Hybrid DCAC converter Control for Inrush Current Protection

S. Mastelone, S. Almer, D. Martini, P. Raboni

filed 2016 // 29 June 2017

DC/AC converter comprising a new control device and method for managing inrush current transients during operation without requiring additional external devices or emergency operating modes forcing the DC/AC converter to shut down. The present invention is particularly, but not exclusively, aimed at grid connected DC/AC, converters, often subject to inrush current phenomena due to perturbations in the grid.

Photovotaic Inverter control method for operating an hybrid PV and genset microgrid without interphering with genset controller

P. Raboni, D. Martini

filed 2015 // 1 July 2017

A hybrid power system (1 ) is disclosed. The system comprises a plurality of dispatchable power resources (7.i; 7.j) and a first controller (12) for controlling the dispatchable power resources. The system further comprises a power resource section (5) responsive to environment conditions and a second controller (17) for controlling the power resource* section (5) responsive to environment conditions. The second controller (17) is configured and arranged for measuring a total power (∑Pgenset) generated by the plurality of dispatchable power resources (7.i; 7,j) and a total power (∑PPV) generated by the power resource section (5) responsive to environment conditions; and for generating a power curtailment signal (PVcurtailment), when required, for reducing the total power (ΣΡPV) generated by the power resource section (5) responsive to environment conditions.

Identification of the best power plant layout for large offshore wind farms connected via VSC-HVDC links

F. Careri, C. Genesi, P. Marannino, M. Montagna, P. Raboni, S. Rossi, et al.

Energy Market (EEM), 2011 8th International Conference on the European, 2011, pp. 281-287. // 27 May 2011

The overall purpose of this paper is to find out the best choice in terms of generator technology for large offshore wind farms connected to the EHV network by means of VSC-HVDC transmission: besides the traditional benefits compared with HVAC and line commutated HVDC, this technology allows variable frequency operation, in order to increase the overall production of the wind farm. Two different types of generators - Squirrel Cage Induction Generator (SCIG) and Doubly-Fed Induction Generator (DFIG) - are compared in variable frequency operation: the related power plant layouts are analyzed, for different wind speed conditions, by means of a slightly modified version of an online optimization algorithm. The resulting programs are coded in MATLAB® environment and some tests are carried out.

Intelligent control for a DC micro-grid system

M. Martino, Y. F. Quinones, P. Raboni, and Z. Chen

Universities Power Engineering Conference (UPEC), 2012 47th International, 2012, pp. 1-5 // 7 September 2012

This paper presents the dynamic response of a DC - micro-grid (DC-MG) controlled in master-slave mode. The benefits of the micro-grids (MGs) are the low cost in terms of power electronics converters and the high reliability and quality, even in case of loss connection to the transmission system. A DC-MG in fact can survive in standalone mode if properly managed. The considered system is made by a photovoltaic array (PV), a wind turbine (WT), a gas engine (GE) and an energy storage system (ESS). The DC-MG behavior is analyzed in different scenarios to demonstrate the efficacy of the control for all the units, especially in case of variable weather conditions with different DC loads. Thus the voltage level of the system and the power flow are shown, out of a detailed description of the power electronic interfaces featuring the distributed generators (DGs).

Reduction method for active distribution networks

P. Raboni and Z. Chen

2013 IEEE Grenoble, 2013, pp. 1-6 // 20 June 2013

On-line security assessment is traditionally performed by Transmission System Operators at the transmission level, ignoring the effective response of distributed generators and small loads. On the other hand the required computation time and amount of real time data for including Distribution Networks also would be too large. In this paper an adaptive aggregation method for subsystems with power electronic interfaced generators and voltage dependant loads is proposed. With this tool may be relatively easier including distribution networks into security assessment. The method is validated by comparing the results obtained in PSCAD® with the detailed network model and with the reduced one. Moreover the control schemes of a wind turbine and a photovoltaic plant included in the detailed network model are described.

Modeling and control of low voltage flexible units for enhanced operation of distribution feeders

P. Raboni, H. Weihao, S. K. Chaudhary, and Z. Chen

IECON 2013 - 39th Annual Conference of the IEEE, 2013, pp. 7469-7474 // 13 November 2013

In some networks Distributed Generators (DGs) are phasing out conventional power plants in terms of power production but still large efforts are required for providing ancillary services. In this paper the usage of fast response units like a Micro Turbines (MT) and a stationary Electric Vehicle Battery (EVB) is proposed for providing primary regulation in grid connected mode and for hierarchically manage an islanded LV distribution feeder. The unit models are described and a novel EVB model directly based on manufacturer's data is proposed and evaluated comparing its performances with SimPowerSystems library block. Moreover a voltage dependant power term is applied to the Voltage-Source Converter (VSC) control scheme of the EVB for improving the performances of the islanded feeder. The control is tested in case of under frequency and consequent load shedding occurring at the residential feeder of CIGRE C6.04.02 benchmark network.

