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An open Δ delta connection transformer uses two single-phase transformers to supply a three-phase load. Also known as a V-V system, this setup is typically used only in emergencies because it is less efficient than a closed Δ (Δ-Δ) system used in normal operations. In essence, one side of the secondary Δ is physically missing but a 3-phase measurement is still possible phase to phase.
This power configuration saves the consumer and utility company the cost of using one less transformer and therefore can be found at locations where the majority of the load uses single phase. As a result, an open Δ power system is not a true 3-phase balanced system and thus provides an unbalanced power supply.
Imagine you have three single-phase transformers, each rated at 10kVA. If they are connected in a Δ connection on both primary and secondary sides, this forms a closed Δ system.
How much 3ø balanced load can be supplied by this combination?
This combination can supply 30kVA, three-phase balanced load.
If one transformer is damaged and disconnected for repairs, the remaining system functions as an open Δ system with only two single-phase transformers.
Now, how much 3ø balanced load can be supplied by this combination?
Now we do have two single-phase transformers of 10kVA, but we are not able to supply 20kVA, 3ø balanced load.
This combination can supply a maximum of 17.32kVA three-phase balanced load (30kVA÷√3).
In this case, the voltages are as follows:
Between phases 240V;
Between split-phases and neutral 120V;
Between high leg (L3) and neutral 208V.
Flip on the SPST switches to turn on primary high voltage.
Switch between the voltmeters to see the different voltages.
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