Andrea Morini

Implementation and comparison of different under frequency load-shedding schemes

In the context of power system restructuring, the maintaining of adequate levels of security and reliability will be operated also through direct load control, thus considering load as the potential provider of ancillary services such as regulation, load-following, frequency responsive spinning reserve. In any case load-shedding still remains the ultimate resource for emergency conditions. In the paper several load-shedding schemes for underfrequency operation are examined. Both traditional schemes, based only on frequency thresholds, and adaptive schemes, based on frequency and on its rate of change, are considered. An IEEE test system for reliability analysis is used to compare the behavior of the proposed schemes when selecting different thresholds and percentages of load to be disconnected. Results are reported into detail and considerations on possible advantages and drawbacks also related to the framework provided by the electricity market are presented.

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.

An Energy Resource Scheduler Implemented in the Automatic Management System of a Microgrid Test Facility

The paper deals with a microgrid test facility, settled at the CESI RICERCA laboratories, equipped with various distributed energy resources (DERs) and connected to a LV network. The operation of such a microgrid is supervised by a central automatic microgrid management system (MMS). The paper first reports and discusses some experimental results carried out for the assessment of DERs performance and for the implementation of specific MMS functions. Then, it describes the functions of the energy resources scheduler (ERS) implemented in the MMS. The scheduler periodically updates the set points of DERs regulators in order to achieve economic, reliability and power quality objectives, starting form the load and renewable production forecasts and from the results of the system state estimation. The ERS is composed by two main parts, namely: a day-ahead economic scheduler of active power set points during the following day for the minimization of the overall costs, and an intra-day scheduler that every 15 minutes settles the set points of the DERs regulators to optimize the voltage profile at the grid buses, taking into account both technical constraints and the day-ahead economic schedules.

A Two-Stage Scheduler of Distributed Energy Resources

The paper presents a two-stage scheduler for the optimal scheduling of distributed energy resources (DER) connected to a distribution network. The proposed approach integrates a centralized day-ahead scheduler with an intra-day one. On the one hand, the day-ahead scheduler calculates the active power set points during the following day taking into account both technical and economic objectives and constraints. On the other hand, the intra-day scheduler takes into account the results of the day-ahead one and updates every 15 minutes the DERs set points for the optimization of the network voltage profiles. The paper describes the implementation of the proposed two-stage scheduler algorithm into a computer code based on a JavaScript link between Matlab and EMTP-RV simulation environments and presents the results obtained for the case of a network configuration adapted from the IEEE 34-node test distribution feeder.

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.

A combined ANN/simulation tool for electric power system dynamic security assessment

The increasing complexity of power systems due to their growth, operation close to technical limits, economical constraints and new regulations, calls for the inclusion of dynamic security assessment (DSA) into energy management systems where the overall system control is performed. Recent researches on DSA have shown a significant interest in the accuracy of tools based on hybrid methods, a combination of time domain simulation and transient energy functions. A synthetic review of a hybrid method based procedure, implemented by the authors using a suitably adapted simulation code, is first recalled. Successively the possible application of a tool based on artificial neural networks (ANN) for operating a contingency pre-screening by identifying stable and unstable situations is proposed. The ANN capability of identifying the stable and unstable clusters which may originate after a perturbation is also investigated. The proposed ANN structure has been trained and validated by simulating properly arranged scenarios from a test system constituted by a real size HV network of the Mediterranean area.

Bifurcation analysis and Chaos detection in power systems

Power systems show a complex non-linear behaviour due to different phenomena widespread over different time frames. Thus, power system stability is a significant research issue. Bifurcation theory can contribute to face these power system security problems. The study of local bifurcations and chaos through the analysis of power system equations can provide a deep insight into the complex behaviour of such large and interconnected systems. This paper describes the numerical continuation methodology and it shows an application example to voltage control in power systems.

Modeling of DFIG Wind Turbine and Lithium Ion Energy Storage System

The paper is aimed at describing the dynamic models of DFIG equipped wind turbine and Lithium Ion Energy System. The purpose of the energy storage system is to be coupled to the wind generation system in order to smooth its power output. Depending on the size of the storage system different task can be performed, starting from short term fluctuation leveling and power quality, getting to primary frequency-power regulation and, in case on large sizing, granting day-ahead turbines dispatching.

A Solid Oxide Fuel Cell model to investigate load following and stability issues in distribution networks

The increase of DG penetration in the distribution networks makes more and more interesting the investigation of security issues which are typical of the large transmission grids (transient stability, voltage stability and small signal stability) as well as the study of the DG responses to load changes in the grid. This paper presents the model of a Solid Oxide Fuel Cell (SOFC) dedicated to perform dynamic simulations on MV distribution networks. In particular in the present paper transient stability and load following issues are tackled. The first part of the paper is aimed at describing the model of the SOFC which is then used in the test system. As also a gas turbine is inserted into the test system, a brief look at the gas turbine model is mandatory. The aforementioned models include both the slow dynamics (GT prime mover, electrochemical system time constants for SOFC) necessary to study the DG response to load variations and the fast dynamics (e.g the power converter control and protection systems for SOFCs) necessary to study the transient stability related issues. The results of the analysis (carried out on a test MV distribution network) are reported and some conclusions are drawn.

Load-frequency control: new perspectives in the open access environment

The paper presents the fundamental concepts of load-frequency control (LFC) in the new context of open access for electrical energy market implementation. LFC is an essential ancillary service for maintaining system security and reliability. Two possible approaches can be considered: the implementation of LFC schemes for centralized, hierarchical and pluralistic control with the derivation and inclusion of autonomy operating conditions for each control area and the adoption of dynamic bilateral contracts between generation and distribution companies. The former approach is the object of this paper. The guidelines of the UCTE Organism have been considered in implementing the LFC schemes. A three area, 73 bus test system has been used for comparing the different LFC schemes. Results are reported and commented also in view of possible indications on accounting of inadvertent interchanges.