Insulation Coordination Studies on the Future Italian 36 kV Network - Transient Overvoltages

The new 36 kV standard voltage level introduced by Terna, the Italian Transmission System Operator (TSO), allows a better and faster integration of renewable energy sources into the Italian grid. Underground cables are foreseen for the connection of renewable energy sources (RESs) to the new 400 kV/36 kV substation. To evaluate the main design parameters of the new equipment and power transformers, the insulation coordination study is performed, evaluating the maximum switching and lightning overvoltage stresses. Symmetric and dissymmetric faults analysis are performed, considering different line lengths, from 10 km to 25 km, and fault clearing times, from 100 ms to 500 ms for the phase-to-phase and three- phases faults. The Alternative Transients Program (ATP) software is used to evaluate short-circuit currents, temporary overvoltages and specific energies. Switching and lightning overvoltages transferred from the primary winding to the secondary winding of 400 kV/36 kV step-up transformer are evaluated according to the simplified approach of IEC 60071-1.