Modelling, Control and Integration of Distributed Generators for Enhanced Ancillary Services

P. Raboni

PhD Thesis, Department of Energy Technology, Aalborg University, Aalborg Universitetsforlag, 2016 // 31 January 2016

The increasing share of Distributed Generators (DGs) poses concerns about network stability and their fair participation to the electricity market. Their aggregation in Virtual Power Plants, sometimes in combination with load prosumers, could represent an answer to these issues. Nevertheless Technical Virtual Power Plant still offer limited ancillary services, as reactive power management and spinning reserve. Focus of this thesis is on the control of DGs for providing further and higher quality ancillary services. In particular LV dispatchable DGs, hereinafter called flexible DGs, as stationary battery units (SBUs), microturbines (MTs) and Diesel Engines (DEs) appear promising solutions. The models of a SBU derived from a commercial Electric Vehicle battery and a MT are proposed. Both units are Voltage Source Converter (VSC) interfaced and, following recent Power Electronics state of the art, they are equipped with an LCL output filter. The thesis starts deepening the design of current control scheme for a stable operation in grid connected mode. Such flexible units offer on the other hand the possibility to survive a LV feeder in the future case of intentional islanding or for black start service. Therefore the design of the voltage regulator is developed proposing the compensation of the voltage drop on to grid side inductance. Moreover considering the hierarchical control a comparison between direct and reverse droop configurations is proposed. The participation to local reactive power market implies the precise control of reactive power through DGs. While reactive power regulator tuning is straightforward in case of current controlled VSCs, it is more challenging in case of synchronous generator interfaced DGs, as DEs and small CHP units, due to the high system order introduced considering the electrical machines, real exciter and network equations. The analytical tuning of the same regulator is here developed starting from small signal analyses of the plant, with an approach fitting to any reactance to resistance ratio of the equivalent Thevenin model regarded for representing the network. Such time consuming tuning procedure is then compared with an automated regulator design based on multiple time domain runs driven by a Simplex Optimisation. In particular some novel objective functions relying on typical step response indexes are proposed and assessed.

Download the document: download

Design of reactive power regulator of synchronous generators by considering grid impedance angle for characteristic index objectives

P. Raboni, S. Chaudhary, and Z. Chen

Design of reactive power regulator of synchronous generators by considering grid impedance angle for characteristic index objectives // 27 October 2016

Effects of low reactance to resistance ratio in distribution networks are widely studied but little work dealing with the tuning of voltage and reactive power regulators of small synchronous generators has been reported. This study endeavours the design of a proportional integral controller for the reactive power regulator to fulfil the characteristic index (CI) objectives, which are based on the step-response characteristics. Two small-signal models of the reactive power regulator have been developed from the well-known k-parameter model and they are validated. Apart from offline tuning of the regulator, these models can be used to assess system robustness and stability. In addition to the analytical approach, the same regulator is tuned optimally by means of multiple simulations. Proposed objective functions rely on the CIs tracked during the sequence of runs, whereas the traditional cost functions are formulated on the basis of the integral of an error. This difference makes them suitable for the cases where the entire step-response data series are unavailable. The performances of differently tuned regulators are compared considering a test system including a 100 kW Diesel Generator Set equipped with a brushless exciter.

Energy transition technology roadmap. Distinguishing hype from reality – 2020 strategic technological plan and business targets

Daniele Rosati, Pietro Raboni, Carlalberto Guglielminotti

White paper // 23 may 2017

Climate change and global warming in particular may be a controversial scientific theory, particularly in the US, but its growing impact on business is undeniable. The energy generation landscape, to begin with, is being disrupted by the growth of renewables; transport is witnessing the affirmation of the electric car, in its pure form or as a hybrid, and, most pervasive of all, the search for energy efficiency is creating new businesses – think for example smart metering or behind-the-meter distributed generation – and is modifying existing ones in a variety of ways. Our vision of the macrotrends shaping the energy transition is consistent with the narrative of Bloomberg New Energy Finance's New Energy Outlook, to which we make extensive reference in this overview.

