What is an open delta(V-V) Connection?
The open delta or V-V connection is used to produce three-phase power using two single-phase transformers. This method is called the V-V connection, an open delta connection.
In the substation, three single-phase transformers are connected in star- star Configuration and supplied three-phase power to consumers. If one of the transformers from the three transformers is failed then three-phase power that can not be supplied to the consumer up to the failed transformer is repaired or replaced.
For eliminated this condiction, two single-phase transformers are connected in the Delta delta connection. If one transformer failed from three single-phase transformer, three-phase power is supplied to the consumer by maintaining the correct voltage and phase relation in the secondary of the transformer. The open delta connection is used in case of serious emergency only because the efficiency of the open delta connection is very poor as compared to the closed delta connection.
With an Open delta connection with two single-phase transformers capacity of the bank is reduced to 57.7% as compared to the closed delta connection.
How does V-V or Open delta connection Work?
If one transformer is failed from three single-phase transformers, the remaining two single-phase transformer connections become V-V or open delta connections.In a closed delta circuit, the load of each phase is equal and opposite to the sum of the voltages of the other two phases. If the transformer is in no-load condition and one single-phase transformer is removed from the back of the three-phase transformers, the other two transformers maintain the same line voltage on the secondary side of the two transformers.
Under the loaded conditions of the transformer and one single-phase transformer is removed from the three transformers, the voltages of the three phases become unequal because of the impedance drop in transformers.
For example to understand the open delta or V-V connections
If there are three transformers connected in the three-phase bank and supply a three-phase power to consumers. Each transformer is ratted 20Kva.All three transformers are connected in closed delta circuits.
The total rating of the three transformers is 60Kva. Now One transformer is failed from three single-phase transformers and two transformers supply three-phase power to a three-phase consumer in an open delta or V-V connection. Now the capacity of transformers becomes 40Kva.
Now Total capacity of transformer in open delta condition is 0.577% x 40Kva = 23.08Kva.
Application of Open delta or V-V connection
1. If one single-phase transformer is failed from the bank of three single-phase transformers, an open delta connection is used in emergency conditions for supply three-phase power to consumers.
2. If the primary or secondary winding of one of the transformers is failed closed delta circuit, then the V-V connection supplies three-phase power to the three-phase consumer without any interruption.