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The increasing penetration of inverter-based resources is affecting the overall power system security, so that effective tools are needed to provide system awareness and identify the most suited countermeasures. To tackle this problem, real-time monitoring and assessment of transmission grids based on synchrophasor measurements have drawn the attention of many researchers over the last decade, thanks to the high temporal resolution of data provided by phasor measurement units. In this study, a new version of the dynamic mode decomposition (DMD) is proposed as a tool for the real-time identification of electromechanical oscillations. This approach is able to provide information on the dynamic characteristics (frequency and damping) and the spatial correlation (mode shape) of the modes identified. Besides, an improved criterion is used to track and discern the dominant modes. The effectiveness of the DMD method has been tested on the two-area Kundur system and validated using data from a real event on the European synchronous grid, giving promising results. Thanks to its reliability and robustness, the DMD is now implemented in the Terna (the Italian transmission system operator) Wide Area Measurement System in use in the control room.

Northern electric grid of Italy was involved in a black start mock drill. This test took place on November 2016 in order to check the coordination and efficiency of all the operational staffs and the automatic regulators involved in the case of a real blackout scenario. This paper presents the results of a research carried out in collaboration with Terna Rete Italia (the Italian Transmission System Operator), in order to find strengths and weaknesses and eventually to upgrade the existing restoration plan. Starting from the measures coming from generating plants and substations bars, the model of frequency regulation system for both the participating hydro and thermal power plants is developed. This work gives the system operator an essential tool to understand many complex dynamics and phenomena occurred during the test. At the same time, a robust model simulating the system behavior with different grid configurations is presented: it gives the opportunity to enhance both the classical power system control theory and the black start practices.

The present paper analyzes the main findings collected from the first year of operation of the MontenegroItaly interconnector, i.e., MON.ITA HVDC. Briefly the main technical characteristics of the asset are provided. A focus on the hourly commercial programs on day ahead market experienced during the 2020 are analyzed and compared with the real physical flows. Then active and reactive power control are described with the aid of some real use cases collected from both the commissioning and the first year of operation. Finally, general concluding remarks are given together with an outlook on future role of HVDC technology within the framework of renewable energy sources and European market integration.

The paper presents an electromagnetic-transient (EMT) analysis of a multi-infeed HVDC cable system in order to assess the transient overvoltages on the cables due to different fault conditions. It is well known that the HVDC-MMC configuration needs surge arresters to reduce the transient overvoltages on the DC cable system. This work highlights how the voltage perturbations on the HVDC-LCC link due to the faults on the HVDC-MMC link are not negligible and how a reduction of transient overvoltage is necessary for both the HVDC systems. The results of this study can be useful to improve the insulation coordination of the multi-infeed HVDC system and their reliable operation.

An accurate monitoring of power system behavior is a hot-topic for modern grid operation. Low-frequency oscillations (LFO), such as inter-area electromechanical oscillations, are detrimental phenomena impairing the development of the grid itself and also the integration of renewable sources. An interesting countermeasure to prevent the occurrence of such oscillations is to continuously identify their characteristic electromechanical mode parameters, possibly realizing an online monitoring system. In this paper an attempt to develop an online modal parameters identification system is done using machine learning techniques. An approach based on the development of a proper artificial neural network exploiting the frequency measurements coming from actual PMU devices is presented. The specifically developed offline training stage is fully detailed. The output results from the dynamic mode decomposition method are considered as reference in order to validate the machine learning approach. Some results are presented in order to validate the effectiveness of the proposed approach on data coming from recordings of real grid events. The main key points affecting the performance of the proposed technique are discussed by means of proper validation scenarios. This contribution is the first step of a more extended project whose final aim is the development of an artificial neural networks (ANN) architecture able to predict the system behavior (in a given time span) in terms of LFO modal parameters, and to classify the contingencies/disturbances based on an online training that has memory of the passed training samples.

The aim of this paper is showing the potential and the use limits of a matrix-based linearized electromechanical dynamics by means of the extensive comparison with a commercial power system software. Since the schematic grid in the matrix-based linearized electromechanical dynamics is a simple graph with only branches, nodes and connected electric elements, and the commercial software grid maintains the complexity of a real electric network, an interface procedure is implemented in order to maintain the correct topological structure. Results from the real dynamics and the linearized one are compared each other and are discussed. The implemented algorithm seems to be an effective tool for both research and power education.

