First, we will see two results:
1) For a particular line voltage, the voltage across phase (windings) of star-connected windings will be 1/√3 (~57.8%) times the line voltage, and the voltage across phase (windings) in delta-connected windings will be equal to the line voltage.
2) For a particular line current, the current through phase (windings) of delta-connected windings will be 1/√3 (~57.8%) times the line current, and the current through phase (windings) in star-connected windings will be equal to the line current.
From the first result, we can say that for the same line voltage level, the insulation level required in windings for star connection will be less therefore cost of insulation is reduced. So, we use star-connected windings on the HV side.
From the second result we can say that for the same line current, the current carrying capacity of windings for a delta connection will be less therefore cost of copper is reduced. So, we use delta-connected windings on the high current side i.e. the LV side.
Application of the above concepts:
At generating stations: we need to step up voltage for transmission, so we use delta on the generating or LV side and star on the transmission or HV side.
At the distribution station: we need to step down voltage but still here we use star on the LV side because at the consumer end, we need to have neutral available for single-phase loads. We use delta on the HV side here to provide a closed path for third harmonic currents in case of unbalanced loads.
In case we need continuity of service we use delta-delta transformers as the transformer can still run in case one phase goes out.
Advantages of Star & Delta Connection
- Delta–Star connection type three-phase transformer is used for both large and low voltage rating transformers.
- Delta–star transformer is used at the generator side to step up the voltage levels and Star – delta transformer is used at the load side of distribution systems to step down the voltage levels.
- Star–star connection transformers are used for small, high-voltage transformers.
- Delta–delta connection transformers are used for large, low-voltage transformers.