In this paper, after a short review of the main characteristics of the NPC-GTO Double Converter jointly developed by EPRI and Ansaldo, the studies performed for the modulation techniques to be used in an NPC double converter are presented. Both Space Vector Modulation (SVM) and Harmonic Reduction Modulation (HRM) have been studied and tested. A third modulation techniques, Optimal Space Vector Modulation (OSVM) has been studied. The optimal space vector modulation method has some of the benefits of both SVR and HRM. In the OSVM technique the durations of switching cycles are the optimization variables. The method is based on converting the reference voltage vector v-ref(t) into a sequence of discrete sampled values v-ref(ts), with the sampling instants ts no more regularly space as conventional scheme. This optimal technique is applied to a 3 level inverter, in order to reduce harmonic currents at given switching frequencies. The optimal scheme can be earned out off-line and it can be implemented at the same cost as the conventional scheme.