This paper presents a model for the study of bottom-up power system restoration tests. The model is built for quick and suitable validation of power-frequency correlation during a restoration test, the most stressed process into a system with low regulation energy. All involved components are modelled by SISO systems describing both power generation controls and load behaviours. The simulation results are widely discussed through the analyses of four load scenarios, with the aim of scanning all possible worst cases in terms of load steps.

The paper deals with a novel method for the analysis of low-frequency inter-area oscillations, a harmful phenomenon that can be more and more frequent in electrical power systems due to the expansion of interconnected power grid. In particular, inter-area oscillations are electromechanical oscillations involving groups of generators geographically far from each other; correctly and timely estimating their parameters is a fundamental task to preserve the stability of the electrical power system. Phasor Measurement Unit (PMU) technology has become the key element in Monitoring and Control systems of Transmission System Operators offering new opportunities for improving their situational awareness. Proposed method processes measures provided by the PMUs to extract the oscillating components and, for each of them, estimates characteristic parameters such as frequency and damping. To this aim, the method exploits a combination of the Hilbert Transform and some optimization algorithms, thus gaining the following improvements: (a) enhanced capability of separating oscillatory components, thanks to a new strategy for the choice of bisecting frequency, (b) possibility of separating components characterized by frequency difference lower than the spectral resolution, (c) remarkable accuracy associated with the damping estimates of each oscillation and (d) robustness to the noise affecting the input signal. Results obtained in tests involving either synthesized signals or simulated electrical systems finally assessed the promising performance of the proposed method.

This paper presents a novel approach for the optimal design of the control system for battery energy storage systems adopted in power systems in the framework of the primary frequency control. The challenge issued by the use of batteries for frequency control in modern power systems is related to the need of the repeatedly deep charge and discharge phases. The presence of intermittent loads and generators, in fact, imposes very stressful power profiles to the battery which imply high rate of degradation to its lifetime. Based on the stress model of the battery, the proposed approach aims to design a control system which allows for containing the state of charge time variation within intervals able to match the requirements of frequency profile improvements and lifetime preservation. At this aim, the typical control scheme for batteries adopted for primary frequency control is analyzed and elaborated to address stable dynamic behavior of the regulator. Moreover, starting from the observation of the statistical nature of the power request for the pure frequency regulation service, a statistical study is conducted to perform an optimal design of the regulator in terms of both lifetime duration and primary frequency regulation service.

Large-scale penetration of renewable energy sources in power systems is essentially related to the need of reducing the environmental impact caused by the fossil-fuel. As well known, the interface between the power grid and this kind of energy resource is achieved by power converters, with a consequent dynamic behavior quite different from the synchronous generators. This matter involves negative impacts on the operating conditions of power systems. In this context, it is crucial to individuate estimation techniques able to predict promptly critical conditions which could, in extreme cases, compromise the stability of whole system. In this paper the authors employ an auto-regressive model which can describe the dynamic evolution of the power system inertia. The core of the procedure relies on an inertia model conceived as the sum of a periodic component and a noise stochastic process distributed according a Logistic model. The robustness of a novel estimation procedure, able to capture the dynamic evolution of the inertia, is investigated by testing two scenarios of Italian Transmission Network. The assumptions in terms of the obtained numerical results show the validity of the estimation technique and of the probabilistic characterization of the noise.

Nowadays modern power system are much more prone to the establishment of wide instability phenomena that could jeopardize overall security. Transmission System Operators (TSOs) must guarantee a stable and reliable operation of the managed power systems. A valid support in suppressing sudden power system oscillations can be provided by High Voltage Direct Current (HVDC) systems though the variation of the transmitted active power or the injected reactive power into the grid. Montenegro-Italy interconnector, a.k.a. MON.ITA, currently in phase of commissioning, is equipped with a power oscillation damping (POD) device. The paper presents the stability analyses devoted to the optimal tuning of MON.ITA POD controller and the validation of its goodness as well as the simulations aimed at demonstrating the aid provided by HVDC in suppression of undamped oscillations.

In this study, a new PMU-based method of identification of electromechanical oscillations is presented exploiting Principal Components Analysis (PCA). The proposed method makes it possible to carry out analysis on a limited number of the so-called Princial Components (PCs), obtaining information on the behavior of voltages and currents at buses following a small perturbation. The analysis is completed using the Prony analysis on the PCs, which is able to determine the amplitude, frequency and damping of a mode. To assess the effectiveness of the method, the algorithm has been applied on a test system based on the Kundur Two-area system as well as on a real event.

