A delta-delta connection is where both the three phase primary and secondary windings are connected in delta. This connection proves to be economical for large low-voltage transformers because it increases the number of turns per phase.
According to Electrical Engineering Portal, the key points for this type of connection include the following: The primary side line voltage equals the secondary side line voltage, the primary side phase voltage equals the secondary side phase voltage, and there is no phase shift between primary and secondary voltages. The voltages on the primary and the secondary sides are shown on the phasor diagram. This type of connection is normally uncommon but is used in some industries to reduce the impact of SLG faults on the primary system. It is generally used in systems that carry large currents on low voltages, especially when continuity of service is to be maintained if one of the phases develops fault. There are some disadvantages of this type of connection: Due to the absence of a neutral point, it is not suitable for a three phase four wire system. More insulation is required, and the voltage appearing between the windings and the core will be equal to full line voltage in case of an earth fault on one phase.