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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.

The electrical power system is being significantly affected by the climate change mitigation actions. The power generation, originally centralized, is transitioning towards a more decentralized paradigm due to the coal-fired power plants shut off and the increase in renewable power. Issues in transmission system's voltage control may arise, if the voltage regulation architecture is not modified accordingly. To this aim, in this paper it is investigated the use of a Linear Quadratic Regulator with Integral action (LQRI) for the secondary voltage regulation, aimed at exploiting several reactive power sources as actuators. Being the LQR class of regulators requiring the system state to correctly operate, and being a transmission system a complex system, a critical investigation must be done. In particular, it is needed to identify the variables that are directly measured in a real system, determine if they can be useful for the LQRI state feedback, and finally study the effect of the different possible feedback selection on the regulation performance.

This work shows the integration process of the information provided by the power quality meters and digital fault recorders with a wide area monitoring system based on synchronized measurements provided by the phasor measurement units (PMUs). The goal of the integration is to correlate the data based on power quality and fault events with the streams of information provided by the PMUs installed in the area of interest. The applications implemented in the prototype of the integration system provide new functionalities to increase the awareness of the state of the electric grid with the data provided by different measurement devices.

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 forced switching converters are the kernel of the modern paradigm of VSC-HVDC system. It is widely recognized that appropriate control laws make these converters more advantageous than the traditional line commutated ones. They can play a versatile role by properly controlling active and reactive powers between electrical networks, improving the dynamic performances of the whole system. Besides this is more and more motivated by the wide spreading penetration of renewable resources. In this paper after a few references to basic concepts, a possible control strategy of VSC-HVDC transmission system is discussed. With respect to a two-area system with AC-DC link, a combined control law is investigated for improving inertial and primary frequency response. The control law is based upon the equivalent HVDC tie link synchronization coefficient which allows determining the reference value of the AC voltage reference angles of the converters. A damping action is further added for improving the power swings damping. The simulations performed with a simple but quite general case study permit to confirm the validity and the goodness of the proposed control strategy.

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 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.

This paper deals with the inertia estimation of power systems in presence of significant contribution provided by renewable energy sources. The efforts to decommission conventional power plants and replace them with renewable resources could affect the robust and reliable operation of future power systems. The main issue related to the integration of renewable-based generation in the networks deals with the substitution of conventional synchronous generators connected to the grid and equipped by rotating masses, with converter-based generator which have not rotating masses, or they are decoupled by the grid due to the presence of interfacing power converters. Since the system inertia is the inherent ability of the online synchronous machines to oppose sudden changes in generation or load, the increasing share of renewable generators, such as wind and solar, the overall system inertia could decrease. This leading network operators to introduce new tools for monitoring the system inertia values, to contain them within admissible values, and to better prepare for different kinds of operational scenarios with undesired reduction of the system inertia. At this purpose, in this paper a tool for the analysis and prediction of inertia is proposed. The proposed method is based on a minimum variance harmonic finite impulse response and can consider the actual penetration of renewable energy sources. The proposed approach is tailored for online monitoring and exploits the real time measurements of relevant features of the network. The method is tested with actual data of the Italian Transmission Network and its effectiveness is studied by comparisons with some popular and well-established method of analysis and prediction.

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.

In this paper, a procedural approach for studying the feasibility of a restoration backbone is presented. The approach uses a Simulink model for the study of frequency and voltage dynamic behaviors in order to obtain a backbone simulation for different load scenarios. The method is then applied to a restoration path involving a portion of Italy transmission grid, focusing on the study of power-frequency behavior. Some details on the modeling of load and distributed generation are given, so to represent the real behavior of a primary substation in terms of active power transit.

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.

Dynamic thermal rating (DTR) assessment of overhead lines is recognized as an enabling technology for reliably improving the exploitation of existing power transmission assets. In this context, the conceptualization of decentralized sensors networks aimed at performing all the DTR functions in an easy-to-use and well-defined way, represents one of the most promising research direction. To address this issue, this paper proposes a self-organizing computing framework based on a network of cooperative smart sensors, which solves the synchronization, identification and prediction problems required for DTR assessment, without the need for centralized data acquisition and processing. Experimental results obtained on a real test-bed are presented and discussed in order to demonstrate the effectiveness of the proposed framework.

This paper describes protection systems currently used by Terna on National Transmission Grid. It deals with distance protections, line differential protections and protections against high resistive faults (in the following GAR from Italian acronym of Guasti Altamente Resistivi), with reference to issues related with high resistance single-phase to ground faults. The characteristics of the latter are analyzed, showing the range of compensated fault resistance and illustrating Terna's current application on some critical 132 kV and 220 kV lines. A possible evolution, which overcomes some current limitations of GAR protection (e.g. application to double circuit lines) are described in last part

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.

Traditionally, the rating of an overhead transmission line is determined under a set of specified and standardized conditions. However, weather conditions along the line change during operation. Therefore, the standard rating of the line might be either underestimated, leading to inefficient utilization of the line, or overestimated, leading to unsecure operation. This is the major drawback of the traditional approach: the so-called dynamic thermal rating (DTR), that takes into account the actual operating conditions along the line to determine the rating, is today a critical need. In this paper, we develop a comprehensive methodology for exploring all necessary information about stochastic processes of environmental variables surrounding and along the line using available data. The results can be used as input to determine the actual rating of the considered transmission line to enhance the determination of the rating for transmission lines.

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.

In this work preliminary yet comprehensive considerations are reported on the possibility for remote monitoring of degraded compression and bolted joints on high voltage overhead power lines. Measurements are performed on new, naturally aged, and artificially degraded joints to quantify the impedance discontinuity introduced along typical HV overhead lines. Numerical simulations, a preliminary time domain reflectometry (TDR) measurement campaign, and circuit simulations are performed on a real power line to study the TDR-based capabilities for the identification of the investigated joint impedances. This study sets the basic concepts to the development of a remote monitoring system for quantifying and locating degraded joints.

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).

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.

Wide-Area Monitoring Systems play a key role in ensuring reliability and security of modern electric power system operation. Phasor Measurement Units are the enabling technology for WAMS, since they provide synchronized measurements in spread points of the system characterized by higher data flow rate compared to traditional SCADA systems. An interesting application of WAMSs is the Dynamic Thermal Rating assessment of power lines, that allows fully exploit the benefits deriving by massive penetration of Renewable Power Generators by dynamically evaluating the interconnections’ transfer capabilities, reducing congestions in the system.Despite these benefits, these systems are vulnerable to Time Synchronization Attacks, since the majority of PMUs uses GPS signals as time reference for the synchronization procedure.In this paper the impact of TSAs on the reliability of DTR assessment will be discussed by analyzing the first results obtained on a realistic case study.

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.

In 2013, Switzerland and Italy started using wide area monitoring (WAM) systems as an additional tool for power-system operation and analysis. One of the driving forces behind this was the need for the comprehensive observation of highly loaded transmission corridors. Another important target was setting up a systemwide stability-monitoring tool with respect to interarea oscillations.