This paper presents the design and demonstration through simulations of a wide-area oscillations damping controller (WADC) for the Italian grid. A simple, linear transfer function model is built using collected ring-down measurements to capture the system’s oscillatory behavior. This constructed transfer function model is utilized for the controller design, instead of a complicated dynamic grid model. A measurement-driven approach is used to tune the parameters of the controller, thus the transfer function model can be updated online to track the variations of the system operating condition. The proposed approach is validated through a case study on the Italian power grid. The optimal observation signal for the controller is selected using Fast Fourier Transform (FFT) analysis of ring-down measurements. Two Compensation Devices (CDs) are selected as the actuators for the WADC. The control effect of the designed controller is shown for an actual undamped oscillation event that took place in December 2017.

Synchrophasor technology is the backbone of the most advanced Wide Area Monitoring Systems. The two main components of such systems are the Phasor Measurement Unit (PMU), which is the sensing element of the measurement chain, and the Phasor Data Concentrator (PDC), whose primary goal is to collect and time-align the synchronized measurements provided by the PMUs installed in the grid. A hierarchical structure, composed of different levels of PDCs, can be also implemented in large power systems. Transmission System Operators are interested to better exploit the potentialities offered by the synchrophasor technology, moving in the direction of synchrophasor-based Wide Area Monitoring Protection And Control (WAMPAC) systems. In this perspective, by considering that the PDC is the first element of a synchrophasor measurement system with a standpoint on the overall state of the electrical grid, this device can be a perfect candidate to play an active role, by suitably handling the data coming from several PMUs or lower level PDCs. Following these criteria, this work explores the feasibility of an active PDC whose functionalities allow the definition of specific protection and control logics.

This paper proposes a methodology to estimate equivalent inertia in large power systems based on measurements provided by Phasor Measurement Units (PMUs). Considering the availability of PMUs spread on the system, the Ward equivalent method is proposed to reduce the system around the measurement points, and dynamic equivalents are built around the selected buses. Simulations are performed with a 65-bus, 16 generators test-system to provide synthetic data for the application of the proposed method, and results considering two study cases are presented to illustrate and validate the approach.

This is a shrunken version of a full paper presented in IEEE Access [1]. This paper presents the results of a research carried out in collaboration with Terna Rete Italia (the Italian Transmission System Operator), in order to find strengths, weaknesses and eventually to upgrade the existing restoration plan. The aim of this research is the creation of a Simulink model in order to study the frequency behaviour during a real restoration test that involved northern Italy electric grid. Starting from the measures coming from generating plants and substations bars, the model of frequency regulation system for both the participating hydro and thermal power plants is developed. This work gives the system operator an essential tool to understand many complex dynamics and phenomena occurred during the test.

The Under Frequency Load Shedding Plan (UFLSP), also called Low Frequency Demand Disconnection Plan in EU Regulation 2017/2196 (LFDD Plan), is a fundamental element of the Defense Plan of the Electrical Power System. Its purpose is to avoid that the active power imbalances due to network separations lead to the disconnection of the generation groups and therefore to the blackout. The increase in production by the plants connected to the distribution networks in medium and low voltage makes the management of the UFLSP increasingly complex, whose optimization process started at TERNA in 2014 to ensure compliance with the required performances. In this paper an Adaptive UFLSP (A-UFLSP) is presented, based on the dynamic selection of the medium voltage lines to be disconnected. After defining a general algorithm, the main characteristics of the devices necessary for its implementation are established. The application to a real case demonstrates the capability of the A-UFLSP to remain equally effective as the load and generation conditions change in the distribution networks, even in the face of further foreseeable increases in production from Renewable Energy Sources (RES) through Distributed Generation (DG).

High Voltage Direct Current (HVDC) links help to integrate large amounts of power from Renewable Energy Sources (RES), however they may interact in complex ways with the control, defense, and protection systems of the power grid. The paper analyses some of the stability phenomena and interactions that may affect small systems weakly connected with the bulk power system via HVDC links, and characterized by low inertia, low short circuit power, and high production from RES. To this purpose, detailed time-domain simulations are performed, based on phasor models, to study severe disturbances such as short circuits close to the AC terminals of HVDC link. The stability outcome is significantly dependent on the control and defense system parameters. A sensitivity analysis is performed and the resulting changes in system performance indicators are discussed.

