Open-Delta Connection


Open Delta Connection

Open delta connection also known as the V-V connection. A three phase transformation of energy is possible using only two single phase transformers. This connection called open-delta or V-V connection.

When one phase of the three single phase transformers connected in delta-delta bank is unable to operate or damaged. Then supply to the load can be continued with the remaining two phase of transformers. This connection thus obtained is called Open Delta connection.

Open delta connection is that one connection, by using two single phase transformers we can provide a temporary three phase supply to the load. fig shows the three phase supply connected to the primaries, then on the secondary side, on no-load three equal voltage will be available.Untitled
This connection is most helpful in emergency conditions. When one of three single phase transformers connected in delta-delta bank will damage.
The total load carried by open delata connection is 57.7% of a detla delta connection.
When it connected in delta detla so VA rating is given by,
Sdd= √3 VL IL     Sdd= √3 VL (√3Iph)
Sdd= 3 VL Ip

When it connected in Open-Delta so VA raing is given by,
Sod = √3 VL IL
Sod = √3 VL Iph
Sod/Sdd = √3 VL Iph / 3 VL Iph
= 1 / √3
= 0.577

Therefore, load carried by the open delta is 57.7% od the original load carried bybthe delta delta bank.
If there are three single phase transformers are connected in delta delta connection. When one transformer become damage or removed other remaining two will overloaded. This overloaded of each transformer is given by,

Total load of open delat / VA rating of each xmer = √3 VL Iph/ VL Iph
= 1.73

This overload can be carried temporary if provision is made to reduce the load otherwise overheating and breakdown of the other two transformer would take place.
When three phase supply is given to the open delta connection. That both transformer operate on different power factor.
One is cos (30° – ø) and delivered power is KVA cos (30° – ø ).
Other is cos (30° + ø ) and delivered power is cos (30° + ø ).

It is employed :

  1. When the three-phase load is too small to warrant the installation of full three-phase transformer bank.
  2. When one of the transformers in a D – D bank is disabled, so that service is continued although at reduced capacity, till the faulty transformer is repaired or a new one is substituted.
  3. It is anticipated that in future the load will increase necessitating the closing of open delta.

Limitations :-

  1. Voltage become unbalanced at the secondary terminal.
  2. Remaining two transformers operate on different power factor.
  3. Power handling capacity of the transformer will be reduced. Reduction is approximately 15.5%

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