E. Telaretti

An Innovative Conversion Device to the Grid Interface of Combined RES-Based Generators and Electric Storage Systems

This paper is focused on the development of an innovative device, which is based on a bidirectional converter, for the interface to the supply utility grid of combined renewable-energy-source-based generators and electric storage systems. The device can be controlled so as to ease the interface between the low-voltage grid and photovoltaic or wind generators combined with lithium-ion LiFePO4 batteries, taking into account the requirements of the reference technical standards for users connection and offering different ancillary services. The operational functioning of the device, the architecture, and its electronic components, as well as laboratory and field test activities and results are described. The conversion device has been developed, and the main results that have been achieved are detailed.

Transient stability simulation of a fixed speed wind turbine by Matlab/Simulink

This paper describes the dynamic behavior of a typical fixed speed wind turbine connected to the grid; the model is developed in the simulation tool Matlab/Simulink and created as a modular structure. The pitch control system is used for stabilization of the wind turbine at grid faults. In this way, voltage stability of the system with grid-connected wind turbines can be improved by using blade-angle control for a temporary reduction of the wind turbine power during a short-circuit fault in the grid. This paper shows a new variable control for maintaining of voltage stability, when a three-phase fault is applied close to the wind turbine and cleared by disconnecting the affected line. In this way, a better re-establishing of the voltage after the fault is obtained.

Financial measures for supporting wind power systems in Europe: A comparison between green tags and feed’in tariffs

Distributed Generation demonstrates to be able to significantly influence the electric power system, bringing considerable benefits to the quality of service and improving the energetic efficiency of the distribution network. In particular, in the last few years, Distributed Generation from renewable energy sources is considerably increased thanks to the many support policies promoted worldwide with the aim of facing the environmental issue and the problem of the permanent rise of prices of the fossil energy. In this contest, after a brief review on the financing strategies carried out in Europe in the last decade for promoting the recourse to renewable energy, the paper presents a comparison among the main support strategies for Wind Farms (Feed-in tariffs and Green Tags), taking into account the situation in some European Countries.

A new device for the control and the connection to the grid of combined RES-based generators and electric storage systems

In this paper a new device for the control and the connection to the supply utility grid of combined RES-based generators and electric storage systems is presented. The device is a bidirectional converter that can be controlled so as to facilitate the interface between the low voltage grid and photovoltaic or wind generators combined with batteries, contemporary addressing the requirements of the reference technical standards for users connection and providing different ancillary services. In the paper the functioning of the device and its circuit diagram are described, the single parts are shown and the potentiality of its control system are commented.

Economic impact of medium-scale battery storage systems in presence of flexible electricity tariffs for end-user applications

This article addresses the question if it is cost-effective to use medium-scale battery storage systems for time-of-use (TMO) energy cost applications at a consumer level, assuming flexible electricity tariffs in the Italian electricity market. TMO application allows customers to reduce their electricity bill charging the storage when off-peak time periods are applied and discharge it during on-peak periods, when electricity energy prices are high. The analysis is carried out taking into account three different electrochemical storage technologies. The economical comparison shows that, at the current costs of storage technologies and at the electricity tariffs currently offered in Italy, the use of battery systems for TMO energy cost applications is not economically advantageous for a medium-scale public institution facility. However, the aggregation of different benefits for the same storage technology, such as voltage support, deferral of investment in T&D network upgrades, reducing T&D losses and so on could modify this assumption.

An Innovative Measurement Approach for Load Flow Analysis in MV Smart Grids

This paper proposes a novel measurement procedure for load flow analysis in medium voltage (MV) smart grids. The new approach is based on power measurements at low voltage (LV) side of MV/LV distribution substations. Such measurements are carried out by means of power quality analyzers. If compared with the load flow methods based on measurements at the MV side, the proposed approach has a lower cost and it offers an equally reliable measurement system. The proposed algorithm takes the basic concept of the current summation backward/forward method; some modifications are introduced to use LV load powers measurements. To this aim, the MV/LV power transformer is properly modeled and taken into account in the algorithm. The proposed approach is validated by means of both simulations and experimental measurements; the test system is the MV distribution grid of Ustica Island, Italy. First, the proposed algorithm is compared with the MATLAB/Simulink load flow tool, which is based on the traditional Newton-Raphson method. Second, a comparison is carried out between the proposed method and the measurement data collected in the actual MV network of Ustica. Such comparison takes into account the uncertainties of both proposed method, and experimental active and reactive MV power measurements.

