Costantino Citro

Smart Operation of Wind Turbines and Diesel Generators According to Economic Criteria

This paper proposes an innovative system for Smart Grid (SG) management aiming at minimizing the total costs supported for carrying out the delivery of energy to consumers. These costs include the production costs of distributed generators, the cost of the power provided by the primary substation, and the cost associated with grid power losses. After a brief overview on the main SG aspects, this paper describes the proposed approach that makes use of an optimal power flow algorithm and the active management schemes. The efficiency of the method is verified on a distribution system comprising wind turbines and diesel generators, considering the time-varying characteristics of the load demand and wind power generation.

Control of grid-connected power converters based on a virtual admittance control loop

The connection of electronic power converters to the electrical network is increasing mainly due to massive integration of renewable energy systems. However, the electrical dynamic performance of these converters does not match the behavior of the network, which is mainly formed by generation facilities based on big synchronous generation systems. Depending on the desired electrical operation mode different control structures can be implemented in the converters in order to get adapted with the grid conditions. However, changing between different control structures and operation is not an optimal solution, as the resulting system results complex and is not highly robust. As an alternative, this paper presents a new control technique for grid connected power converters based on the concept of virtual admittance. The proposed control permits to emulate the electrical performance of generation facilities based on classical synchronous generators with a power converter, with no need of implementing different control structures, giving rise to a system that provides a friendly and robust operation with the network.

Decentralized Primary Control of MTDC Networks With Energy Storage and Distributed Generation

Multiterminal dc networks are drawing a lot of interest lately in applications related to distributed generation, particularly in those that also integrate energy storage (ES). A few approaches for controlling the operation of such systems have been proposed in the literature; however, the existing structures can be significantly enhanced. This paper proposes an improved primary control layer, based on custom droop characteristics obtained by combining concepts of droop and dc-bus signaling control. This approach is designed to be generic and takes into account the various operating states of the network. Five operating bands, similar to the operating states of the ac grids, as well as various droop characteristics for different elements connected to the dc network, are defined. For the ES, the state of charge is taken into account at the primary control level and included in the droop characteristic, creating a two-variable droop surface. The proposed control strategy is validated through simulation and experimental results obtained from a case study that involves a micro dc network composed of a photovoltaic generator, a lead-acid battery, and a connection point to the ac grid.

Overview of power processing structures for embedding Energy Storage in PV power converters

The level of penetration of grid-connected photovoltaic (PV) systems has grown very quickly in recent years. Nonetheless, network stresses related to the recurrent voltage and frequency oscillations caused by their irregular power production could be a limiting factor on the future expansion of these applications. With the capability of storing and releasing electrical energy on demand, Energy Storage Devices (ESDs) may play a key role in finding an effective solution to overcome these kinds of problems. The aim of this paper is to provide a description of the state-of-the-art power conversion systems used to combine ESDs and grid-connected PV plants. The work has been edited after an accurate analysis of the current scientific literature, focusing particular attention on medium-high-power PV applications utilizing batteries and/or supercapacitors. A detailed analysis on advantages, disadvantages and range of application is provided for each of the reported conversion systems.

Designing Inverters’ Current Controllers With Resonance Frequencies Cancellation

This paper presents an innovative and effective linear current regulator for grid-connected converters interfaced by an LCL filter. The proposed scheme entails the use of voltages measured at the terminals of the output filter capacitors as feedforward signals to be included in a classical structure of proportional-resonant (PR) controller. Processing these signals by an opportune filter K(s), the resonance effects characterizing the LCL filter are neutralized and the impact of the grid impedances variations on the converters performances is greatly reduced. The proposed solution simplifies the controllers design, whose gains are obtained by a straightforward mathematical formula which prevents complex root locus analyses. Moreover, for a given phase margin, the proposed technique ensures the achievement of maximum possible bandwidth of closed-loop system, sidestepping the guesswork typical of the conventional methods used for selecting the crossover frequency. Conclusions are supported by a detailed mathematical analysis and validated by laboratory results obtained with a 7.5-kW three-phase inverter prototype connected to a power grid with time-variant impedance characteristics.

