A. Berizzi

Comparison of voltage security constrained optimal power flow techniques

This paper compares two different optimal power flow (OPF) formulations that consider voltage security in power systems. The techniques are both based on multi-objective optimization methodologies, so that operating costs and losses can be minimized while maximizing the distance to voltage collapse. The techniques are described in detail and compared to study their similarities, as well as advantages and disadvantages. The comparisons are based on the results obtained by applying these two methods to a modified version of the 118-bus IEEE test system.

ORPF procedures for voltage security in a market framework

The paper deals with the automatic procedures used for the definition of voltage profiles to be adopted in the operation of bulk power systems with the aim of optimizing both the economic and security issues. After reviewing the most important automatic algorithms for optimal reactive power flow (ORPF) existing in the technical literature, some new ORPF procedures are proposed. They are based on modern optimization techniques and on a new formulation of the objective function tailored for the operation of power systems in a deregulated framework. The final aim of the paper is to demonstrate the effectiveness of ORPF procedures for the critical evaluation of the working point of power systems obtained from the day-ahead market auction. Furthermore, the tests reported show that, even at the day-ahead stage of energy market, it is possible to define the optimal voltage set points for generating units to be adopted for hierarchical voltage control in real time operation.

A probabilistic approach to power system security assessment under uncertainty

The deepening penetration of renewable resources, such as wind and photovoltaic solar, has introduced additional uncertainty into power system operation and control. This added uncertainty, together with the conventional sources of uncertainty, the loads and the availability of resources and transmission assets, makes clear the limitations of the conventional deterministic power flow in power system analysis and security assessment applications. Therefore, the explicit consideration of uncertainty requires the deployment of probabilistic approaches so as to provide the ability to manage the wide spectrum of all possible values of the input and state variables. In this paper, we make use of cumulant-based probabilistic power flow methodology to account for correlations among the input random variables. Extensive testing indicates good performance of probabilistic power flow. We illustrate application of the probabilistic power flow on the 14-bus IEEE test system and present a comparison with the result obtained by the computationally more demanding Monte Carlo approach. The probabilistic power flow results provide valuable information for power system analysis and security assessment and, in particular, provide insights into issues associated with line overloading, over-/under-voltage, and the critical ramping requirements from conventional generators in system with deep penetration of highly variable resources, such as wind farms.

Coordination of the hierarchical voltage control with the reactive power economic compensation

Voltage control and reactive power management are usually studied and applied mainly by the technical point of view. System operators and planners have developed complex control schemes, in order to achieve a secure and efficient operation. However, the deregulation of the electric systems requires the definition of market procedures ensuring an effective management of reactive resources on which the voltage regulation relies. If the economic issues related to the transmission network (and to the system control) have to be managed separately from the generation, the definition of payments for participating to the voltage regulation becomes a difficult task, bound by conflicting objectives, from both the technical and the economic viewpoints. Such objectives are: maintaining an adequate security level, defining correct economic signals for power plants equipment, providing a simple and transparent structure, ensuring market equity, and, last but not least, avoiding burdening on the final energy price. Whereas in a vertically integrated utility it should be sufficient to analyse the global behaviour of the system (transmission plus generation), within a competitive framework it is crucial to determine and meter the performance of each generator. After a brief review of the main procedures of reactive pricing available in literature, a simple scheme, aimed at enhancing the reactive resources management in the Italian market, is proposed, based on the economic quantification of the actual charges related to the voltage regulation.

A conjectural supply function model for the Italian electricity market

This paper presents a detailed electricity market model for analyzing the price development in the Italian electricity market under varying degrees of competitiveness in the market. The proposed model is more realistic than the models proposed for the Italian market by other studies, as it can be used when price elasticity of electricity demand is zero (the short term electricity demand in Italy is almost price inelastic) and it takes into account the Italian large transmission network with its constraints.

Expansion var planning model in a meshed/mixed AC/DC network

The importance of wind power generation has risen in the last years so much to be considered the most important and affordable renewable energy source with a growing rate faster than other mainstream technologies in the word. Due to the lack of land resources and some environmentalist opposition, the wind strategy is now looking at offshore installations. This requires new criteria to set out the appropriate architecture for future offshore grids. The use of HVAC Cables and LCC (Line Commutated Convertion) and VSC (Voltage Source Conversion) technologies stress the importance of extending the long-term expansion planning to the var support. Coping adequately with the problem of the voltage security assessment in a long-term expansion planning, is the purpose of this work. In this paper a model for the reactive expansion planning is proposed in the presence of a meshed AC/DC grid.

Reactive power pricing: a proposal for the Italian market

Voltage control and reactive power support have been deeply investigated by the technical point of view. System operators and planners have developed complex control schemes, in order to achieve a secure and efficient operation. After the enforcement of market structures, new issues have appeared. Nowadays, the main subjects related with the reactive power management are: maintaining an adequate security level, defining correct economic signals, providing a simple and transparent structure, ensuring market equity, and avoiding additional charges on the final energy price. The economic issues related to the transmission network (and to the system control) have to be managed separately from the real power generation and this results in the need for defining fair payments for participating to the voltage regulation. The evaluation of the costs of the reactive power demand and the benefits of the reactive power production can be performed by means of different methods: a brief summary of the approaches proposed in literature is reported, together with some results obtained by the use of a suitable optimal reactive power flow (ORPF), which embeds the hierarchical voltage control (HVC) scheme designed for the Italian EHV transmission system. The results can be used for setting the presented proposal for the reactive power pricing.

Islanding feasibility considering reactive power in the subtransmission systems

Nowadays, the penetration of Renewable Energy Sources (RES) in electrical networks is growing significantly. In spite of many problems it introduces, the connection of RES in the subtransmission systems also gives the opportunity of exploiting them for the benefit of the network. In this paper, this is done for islanding conditions and a Genetic Algorithm (GA) based procedure to make a stable islanded operation is proposed. The results are evaluated successfully by tests carried out on a real system.

Voltage security in modern power systems

This chapter deals with the computation of an optimal voltage profile using different optimisation strategies. For this purpose, the mathematical model of the optimisation problem is defined and described considering two issues: defining the constraints of the optimisation problem in order to fulfil the actual operating condition of the SVC system and testing different objective functions. A primal dual interior point method is proposed to solve the OPF problem and the structure of the matrices used by the method is described in detail. In particular, in the OPF models, a quadratic formulation of the PF equations is adopted. In this way, no trigonometrical equations are adopted. The main advantage of this formulation is the robustness of the algorithm.

Locational signal for TTC investment by sensitivity calculation

The zonal approach is now broadly adopted by electricity markets worldwide, since it provides the participants with an easily understandable representation of the transmission constraints. The zonal approach and the total transmission capability (TTC) interact in the sense that the higher is the TTC, the lower is the influence of transmission constraints on market results. As a consequence the TTC value is one of the most important issues in a liberalized environment. Its improvement is crucial for the market: it is typically pursued by investments on new branches. The effectiveness of a new line can be measured by evaluating its ability to relieve the transmission constraints that currently limit the TTC: since an exhaustive analysis of this problem would entail an unacceptable computation burden, a new approach based on a sensitivity approach is used to identify possible new lines for planning purposes The related benefits are eventually evaluated by effectively computing the relevant TTC. The effectiveness of this new approach is successfully proved on the Italian transmission network.