New measurement procedure for load flow evaluation in medium voltage smart grids

The aim of this paper is to present a new approach for the medium voltage (MV) distribution network load flow analysis, mainly based on power measurement at the low voltage (LV) level of MV/LV distribution substations. This allows to use measurement instruments, usually already installed in the secondary substations, thus achieving an equally reliable measurement system with a lower cost compared to measurements at the MV side. The new approach can be applied using a proper communication system to collect the nodal measurements and an iterative algorithm based on ladder iterative technique (LIT) to compute the load flow. The validity of this method is presented and discussed on the basis of the simulation results of Ustica islands distribution network.

Economic impact of Ice Thermal Energy Storage Systems in residential buildings in presence of double-tariffs contracts for electricity

The current paper presents a study on the effects of the installation of Ice Thermal Energy Storage Systems for cooling on the power daily profile of residential buildings, and examines the economic repercussions on the electricity billing. With reference to this second theme, the paper examines, in particular, the convenience of the installation of a domestic Thermal Energy Storage Systems, in presence of a doubletariff contract for electricity. For the estimation of the daily cooling request and power profile a Monte Carlo approach is used.

Economic feasibility of a customer-side energy storage in the Italian electricity market

Electricity prices show significant short-term variations during the day due to the need of balancing supply and demand in real time. Normally, customers are not exposed to these variations but pay a constant electricity price. In an attempt to reduce the volatility of the wholesale prices, several utilities are moving from conventional fixed-rate pricing schemes to new market-based models, where the electricity price can fluctuate during the day depending on the market conditions. Examples of time-dependent pricing schemes are Time-Of-Use (TOU) tariffs, where the electricity price can take two or three price levels during the day, or Real-Time Pricing (RTP) tariffs, where the energy price can vary hourly depending on the wholesale market. Electricity customers can thus take profit from the installation of storage technologies, storing electricity when the price is low and reselling it back to the grid during peak hours, at a significantly higher price. In this paper, a sodium-sulfur (NaS) battery is considered to be added to a medium-scale public facility, in order to reduce the electricity bill. The storage is operated exploiting a simple strategy (charging during off-peak, discharging during on-peak prices) with the aim of maximizing the saving for the end-user. The economic viability of using the storage is evaluated and discussed, referring both to the hourly national prices and to the zonal electricity prices of the Italian energy market. The results show that the payback period (PBP) is greater than the battery life when the customer prices are assumed proportional to the single national prices. The situation could change in the near future, thanks to the cost reduction of storage technologies expected in coming years.

Battery storage systems for peak load shaving applications: Part 1: Operating strategy and modification of the power diagram

Battery Energy Storage Systems (BESSs) are gaining more and more attention in the current global energy landscape, thanks to some peculiar characteristics such as the versatility, modularity, scalability, fast response time, etc. BESSs can advantageously be used in the ancillary services market, in order to provide all the services necessary to ensure the stability and the reliability of the power system. Among other applications, BESS can also be used to shift electricity demand from on-peak to off-peak periods, also contributing to relieve peak demand charges, thus ensuring a saving for the customers. The BESS is charged when the electricity prices are the lowest (during the night) and discharged when the prices are the highest (during the daily hours), contemporary achieving a leveling of the daily usage pattern. This paper is the first of a two-part series, aiming to evaluate the economic feasibility of electrochemical storage systems in peak load shaving applications. The sizing and the operating strategy of the BESS are defined, so as to obtain, at the same time, the flattest daily usage pattern and the maximum benefit for the customer. A case study is analyzed, showing the modification of the power diagram when the BESS is added. The simulation results show a customer daily power profile perfectly leveled in shoulder seasons and a peak shaving effect in all other days.