Power system planning methods and experiences in the energy transition framework

In recent years, the unbundling of the electricity market together with the profound “energy landscape” transformation have made the transmission network development planning a very complex multi-objective problem. The climate and energy objectives defined at the European level aim for a deepening integration of the European power markets and the electricity sector is recognized as one of the main contributors to the energy transition from a thermal-based power system to a renewable-based one. In the deregulated framework, network planners have to satisfy multiple different objectives, including: facilitating competition between market participants, providing non-discriminatory access to all generation resources for all customers, including green resources, mitigating transmission congestions, efficiently allocating the network development actions, minimizing risks associated with investments, enhancing power system security and reliability and minimizing the transmission infrastructure environmental impact. Further complexities are related to the significant uncertainty about future energy scenarios and policy rules. In particular, the increasing distributed renewable energy source integration dictated by the European energy targets, raises several issues in terms of future power flow patterns, power system flexibility and inertia requirements, and cost-effective development strategies identification. The thesis aims to investigate various aspects concerning the transmission network planning, with particular reference to the Italian power system and the experience gained working in the “Grid Planning and Interconnections Department” of Terna, the Italian Transmission System Operator. One of the main topics of this work is the use of the series compensation to exploit operating limits of underused portions of the HV – EHV transmission network in parallel to critically loaded ones, in order to control and provide alternative paths for power flows. The purpose is to extend the allowable transmission capacity across internal market sections. To this aim, a specific application of series compensation (together with reconductoring) to exploit the transfer capacity of a 250 km long, 230 kV-50 Hz transmission backbone spanning the critical section Centre South – Centre North is illustrated. The results are validated by means of static assessment and similar applications could be hypothesized for grid portions in the South of Italy where the primary network is mainly unloaded whereas the sub-transmission network reaches high levels of loading because of the huge renewable generation capacity situated there. A further characteristic of modern power systems is the need to integrate high levels of renewable energies while fulfilling reliability and security requirements. The offshore wind farms perspectives in the Italian transmission system are evaluated, considering policies, environmental and technical aspects. Furthermore, the adoption of the HVDC technology in parallel to the AC traditional system topic is addressed: planning static and dynamic studies involving a real HVDC Italian project are proposed. In particular, the impact of the planned HVDC link on the loadability and the dynamic performance of the system is investigated in medium and in long-term future planning scenarios. The evaluation of the thermal performance of a specific grid portion in the South of Italy affected by significant increase of power generation by variable energy sources is proposed both in the current situation and in the future scenarios in order to highlight the benefits related to the presence of the planned network reinforcements. Finally, some issues of the prospective reduced inertia systems are illustrated and a possible methodology to evaluate the economic impact of inertia constraints in long-term market studies is proposed. In the light of the emerging concept of power system flexibility, traditional planning evolved to assess the ability of the system to employ its resources when dealing with the changes in load demand and variable generation. Flexibility analyses of the Italian power system, carried out in terms of some market studies-based metrics and grid infrastructure-based indexes, are provided. The flexibility requirements assessment in planning scenarios are of interest to evaluate the impact of network development actions and have been included in the yearly National Development Plan. The last research topic involves the cost-effective target capacity assessment methodology developed by Terna in compliance with the Regulator directives presented together with the results yielded by its application to each significant market section of the Italian power system. The methodology has been positively evaluated from academic independent expert reviewers, and its outputs are relevant for the policy makers, regulatory authority and market participant to assess and co-design the energy transition plan of a future European interconnected power system

Technical and Economic Impact of the Inertia Constraints on Power Plant Unit Commitment

The whole interconnected European network is involved in the energy transition towards power systems based on renewable power electronics interfaced generation. In this context, the major concerns for both network planning and operation are the inertia reduction and the frequency control due to the progressive decommissioning of thermal power plants with synchronous generators. This paper investigates the impact of different frequency control constraints on the unit commitment of power plants resulting from market simulations. The market outputs are compared in terms of system costs, and of frequency stability performance evaluated on the basis of the rate of change of frequency and the maximum frequency excursion. The best compromise solution is found using a multiple-criteria decision analysis method, depending on the choice of the decision maker’s weighting factors. The proposed approach is tested on a real case taken from one of the most relevant future scenarios of the Italian transmission system operator. The results show how the best compromise solution that can be found depends on the decision maker preference towards cost-based or frequency stability-based criteria

A novel Target Capacities Identification Methodology: the Italian Approach

In order to ensure power system reliability and security in long term scenario, the identification of the costeffective interconnection levels is one of the main objectives in the modern network planning framework. Terna, the Italian Transmission System Operator (TSO), with the goal of defining the efficient target capacities for each significant market section of the Italian power system developed a novel iterative methodology. The aim to this paper is to provide an exhaustive presentation of the procedure and to show the results obtained for the critical section Centre South–South. In particular, the price spread and the number of congestion hours resulting from market simulations are the main parameters investigated to decide which sections/borders are suitable for additional transmission capacity implementation. The corresponding benefits must be verified by the means of market and network analysis in order to demonstrate the cost-efficiency for the system. The analyses were carried out in two different contrasting future energy scenarios in order to obtain the related different development strategies

