F. Zanellini

Steady-state and dynamic approaches for the evaluation of loadability margins in the presence of secondary voltage regulation

The electricity markets are changing the power system operation. The increasing power exchanges make it necessary to operate the transmission grids closer and closer to security limits. A problem posed to power system engineers is therefore to find a suitable methodology to combine the results of both steady-state and dynamic tools: this can reduce the overall computational effort and the difficulty in the interpretation of results. The paper presents a detailed comparison of the computations that can be performed through steady-state and dynamic procedures regarding the power system security. In particular, the analysis of the loadability margins available on the corridor between the Italian power system and the Union for the Coordination of the Transmission of Electricity (UCTE) grids is carried out using both steady-state and dynamic tools; the results are compared, pointing out also the security enhancement given by a hierarchical voltage control.

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.

Impact of real losses and operational constraints on the reactive support value in presence of a hierarchical voltage control

The aim of the paper is to assess the impact of the real power losses and of the satisfaction of some operational constraints on the value and the consequent price of the reactive power support in a new competitive electricity market. The evaluation of the marginal costs of the reactive power demand and the marginal benefits of the reactive power production is performed by the use of a suitable optimal reactive power flow program (ORPF) which takes in the due account the hierarchical voltage control (HVC) scheme designed for the Italian EHV transmission system. The marginal costs (benefits) are determined by a linear combination of the real losses gradient and of the active constraints sensitivities with respect to the nodal reactive power injections in the ORPF solution point Therefore the impact of the operational constraints adopted by the transmission system operators (TSO) on the reactive support value is strictly tied to the Lagrange multipliers of the active constraints of the problem. The ORPF procedure has been applied to the analysis of the operation of the EHV Italian system for different voltage control structures. The results show that an adequate HVC scheme is suitable to exploit the local nature of reactive power allowing the introduction of a regional reactive market, obviously related to the control areas of the secondary voltage regulation (SVR).

Cross border transmission capacity allocation by multilateral market coupling

The problem of cross border interchange and of allocation of exchange capacities between interconnected countries of the European community has gained an ever increasing importance during the most recent years. The reasons for that are to be found in the existing price gap between different countries as well as in the limited exchange capacity of some interconnection corridors. As a result, the European transmission system operators (ETSO) committee has issued a sequence of documents illustrating the guidelines for efficient and economic assignment of the available cross border capacity. In addition to the ETSOpsilas efforts, an example of successful multi-market coordination is given by the trilateral market coupling system developed by the transmission system operators of France, Belgium and the Netherlands. In the present paper, the idea of multilateral market coupling is re-considered and extended to an arbitrary number of interconnected countries. The proposed procedure elaborates the net export curves, relative to each country, by means of a suitable coordinating procedure based on linear programming. Examples of the application of the proposed procedure are given with reference to a small test network as well as to an equivalent model of the European (UCTE) system.

Identification of Critical Outages Leading to Cascading Failures in Electrical Power Systems

The extended black-outs that have occurred in the near past, have stimulated a renewed interest in the assessment of power system vulnerability. Accurate post-mortem analyses of the causes involved in a major black-out have shown that, in most cases, the outage of a line or a generator is the initiating event of cascading faults that may rapidly lead to a catastrophic failure. The role of hidden faults has been emphasized, but a systematic screening of the protection system in search for hidden faults remains a formidable and almost impossible task for today power engineers. The aim of the present paper is that of carrying out a more effective screening of single and multiple contingencies looking for those situations which are likely to jeopardize system security and to expose possible hidden faults. The problem of finding the worst branch or generator outages is recast in the form of a linear programming problem with mixed (real and binary) variables which is solved by efficient branch-and-bound techniques implemented in the CPLEX optimization package. Tests carried out with reference to the Italian EHV network and to the European UCTE system, confirm the effectiveness of the proposed contingency screening approach.

A multilateral market coupling approach for the allocation of cross border transmission capacity

The problem of cross border interchange and of exchange capacity allocation has gained an increasing importance because of existing price gaps between the different countries and of limited exchange capacity. The European Transmission System Operators (ETSO) committee has issued a sequence of documents illustrating the guidelines for efficient and economic assignment of the available cross border capacity. An example of successful multi-market coordination is given by the trilateral market coupling system developed by the TSOs of France, Belgium and the Netherlands. The aim of the present paper is that of finding a procedure able to make results from market coupling as close as possible to those furnished by single electricity pool.

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.

Steady state and dynamic approaches for the evaluation of loadability margins in the presence of secondary voltage regulation

Summary form only given. The electricity markets are changing the power system operation. The increasing power exchanges make it necessary to operate the transmission grids closer and closer to security limits. A problem posed to power system engineers is therefore to find a suitable methodology to combine the results of both steady state and dynamic tools: this can reduce the overall computational effort and the difficulty in the interpretation of results. The paper presents a detailed comparison of the computations that can be performed through steady state and dynamic procedures regarding the power system security. In particular, the analysis of the loadability margins available on the corridor between the Italian power system and the UCTE (Union for the Coordination of the Transmission of Electricity) grids is carried out using both steady state and dynamic tools; the results are compared, pointing out also the security enhancement given by a hierarchical voltage control.

Load and LTC modeling for voltage stability assessment in EHV network studies

In the last years electric systems, all around the world, experienced voltage stability problems, basically related to their exploitation, near to the limits, caused by market opportunities and progressive growth of loads with low power factor (air-conditioning, electronic devices, etc.). In this paper it is investigated the problem of modeling some specific components of the electric power system, for the simulation of its dynamic response to disturbances, with particular attention to the voltage instability and collapse phenomena. The paper focuses on the analysis of different types of load models and their influence on the voltage stability assessment of electric power systems. Also the effects of LTCs are investigated.