Terna (Italy)

Abstract This paper presents an original deterministic approach to improve the success rate of the synchronization process of a synchronous condenser. In fact, the start-up process of a synchronous condenser cannot rely on the presence of a supply system to regulate its speed during the synchronization process, as happens with traditional synchronous generators. A static frequency converter is used to start the machine, but it should be disconnected before the grid synchronization to avoid damage, making the process uncontrollable. The proposed method aims to analytically determine the optimal frequency and phase angle to disconnect the converter to ensure synchronization with the grid within proper thresholds. The accuracy of the method is confirmed by a comparison with on-field recording of real synchronous condenser start-up processes. This procedure enables significantly reducing the frequency threshold for the synchronization so to limit the thermal and mechanical stresses of the machine and to increase its lifecycle.

In this study, the temporal evolution of the dynamic fractal dimension of lightning leaders is estimated using the box-counting method implemented in MATLAB, applied to a dataset of high-speed video recordings of natural negative downward lightning flashes and supported by image processing techniques. In an attempt to better understand the physical processes underlying lightning propagation, potential explanations for the observed trends are proposed and critically discussed, based on both the analysis of the results and the physical principles governing lightning attachment. The findings of this work may contribute to the development of more accurate lightning attachment models, ultimately enhancing the effectiveness of lightning protection systems in power networks.

The Multi-purpose Interconnectors (MPIs) represent a promising solution to integrate significant amounts of offshore wind generation while simultaneously increasing transmission capacity between market zones, within and between Member States, in the view of decarbonization targets established at European level. Furthermore, other relevant resources (i.e. storage and electrolysers) could be connected in synergic projects to enable higher system benefits. In this paper, an extensive analysis on a test system in the very long-term scenario is presented: benefits in terms of overall system cost and Renewable Energy Sources (RES) curtailment (overgeneration) reduction are evaluated by the means of market simulations considering different configurations of offshore network (radial or meshed offshore bidding zones connection) and different combination of resources connected to the offshore bidding zones (offshore wind generation alone or in combination with storage and electrolysers resources). The analyses with hourly detail over an entire year are performed to assess the system performance also in comparison with the traditional Home Market design, connecting the offshore generation radially to the onshore market zones. The output of the study is also interesting for understanding the possible developments of market mechanisms supporting the energy transition.

Electrical interconnections are crucial to achieving those levels of safety, reliability, and flexibility required in future decarbonized energy systems. In fact, increased interconnection levels enable more efficient energy demand and supply balancing by integrating intermittent RES generation and promoting the clean energy exchange between different Countries, optimizing the use of renewable generation and storage resources available locally. In planning studies, the operation of a potential interconnection is evaluated through indepth analyses in prospective evolving energy scenarios, whose reference assumptions-especially over the very long-term horizon-are subject to high uncertainty. This study presents the approach used by the Italian TSO to evaluate the different possible configurations for the implementation of a future crossborder interconnection in a planning scenario at 2040 year horizon: the solution maximizing system benefits is then assessed under has been then evaluated under conditions where the most significant input data deviated from the scenario assumptions, providing important insights into the potential performance of the future interconnection under alternative, yet still plausible, hypothesis.

In 2021, Terna S.p.A. introduced a 36 kV connection standard for renewable energy plants (10-100 MW), optimizing costs, simplifying connections, and reducing substation space requirements. This allows multiple producers to connect to shared bays near 380-60kV/36kV substations, promoting efficiency and scalability in renewable energy integration. The initiative was prompted by the increasing demand for renewable energy plants to connect to the National Transmission Grid (RTN). To regulate AC voltage at the 36 kV Point of Connection (PoC), this paper proposes a transmission-type STATCOM with an open-end winding multilevel converter. The design uses silicon IGBTs in a five-level neutral point clamped (NPC) converter, switching at line frequency (50 Hz) to minimize switching losses, and SiC MOSFETs in a two-level converter, enhancing efficiency due to their low switching and conduction losses. The STATCOM includes a coupling transformer with an open-end secondary winding, eliminating the need for additional series filter reactors. This solution aims to facilitate the integration of wide-bandgap (WBG) devices in grid-connected renewable energy systems like SiC MOSFETs limited at 3.3kV, 1kA. Preliminary simulations show that the design, when applied to a 12 MVAR, 11 kV STATCOM connected to a 36 kV transmission line, exhibits high efficiency, compactness, and enhanced performance, paving the way for the adoption of WBG technology in renewable energy applications.

This work addresses the lightning performance evaluation of the new <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 500 \text{kV}$</tex> HVDC overhead lines of the Italian transmission grid, part of the Hypergrid project, specifically focusing on direct lightning strikes to the pole conductors and the towers/shield wires of the line. The lightning performance evaluation procedures outlined by the international standard IEEE Std 1243 and the technical guidelines of CIGRE TB 63 are employed to estimate the shielding failure flashover and backflashover rate of the line. A comparison is made between different electrogeometric models regarding the shielding efficiency of the overhead line against direct lightning strikes. The results of this study indicate that, due to the relative positioning of the shield wires and pole conductors, forming a negative shielding angle, the shielding failure flashover rate of the line is practically zero. Furthermore, a negligible backflashover rate to the line is deduced for a reasonable range of lightning peak currents, due to the positioning of the underbuilt metallic return conductors, which provide additional protection to the pole conductor in the event of a backflashover scenario.

Meshed HVDC systems, particularly those employing voltage source modular multilevel converters in half-bridge configurations, are expected to play a pivotal role in advancing future grid interoperability and strength. However, these systems require more complex protection schemes due to the inherent lack of dc short circuit interruption capability. Dc circuit breakers represent a promising solution, but require careful assessment of switching overvoltages and voltage polarity reversals phenomena. The novel device Polarity Reversal Inhibitor (PRI) aims to address these challenges. Combining both passive and power electronic components in an innovative modular structure, without the need of complex gating circuits and cooling systems, it can be easily adapted to any voltage level and system topology. In this paper, PRI’s sizing criteria for the optimal operation on a 500 kV mixed dc link have been envisaged. Results confirmed that candidate devices can be selected among commercially available ones, significantly reducing the cost and footprint.

In the light of the decarbonization process, modern power systems pose multiple challenges to transmission network planners. In the Italian context, increasing network congestions are expected in the medium/long-term horizon across long corridors from South, where connection applications for new renewable power plants are concentrated, to North, where main load consumptions are located: in these scenarios, the angle stability constraint become an additional subject to be addressed in the EHV transmission system planning. For this purpose, the reduction of equivalent reactance of strategic transmission paths enables operational limits extension and becomes a crucial topic in planning framework. In this paper, the innovative "5-phases" solution, developed by Terna (the Italian TSO), is combined with the capacitive series compensation in a wider pilot network affected by stressful power transfer conditions. The main benefits in terms of transfer capacity increase between relevant market section and voltage phase shifts reduction are assessed in different cases study both in normal conditions and during critical contingencies.