Open Delta Connection:
An open-delta connection is a method of providing a three-phase supply, using two single-phase transformers. It is particularly useful if , say, one single-phase transformer, part of a three single-phase transformers forming a three-phase transformer bank, becomes damaged -allowing the two remaining transformers to provide a temporary three-phase supply to the load.
The drawback with this connection is that the capacity of the transformer bank is reduced, and it can only provide a lower load current.
It should be noted that the output power of an open delta connection is only 87% of the rated power of the two transformers. For example, assume two transformers, each having a capacity of 25 kVA, are connected in an open delta connection. The total output power of this connection is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA).
Another figure given for this calculation is 58%. This percentage assumes a closed delta bank containing 3 transformers. If three 25 kVA transformers were connected to form a closed delta connection, the total output would be 75 kVA (3 x 25 = 75 kVA). If one of these transformers were removed and the transformer bank operated as an open delta connection, the output power would be reduced to 58% of its original capacity of 75 kVA. The output capacity of the open delta bank is 43.5 kVA (75 kVA x .58% = 43.5 kVA).
The voltage and current values of an open delta connection are computed in the same manner as a standard delta-delta connection when three transformers are employed. The voltage and current rules for a delta connection must be used when determining line and phase values of voltage current.
Closing a Delta:
When closing a delta system, connections should be checked for proper polarity before making the final connection and applying power. If the phase winding of one transformer is reversed, an extremely high current will flow when power is applied. Proper phasing can be checked with a voltmeter at delta opening. If power is applied to the transformer bank before the delta connection is closed, the voltmeter should indicate 0 volts. If one phase winding has been reversed, however, the voltmeter will indicate double the amount of voltage. It should be noted that a voltmeter is a high impedance device. It is not unusual for a voltmeter to indicate some amount of voltage before the delta is closed, especially if the primary has been connected as a wye and the secondary as a delta. When this is the case, the voltmeter will generally indicate close to the normal output voltage if the connection is correct and double the output voltage if the connection is incorrect
Calculation on how 58% arrived for open delta transformers:
There are two biggest advantages:
1) In an unearthed system like capacitor banks, it is possible to detect ground faults though earth path is not available.
2) In case of bulk power transmission where transformer banks are used, their advantage is that if one leg is out for maintenance/replacement than also approx. 57% power can be transmitted.