Andrea Pitto

A Gas Turbine Model for Studies on Distributed Generation Penetration Into Distribution Networks

The paper proposes a detailed model of a gas turbine for a combined heat and power (CHP) plant. The prime mover is endowed with several controls: first of all, a speed control which can be either droop type or isochronous type according to the status of connection to the distribution grid. An acceleration limiter and an exhaust gases temperature limiter intervene to cut the fuel request, respectively, in case of an excessive rotor acceleration and of a too high temperature of the exhaust gases. A further control regulates inlet guide vanes (IGV) in order to keep the exhaust temperature almost constant. This is important in a cogeneration plant where the heat recovery from the exhaust gases is used to satisfy the demand of the thermal load. Some validation simulations on a model of cogeneration plant connected to a MV distribution grid are carried out and commented.

Microturbine Control Modeling to Investigate the Effects of Distributed Generation in Electric Energy Networks

The great attention of research initiatives to sustainable energy systems calls for contributions in modeling new small size generation plants exploiting innovative technologies. In such a context, the paper presents detailed models of the components and controls forming the thermo-mechanical and electric subsystems of a microturbine power plantP. The modeled thermo-mechanical subsystem includes different control loops: a speed controller for primary frequency control (droop control), an acceleration control loop, which limits the rotor acceleration in case of sudden loss of load or in case of start-up, and a controller to limit the temperature of the exhaust gases below the maximum admissible temperature. The modeling of control schemes in the electric subsystem is another key issue of the paper in view of providing efficient energy production from distributed generation: an active power-voltage (PV) control is adopted for the inverter in case of the operation of a microturbine connected to the grid. This control scheme provides an innovative contribution with respect to the usually adopted active power-reactive power (PQ) control scheme. The adoption of the PV control scheme allows to evaluate the contribution of micro-turbines to voltage support in electric distribution grids. In case of isolated operation of the generation source a voltage-frequency (VF) control scheme is proposed. A test grid is set up for model validation and the simulation results are described and discussed.

Distance Protection of AC Grid With HVDC-Connected Offshore Wind Generators

When a transmission line close to points of common coupling (PCCs) experiences a short-circuit (SC) fault, the fast reactive power control of voltage-source converter-HVDC (VSC-HVDC) is likely to affect the protective relay operation of transmission lines. To study the performance of distance relays on an ac grid with an offshore wind HVDC network, this paper presents an apparent impedance calculation method, which utilizes the bus impedance matrix (Zbus) to calculate the impedances viewed by distance relays during a three-phase SC fault. The proposed method is used to identify the potential miscoordinated Zone 2 relays in the proposed combined ac/dc system. The analysis is verified by software simulation results. It is shown that the proposed method results in accurate impedances viewed by distance relays. It also identifies the protective device settings on the ac grid that need to be adjusted due to HVDC control of offshore wind generators.

Mitigation and prevention of cascading outages: Methodologies and practical applications

Interconnected power grids throughout the world are very reliable but occasionally suffer massive blackouts with multibillion dollar costs to society. Cascading failures present severe threats to power grid reliability, and thus reducing their likelihood, mitigation and prevention is of significant importance. This paper is one in a series presented by Cascading Failures Task Force, under the IEEE PES Computer Analytical Methods Subcommittee (CAMS) with primary focus on mitigation and prevention of cascading outages. The paper presents the basic methodologies for mitigation, summarizes currently deployed special protection schemes, and lists cases of successful and unsuccessful mitigation of cascading outages and lessons learned. Future developments and challenges in the area of mitigating cascading outages are also discussed.

An Integrated Platform for Power System Security Assessment Implementing Probabilistic and Deterministic Methodologies

Power system security assessment both for planning (off-line) studies and for operational (on-line) applications is performed by various analysis methods which highlight different phenomena (steady-state violations, angle stability, voltage stability, etc.) in order to retrieve an exhaustive vision of the problems. Besides traditional deterministic tools, recently proposed probabilistic tools may highlight new interesting security aspects by introducing the concepts of probability and quantifying the risk associated with the contingencies. This paper proposes the overall architecture of a security assessment platform which integrates both probabilistic and deterministic methodologies in a simple environment.