Cost-Effective Target Capacity Assessment in the Energy Transition and sensitivity analysis on the main inputs

The cost-effectiveness of new transmission grid infrastructures became a more complex issue in the liberalized electricity market. The Transmission System Operators (TSOs) have to manage relevant uncertainties in long-term planning studies by the means of new approaches able to identify the power system needs in future energy scenarios. In view of the above, the Italian TSO developed a novel methodology to assess the target capacity that evaluates the additional cost-effective transfer capacity between internal bidding zones and at borders of the Italian power system in different planning scenarios. The aim of this paper is to present the results obtained from the application of this methodology to the Italian power system in two contrasting energy scenarios for 2030 horizon. Furthermore, the impact of some input data and other significant parameters variation, is assessed in sensitivity analysis performed by the means of market simulations

Series Compensation of an Uprated 230 kV – 50 Hz Backbone in Central Italy: Impact on Fault Quantities

The predominant reason for adding series capacitors in power systems is to increase power transfer capacity enhancing both the voltage and angle stability margins. The resulting decrease in series reactance can determine also undesired effects during faults: the fault current increases in dependence of the compensation degree and of the fault location in the system. This paper investigates the case of an Italian 230 kV – 50 Hz backbone spanning the Centre North – Centre South section, for which re-equipment with high-temperature conductors in conjunction with the installation of 60% capacitive series compensation are being evaluated. The impact of series compensation on faults levels (both phase-to-phase and phase-to-ground) and on the earth fault factors is systematically studied, within the backbone and in the nearby area. A parametric analysis is carried out, considering different generation, load and network scenarios evidencing the impact of series compensation is compatible with present equipment and system operation

Integration of Wind Offshore Generation into the Italian Transmission Network: connection solutions and case study

Offshore wind is a rapidly maturing renewable energy technology that is set to play a primary role in future energy systems. So far, offshore wind provided a small fraction of global electricity demand, but it is set to expand strongly over the next two decade. Turbines are growing in size and in terms of the power capacity they can provide, which in turn is delivering major performance and cost improvements for offshore wind farms. The growth of offshore wind projects poses numerous technical and operational challenges to Transmission System Operators (TSOs). One of those issues concerns fact that the floating technology is specifically applied in deep seabed, such as the Mediterranean Sea. And for this reason, over the last two years an unprecedented volume of offshore connection requests is affecting Southern Italian regions, requiring extensive planning studies in order to provide coordinated, secure and cost-effective connection solutions. Present paper describes the connection schemes adopted by Terna to meet the challenges of the future Italian power system, to facilitate larger amount of variable renewable generation while providing a secure, cost effective and reliable electrical power supply. A representative case study on the connection of a high share of RES to a regional power system with a weak link to the main transmission grid is assessed

Evaluation of thermal performance for power system development

The remarkable renewable generation capacity installed in the South of Italy can determine high loading conditions of the 400 kV – 50 Hz backbones located in the area. The resultant conductors’ temperature increase accelerates components deterioration with negative effects in terms of reliability and economy of the system performance. In this paper the thermal analysis of a selected set of EHV transmission lines spanning the critical section Centre South -South is carried out both in the current and in the future energy scenario at 2040 year horizon in presence and in absence of planned developments. The conductors’ temperature is calculated via the heat balance equation and corresponding resistance and losses increases are obtained. The results highlight the benefits related to the presence of the planned reinforcements

Transmission capacity and static stability improvement by series compensation: case study on South-North corridor of Italy

The European power system is undergoing significant change, driven particularly by the sustained growth in electricity generation from variable Renewable Energy Sources (RES). In this context, securing the power system stability becomes more complex for the Transmission System Operators (TSOs), due to the unpredictability in energy flows and the long corridors between primary sources and load centres. Present paper addresses the series capacitive compensation of Extra High Voltage (EHV) transmission system with hight share of RES installations and focuses on how static stability and transmission capacity can be enhanced by power lines reactance reduction. In more detail, the study assesses the impact of series compensation on a pilot grid in terms of voltage phase shifts, and quantifies the benefits provided by series capacitors as compared to an uncompensated base case

Transmission network expansion planning: towards enhanced renewable integration

Recent development of renewable distributed generation, strongly affected by the need of reaching objectives established by European Energy and Climate policies, has an important impact on planning and operation of a power system. In this context, TSO has to guarantee major penetration of RES generation ensuring security of supply, efficiency and level of service. In literature, the ability of the electrical system to employ its resources dealing with the changes in load demand and variable generation, is defined as the flexibility of the system. Indexes for evaluation of system’s flexibility, defined by European Network Transmission System Operator, are Residual Load and RES ramping. The aim of this paper is to present a methodology to assess the amount of RES generation which must be curtailed to avoid overloads due to existent grid congestions, by extending ENTSO-E indexes. In fact, in the planning framework it is crucial to quantify RES production limits in terms of congestion and system overgeneration. Furthermore, major technical and economic benefits yielded by employing the proposed methodology in planning design of power systems under different scenarios are illustrated