Decentralized control of MTDC networks with energy storage and distributed generation

Multi-terminal dc (MTDC) networks are drawing a lot of interest lately in applications related to distributed generation, especially in those that integrate energy storage. Several approaches for controlling the operation of such systems have been proposed in the literature; however the existing structures can be significantly enhanced. This paper proposes an improved primary control layer, based on droop and dc-bus signaling to serve as a base framework for implementing a hierarchical control structure in a MTDC system. As it will be further discussed in this work, five operating bands as well as various droop characteristics for different elements connected to the dc-bus were defined. For the energy storage the state of charge (SoC) was taken into account at the primary control level and it was included in the droop characteristic, creating a two variables droop surface. The proposed control strategy was validated through simulation and experimental results obtained from a case study that involves a micro dc network composed of a PV generator, a lead-acid battery and one connection point to the ac grid.

Flexible grid connection and islanding of SPC-based PV power converters

At the present time, distributed generation systems are required to disconnect from the main grid when there is an outage. In order to fulfill this requirement, photovoltaic (PV) power plants are equipped with anti-islanding algorithms, embedded in the converters controller, to avoid the island operation. However, the current trends in the development of the future electrical networks evidence that it is technically feasible and economically advantageous to keep feeding islanded systems under these situations, without cutting the power supply to the loads connected to the network. Nevertheless, commercial PV power converters are programmed as grid-feeding converters and they are unable to work in island mode if there is not an agent forming the grid. In order to overcome this problem, the synchronous power controller (SPC) is presented in this paper as a suitable alternative for controlling PV inverters. As will be further discussed, this controller permits PV plants to operate seamlessly in grid-connected and island mode, with no need of changing the control structure in either case. Moreover, the participation of SPC-based power converters integrating energy storage enables other grid-feeding systems to contribute to the grid operation in island conditions. The good results achieved with the SPC in different conditions will be shown in simulations, and also with experiments considering a real PV power plant combining SPC and commercial PV inverters.

Tuning of proportional resonant controllers for three phase PV power converters with LCL+trap filter

Power generation facilities based on renewable energy sources, such as PV or wind, are frequently connected to the grid through three-phase power converters. As the power of the plant increases, the design of the grid connection filters becomes critical, as a fast dynamic performance is needed while the injection of current harmonics to the network should be prevented. LCL filters, together with trap filters tuned at the switching frequency, provide a good trade-off between both needs. However, the tuning of the controllers becomes more complex as the LCL+trap filter lead to a considerably higher order system. In this paper a tuning method for determining the parameters of proportional resonant current controllers for three-phase high power converters linked to the grid through a LCL+trap filter is proposed and analyzed. In order to validate the proposed solution, different results taken in different scenarios are shown.

A simple approach for fast controller prototyping for a three phase interleaved DC-DC converter

Initial control prototyping in power electronics is often a matter of trial and error because of the complicated mathematics involved in modeling systems based on their internal structure. Moreover, in practice, small changes as the introduction of a measurement filter or the change of the sensing system appear all the time. If the controller design method was based on the internal structure of the system, then these small changes would mean a re-iteration through all the cumbersome equations. This paper presents a simple methodology for fast control prototyping based on the approximation of the Bode plots. As prerequisites, a power electronics simulation software as PSIM is needed. The method is presented by taking as a study case the design of a current control loop of a 30kW three phase interleaved DC-DC converter. Simulations are presented for the different considered controllers, and finally the method is validated by experimental results.

Smart Grids Operation with Distributed Generation and Demand Side Management

The integration of Distributed Generation (DG) based on renewable sources in the Smart Grids (SGs) is considered a challenging task because of the problems arising for the intermittent nature of the sources (e.g. wind or sun), only partly predictable. Another important issue concerning the design of SGs is how to support the consumers’ participation in the electricity market minimizing the costs of the global energy consumption. This chapter proposes an Energy Management System for SGs using Optimal Power Flow and integrating the demand side management mechanism and the active management schemes for the optimization of a SG in a competitive power market. The idea is that they can achieve a better integration of different types of DG improving flexibility while reducing the costs of energy for customers. The efficiency of the proposed technique is verified on a 19-bus 11 kV distribution network.