Operational Risk Assessment and control: A probabilistic approach

Probabilistic Risk Assessment (PRA) techniques are raising more and more interest in the context of the operation of high voltage transmission networks because they represent a new approach to the security assessment in power system operation. The classical deterministic methods, which compare the power system performance to a predefined set of requirements (e.g. no current and voltage violations, no stability problems), do not take into account the probability of occurrence and the magnitude of events and do not perform a quantitative characterization of the impact of the contingencies. The fulfillment of the security requirements also for the most critical contingencies determines large security margins, thus higher operational and planning costs. On the other hand the risk of other contingencies may be underestimated. The paper starts by illustrating a methodology to assess the risk of loss of load caused by conventional (N-1) or multiple dependent contingencies resulting into a cascading process. A control strategy is then introduced, aimed to reduce operational risk by optimal preventive redispatching of conventional generators. Simulation results of the application of the approach to an IEEE test system and to a model of the Italian EHV transmission grid are illustrated and discussed.

Benchmarking and Validation of Cascading Failure Analysis Tools

Cascading failure in electric power systems is a complicated problem for which a variety of models, software tools, and analytical tools have been proposed but are difficult to verify. Benchmarking and validation are necessary to understand how closely a particular modeling method corresponds to reality, what engineering conclusions may be drawn from a particular tool, and what improvements need to be made to the tool in order to reach valid conclusions. The community needs to develop the test cases tailored to cascading that are central to practical benchmarking and validation. In this paper, the IEEE PES working group on cascading failure reviews and synthesizes how benchmarking and validation can be done for cascading failure analysis, summarizes and reviews the cascading test cases that are available to the international community, and makes recommendations for improving the state of the art.

Evaluation of some indices for voltage stability assessment

The liberalization of the electricity markets causes power systems to work closer and closer to their limits. The online assessment of system security becomes a topic of paramount importance in control centers. In this context fast indices to quickly assess system security are fundamental in an on-line DSA session. This paper considers three voltage stability indices recently proposed in literature and carries out some comparisons. In the first part the authors briefly describe the theoretical background of each index. The second part is devoted to two comparisons: at first, the three indices are compared by using an IEEE test system. Secondly, the FVSI (Fast Voltage Stability Index) and the VCI (Voltage Collapse Index) are compared by adopting a model of the Italian HV transmission grid. The comparisons will show the peculiar information provided by each of the considered indices and will assess the performance of these indices also on a realistic power system. Final remarks are reported and discussed.

Probabilistic Risk-Based Security Assessment of Power Systems Considering Incumbent Threats and Uncertainties

In-depth security analyses of power systems (PSs) require to consider the vulnerabilities to natural and human-related threats, which may cause multiple dependent contingencies. On the other hand, such events often lead to high impact on the system, so that decision-making aimed to enhance security may become difficult. Introducing the uncertainty, the risk associated to each contingency can be evaluated, thus allowing to perform effective contingency ranking. This paper describes an in-depth security assessment methodology, based on an “extended” definition of risk (including threats, vulnerability, contingency, and impact) aimed to perform the risk assessment of the integrated power and Information and Communication Technology (ICT) systems. The results of the application to test cases and realistic PSs show the added value of the proposed approach with respect to conventional security analyses in dealing with uncertainty of threats, vulnerabilities, and system response.

Modeling of doubly fed induction generator (DFIG) equipped wind turbine for dynamic studies

The paper is aimed at presenting the dynamic behaviour of a wind turbine equipped with a doubly fed induction generator (DFIG) in case of disturbances in the interconnected grid. A model of a DFIG is presented and adapted for analyzing the response of the wind generator to voltage control, frequency control, voltage sags and wind variations. Simulations, performed by a widely used power system analysis code, are reported and commented.