limit. It is proposed a modified droop control to implement this task.

Small renewable hybrid systems for stand alone applications

To provide renewable energy to stand alone applications the generation system should be composed of different types of energy sources to make better use of the natural resources in these applications. A common practice is to combine wind and solar PV energies in the so called hybrid renewable systems. Due to the long distance, and difficult access the overall system used in these applications must be reliable. And the reliability of the system, specially the inverter used to regulate the AC voltage, is one of the main problems associated to these systems, and is responsible for the lack of confidence in renewable systems at several locations in Brazil. This paper shows the initial results of using renewable hybrid systems specially designed for isolated areas, focusing attention on reliability, efficiency and expansion flexibility. It presents the system description, mode of operation, inverter design, and experimental results measured in a pilot plant located in Lençóis Island, a small isolated community in the northeast region of Brazil.

Power control in isolated Microgrids with renewable distributed energy sources and battey banks

This paper presents a new strategy to control the power generation from existing energy sources in autonomous and isolated Microgrids. In this study, the Microgrid is composed of a power electronic converter, supplied by a battery bank, which is used to form the AC network (grid-forming power converter), an energy source based on a wind turbine and its respective power electronic converter (grid supplier converter), and the loads. The primary subject of the proposed control strategy is to keep the energy balance into the Microgrid, in order to control the battery bank state of charge even when more power can be generated than loads can consume. This goal is achieved controlling the generated power into the Microgrid, without using dump load or any physical communication with the power electronic converters or individual energy source controls. The electrical frequency of the Microgrid is used for dictating the amount of power the energy sources need to generate in order to maintain the battery-bank state of charge below its maximum permissible value. A modified droop control to implement this work is proposed.

Operation of three-phase rectifiers with Diesel Generator Sets of similar power: Practical solution based on passive filters

This work proposes a solution to enable the operation of diesel generators that supply line-frequency phase-controlled rectifiers to charge battery banks in isolated microgrids. The solution consists of the use of resonant filters, tuned to the harmonic frequencies that must be suppressed from the AC source line currents. In addition to the resonant filters, a circuit topology is proposed to attenuate the notches in the line terminal voltages due to the operation of the rectifiers fed by AC sources that have equivalent non-negligible series inductances. The reactive power due to the proposed solution can self-excite the generator and interfere with its voltage regulation. The work proposes to compensate for this capacitive reactive power through a bank of inductors installed in the generator terminals. This solution has been used successfully in two isolated microgrids installed in two islands.

A wind energy battery charging system with dynamic current limitation for output power limiting

One of the problems related to isolated microgrids with battery storage systems (BSS) and small wind turbines is related to power balance among generation, BSS, and loads, especially when the generation is higher than the consumption and the BSS is charged. This paper presents the development of a wind-powered battery charging system for isolated microgrids. The system implements Maximum Power Point Tracking (MPPT) for optimization of the generated power and a current-limiting loop for battery storage system (BSS) voltage regulation and safe operation. This current-limiting loop is based on a dynamic battery current limitation. That approach has the advantage of being able to connect several systems in parallel, as the BSS voltage is not tightly controlled, but limited. Simulation on MATLAB/Simulink environment are presented to validate the proposed strategy. The developed control algorithm provides a smooth transition between the operation modes of the non-inverting buck-boost converter. The system is being experimentally tested on a 10kW non-inverting buck-boost based converter connected to a 240V/1200Ah OPZv BSS.

Comparison analysis of resonant controllers in discrete domain taking into account the computational delay

This paper presents the study, analysis and design of proportional-resonant classic (classic PR) and vector-resonant (VR) controllers. The vector resonant controller is more robust to parametric and frequency variations than the classic PR. This becomes more significant in applications that require selective harmonics compensation, since the increasing the order of frequency to be compensated decreases the system performance for both controllers. This situation is aggravated when the design is carried out in the discrete domain due to the effect of computational delay. Thus, in this paper it will be carried out a comparative analysis of classic PR and vector-resonant controllers in discrete domain with and without delay compensation. It is noted that there is an improvement in system performance when a computational delay compensator is used to compensate harmonics of high order.

An optimized battery charging system based on a modified MPPT strategy for isolated PV energy generation

This paper describes the development of a high power battery charging system optimized for photovoltaic (PV) isolated microgrids. The charger implements a modified Maximum Power Point Tracking (MPPT) strategy that is suitable for isolated photovoltaic applications with battery storage systems (BSS). The proposed strategy is able to extract maximum power when possible and controls the power delivered to the BSS in order to supply the maximum possible energy without overcharging the batteries, enabling the operation with other current sources battery chargers. The proposed system is first simulated in the MATLAB/Simulink environment and then implemented in a 35kW charge controller, based on ten paralleled buck converters, connected to a 120V 600Ah OPZv BSS. The system hardware is designed to be very reliable and to be operated at extreme ambient conditions.

Parallel-connected inverters applied in renewable energy systems

This paper presents the results of inverter development for standalone renewable energy systems used in isolated communities. One of the main problems of such systems, located in isolated areas of difficult access, is the inverters. The mean time before failure of these equipments has been shown smaller than expected. In these applications the inverters should have the following features: expansion flexibility and robustness; high efficiency; and adequacy to operate in adverse environmental conditions. The results presented in this paper are related to inverters development taking into account these requirements. The paper will present the mode of operation of the system, showing its robustness and viability for utilization of the inverters in standalone renewable energy systems.