Is it used for protection or ground reference? A usual fixture in a switchboard is a GPT = ground protection transformer! Its composed of three transformers connected in Wye-Delta but a relay is inserted at one corner of the delta secondary, that relay only becomes energized when one of the phase in the primary side gets grounded (broken-delta)! Most relay settings are set 10% of the possible broken-delta voltage.Can someone please explain why one would want a ‘grounding transformer’? It appears it’s a Wye-Delta but nothing is hooked up on the secondary Delta side. ETA: in application it’s fed from a switchboard
One way to think how this wye-delta works is the following: The voltage across each winding of the delta secondary is proportional to the voltage across the corresponding winding on the wye primary. The delta has all of its windings in series to make a closed circuit loop, and so all of the winding voltages sum to zero via Kirchoff's voltage law. Now as long as the voltages on the wye are balanced then so are the delta voltages. With balanced 3-phase voltages the sum of all three voltages is zero, and so in this case the voltages across the delta windings would sum to zero even if the loop was broken. Therefore if the voltages are balanced then no current will flow even when the delta loop is closed. However, if there are loads that tend to unbalance the voltages on the wye, then as Jon mentioned this will induce a circulating current in the delta loop. This delta current will be reflected back as a common current that flows through all three of the windings of the wye and which compensates for the unbalanced load current, improving the balance of the primary voltages relative to the neutral. As a result this also creates a lower impedance for "zero sequence" load currents and ground faults than a wye without a delta would, as Jon mentioned above.Another use of a wye-delta transformer is to have a reference ground with the delta side just left unloaded. A ground connection is derived at the primary winding neutral, while the delta acts to stabilize the voltage due to any unbalanced loading on the primary side.