This work presents the feasibility study of an on-line monitoring technique aimed to discover unwanted variations of longitudinal impedance along the line (also named &ldquo, impedance discontinuities&rdquo, ) and, possibly, incipient faults typically occurring on high voltage power transmission lines, like those generated by oxidated midspan joints or bolted joints usually present on such lines. In this paper, the focus is placed on the application and proper customization of a technique based on the time-domain reflectometry (TDR) technique when applied to an in-service high-voltage overhead line. An extensive set of numerical simulations are provided in order to highlight the critical points of this particular application scenario, especially those that concern the modeling of both the TDR signal injection strategy and the required high-voltage coupling devices, and to plan a measurement activity. The modeling and simulation approach followed for the study of either the overhead line or the on-line TDR system is fully detailed, discussing three main strategies. Furthermore, some measurement data that were used to characterize the specific coupling device selected for this application at high frequency&mdash, that is, a capacitive voltage transformer (CVT)&mdash, are presented and discussed too. This work sets the basic concepts underlying the implementation of an on-line remote monitoring system based on reflectometric principles for in-service lines, showing how much impact is introduced by the high-voltage coupling strategy on the amplitude of the detected reflected voltage waves (also named &ldquo, voltage echoes&rdquo, ).

The goal of this work is to study the different dynamic behavior of the Italian electric power system, when subject to large disturbances, in relation to adopted load model and reactive power compensation devices considered for the network. Effectiveness of Defence Plans has already been proved by system Operators, but little attention is paid sometimes to the influence of used models. The adoption of a realistic load model implies a better representation of the system: in particular, the conclusions are that if the load is modeled as dynamic and voltage and frequency dependent, frequency perturbations resulting from the simulated contingencies are less dangerous as compared to more simplified models. Furthermore, the possibility to support voltages at particular buses damps voltage transient consequent to large perturbations and avoids further disconnection of loads, keeping the same security level. The conclusions are supported by tests carried out both on a relatively small portion of the Italian system (Sicily) and on a larger area (central and southern part).

This paper presents a dynamical model for simulating combined cycle power plants to be used for assessing the plant behavior during large transients and for restoration procedures. The 80th dynamic order model considers the complex structure of the heat recovery steam generator and calculates the main variables that are critical during these transients. The model has the double purpose of studying both the plant behavior under the usual control systems as well as innovative logics and control strategies for increasing its performances in terms of regulation and emergency services. The possible use of combined cycle power plants for supplying a restoration area is finally investigated adopting different control strategies that also involve the regulating capacity of the steam turbine.

The paper describes how gas turbine based plants (open cycle and combined cycle) can be profitably used in power system restoration for supplying restoration areas. In recent times, in fact, several gas turbine sections entered the power system due to the improved efficiency of gas turbines and to the development of high efficiency combined-cycle plants. These units can be easily improved to provide black-start capability and can therefore largely increase the black-start capacity of the entire system. Restoration islands to be used for minimizing the time to supply critical areas, such as urban and industrial zones, can support the usual restoration paths designed to provide cranking power to large steam units. The paper presents the defining criteria for the procedures to be followed during restoration. An example referred to as an urban area is reported with simulation results. The Italian System Operator recently carried out some tests on an open cycle gas turbine aimed at checking the island operation of the unit. Some results are described.

The present paper reports the main results of a comprehensive international survey that, on behalf of the Italian independent system operator, the authors have carried out into the main strategies, critical issues and practical experiences related to the restoration plans of a wide set of deregulated systems worldwide. For many ISOs, people directly involved in designing, testing and updating restoration plans have been contacted and interviewed, in order to focus general issues and to outline common improvement trends. Taking the international survey as a starting point, the paper also proposes and discusses possible innovative enhancements being studied for the Italian system, aimed at increasing the effectiveness of the restoration service.

Power system restoration plans worldwide have often been based on black start units capable of supplying cranking power to bulk generation plants. The increase in the number and size of CCGT plants and other forms of generation with low regulating capacity and requiring a restless growing power for the cranking phase needs that new strategies and tools be developed for facing the unlucky event of a black-out. The study presents the possibility of using HVDC links with a synchronous compensator at the receiving end. Such systems have high regulating capacity and reliability which are precious characteristics during the critical phases of the restoration. A detailed stability analysis is developed for checking the correct size of the compensator. Electromagnetic simulations prove the validity of the solution.