Riccardo Vailati

HVDC Connection of Offshore Wind Farms to the Transmission System

This paper presents a technical and economic analysis to evaluate the benefits and drawbacks of grid connecting offshore wind farms through a dc link. A first case, concerning a 100-MW wind farm, is thoroughly investigated and cases of larger wind farms (200 and 500 MW) are presented. Three different transmission solutions are compared: 150-kV ac, 400-kV ac, and high-voltage dc based on voltage sourced converters (VSC-HVDC). After a brief overview of the features of these connection solutions, the related operational aspects are evaluated. An economic assessment compares the dc connection option to the ac alternatives, taking into account the investment, operation, and maintenance costs, and the negative valorization of losses and energy not supplied. Economic assessment includes sensitivity analyses of parameters, which could impact the 100-MW wind farm: distance, component costs, dc converter reliability, and dc converter losses

A genetic algorithm-based procedure to optimize system topology against parallel flows

Parallel (or loop) flows consist in the undesired circulation of power flows through certain interconnection corridors. Remedial actions available to transmission system operators or system planners include installation and operation of phase-shifting transformers and of dc transmission systems. Moreover, the invaluable experience of transmission system operators has shown that the network can be operated so as to reduce parallel flows also by properly selecting the topology of the system. In the present paper, a genetic algorithm-based procedure is designed for the topological optimization of the network against parallel flows. The control variables considered are the status of substation breakers and the location (and angle) of phase-shifting transformers. The problem is formulated as a multiobjective optimization. The main objective is that of reducing the power transfer distribution factor of an assigned transaction with reference to a set of lines; N and N-1 security levels are accounted for by means of subsidiary objective functions. The procedure is tested on a small CIGRE sample system and on a 4500-bus network representative of the European electric system (UCTE).

OPF tools for optimal pricing and congestion management in a two sided auction market structure

This paper presents the application to large-scale systems of a new OPF model, which is characterized by the introduction of a two sided auction market structure, with power demand elasticity. This means that the load demand is no longer fixed and each customer presents a demand bid, giving the ISO additional degrees of freedom in managing congestion conditions. The use of the OPF is envisaged in a pool model where the independent system operator (ISO) has a centralized dispatch function and he is also responsible for the security and the quality of operation. The ISO runs an OPF to determine the optimal solution, taking into account the network constraints; the byproducts of this optimization are the electricity prices at each bus of the network. In the competitive environment the opening of the transmission system to the market players is leading more than in the past to congestion conditions, with electricity price volatility and price spikes. In the work we show the capability of the OPF based on a two sided auction structure to furnish to the ISO a solution that reduces nodal price volatility and allows congestion relief. A CIGRE 63-bus test system with 5 areas is adopted for an easy comprehension of the usability of the proposed tool. Besides, some important analyses of different scenarios of the Italian market are examined. The results of the investigation put in evidence the presence of bottlenecks in the transmission system in some of the border areas, which limit the TTC of the interconnection with the UCTE networks. The economic signals provided by the nodal price distribution and by the congestion costs are envisaged as useful tools for the Italian ISO (GRTN), market administrator (PX or GME) and market players for planning, operational planning and short term operation.

Transmission expansion issues for offshore wind farms integration in Europe

This paper presents an overview of the major issues concerning the expansion of the European transmission system for the connection and for the integration of offshore wind farms. These issues are basically related to: i) the connection solutions for offshore wind farms (infrastructures at sea), ii) the reinforcement of the onshore transmission system for accommodating the power produced by the wind farms, while guaranteeing system security, and iii) the options for facilitating transmission investments (definition of priority projects, funding support schemes, regulatory frameworks). Concerning the first issue, we compare two connection solutions with reference to a 100-MW offshore wind farm: 150 kV alternating current (AC) and high voltage direct current based on voltage sourced converters (VSC-HVDC). An economic assessment takes into account the investment, operation and maintenance costs, and the negative valorization of losses and energy not supplied. As far as the second issue is concerned, we describe the prospects for the development of offshore wind farms and, taking into account the characteristics of the existing European wind and hydro pumped-storage facilities and a possible wind-hydro combination, we discuss the need for system reinforcement. With regard to the third issue, we present the options and instruments which exist and which are under discussion in order to facilitate transmission expansion investments, such as the list of electricity projects of common interest prepared by the European Commission and the regulatory framework of tenders for licensing offshore electricity transmission in the United Kingdom (UK).

Impact of greenhouse gas emission constraints on the electrical system of continental Europe

The purely economic approach world-wide followed by electric market actors in the recent years led to a significant increase of cross border power trades. But nowadays, after the entry into force of the Kyoto protocol, it is possible that new environmental obligations concerning the emission of greenhouse gases (GHG) could partially modify this approach. Two main impacts on the electric sector could be envisaged: first, an increase of the electricity price for final customers, due to higher costs in power generation and, second, a different distribution of the power flows among the countries, related to different national behaviours and needs in respecting the GHG constraints. With the aim of preventing high burdens, the European union promoted the so-called emission trading scheme (EU-ETS). This mechanism allows some industrial activities, that cause GHG pollution, to develop a trading of allowances within the Community in order to promote reductions of emissions in a cost-effective and economically efficient manner. The paper will assess the impact of Kyoto constraints and of the proposed EU-ETS scheme on the European power system presenting a suitable multi-area optimisation tool and its application to study the optimal operation of the interconnected network of Western Europe.