What will happen if a single line to ground fault occurs in a delta side of a transformer?

Delta -Star Connection:

Star-connected winding or load:

(1) Line current is the same as phase current.

(2) Phase voltage is only 58% of line voltage. Therefore, star connection is advantageous in the case of generator or transformer high voltage windings because it reduces insulation cost as well as reduces space occupied by insulation in phase windings.

(3) There is 30 degrees phase shift between phase voltage and line voltage. Thus, there is 90-degree phase shift between Phase voltage & Neutral Voltage or Phase-to-phase Voltage.  This property can be utilized in many applications.

(4) If neutral is grounded, it provides a path for third harmonic currents as well as zero sequence currents.

(5) With no neutral wire, star load has to be balanced. Else, there will be unequal voltages in phases.

Delta connected winding or load:

(1) Line voltage is the same as phase voltage

(2) Phase current is only 58% of line current. Therefore, delta connection is advantageous in case of low voltage high current generators or low voltage winding of a transformer because the phase windings have to carry only 58% of the line current.

(3) There is 30 degrees phase shift between phase current and line current.

(4) Delta being a closed loop winding, it provides a path for circulation of third harmonic currents as well as zero sequence currents.

(5) In case of an unbalanced load, delta connection works better. This is because the phases continue to get equal voltages. At the generator end, open delta can be operated, if need be.

      If a single line-to-ground fault occurs in an electrical system fed from an ungrounded delta winding in a transformer, the ungrounded delta will in essence become a corner-grounded delta. No fault current will flow because the winding is not connected to the ground. If a second line-to-ground fault occurs on another phase, then there will be a phase-to-phase fault and fault current will flow. so, the value of the voltage of other lines will increase.