An open delta connection transformer uses two single-phase transformers to provide a three-phase supply to the load. An open delta connection system is also called a V-V system. Open delta connection systems are usually only used in emergency conditions, as their efficiency is low when compared to delta-delta (closed delta) systems (which are used during standard operations). electrical machines | signal phase transformer | three phase transformer | V V system | electrical machines| gate 2020 | gate 2021