Single-phase-to-ground faults may cause substantial temporary overvoltages (TOVs) in large radial medium-voltage networks with isolated neutral, even over 3-p.u. phase to ground. Resonant neutral earthing limits these overvoltages to 1.8 p.u. but credible earthing apparatus failures might trigger TOVs up to 2.4 p.u. This paper presents the ground fault study of an Italian 20-kV ENEL Distribuzione network. Analytical evaluations in a wide parametric range of neutral earthing arrangements, include isolated neutral and ENEL resonant earthing with parallel resistance, as evidence of 2.4-p.u. TOVs with isolated neutral, 1.8 p.u. with resonant earthing, and more than 2.0 p.u. with partial compensation. Recordings of ground faults staged in the same network are presented, showing excellent agreement between analytical predictions and experimental test. The tests confirm TOVs of more than 2.3 p.u. with isolated neutral, sometimes evolving into cross-country faults (possibly explaining unforeseen cable fault rates), and the effectiveness of the ENEL neutral earthing practices in suppressing these TOVs.

The use of diesel generators to supply electricity to small remote islands is both costly and environmentally troublesome. On the other hand, small island networks pose peculiar stability and reliability problems when renewable energy sources are added. The paper regards the installation of an hybrid power plant (phortovoltaic, PV, plus lithium battery and synchronous condenser) in Giglio island, to replace existing diesel units. The paper discusses the general overview of the project and presents steady-state and dynamic simulations of the proposed energy system. Results evidence the advantages of the proposed Giglio hybrid power plant over existing diesel generators in terms of reduction of both pollution and energy costs, also allowing to fulfill the N-1 network security criterion. At the same time, results underline the need to install a properly sized flywheel in order to assure the stability of supply.

In a free-market scenario, power system planning is affected by a significant degree of uncertainty about time and location of generating assets expansion. The paper contains a methodology for the evaluation of the flexibility, defined as the attitude of the transmission system to adapt, quickly and with limited costs, to every change, from the initial planning conditions, with a particular regard to the changes in generation. The method provides new global and local flexibility indices, based on technical and economical parameters, defined as Technical Uncertainty Scenarios Flexibility Index (T-USFI) and Technical Economical Uncertainty Scenarios Flexibility Index (TE-USFI). The T-USFI computation segment of methodology has been tested on simple networks.

In a free-market scenario, power system planning has to deal with a significant degree of uncertainty about time and location of generating assets expansion. A methodology for the evaluation of the attitude of the transmission system to keep up a desired standard of reliability under such uncertainty (that is, flexibility) is presented. The method, relying on the Monte Carlo simulation approach, is applied to check the effectiveness of a flexibility index, based on structural as well as operational network parameters. The results of tests carried out on simple networks and on IEEE RTS are reported showing the encouraging agreement between the probabilistic simulations and the proposed index.