Alternatives for blocking direct current in AC system neutrals at the Radisson/LG2 complex

The authors document the development of alternative approaches to the design and application of blocking devices placed between transformer neutrals and the substation ground. System constraints on the impedance and overvoltage limitation of the neutral blocking device (NBD) are covered. Three alternative NBD schemes are developed and optimized. System performance of the NBD is discussed, as are the practical implementation considerations for this unconventional equipment application. Although the focus is on the NBD requirements of Hydro-Quebec´s Radisson/LG2 complex, the fundamental information is applicable to any situation where DC must be blocked from a power transformer neutral in a system designed for effectively grounded operation