Stefano Barsali

Bio: Stefano Barsali is an academic researcher from University of Pisa. The author has contributed to research in topics: Energy storage & Electric power system. The author has an hindex of 13, co-authored 55 publications receiving 1124 citations.

Control techniques of Dispersed Generators to improve the continuity of electricity supply

Abstract: It is expected that dispersed generation (DG) will play an increasing role in electric power systems in the near future. Among the benefits that DG can give to the power system operators and to the electricity customers, one of the most attractive is the possibility of improving the continuity of power supply. DG plants can be designed to supply portions of the distribution grid in the event of an upstream supply outage. Techniques for controlling DG plants that use feedback of only locally measurable variables are presented. This solution allows correct system operation and switching between parallel and isolated modes without needing online communication of control signals between the generators. The control technique is described with particular reference to inverter-interfaced systems (micro-turbines, fuel cells). Simulations of sample cases including different size and type of generators are presented.

Dynamical Models of Lead-Acid Batteries: Implementation Issues

Abstract: This paper explains how the lead-acid models described in a previous paper (see M. Ceraolo, IEEE Trans. Power Syst., vol.15, p.1184-90, 2000) can be utilized in practice. Two main issues are opened by that paper: (1) the paper does not supply detailed information on how to identify the several parameters of the proposed models, and (2) it defines a whole family of models, but does not discuss which model of the family is adequate for a given purpose. These two issues are tackled in this paper. For the first issue, the more complex one, two options are proposed and discussed: (1) a complete identification procedure involving extensive lab tests, and (2) a simplified one that combines information from lab tests and data, supplied by the manufacturer. In addition, further simplifications applicable in cases of batteries belonging to the same family are presented.

Techniques to control the electricity generation in a series hybrid electrical vehicle

Abstract: In a series hybrid electric vehicle (SHEV), an electric generator feeds a DC busbar (containing an electrochemical accumulator), which, in turns, feeds the vehicle traction system. A very important part of the vehicle is its control system, which has to maximize the vehicle efficiency while keeping the emissions within predetermined limits. To attain this goal, it can act in two ways: (1) it can switch the electric generator ON or OFF, and (2) when in the ON state, it can impose a given power to be delivered. To choose the control actions to perform it needs (a) an algorithm to understand the behavior of the battery (determine the state-of-charge, maximum power, losses) and (b) an algorithm to make some forecast of the system load (i.e., the power required in the future of the trip, as a function of time). This paper discusses the main problems that arise when trying to synthesize the control system of the vehicle, taking into account issues (a) and (b). Some lab tests are also presented in which some of the proposed control techniques are experimentally checked.

Hierarchical control of droop-controlled DC and AC microgrids — a general approach towards standardization

Abstract: DC and AC Microgrids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems. In the last years, efforts toward the standardization of these Microgrids have been made. In this sense, this paper present the hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to microgrids. The hierarchical control proposed consist of three levels: i) the primary control is based on the droop method, including an output impedance virtual loop; ii) the secondary control allows restoring the deviations produced by the primary control; and iii) the tertiary control manage the power flow between the microgrid and the external electrical distribution system. Results from a hierarchical-controlled microgrid are provided to show the feasibility of the proposed approach.

Ultracapacitors: Why, How, and Where is the Technology

Abstract: The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Defining control strategies for MicroGrids islanded operation

Abstract: This paper describes and evaluates the feasibility of control strategies to be adopted for the operation of a microgrid when it becomes isolated. Normally, the microgrid operates in interconnected mode with the medium voltage network; however, scheduled or forced isolation can take place. In such conditions, the microgrid must have the ability to operate stably and autonomously. An evaluation of the need of storage devices and load shedding strategies is included in this paper.

Accurate electrical battery model capable of predicting runtime and I-V performance

Abstract: Low power dissipation and maximum battery runtime are crucial in portable electronics. With accurate and efficient circuit and battery models in hand, circuit designers can predict and optimize battery runtime and circuit performance. In this paper, an accurate, intuitive, and comprehensive electrical battery model is proposed and implemented in a Cadence environment. This model accounts for all dynamic characteristics of the battery, from nonlinear open-circuit voltage, current-, temperature-, cycle number-, and storage time-dependent capacity to transient response. A simplified model neglecting the effects of self-discharge, cycle number, and temperature, which are nonconsequential in low-power Li-ion-supplied applications, is validated with experimental data on NiMH and polymer Li-ion batteries. Less than 0.4% runtime error and 30-mV maximum error voltage show that the proposed model predicts both the battery runtime and I-V performance accurately. The model can also be easily extended to other battery and power sourcing technologies.

A wireless controller to enhance dynamic performance of parallel inverters in distributed generation systems

Abstract: This paper presents a novel control strategy for parallel inverters of distributed generation units in an AC distribution system. The proposed control technique, based on the droop control method, uses only locally measurable feedback signals. This method is usually applied to achieve good active and reactive power sharing when communication between the inverters is difficult due to its physical location. However, the conventional voltage and frequency droop methods of achieving load sharing have a slow and oscillating transient response. Moreover, there is no possibility to modify the transient response without the loss of power sharing precision or output-voltage and frequency accuracy. In this work, a great improvement in transient response is achieved by introducing power derivative-integral terms into a conventional droop scheme. Hence, better controllability of the system is obtained and, consequently, correct transient performance can be achieved. In addition, an instantaneous current control loop is also included in the novel controller to ensure correct sharing of harmonic components when supplying nonlinear loads. Simulation and experimental results are presented to prove the validity of this approach, which shows excellent performance as opposed to the conventional one.