Download the document: download

Optimization Models for Islanded Micro-Grids: A Comparative Analysis between Linear Programming and Mixed Integer Programming

Alberto Dolara, Francesco Grimaccia, Giulia Magistrati, Gabriele Marchegiani

Energies 2017, 10(2), 241 // 16 February 2017

"This paper presents a comparison of optimization methods applied to islanded micro-grids including renewable energy sources, diesel generators and battery energy storage systems. In particular, a comparative analysis between an optimization model based on linear programming and a model based on mixed integer programming has been carried out. The general formulation of these models has been presented and applied to a real case study micro-grid installed in Somalia. The case study is an islanded micro-grid supplying the city of Garowe by means of a hybrid power plant, consisting of diesel generators, photovoltaic systems and batteries. In both models the optimization is based on load demand and renewable energy production forecast. The optimized control of the battery state of charge, of the spinning reserve and diesel generators allows harvesting as much renewable power as possible or to minimize the use of fossil fuels in energy production.”

Performance Analysis of a hybrid micro-grid in Somalia

Alberto Dolara, Elisa Donadoni, Sonia Leva, Giulia Magistrati, Gabriele Marchegiani

IEEE PES PowerTech 2017 // 2017

This paper presents a data analysis of an islanded diesel-PV-battery system, placed in Somalia. Operation of each generator is observed, concerning the most relevant performance indicators. Analysis is carried out during the first 11 months of operation, from November 2015 to September 2016. Investigations are divided into two periods, according to a significant load variation due to the connection of a new distribution line to the power plant. Thus, a comparison on load demand sharing among generators is realized between the two periods. In conclusion, benefits in terms of fuel consumption, CO2 emissions and money savings are presented.

Distributed storage for provision of ancillary services to the main grid: project PRESTO

S. Nassuato, G. Magistrati, G. Marchegiani, D. Falabretti, M. Delfanti, M. Merlo

Energy Procedia, vol. 99, pp. 182-193 // November 2016

This paper presents the three-year PRESTO research project (2013-2015). PRESTO is a self-funded project developed by the Department of Energy of Politecnico di Milano in cooperation with FIAMM Storage, Elvi Energy and MCM Energy Lab (an Italian spin-off). Within the project, experimental tests and numerical simulations were performed in order to evaluate the effectiveness of an Energy Storage System (ESS) in the provision of ancillary services to the main grid. This paper focuses specifically on the experimental and numerical analyses carried out in the project to develop an innovative control law for the primary frequency regulation, able to maximize the performances of the regulating service and effectively manage the ESS state of charge.

Optimal Management Algorithm for Battery Energy Storage System Included in an Islanded Micro-Grid

Alberto Dolara, Giulia Magistrati, Francesco Grimaccia and Gabriele Marchegiani

16th International Conference on Environment and Electrical Engineering (EEEIC) // 7-10 June 2016

This work proposes an optimization method for the management of a Battery Energy Storage System (BESS) integrated in an islanded Micro-Grid, including Renewable Energy Systems (RESs) and generators burning fossil fuels. The optimized control of the battery State of Charge (SOC) allows to get the BESS available as much as possible to support the RES power production, hence minimizing the use of fossil fuels. The optimization of the BESS is based on load demand and RES production forecast. The Optimization Algorithm that has been adopted is the Linear Programming, using the method of the Interior Point.

Control loops design in a grid supporting mode inverter connected to a microgrid

Giovanni M. Foglia, Luisa Frosio, Matteo F. Iacchetti, Roberto Perini

2015 17th European Conference on Power Electronics and Applications (EPE'15 ECCE-Europe) // 8-10 September 2015

This paper deals with some design aspects of the control loops in a droop-controlled VSI connected to other inverters of greater ratings or to a strong grid, which sets the frequency. The design of a damping resistor in series with the AC filter capacitor is discussed and the impact of strong feedback between the output current and the capacitor voltage is investigated. Moreover, an analysis of the derivative droop coefficients is carried out through a simplified equivalent circuit.

Hybrid micro-grid experimental application in Tanzania

M. S. Carmeli, M. Mauri, C. Brivio, P. Guidetti, S. Mandelli, M. Merlo, R. Perini, G. Marchegiani, D. Rosati

2015 International Conference on Clean Electrical Power (ICCEP) // 16-18 June 2015

This paper is intended to introduce the project Energy4Growing that aims at studying and setting up a hybrid micro-grid to supply power to a school in rural Tanzania. It describes the results of the early project actions which concerned in: (i) analysing the actual school power supply system through locally metered data; (ii) developing the first step of a new electro-mechanical model which addresses the simulation of operation and dynamics within an off-grid power system; (iii) validating the new model. In particular the paper focuses on describing the development and the first step implementation of the new modelling approach which address: (i) voltage and frequency analyses over medium term period by means of simplified electromechanical models of power sources and power electronics, (ii) the analysis of different dispatch strategies and their consequence on voltage and frequency trends, (iii) the effects of dispatch strategies on the energy performances of particular components such as battery bank or dump loads. The new approach has also been applied in modelling the current school power supply system.

