Figure 1: Open Delta Connection of Transformer.
In delta-delta connection of three 1-ϕ transformers, if one of the transformer is damaged and removed from the circuit and if the primaries are energized by 3-ϕ supply, then the 3-phase voltages obtained at the terminals of secondary will still remain balanced, ensuring continuous supply of 3-phase load with remaining two transformers. The connection thus obtained is known as open-delta (or) V-V connection. The total kVA rating of open delta connection is less than sum of the kVA rating of the remaining two transformers i.e., the total load that can be supplied by V-V bank is less than two-third of the capacity of delta-delta connection.
Open-delta connection of transformer is shown in figure 1. In this type of connection, the line current supplied to the load IL is equal to the phase current in each transformer (Iph). The 3-phase power rating for open delta connection is given by,
\[{{P}_{V.V}}=\sqrt{3}{{V}_{L}}{{I}_{L}}=\sqrt{3}{{V}_{L}}{{I}_{ph}}…(1)\]
Where,
VL = Line voltage across the load.
From delta-delta connection, the 3-phase power rating is given by,
\[{{P}_{\Delta .\Delta }}=\sqrt{3}{{V}_{L}}{{I}_{L}}=\sqrt{3}{{V}_{L}}\left( \sqrt{3}{{I}_{ph}} \right)=3{{V}_{L}}{{I}_{ph}}…(2)\]
Dividing equations (1) & (2), we get,
\[\frac{{{P}_{V.V}}}{{{P}_{\Delta .\Delta }}}=\frac{\sqrt{3}{{V}_{L}}{{I}_{ph}}}{3{{V}_{L}}{{I}_{ph}}}=\frac{\sqrt{3}}{3}=0.577\simeq 58%\]
Thus the power available in open-delta connection is only 58% of the power available in original delta-delta connection.
Advantages of V-V Connections
A V-V Connection can work as automatic standby for a Δ-Δ connection under the failure of any one winding of Δ-Δ connection.
- Power handling capacity of V-V connection is \(\frac{1}{\sqrt{3}}\) times of Δ-Δ connection.
- Hence for small load V-V connection can be used instead of Δ-Δ connection.
- If the future load increases, the open-delta can be closed to increase the rating.
- Though open delta connection leads to small imbalance in voltage yet it is generally, unnoticed by motor loads and other commercial loads.