Electricity tariff reform – Discourages energy efficiency

Giuseppe Artizzu // february 2015

The solution suggested by the authority to resolve problems is to penalise and shift part of the financial burden on to those who produce renewable energy power, which seems to be contrary to what the legislations state. Here follows Giuseppe Artizzu’s opinion on the proposal for reform of the electricity rates of residential customers. Having summarized the proposed reform of the electricity rates for residential customers deeply discussed by the energy authority, we will host a debate about it on the following pages. We have published the opinions of GB Zorzoli the Honorary President of FREE, and of Chicco Testa, the President of Assoelettrica.

Opec had no choice

Giuseppe Artizzu // january 2015

The long-awaited Opec Summit in Vienna on thursday ended with nothing concrete: cartel production in 2015 will remain the current one, despite the collapse of more than 30% of barrel price since this summer. Who has interpreted the decision as a deliberate choice by the Saudis to make a price war on American producers is wrong. Opec had no choice, the financial perspectives are grave for weaker members of the cartel, such as Venezuela and Nigeria, in a context of low oil prices. What determines the excess supply on the international oil market are not the big producers who challenge the hegemony of the Kingdom, but hundreds of small American oil companies, which drill as if there was no tomorrow. For these, the future consequences of petrol supply are not taken into consideration.

Technical Strategies for Voltage Power Regulation in LV Distribution Networks

M. Delfanti, L. Frosio, G. Monfredini, M. Merlo, C. Rosati, D. Rosati, G. Marchegiani

Distributed Generation and Alternative Energy Journal, vol. 30, no.4, pp. 7-14 // 1 January 2015

We report the integration of a novel voltage regulation strategy in the inverter control algorithm for Dispersed Generation (DG) applications. The adoption of a decentralized voltage control based on local curves allows the voltage profile to be controlled at the Point of Common Coupling (PCC) of the generators and, therefore, the capacity of existing distribution networks to host more generation from renewables. The work is focused on a low voltage distribution network and both the reactive power modulation and the real power curtailment are considered in the proposed local strategy. Both numerical and experimental analysis are necessary in order to test and validate the proposed voltage control strategy. Firstly, the performances of the proposed control are tested by a numeral analysis and the impact on both the electrical system and the inverter apparatus is evaluated. Secondly, the benefit of the control strategy is analysed by experimental analysis in a test facility to verify the feasibility of voltage regulation in a real feeder.

Hybrid distributed generation system for a rural village in Africa

M. S. Carmeli, P. Guidetti, M. Mancini, S. Mandelli, M. Mauri, M. Merlo, R. Perini, G. Tomasini, G. Marchegiani, D. Rosati

3rd Renewable Power Generation Conference (RPG 2014), pp. 1-6 // 24-25 September 2014

Nowadays the use of distributed generating systems, which use a renewable energy source, has experienced a fast development. Moreover, their intrinsic intermittent nature can be overcome using hybrid systems which combine more energy sources. This paper proposes the study and design of a hybrid generation system able to cover the energy needs of a school in a rural village in Africa (Tanzania), combining renewable and/or alternative sources and storage systems. The final results consist in the definition of an electric grid architecture suitable to interface renewable and energy needs in the most reliable way, exploiting advanced regulation and control techniques.

Why solar energy has systemic relevance

Giuseppe Artizzu // june 2014

Photovoltaics has assumed a role, sometimes that is a dominant one in the coverage of daily requirements. Its cost, especially in southern Italy, is competitive with that of fossil fuels. The near future is auto-consumption and accumulation. How can we change the logic of the electricity market? Giuseppe Artizzu’s article published in the latest issue of the magazine QualEnergia. In 2013 photovoltaics covered 7% of Italian electric requirements, a share that has no equal in the world. But due to the seasonal profile and time, this figure is not representative of the actual role that technology has taken. However in winter, there is a tangible impact on the wholesale price of electricity, the weight on the need is still limited, in summer and even more in the late spring technology has transformed the market, arriving to cover in a structural way 15-20% of the working day (significantly more for the holidays).